compiler, runtime: stop using __go_runtime_error

[official-gcc.git] / libgo / go / runtime / proc.go

// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package runtime

import (
        "internal/cpu"
        "runtime/internal/atomic"
        "runtime/internal/sys"
        "unsafe"
)

// Functions called by C code.
//go:linkname main
//go:linkname goparkunlock
//go:linkname newextram
//go:linkname acquirep
//go:linkname releasep
//go:linkname incidlelocked
//go:linkname ginit
//go:linkname schedinit
//go:linkname ready
//go:linkname stopm
//go:linkname handoffp
//go:linkname wakep
//go:linkname stoplockedm
//go:linkname schedule
//go:linkname execute
//go:linkname goexit1
//go:linkname reentersyscall
//go:linkname reentersyscallblock
//go:linkname exitsyscall
//go:linkname gfget
//go:linkname kickoff
//go:linkname mstart1
//go:linkname mexit
//go:linkname globrunqput
//go:linkname pidleget

// Exported for test (see runtime/testdata/testprogcgo/dropm_stub.go).
//go:linkname getm

// Function called by misc/cgo/test.
//go:linkname lockedOSThread

// C functions for thread and context management.
func newosproc(*m)

//go:noescape
func malg(bool, bool, *unsafe.Pointer, *uintptr) *g

//go:noescape
func resetNewG(*g, *unsafe.Pointer, *uintptr)
func gogo(*g)
func setGContext()
func makeGContext(*g, unsafe.Pointer, uintptr)
func getTraceback(me, gp *g)
func gtraceback(*g)
func _cgo_notify_runtime_init_done()
func alreadyInCallers() bool
func stackfree(*g)

// Functions created by the compiler.
//extern __go_init_main
func main_init()

//extern main.main
func main_main()

var buildVersion = sys.TheVersion

// set using cmd/go/internal/modload.ModInfoProg
var modinfo string

// Goroutine scheduler
// The scheduler's job is to distribute ready-to-run goroutines over worker threads.
//
// The main concepts are:
// G - goroutine.
// M - worker thread, or machine.
// P - processor, a resource that is required to execute Go code.
//     M must have an associated P to execute Go code, however it can be
//     blocked or in a syscall w/o an associated P.
//
// Design doc at https://golang.org/s/go11sched.

// Worker thread parking/unparking.
// We need to balance between keeping enough running worker threads to utilize
// available hardware parallelism and parking excessive running worker threads
// to conserve CPU resources and power. This is not simple for two reasons:
// (1) scheduler state is intentionally distributed (in particular, per-P work
// queues), so it is not possible to compute global predicates on fast paths;
// (2) for optimal thread management we would need to know the future (don't park
// a worker thread when a new goroutine will be readied in near future).
//
// Three rejected approaches that would work badly:
// 1. Centralize all scheduler state (would inhibit scalability).
// 2. Direct goroutine handoff. That is, when we ready a new goroutine and there
//    is a spare P, unpark a thread and handoff it the thread and the goroutine.
//    This would lead to thread state thrashing, as the thread that readied the
//    goroutine can be out of work the very next moment, we will need to park it.
//    Also, it would destroy locality of computation as we want to preserve
//    dependent goroutines on the same thread; and introduce additional latency.
// 3. Unpark an additional thread whenever we ready a goroutine and there is an
//    idle P, but don't do handoff. This would lead to excessive thread parking/
//    unparking as the additional threads will instantly park without discovering
//    any work to do.
//
// The current approach:
// We unpark an additional thread when we ready a goroutine if (1) there is an
// idle P and there are no "spinning" worker threads. A worker thread is considered
// spinning if it is out of local work and did not find work in global run queue/
// netpoller; the spinning state is denoted in m.spinning and in sched.nmspinning.
// Threads unparked this way are also considered spinning; we don't do goroutine
// handoff so such threads are out of work initially. Spinning threads do some
// spinning looking for work in per-P run queues before parking. If a spinning
// thread finds work it takes itself out of the spinning state and proceeds to
// execution. If it does not find work it takes itself out of the spinning state
// and then parks.
// If there is at least one spinning thread (sched.nmspinning>1), we don't unpark
// new threads when readying goroutines. To compensate for that, if the last spinning
// thread finds work and stops spinning, it must unpark a new spinning thread.
// This approach smooths out unjustified spikes of thread unparking,
// but at the same time guarantees eventual maximal CPU parallelism utilization.
//
// The main implementation complication is that we need to be very careful during
// spinning->non-spinning thread transition. This transition can race with submission
// of a new goroutine, and either one part or another needs to unpark another worker
// thread. If they both fail to do that, we can end up with semi-persistent CPU
// underutilization. The general pattern for goroutine readying is: submit a goroutine
// to local work queue, #StoreLoad-style memory barrier, check sched.nmspinning.
// The general pattern for spinning->non-spinning transition is: decrement nmspinning,
// #StoreLoad-style memory barrier, check all per-P work queues for new work.
// Note that all this complexity does not apply to global run queue as we are not
// sloppy about thread unparking when submitting to global queue. Also see comments
// for nmspinning manipulation.

var (
        m0           m
        g0           g
        raceprocctx0 uintptr
)

// main_init_done is a signal used by cgocallbackg that initialization
// has been completed. It is made before _cgo_notify_runtime_init_done,
// so all cgo calls can rely on it existing. When main_init is complete,
// it is closed, meaning cgocallbackg can reliably receive from it.
var main_init_done chan bool

// mainStarted indicates that the main M has started.
var mainStarted bool

// runtimeInitTime is the nanotime() at which the runtime started.
var runtimeInitTime int64

// Value to use for signal mask for newly created M's.
var initSigmask sigset

// The main goroutine.
func main(unsafe.Pointer) {
        g := getg()

        // Max stack size is 1 GB on 64-bit, 250 MB on 32-bit.
        // Using decimal instead of binary GB and MB because
        // they look nicer in the stack overflow failure message.
        if sys.PtrSize == 8 {
                maxstacksize = 1000000000
        } else {
                maxstacksize = 250000000
        }

        // Allow newproc to start new Ms.
        mainStarted = true

        if GOARCH != "wasm" { // no threads on wasm yet, so no sysmon
                systemstack(func() {
                        newm(sysmon, nil)
                })
        }

        // Lock the main goroutine onto this, the main OS thread,
        // during initialization. Most programs won't care, but a few
        // do require certain calls to be made by the main thread.
        // Those can arrange for main.main to run in the main thread
        // by calling runtime.LockOSThread during initialization
        // to preserve the lock.
        lockOSThread()

        if g.m != &m0 {
                throw("runtime.main not on m0")
        }

        if nanotime() == 0 {
                throw("nanotime returning zero")
        }

        // Defer unlock so that runtime.Goexit during init does the unlock too.
        needUnlock := true
        defer func() {
                if needUnlock {
                        unlockOSThread()
                }
        }()

        // Record when the world started.
        runtimeInitTime = nanotime()

        main_init_done = make(chan bool)
        if iscgo {
                // Start the template thread in case we enter Go from
                // a C-created thread and need to create a new thread.
                startTemplateThread()
                _cgo_notify_runtime_init_done()
        }

        fn := main_init // make an indirect call, as the linker doesn't know the address of the main package when laying down the runtime
        fn()
        createGcRootsIndex()
        close(main_init_done)

        // For gccgo we have to wait until after main is initialized
        // to enable GC, because initializing main registers the GC roots.
        gcenable()

        needUnlock = false
        unlockOSThread()

        if isarchive || islibrary {
                // A program compiled with -buildmode=c-archive or c-shared
                // has a main, but it is not executed.
                return
        }
        fn = main_main // make an indirect call, as the linker doesn't know the address of the main package when laying down the runtime
        fn()
        if raceenabled {
                racefini()
        }

        // Make racy client program work: if panicking on
        // another goroutine at the same time as main returns,
        // let the other goroutine finish printing the panic trace.
        // Once it does, it will exit. See issues 3934 and 20018.
        if atomic.Load(&runningPanicDefers) != 0 {
                // Running deferred functions should not take long.
                for c := 0; c < 1000; c++ {
                        if atomic.Load(&runningPanicDefers) == 0 {
                                break
                        }
                        Gosched()
                }
        }
        if atomic.Load(&panicking) != 0 {
                gopark(nil, nil, waitReasonPanicWait, traceEvGoStop, 1)
        }

        exit(0)
        for {
                var x *int32
                *x = 0
        }
}

// os_beforeExit is called from os.Exit(0).
//go:linkname os_beforeExit os.runtime_beforeExit
func os_beforeExit() {
        if raceenabled {
                racefini()
        }
}

// start forcegc helper goroutine
func init() {
        expectSystemGoroutine()
        go forcegchelper()
}

func forcegchelper() {
        setSystemGoroutine()

        forcegc.g = getg()
        for {
                lock(&forcegc.lock)
                if forcegc.idle != 0 {
                        throw("forcegc: phase error")
                }
                atomic.Store(&forcegc.idle, 1)
                goparkunlock(&forcegc.lock, waitReasonForceGGIdle, traceEvGoBlock, 1)
                // this goroutine is explicitly resumed by sysmon
                if debug.gctrace > 0 {
                        println("GC forced")
                }
                // Time-triggered, fully concurrent.
                gcStart(gcTrigger{kind: gcTriggerTime, now: nanotime()})
        }
}

//go:nosplit

// Gosched yields the processor, allowing other goroutines to run. It does not
// suspend the current goroutine, so execution resumes automatically.
func Gosched() {
        checkTimeouts()
        mcall(gosched_m)
}

// goschedguarded yields the processor like gosched, but also checks
// for forbidden states and opts out of the yield in those cases.
//go:nosplit
func goschedguarded() {
        mcall(goschedguarded_m)
}

// Puts the current goroutine into a waiting state and calls unlockf.
// If unlockf returns false, the goroutine is resumed.
// unlockf must not access this G's stack, as it may be moved between
// the call to gopark and the call to unlockf.
// Reason explains why the goroutine has been parked.
// It is displayed in stack traces and heap dumps.
// Reasons should be unique and descriptive.
// Do not re-use reasons, add new ones.
func gopark(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason waitReason, traceEv byte, traceskip int) {
        if reason != waitReasonSleep {
                checkTimeouts() // timeouts may expire while two goroutines keep the scheduler busy
        }
        mp := acquirem()
        gp := mp.curg
        status := readgstatus(gp)
        if status != _Grunning && status != _Gscanrunning {
                throw("gopark: bad g status")
        }
        mp.waitlock = lock
        mp.waitunlockf = unlockf
        gp.waitreason = reason
        mp.waittraceev = traceEv
        mp.waittraceskip = traceskip
        releasem(mp)
        // can't do anything that might move the G between Ms here.
        mcall(park_m)
}

// Puts the current goroutine into a waiting state and unlocks the lock.
// The goroutine can be made runnable again by calling goready(gp).
func goparkunlock(lock *mutex, reason waitReason, traceEv byte, traceskip int) {
        gopark(parkunlock_c, unsafe.Pointer(lock), reason, traceEv, traceskip)
}

func goready(gp *g, traceskip int) {
        systemstack(func() {
                ready(gp, traceskip, true)
        })
}

//go:nosplit
func acquireSudog() *sudog {
        // Delicate dance: the semaphore implementation calls
        // acquireSudog, acquireSudog calls new(sudog),
        // new calls malloc, malloc can call the garbage collector,
        // and the garbage collector calls the semaphore implementation
        // in stopTheWorld.
        // Break the cycle by doing acquirem/releasem around new(sudog).
        // The acquirem/releasem increments m.locks during new(sudog),
        // which keeps the garbage collector from being invoked.
        mp := acquirem()
        pp := mp.p.ptr()
        if len(pp.sudogcache) == 0 {
                lock(&sched.sudoglock)
                // First, try to grab a batch from central cache.
                for len(pp.sudogcache) < cap(pp.sudogcache)/2 && sched.sudogcache != nil {
                        s := sched.sudogcache
                        sched.sudogcache = s.next
                        s.next = nil
                        pp.sudogcache = append(pp.sudogcache, s)
                }
                unlock(&sched.sudoglock)
                // If the central cache is empty, allocate a new one.
                if len(pp.sudogcache) == 0 {
                        pp.sudogcache = append(pp.sudogcache, new(sudog))
                }
        }
        n := len(pp.sudogcache)
        s := pp.sudogcache[n-1]
        pp.sudogcache[n-1] = nil
        pp.sudogcache = pp.sudogcache[:n-1]
        if s.elem != nil {
                throw("acquireSudog: found s.elem != nil in cache")
        }
        releasem(mp)
        return s
}

//go:nosplit
func releaseSudog(s *sudog) {
        if s.elem != nil {
                throw("runtime: sudog with non-nil elem")
        }
        if s.isSelect {
                throw("runtime: sudog with non-false isSelect")
        }
        if s.next != nil {
                throw("runtime: sudog with non-nil next")
        }
        if s.prev != nil {
                throw("runtime: sudog with non-nil prev")
        }
        if s.waitlink != nil {
                throw("runtime: sudog with non-nil waitlink")
        }
        if s.c != nil {
                throw("runtime: sudog with non-nil c")
        }
        gp := getg()
        if gp.param != nil {
                throw("runtime: releaseSudog with non-nil gp.param")
        }
        mp := acquirem() // avoid rescheduling to another P
        pp := mp.p.ptr()
        if len(pp.sudogcache) == cap(pp.sudogcache) {
                // Transfer half of local cache to the central cache.
                var first, last *sudog
                for len(pp.sudogcache) > cap(pp.sudogcache)/2 {
                        n := len(pp.sudogcache)
                        p := pp.sudogcache[n-1]
                        pp.sudogcache[n-1] = nil
                        pp.sudogcache = pp.sudogcache[:n-1]
                        if first == nil {
                                first = p
                        } else {
                                last.next = p
                        }
                        last = p
                }
                lock(&sched.sudoglock)
                last.next = sched.sudogcache
                sched.sudogcache = first
                unlock(&sched.sudoglock)
        }
        pp.sudogcache = append(pp.sudogcache, s)
        releasem(mp)
}

// funcPC returns the entry PC of the function f.
// It assumes that f is a func value. Otherwise the behavior is undefined.
// CAREFUL: In programs with plugins, funcPC can return different values
// for the same function (because there are actually multiple copies of
// the same function in the address space). To be safe, don't use the
// results of this function in any == expression. It is only safe to
// use the result as an address at which to start executing code.
//
// For gccgo note that this differs from the gc implementation; the gc
// implementation adds sys.PtrSize to the address of the interface
// value, but GCC's alias analysis decides that that can not be a
// reference to the second field of the interface, and in some cases
// it drops the initialization of the second field as a dead store.
//go:nosplit
func funcPC(f interface{}) uintptr {
        i := (*iface)(unsafe.Pointer(&f))
        r := *(*uintptr)(i.data)
        if cpu.FunctionDescriptors {
                // With PPC64 ELF ABI v1 function descriptors the
                // function address is a pointer to a struct whose
                // first field is the actual PC.
                r = *(*uintptr)(unsafe.Pointer(r))
        }
        return r
}

func lockedOSThread() bool {
        gp := getg()
        return gp.lockedm != 0 && gp.m.lockedg != 0
}

var (
        allgs    []*g
        allglock mutex
)

func allgadd(gp *g) {
        if readgstatus(gp) == _Gidle {
                throw("allgadd: bad status Gidle")
        }

        lock(&allglock)
        allgs = append(allgs, gp)
        allglen = uintptr(len(allgs))
        unlock(&allglock)
}

const (
        // Number of goroutine ids to grab from sched.goidgen to local per-P cache at once.
        // 16 seems to provide enough amortization, but other than that it's mostly arbitrary number.
        _GoidCacheBatch = 16
)

// cpuinit extracts the environment variable GODEBUG from the environment on
// Unix-like operating systems and calls internal/cpu.Initialize.
func cpuinit() {
        const prefix = "GODEBUG="
        var env string

        switch GOOS {
        case "aix", "darwin", "dragonfly", "freebsd", "netbsd", "openbsd", "illumos", "solaris", "linux":
                cpu.DebugOptions = true

                // Similar to goenv_unix but extracts the environment value for
                // GODEBUG directly.
                // TODO(moehrmann): remove when general goenvs() can be called before cpuinit()
                n := int32(0)
                for argv_index(argv, argc+1+n) != nil {
                        n++
                }

                for i := int32(0); i < n; i++ {
                        p := argv_index(argv, argc+1+i)
                        s := *(*string)(unsafe.Pointer(&stringStruct{unsafe.Pointer(p), findnull(p)}))

                        if hasPrefix(s, prefix) {
                                env = gostring(p)[len(prefix):]
                                break
                        }
                }
        }

        cpu.Initialize(env)
}

func ginit() {
        _m_ := &m0
        _g_ := &g0
        _m_.g0 = _g_
        _m_.curg = _g_
        _g_.m = _m_
        setg(_g_)
}

// The bootstrap sequence is:
//
//      call osinit
//      call schedinit
//      make & queue new G
//      call runtime·mstart
//
// The new G calls runtime·main.
func schedinit() {
        _g_ := getg()
        sched.maxmcount = 10000

        usestackmaps = probestackmaps()

        mallocinit()
        mcommoninit(_g_.m)
        cpuinit() // must run before alginit
        alginit() // maps must not be used before this call

        msigsave(_g_.m)
        initSigmask = _g_.m.sigmask

        goargs()
        goenvs()
        parsedebugvars()
        gcinit()

        sched.lastpoll = uint64(nanotime())
        procs := ncpu
        if n, ok := atoi32(gogetenv("GOMAXPROCS")); ok && n > 0 {
                procs = n
        }
        if procresize(procs) != nil {
                throw("unknown runnable goroutine during bootstrap")
        }

        // For cgocheck > 1, we turn on the write barrier at all times
        // and check all pointer writes. We can't do this until after
        // procresize because the write barrier needs a P.
        if debug.cgocheck > 1 {
                writeBarrier.cgo = true
                writeBarrier.enabled = true
                for _, p := range allp {
                        p.wbBuf.reset()
                }
        }

        if buildVersion == "" {
                // Condition should never trigger. This code just serves
                // to ensure runtime·buildVersion is kept in the resulting binary.
                buildVersion = "unknown"
        }
        if len(modinfo) == 1 {
                // Condition should never trigger. This code just serves
                // to ensure runtime·modinfo is kept in the resulting binary.
                modinfo = ""
        }
}

func dumpgstatus(gp *g) {
        _g_ := getg()
        print("runtime: gp: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n")
        print("runtime:  g:  g=", _g_, ", goid=", _g_.goid, ",  g->atomicstatus=", readgstatus(_g_), "\n")
}

func checkmcount() {
        // sched lock is held
        if mcount() > sched.maxmcount {
                print("runtime: program exceeds ", sched.maxmcount, "-thread limit\n")
                throw("thread exhaustion")
        }
}

func mcommoninit(mp *m) {
        _g_ := getg()

        // g0 stack won't make sense for user (and is not necessary unwindable).
        if _g_ != _g_.m.g0 {
                callers(1, mp.createstack[:])
        }

        lock(&sched.lock)
        if sched.mnext+1 < sched.mnext {
                throw("runtime: thread ID overflow")
        }
        mp.id = sched.mnext
        sched.mnext++
        checkmcount()

        mp.fastrand[0] = 1597334677 * uint32(mp.id)
        mp.fastrand[1] = uint32(cputicks())
        if mp.fastrand[0]|mp.fastrand[1] == 0 {
                mp.fastrand[1] = 1
        }

        mpreinit(mp)

        // Add to allm so garbage collector doesn't free g->m
        // when it is just in a register or thread-local storage.
        mp.alllink = allm

        // NumCgoCall() iterates over allm w/o schedlock,
        // so we need to publish it safely.
        atomicstorep(unsafe.Pointer(&allm), unsafe.Pointer(mp))
        unlock(&sched.lock)
}

// Mark gp ready to run.
func ready(gp *g, traceskip int, next bool) {
        if trace.enabled {
                traceGoUnpark(gp, traceskip)
        }

        status := readgstatus(gp)

        // Mark runnable.
        _g_ := getg()
        mp := acquirem() // disable preemption because it can be holding p in a local var
        if status&^_Gscan != _Gwaiting {
                dumpgstatus(gp)
                throw("bad g->status in ready")
        }

        // status is Gwaiting or Gscanwaiting, make Grunnable and put on runq
        casgstatus(gp, _Gwaiting, _Grunnable)
        runqput(_g_.m.p.ptr(), gp, next)
        if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {
                wakep()
        }
        releasem(mp)
}

// freezeStopWait is a large value that freezetheworld sets
// sched.stopwait to in order to request that all Gs permanently stop.
const freezeStopWait = 0x7fffffff

// freezing is set to non-zero if the runtime is trying to freeze the
// world.
var freezing uint32

// Similar to stopTheWorld but best-effort and can be called several times.
// There is no reverse operation, used during crashing.
// This function must not lock any mutexes.
func freezetheworld() {
        atomic.Store(&freezing, 1)
        // stopwait and preemption requests can be lost
        // due to races with concurrently executing threads,
        // so try several times
        for i := 0; i < 5; i++ {
                // this should tell the scheduler to not start any new goroutines
                sched.stopwait = freezeStopWait
                atomic.Store(&sched.gcwaiting, 1)
                // this should stop running goroutines
                if !preemptall() {
                        break // no running goroutines
                }
                usleep(1000)
        }
        // to be sure
        usleep(1000)
        preemptall()
        usleep(1000)
}

// All reads and writes of g's status go through readgstatus, casgstatus
// castogscanstatus, casfrom_Gscanstatus.
//go:nosplit
func readgstatus(gp *g) uint32 {
        return atomic.Load(&gp.atomicstatus)
}

// Ownership of gcscanvalid:
//
// If gp is running (meaning status == _Grunning or _Grunning|_Gscan),
// then gp owns gp.gcscanvalid, and other goroutines must not modify it.
//
// Otherwise, a second goroutine can lock the scan state by setting _Gscan
// in the status bit and then modify gcscanvalid, and then unlock the scan state.
//
// Note that the first condition implies an exception to the second:
// if a second goroutine changes gp's status to _Grunning|_Gscan,
// that second goroutine still does not have the right to modify gcscanvalid.

// The Gscanstatuses are acting like locks and this releases them.
// If it proves to be a performance hit we should be able to make these
// simple atomic stores but for now we are going to throw if
// we see an inconsistent state.
func casfrom_Gscanstatus(gp *g, oldval, newval uint32) {
        success := false

        // Check that transition is valid.
        switch oldval {
        default:
                print("runtime: casfrom_Gscanstatus bad oldval gp=", gp, ", oldval=", hex(oldval), ", newval=", hex(newval), "\n")
                dumpgstatus(gp)
                throw("casfrom_Gscanstatus:top gp->status is not in scan state")
        case _Gscanrunnable,
                _Gscanwaiting,
                _Gscanrunning,
                _Gscansyscall:
                if newval == oldval&^_Gscan {
                        success = atomic.Cas(&gp.atomicstatus, oldval, newval)
                }
        }
        if !success {
                print("runtime: casfrom_Gscanstatus failed gp=", gp, ", oldval=", hex(oldval), ", newval=", hex(newval), "\n")
                dumpgstatus(gp)
                throw("casfrom_Gscanstatus: gp->status is not in scan state")
        }
}

// This will return false if the gp is not in the expected status and the cas fails.
// This acts like a lock acquire while the casfromgstatus acts like a lock release.
func castogscanstatus(gp *g, oldval, newval uint32) bool {
        switch oldval {
        case _Grunnable,
                _Grunning,
                _Gwaiting,
                _Gsyscall:
                if newval == oldval|_Gscan {
                        return atomic.Cas(&gp.atomicstatus, oldval, newval)
                }
        }
        print("runtime: castogscanstatus oldval=", hex(oldval), " newval=", hex(newval), "\n")
        throw("castogscanstatus")
        panic("not reached")
}

// If asked to move to or from a Gscanstatus this will throw. Use the castogscanstatus
// and casfrom_Gscanstatus instead.
// casgstatus will loop if the g->atomicstatus is in a Gscan status until the routine that
// put it in the Gscan state is finished.
//go:nosplit
func casgstatus(gp *g, oldval, newval uint32) {
        if (oldval&_Gscan != 0) || (newval&_Gscan != 0) || oldval == newval {
                systemstack(func() {
                        print("runtime: casgstatus: oldval=", hex(oldval), " newval=", hex(newval), "\n")
                        throw("casgstatus: bad incoming values")
                })
        }

        if oldval == _Grunning && gp.gcscanvalid {
                // If oldvall == _Grunning, then the actual status must be
                // _Grunning or _Grunning|_Gscan; either way,
                // we own gp.gcscanvalid, so it's safe to read.
                // gp.gcscanvalid must not be true when we are running.
                systemstack(func() {
                        print("runtime: casgstatus ", hex(oldval), "->", hex(newval), " gp.status=", hex(gp.atomicstatus), " gp.gcscanvalid=true\n")
                        throw("casgstatus")
                })
        }

        // See https://golang.org/cl/21503 for justification of the yield delay.
        const yieldDelay = 5 * 1000
        var nextYield int64

        // loop if gp->atomicstatus is in a scan state giving
        // GC time to finish and change the state to oldval.
        for i := 0; !atomic.Cas(&gp.atomicstatus, oldval, newval); i++ {
                if oldval == _Gwaiting && gp.atomicstatus == _Grunnable {
                        throw("casgstatus: waiting for Gwaiting but is Grunnable")
                }
                // Help GC if needed.
                // if gp.preemptscan && !gp.gcworkdone && (oldval == _Grunning || oldval == _Gsyscall) {
                //      gp.preemptscan = false
                //      systemstack(func() {
                //              gcphasework(gp)
                //      })
                // }
                // But meanwhile just yield.
                if i == 0 {
                        nextYield = nanotime() + yieldDelay
                }
                if nanotime() < nextYield {
                        for x := 0; x < 10 && gp.atomicstatus != oldval; x++ {
                                procyield(1)
                        }
                } else {
                        osyield()
                        nextYield = nanotime() + yieldDelay/2
                }
        }
        if newval == _Grunning {
                gp.gcscanvalid = false
        }
}

// scang blocks until gp's stack has been scanned.
// It might be scanned by scang or it might be scanned by the goroutine itself.
// Either way, the stack scan has completed when scang returns.
func scang(gp *g, gcw *gcWork) {
        // Invariant; we (the caller, markroot for a specific goroutine) own gp.gcscandone.
        // Nothing is racing with us now, but gcscandone might be set to true left over
        // from an earlier round of stack scanning (we scan twice per GC).
        // We use gcscandone to record whether the scan has been done during this round.

        gp.gcscandone = false

        // See https://golang.org/cl/21503 for justification of the yield delay.
        const yieldDelay = 10 * 1000
        var nextYield int64

        // Endeavor to get gcscandone set to true,
        // either by doing the stack scan ourselves or by coercing gp to scan itself.
        // gp.gcscandone can transition from false to true when we're not looking
        // (if we asked for preemption), so any time we lock the status using
        // castogscanstatus we have to double-check that the scan is still not done.
loop:
        for i := 0; !gp.gcscandone; i++ {
                switch s := readgstatus(gp); s {
                default:
                        dumpgstatus(gp)
                        throw("stopg: invalid status")

                case _Gdead:
                        // No stack.
                        gp.gcscandone = true
                        break loop

                case _Gcopystack:
                // Stack being switched. Go around again.

                case _Gsyscall:
                        if usestackmaps {
                                // Claim goroutine by setting scan bit.
                                // Racing with execution or readying of gp.
                                // The scan bit keeps them from running
                                // the goroutine until we're done.
                                if castogscanstatus(gp, s, s|_Gscan) {
                                        if gp.scanningself {
                                                // Don't try to scan the stack
                                                // if the goroutine is going to do
                                                // it itself.
                                                // FIXME: can this happen?
                                                restartg(gp)
                                                break
                                        }
                                        if !gp.gcscandone {
                                                // Send a signal to let the goroutine scan
                                                // itself. This races with enter/exitsyscall.
                                                // If the goroutine is not stopped at a safepoint,
                                                // it will not scan the stack and we'll try again.
                                                mp := gp.m
                                                noteclear(&mp.scannote)
                                                gp.scangcw = uintptr(unsafe.Pointer(gcw))
                                                tgkill(getpid(), _pid_t(mp.procid), _SIGURG)

                                                // Wait for gp to scan its own stack.
                                                notesleep(&mp.scannote)

                                                if !gp.gcscandone {
                                                        // The signal delivered at a bad time.
                                                        // Try again.
                                                        restartg(gp)
                                                        break
                                                }
                                        }
                                        restartg(gp)
                                        break loop
                                }
                                break
                        }
                        fallthrough

                case _Grunnable, _Gwaiting:
                        // Claim goroutine by setting scan bit.
                        // Racing with execution or readying of gp.
                        // The scan bit keeps them from running
                        // the goroutine until we're done.
                        if castogscanstatus(gp, s, s|_Gscan) {
                                if gp.scanningself {
                                        // Don't try to scan the stack
                                        // if the goroutine is going to do
                                        // it itself.
                                        restartg(gp)
                                        break
                                }
                                if !gp.gcscandone {
                                        scanstack(gp, gcw)
                                        gp.gcscandone = true
                                }
                                restartg(gp)
                                break loop
                        }

                case _Gexitingsyscall:
                        // This is a transient state during which we should not scan its stack.
                        // Try again.

                case _Gscanwaiting:
                        // newstack is doing a scan for us right now. Wait.

                case _Gscanrunning:
                        // checkPreempt is scanning. Wait.

                case _Grunning:
                        // Goroutine running. Try to preempt execution so it can scan itself.
                        // The preemption handler (in newstack) does the actual scan.

                        // Optimization: if there is already a pending preemption request
                        // (from the previous loop iteration), don't bother with the atomics.
                        if gp.preemptscan && gp.preempt {
                                break
                        }

                        // Ask for preemption and self scan.
                        if castogscanstatus(gp, _Grunning, _Gscanrunning) {
                                if !gp.gcscandone {
                                        gp.preemptscan = true
                                        gp.preempt = true
                                }
                                casfrom_Gscanstatus(gp, _Gscanrunning, _Grunning)
                        }
                }

                if i == 0 {
                        nextYield = nanotime() + yieldDelay
                }
                if nanotime() < nextYield {
                        procyield(10)
                } else {
                        osyield()
                        nextYield = nanotime() + yieldDelay/2
                }
        }

        gp.preemptscan = false // cancel scan request if no longer needed
}

// The GC requests that this routine be moved from a scanmumble state to a mumble state.
func restartg(gp *g) {
        if gp.scang != 0 || gp.scangcw != 0 {
                print("g ", gp.goid, "is being scanned scang=", gp.scang, " scangcw=", gp.scangcw, "\n")
                throw("restartg: being scanned")
        }

        s := readgstatus(gp)
        switch s {
        default:
                dumpgstatus(gp)
                throw("restartg: unexpected status")

        case _Gdead:
        // ok

        case _Gscanrunnable,
                _Gscanwaiting,
                _Gscansyscall:
                casfrom_Gscanstatus(gp, s, s&^_Gscan)
        }
}

// stopTheWorld stops all P's from executing goroutines, interrupting
// all goroutines at GC safe points and records reason as the reason
// for the stop. On return, only the current goroutine's P is running.
// stopTheWorld must not be called from a system stack and the caller
// must not hold worldsema. The caller must call startTheWorld when
// other P's should resume execution.
//
// stopTheWorld is safe for multiple goroutines to call at the
// same time. Each will execute its own stop, and the stops will
// be serialized.
//
// This is also used by routines that do stack dumps. If the system is
// in panic or being exited, this may not reliably stop all
// goroutines.
func stopTheWorld(reason string) {
        semacquire(&worldsema)
        getg().m.preemptoff = reason
        systemstack(stopTheWorldWithSema)
}

// startTheWorld undoes the effects of stopTheWorld.
func startTheWorld() {
        systemstack(func() { startTheWorldWithSema(false) })
        // worldsema must be held over startTheWorldWithSema to ensure
        // gomaxprocs cannot change while worldsema is held.
        semrelease(&worldsema)
        getg().m.preemptoff = ""
}

// Holding worldsema grants an M the right to try to stop the world
// and prevents gomaxprocs from changing concurrently.
var worldsema uint32 = 1

// stopTheWorldWithSema is the core implementation of stopTheWorld.
// The caller is responsible for acquiring worldsema and disabling
// preemption first and then should stopTheWorldWithSema on the system
// stack:
//
//      semacquire(&worldsema, 0)
//      m.preemptoff = "reason"
//      systemstack(stopTheWorldWithSema)
//
// When finished, the caller must either call startTheWorld or undo
// these three operations separately:
//
//      m.preemptoff = ""
//      systemstack(startTheWorldWithSema)
//      semrelease(&worldsema)
//
// It is allowed to acquire worldsema once and then execute multiple
// startTheWorldWithSema/stopTheWorldWithSema pairs.
// Other P's are able to execute between successive calls to
// startTheWorldWithSema and stopTheWorldWithSema.
// Holding worldsema causes any other goroutines invoking
// stopTheWorld to block.
func stopTheWorldWithSema() {
        _g_ := getg()

        // If we hold a lock, then we won't be able to stop another M
        // that is blocked trying to acquire the lock.
        if _g_.m.locks > 0 {
                throw("stopTheWorld: holding locks")
        }

        lock(&sched.lock)
        sched.stopwait = gomaxprocs
        atomic.Store(&sched.gcwaiting, 1)
        preemptall()
        // stop current P
        _g_.m.p.ptr().status = _Pgcstop // Pgcstop is only diagnostic.
        sched.stopwait--
        // try to retake all P's in Psyscall status
        for _, p := range allp {
                s := p.status
                if s == _Psyscall && atomic.Cas(&p.status, s, _Pgcstop) {
                        if trace.enabled {
                                traceGoSysBlock(p)
                                traceProcStop(p)
                        }
                        p.syscalltick++
                        sched.stopwait--
                }
        }
        // stop idle P's
        for {
                p := pidleget()
                if p == nil {
                        break
                }
                p.status = _Pgcstop
                sched.stopwait--
        }
        wait := sched.stopwait > 0
        unlock(&sched.lock)

        // wait for remaining P's to stop voluntarily
        if wait {
                for {
                        // wait for 100us, then try to re-preempt in case of any races
                        if notetsleep(&sched.stopnote, 100*1000) {
                                noteclear(&sched.stopnote)
                                break
                        }
                        preemptall()
                }
        }

        // sanity checks
        bad := ""
        if sched.stopwait != 0 {
                bad = "stopTheWorld: not stopped (stopwait != 0)"
        } else {
                for _, p := range allp {
                        if p.status != _Pgcstop {
                                bad = "stopTheWorld: not stopped (status != _Pgcstop)"
                        }
                }
        }
        if atomic.Load(&freezing) != 0 {
                // Some other thread is panicking. This can cause the
                // sanity checks above to fail if the panic happens in
                // the signal handler on a stopped thread. Either way,
                // we should halt this thread.
                lock(&deadlock)
                lock(&deadlock)
        }
        if bad != "" {
                throw(bad)
        }
}

func startTheWorldWithSema(emitTraceEvent bool) int64 {
        mp := acquirem() // disable preemption because it can be holding p in a local var
        if netpollinited() {
                list := netpoll(false) // non-blocking
                injectglist(&list)
        }
        lock(&sched.lock)

        procs := gomaxprocs
        if newprocs != 0 {
                procs = newprocs
                newprocs = 0
        }
        p1 := procresize(procs)
        sched.gcwaiting = 0
        if sched.sysmonwait != 0 {
                sched.sysmonwait = 0
                notewakeup(&sched.sysmonnote)
        }
        unlock(&sched.lock)

        for p1 != nil {
                p := p1
                p1 = p1.link.ptr()
                if p.m != 0 {
                        mp := p.m.ptr()
                        p.m = 0
                        if mp.nextp != 0 {
                                throw("startTheWorld: inconsistent mp->nextp")
                        }
                        mp.nextp.set(p)
                        notewakeup(&mp.park)
                } else {
                        // Start M to run P.  Do not start another M below.
                        newm(nil, p)
                }
        }

        // Capture start-the-world time before doing clean-up tasks.
        startTime := nanotime()
        if emitTraceEvent {
                traceGCSTWDone()
        }

        // Wakeup an additional proc in case we have excessive runnable goroutines
        // in local queues or in the global queue. If we don't, the proc will park itself.
        // If we have lots of excessive work, resetspinning will unpark additional procs as necessary.
        if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {
                wakep()
        }

        releasem(mp)

        return startTime
}

// First function run by a new goroutine.
// This is passed to makecontext.
func kickoff() {
        gp := getg()

        if gp.traceback != 0 {
                gtraceback(gp)
        }

        fv := gp.entry
        param := gp.param

        // When running on the g0 stack we can wind up here without a p,
        // for example from mcall(exitsyscall0) in exitsyscall, in
        // which case we can not run a write barrier.
        // It is also possible for us to get here from the systemstack
        // call in wbBufFlush, at which point the write barrier buffer
        // is full and we can not run a write barrier.
        // Setting gp.entry = nil or gp.param = nil will try to run a
        // write barrier, so if we are on the g0 stack due to mcall
        // (systemstack calls mcall) then clear the field using uintptr.
        // This is OK when gp.param is gp.m.curg, as curg will be kept
        // alive elsewhere, and gp.entry always points into g, or
        // to a statically allocated value, or (in the case of mcall)
        // to the stack.
        if gp == gp.m.g0 && gp.param == unsafe.Pointer(gp.m.curg) {
                *(*uintptr)(unsafe.Pointer(&gp.entry)) = 0
                *(*uintptr)(unsafe.Pointer(&gp.param)) = 0
        } else if gp.m.p == 0 {
                throw("no p in kickoff")
        } else {
                gp.entry = nil
                gp.param = nil
        }

        // Record the entry SP to help stack scan.
        gp.entrysp = getsp()

        fv(param)
        goexit1()
}

func mstart1() {
        _g_ := getg()

        if _g_ != _g_.m.g0 {
                throw("bad runtime·mstart")
        }

        asminit()

        // Install signal handlers; after minit so that minit can
        // prepare the thread to be able to handle the signals.
        // For gccgo minit was called by C code.
        if _g_.m == &m0 {
                mstartm0()
        }

        if fn := _g_.m.mstartfn; fn != nil {
                fn()
        }

        if _g_.m != &m0 {
                acquirep(_g_.m.nextp.ptr())
                _g_.m.nextp = 0
        }
        schedule()
}

// mstartm0 implements part of mstart1 that only runs on the m0.
//
// Write barriers are allowed here because we know the GC can't be
// running yet, so they'll be no-ops.
//
//go:yeswritebarrierrec
func mstartm0() {
        // Create an extra M for callbacks on threads not created by Go.
        // An extra M is also needed on Windows for callbacks created by
        // syscall.NewCallback. See issue #6751 for details.
        if (iscgo || GOOS == "windows") && !cgoHasExtraM {
                cgoHasExtraM = true
                newextram()
        }
        initsig(false)
}

// mexit tears down and exits the current thread.
//
// Don't call this directly to exit the thread, since it must run at
// the top of the thread stack. Instead, use gogo(&_g_.m.g0.sched) to
// unwind the stack to the point that exits the thread.
//
// It is entered with m.p != nil, so write barriers are allowed. It
// will release the P before exiting.
//
//go:yeswritebarrierrec
func mexit(osStack bool) {
        g := getg()
        m := g.m

        if m == &m0 {
                // This is the main thread. Just wedge it.
                //
                // On Linux, exiting the main thread puts the process
                // into a non-waitable zombie state. On Plan 9,
                // exiting the main thread unblocks wait even though
                // other threads are still running. On Solaris we can
                // neither exitThread nor return from mstart. Other
                // bad things probably happen on other platforms.
                //
                // We could try to clean up this M more before wedging
                // it, but that complicates signal handling.
                handoffp(releasep())
                lock(&sched.lock)
                sched.nmfreed++
                checkdead()
                unlock(&sched.lock)
                notesleep(&m.park)
                throw("locked m0 woke up")
        }

        sigblock()
        unminit()

        // Free the gsignal stack.
        if m.gsignal != nil {
                stackfree(m.gsignal)
        }

        // Remove m from allm.
        lock(&sched.lock)
        for pprev := &allm; *pprev != nil; pprev = &(*pprev).alllink {
                if *pprev == m {
                        *pprev = m.alllink
                        goto found
                }
        }
        throw("m not found in allm")
found:
        if !osStack {
                // Delay reaping m until it's done with the stack.
                //
                // If this is using an OS stack, the OS will free it
                // so there's no need for reaping.
                atomic.Store(&m.freeWait, 1)
                // Put m on the free list, though it will not be reaped until
                // freeWait is 0. Note that the free list must not be linked
                // through alllink because some functions walk allm without
                // locking, so may be using alllink.
                m.freelink = sched.freem
                sched.freem = m
        }
        unlock(&sched.lock)

        // Release the P.
        handoffp(releasep())
        // After this point we must not have write barriers.

        // Invoke the deadlock detector. This must happen after
        // handoffp because it may have started a new M to take our
        // P's work.
        lock(&sched.lock)
        sched.nmfreed++
        checkdead()
        unlock(&sched.lock)

        if osStack {
                // Return from mstart and let the system thread
                // library free the g0 stack and terminate the thread.
                return
        }

        // mstart is the thread's entry point, so there's nothing to
        // return to. Exit the thread directly. exitThread will clear
        // m.freeWait when it's done with the stack and the m can be
        // reaped.
        exitThread(&m.freeWait)
}

// forEachP calls fn(p) for every P p when p reaches a GC safe point.
// If a P is currently executing code, this will bring the P to a GC
// safe point and execute fn on that P. If the P is not executing code
// (it is idle or in a syscall), this will call fn(p) directly while
// preventing the P from exiting its state. This does not ensure that
// fn will run on every CPU executing Go code, but it acts as a global
// memory barrier. GC uses this as a "ragged barrier."
//
// The caller must hold worldsema.
//
//go:systemstack
func forEachP(fn func(*p)) {
        mp := acquirem()
        _p_ := getg().m.p.ptr()

        lock(&sched.lock)
        if sched.safePointWait != 0 {
                throw("forEachP: sched.safePointWait != 0")
        }
        sched.safePointWait = gomaxprocs - 1
        sched.safePointFn = fn

        // Ask all Ps to run the safe point function.
        for _, p := range allp {
                if p != _p_ {
                        atomic.Store(&p.runSafePointFn, 1)
                }
        }
        preemptall()

        // Any P entering _Pidle or _Psyscall from now on will observe
        // p.runSafePointFn == 1 and will call runSafePointFn when
        // changing its status to _Pidle/_Psyscall.

        // Run safe point function for all idle Ps. sched.pidle will
        // not change because we hold sched.lock.
        for p := sched.pidle.ptr(); p != nil; p = p.link.ptr() {
                if atomic.Cas(&p.runSafePointFn, 1, 0) {
                        fn(p)
                        sched.safePointWait--
                }
        }

        wait := sched.safePointWait > 0
        unlock(&sched.lock)

        // Run fn for the current P.
        fn(_p_)

        // Force Ps currently in _Psyscall into _Pidle and hand them
        // off to induce safe point function execution.
        for _, p := range allp {
                s := p.status
                if s == _Psyscall && p.runSafePointFn == 1 && atomic.Cas(&p.status, s, _Pidle) {
                        if trace.enabled {
                                traceGoSysBlock(p)
                                traceProcStop(p)
                        }
                        p.syscalltick++
                        handoffp(p)
                }
        }

        // Wait for remaining Ps to run fn.
        if wait {
                for {
                        // Wait for 100us, then try to re-preempt in
                        // case of any races.
                        //
                        // Requires system stack.
                        if notetsleep(&sched.safePointNote, 100*1000) {
                                noteclear(&sched.safePointNote)
                                break
                        }
                        preemptall()
                }
        }
        if sched.safePointWait != 0 {
                throw("forEachP: not done")
        }
        for _, p := range allp {
                if p.runSafePointFn != 0 {
                        throw("forEachP: P did not run fn")
                }
        }

        lock(&sched.lock)
        sched.safePointFn = nil
        unlock(&sched.lock)
        releasem(mp)
}

// runSafePointFn runs the safe point function, if any, for this P.
// This should be called like
//
//     if getg().m.p.runSafePointFn != 0 {
//         runSafePointFn()
//     }
//
// runSafePointFn must be checked on any transition in to _Pidle or
// _Psyscall to avoid a race where forEachP sees that the P is running
// just before the P goes into _Pidle/_Psyscall and neither forEachP
// nor the P run the safe-point function.
func runSafePointFn() {
        p := getg().m.p.ptr()
        // Resolve the race between forEachP running the safe-point
        // function on this P's behalf and this P running the
        // safe-point function directly.
        if !atomic.Cas(&p.runSafePointFn, 1, 0) {
                return
        }
        sched.safePointFn(p)
        lock(&sched.lock)
        sched.safePointWait--
        if sched.safePointWait == 0 {
                notewakeup(&sched.safePointNote)
        }
        unlock(&sched.lock)
}

// Allocate a new m unassociated with any thread.
// Can use p for allocation context if needed.
// fn is recorded as the new m's m.mstartfn.
//
// This function is allowed to have write barriers even if the caller
// isn't because it borrows _p_.
//
//go:yeswritebarrierrec
func allocm(_p_ *p, fn func(), allocatestack bool) (mp *m, g0Stack unsafe.Pointer, g0StackSize uintptr) {
        _g_ := getg()
        acquirem() // disable GC because it can be called from sysmon
        if _g_.m.p == 0 {
                acquirep(_p_) // temporarily borrow p for mallocs in this function
        }

        // Release the free M list. We need to do this somewhere and
        // this may free up a stack we can use.
        if sched.freem != nil {
                lock(&sched.lock)
                var newList *m
                for freem := sched.freem; freem != nil; {
                        if freem.freeWait != 0 {
                                next := freem.freelink
                                freem.freelink = newList
                                newList = freem
                                freem = next
                                continue
                        }
                        stackfree(freem.g0)
                        freem = freem.freelink
                }
                sched.freem = newList
                unlock(&sched.lock)
        }

        mp = new(m)
        mp.mstartfn = fn
        mcommoninit(mp)

        mp.g0 = malg(allocatestack, false, &g0Stack, &g0StackSize)
        mp.g0.m = mp

        if _p_ == _g_.m.p.ptr() {
                releasep()
        }
        releasem(_g_.m)

        return mp, g0Stack, g0StackSize
}

// needm is called when a cgo callback happens on a
// thread without an m (a thread not created by Go).
// In this case, needm is expected to find an m to use
// and return with m, g initialized correctly.
// Since m and g are not set now (likely nil, but see below)
// needm is limited in what routines it can call. In particular
// it can only call nosplit functions (textflag 7) and cannot
// do any scheduling that requires an m.
//
// In order to avoid needing heavy lifting here, we adopt
// the following strategy: there is a stack of available m's
// that can be stolen. Using compare-and-swap
// to pop from the stack has ABA races, so we simulate
// a lock by doing an exchange (via Casuintptr) to steal the stack
// head and replace the top pointer with MLOCKED (1).
// This serves as a simple spin lock that we can use even
// without an m. The thread that locks the stack in this way
// unlocks the stack by storing a valid stack head pointer.
//
// In order to make sure that there is always an m structure
// available to be stolen, we maintain the invariant that there
// is always one more than needed. At the beginning of the
// program (if cgo is in use) the list is seeded with a single m.
// If needm finds that it has taken the last m off the list, its job
// is - once it has installed its own m so that it can do things like
// allocate memory - to create a spare m and put it on the list.
//
// Each of these extra m's also has a g0 and a curg that are
// pressed into service as the scheduling stack and current
// goroutine for the duration of the cgo callback.
//
// When the callback is done with the m, it calls dropm to
// put the m back on the list.
//go:nosplit
func needm(x byte) {
        if (iscgo || GOOS == "windows") && !cgoHasExtraM {
                // Can happen if C/C++ code calls Go from a global ctor.
                // Can also happen on Windows if a global ctor uses a
                // callback created by syscall.NewCallback. See issue #6751
                // for details.
                //
                // Can not throw, because scheduler is not initialized yet.
                write(2, unsafe.Pointer(&earlycgocallback[0]), int32(len(earlycgocallback)))
                exit(1)
        }

        // Lock extra list, take head, unlock popped list.
        // nilokay=false is safe here because of the invariant above,
        // that the extra list always contains or will soon contain
        // at least one m.
        mp := lockextra(false)

        // Set needextram when we've just emptied the list,
        // so that the eventual call into cgocallbackg will
        // allocate a new m for the extra list. We delay the
        // allocation until then so that it can be done
        // after exitsyscall makes sure it is okay to be
        // running at all (that is, there's no garbage collection
        // running right now).
        mp.needextram = mp.schedlink == 0
        extraMCount--
        unlockextra(mp.schedlink.ptr())

        // Save and block signals before installing g.
        // Once g is installed, any incoming signals will try to execute,
        // but we won't have the sigaltstack settings and other data
        // set up appropriately until the end of minit, which will
        // unblock the signals. This is the same dance as when
        // starting a new m to run Go code via newosproc.
        msigsave(mp)
        sigblock()

        // Install g (= m->curg).
        setg(mp.curg)

        // Initialize this thread to use the m.
        asminit()
        minit()

        setGContext()

        // mp.curg is now a real goroutine.
        casgstatus(mp.curg, _Gdead, _Gsyscall)
        atomic.Xadd(&sched.ngsys, -1)
}

var earlycgocallback = []byte("fatal error: cgo callback before cgo call\n")

// newextram allocates m's and puts them on the extra list.
// It is called with a working local m, so that it can do things
// like call schedlock and allocate.
func newextram() {
        c := atomic.Xchg(&extraMWaiters, 0)
        if c > 0 {
                for i := uint32(0); i < c; i++ {
                        oneNewExtraM()
                }
        } else {
                // Make sure there is at least one extra M.
                mp := lockextra(true)
                unlockextra(mp)
                if mp == nil {
                        oneNewExtraM()
                }
        }
}

// oneNewExtraM allocates an m and puts it on the extra list.
func oneNewExtraM() {
        // Create extra goroutine locked to extra m.
        // The goroutine is the context in which the cgo callback will run.
        // The sched.pc will never be returned to, but setting it to
        // goexit makes clear to the traceback routines where
        // the goroutine stack ends.
        mp, g0SP, g0SPSize := allocm(nil, nil, true)
        gp := malg(true, false, nil, nil)
        gp.gcscanvalid = true
        gp.gcscandone = true
        // malg returns status as _Gidle. Change to _Gdead before
        // adding to allg where GC can see it. We use _Gdead to hide
        // this from tracebacks and stack scans since it isn't a
        // "real" goroutine until needm grabs it.
        casgstatus(gp, _Gidle, _Gdead)
        gp.m = mp
        mp.curg = gp
        mp.lockedInt++
        mp.lockedg.set(gp)
        gp.lockedm.set(mp)
        gp.goid = int64(atomic.Xadd64(&sched.goidgen, 1))
        // put on allg for garbage collector
        allgadd(gp)

        // The context for gp will be set up in needm.
        // Here we need to set the context for g0.
        makeGContext(mp.g0, g0SP, g0SPSize)

        // gp is now on the allg list, but we don't want it to be
        // counted by gcount. It would be more "proper" to increment
        // sched.ngfree, but that requires locking. Incrementing ngsys
        // has the same effect.
        atomic.Xadd(&sched.ngsys, +1)

        // Add m to the extra list.
        mnext := lockextra(true)
        mp.schedlink.set(mnext)
        extraMCount++
        unlockextra(mp)
}

// dropm is called when a cgo callback has called needm but is now
// done with the callback and returning back into the non-Go thread.
// It puts the current m back onto the extra list.
//
// The main expense here is the call to signalstack to release the
// m's signal stack, and then the call to needm on the next callback
// from this thread. It is tempting to try to save the m for next time,
// which would eliminate both these costs, but there might not be
// a next time: the current thread (which Go does not control) might exit.
// If we saved the m for that thread, there would be an m leak each time
// such a thread exited. Instead, we acquire and release an m on each
// call. These should typically not be scheduling operations, just a few
// atomics, so the cost should be small.
//
// TODO(rsc): An alternative would be to allocate a dummy pthread per-thread
// variable using pthread_key_create. Unlike the pthread keys we already use
// on OS X, this dummy key would never be read by Go code. It would exist
// only so that we could register at thread-exit-time destructor.
// That destructor would put the m back onto the extra list.
// This is purely a performance optimization. The current version,
// in which dropm happens on each cgo call, is still correct too.
// We may have to keep the current version on systems with cgo
// but without pthreads, like Windows.
//
// CgocallBackDone calls this after releasing p, so no write barriers.
//go:nowritebarrierrec
func dropm() {
        // Clear m and g, and return m to the extra list.
        // After the call to setg we can only call nosplit functions
        // with no pointer manipulation.
        mp := getg().m

        // Return mp.curg to dead state.
        casgstatus(mp.curg, _Gsyscall, _Gdead)
        atomic.Xadd(&sched.ngsys, +1)

        // Block signals before unminit.
        // Unminit unregisters the signal handling stack (but needs g on some systems).
        // Setg(nil) clears g, which is the signal handler's cue not to run Go handlers.
        // It's important not to try to handle a signal between those two steps.
        sigmask := mp.sigmask
        sigblock()
        unminit()

        // gccgo sets the stack to Gdead here, because the splitstack
        // context is not initialized.
        atomic.Store(&mp.curg.atomicstatus, _Gdead)
        mp.curg.gcstack = 0
        mp.curg.gcnextsp = 0

        mnext := lockextra(true)
        extraMCount++
        mp.schedlink.set(mnext)

        setg(nil)

        // Commit the release of mp.
        unlockextra(mp)

        msigrestore(sigmask)
}

// A helper function for EnsureDropM.
func getm() uintptr {
        return uintptr(unsafe.Pointer(getg().m))
}

var extram uintptr
var extraMCount uint32 // Protected by lockextra
var extraMWaiters uint32

// lockextra locks the extra list and returns the list head.
// The caller must unlock the list by storing a new list head
// to extram. If nilokay is true, then lockextra will
// return a nil list head if that's what it finds. If nilokay is false,
// lockextra will keep waiting until the list head is no longer nil.
//go:nosplit
//go:nowritebarrierrec
func lockextra(nilokay bool) *m {
        const locked = 1

        incr := false
        for {
                old := atomic.Loaduintptr(&extram)
                if old == locked {
                        yield := osyield
                        yield()
                        continue
                }
                if old == 0 && !nilokay {
                        if !incr {
                                // Add 1 to the number of threads
                                // waiting for an M.
                                // This is cleared by newextram.
                                atomic.Xadd(&extraMWaiters, 1)
                                incr = true
                        }
                        usleep(1)
                        continue
                }
                if atomic.Casuintptr(&extram, old, locked) {
                        return (*m)(unsafe.Pointer(old))
                }
                yield := osyield
                yield()
                continue
        }
}

//go:nosplit
//go:nowritebarrierrec
func unlockextra(mp *m) {
        atomic.Storeuintptr(&extram, uintptr(unsafe.Pointer(mp)))
}

// execLock serializes exec and clone to avoid bugs or unspecified behaviour
// around exec'ing while creating/destroying threads.  See issue #19546.
var execLock rwmutex

// newmHandoff contains a list of m structures that need new OS threads.
// This is used by newm in situations where newm itself can't safely
// start an OS thread.
var newmHandoff struct {
        lock mutex

        // newm points to a list of M structures that need new OS
        // threads. The list is linked through m.schedlink.
        newm muintptr

        // waiting indicates that wake needs to be notified when an m
        // is put on the list.
        waiting bool
        wake    note

        // haveTemplateThread indicates that the templateThread has
        // been started. This is not protected by lock. Use cas to set
        // to 1.
        haveTemplateThread uint32
}

// Create a new m. It will start off with a call to fn, or else the scheduler.
// fn needs to be static and not a heap allocated closure.
// May run with m.p==nil, so write barriers are not allowed.
//go:nowritebarrierrec
func newm(fn func(), _p_ *p) {
        mp, _, _ := allocm(_p_, fn, false)
        mp.nextp.set(_p_)
        mp.sigmask = initSigmask
        if gp := getg(); gp != nil && gp.m != nil && (gp.m.lockedExt != 0 || gp.m.incgo) && GOOS != "plan9" {
                // We're on a locked M or a thread that may have been
                // started by C. The kernel state of this thread may
                // be strange (the user may have locked it for that
                // purpose). We don't want to clone that into another
                // thread. Instead, ask a known-good thread to create
                // the thread for us.
                //
                // This is disabled on Plan 9. See golang.org/issue/22227.
                //
                // TODO: This may be unnecessary on Windows, which
                // doesn't model thread creation off fork.
                lock(&newmHandoff.lock)
                if newmHandoff.haveTemplateThread == 0 {
                        throw("on a locked thread with no template thread")
                }
                mp.schedlink = newmHandoff.newm
                newmHandoff.newm.set(mp)
                if newmHandoff.waiting {
                        newmHandoff.waiting = false
                        notewakeup(&newmHandoff.wake)
                }
                unlock(&newmHandoff.lock)
                return
        }
        newm1(mp)
}

func newm1(mp *m) {
        execLock.rlock() // Prevent process clone.
        newosproc(mp)
        execLock.runlock()
}

// startTemplateThread starts the template thread if it is not already
// running.
//
// The calling thread must itself be in a known-good state.
func startTemplateThread() {
        if GOARCH == "wasm" { // no threads on wasm yet
                return
        }
        if !atomic.Cas(&newmHandoff.haveTemplateThread, 0, 1) {
                return
        }
        newm(templateThread, nil)
}

// templateThread is a thread in a known-good state that exists solely
// to start new threads in known-good states when the calling thread
// may not be in a good state.
//
// Many programs never need this, so templateThread is started lazily
// when we first enter a state that might lead to running on a thread
// in an unknown state.
//
// templateThread runs on an M without a P, so it must not have write
// barriers.
//
//go:nowritebarrierrec
func templateThread() {
        lock(&sched.lock)
        sched.nmsys++
        checkdead()
        unlock(&sched.lock)

        for {
                lock(&newmHandoff.lock)
                for newmHandoff.newm != 0 {
                        newm := newmHandoff.newm.ptr()
                        newmHandoff.newm = 0
                        unlock(&newmHandoff.lock)
                        for newm != nil {
                                next := newm.schedlink.ptr()
                                newm.schedlink = 0
                                newm1(newm)
                                newm = next
                        }
                        lock(&newmHandoff.lock)
                }
                newmHandoff.waiting = true
                noteclear(&newmHandoff.wake)
                unlock(&newmHandoff.lock)
                notesleep(&newmHandoff.wake)
        }
}

// Stops execution of the current m until new work is available.
// Returns with acquired P.
func stopm() {
        _g_ := getg()

        if _g_.m.locks != 0 {
                throw("stopm holding locks")
        }
        if _g_.m.p != 0 {
                throw("stopm holding p")
        }
        if _g_.m.spinning {
                throw("stopm spinning")
        }

        lock(&sched.lock)
        mput(_g_.m)
        unlock(&sched.lock)
        notesleep(&_g_.m.park)
        noteclear(&_g_.m.park)
        acquirep(_g_.m.nextp.ptr())
        _g_.m.nextp = 0
}

func mspinning() {
        // startm's caller incremented nmspinning. Set the new M's spinning.
        getg().m.spinning = true
}

// Schedules some M to run the p (creates an M if necessary).
// If p==nil, tries to get an idle P, if no idle P's does nothing.
// May run with m.p==nil, so write barriers are not allowed.
// If spinning is set, the caller has incremented nmspinning and startm will
// either decrement nmspinning or set m.spinning in the newly started M.
//go:nowritebarrierrec
func startm(_p_ *p, spinning bool) {
        lock(&sched.lock)
        if _p_ == nil {
                _p_ = pidleget()
                if _p_ == nil {
                        unlock(&sched.lock)
                        if spinning {
                                // The caller incremented nmspinning, but there are no idle Ps,
                                // so it's okay to just undo the increment and give up.
                                if int32(atomic.Xadd(&sched.nmspinning, -1)) < 0 {
                                        throw("startm: negative nmspinning")
                                }
                        }
                        return
                }
        }
        mp := mget()
        unlock(&sched.lock)
        if mp == nil {
                var fn func()
                if spinning {
                        // The caller incremented nmspinning, so set m.spinning in the new M.
                        fn = mspinning
                }
                newm(fn, _p_)
                return
        }
        if mp.spinning {
                throw("startm: m is spinning")
        }
        if mp.nextp != 0 {
                throw("startm: m has p")
        }
        if spinning && !runqempty(_p_) {
                throw("startm: p has runnable gs")
        }
        // The caller incremented nmspinning, so set m.spinning in the new M.
        mp.spinning = spinning
        mp.nextp.set(_p_)
        notewakeup(&mp.park)
}

// Hands off P from syscall or locked M.
// Always runs without a P, so write barriers are not allowed.
//go:nowritebarrierrec
func handoffp(_p_ *p) {
        // handoffp must start an M in any situation where
        // findrunnable would return a G to run on _p_.

        // if it has local work, start it straight away
        if !runqempty(_p_) || sched.runqsize != 0 {
                startm(_p_, false)
                return
        }
        // if it has GC work, start it straight away
        if gcBlackenEnabled != 0 && gcMarkWorkAvailable(_p_) {
                startm(_p_, false)
                return
        }
        // no local work, check that there are no spinning/idle M's,
        // otherwise our help is not required
        if atomic.Load(&sched.nmspinning)+atomic.Load(&sched.npidle) == 0 && atomic.Cas(&sched.nmspinning, 0, 1) { // TODO: fast atomic
                startm(_p_, true)
                return
        }
        lock(&sched.lock)
        if sched.gcwaiting != 0 {
                _p_.status = _Pgcstop
                sched.stopwait--
                if sched.stopwait == 0 {
                        notewakeup(&sched.stopnote)
                }
                unlock(&sched.lock)
                return
        }
        if _p_.runSafePointFn != 0 && atomic.Cas(&_p_.runSafePointFn, 1, 0) {
                sched.safePointFn(_p_)
                sched.safePointWait--
                if sched.safePointWait == 0 {
                        notewakeup(&sched.safePointNote)
                }
        }
        if sched.runqsize != 0 {
                unlock(&sched.lock)
                startm(_p_, false)
                return
        }
        // If this is the last running P and nobody is polling network,
        // need to wakeup another M to poll network.
        if sched.npidle == uint32(gomaxprocs-1) && atomic.Load64(&sched.lastpoll) != 0 {
                unlock(&sched.lock)
                startm(_p_, false)
                return
        }
        pidleput(_p_)
        unlock(&sched.lock)
}

// Tries to add one more P to execute G's.
// Called when a G is made runnable (newproc, ready).
func wakep() {
        // be conservative about spinning threads
        if !atomic.Cas(&sched.nmspinning, 0, 1) {
                return
        }
        startm(nil, true)
}

// Stops execution of the current m that is locked to a g until the g is runnable again.
// Returns with acquired P.
func stoplockedm() {
        _g_ := getg()

        if _g_.m.lockedg == 0 || _g_.m.lockedg.ptr().lockedm.ptr() != _g_.m {
                throw("stoplockedm: inconsistent locking")
        }
        if _g_.m.p != 0 {
                // Schedule another M to run this p.
                _p_ := releasep()
                handoffp(_p_)
        }
        incidlelocked(1)
        // Wait until another thread schedules lockedg again.
        notesleep(&_g_.m.park)
        noteclear(&_g_.m.park)
        status := readgstatus(_g_.m.lockedg.ptr())
        if status&^_Gscan != _Grunnable {
                print("runtime:stoplockedm: g is not Grunnable or Gscanrunnable\n")
                dumpgstatus(_g_)
                throw("stoplockedm: not runnable")
        }
        acquirep(_g_.m.nextp.ptr())
        _g_.m.nextp = 0
}

// Schedules the locked m to run the locked gp.
// May run during STW, so write barriers are not allowed.
//go:nowritebarrierrec
func startlockedm(gp *g) {
        _g_ := getg()

        mp := gp.lockedm.ptr()
        if mp == _g_.m {
                throw("startlockedm: locked to me")
        }
        if mp.nextp != 0 {
                throw("startlockedm: m has p")
        }
        // directly handoff current P to the locked m
        incidlelocked(-1)
        _p_ := releasep()
        mp.nextp.set(_p_)
        notewakeup(&mp.park)
        stopm()
}

// Stops the current m for stopTheWorld.
// Returns when the world is restarted.
func gcstopm() {
        _g_ := getg()

        if sched.gcwaiting == 0 {
                throw("gcstopm: not waiting for gc")
        }
        if _g_.m.spinning {
                _g_.m.spinning = false
                // OK to just drop nmspinning here,
                // startTheWorld will unpark threads as necessary.
                if int32(atomic.Xadd(&sched.nmspinning, -1)) < 0 {
                        throw("gcstopm: negative nmspinning")
                }
        }
        _p_ := releasep()
        lock(&sched.lock)
        _p_.status = _Pgcstop
        sched.stopwait--
        if sched.stopwait == 0 {
                notewakeup(&sched.stopnote)
        }
        unlock(&sched.lock)
        stopm()
}

// Schedules gp to run on the current M.
// If inheritTime is true, gp inherits the remaining time in the
// current time slice. Otherwise, it starts a new time slice.
// Never returns.
//
// Write barriers are allowed because this is called immediately after
// acquiring a P in several places.
//
//go:yeswritebarrierrec
func execute(gp *g, inheritTime bool) {
        _g_ := getg()

        casgstatus(gp, _Grunnable, _Grunning)
        gp.waitsince = 0
        gp.preempt = false
        if !inheritTime {
                _g_.m.p.ptr().schedtick++
        }
        _g_.m.curg = gp
        gp.m = _g_.m

        // Check whether the profiler needs to be turned on or off.
        hz := sched.profilehz
        if _g_.m.profilehz != hz {
                setThreadCPUProfiler(hz)
        }

        if trace.enabled {
                // GoSysExit has to happen when we have a P, but before GoStart.
                // So we emit it here.
                if gp.syscallsp != 0 && gp.sysblocktraced {
                        traceGoSysExit(gp.sysexitticks)
                }
                traceGoStart()
        }

        gogo(gp)
}

// Finds a runnable goroutine to execute.
// Tries to steal from other P's, get g from global queue, poll network.
func findrunnable() (gp *g, inheritTime bool) {
        _g_ := getg()

        // The conditions here and in handoffp must agree: if
        // findrunnable would return a G to run, handoffp must start
        // an M.

top:
        _p_ := _g_.m.p.ptr()
        if sched.gcwaiting != 0 {
                gcstopm()
                goto top
        }
        if _p_.runSafePointFn != 0 {
                runSafePointFn()
        }
        if fingwait && fingwake {
                if gp := wakefing(); gp != nil {
                        ready(gp, 0, true)
                }
        }
        if *cgo_yield != nil {
                asmcgocall(*cgo_yield, nil)
        }

        // local runq
        if gp, inheritTime := runqget(_p_); gp != nil {
                return gp, inheritTime
        }

        // global runq
        if sched.runqsize != 0 {
                lock(&sched.lock)
                gp := globrunqget(_p_, 0)
                unlock(&sched.lock)
                if gp != nil {
                        return gp, false
                }
        }

        // Poll network.
        // This netpoll is only an optimization before we resort to stealing.
        // We can safely skip it if there are no waiters or a thread is blocked
        // in netpoll already. If there is any kind of logical race with that
        // blocked thread (e.g. it has already returned from netpoll, but does
        // not set lastpoll yet), this thread will do blocking netpoll below
        // anyway.
        if netpollinited() && atomic.Load(&netpollWaiters) > 0 && atomic.Load64(&sched.lastpoll) != 0 {
                if list := netpoll(false); !list.empty() { // non-blocking
                        gp := list.pop()
                        injectglist(&list)
                        casgstatus(gp, _Gwaiting, _Grunnable)
                        if trace.enabled {
                                traceGoUnpark(gp, 0)
                        }
                        return gp, false
                }
        }

        // Steal work from other P's.
        procs := uint32(gomaxprocs)
        if atomic.Load(&sched.npidle) == procs-1 {
                // Either GOMAXPROCS=1 or everybody, except for us, is idle already.
                // New work can appear from returning syscall/cgocall, network or timers.
                // Neither of that submits to local run queues, so no point in stealing.
                goto stop
        }
        // If number of spinning M's >= number of busy P's, block.
        // This is necessary to prevent excessive CPU consumption
        // when GOMAXPROCS>>1 but the program parallelism is low.
        if !_g_.m.spinning && 2*atomic.Load(&sched.nmspinning) >= procs-atomic.Load(&sched.npidle) {
                goto stop
        }
        if !_g_.m.spinning {
                _g_.m.spinning = true
                atomic.Xadd(&sched.nmspinning, 1)
        }
        for i := 0; i < 4; i++ {
                for enum := stealOrder.start(fastrand()); !enum.done(); enum.next() {
                        if sched.gcwaiting != 0 {
                                goto top
                        }
                        stealRunNextG := i > 2 // first look for ready queues with more than 1 g
                        if gp := runqsteal(_p_, allp[enum.position()], stealRunNextG); gp != nil {
                                return gp, false
                        }
                }
        }

stop:

        // We have nothing to do. If we're in the GC mark phase, can
        // safely scan and blacken objects, and have work to do, run
        // idle-time marking rather than give up the P.
        if gcBlackenEnabled != 0 && _p_.gcBgMarkWorker != 0 && gcMarkWorkAvailable(_p_) {
                _p_.gcMarkWorkerMode = gcMarkWorkerIdleMode
                gp := _p_.gcBgMarkWorker.ptr()
                casgstatus(gp, _Gwaiting, _Grunnable)
                if trace.enabled {
                        traceGoUnpark(gp, 0)
                }
                return gp, false
        }

        // wasm only:
        // If a callback returned and no other goroutine is awake,
        // then pause execution until a callback was triggered.
        if beforeIdle() {
                // At least one goroutine got woken.
                goto top
        }

        // Before we drop our P, make a snapshot of the allp slice,
        // which can change underfoot once we no longer block
        // safe-points. We don't need to snapshot the contents because
        // everything up to cap(allp) is immutable.
        allpSnapshot := allp

        // return P and block
        lock(&sched.lock)
        if sched.gcwaiting != 0 || _p_.runSafePointFn != 0 {
                unlock(&sched.lock)
                goto top
        }
        if sched.runqsize != 0 {
                gp := globrunqget(_p_, 0)
                unlock(&sched.lock)
                return gp, false
        }
        if releasep() != _p_ {
                throw("findrunnable: wrong p")
        }
        pidleput(_p_)
        unlock(&sched.lock)

        // Delicate dance: thread transitions from spinning to non-spinning state,
        // potentially concurrently with submission of new goroutines. We must
        // drop nmspinning first and then check all per-P queues again (with
        // #StoreLoad memory barrier in between). If we do it the other way around,
        // another thread can submit a goroutine after we've checked all run queues
        // but before we drop nmspinning; as the result nobody will unpark a thread
        // to run the goroutine.
        // If we discover new work below, we need to restore m.spinning as a signal
        // for resetspinning to unpark a new worker thread (because there can be more
        // than one starving goroutine). However, if after discovering new work
        // we also observe no idle Ps, it is OK to just park the current thread:
        // the system is fully loaded so no spinning threads are required.
        // Also see "Worker thread parking/unparking" comment at the top of the file.
        wasSpinning := _g_.m.spinning
        if _g_.m.spinning {
                _g_.m.spinning = false
                if int32(atomic.Xadd(&sched.nmspinning, -1)) < 0 {
                        throw("findrunnable: negative nmspinning")
                }
        }

        // check all runqueues once again
        for _, _p_ := range allpSnapshot {
                if !runqempty(_p_) {
                        lock(&sched.lock)
                        _p_ = pidleget()
                        unlock(&sched.lock)
                        if _p_ != nil {
                                acquirep(_p_)
                                if wasSpinning {
                                        _g_.m.spinning = true
                                        atomic.Xadd(&sched.nmspinning, 1)
                                }
                                goto top
                        }
                        break
                }
        }

        // Check for idle-priority GC work again.
        if gcBlackenEnabled != 0 && gcMarkWorkAvailable(nil) {
                lock(&sched.lock)
                _p_ = pidleget()
                if _p_ != nil && _p_.gcBgMarkWorker == 0 {
                        pidleput(_p_)
                        _p_ = nil
                }
                unlock(&sched.lock)
                if _p_ != nil {
                        acquirep(_p_)
                        if wasSpinning {
                                _g_.m.spinning = true
                                atomic.Xadd(&sched.nmspinning, 1)
                        }
                        // Go back to idle GC check.
                        goto stop
                }
        }

        // poll network
        if netpollinited() && atomic.Load(&netpollWaiters) > 0 && atomic.Xchg64(&sched.lastpoll, 0) != 0 {
                if _g_.m.p != 0 {
                        throw("findrunnable: netpoll with p")
                }
                if _g_.m.spinning {
                        throw("findrunnable: netpoll with spinning")
                }
                list := netpoll(true) // block until new work is available
                atomic.Store64(&sched.lastpoll, uint64(nanotime()))
                if !list.empty() {
                        lock(&sched.lock)
                        _p_ = pidleget()
                        unlock(&sched.lock)
                        if _p_ != nil {
                                acquirep(_p_)
                                gp := list.pop()
                                injectglist(&list)
                                casgstatus(gp, _Gwaiting, _Grunnable)
                                if trace.enabled {
                                        traceGoUnpark(gp, 0)
                                }
                                return gp, false
                        }
                        injectglist(&list)
                }
        }
        stopm()
        goto top
}

// pollWork reports whether there is non-background work this P could
// be doing. This is a fairly lightweight check to be used for
// background work loops, like idle GC. It checks a subset of the
// conditions checked by the actual scheduler.
func pollWork() bool {
        if sched.runqsize != 0 {
                return true
        }
        p := getg().m.p.ptr()
        if !runqempty(p) {
                return true
        }
        if netpollinited() && atomic.Load(&netpollWaiters) > 0 && sched.lastpoll != 0 {
                if list := netpoll(false); !list.empty() {
                        injectglist(&list)
                        return true
                }
        }
        return false
}

func resetspinning() {
        _g_ := getg()
        if !_g_.m.spinning {
                throw("resetspinning: not a spinning m")
        }
        _g_.m.spinning = false
        nmspinning := atomic.Xadd(&sched.nmspinning, -1)
        if int32(nmspinning) < 0 {
                throw("findrunnable: negative nmspinning")
        }
        // M wakeup policy is deliberately somewhat conservative, so check if we
        // need to wakeup another P here. See "Worker thread parking/unparking"
        // comment at the top of the file for details.
        if nmspinning == 0 && atomic.Load(&sched.npidle) > 0 {
                wakep()
        }
}

// Injects the list of runnable G's into the scheduler and clears glist.
// Can run concurrently with GC.
func injectglist(glist *gList) {
        if glist.empty() {
                return
        }
        if trace.enabled {
                for gp := glist.head.ptr(); gp != nil; gp = gp.schedlink.ptr() {
                        traceGoUnpark(gp, 0)
                }
        }
        lock(&sched.lock)
        var n int
        for n = 0; !glist.empty(); n++ {
                gp := glist.pop()
                casgstatus(gp, _Gwaiting, _Grunnable)
                globrunqput(gp)
        }
        unlock(&sched.lock)
        for ; n != 0 && sched.npidle != 0; n-- {
                startm(nil, false)
        }
        *glist = gList{}
}

// One round of scheduler: find a runnable goroutine and execute it.
// Never returns.
func schedule() {
        _g_ := getg()

        if _g_.m.locks != 0 {
                throw("schedule: holding locks")
        }

        if _g_.m.lockedg != 0 {
                stoplockedm()
                execute(_g_.m.lockedg.ptr(), false) // Never returns.
        }

        // We should not schedule away from a g that is executing a cgo call,
        // since the cgo call is using the m's g0 stack.
        if _g_.m.incgo {
                throw("schedule: in cgo")
        }

top:
        if sched.gcwaiting != 0 {
                gcstopm()
                goto top
        }
        if _g_.m.p.ptr().runSafePointFn != 0 {
                runSafePointFn()
        }

        var gp *g
        var inheritTime bool

        // Normal goroutines will check for need to wakeP in ready,
        // but GCworkers and tracereaders will not, so the check must
        // be done here instead.
        tryWakeP := false
        if trace.enabled || trace.shutdown {
                gp = traceReader()
                if gp != nil {
                        casgstatus(gp, _Gwaiting, _Grunnable)
                        traceGoUnpark(gp, 0)
                        tryWakeP = true
                }
        }
        if gp == nil && gcBlackenEnabled != 0 {
                gp = gcController.findRunnableGCWorker(_g_.m.p.ptr())
                tryWakeP = tryWakeP || gp != nil
        }
        if gp == nil {
                // Check the global runnable queue once in a while to ensure fairness.
                // Otherwise two goroutines can completely occupy the local runqueue
                // by constantly respawning each other.
                if _g_.m.p.ptr().schedtick%61 == 0 && sched.runqsize > 0 {
                        lock(&sched.lock)
                        gp = globrunqget(_g_.m.p.ptr(), 1)
                        unlock(&sched.lock)
                }
        }
        if gp == nil {
                gp, inheritTime = runqget(_g_.m.p.ptr())
                if gp != nil && _g_.m.spinning {
                        throw("schedule: spinning with local work")
                }

                // Because gccgo does not implement preemption as a stack check,
                // we need to check for preemption here for fairness.
                // Otherwise goroutines on the local queue may starve
                // goroutines on the global queue.
                // Since we preempt by storing the goroutine on the global
                // queue, this is the only place we need to check preempt.
                // This does not call checkPreempt because gp is not running.
                if gp != nil && gp.preempt {
                        gp.preempt = false
                        lock(&sched.lock)
                        globrunqput(gp)
                        unlock(&sched.lock)
                        goto top
                }
        }
        if gp == nil {
                gp, inheritTime = findrunnable() // blocks until work is available
        }

        // This thread is going to run a goroutine and is not spinning anymore,
        // so if it was marked as spinning we need to reset it now and potentially
        // start a new spinning M.
        if _g_.m.spinning {
                resetspinning()
        }

        if sched.disable.user && !schedEnabled(gp) {
                // Scheduling of this goroutine is disabled. Put it on
                // the list of pending runnable goroutines for when we
                // re-enable user scheduling and look again.
                lock(&sched.lock)
                if schedEnabled(gp) {
                        // Something re-enabled scheduling while we
                        // were acquiring the lock.
                        unlock(&sched.lock)
                } else {
                        sched.disable.runnable.pushBack(gp)
                        sched.disable.n++
                        unlock(&sched.lock)
                        goto top
                }
        }

        // If about to schedule a not-normal goroutine (a GCworker or tracereader),
        // wake a P if there is one.
        if tryWakeP {
                if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {
                        wakep()
                }
        }
        if gp.lockedm != 0 {
                // Hands off own p to the locked m,
                // then blocks waiting for a new p.
                startlockedm(gp)
                goto top
        }

        execute(gp, inheritTime)
}

// dropg removes the association between m and the current goroutine m->curg (gp for short).
// Typically a caller sets gp's status away from Grunning and then
// immediately calls dropg to finish the job. The caller is also responsible
// for arranging that gp will be restarted using ready at an
// appropriate time. After calling dropg and arranging for gp to be
// readied later, the caller can do other work but eventually should
// call schedule to restart the scheduling of goroutines on this m.
func dropg() {
        _g_ := getg()

        setMNoWB(&_g_.m.curg.m, nil)
        setGNoWB(&_g_.m.curg, nil)
}

func parkunlock_c(gp *g, lock unsafe.Pointer) bool {
        unlock((*mutex)(lock))
        return true
}

// park continuation on g0.
func park_m(gp *g) {
        _g_ := getg()

        if trace.enabled {
                traceGoPark(_g_.m.waittraceev, _g_.m.waittraceskip)
        }

        dropg()
        casgstatus(gp, _Grunning, _Gwaiting)

        if fn := _g_.m.waitunlockf; fn != nil {
                ok := fn(gp, _g_.m.waitlock)
                _g_.m.waitunlockf = nil
                _g_.m.waitlock = nil
                if !ok {
                        if trace.enabled {
                                traceGoUnpark(gp, 2)
                        }
                        casgstatus(gp, _Gwaiting, _Grunnable)
                        execute(gp, true) // Schedule it back, never returns.
                }
        }
        schedule()
}

func goschedImpl(gp *g) {
        status := readgstatus(gp)
        if status&^_Gscan != _Grunning {
                dumpgstatus(gp)
                throw("bad g status")
        }
        dropg()
        casgstatus(gp, _Grunning, _Grunnable)
        lock(&sched.lock)
        globrunqput(gp)
        unlock(&sched.lock)

        schedule()
}

// Gosched continuation on g0.
func gosched_m(gp *g) {
        if trace.enabled {
                traceGoSched()
        }
        goschedImpl(gp)
}

// goschedguarded is a forbidden-states-avoided version of gosched_m
func goschedguarded_m(gp *g) {

        if gp.m.locks != 0 || gp.m.mallocing != 0 || gp.m.preemptoff != "" || gp.m.p.ptr().status != _Prunning {
                gogo(gp) // never return
        }

        if trace.enabled {
                traceGoSched()
        }
        goschedImpl(gp)
}

func gopreempt_m(gp *g) {
        if trace.enabled {
                traceGoPreempt()
        }
        goschedImpl(gp)
}

// Finishes execution of the current goroutine.
func goexit1() {
        if trace.enabled {
                traceGoEnd()
        }
        mcall(goexit0)
}

// goexit continuation on g0.
func goexit0(gp *g) {
        _g_ := getg()

        casgstatus(gp, _Grunning, _Gdead)
        if isSystemGoroutine(gp, false) {
                atomic.Xadd(&sched.ngsys, -1)
                gp.isSystemGoroutine = false
        }
        gp.m = nil
        locked := gp.lockedm != 0
        gp.lockedm = 0
        _g_.m.lockedg = 0
        gp.entry = nil
        gp.paniconfault = false
        gp._defer = nil // should be true already but just in case.
        gp._panic = nil // non-nil for Goexit during panic. points at stack-allocated data.
        gp.writebuf = nil
        gp.waitreason = 0
        gp.param = nil
        gp.labels = nil
        gp.timer = nil

        if gcBlackenEnabled != 0 && gp.gcAssistBytes > 0 {
                // Flush assist credit to the global pool. This gives
                // better information to pacing if the application is
                // rapidly creating an exiting goroutines.
                scanCredit := int64(gcController.assistWorkPerByte * float64(gp.gcAssistBytes))
                atomic.Xaddint64(&gcController.bgScanCredit, scanCredit)
                gp.gcAssistBytes = 0
        }

        // Note that gp's stack scan is now "valid" because it has no
        // stack.
        gp.gcscanvalid = true
        dropg()

        if GOARCH == "wasm" { // no threads yet on wasm
                gfput(_g_.m.p.ptr(), gp)
                schedule() // never returns
        }

        if _g_.m.lockedInt != 0 {
                print("invalid m->lockedInt = ", _g_.m.lockedInt, "\n")
                throw("internal lockOSThread error")
        }
        gfput(_g_.m.p.ptr(), gp)
        if locked {
                // The goroutine may have locked this thread because
                // it put it in an unusual kernel state. Kill it
                // rather than returning it to the thread pool.

                // Return to mstart, which will release the P and exit
                // the thread.
                if GOOS != "plan9" { // See golang.org/issue/22227.
                        _g_.m.exiting = true
                        gogo(_g_.m.g0)
                } else {
                        // Clear lockedExt on plan9 since we may end up re-using
                        // this thread.
                        _g_.m.lockedExt = 0
                }
        }
        schedule()
}

// The goroutine g is about to enter a system call.
// Record that it's not using the cpu anymore.
// This is called only from the go syscall library and cgocall,
// not from the low-level system calls used by the runtime.
//
// The entersyscall function is written in C, so that it can save the
// current register context so that the GC will see them.
// It calls reentersyscall.
//
// Syscall tracing:
// At the start of a syscall we emit traceGoSysCall to capture the stack trace.
// If the syscall does not block, that is it, we do not emit any other events.
// If the syscall blocks (that is, P is retaken), retaker emits traceGoSysBlock;
// when syscall returns we emit traceGoSysExit and when the goroutine starts running
// (potentially instantly, if exitsyscallfast returns true) we emit traceGoStart.
// To ensure that traceGoSysExit is emitted strictly after traceGoSysBlock,
// we remember current value of syscalltick in m (_g_.m.syscalltick = _g_.m.p.ptr().syscalltick),
// whoever emits traceGoSysBlock increments p.syscalltick afterwards;
// and we wait for the increment before emitting traceGoSysExit.
// Note that the increment is done even if tracing is not enabled,
// because tracing can be enabled in the middle of syscall. We don't want the wait to hang.
//
//go:nosplit
//go:noinline
func reentersyscall(pc, sp uintptr) {
        _g_ := getg()

        // Disable preemption because during this function g is in Gsyscall status,
        // but can have inconsistent g->sched, do not let GC observe it.
        _g_.m.locks++

        _g_.syscallsp = sp
        _g_.syscallpc = pc
        casgstatus(_g_, _Grunning, _Gsyscall)

        if trace.enabled {
                systemstack(traceGoSysCall)
        }

        if atomic.Load(&sched.sysmonwait) != 0 {
                systemstack(entersyscall_sysmon)
        }

        if _g_.m.p.ptr().runSafePointFn != 0 {
                // runSafePointFn may stack split if run on this stack
                systemstack(runSafePointFn)
        }

        _g_.m.syscalltick = _g_.m.p.ptr().syscalltick
        _g_.sysblocktraced = true
        _g_.m.mcache = nil
        pp := _g_.m.p.ptr()
        pp.m = 0
        _g_.m.oldp.set(pp)
        _g_.m.p = 0
        atomic.Store(&pp.status, _Psyscall)
        if sched.gcwaiting != 0 {
                systemstack(entersyscall_gcwait)
        }

        _g_.m.locks--
}

func entersyscall_sysmon() {
        lock(&sched.lock)
        if atomic.Load(&sched.sysmonwait) != 0 {
                atomic.Store(&sched.sysmonwait, 0)
                notewakeup(&sched.sysmonnote)
        }
        unlock(&sched.lock)
}

func entersyscall_gcwait() {
        _g_ := getg()
        _p_ := _g_.m.oldp.ptr()

        lock(&sched.lock)
        if sched.stopwait > 0 && atomic.Cas(&_p_.status, _Psyscall, _Pgcstop) {
                if trace.enabled {
                        traceGoSysBlock(_p_)
                        traceProcStop(_p_)
                }
                _p_.syscalltick++
                if sched.stopwait--; sched.stopwait == 0 {
                        notewakeup(&sched.stopnote)
                }
        }
        unlock(&sched.lock)
}

func reentersyscallblock(pc, sp uintptr) {
        _g_ := getg()

        _g_.m.locks++ // see comment in entersyscall
        _g_.throwsplit = true
        _g_.m.syscalltick = _g_.m.p.ptr().syscalltick
        _g_.sysblocktraced = true
        _g_.m.p.ptr().syscalltick++

        // Leave SP around for GC and traceback.
        _g_.syscallsp = sp
        _g_.syscallpc = pc
        casgstatus(_g_, _Grunning, _Gsyscall)
        systemstack(entersyscallblock_handoff)

        _g_.m.locks--
}

func entersyscallblock_handoff() {
        if trace.enabled {
                traceGoSysCall()
                traceGoSysBlock(getg().m.p.ptr())
        }
        handoffp(releasep())
}

// The goroutine g exited its system call.
// Arrange for it to run on a cpu again.
// This is called only from the go syscall library, not
// from the low-level system calls used by the runtime.
//
// Write barriers are not allowed because our P may have been stolen.
//
//go:nosplit
//go:nowritebarrierrec
func exitsyscall() {
        _g_ := getg()

        _g_.m.locks++ // see comment in entersyscall

        _g_.waitsince = 0
        oldp := _g_.m.oldp.ptr()
        _g_.m.oldp = 0
        if exitsyscallfast(oldp) {
                if _g_.m.mcache == nil {
                        throw("lost mcache")
                }
                if trace.enabled {
                        if oldp != _g_.m.p.ptr() || _g_.m.syscalltick != _g_.m.p.ptr().syscalltick {
                                systemstack(traceGoStart)
                        }
                }
                // There's a cpu for us, so we can run.
                _g_.m.p.ptr().syscalltick++
                // We need to cas the status and scan before resuming...
                casgstatus(_g_, _Gsyscall, _Grunning)

                exitsyscallclear(_g_)
                _g_.m.locks--
                _g_.throwsplit = false

                // Check preemption, since unlike gc we don't check on
                // every call.
                if getg().preempt {
                        checkPreempt()
                }
                _g_.throwsplit = false

                if sched.disable.user && !schedEnabled(_g_) {
                        // Scheduling of this goroutine is disabled.
                        Gosched()
                }

                return
        }

        _g_.sysexitticks = 0
        if trace.enabled {
                // Wait till traceGoSysBlock event is emitted.
                // This ensures consistency of the trace (the goroutine is started after it is blocked).
                for oldp != nil && oldp.syscalltick == _g_.m.syscalltick {
                        osyield()
                }
                // We can't trace syscall exit right now because we don't have a P.
                // Tracing code can invoke write barriers that cannot run without a P.
                // So instead we remember the syscall exit time and emit the event
                // in execute when we have a P.
                _g_.sysexitticks = cputicks()
        }

        _g_.m.locks--

        // Call the scheduler.
        mcall(exitsyscall0)

        if _g_.m.mcache == nil {
                throw("lost mcache")
        }

        // Scheduler returned, so we're allowed to run now.
        // Delete the syscallsp information that we left for
        // the garbage collector during the system call.
        // Must wait until now because until gosched returns
        // we don't know for sure that the garbage collector
        // is not running.
        exitsyscallclear(_g_)

        _g_.m.p.ptr().syscalltick++
        _g_.throwsplit = false
}

//go:nosplit
func exitsyscallfast(oldp *p) bool {
        _g_ := getg()

        // Freezetheworld sets stopwait but does not retake P's.
        if sched.stopwait == freezeStopWait {
                return false
        }

        // Try to re-acquire the last P.
        if oldp != nil && oldp.status == _Psyscall && atomic.Cas(&oldp.status, _Psyscall, _Pidle) {
                // There's a cpu for us, so we can run.
                wirep(oldp)
                exitsyscallfast_reacquired()
                return true
        }

        // Try to get any other idle P.
        if sched.pidle != 0 {
                var ok bool
                systemstack(func() {
                        ok = exitsyscallfast_pidle()
                        if ok && trace.enabled {
                                if oldp != nil {
                                        // Wait till traceGoSysBlock event is emitted.
                                        // This ensures consistency of the trace (the goroutine is started after it is blocked).
                                        for oldp.syscalltick == _g_.m.syscalltick {
                                                osyield()
                                        }
                                }
                                traceGoSysExit(0)
                        }
                })
                if ok {
                        return true
                }
        }
        return false
}

// exitsyscallfast_reacquired is the exitsyscall path on which this G
// has successfully reacquired the P it was running on before the
// syscall.
//
//go:nosplit
func exitsyscallfast_reacquired() {
        _g_ := getg()
        if _g_.m.syscalltick != _g_.m.p.ptr().syscalltick {
                if trace.enabled {
                        // The p was retaken and then enter into syscall again (since _g_.m.syscalltick has changed).
                        // traceGoSysBlock for this syscall was already emitted,
                        // but here we effectively retake the p from the new syscall running on the same p.
                        systemstack(func() {
                                // Denote blocking of the new syscall.
                                traceGoSysBlock(_g_.m.p.ptr())
                                // Denote completion of the current syscall.
                                traceGoSysExit(0)
                        })
                }
                _g_.m.p.ptr().syscalltick++
        }
}

func exitsyscallfast_pidle() bool {
        lock(&sched.lock)
        _p_ := pidleget()
        if _p_ != nil && atomic.Load(&sched.sysmonwait) != 0 {
                atomic.Store(&sched.sysmonwait, 0)
                notewakeup(&sched.sysmonnote)
        }
        unlock(&sched.lock)
        if _p_ != nil {
                acquirep(_p_)
                return true
        }
        return false
}

// exitsyscall slow path on g0.
// Failed to acquire P, enqueue gp as runnable.
//
//go:nowritebarrierrec
func exitsyscall0(gp *g) {
        _g_ := getg()

        casgstatus(gp, _Gsyscall, _Gexitingsyscall)
        dropg()
        casgstatus(gp, _Gexitingsyscall, _Grunnable)
        lock(&sched.lock)
        var _p_ *p
        if schedEnabled(_g_) {
                _p_ = pidleget()
        }
        if _p_ == nil {
                globrunqput(gp)
        } else if atomic.Load(&sched.sysmonwait) != 0 {
                atomic.Store(&sched.sysmonwait, 0)
                notewakeup(&sched.sysmonnote)
        }
        unlock(&sched.lock)
        if _p_ != nil {
                acquirep(_p_)
                execute(gp, false) // Never returns.
        }
        if _g_.m.lockedg != 0 {
                // Wait until another thread schedules gp and so m again.
                stoplockedm()
                execute(gp, false) // Never returns.
        }
        stopm()
        schedule() // Never returns.
}

// exitsyscallclear clears GC-related information that we only track
// during a syscall.
func exitsyscallclear(gp *g) {
        // Garbage collector isn't running (since we are), so okay to
        // clear syscallsp.
        gp.syscallsp = 0

        gp.gcstack = 0
        gp.gcnextsp = 0
        memclrNoHeapPointers(unsafe.Pointer(&gp.gcregs), unsafe.Sizeof(gp.gcregs))
}

// Code generated by cgo, and some library code, calls syscall.Entersyscall
// and syscall.Exitsyscall.

//go:linkname syscall_entersyscall syscall.Entersyscall
//go:nosplit
func syscall_entersyscall() {
        entersyscall()
}

//go:linkname syscall_exitsyscall syscall.Exitsyscall
//go:nosplit
func syscall_exitsyscall() {
        exitsyscall()
}

func beforefork() {
        gp := getg().m.curg

        // Block signals during a fork, so that the child does not run
        // a signal handler before exec if a signal is sent to the process
        // group. See issue #18600.
        gp.m.locks++
        msigsave(gp.m)
        sigblock()
}

// Called from syscall package before fork.
//go:linkname syscall_runtime_BeforeFork syscall.runtime_BeforeFork
//go:nosplit
func syscall_runtime_BeforeFork() {
        systemstack(beforefork)
}

func afterfork() {
        gp := getg().m.curg

        msigrestore(gp.m.sigmask)

        gp.m.locks--
}

// Called from syscall package after fork in parent.
//go:linkname syscall_runtime_AfterFork syscall.runtime_AfterFork
//go:nosplit
func syscall_runtime_AfterFork() {
        systemstack(afterfork)
}

// inForkedChild is true while manipulating signals in the child process.
// This is used to avoid calling libc functions in case we are using vfork.
var inForkedChild bool

// Called from syscall package after fork in child.
// It resets non-sigignored signals to the default handler, and
// restores the signal mask in preparation for the exec.
//
// Because this might be called during a vfork, and therefore may be
// temporarily sharing address space with the parent process, this must
// not change any global variables or calling into C code that may do so.
//
//go:linkname syscall_runtime_AfterForkInChild syscall.runtime_AfterForkInChild
//go:nosplit
//go:nowritebarrierrec
func syscall_runtime_AfterForkInChild() {
        // It's OK to change the global variable inForkedChild here
        // because we are going to change it back. There is no race here,
        // because if we are sharing address space with the parent process,
        // then the parent process can not be running concurrently.
        inForkedChild = true

        clearSignalHandlers()

        // When we are the child we are the only thread running,
        // so we know that nothing else has changed gp.m.sigmask.
        msigrestore(getg().m.sigmask)

        inForkedChild = false
}

// Called from syscall package before Exec.
//go:linkname syscall_runtime_BeforeExec syscall.runtime_BeforeExec
func syscall_runtime_BeforeExec() {
        // Prevent thread creation during exec.
        execLock.lock()
}

// Called from syscall package after Exec.
//go:linkname syscall_runtime_AfterExec syscall.runtime_AfterExec
func syscall_runtime_AfterExec() {
        execLock.unlock()
}

// panicgonil is used for gccgo as we need to use a compiler check for
// a nil func, in case we have to build a thunk.
//go:linkname panicgonil
func panicgonil() {
        getg().m.throwing = -1 // do not dump full stacks
        throw("go of nil func value")
}

// Create a new g running fn passing arg as the single argument.
// Put it on the queue of g's waiting to run.
// The compiler turns a go statement into a call to this.
//go:linkname newproc __go_go
func newproc(fn uintptr, arg unsafe.Pointer) *g {
        _g_ := getg()

        if fn == 0 {
                _g_.m.throwing = -1 // do not dump full stacks
                throw("go of nil func value")
        }
        acquirem() // disable preemption because it can be holding p in a local var

        _p_ := _g_.m.p.ptr()
        newg := gfget(_p_)
        var (
                sp     unsafe.Pointer
                spsize uintptr
        )
        if newg == nil {
                newg = malg(true, false, &sp, &spsize)
                casgstatus(newg, _Gidle, _Gdead)
                allgadd(newg) // publishes with a g->status of Gdead so GC scanner doesn't look at uninitialized stack.
        } else {
                resetNewG(newg, &sp, &spsize)
        }
        newg.traceback = 0

        if readgstatus(newg) != _Gdead {
                throw("newproc1: new g is not Gdead")
        }

        // Store the C function pointer into entryfn, take the address
        // of entryfn, convert it to a Go function value, and store
        // that in entry.
        newg.entryfn = fn
        var entry func(unsafe.Pointer)
        *(*unsafe.Pointer)(unsafe.Pointer(&entry)) = unsafe.Pointer(&newg.entryfn)
        newg.entry = entry

        newg.param = arg
        newg.gopc = getcallerpc()
        newg.ancestors = saveAncestors(_g_)
        newg.startpc = fn
        if _g_.m.curg != nil {
                newg.labels = _g_.m.curg.labels
        }
        if isSystemGoroutine(newg, false) {
                atomic.Xadd(&sched.ngsys, +1)
        }
        newg.gcscanvalid = false
        casgstatus(newg, _Gdead, _Grunnable)

        if _p_.goidcache == _p_.goidcacheend {
                // Sched.goidgen is the last allocated id,
                // this batch must be [sched.goidgen+1, sched.goidgen+GoidCacheBatch].
                // At startup sched.goidgen=0, so main goroutine receives goid=1.
                _p_.goidcache = atomic.Xadd64(&sched.goidgen, _GoidCacheBatch)
                _p_.goidcache -= _GoidCacheBatch - 1
                _p_.goidcacheend = _p_.goidcache + _GoidCacheBatch
        }
        newg.goid = int64(_p_.goidcache)
        _p_.goidcache++
        if trace.enabled {
                traceGoCreate(newg, newg.startpc)
        }

        makeGContext(newg, sp, spsize)

        runqput(_p_, newg, true)

        if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 && mainStarted {
                wakep()
        }
        releasem(_g_.m)
        return newg
}

// expectedSystemGoroutines counts the number of goroutines expected
// to mark themselves as system goroutines. After they mark themselves
// by calling setSystemGoroutine, this is decremented. NumGoroutines
// uses this to wait for all system goroutines to mark themselves
// before it counts them.
var expectedSystemGoroutines uint32

// expectSystemGoroutine is called when starting a goroutine that will
// call setSystemGoroutine. It increments expectedSystemGoroutines.
func expectSystemGoroutine() {
        atomic.Xadd(&expectedSystemGoroutines, +1)
}

// waitForSystemGoroutines waits for all currently expected system
// goroutines to register themselves.
func waitForSystemGoroutines() {
        for atomic.Load(&expectedSystemGoroutines) > 0 {
                Gosched()
                osyield()
        }
}

// setSystemGoroutine marks this goroutine as a "system goroutine".
// In the gc toolchain this is done by comparing startpc to a list of
// saved special PCs. In gccgo that approach does not work as startpc
// is often a thunk that invokes the real function with arguments,
// so the thunk address never matches the saved special PCs. Instead,
// since there are only a limited number of "system goroutines",
// we force each one to mark itself as special.
func setSystemGoroutine() {
        getg().isSystemGoroutine = true
        atomic.Xadd(&sched.ngsys, +1)
        atomic.Xadd(&expectedSystemGoroutines, -1)
}

// saveAncestors copies previous ancestors of the given caller g and
// includes infor for the current caller into a new set of tracebacks for
// a g being created.
func saveAncestors(callergp *g) *[]ancestorInfo {
        // Copy all prior info, except for the root goroutine (goid 0).
        if debug.tracebackancestors <= 0 || callergp.goid == 0 {
                return nil
        }
        var callerAncestors []ancestorInfo
        if callergp.ancestors != nil {
                callerAncestors = *callergp.ancestors
        }
        n := int32(len(callerAncestors)) + 1
        if n > debug.tracebackancestors {
                n = debug.tracebackancestors
        }
        ancestors := make([]ancestorInfo, n)
        copy(ancestors[1:], callerAncestors)

        var pcs [_TracebackMaxFrames]uintptr
        // FIXME: This should get a traceback of callergp.
        // npcs := gcallers(callergp, 0, pcs[:])
        npcs := 0
        ipcs := make([]uintptr, npcs)
        copy(ipcs, pcs[:])
        ancestors[0] = ancestorInfo{
                pcs:  ipcs,
                goid: callergp.goid,
                gopc: callergp.gopc,
        }

        ancestorsp := new([]ancestorInfo)
        *ancestorsp = ancestors
        return ancestorsp
}

// Put on gfree list.
// If local list is too long, transfer a batch to the global list.
func gfput(_p_ *p, gp *g) {
        if readgstatus(gp) != _Gdead {
                throw("gfput: bad status (not Gdead)")
        }

        _p_.gFree.push(gp)
        _p_.gFree.n++
        if _p_.gFree.n >= 64 {
                lock(&sched.gFree.lock)
                for _p_.gFree.n >= 32 {
                        _p_.gFree.n--
                        gp = _p_.gFree.pop()
                        sched.gFree.list.push(gp)
                        sched.gFree.n++
                }
                unlock(&sched.gFree.lock)
        }
}

// Get from gfree list.
// If local list is empty, grab a batch from global list.
func gfget(_p_ *p) *g {
retry:
        if _p_.gFree.empty() && !sched.gFree.list.empty() {
                lock(&sched.gFree.lock)
                // Move a batch of free Gs to the P.
                for _p_.gFree.n < 32 {
                        gp := sched.gFree.list.pop()
                        if gp == nil {
                                break
                        }
                        sched.gFree.n--
                        _p_.gFree.push(gp)
                        _p_.gFree.n++
                }
                unlock(&sched.gFree.lock)
                goto retry
        }
        gp := _p_.gFree.pop()
        if gp == nil {
                return nil
        }
        _p_.gFree.n--
        return gp
}

// Purge all cached G's from gfree list to the global list.
func gfpurge(_p_ *p) {
        lock(&sched.gFree.lock)
        for !_p_.gFree.empty() {
                gp := _p_.gFree.pop()
                _p_.gFree.n--
                sched.gFree.list.push(gp)
                sched.gFree.n++
        }
        unlock(&sched.gFree.lock)
}

// Breakpoint executes a breakpoint trap.
func Breakpoint() {
        breakpoint()
}

// dolockOSThread is called by LockOSThread and lockOSThread below
// after they modify m.locked. Do not allow preemption during this call,
// or else the m might be different in this function than in the caller.
//go:nosplit
func dolockOSThread() {
        if GOARCH == "wasm" {
                return // no threads on wasm yet
        }
        _g_ := getg()
        _g_.m.lockedg.set(_g_)
        _g_.lockedm.set(_g_.m)
}

//go:nosplit

// LockOSThread wires the calling goroutine to its current operating system thread.
// The calling goroutine will always execute in that thread,
// and no other goroutine will execute in it,
// until the calling goroutine has made as many calls to
// UnlockOSThread as to LockOSThread.
// If the calling goroutine exits without unlocking the thread,
// the thread will be terminated.
//
// All init functions are run on the startup thread. Calling LockOSThread
// from an init function will cause the main function to be invoked on
// that thread.
//
// A goroutine should call LockOSThread before calling OS services or
// non-Go library functions that depend on per-thread state.
func LockOSThread() {
        if atomic.Load(&newmHandoff.haveTemplateThread) == 0 && GOOS != "plan9" {
                // If we need to start a new thread from the locked
                // thread, we need the template thread. Start it now
                // while we're in a known-good state.
                startTemplateThread()
        }
        _g_ := getg()
        _g_.m.lockedExt++
        if _g_.m.lockedExt == 0 {
                _g_.m.lockedExt--
                panic("LockOSThread nesting overflow")
        }
        dolockOSThread()
}

//go:nosplit
func lockOSThread() {
        getg().m.lockedInt++
        dolockOSThread()
}

// dounlockOSThread is called by UnlockOSThread and unlockOSThread below
// after they update m->locked. Do not allow preemption during this call,
// or else the m might be in different in this function than in the caller.
//go:nosplit
func dounlockOSThread() {
        if GOARCH == "wasm" {
                return // no threads on wasm yet
        }
        _g_ := getg()
        if _g_.m.lockedInt != 0 || _g_.m.lockedExt != 0 {
                return
        }
        _g_.m.lockedg = 0
        _g_.lockedm = 0
}

//go:nosplit

// UnlockOSThread undoes an earlier call to LockOSThread.
// If this drops the number of active LockOSThread calls on the
// calling goroutine to zero, it unwires the calling goroutine from
// its fixed operating system thread.
// If there are no active LockOSThread calls, this is a no-op.
//
// Before calling UnlockOSThread, the caller must ensure that the OS
// thread is suitable for running other goroutines. If the caller made
// any permanent changes to the state of the thread that would affect
// other goroutines, it should not call this function and thus leave
// the goroutine locked to the OS thread until the goroutine (and
// hence the thread) exits.
func UnlockOSThread() {
        _g_ := getg()
        if _g_.m.lockedExt == 0 {
                return
        }
        _g_.m.lockedExt--
        dounlockOSThread()
}

//go:nosplit
func unlockOSThread() {
        _g_ := getg()
        if _g_.m.lockedInt == 0 {
                systemstack(badunlockosthread)
        }
        _g_.m.lockedInt--
        dounlockOSThread()
}

func badunlockosthread() {
        throw("runtime: internal error: misuse of lockOSThread/unlockOSThread")
}

func gcount() int32 {
        n := int32(allglen) - sched.gFree.n - int32(atomic.Load(&sched.ngsys))
        for _, _p_ := range allp {
                n -= _p_.gFree.n
        }

        // All these variables can be changed concurrently, so the result can be inconsistent.
        // But at least the current goroutine is running.
        if n < 1 {
                n = 1
        }
        return n
}

func mcount() int32 {
        return int32(sched.mnext - sched.nmfreed)
}

var prof struct {
        signalLock uint32
        hz         int32
}

func _System()                    { _System() }
func _ExternalCode()              { _ExternalCode() }
func _LostExternalCode()          { _LostExternalCode() }
func _GC()                        { _GC() }
func _LostSIGPROFDuringAtomic64() { _LostSIGPROFDuringAtomic64() }
func _VDSO()                      { _VDSO() }

var _SystemPC = funcPC(_System)
var _ExternalCodePC = funcPC(_ExternalCode)
var _LostExternalCodePC = funcPC(_LostExternalCode)
var _GCPC = funcPC(_GC)
var _LostSIGPROFDuringAtomic64PC = funcPC(_LostSIGPROFDuringAtomic64)

// Called if we receive a SIGPROF signal.
// Called by the signal handler, may run during STW.
//go:nowritebarrierrec
func sigprof(pc uintptr, gp *g, mp *m) {
        if prof.hz == 0 {
                return
        }

        // Profiling runs concurrently with GC, so it must not allocate.
        // Set a trap in case the code does allocate.
        // Note that on windows, one thread takes profiles of all the
        // other threads, so mp is usually not getg().m.
        // In fact mp may not even be stopped.
        // See golang.org/issue/17165.
        getg().m.mallocing++

        traceback := true

        // If SIGPROF arrived while already fetching runtime callers
        // we can have trouble on older systems because the unwind
        // library calls dl_iterate_phdr which was not reentrant in
        // the past. alreadyInCallers checks for that.
        if gp == nil || alreadyInCallers() {
                traceback = false
        }

        var stk [maxCPUProfStack]uintptr
        n := 0
        if traceback {
                var stklocs [maxCPUProfStack]location
                n = callers(0, stklocs[:])

                // Issue 26595: the stack trace we've just collected is going
                // to include frames that we don't want to report in the CPU
                // profile, including signal handler frames. Here is what we
                // might typically see at the point of "callers" above for a
                // signal delivered to the application routine "interesting"
                // called by "main".
                //
                //  0: runtime.sigprof
                //  1: runtime.sighandler
                //  2: runtime.sigtrampgo
                //  3: runtime.sigtramp
                //  4: <signal handler called>
                //  5: main.interesting_routine
                //  6: main.main
                //
                // To ensure a sane profile, walk through the frames in
                // "stklocs" until we find the "runtime.sigtramp" frame, then
                // report only those frames below the frame one down from
                // that. On systems that don't split stack, "sigtramp" can
                // do a sibling call to "sigtrampgo", so use "sigtrampgo"
                // if we don't find "sigtramp". If for some reason
                // neither "runtime.sigtramp" nor "runtime.sigtrampgo" is
                // present, don't make any changes.
                framesToDiscard := 0
                for i := 0; i < n; i++ {
                        if stklocs[i].function == "runtime.sigtrampgo" && i+2 < n {
                                framesToDiscard = i + 2
                        }
                        if stklocs[i].function == "runtime.sigtramp" && i+2 < n {
                                framesToDiscard = i + 2
                                break
                        }
                }
                n -= framesToDiscard
                for i := 0; i < n; i++ {
                        stk[i] = stklocs[i+framesToDiscard].pc
                }
        }

        if n <= 0 {
                // Normal traceback is impossible or has failed.
                // Account it against abstract "System" or "GC".
                n = 2
                stk[0] = pc
                if mp.preemptoff != "" {
                        stk[1] = _GCPC + sys.PCQuantum
                } else {
                        stk[1] = _SystemPC + sys.PCQuantum
                }
        }

        if prof.hz != 0 {
                cpuprof.add(gp, stk[:n])
        }
        getg().m.mallocing--
}

// Use global arrays rather than using up lots of stack space in the
// signal handler. This is safe since while we are executing a SIGPROF
// signal other SIGPROF signals are blocked.
var nonprofGoStklocs [maxCPUProfStack]location
var nonprofGoStk [maxCPUProfStack]uintptr

// sigprofNonGo is called if we receive a SIGPROF signal on a non-Go thread,
// and the signal handler collected a stack trace in sigprofCallers.
// When this is called, sigprofCallersUse will be non-zero.
// g is nil, and what we can do is very limited.
//go:nosplit
//go:nowritebarrierrec
func sigprofNonGo(pc uintptr) {
        if prof.hz != 0 {
                n := callers(0, nonprofGoStklocs[:])

                for i := 0; i < n; i++ {
                        nonprofGoStk[i] = nonprofGoStklocs[i].pc
                }

                if n <= 0 {
                        n = 2
                        nonprofGoStk[0] = pc
                        nonprofGoStk[1] = _ExternalCodePC + sys.PCQuantum
                }

                cpuprof.addNonGo(nonprofGoStk[:n])
        }
}

// sigprofNonGoPC is called when a profiling signal arrived on a
// non-Go thread and we have a single PC value, not a stack trace.
// g is nil, and what we can do is very limited.
//go:nosplit
//go:nowritebarrierrec
func sigprofNonGoPC(pc uintptr) {
        if prof.hz != 0 {
                stk := []uintptr{
                        pc,
                        _ExternalCodePC + sys.PCQuantum,
                }
                cpuprof.addNonGo(stk)
        }
}

// setcpuprofilerate sets the CPU profiling rate to hz times per second.
// If hz <= 0, setcpuprofilerate turns off CPU profiling.
func setcpuprofilerate(hz int32) {
        // Force sane arguments.
        if hz < 0 {
                hz = 0
        }

        // Disable preemption, otherwise we can be rescheduled to another thread
        // that has profiling enabled.
        _g_ := getg()
        _g_.m.locks++

        // Stop profiler on this thread so that it is safe to lock prof.
        // if a profiling signal came in while we had prof locked,
        // it would deadlock.
        setThreadCPUProfiler(0)

        for !atomic.Cas(&prof.signalLock, 0, 1) {
                osyield()
        }
        if prof.hz != hz {
                setProcessCPUProfiler(hz)
                prof.hz = hz
        }
        atomic.Store(&prof.signalLock, 0)

        lock(&sched.lock)
        sched.profilehz = hz
        unlock(&sched.lock)

        if hz != 0 {
                setThreadCPUProfiler(hz)
        }

        _g_.m.locks--
}

// init initializes pp, which may be a freshly allocated p or a
// previously destroyed p, and transitions it to status _Pgcstop.
func (pp *p) init(id int32) {
        pp.id = id
        pp.status = _Pgcstop
        pp.sudogcache = pp.sudogbuf[:0]
        pp.deferpool = pp.deferpoolbuf[:0]
        pp.wbBuf.reset()
        if pp.mcache == nil {
                if id == 0 {
                        if getg().m.mcache == nil {
                                throw("missing mcache?")
                        }
                        pp.mcache = getg().m.mcache // bootstrap
                } else {
                        pp.mcache = allocmcache()
                }
        }
        if raceenabled && pp.raceprocctx == 0 {
                if id == 0 {
                        pp.raceprocctx = raceprocctx0
                        raceprocctx0 = 0 // bootstrap
                } else {
                        pp.raceprocctx = raceproccreate()
                }
        }
}

// destroy releases all of the resources associated with pp and
// transitions it to status _Pdead.
//
// sched.lock must be held and the world must be stopped.
func (pp *p) destroy() {
        // Move all runnable goroutines to the global queue
        for pp.runqhead != pp.runqtail {
                // Pop from tail of local queue
                pp.runqtail--
                gp := pp.runq[pp.runqtail%uint32(len(pp.runq))].ptr()
                // Push onto head of global queue
                globrunqputhead(gp)
        }
        if pp.runnext != 0 {
                globrunqputhead(pp.runnext.ptr())
                pp.runnext = 0
        }
        // If there's a background worker, make it runnable and put
        // it on the global queue so it can clean itself up.
        if gp := pp.gcBgMarkWorker.ptr(); gp != nil {
                casgstatus(gp, _Gwaiting, _Grunnable)
                if trace.enabled {
                        traceGoUnpark(gp, 0)
                }
                globrunqput(gp)
                // This assignment doesn't race because the
                // world is stopped.
                pp.gcBgMarkWorker.set(nil)
        }
        // Flush p's write barrier buffer.
        if gcphase != _GCoff {
                wbBufFlush1(pp)
                pp.gcw.dispose()
        }
        for i := range pp.sudogbuf {
                pp.sudogbuf[i] = nil
        }
        pp.sudogcache = pp.sudogbuf[:0]
        for i := range pp.deferpoolbuf {
                pp.deferpoolbuf[i] = nil
        }
        pp.deferpool = pp.deferpoolbuf[:0]
        freemcache(pp.mcache)
        pp.mcache = nil
        gfpurge(pp)
        traceProcFree(pp)
        if raceenabled {
                raceprocdestroy(pp.raceprocctx)
                pp.raceprocctx = 0
        }
        pp.gcAssistTime = 0
        pp.status = _Pdead
}

// Change number of processors. The world is stopped, sched is locked.
// gcworkbufs are not being modified by either the GC or
// the write barrier code.
// Returns list of Ps with local work, they need to be scheduled by the caller.
func procresize(nprocs int32) *p {
        old := gomaxprocs
        if old < 0 || nprocs <= 0 {
                throw("procresize: invalid arg")
        }
        if trace.enabled {
                traceGomaxprocs(nprocs)
        }

        // update statistics
        now := nanotime()
        if sched.procresizetime != 0 {
                sched.totaltime += int64(old) * (now - sched.procresizetime)
        }
        sched.procresizetime = now

        // Grow allp if necessary.
        if nprocs > int32(len(allp)) {
                // Synchronize with retake, which could be running
                // concurrently since it doesn't run on a P.
                lock(&allpLock)
                if nprocs <= int32(cap(allp)) {
                        allp = allp[:nprocs]
                } else {
                        nallp := make([]*p, nprocs)
                        // Copy everything up to allp's cap so we
                        // never lose old allocated Ps.
                        copy(nallp, allp[:cap(allp)])
                        allp = nallp
                }
                unlock(&allpLock)
        }

        // initialize new P's
        for i := old; i < nprocs; i++ {
                pp := allp[i]
                if pp == nil {
                        pp = new(p)
                }
                pp.init(i)
                atomicstorep(unsafe.Pointer(&allp[i]), unsafe.Pointer(pp))
        }

        _g_ := getg()
        if _g_.m.p != 0 && _g_.m.p.ptr().id < nprocs {
                // continue to use the current P
                _g_.m.p.ptr().status = _Prunning
                _g_.m.p.ptr().mcache.prepareForSweep()
        } else {
                // release the current P and acquire allp[0].
                //
                // We must do this before destroying our current P
                // because p.destroy itself has write barriers, so we
                // need to do that from a valid P.
                if _g_.m.p != 0 {
                        if trace.enabled {
                                // Pretend that we were descheduled
                                // and then scheduled again to keep
                                // the trace sane.
                                traceGoSched()
                                traceProcStop(_g_.m.p.ptr())
                        }
                        _g_.m.p.ptr().m = 0
                }
                _g_.m.p = 0
                _g_.m.mcache = nil
                p := allp[0]
                p.m = 0
                p.status = _Pidle
                acquirep(p)
                if trace.enabled {
                        traceGoStart()
                }
        }

        // release resources from unused P's
        for i := nprocs; i < old; i++ {
                p := allp[i]
                p.destroy()
                // can't free P itself because it can be referenced by an M in syscall
        }

        // Trim allp.
        if int32(len(allp)) != nprocs {
                lock(&allpLock)
                allp = allp[:nprocs]
                unlock(&allpLock)
        }

        var runnablePs *p
        for i := nprocs - 1; i >= 0; i-- {
                p := allp[i]
                if _g_.m.p.ptr() == p {
                        continue
                }
                p.status = _Pidle
                if runqempty(p) {
                        pidleput(p)
                } else {
                        p.m.set(mget())
                        p.link.set(runnablePs)
                        runnablePs = p
                }
        }
        stealOrder.reset(uint32(nprocs))
        var int32p *int32 = &gomaxprocs // make compiler check that gomaxprocs is an int32
        atomic.Store((*uint32)(unsafe.Pointer(int32p)), uint32(nprocs))
        return runnablePs
}

// Associate p and the current m.
//
// This function is allowed to have write barriers even if the caller
// isn't because it immediately acquires _p_.
//
//go:yeswritebarrierrec
func acquirep(_p_ *p) {
        // Do the part that isn't allowed to have write barriers.
        wirep(_p_)

        // Have p; write barriers now allowed.

        // Perform deferred mcache flush before this P can allocate
        // from a potentially stale mcache.
        _p_.mcache.prepareForSweep()

        if trace.enabled {
                traceProcStart()
        }
}

// wirep is the first step of acquirep, which actually associates the
// current M to _p_. This is broken out so we can disallow write
// barriers for this part, since we don't yet have a P.
//
//go:nowritebarrierrec
//go:nosplit
func wirep(_p_ *p) {
        _g_ := getg()

        if _g_.m.p != 0 || _g_.m.mcache != nil {
                throw("wirep: already in go")
        }
        if _p_.m != 0 || _p_.status != _Pidle {
                id := int64(0)
                if _p_.m != 0 {
                        id = _p_.m.ptr().id
                }
                print("wirep: p->m=", _p_.m, "(", id, ") p->status=", _p_.status, "\n")
                throw("wirep: invalid p state")
        }
        _g_.m.mcache = _p_.mcache
        _g_.m.p.set(_p_)
        _p_.m.set(_g_.m)
        _p_.status = _Prunning
}

// Disassociate p and the current m.
func releasep() *p {
        _g_ := getg()

        if _g_.m.p == 0 || _g_.m.mcache == nil {
                throw("releasep: invalid arg")
        }
        _p_ := _g_.m.p.ptr()
        if _p_.m.ptr() != _g_.m || _p_.mcache != _g_.m.mcache || _p_.status != _Prunning {
                print("releasep: m=", _g_.m, " m->p=", _g_.m.p.ptr(), " p->m=", hex(_p_.m), " m->mcache=", _g_.m.mcache, " p->mcache=", _p_.mcache, " p->status=", _p_.status, "\n")
                throw("releasep: invalid p state")
        }
        if trace.enabled {
                traceProcStop(_g_.m.p.ptr())
        }
        _g_.m.p = 0
        _g_.m.mcache = nil
        _p_.m = 0
        _p_.status = _Pidle
        return _p_
}

func incidlelocked(v int32) {
        lock(&sched.lock)
        sched.nmidlelocked += v
        if v > 0 {
                checkdead()
        }
        unlock(&sched.lock)
}

// Check for deadlock situation.
// The check is based on number of running M's, if 0 -> deadlock.
// sched.lock must be held.
func checkdead() {
        // For -buildmode=c-shared or -buildmode=c-archive it's OK if
        // there are no running goroutines. The calling program is
        // assumed to be running.
        if islibrary || isarchive {
                return
        }

        // If we are dying because of a signal caught on an already idle thread,
        // freezetheworld will cause all running threads to block.
        // And runtime will essentially enter into deadlock state,
        // except that there is a thread that will call exit soon.
        if panicking > 0 {
                return
        }

        // If we are not running under cgo, but we have an extra M then account
        // for it. (It is possible to have an extra M on Windows without cgo to
        // accommodate callbacks created by syscall.NewCallback. See issue #6751
        // for details.)
        var run0 int32
        if !iscgo && cgoHasExtraM {
                mp := lockextra(true)
                haveExtraM := extraMCount > 0
                unlockextra(mp)
                if haveExtraM {
                        run0 = 1
                }
        }

        run := mcount() - sched.nmidle - sched.nmidlelocked - sched.nmsys
        if run > run0 {
                return
        }
        if run < 0 {
                print("runtime: checkdead: nmidle=", sched.nmidle, " nmidlelocked=", sched.nmidlelocked, " mcount=", mcount(), " nmsys=", sched.nmsys, "\n")
                throw("checkdead: inconsistent counts")
        }

        grunning := 0
        lock(&allglock)
        for i := 0; i < len(allgs); i++ {
                gp := allgs[i]
                if isSystemGoroutine(gp, false) {
                        continue
                }
                s := readgstatus(gp)
                switch s &^ _Gscan {
                case _Gwaiting:
                        grunning++
                case _Grunnable,
                        _Grunning,
                        _Gsyscall:
                        unlock(&allglock)
                        print("runtime: checkdead: find g ", gp.goid, " in status ", s, "\n")
                        throw("checkdead: runnable g")
                }
        }
        unlock(&allglock)
        if grunning == 0 { // possible if main goroutine calls runtime·Goexit()
                throw("no goroutines (main called runtime.Goexit) - deadlock!")
        }

        // Maybe jump time forward for playground.
        gp := timejump()
        if gp != nil {
                casgstatus(gp, _Gwaiting, _Grunnable)
                globrunqput(gp)
                _p_ := pidleget()
                if _p_ == nil {
                        throw("checkdead: no p for timer")
                }
                mp := mget()
                if mp == nil {
                        // There should always be a free M since
                        // nothing is running.
                        throw("checkdead: no m for timer")
                }
                mp.nextp.set(_p_)
                notewakeup(&mp.park)
                return
        }

        getg().m.throwing = -1 // do not dump full stacks
        throw("all goroutines are asleep - deadlock!")
}

// forcegcperiod is the maximum time in nanoseconds between garbage
// collections. If we go this long without a garbage collection, one
// is forced to run.
//
// This is a variable for testing purposes. It normally doesn't change.
var forcegcperiod int64 = 2 * 60 * 1e9

// Always runs without a P, so write barriers are not allowed.
//
//go:nowritebarrierrec
func sysmon() {
        lock(&sched.lock)
        sched.nmsys++
        checkdead()
        unlock(&sched.lock)

        lasttrace := int64(0)
        idle := 0 // how many cycles in succession we had not wokeup somebody
        delay := uint32(0)
        for {
                if idle == 0 { // start with 20us sleep...
                        delay = 20
                } else if idle > 50 { // start doubling the sleep after 1ms...
                        delay *= 2
                }
                if delay > 10*1000 { // up to 10ms
                        delay = 10 * 1000
                }
                usleep(delay)
                if debug.schedtrace <= 0 && (sched.gcwaiting != 0 || atomic.Load(&sched.npidle) == uint32(gomaxprocs)) {
                        lock(&sched.lock)
                        if atomic.Load(&sched.gcwaiting) != 0 || atomic.Load(&sched.npidle) == uint32(gomaxprocs) {
                                atomic.Store(&sched.sysmonwait, 1)
                                unlock(&sched.lock)
                                // Make wake-up period small enough
                                // for the sampling to be correct.
                                maxsleep := forcegcperiod / 2
                                shouldRelax := true
                                if osRelaxMinNS > 0 {
                                        next := timeSleepUntil()
                                        now := nanotime()
                                        if next-now < osRelaxMinNS {
                                                shouldRelax = false
                                        }
                                }
                                if shouldRelax {
                                        osRelax(true)
                                }
                                notetsleep(&sched.sysmonnote, maxsleep)
                                if shouldRelax {
                                        osRelax(false)
                                }
                                lock(&sched.lock)
                                atomic.Store(&sched.sysmonwait, 0)
                                noteclear(&sched.sysmonnote)
                                idle = 0
                                delay = 20
                        }
                        unlock(&sched.lock)
                }
                // trigger libc interceptors if needed
                if *cgo_yield != nil {
                        asmcgocall(*cgo_yield, nil)
                }
                // poll network if not polled for more than 10ms
                lastpoll := int64(atomic.Load64(&sched.lastpoll))
                now := nanotime()
                if netpollinited() && lastpoll != 0 && lastpoll+10*1000*1000 < now {
                        atomic.Cas64(&sched.lastpoll, uint64(lastpoll), uint64(now))
                        list := netpoll(false) // non-blocking - returns list of goroutines
                        if !list.empty() {
                                // Need to decrement number of idle locked M's
                                // (pretending that one more is running) before injectglist.
                                // Otherwise it can lead to the following situation:
                                // injectglist grabs all P's but before it starts M's to run the P's,
                                // another M returns from syscall, finishes running its G,
                                // observes that there is no work to do and no other running M's
                                // and reports deadlock.
                                incidlelocked(-1)
                                injectglist(&list)
                                incidlelocked(1)
                        }
                }
                // retake P's blocked in syscalls
                // and preempt long running G's
                if retake(now) != 0 {
                        idle = 0
                } else {
                        idle++
                }
                // check if we need to force a GC
                if t := (gcTrigger{kind: gcTriggerTime, now: now}); t.test() && atomic.Load(&forcegc.idle) != 0 {
                        lock(&forcegc.lock)
                        forcegc.idle = 0
                        var list gList
                        list.push(forcegc.g)
                        injectglist(&list)
                        unlock(&forcegc.lock)
                }
                if debug.schedtrace > 0 && lasttrace+int64(debug.schedtrace)*1000000 <= now {
                        lasttrace = now
                        schedtrace(debug.scheddetail > 0)
                }
        }
}

type sysmontick struct {
        schedtick   uint32
        schedwhen   int64
        syscalltick uint32
        syscallwhen int64
}

// forcePreemptNS is the time slice given to a G before it is
// preempted.
const forcePreemptNS = 10 * 1000 * 1000 // 10ms

func retake(now int64) uint32 {
        n := 0
        // Prevent allp slice changes. This lock will be completely
        // uncontended unless we're already stopping the world.
        lock(&allpLock)
        // We can't use a range loop over allp because we may
        // temporarily drop the allpLock. Hence, we need to re-fetch
        // allp each time around the loop.
        for i := 0; i < len(allp); i++ {
                _p_ := allp[i]
                if _p_ == nil {
                        // This can happen if procresize has grown
                        // allp but not yet created new Ps.
                        continue
                }
                pd := &_p_.sysmontick
                s := _p_.status
                sysretake := false
                if s == _Prunning || s == _Psyscall {
                        // Preempt G if it's running for too long.
                        t := int64(_p_.schedtick)
                        if int64(pd.schedtick) != t {
                                pd.schedtick = uint32(t)
                                pd.schedwhen = now
                        } else if pd.schedwhen+forcePreemptNS <= now {
                                preemptone(_p_)
                                // In case of syscall, preemptone() doesn't
                                // work, because there is no M wired to P.
                                sysretake = true
                        }
                }
                if s == _Psyscall {
                        // Retake P from syscall if it's there for more than 1 sysmon tick (at least 20us).
                        t := int64(_p_.syscalltick)
                        if !sysretake && int64(pd.syscalltick) != t {
                                pd.syscalltick = uint32(t)
                                pd.syscallwhen = now
                                continue
                        }
                        // On the one hand we don't want to retake Ps if there is no other work to do,
                        // but on the other hand we want to retake them eventually
                        // because they can prevent the sysmon thread from deep sleep.
                        if runqempty(_p_) && atomic.Load(&sched.nmspinning)+atomic.Load(&sched.npidle) > 0 && pd.syscallwhen+10*1000*1000 > now {
                                continue
                        }
                        // Drop allpLock so we can take sched.lock.
                        unlock(&allpLock)
                        // Need to decrement number of idle locked M's
                        // (pretending that one more is running) before the CAS.
                        // Otherwise the M from which we retake can exit the syscall,
                        // increment nmidle and report deadlock.
                        incidlelocked(-1)
                        if atomic.Cas(&_p_.status, s, _Pidle) {
                                if trace.enabled {
                                        traceGoSysBlock(_p_)
                                        traceProcStop(_p_)
                                }
                                n++
                                _p_.syscalltick++
                                handoffp(_p_)
                        }
                        incidlelocked(1)
                        lock(&allpLock)
                }
        }
        unlock(&allpLock)
        return uint32(n)
}

// Tell all goroutines that they have been preempted and they should stop.
// This function is purely best-effort. It can fail to inform a goroutine if a
// processor just started running it.
// No locks need to be held.
// Returns true if preemption request was issued to at least one goroutine.
func preemptall() bool {
        res := false
        for _, _p_ := range allp {
                if _p_.status != _Prunning {
                        continue
                }
                if preemptone(_p_) {
                        res = true
                }
        }
        return res
}

// Tell the goroutine running on processor P to stop.
// This function is purely best-effort. It can incorrectly fail to inform the
// goroutine. It can send inform the wrong goroutine. Even if it informs the
// correct goroutine, that goroutine might ignore the request if it is
// simultaneously executing newstack.
// No lock needs to be held.
// Returns true if preemption request was issued.
// The actual preemption will happen at some point in the future
// and will be indicated by the gp->status no longer being
// Grunning
func preemptone(_p_ *p) bool {
        mp := _p_.m.ptr()
        if mp == nil || mp == getg().m {
                return false
        }
        gp := mp.curg
        if gp == nil || gp == mp.g0 {
                return false
        }

        gp.preempt = true

        // At this point the gc implementation sets gp.stackguard0 to
        // a value that causes the goroutine to suspend itself.
        // gccgo has no support for this, and it's hard to support.
        // The split stack code reads a value from its TCB.
        // We have no way to set a value in the TCB of a different thread.
        // And, of course, not all systems support split stack anyhow.
        // Checking the field in the g is expensive, since it requires
        // loading the g from TLS.  The best mechanism is likely to be
        // setting a global variable and figuring out a way to efficiently
        // check that global variable.
        //
        // For now we check gp.preempt in schedule, mallocgc, selectgo,
        // and a few other places, which is at least better than doing
        // nothing at all.

        return true
}

var starttime int64

func schedtrace(detailed bool) {
        now := nanotime()
        if starttime == 0 {
                starttime = now
        }

        lock(&sched.lock)
        print("SCHED ", (now-starttime)/1e6, "ms: gomaxprocs=", gomaxprocs, " idleprocs=", sched.npidle, " threads=", mcount(), " spinningthreads=", sched.nmspinning, " idlethreads=", sched.nmidle, " runqueue=", sched.runqsize)
        if detailed {
                print(" gcwaiting=", sched.gcwaiting, " nmidlelocked=", sched.nmidlelocked, " stopwait=", sched.stopwait, " sysmonwait=", sched.sysmonwait, "\n")
        }
        // We must be careful while reading data from P's, M's and G's.
        // Even if we hold schedlock, most data can be changed concurrently.
        // E.g. (p->m ? p->m->id : -1) can crash if p->m changes from non-nil to nil.
        for i, _p_ := range allp {
                mp := _p_.m.ptr()
                h := atomic.Load(&_p_.runqhead)
                t := atomic.Load(&_p_.runqtail)
                if detailed {
                        id := int64(-1)
                        if mp != nil {
                                id = mp.id
                        }
                        print("  P", i, ": status=", _p_.status, " schedtick=", _p_.schedtick, " syscalltick=", _p_.syscalltick, " m=", id, " runqsize=", t-h, " gfreecnt=", _p_.gFree.n, "\n")
                } else {
                        // In non-detailed mode format lengths of per-P run queues as:
                        // [len1 len2 len3 len4]
                        print(" ")
                        if i == 0 {
                                print("[")
                        }
                        print(t - h)
                        if i == len(allp)-1 {
                                print("]\n")
                        }
                }
        }

        if !detailed {
                unlock(&sched.lock)
                return
        }

        for mp := allm; mp != nil; mp = mp.alllink {
                _p_ := mp.p.ptr()
                gp := mp.curg
                lockedg := mp.lockedg.ptr()
                id1 := int32(-1)
                if _p_ != nil {
                        id1 = _p_.id
                }
                id2 := int64(-1)
                if gp != nil {
                        id2 = gp.goid
                }
                id3 := int64(-1)
                if lockedg != nil {
                        id3 = lockedg.goid
                }
                print("  M", mp.id, ": p=", id1, " curg=", id2, " mallocing=", mp.mallocing, " throwing=", mp.throwing, " preemptoff=", mp.preemptoff, ""+" locks=", mp.locks, " dying=", mp.dying, " spinning=", mp.spinning, " blocked=", mp.blocked, " lockedg=", id3, "\n")
        }

        lock(&allglock)
        for gi := 0; gi < len(allgs); gi++ {
                gp := allgs[gi]
                mp := gp.m
                lockedm := gp.lockedm.ptr()
                id1 := int64(-1)
                if mp != nil {
                        id1 = mp.id
                }
                id2 := int64(-1)
                if lockedm != nil {
                        id2 = lockedm.id
                }
                print("  G", gp.goid, ": status=", readgstatus(gp), "(", gp.waitreason.String(), ") m=", id1, " lockedm=", id2, "\n")
        }
        unlock(&allglock)
        unlock(&sched.lock)
}

// schedEnableUser enables or disables the scheduling of user
// goroutines.
//
// This does not stop already running user goroutines, so the caller
// should first stop the world when disabling user goroutines.
func schedEnableUser(enable bool) {
        lock(&sched.lock)
        if sched.disable.user == !enable {
                unlock(&sched.lock)
                return
        }
        sched.disable.user = !enable
        if enable {
                n := sched.disable.n
                sched.disable.n = 0
                globrunqputbatch(&sched.disable.runnable, n)
                unlock(&sched.lock)
                for ; n != 0 && sched.npidle != 0; n-- {
                        startm(nil, false)
                }
        } else {
                unlock(&sched.lock)
        }
}

// schedEnabled reports whether gp should be scheduled. It returns
// false is scheduling of gp is disabled.
func schedEnabled(gp *g) bool {
        if sched.disable.user {
                return isSystemGoroutine(gp, true)
        }
        return true
}

// Put mp on midle list.
// Sched must be locked.
// May run during STW, so write barriers are not allowed.
//go:nowritebarrierrec
func mput(mp *m) {
        mp.schedlink = sched.midle
        sched.midle.set(mp)
        sched.nmidle++
        checkdead()
}

// Try to get an m from midle list.
// Sched must be locked.
// May run during STW, so write barriers are not allowed.
//go:nowritebarrierrec
func mget() *m {
        mp := sched.midle.ptr()
        if mp != nil {
                sched.midle = mp.schedlink
                sched.nmidle--
        }
        return mp
}

// Put gp on the global runnable queue.
// Sched must be locked.
// May run during STW, so write barriers are not allowed.
//go:nowritebarrierrec
func globrunqput(gp *g) {
        sched.runq.pushBack(gp)
        sched.runqsize++
}

// Put gp at the head of the global runnable queue.
// Sched must be locked.
// May run during STW, so write barriers are not allowed.
//go:nowritebarrierrec
func globrunqputhead(gp *g) {
        sched.runq.push(gp)
        sched.runqsize++
}

// Put a batch of runnable goroutines on the global runnable queue.
// This clears *batch.
// Sched must be locked.
func globrunqputbatch(batch *gQueue, n int32) {
        sched.runq.pushBackAll(*batch)
        sched.runqsize += n
        *batch = gQueue{}
}

// Try get a batch of G's from the global runnable queue.
// Sched must be locked.
func globrunqget(_p_ *p, max int32) *g {
        if sched.runqsize == 0 {
                return nil
        }

        n := sched.runqsize/gomaxprocs + 1
        if n > sched.runqsize {
                n = sched.runqsize
        }
        if max > 0 && n > max {
                n = max
        }
        if n > int32(len(_p_.runq))/2 {
                n = int32(len(_p_.runq)) / 2
        }

        sched.runqsize -= n

        gp := sched.runq.pop()
        n--
        for ; n > 0; n-- {
                gp1 := sched.runq.pop()
                runqput(_p_, gp1, false)
        }
        return gp
}

// Put p to on _Pidle list.
// Sched must be locked.
// May run during STW, so write barriers are not allowed.
//go:nowritebarrierrec
func pidleput(_p_ *p) {
        if !runqempty(_p_) {
                throw("pidleput: P has non-empty run queue")
        }
        _p_.link = sched.pidle
        sched.pidle.set(_p_)
        atomic.Xadd(&sched.npidle, 1) // TODO: fast atomic
}

// Try get a p from _Pidle list.
// Sched must be locked.
// May run during STW, so write barriers are not allowed.
//go:nowritebarrierrec
func pidleget() *p {
        _p_ := sched.pidle.ptr()
        if _p_ != nil {
                sched.pidle = _p_.link
                atomic.Xadd(&sched.npidle, -1) // TODO: fast atomic
        }
        return _p_
}

// runqempty reports whether _p_ has no Gs on its local run queue.
// It never returns true spuriously.
func runqempty(_p_ *p) bool {
        // Defend against a race where 1) _p_ has G1 in runqnext but runqhead == runqtail,
        // 2) runqput on _p_ kicks G1 to the runq, 3) runqget on _p_ empties runqnext.
        // Simply observing that runqhead == runqtail and then observing that runqnext == nil
        // does not mean the queue is empty.
        for {
                head := atomic.Load(&_p_.runqhead)
                tail := atomic.Load(&_p_.runqtail)
                runnext := atomic.Loaduintptr((*uintptr)(unsafe.Pointer(&_p_.runnext)))
                if tail == atomic.Load(&_p_.runqtail) {
                        return head == tail && runnext == 0
                }
        }
}

// To shake out latent assumptions about scheduling order,
// we introduce some randomness into scheduling decisions
// when running with the race detector.
// The need for this was made obvious by changing the
// (deterministic) scheduling order in Go 1.5 and breaking
// many poorly-written tests.
// With the randomness here, as long as the tests pass
// consistently with -race, they shouldn't have latent scheduling
// assumptions.
const randomizeScheduler = raceenabled

// runqput tries to put g on the local runnable queue.
// If next is false, runqput adds g to the tail of the runnable queue.
// If next is true, runqput puts g in the _p_.runnext slot.
// If the run queue is full, runnext puts g on the global queue.
// Executed only by the owner P.
func runqput(_p_ *p, gp *g, next bool) {
        if randomizeScheduler && next && fastrand()%2 == 0 {
                next = false
        }

        if next {
        retryNext:
                oldnext := _p_.runnext
                if !_p_.runnext.cas(oldnext, guintptr(unsafe.Pointer(gp))) {
                        goto retryNext
                }
                if oldnext == 0 {
                        return
                }
                // Kick the old runnext out to the regular run queue.
                gp = oldnext.ptr()
        }

retry:
        h := atomic.LoadAcq(&_p_.runqhead) // load-acquire, synchronize with consumers
        t := _p_.runqtail
        if t-h < uint32(len(_p_.runq)) {
                _p_.runq[t%uint32(len(_p_.runq))].set(gp)
                atomic.StoreRel(&_p_.runqtail, t+1) // store-release, makes the item available for consumption
                return
        }
        if runqputslow(_p_, gp, h, t) {
                return
        }
        // the queue is not full, now the put above must succeed
        goto retry
}

// Put g and a batch of work from local runnable queue on global queue.
// Executed only by the owner P.
func runqputslow(_p_ *p, gp *g, h, t uint32) bool {
        var batch [len(_p_.runq)/2 + 1]*g

        // First, grab a batch from local queue.
        n := t - h
        n = n / 2
        if n != uint32(len(_p_.runq)/2) {
                throw("runqputslow: queue is not full")
        }
        for i := uint32(0); i < n; i++ {
                batch[i] = _p_.runq[(h+i)%uint32(len(_p_.runq))].ptr()
        }
        if !atomic.CasRel(&_p_.runqhead, h, h+n) { // cas-release, commits consume
                return false
        }
        batch[n] = gp

        if randomizeScheduler {
                for i := uint32(1); i <= n; i++ {
                        j := fastrandn(i + 1)
                        batch[i], batch[j] = batch[j], batch[i]
                }
        }

        // Link the goroutines.
        for i := uint32(0); i < n; i++ {
                batch[i].schedlink.set(batch[i+1])
        }
        var q gQueue
        q.head.set(batch[0])
        q.tail.set(batch[n])

        // Now put the batch on global queue.
        lock(&sched.lock)
        globrunqputbatch(&q, int32(n+1))
        unlock(&sched.lock)
        return true
}

// Get g from local runnable queue.
// If inheritTime is true, gp should inherit the remaining time in the
// current time slice. Otherwise, it should start a new time slice.
// Executed only by the owner P.
func runqget(_p_ *p) (gp *g, inheritTime bool) {
        // If there's a runnext, it's the next G to run.
        for {
                next := _p_.runnext
                if next == 0 {
                        break
                }
                if _p_.runnext.cas(next, 0) {
                        return next.ptr(), true
                }
        }

        for {
                h := atomic.LoadAcq(&_p_.runqhead) // load-acquire, synchronize with other consumers
                t := _p_.runqtail
                if t == h {
                        return nil, false
                }
                gp := _p_.runq[h%uint32(len(_p_.runq))].ptr()
                if atomic.CasRel(&_p_.runqhead, h, h+1) { // cas-release, commits consume
                        return gp, false
                }
        }
}

// Grabs a batch of goroutines from _p_'s runnable queue into batch.
// Batch is a ring buffer starting at batchHead.
// Returns number of grabbed goroutines.
// Can be executed by any P.
func runqgrab(_p_ *p, batch *[256]guintptr, batchHead uint32, stealRunNextG bool) uint32 {
        for {
                h := atomic.LoadAcq(&_p_.runqhead) // load-acquire, synchronize with other consumers
                t := atomic.LoadAcq(&_p_.runqtail) // load-acquire, synchronize with the producer
                n := t - h
                n = n - n/2
                if n == 0 {
                        if stealRunNextG {
                                // Try to steal from _p_.runnext.
                                if next := _p_.runnext; next != 0 {
                                        if _p_.status == _Prunning {
                                                // Sleep to ensure that _p_ isn't about to run the g
                                                // we are about to steal.
                                                // The important use case here is when the g running
                                                // on _p_ ready()s another g and then almost
                                                // immediately blocks. Instead of stealing runnext
                                                // in this window, back off to give _p_ a chance to
                                                // schedule runnext. This will avoid thrashing gs
                                                // between different Ps.
                                                // A sync chan send/recv takes ~50ns as of time of
                                                // writing, so 3us gives ~50x overshoot.
                                                if GOOS != "windows" {
                                                        usleep(3)
                                                } else {
                                                        // On windows system timer granularity is
                                                        // 1-15ms, which is way too much for this
                                                        // optimization. So just yield.
                                                        osyield()
                                                }
                                        }
                                        if !_p_.runnext.cas(next, 0) {
                                                continue
                                        }
                                        batch[batchHead%uint32(len(batch))] = next
                                        return 1
                                }
                        }
                        return 0
                }
                if n > uint32(len(_p_.runq)/2) { // read inconsistent h and t
                        continue
                }
                for i := uint32(0); i < n; i++ {
                        g := _p_.runq[(h+i)%uint32(len(_p_.runq))]
                        batch[(batchHead+i)%uint32(len(batch))] = g
                }
                if atomic.CasRel(&_p_.runqhead, h, h+n) { // cas-release, commits consume
                        return n
                }
        }
}

// Steal half of elements from local runnable queue of p2
// and put onto local runnable queue of p.
// Returns one of the stolen elements (or nil if failed).
func runqsteal(_p_, p2 *p, stealRunNextG bool) *g {
        t := _p_.runqtail
        n := runqgrab(p2, &_p_.runq, t, stealRunNextG)
        if n == 0 {
                return nil
        }
        n--
        gp := _p_.runq[(t+n)%uint32(len(_p_.runq))].ptr()
        if n == 0 {
                return gp
        }
        h := atomic.LoadAcq(&_p_.runqhead) // load-acquire, synchronize with consumers
        if t-h+n >= uint32(len(_p_.runq)) {
                throw("runqsteal: runq overflow")
        }
        atomic.StoreRel(&_p_.runqtail, t+n) // store-release, makes the item available for consumption
        return gp
}

// A gQueue is a dequeue of Gs linked through g.schedlink. A G can only
// be on one gQueue or gList at a time.
type gQueue struct {
        head guintptr
        tail guintptr
}

// empty reports whether q is empty.
func (q *gQueue) empty() bool {
        return q.head == 0
}

// push adds gp to the head of q.
func (q *gQueue) push(gp *g) {
        gp.schedlink = q.head
        q.head.set(gp)
        if q.tail == 0 {
                q.tail.set(gp)
        }
}

// pushBack adds gp to the tail of q.
func (q *gQueue) pushBack(gp *g) {
        gp.schedlink = 0
        if q.tail != 0 {
                q.tail.ptr().schedlink.set(gp)
        } else {
                q.head.set(gp)
        }
        q.tail.set(gp)
}

// pushBackAll adds all Gs in l2 to the tail of q. After this q2 must
// not be used.
func (q *gQueue) pushBackAll(q2 gQueue) {
        if q2.tail == 0 {
                return
        }
        q2.tail.ptr().schedlink = 0
        if q.tail != 0 {
                q.tail.ptr().schedlink = q2.head
        } else {
                q.head = q2.head
        }
        q.tail = q2.tail
}

// pop removes and returns the head of queue q. It returns nil if
// q is empty.
func (q *gQueue) pop() *g {
        gp := q.head.ptr()
        if gp != nil {
                q.head = gp.schedlink
                if q.head == 0 {
                        q.tail = 0
                }
        }
        return gp
}

// popList takes all Gs in q and returns them as a gList.
func (q *gQueue) popList() gList {
        stack := gList{q.head}
        *q = gQueue{}
        return stack
}

// A gList is a list of Gs linked through g.schedlink. A G can only be
// on one gQueue or gList at a time.
type gList struct {
        head guintptr
}

// empty reports whether l is empty.
func (l *gList) empty() bool {
        return l.head == 0
}

// push adds gp to the head of l.
func (l *gList) push(gp *g) {
        gp.schedlink = l.head
        l.head.set(gp)
}

// pushAll prepends all Gs in q to l.
func (l *gList) pushAll(q gQueue) {
        if !q.empty() {
                q.tail.ptr().schedlink = l.head
                l.head = q.head
        }
}

// pop removes and returns the head of l. If l is empty, it returns nil.
func (l *gList) pop() *g {
        gp := l.head.ptr()
        if gp != nil {
                l.head = gp.schedlink
        }
        return gp
}

//go:linkname setMaxThreads runtime..z2fdebug.setMaxThreads
func setMaxThreads(in int) (out int) {
        lock(&sched.lock)
        out = int(sched.maxmcount)
        if in > 0x7fffffff { // MaxInt32
                sched.maxmcount = 0x7fffffff
        } else {
                sched.maxmcount = int32(in)
        }
        checkmcount()
        unlock(&sched.lock)
        return
}

func haveexperiment(name string) bool {
        // The gofrontend does not support experiments.
        return false
}

//go:nosplit
func procPin() int {
        _g_ := getg()
        mp := _g_.m

        mp.locks++
        return int(mp.p.ptr().id)
}

//go:nosplit
func procUnpin() {
        _g_ := getg()
        _g_.m.locks--
}

//go:linkname sync_runtime_procPin sync.runtime_procPin
//go:nosplit
func sync_runtime_procPin() int {
        return procPin()
}

//go:linkname sync_runtime_procUnpin sync.runtime_procUnpin
//go:nosplit
func sync_runtime_procUnpin() {
        procUnpin()
}

//go:linkname sync_atomic_runtime_procPin sync..z2fatomic.runtime_procPin
//go:nosplit
func sync_atomic_runtime_procPin() int {
        return procPin()
}

//go:linkname sync_atomic_runtime_procUnpin sync..z2fatomic.runtime_procUnpin
//go:nosplit
func sync_atomic_runtime_procUnpin() {
        procUnpin()
}

// Active spinning for sync.Mutex.
//go:linkname sync_runtime_canSpin sync.runtime_canSpin
//go:nosplit
func sync_runtime_canSpin(i int) bool {
        // sync.Mutex is cooperative, so we are conservative with spinning.
        // Spin only few times and only if running on a multicore machine and
        // GOMAXPROCS>1 and there is at least one other running P and local runq is empty.
        // As opposed to runtime mutex we don't do passive spinning here,
        // because there can be work on global runq or on other Ps.
        if i >= active_spin || ncpu <= 1 || gomaxprocs <= int32(sched.npidle+sched.nmspinning)+1 {
                return false
        }
        if p := getg().m.p.ptr(); !runqempty(p) {
                return false
        }
        return true
}

//go:linkname sync_runtime_doSpin sync.runtime_doSpin
//go:nosplit
func sync_runtime_doSpin() {
        procyield(active_spin_cnt)
}

var stealOrder randomOrder

// randomOrder/randomEnum are helper types for randomized work stealing.
// They allow to enumerate all Ps in different pseudo-random orders without repetitions.
// The algorithm is based on the fact that if we have X such that X and GOMAXPROCS
// are coprime, then a sequences of (i + X) % GOMAXPROCS gives the required enumeration.
type randomOrder struct {
        count    uint32
        coprimes []uint32
}

type randomEnum struct {
        i     uint32
        count uint32
        pos   uint32
        inc   uint32
}

func (ord *randomOrder) reset(count uint32) {
        ord.count = count
        ord.coprimes = ord.coprimes[:0]
        for i := uint32(1); i <= count; i++ {
                if gcd(i, count) == 1 {
                        ord.coprimes = append(ord.coprimes, i)
                }
        }
}

func (ord *randomOrder) start(i uint32) randomEnum {
        return randomEnum{
                count: ord.count,
                pos:   i % ord.count,
                inc:   ord.coprimes[i%uint32(len(ord.coprimes))],
        }
}

func (enum *randomEnum) done() bool {
        return enum.i == enum.count
}

func (enum *randomEnum) next() {
        enum.i++
        enum.pos = (enum.pos + enum.inc) % enum.count
}

func (enum *randomEnum) position() uint32 {
        return enum.pos
}

func gcd(a, b uint32) uint32 {
        for b != 0 {
                a, b = b, a%b
        }
        return a
}