libstdc++: Avoid -Wmaybe-uninitialized warnings in text_encoding.cc

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

// Copyright 2019 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.

// Goroutine preemption
//
// A goroutine can be preempted at any safe-point. Currently, there
// are a few categories of safe-points:
//
// 1. A blocked safe-point occurs for the duration that a goroutine is
//    descheduled, blocked on synchronization, or in a system call.
//
// 2. Synchronous safe-points occur when a running goroutine checks
//    for a preemption request.
//
// 3. Asynchronous safe-points occur at any instruction in user code
//    where the goroutine can be safely paused and a conservative
//    stack and register scan can find stack roots. The runtime can
//    stop a goroutine at an async safe-point using a signal.
//
// At both blocked and synchronous safe-points, a goroutine's CPU
// state is minimal and the garbage collector has complete information
// about its entire stack. This makes it possible to deschedule a
// goroutine with minimal space, and to precisely scan a goroutine's
// stack.
//
// Synchronous safe-points are implemented by overloading the stack
// bound check in function prologues. To preempt a goroutine at the
// next synchronous safe-point, the runtime poisons the goroutine's
// stack bound to a value that will cause the next stack bound check
// to fail and enter the stack growth implementation, which will
// detect that it was actually a preemption and redirect to preemption
// handling.
//
// Preemption at asynchronous safe-points is implemented by suspending
// the thread using an OS mechanism (e.g., signals) and inspecting its
// state to determine if the goroutine was at an asynchronous
// safe-point. Since the thread suspension itself is generally
// asynchronous, it also checks if the running goroutine wants to be
// preempted, since this could have changed. If all conditions are
// satisfied, it adjusts the signal context to make it look like the
// signaled thread just called asyncPreempt and resumes the thread.
// asyncPreempt spills all registers and enters the scheduler.
//
// (An alternative would be to preempt in the signal handler itself.
// This would let the OS save and restore the register state and the
// runtime would only need to know how to extract potentially
// pointer-containing registers from the signal context. However, this
// would consume an M for every preempted G, and the scheduler itself
// is not designed to run from a signal handler, as it tends to
// allocate memory and start threads in the preemption path.)

package runtime

import (
        "runtime/internal/atomic"
)

type suspendGState struct {
        g *g

        // dead indicates the goroutine was not suspended because it
        // is dead. This goroutine could be reused after the dead
        // state was observed, so the caller must not assume that it
        // remains dead.
        dead bool

        // stopped indicates that this suspendG transitioned the G to
        // _Gwaiting via g.preemptStop and thus is responsible for
        // readying it when done.
        stopped bool
}

// suspendG suspends goroutine gp at a safe-point and returns the
// state of the suspended goroutine. The caller gets read access to
// the goroutine until it calls resumeG.
//
// It is safe for multiple callers to attempt to suspend the same
// goroutine at the same time. The goroutine may execute between
// subsequent successful suspend operations. The current
// implementation grants exclusive access to the goroutine, and hence
// multiple callers will serialize. However, the intent is to grant
// shared read access, so please don't depend on exclusive access.
//
// This must be called from the system stack and the user goroutine on
// the current M (if any) must be in a preemptible state. This
// prevents deadlocks where two goroutines attempt to suspend each
// other and both are in non-preemptible states. There are other ways
// to resolve this deadlock, but this seems simplest.
//
// TODO(austin): What if we instead required this to be called from a
// user goroutine? Then we could deschedule the goroutine while
// waiting instead of blocking the thread. If two goroutines tried to
// suspend each other, one of them would win and the other wouldn't
// complete the suspend until it was resumed. We would have to be
// careful that they couldn't actually queue up suspend for each other
// and then both be suspended. This would also avoid the need for a
// kernel context switch in the synchronous case because we could just
// directly schedule the waiter. The context switch is unavoidable in
// the signal case.
//
//go:systemstack
func suspendG(gp *g) suspendGState {
        if mp := getg().m; mp.curg != nil && readgstatus(mp.curg) == _Grunning {
                // Since we're on the system stack of this M, the user
                // G is stuck at an unsafe point. If another goroutine
                // were to try to preempt m.curg, it could deadlock.
                throw("suspendG from non-preemptible goroutine")
        }

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

        // Drive the goroutine to a preemption point.
        stopped := false
        var asyncM *m
        var asyncGen uint32
        var nextPreemptM int64
        for i := 0; ; i++ {
                switch s := readgstatus(gp); s {
                default:
                        if s&_Gscan != 0 {
                                // Someone else is suspending it. Wait
                                // for them to finish.
                                //
                                // TODO: It would be nicer if we could
                                // coalesce suspends.
                                break
                        }

                        dumpgstatus(gp)
                        throw("invalid g status")

                case _Gdead:
                        // Nothing to suspend.
                        //
                        // preemptStop may need to be cleared, but
                        // doing that here could race with goroutine
                        // reuse. Instead, goexit0 clears it.
                        return suspendGState{dead: true}

                case _Gcopystack:
                        // The stack is being copied. We need to wait
                        // until this is done.

                case _Gexitingsyscall:
                        // This is a transient state.  Try again.

                case _Gpreempted:
                        // We (or someone else) suspended the G. Claim
                        // ownership of it by transitioning it to
                        // _Gwaiting.
                        if !casGFromPreempted(gp, _Gpreempted, _Gwaiting) {
                                break
                        }

                        // We stopped the G, so we have to ready it later.
                        stopped = true

                        s = _Gwaiting
                        fallthrough

                case _Grunnable, _Gsyscall, _Gwaiting:
                        // Claim goroutine by setting scan bit.
                        // This may race with execution or readying of gp.
                        // The scan bit keeps it from transition state.
                        if !castogscanstatus(gp, s, s|_Gscan) {
                                break
                        }

                        // Clear the preemption request. It's safe to
                        // reset the stack guard because we hold the
                        // _Gscan bit and thus own the stack.
                        gp.preemptStop = false
                        gp.preempt = false

                        // The goroutine was already at a safe-point
                        // and we've now locked that in.
                        //
                        // TODO: It would be much better if we didn't
                        // leave it in _Gscan, but instead gently
                        // prevented its scheduling until resumption.
                        // Maybe we only use this to bump a suspended
                        // count and the scheduler skips suspended
                        // goroutines? That wouldn't be enough for
                        // {_Gsyscall,_Gwaiting} -> _Grunning. Maybe
                        // for all those transitions we need to check
                        // suspended and deschedule?
                        return suspendGState{g: gp, stopped: stopped}

                case _Grunning:
                        // Optimization: if there is already a pending preemption request
                        // (from the previous loop iteration), don't bother with the atomics.
                        if asyncM != nil && gp.preemptStop && gp.preempt && asyncM == gp.m && atomic.Load(&asyncM.preemptGen) == asyncGen {
                                break
                        }

                        // Temporarily block state transitions.
                        if !castogscanstatus(gp, _Grunning, _Gscanrunning) {
                                break
                        }

                        // Request synchronous preemption.
                        gp.preemptStop = true
                        gp.preempt = true

                        // Prepare for asynchronous preemption.
                        asyncM2 := gp.m
                        asyncGen2 := atomic.Load(&asyncM2.preemptGen)
                        needAsync := asyncM != asyncM2 || asyncGen != asyncGen2
                        asyncM = asyncM2
                        asyncGen = asyncGen2

                        casfrom_Gscanstatus(gp, _Gscanrunning, _Grunning)

                        // Send asynchronous preemption. We do this
                        // after CASing the G back to _Grunning
                        // because preemptM may be synchronous and we
                        // don't want to catch the G just spinning on
                        // its status.
                        if preemptMSupported && debug.asyncpreemptoff == 0 && needAsync {
                                // Rate limit preemptM calls. This is
                                // particularly important on Windows
                                // where preemptM is actually
                                // synchronous and the spin loop here
                                // can lead to live-lock.
                                now := nanotime()
                                if now >= nextPreemptM {
                                        nextPreemptM = now + yieldDelay/2
                                        preemptM(asyncM)
                                }
                        }
                }

                // TODO: Don't busy wait. This loop should really only
                // be a simple read/decide/CAS loop that only fails if
                // there's an active race. Once the CAS succeeds, we
                // should queue up the preemption (which will require
                // it to be reliable in the _Grunning case, not
                // best-effort) and then sleep until we're notified
                // that the goroutine is suspended.
                if i == 0 {
                        nextYield = nanotime() + yieldDelay
                }
                if nanotime() < nextYield {
                        procyield(10)
                } else {
                        osyield()
                        nextYield = nanotime() + yieldDelay/2
                }
        }
}

// resumeG undoes the effects of suspendG, allowing the suspended
// goroutine to continue from its current safe-point.
func resumeG(state suspendGState) {
        if state.dead {
                // We didn't actually stop anything.
                return
        }

        gp := state.g
        switch s := readgstatus(gp); s {
        default:
                dumpgstatus(gp)
                throw("unexpected g status")

        case _Grunnable | _Gscan,
                _Gwaiting | _Gscan,
                _Gsyscall | _Gscan:
                casfrom_Gscanstatus(gp, s, s&^_Gscan)
        }

        if state.stopped {
                // We stopped it, so we need to re-schedule it.
                ready(gp, 0, true)
        }
}

// canPreemptM reports whether mp is in a state that is safe to preempt.
//
// It is nosplit because it has nosplit callers.
//
//go:nosplit
func canPreemptM(mp *m) bool {
        return mp.locks == 0 && mp.mallocing == 0 && mp.preemptoff == "" && mp.p.ptr().status == _Prunning
}

//go:generate go run mkpreempt.go

// asyncPreempt saves all user registers and calls asyncPreempt2.
//
// When stack scanning encounters an asyncPreempt frame, it scans that
// frame and its parent frame conservatively.
//
// asyncPreempt is implemented in assembly.
func asyncPreempt()

//go:nosplit
func asyncPreempt2() {
        gp := getg()
        gp.asyncSafePoint = true
        if gp.preemptStop {
                mcall(preemptPark)
        } else {
                mcall(gopreempt_m)
        }
        gp.asyncSafePoint = false
}

// wantAsyncPreempt returns whether an asynchronous preemption is
// queued for gp.
func wantAsyncPreempt(gp *g) bool {
        // Check both the G and the P.
        return (gp.preempt || gp.m.p != 0 && gp.m.p.ptr().preempt) && readgstatus(gp)&^_Gscan == _Grunning
}

// isAsyncSafePoint reports whether gp at instruction PC is an
// asynchronous safe point. This indicates that:
//
// 1. It's safe to suspend gp and conservatively scan its stack and
// registers. There are no potentially hidden pointer values and it's
// not in the middle of an atomic sequence like a write barrier.
//
// 2. gp has enough stack space to inject the asyncPreempt call.
//
// 3. It's generally safe to interact with the runtime, even if we're
// in a signal handler stopped here. For example, there are no runtime
// locks held, so acquiring a runtime lock won't self-deadlock.
//
// In some cases the PC is safe for asynchronous preemption but it
// also needs to adjust the resumption PC. The new PC is returned in
// the second result.
func isAsyncSafePoint(gp *g, pc uintptr) (bool, uintptr) {
        mp := gp.m

        // Only user Gs can have safe-points. We check this first
        // because it's extremely common that we'll catch mp in the
        // scheduler processing this G preemption.
        if mp.curg != gp {
                return false, 0
        }

        // Check M state.
        if mp.p == 0 || !canPreemptM(mp) {
                return false, 0
        }

        // Check if PC is an unsafe-point.
        f := FuncForPC(pc)
        if f == nil {
                // Not Go code.
                return false, 0
        }
        name := f.Name()
        if hasPrefix(name, "runtime.") ||
                hasPrefix(name, "runtime_1internal_1") ||
                hasPrefix(name, "reflect.") {
                // For now we never async preempt the runtime or
                // anything closely tied to the runtime. Known issues
                // include: various points in the scheduler ("don't
                // preempt between here and here"), much of the defer
                // implementation (untyped info on stack), bulk write
                // barriers (write barrier check),
                // reflect.{makeFuncStub,methodValueCall}.
                //
                // TODO(austin): We should improve this, or opt things
                // in incrementally.
                return false, 0
        }
        return true, pc
}