Do not error when -E provided (PR pch/78970).

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

// Copyright 2009 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.

#include <limits.h>
#include <signal.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>

#include "config.h"

#ifdef HAVE_DL_ITERATE_PHDR
#include <link.h>
#endif

#include "runtime.h"
#include "arch.h"
#include "defs.h"
#include "malloc.h"
#include "go-type.h"

#ifdef USING_SPLIT_STACK

/* FIXME: These are not declared anywhere.  */

extern void __splitstack_getcontext(void *context[10]);

extern void __splitstack_setcontext(void *context[10]);

extern void *__splitstack_makecontext(size_t, void *context[10], size_t *);

extern void * __splitstack_resetcontext(void *context[10], size_t *);

extern void *__splitstack_find(void *, void *, size_t *, void **, void **,
                               void **);

extern void __splitstack_block_signals (int *, int *);

extern void __splitstack_block_signals_context (void *context[10], int *,
                                                int *);

#endif

#ifndef PTHREAD_STACK_MIN
# define PTHREAD_STACK_MIN 8192
#endif

#if defined(USING_SPLIT_STACK) && defined(LINKER_SUPPORTS_SPLIT_STACK)
# define StackMin PTHREAD_STACK_MIN
#else
# define StackMin ((sizeof(char *) < 8) ? 2 * 1024 * 1024 : 4 * 1024 * 1024)
#endif

uintptr runtime_stacks_sys;

static void gtraceback(G*);

#ifdef __rtems__
#define __thread
#endif

static __thread G *g;

#ifndef SETCONTEXT_CLOBBERS_TLS

static inline void
initcontext(void)
{
}

static inline void
fixcontext(ucontext_t *c __attribute__ ((unused)))
{
}

#else

# if defined(__x86_64__) && defined(__sun__)

// x86_64 Solaris 10 and 11 have a bug: setcontext switches the %fs
// register to that of the thread which called getcontext.  The effect
// is that the address of all __thread variables changes.  This bug
// also affects pthread_self() and pthread_getspecific.  We work
// around it by clobbering the context field directly to keep %fs the
// same.

static __thread greg_t fs;

static inline void
initcontext(void)
{
        ucontext_t c;

        getcontext(&c);
        fs = c.uc_mcontext.gregs[REG_FSBASE];
}

static inline void
fixcontext(ucontext_t* c)
{
        c->uc_mcontext.gregs[REG_FSBASE] = fs;
}

# elif defined(__NetBSD__)

// NetBSD has a bug: setcontext clobbers tlsbase, we need to save
// and restore it ourselves.

static __thread __greg_t tlsbase;

static inline void
initcontext(void)
{
        ucontext_t c;

        getcontext(&c);
        tlsbase = c.uc_mcontext._mc_tlsbase;
}

static inline void
fixcontext(ucontext_t* c)
{
        c->uc_mcontext._mc_tlsbase = tlsbase;
}

# elif defined(__sparc__)

static inline void
initcontext(void)
{
}

static inline void
fixcontext(ucontext_t *c)
{
        /* ??? Using 
             register unsigned long thread __asm__("%g7");
             c->uc_mcontext.gregs[REG_G7] = thread;
           results in
             error: variable ‘thread’ might be clobbered by \
                ‘longjmp’ or ‘vfork’ [-Werror=clobbered]
           which ought to be false, as %g7 is a fixed register.  */

        if (sizeof (c->uc_mcontext.gregs[REG_G7]) == 8)
                asm ("stx %%g7, %0" : "=m"(c->uc_mcontext.gregs[REG_G7]));
        else
                asm ("st %%g7, %0" : "=m"(c->uc_mcontext.gregs[REG_G7]));
}

# else

#  error unknown case for SETCONTEXT_CLOBBERS_TLS

# endif

#endif

// ucontext_arg returns a properly aligned ucontext_t value.  On some
// systems a ucontext_t value must be aligned to a 16-byte boundary.
// The g structure that has fields of type ucontext_t is defined in
// Go, and Go has no simple way to align a field to such a boundary.
// So we make the field larger in runtime2.go and pick an appropriate
// offset within the field here.
static ucontext_t*
ucontext_arg(void** go_ucontext)
{
        uintptr_t p = (uintptr_t)go_ucontext;
        size_t align = __alignof__(ucontext_t);
        if(align > 16) {
                // We only ensured space for up to a 16 byte alignment
                // in libgo/go/runtime/runtime2.go.
                runtime_throw("required alignment of ucontext_t too large");
        }
        p = (p + align - 1) &~ (uintptr_t)(align - 1);
        return (ucontext_t*)p;
}

// We can not always refer to the TLS variables directly.  The
// compiler will call tls_get_addr to get the address of the variable,
// and it may hold it in a register across a call to schedule.  When
// we get back from the call we may be running in a different thread,
// in which case the register now points to the TLS variable for a
// different thread.  We use non-inlinable functions to avoid this
// when necessary.

G* runtime_g(void) __attribute__ ((noinline, no_split_stack));

G*
runtime_g(void)
{
        return g;
}

M* runtime_m(void) __attribute__ ((noinline, no_split_stack));

M*
runtime_m(void)
{
        if(g == nil)
                return nil;
        return g->m;
}

// Set g.
void
runtime_setg(G* gp)
{
        g = gp;
}

// Start a new thread.
static void
runtime_newosproc(M *mp)
{
        pthread_attr_t attr;
        sigset_t clear, old;
        pthread_t tid;
        int ret;

        if(pthread_attr_init(&attr) != 0)
                runtime_throw("pthread_attr_init");
        if(pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED) != 0)
                runtime_throw("pthread_attr_setdetachstate");

        // Block signals during pthread_create so that the new thread
        // starts with signals disabled.  It will enable them in minit.
        sigfillset(&clear);

#ifdef SIGTRAP
        // Blocking SIGTRAP reportedly breaks gdb on Alpha GNU/Linux.
        sigdelset(&clear, SIGTRAP);
#endif

        sigemptyset(&old);
        pthread_sigmask(SIG_BLOCK, &clear, &old);
        ret = pthread_create(&tid, &attr, runtime_mstart, mp);
        pthread_sigmask(SIG_SETMASK, &old, nil);

        if (ret != 0)
                runtime_throw("pthread_create");
}

// First function run by a new goroutine.  This replaces gogocall.
static void
kickoff(void)
{
        void (*fn)(void*);
        void *param;

        if(g->traceback != nil)
                gtraceback(g);

        fn = (void (*)(void*))(g->entry);
        param = g->param;
        g->param = nil;
        fn(param);
        runtime_goexit1();
}

// Switch context to a different goroutine.  This is like longjmp.
void runtime_gogo(G*) __attribute__ ((noinline));
void
runtime_gogo(G* newg)
{
#ifdef USING_SPLIT_STACK
        __splitstack_setcontext(&newg->stackcontext[0]);
#endif
        g = newg;
        newg->fromgogo = true;
        fixcontext(ucontext_arg(&newg->context[0]));
        setcontext(ucontext_arg(&newg->context[0]));
        runtime_throw("gogo setcontext returned");
}

// Save context and call fn passing g as a parameter.  This is like
// setjmp.  Because getcontext always returns 0, unlike setjmp, we use
// g->fromgogo as a code.  It will be true if we got here via
// setcontext.  g == nil the first time this is called in a new m.
void runtime_mcall(void (*)(G*)) __attribute__ ((noinline));
void
runtime_mcall(void (*pfn)(G*))
{
        M *mp;
        G *gp;
#ifndef USING_SPLIT_STACK
        void *afterregs;
#endif

        // Ensure that all registers are on the stack for the garbage
        // collector.
        __builtin_unwind_init();

        gp = g;
        mp = gp->m;
        if(gp == mp->g0)
                runtime_throw("runtime: mcall called on m->g0 stack");

        if(gp != nil) {

#ifdef USING_SPLIT_STACK
                __splitstack_getcontext(&g->stackcontext[0]);
#else
                // We have to point to an address on the stack that is
                // below the saved registers.
                gp->gcnextsp = &afterregs;
#endif
                gp->fromgogo = false;
                getcontext(ucontext_arg(&gp->context[0]));

                // When we return from getcontext, we may be running
                // in a new thread.  That means that g may have
                // changed.  It is a global variables so we will
                // reload it, but the address of g may be cached in
                // our local stack frame, and that address may be
                // wrong.  Call the function to reload the value for
                // this thread.
                gp = runtime_g();
                mp = gp->m;

                if(gp->traceback != nil)
                        gtraceback(gp);
        }
        if (gp == nil || !gp->fromgogo) {
#ifdef USING_SPLIT_STACK
                __splitstack_setcontext(&mp->g0->stackcontext[0]);
#endif
                mp->g0->entry = (byte*)pfn;
                mp->g0->param = gp;

                // It's OK to set g directly here because this case
                // can not occur if we got here via a setcontext to
                // the getcontext call just above.
                g = mp->g0;

                fixcontext(ucontext_arg(&mp->g0->context[0]));
                setcontext(ucontext_arg(&mp->g0->context[0]));
                runtime_throw("runtime: mcall function returned");
        }
}

// 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 http://golang.org/s/go11sched.

enum
{
        // Number of goroutine ids to grab from runtime_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,
};

extern Sched* runtime_getsched() __asm__ (GOSYM_PREFIX "runtime.getsched");
extern bool* runtime_getCgoHasExtraM()
  __asm__ (GOSYM_PREFIX "runtime.getCgoHasExtraM");
extern P** runtime_getAllP()
  __asm__ (GOSYM_PREFIX "runtime.getAllP");
extern G* allocg(void)
  __asm__ (GOSYM_PREFIX "runtime.allocg");

Sched*  runtime_sched;
int32   runtime_gomaxprocs;
M       runtime_m0;
G       runtime_g0;     // idle goroutine for m0
G*      runtime_lastg;
M*      runtime_allm;
P**     runtime_allp;
int8*   runtime_goos;
int32   runtime_ncpu;
bool    runtime_precisestack;
static int32    newprocs;

bool    runtime_isarchive;

void* runtime_mstart(void*);
static void runqput(P*, G*);
static G* runqget(P*);
static bool runqputslow(P*, G*, uint32, uint32);
static G* runqsteal(P*, P*);
static void mput(M*);
static M* mget(void);
static void mcommoninit(M*);
static void schedule(void);
static void procresize(int32);
static void acquirep(P*);
static P* releasep(void);
static void newm(void(*)(void), P*);
static void stopm(void);
static void startm(P*, bool);
static void handoffp(P*);
static void wakep(void);
static void stoplockedm(void);
static void startlockedm(G*);
static void sysmon(void);
static uint32 retake(int64);
static void incidlelocked(int32);
static void exitsyscall0(G*);
static void park0(G*);
static void goexit0(G*);
static void gfput(P*, G*);
static G* gfget(P*);
static void gfpurge(P*);
static void globrunqput(G*);
static void globrunqputbatch(G*, G*, int32);
static G* globrunqget(P*, int32);
static P* pidleget(void);
static void pidleput(P*);
static void injectglist(G*);
static bool preemptall(void);
static bool exitsyscallfast(void);

void allgadd(G*)
  __asm__(GOSYM_PREFIX "runtime.allgadd");
void checkdead(void)
  __asm__(GOSYM_PREFIX "runtime.checkdead");

bool runtime_isstarted;

// The bootstrap sequence is:
//
//      call osinit
//      call schedinit
//      make & queue new G
//      call runtime_mstart
//
// The new G calls runtime_main.
void
runtime_schedinit(void)
{
        M *m;
        int32 n, procs;
        String s;
        const byte *p;
        Eface i;

        runtime_sched = runtime_getsched();

        m = &runtime_m0;
        g = &runtime_g0;
        m->g0 = g;
        m->curg = g;
        g->m = m;

        initcontext();

        runtime_sched->maxmcount = 10000;
        runtime_precisestack = 0;

        // runtime_symtabinit();
        runtime_mallocinit();
        mcommoninit(m);
        runtime_alginit(); // maps must not be used before this call

        // Initialize the itable value for newErrorCString,
        // so that the next time it gets called, possibly
        // in a fault during a garbage collection, it will not
        // need to allocated memory.
        runtime_newErrorCString(0, &i);
        
        // Initialize the cached gotraceback value, since
        // gotraceback calls getenv, which mallocs on Plan 9.
        runtime_gotraceback(nil);

        runtime_goargs();
        runtime_goenvs();
        runtime_parsedebugvars();

        runtime_sched->lastpoll = runtime_nanotime();
        procs = 1;
        s = runtime_getenv("GOMAXPROCS");
        p = s.str;
        if(p != nil && (n = runtime_atoi(p, s.len)) > 0) {
                if(n > _MaxGomaxprocs)
                        n = _MaxGomaxprocs;
                procs = n;
        }
        runtime_allp = runtime_getAllP();
        procresize(procs);

        // Can not enable GC until all roots are registered.
        // mstats()->enablegc = 1;
}

extern void main_init(void) __asm__ (GOSYM_PREFIX "__go_init_main");
extern void main_main(void) __asm__ (GOSYM_PREFIX "main.main");

// Used to determine the field alignment.

struct field_align
{
  char c;
  Hchan *p;
};

static void
initDone(void *arg __attribute__ ((unused))) {
        runtime_unlockOSThread();
};

// The main goroutine.
// Note: C frames in general are not copyable during stack growth, for two reasons:
//   1) We don't know where in a frame to find pointers to other stack locations.
//   2) There's no guarantee that globals or heap values do not point into the frame.
//
// The C frame for runtime.main is copyable, because:
//   1) There are no pointers to other stack locations in the frame
//      (d.fn points at a global, d.link is nil, d.argp is -1).
//   2) The only pointer into this frame is from the defer chain,
//      which is explicitly handled during stack copying.
void
runtime_main(void* dummy __attribute__((unused)))
{
        Defer d;
        _Bool frame;
        
        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.
        runtime_lockOSThread();
        
        // Defer unlock so that runtime.Goexit during init does the unlock too.
        d.pfn = (uintptr)(void*)initDone;
        d.link = g->_defer;
        d.arg = (void*)-1;
        d._panic = g->_panic;
        d.retaddr = 0;
        d.makefunccanrecover = 0;
        d.frame = &frame;
        d.special = true;
        g->_defer = &d;

        if(g->m != &runtime_m0)
                runtime_throw("runtime_main not on m0");
        __go_go(runtime_MHeap_Scavenger, nil);

        makeMainInitDone();

        _cgo_notify_runtime_init_done();

        main_init();

        closeMainInitDone();

        if(g->_defer != &d || (void*)d.pfn != initDone)
                runtime_throw("runtime: bad defer entry after init");
        g->_defer = d.link;
        runtime_unlockOSThread();

        // For gccgo we have to wait until after main is initialized
        // to enable GC, because initializing main registers the GC
        // roots.
        mstats()->enablegc = 1;

        if(runtime_isarchive) {
                // This is not a complete program, but is instead a
                // library built using -buildmode=c-archive or
                // c-shared.  Now that we are initialized, there is
                // nothing further to do.
                return;
        }

        main_main();

        // 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 issue 3934.
        if(runtime_panicking())
                runtime_park(nil, nil, "panicwait");

        runtime_exit(0);
        for(;;)
                *(int32*)0 = 0;
}

void getTraceback(G*, G*) __asm__(GOSYM_PREFIX "runtime.getTraceback");

// getTraceback stores a traceback of gp in the g's traceback field
// and then returns to me.  We expect that gp's traceback is not nil.
// It works by saving me's current context, and checking gp's traceback field.
// If gp's traceback field is not nil, it starts running gp.
// In places where we call getcontext, we check the traceback field.
// If it is not nil, we collect a traceback, and then return to the
// goroutine stored in the traceback field, which is me.
void getTraceback(G* me, G* gp)
{
#ifdef USING_SPLIT_STACK
        __splitstack_getcontext(&me->stackcontext[0]);
#endif
        getcontext(ucontext_arg(&me->stackcontext[0]));

        if (gp->traceback != nil) {
                runtime_gogo(gp);
        }
}

static void
checkmcount(void)
{
        // sched lock is held
        if(runtime_sched->mcount > runtime_sched->maxmcount) {
                runtime_printf("runtime: program exceeds %d-thread limit\n", runtime_sched->maxmcount);
                runtime_throw("thread exhaustion");
        }
}

// Do a stack trace of gp, and then restore the context to
// gp->dotraceback.

static void
gtraceback(G* gp)
{
        Traceback* traceback;

        traceback = gp->traceback;
        gp->traceback = nil;
        if(gp->m != nil)
                runtime_throw("gtraceback: m is not nil");
        gp->m = traceback->gp->m;
        traceback->c = runtime_callers(1, traceback->locbuf,
                sizeof traceback->locbuf / sizeof traceback->locbuf[0], false);
        gp->m = nil;
        runtime_gogo(traceback->gp);
}

static void
mcommoninit(M *mp)
{
        // If there is no mcache runtime_callers() will crash,
        // and we are most likely in sysmon thread so the stack is senseless anyway.
        if(g->m->mcache)
                runtime_callers(1, mp->createstack, nelem(mp->createstack), false);

        mp->fastrand = 0x49f6428aUL + mp->id + runtime_cputicks();

        runtime_lock(&runtime_sched->lock);
        mp->id = runtime_sched->mcount++;
        checkmcount();
        runtime_mpreinit(mp);

        // Add to runtime_allm so garbage collector doesn't free m
        // when it is just in a register or thread-local storage.
        mp->alllink = runtime_allm;
        // runtime_NumCgoCall() iterates over allm w/o schedlock,
        // so we need to publish it safely.
        runtime_atomicstorep(&runtime_allm, mp);
        runtime_unlock(&runtime_sched->lock);
}

// Mark gp ready to run.
void
runtime_ready(G *gp)
{
        // Mark runnable.
        g->m->locks++;  // disable preemption because it can be holding p in a local var
        if(gp->atomicstatus != _Gwaiting) {
                runtime_printf("goroutine %D has status %d\n", gp->goid, gp->atomicstatus);
                runtime_throw("bad g->atomicstatus in ready");
        }
        gp->atomicstatus = _Grunnable;
        runqput((P*)g->m->p, gp);
        if(runtime_atomicload(&runtime_sched->npidle) != 0 && runtime_atomicload(&runtime_sched->nmspinning) == 0)  // TODO: fast atomic
                wakep();
        g->m->locks--;
}

void goready(G*, int) __asm__ (GOSYM_PREFIX "runtime.goready");

void
goready(G* gp, int traceskip __attribute__ ((unused)))
{
        runtime_ready(gp);
}

int32
runtime_gcprocs(void)
{
        int32 n;

        // Figure out how many CPUs to use during GC.
        // Limited by gomaxprocs, number of actual CPUs, and MaxGcproc.
        runtime_lock(&runtime_sched->lock);
        n = runtime_gomaxprocs;
        if(n > runtime_ncpu)
                n = runtime_ncpu > 0 ? runtime_ncpu : 1;
        if(n > MaxGcproc)
                n = MaxGcproc;
        if(n > runtime_sched->nmidle+1) // one M is currently running
                n = runtime_sched->nmidle+1;
        runtime_unlock(&runtime_sched->lock);
        return n;
}

static bool
needaddgcproc(void)
{
        int32 n;

        runtime_lock(&runtime_sched->lock);
        n = runtime_gomaxprocs;
        if(n > runtime_ncpu)
                n = runtime_ncpu;
        if(n > MaxGcproc)
                n = MaxGcproc;
        n -= runtime_sched->nmidle+1; // one M is currently running
        runtime_unlock(&runtime_sched->lock);
        return n > 0;
}

void
runtime_helpgc(int32 nproc)
{
        M *mp;
        int32 n, pos;

        runtime_lock(&runtime_sched->lock);
        pos = 0;
        for(n = 1; n < nproc; n++) {  // one M is currently running
                if(runtime_allp[pos]->mcache == g->m->mcache)
                        pos++;
                mp = mget();
                if(mp == nil)
                        runtime_throw("runtime_gcprocs inconsistency");
                mp->helpgc = n;
                mp->mcache = runtime_allp[pos]->mcache;
                pos++;
                runtime_notewakeup(&mp->park);
        }
        runtime_unlock(&runtime_sched->lock);
}

// 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.
void
runtime_freezetheworld(void)
{
        int32 i;

        if(runtime_gomaxprocs == 1)
                return;
        // 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
                runtime_sched->stopwait = 0x7fffffff;
                runtime_atomicstore((uint32*)&runtime_sched->gcwaiting, 1);
                // this should stop running goroutines
                if(!preemptall())
                        break;  // no running goroutines
                runtime_usleep(1000);
        }
        // to be sure
        runtime_usleep(1000);
        preemptall();
        runtime_usleep(1000);
}

void
runtime_stopTheWorldWithSema(void)
{
        int32 i;
        uint32 s;
        P *p;
        bool wait;

        runtime_lock(&runtime_sched->lock);
        runtime_sched->stopwait = runtime_gomaxprocs;
        runtime_atomicstore((uint32*)&runtime_sched->gcwaiting, 1);
        preemptall();
        // stop current P
        ((P*)g->m->p)->status = _Pgcstop;
        runtime_sched->stopwait--;
        // try to retake all P's in _Psyscall status
        for(i = 0; i < runtime_gomaxprocs; i++) {
                p = runtime_allp[i];
                s = p->status;
                if(s == _Psyscall && runtime_cas(&p->status, s, _Pgcstop))
                        runtime_sched->stopwait--;
        }
        // stop idle P's
        while((p = pidleget()) != nil) {
                p->status = _Pgcstop;
                runtime_sched->stopwait--;
        }
        wait = runtime_sched->stopwait > 0;
        runtime_unlock(&runtime_sched->lock);

        // wait for remaining P's to stop voluntarily
        if(wait) {
                runtime_notesleep(&runtime_sched->stopnote);
                runtime_noteclear(&runtime_sched->stopnote);
        }
        if(runtime_sched->stopwait)
                runtime_throw("stoptheworld: not stopped");
        for(i = 0; i < runtime_gomaxprocs; i++) {
                p = runtime_allp[i];
                if(p->status != _Pgcstop)
                        runtime_throw("stoptheworld: not stopped");
        }
}

static void
mhelpgc(void)
{
        g->m->helpgc = -1;
}

void
runtime_startTheWorldWithSema(void)
{
        P *p, *p1;
        M *mp;
        G *gp;
        bool add;

        g->m->locks++;  // disable preemption because it can be holding p in a local var
        gp = runtime_netpoll(false);  // non-blocking
        injectglist(gp);
        add = needaddgcproc();
        runtime_lock(&runtime_sched->lock);
        if(newprocs) {
                procresize(newprocs);
                newprocs = 0;
        } else
                procresize(runtime_gomaxprocs);
        runtime_sched->gcwaiting = 0;

        p1 = nil;
        while((p = pidleget()) != nil) {
                // procresize() puts p's with work at the beginning of the list.
                // Once we reach a p without a run queue, the rest don't have one either.
                if(p->runqhead == p->runqtail) {
                        pidleput(p);
                        break;
                }
                p->m = (uintptr)mget();
                p->link = (uintptr)p1;
                p1 = p;
        }
        if(runtime_sched->sysmonwait) {
                runtime_sched->sysmonwait = false;
                runtime_notewakeup(&runtime_sched->sysmonnote);
        }
        runtime_unlock(&runtime_sched->lock);

        while(p1) {
                p = p1;
                p1 = (P*)p1->link;
                if(p->m) {
                        mp = (M*)p->m;
                        p->m = 0;
                        if(mp->nextp)
                                runtime_throw("startTheWorldWithSema: inconsistent mp->nextp");
                        mp->nextp = (uintptr)p;
                        runtime_notewakeup(&mp->park);
                } else {
                        // Start M to run P.  Do not start another M below.
                        newm(nil, p);
                        add = false;
                }
        }

        if(add) {
                // If GC could have used another helper proc, start one now,
                // in the hope that it will be available next time.
                // It would have been even better to start it before the collection,
                // but doing so requires allocating memory, so it's tricky to
                // coordinate.  This lazy approach works out in practice:
                // we don't mind if the first couple gc rounds don't have quite
                // the maximum number of procs.
                newm(mhelpgc, nil);
        }
        g->m->locks--;
}

// Called to start an M.
void*
runtime_mstart(void* mp)
{
        M *m;

        m = (M*)mp;
        g = m->g0;
        g->m = m;

        initcontext();

        g->entry = nil;
        g->param = nil;

        // Record top of stack for use by mcall.
        // Once we call schedule we're never coming back,
        // so other calls can reuse this stack space.
#ifdef USING_SPLIT_STACK
        __splitstack_getcontext(&g->stackcontext[0]);
#else
        g->gcinitialsp = &mp;
        // Setting gcstacksize to 0 is a marker meaning that gcinitialsp
        // is the top of the stack, not the bottom.
        g->gcstacksize = 0;
        g->gcnextsp = &mp;
#endif
        getcontext(ucontext_arg(&g->context[0]));

        if(g->entry != nil) {
                // Got here from mcall.
                void (*pfn)(G*) = (void (*)(G*))g->entry;
                G* gp = (G*)g->param;
                pfn(gp);
                *(int*)0x21 = 0x21;
        }
        runtime_minit();

#ifdef USING_SPLIT_STACK
        {
                int dont_block_signals = 0;
                __splitstack_block_signals(&dont_block_signals, nil);
        }
#endif

        // Install signal handlers; after minit so that minit can
        // prepare the thread to be able to handle the signals.
        if(m == &runtime_m0) {
                if(runtime_iscgo) {
                        bool* cgoHasExtraM = runtime_getCgoHasExtraM();
                        if(!*cgoHasExtraM) {
                                *cgoHasExtraM = true;
                                runtime_newextram();
                        }
                }
                runtime_initsig(false);
        }
        
        if(m->mstartfn)
                ((void (*)(void))m->mstartfn)();

        if(m->helpgc) {
                m->helpgc = 0;
                stopm();
        } else if(m != &runtime_m0) {
                acquirep((P*)m->nextp);
                m->nextp = 0;
        }
        schedule();

        // TODO(brainman): This point is never reached, because scheduler
        // does not release os threads at the moment. But once this path
        // is enabled, we must remove our seh here.

        return nil;
}

typedef struct CgoThreadStart CgoThreadStart;
struct CgoThreadStart
{
        M *m;
        G *g;
        uintptr *tls;
        void (*fn)(void);
};

M* runtime_allocm(P*, bool, byte**, uintptr*)
        __asm__(GOSYM_PREFIX "runtime.allocm");

// Allocate a new m unassociated with any thread.
// Can use p for allocation context if needed.
M*
runtime_allocm(P *p, bool allocatestack, byte** ret_g0_stack, uintptr* ret_g0_stacksize)
{
        M *mp;

        g->m->locks++;  // disable GC because it can be called from sysmon
        if(g->m->p == 0)
                acquirep(p);  // temporarily borrow p for mallocs in this function
#if 0
        if(mtype == nil) {
                Eface e;
                runtime_gc_m_ptr(&e);
                mtype = ((const PtrType*)e.__type_descriptor)->__element_type;
        }
#endif

        mp = runtime_mal(sizeof *mp);
        mcommoninit(mp);
        mp->g0 = runtime_malg(allocatestack, false, ret_g0_stack, ret_g0_stacksize);
        mp->g0->m = mp;

        if(p == (P*)g->m->p)
                releasep();
        g->m->locks--;

        return mp;
}

void setGContext(void) __asm__ (GOSYM_PREFIX "runtime.setGContext");

// setGContext sets up a new goroutine context for the current g.
void
setGContext()
{
        int val;

        initcontext();
        g->entry = nil;
        g->param = nil;
#ifdef USING_SPLIT_STACK
        __splitstack_getcontext(&g->stackcontext[0]);
        val = 0;
        __splitstack_block_signals(&val, nil);
#else
        g->gcinitialsp = &val;
        g->gcstack = nil;
        g->gcstacksize = 0;
        g->gcnextsp = &val;
#endif
        getcontext(ucontext_arg(&g->context[0]));

        if(g->entry != nil) {
                // Got here from mcall.
                void (*pfn)(G*) = (void (*)(G*))g->entry;
                G* gp = (G*)g->param;
                pfn(gp);
                *(int*)0x22 = 0x22;
        }
}

void makeGContext(G*, byte*, uintptr)
        __asm__(GOSYM_PREFIX "runtime.makeGContext");

// makeGContext makes a new context for a g.
void
makeGContext(G* gp, byte* sp, uintptr spsize) {
        ucontext_t *uc;

        uc = ucontext_arg(&gp->context[0]);
        getcontext(uc);
        uc->uc_stack.ss_sp = sp;
        uc->uc_stack.ss_size = (size_t)spsize;
        makecontext(uc, kickoff, 0);
}

// Create a new m.  It will start off with a call to fn, or else the scheduler.
static void
newm(void(*fn)(void), P *p)
{
        M *mp;

        mp = runtime_allocm(p, false, nil, nil);
        mp->nextp = (uintptr)p;
        mp->mstartfn = (uintptr)(void*)fn;

        runtime_newosproc(mp);
}

// Stops execution of the current m until new work is available.
// Returns with acquired P.
static void
stopm(void)
{
        M* m;

        m = g->m;
        if(m->locks)
                runtime_throw("stopm holding locks");
        if(m->p)
                runtime_throw("stopm holding p");
        if(m->spinning) {
                m->spinning = false;
                runtime_xadd(&runtime_sched->nmspinning, -1);
        }

retry:
        runtime_lock(&runtime_sched->lock);
        mput(m);
        runtime_unlock(&runtime_sched->lock);
        runtime_notesleep(&m->park);
        m = g->m;
        runtime_noteclear(&m->park);
        if(m->helpgc) {
                runtime_gchelper();
                m->helpgc = 0;
                m->mcache = nil;
                goto retry;
        }
        acquirep((P*)m->nextp);
        m->nextp = 0;
}

static void
mspinning(void)
{
        g->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.
static void
startm(P *p, bool spinning)
{
        M *mp;
        void (*fn)(void);

        runtime_lock(&runtime_sched->lock);
        if(p == nil) {
                p = pidleget();
                if(p == nil) {
                        runtime_unlock(&runtime_sched->lock);
                        if(spinning)
                                runtime_xadd(&runtime_sched->nmspinning, -1);
                        return;
                }
        }
        mp = mget();
        runtime_unlock(&runtime_sched->lock);
        if(mp == nil) {
                fn = nil;
                if(spinning)
                        fn = mspinning;
                newm(fn, p);
                return;
        }
        if(mp->spinning)
                runtime_throw("startm: m is spinning");
        if(mp->nextp)
                runtime_throw("startm: m has p");
        mp->spinning = spinning;
        mp->nextp = (uintptr)p;
        runtime_notewakeup(&mp->park);
}

// Hands off P from syscall or locked M.
static void
handoffp(P *p)
{
        // if it has local work, start it straight away
        if(p->runqhead != p->runqtail || runtime_sched->runqsize) {
                startm(p, false);
                return;
        }
        // no local work, check that there are no spinning/idle M's,
        // otherwise our help is not required
        if(runtime_atomicload(&runtime_sched->nmspinning) + runtime_atomicload(&runtime_sched->npidle) == 0 &&  // TODO: fast atomic
                runtime_cas(&runtime_sched->nmspinning, 0, 1)) {
                startm(p, true);
                return;
        }
        runtime_lock(&runtime_sched->lock);
        if(runtime_sched->gcwaiting) {
                p->status = _Pgcstop;
                if(--runtime_sched->stopwait == 0)
                        runtime_notewakeup(&runtime_sched->stopnote);
                runtime_unlock(&runtime_sched->lock);
                return;
        }
        if(runtime_sched->runqsize) {
                runtime_unlock(&runtime_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(runtime_sched->npidle == (uint32)runtime_gomaxprocs-1 && runtime_atomicload64(&runtime_sched->lastpoll) != 0) {
                runtime_unlock(&runtime_sched->lock);
                startm(p, false);
                return;
        }
        pidleput(p);
        runtime_unlock(&runtime_sched->lock);
}

// Tries to add one more P to execute G's.
// Called when a G is made runnable (newproc, ready).
static void
wakep(void)
{
        // be conservative about spinning threads
        if(!runtime_cas(&runtime_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.
static void
stoplockedm(void)
{
        M *m;
        P *p;

        m = g->m;
        if(m->lockedg == nil || m->lockedg->lockedm != m)
                runtime_throw("stoplockedm: inconsistent locking");
        if(m->p) {
                // Schedule another M to run this p.
                p = releasep();
                handoffp(p);
        }
        incidlelocked(1);
        // Wait until another thread schedules lockedg again.
        runtime_notesleep(&m->park);
        m = g->m;
        runtime_noteclear(&m->park);
        if(m->lockedg->atomicstatus != _Grunnable)
                runtime_throw("stoplockedm: not runnable");
        acquirep((P*)m->nextp);
        m->nextp = 0;
}

// Schedules the locked m to run the locked gp.
static void
startlockedm(G *gp)
{
        M *mp;
        P *p;

        mp = gp->lockedm;
        if(mp == g->m)
                runtime_throw("startlockedm: locked to me");
        if(mp->nextp)
                runtime_throw("startlockedm: m has p");
        // directly handoff current P to the locked m
        incidlelocked(-1);
        p = releasep();
        mp->nextp = (uintptr)p;
        runtime_notewakeup(&mp->park);
        stopm();
}

// Stops the current m for stoptheworld.
// Returns when the world is restarted.
static void
gcstopm(void)
{
        P *p;

        if(!runtime_sched->gcwaiting)
                runtime_throw("gcstopm: not waiting for gc");
        if(g->m->spinning) {
                g->m->spinning = false;
                runtime_xadd(&runtime_sched->nmspinning, -1);
        }
        p = releasep();
        runtime_lock(&runtime_sched->lock);
        p->status = _Pgcstop;
        if(--runtime_sched->stopwait == 0)
                runtime_notewakeup(&runtime_sched->stopnote);
        runtime_unlock(&runtime_sched->lock);
        stopm();
}

// Schedules gp to run on the current M.
// Never returns.
static void
execute(G *gp)
{
        int32 hz;

        if(gp->atomicstatus != _Grunnable) {
                runtime_printf("execute: bad g status %d\n", gp->atomicstatus);
                runtime_throw("execute: bad g status");
        }
        gp->atomicstatus = _Grunning;
        gp->waitsince = 0;
        ((P*)g->m->p)->schedtick++;
        g->m->curg = gp;
        gp->m = g->m;

        // Check whether the profiler needs to be turned on or off.
        hz = runtime_sched->profilehz;
        if(g->m->profilehz != hz)
                runtime_resetcpuprofiler(hz);

        runtime_gogo(gp);
}

// Finds a runnable goroutine to execute.
// Tries to steal from other P's, get g from global queue, poll network.
static G*
findrunnable(void)
{
        G *gp;
        P *p;
        int32 i;

top:
        if(runtime_sched->gcwaiting) {
                gcstopm();
                goto top;
        }
        if(runtime_fingwait && runtime_fingwake && (gp = runtime_wakefing()) != nil)
                runtime_ready(gp);
        // local runq
        gp = runqget((P*)g->m->p);
        if(gp)
                return gp;
        // global runq
        if(runtime_sched->runqsize) {
                runtime_lock(&runtime_sched->lock);
                gp = globrunqget((P*)g->m->p, 0);
                runtime_unlock(&runtime_sched->lock);
                if(gp)
                        return gp;
        }
        // poll network
        gp = runtime_netpoll(false);  // non-blocking
        if(gp) {
                injectglist((G*)gp->schedlink);
                gp->atomicstatus = _Grunnable;
                return gp;
        }
        // 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 * runtime_atomicload(&runtime_sched->nmspinning) >= runtime_gomaxprocs - runtime_atomicload(&runtime_sched->npidle))  // TODO: fast atomic
                goto stop;
        if(!g->m->spinning) {
                g->m->spinning = true;
                runtime_xadd(&runtime_sched->nmspinning, 1);
        }
        // random steal from other P's
        for(i = 0; i < 2*runtime_gomaxprocs; i++) {
                if(runtime_sched->gcwaiting)
                        goto top;
                p = runtime_allp[runtime_fastrand1()%runtime_gomaxprocs];
                if(p == (P*)g->m->p)
                        gp = runqget(p);
                else
                        gp = runqsteal((P*)g->m->p, p);
                if(gp)
                        return gp;
        }
stop:
        // return P and block
        runtime_lock(&runtime_sched->lock);
        if(runtime_sched->gcwaiting) {
                runtime_unlock(&runtime_sched->lock);
                goto top;
        }
        if(runtime_sched->runqsize) {
                gp = globrunqget((P*)g->m->p, 0);
                runtime_unlock(&runtime_sched->lock);
                return gp;
        }
        p = releasep();
        pidleput(p);
        runtime_unlock(&runtime_sched->lock);
        if(g->m->spinning) {
                g->m->spinning = false;
                runtime_xadd(&runtime_sched->nmspinning, -1);
        }
        // check all runqueues once again
        for(i = 0; i < runtime_gomaxprocs; i++) {
                p = runtime_allp[i];
                if(p && p->runqhead != p->runqtail) {
                        runtime_lock(&runtime_sched->lock);
                        p = pidleget();
                        runtime_unlock(&runtime_sched->lock);
                        if(p) {
                                acquirep(p);
                                goto top;
                        }
                        break;
                }
        }
        // poll network
        if(runtime_xchg64(&runtime_sched->lastpoll, 0) != 0) {
                if(g->m->p)
                        runtime_throw("findrunnable: netpoll with p");
                if(g->m->spinning)
                        runtime_throw("findrunnable: netpoll with spinning");
                gp = runtime_netpoll(true);  // block until new work is available
                runtime_atomicstore64(&runtime_sched->lastpoll, runtime_nanotime());
                if(gp) {
                        runtime_lock(&runtime_sched->lock);
                        p = pidleget();
                        runtime_unlock(&runtime_sched->lock);
                        if(p) {
                                acquirep(p);
                                injectglist((G*)gp->schedlink);
                                gp->atomicstatus = _Grunnable;
                                return gp;
                        }
                        injectglist(gp);
                }
        }
        stopm();
        goto top;
}

static void
resetspinning(void)
{
        int32 nmspinning;

        if(g->m->spinning) {
                g->m->spinning = false;
                nmspinning = runtime_xadd(&runtime_sched->nmspinning, -1);
                if(nmspinning < 0)
                        runtime_throw("findrunnable: negative nmspinning");
        } else
                nmspinning = runtime_atomicload(&runtime_sched->nmspinning);

        // M wakeup policy is deliberately somewhat conservative (see nmspinning handling),
        // so see if we need to wakeup another P here.
        if (nmspinning == 0 && runtime_atomicload(&runtime_sched->npidle) > 0)
                wakep();
}

// Injects the list of runnable G's into the scheduler.
// Can run concurrently with GC.
static void
injectglist(G *glist)
{
        int32 n;
        G *gp;

        if(glist == nil)
                return;
        runtime_lock(&runtime_sched->lock);
        for(n = 0; glist; n++) {
                gp = glist;
                glist = (G*)gp->schedlink;
                gp->atomicstatus = _Grunnable;
                globrunqput(gp);
        }
        runtime_unlock(&runtime_sched->lock);

        for(; n && runtime_sched->npidle; n--)
                startm(nil, false);
}

// One round of scheduler: find a runnable goroutine and execute it.
// Never returns.
static void
schedule(void)
{
        G *gp;
        uint32 tick;

        if(g->m->locks)
                runtime_throw("schedule: holding locks");

top:
        if(runtime_sched->gcwaiting) {
                gcstopm();
                goto top;
        }

        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.
        tick = ((P*)g->m->p)->schedtick;
        // This is a fancy way to say tick%61==0,
        // it uses 2 MUL instructions instead of a single DIV and so is faster on modern processors.
        if(tick - (((uint64)tick*0x4325c53fu)>>36)*61 == 0 && runtime_sched->runqsize > 0) {
                runtime_lock(&runtime_sched->lock);
                gp = globrunqget((P*)g->m->p, 1);
                runtime_unlock(&runtime_sched->lock);
                if(gp)
                        resetspinning();
        }
        if(gp == nil) {
                gp = runqget((P*)g->m->p);
                if(gp && g->m->spinning)
                        runtime_throw("schedule: spinning with local work");
        }
        if(gp == nil) {
                gp = findrunnable();  // blocks until work is available
                resetspinning();
        }

        if(gp->lockedm) {
                // Hands off own p to the locked m,
                // then blocks waiting for a new p.
                startlockedm(gp);
                goto top;
        }

        execute(gp);
}

// Puts the current goroutine into a waiting state and calls unlockf.
// If unlockf returns false, the goroutine is resumed.
void
runtime_park(bool(*unlockf)(G*, void*), void *lock, const char *reason)
{
        if(g->atomicstatus != _Grunning)
                runtime_throw("bad g status");
        g->m->waitlock = lock;
        g->m->waitunlockf = unlockf;
        g->waitreason = runtime_gostringnocopy((const byte*)reason);
        runtime_mcall(park0);
}

void gopark(FuncVal *, void *, String, byte, int)
  __asm__ ("runtime.gopark");

void
gopark(FuncVal *unlockf, void *lock, String reason,
       byte traceEv __attribute__ ((unused)),
       int traceskip __attribute__ ((unused)))
{
        if(g->atomicstatus != _Grunning)
                runtime_throw("bad g status");
        g->m->waitlock = lock;
        g->m->waitunlockf = unlockf == nil ? nil : (void*)unlockf->fn;
        g->waitreason = reason;
        runtime_mcall(park0);
}

static bool
parkunlock(G *gp, void *lock)
{
        USED(gp);
        runtime_unlock(lock);
        return true;
}

// Puts the current goroutine into a waiting state and unlocks the lock.
// The goroutine can be made runnable again by calling runtime_ready(gp).
void
runtime_parkunlock(Lock *lock, const char *reason)
{
        runtime_park(parkunlock, lock, reason);
}

void goparkunlock(Lock *, String, byte, int)
  __asm__ (GOSYM_PREFIX "runtime.goparkunlock");

void
goparkunlock(Lock *lock, String reason, byte traceEv __attribute__ ((unused)),
             int traceskip __attribute__ ((unused)))
{
        if(g->atomicstatus != _Grunning)
                runtime_throw("bad g status");
        g->m->waitlock = lock;
        g->m->waitunlockf = parkunlock;
        g->waitreason = reason;
        runtime_mcall(park0);
}

// runtime_park continuation on g0.
static void
park0(G *gp)
{
        M *m;
        bool ok;

        m = g->m;
        gp->atomicstatus = _Gwaiting;
        gp->m = nil;
        m->curg = nil;
        if(m->waitunlockf) {
                ok = ((bool (*)(G*, void*))m->waitunlockf)(gp, m->waitlock);
                m->waitunlockf = nil;
                m->waitlock = nil;
                if(!ok) {
                        gp->atomicstatus = _Grunnable;
                        execute(gp);  // Schedule it back, never returns.
                }
        }
        if(m->lockedg) {
                stoplockedm();
                execute(gp);  // Never returns.
        }
        schedule();
}

// Scheduler yield.
void
runtime_gosched(void)
{
        if(g->atomicstatus != _Grunning)
                runtime_throw("bad g status");
        runtime_mcall(runtime_gosched0);
}

// runtime_gosched continuation on g0.
void
runtime_gosched0(G *gp)
{
        M *m;

        m = g->m;
        gp->atomicstatus = _Grunnable;
        gp->m = nil;
        m->curg = nil;
        runtime_lock(&runtime_sched->lock);
        globrunqput(gp);
        runtime_unlock(&runtime_sched->lock);
        if(m->lockedg) {
                stoplockedm();
                execute(gp);  // Never returns.
        }
        schedule();
}

// Finishes execution of the current goroutine.
// Need to mark it as nosplit, because it runs with sp > stackbase (as runtime_lessstack).
// Since it does not return it does not matter.  But if it is preempted
// at the split stack check, GC will complain about inconsistent sp.
void runtime_goexit1(void) __attribute__ ((noinline));
void
runtime_goexit1(void)
{
        if(g->atomicstatus != _Grunning)
                runtime_throw("bad g status");
        runtime_mcall(goexit0);
}

// runtime_goexit1 continuation on g0.
static void
goexit0(G *gp)
{
        M *m;

        m = g->m;
        gp->atomicstatus = _Gdead;
        gp->entry = nil;
        gp->m = nil;
        gp->lockedm = nil;
        gp->paniconfault = 0;
        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.__values = nil;
        gp->writebuf.__count = 0;
        gp->writebuf.__capacity = 0;
        gp->waitreason = runtime_gostringnocopy(nil);
        gp->param = nil;
        m->curg = nil;
        m->lockedg = nil;
        if(m->locked & ~_LockExternal) {
                runtime_printf("invalid m->locked = %d\n", m->locked);
                runtime_throw("internal lockOSThread error");
        }       
        m->locked = 0;
        gfput((P*)m->p, gp);
        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.
//
// Entersyscall cannot split the stack: the runtime_gosave must
// make g->sched refer to the caller's stack segment, because
// entersyscall is going to return immediately after.

void runtime_entersyscall(int32) __attribute__ ((no_split_stack));
static void doentersyscall(uintptr, uintptr)
  __attribute__ ((no_split_stack, noinline));

void
runtime_entersyscall(int32 dummy __attribute__ ((unused)))
{
        // Save the registers in the g structure so that any pointers
        // held in registers will be seen by the garbage collector.
        getcontext(ucontext_arg(&g->gcregs[0]));

        // Do the work in a separate function, so that this function
        // doesn't save any registers on its own stack.  If this
        // function does save any registers, we might store the wrong
        // value in the call to getcontext.
        //
        // FIXME: This assumes that we do not need to save any
        // callee-saved registers to access the TLS variable g.  We
        // don't want to put the ucontext_t on the stack because it is
        // large and we can not split the stack here.
        doentersyscall((uintptr)runtime_getcallerpc(&dummy),
                       (uintptr)runtime_getcallersp(&dummy));
}

static void
doentersyscall(uintptr pc, uintptr sp)
{
        // 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++;

        // Leave SP around for GC and traceback.
#ifdef USING_SPLIT_STACK
        {
          size_t gcstacksize;
          g->gcstack = __splitstack_find(nil, nil, &gcstacksize,
                                         &g->gcnextsegment, &g->gcnextsp,
                                         &g->gcinitialsp);
          g->gcstacksize = (uintptr)gcstacksize;
        }
#else
        {
                void *v;

                g->gcnextsp = (byte *) &v;
        }
#endif

        g->syscallsp = sp;
        g->syscallpc = pc;

        g->atomicstatus = _Gsyscall;

        if(runtime_atomicload(&runtime_sched->sysmonwait)) {  // TODO: fast atomic
                runtime_lock(&runtime_sched->lock);
                if(runtime_atomicload(&runtime_sched->sysmonwait)) {
                        runtime_atomicstore(&runtime_sched->sysmonwait, 0);
                        runtime_notewakeup(&runtime_sched->sysmonnote);
                }
                runtime_unlock(&runtime_sched->lock);
        }

        g->m->mcache = nil;
        ((P*)(g->m->p))->m = 0;
        runtime_atomicstore(&((P*)g->m->p)->status, _Psyscall);
        if(runtime_atomicload(&runtime_sched->gcwaiting)) {
                runtime_lock(&runtime_sched->lock);
                if (runtime_sched->stopwait > 0 && runtime_cas(&((P*)g->m->p)->status, _Psyscall, _Pgcstop)) {
                        if(--runtime_sched->stopwait == 0)
                                runtime_notewakeup(&runtime_sched->stopnote);
                }
                runtime_unlock(&runtime_sched->lock);
        }

        g->m->locks--;
}

// The same as runtime_entersyscall(), but with a hint that the syscall is blocking.
void
runtime_entersyscallblock(int32 dummy __attribute__ ((unused)))
{
        P *p;

        g->m->locks++;  // see comment in entersyscall

        // Leave SP around for GC and traceback.
#ifdef USING_SPLIT_STACK
        {
          size_t gcstacksize;
          g->gcstack = __splitstack_find(nil, nil, &gcstacksize,
                                         &g->gcnextsegment, &g->gcnextsp,
                                         &g->gcinitialsp);
          g->gcstacksize = (uintptr)gcstacksize;
        }
#else
        g->gcnextsp = (byte *) &p;
#endif

        // Save the registers in the g structure so that any pointers
        // held in registers will be seen by the garbage collector.
        getcontext(ucontext_arg(&g->gcregs[0]));

        g->syscallpc = (uintptr)runtime_getcallerpc(&dummy);
        g->syscallsp = (uintptr)runtime_getcallersp(&dummy);

        g->atomicstatus = _Gsyscall;

        p = releasep();
        handoffp(p);
        if(g->isbackground)  // do not consider blocked scavenger for deadlock detection
                incidlelocked(1);

        g->m->locks--;
}

// 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.
void
runtime_exitsyscall(int32 dummy __attribute__ ((unused)))
{
        G *gp;

        gp = g;
        gp->m->locks++;  // see comment in entersyscall

        if(gp->isbackground)  // do not consider blocked scavenger for deadlock detection
                incidlelocked(-1);

        gp->waitsince = 0;
        if(exitsyscallfast()) {
                // There's a cpu for us, so we can run.
                ((P*)gp->m->p)->syscalltick++;
                gp->atomicstatus = _Grunning;
                // Garbage collector isn't running (since we are),
                // so okay to clear gcstack and gcsp.
#ifdef USING_SPLIT_STACK
                gp->gcstack = nil;
#endif
                gp->gcnextsp = nil;
                runtime_memclr(&gp->gcregs[0], sizeof gp->gcregs);
                gp->syscallsp = 0;
                gp->m->locks--;
                return;
        }

        gp->m->locks--;

        // Call the scheduler.
        runtime_mcall(exitsyscall0);

        // Scheduler returned, so we're allowed to run now.
        // Delete the gcstack 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.
#ifdef USING_SPLIT_STACK
        gp->gcstack = nil;
#endif
        gp->gcnextsp = nil;
        runtime_memclr(&gp->gcregs[0], sizeof gp->gcregs);

        gp->syscallsp = 0;

        // Note that this gp->m might be different than the earlier
        // gp->m after returning from runtime_mcall.
        ((P*)gp->m->p)->syscalltick++;
}

static bool
exitsyscallfast(void)
{
        G *gp;
        P *p;

        gp = g;

        // Freezetheworld sets stopwait but does not retake P's.
        if(runtime_sched->stopwait) {
                gp->m->p = 0;
                return false;
        }

        // Try to re-acquire the last P.
        if(gp->m->p && ((P*)gp->m->p)->status == _Psyscall && runtime_cas(&((P*)gp->m->p)->status, _Psyscall, _Prunning)) {
                // There's a cpu for us, so we can run.
                gp->m->mcache = ((P*)gp->m->p)->mcache;
                ((P*)gp->m->p)->m = (uintptr)gp->m;
                return true;
        }
        // Try to get any other idle P.
        gp->m->p = 0;
        if(runtime_sched->pidle) {
                runtime_lock(&runtime_sched->lock);
                p = pidleget();
                if(p && runtime_atomicload(&runtime_sched->sysmonwait)) {
                        runtime_atomicstore(&runtime_sched->sysmonwait, 0);
                        runtime_notewakeup(&runtime_sched->sysmonnote);
                }
                runtime_unlock(&runtime_sched->lock);
                if(p) {
                        acquirep(p);
                        return true;
                }
        }
        return false;
}

// runtime_exitsyscall slow path on g0.
// Failed to acquire P, enqueue gp as runnable.
static void
exitsyscall0(G *gp)
{
        M *m;
        P *p;

        m = g->m;
        gp->atomicstatus = _Grunnable;
        gp->m = nil;
        m->curg = nil;
        runtime_lock(&runtime_sched->lock);
        p = pidleget();
        if(p == nil)
                globrunqput(gp);
        else if(runtime_atomicload(&runtime_sched->sysmonwait)) {
                runtime_atomicstore(&runtime_sched->sysmonwait, 0);
                runtime_notewakeup(&runtime_sched->sysmonnote);
        }
        runtime_unlock(&runtime_sched->lock);
        if(p) {
                acquirep(p);
                execute(gp);  // Never returns.
        }
        if(m->lockedg) {
                // Wait until another thread schedules gp and so m again.
                stoplockedm();
                execute(gp);  // Never returns.
        }
        stopm();
        schedule();  // Never returns.
}

void syscall_entersyscall(void)
  __asm__(GOSYM_PREFIX "syscall.Entersyscall");

void syscall_entersyscall(void) __attribute__ ((no_split_stack));

void
syscall_entersyscall()
{
  runtime_entersyscall(0);
}

void syscall_exitsyscall(void)
  __asm__(GOSYM_PREFIX "syscall.Exitsyscall");

void syscall_exitsyscall(void) __attribute__ ((no_split_stack));

void
syscall_exitsyscall()
{
  runtime_exitsyscall(0);
}

// Called from syscall package before fork.
void syscall_runtime_BeforeFork(void)
  __asm__(GOSYM_PREFIX "syscall.runtime_BeforeFork");
void
syscall_runtime_BeforeFork(void)
{
        // Fork can hang if preempted with signals frequently enough (see issue 5517).
        // Ensure that we stay on the same M where we disable profiling.
        runtime_m()->locks++;
        if(runtime_m()->profilehz != 0)
                runtime_resetcpuprofiler(0);
}

// Called from syscall package after fork in parent.
void syscall_runtime_AfterFork(void)
  __asm__(GOSYM_PREFIX "syscall.runtime_AfterFork");
void
syscall_runtime_AfterFork(void)
{
        int32 hz;

        hz = runtime_sched->profilehz;
        if(hz != 0)
                runtime_resetcpuprofiler(hz);
        runtime_m()->locks--;
}

// Allocate a new g, with a stack big enough for stacksize bytes.
G*
runtime_malg(bool allocatestack, bool signalstack, byte** ret_stack, uintptr* ret_stacksize)
{
        uintptr stacksize;
        G *newg;
        byte* unused_stack;
        uintptr unused_stacksize;
#if USING_SPLIT_STACK
        int dont_block_signals = 0;
        size_t ss_stacksize;
#endif

        if (ret_stack == nil) {
                ret_stack = &unused_stack;
        }
        if (ret_stacksize == nil) {
                ret_stacksize = &unused_stacksize;
        }
        newg = allocg();
        if(allocatestack) {
                stacksize = StackMin;
                if(signalstack) {
                        stacksize = 32 * 1024; // OS X wants >= 8K, GNU/Linux >= 2K
#ifdef SIGSTKSZ
                        if(stacksize < SIGSTKSZ)
                                stacksize = SIGSTKSZ;
#endif
                }

#if USING_SPLIT_STACK
                *ret_stack = __splitstack_makecontext(stacksize,
                                                      &newg->stackcontext[0],
                                                      &ss_stacksize);
                *ret_stacksize = (uintptr)ss_stacksize;
                __splitstack_block_signals_context(&newg->stackcontext[0],
                                                   &dont_block_signals, nil);
#else
                // In 64-bit mode, the maximum Go allocation space is
                // 128G.  Our stack size is 4M, which only permits 32K
                // goroutines.  In order to not limit ourselves,
                // allocate the stacks out of separate memory.  In
                // 32-bit mode, the Go allocation space is all of
                // memory anyhow.
                if(sizeof(void*) == 8) {
                        void *p = runtime_SysAlloc(stacksize, &mstats()->other_sys);
                        if(p == nil)
                                runtime_throw("runtime: cannot allocate memory for goroutine stack");
                        *ret_stack = (byte*)p;
                } else {
                        *ret_stack = runtime_mallocgc(stacksize, 0, FlagNoProfiling|FlagNoGC);
                        runtime_xadd(&runtime_stacks_sys, stacksize);
                }
                *ret_stacksize = (uintptr)stacksize;
                newg->gcinitialsp = *ret_stack;
                newg->gcstacksize = (uintptr)stacksize;
#endif
        }
        return newg;
}

G*
__go_go(void (*fn)(void*), void* arg)
{
        byte *sp;
        size_t spsize;
        G *newg;
        P *p;

//runtime_printf("newproc1 %p %p narg=%d nret=%d\n", fn->fn, argp, narg, nret);
        if(fn == nil) {
                g->m->throwing = -1;  // do not dump full stacks
                runtime_throw("go of nil func value");
        }
        g->m->locks++;  // disable preemption because it can be holding p in a local var

        p = (P*)g->m->p;
        if((newg = gfget(p)) != nil) {
#ifdef USING_SPLIT_STACK
                int dont_block_signals = 0;

                sp = __splitstack_resetcontext(&newg->stackcontext[0],
                                               &spsize);
                __splitstack_block_signals_context(&newg->stackcontext[0],
                                                   &dont_block_signals, nil);
#else
                sp = newg->gcinitialsp;
                spsize = newg->gcstacksize;
                if(spsize == 0)
                        runtime_throw("bad spsize in __go_go");
                newg->gcnextsp = sp;
#endif
        } else {
                uintptr malsize;

                newg = runtime_malg(true, false, &sp, &malsize);
                spsize = (size_t)malsize;
                newg->atomicstatus = _Gdead;
                allgadd(newg);
        }

        newg->entry = (byte*)fn;
        newg->param = arg;
        newg->gopc = (uintptr)__builtin_return_address(0);
        newg->atomicstatus = _Grunnable;
        if(p->goidcache == p->goidcacheend) {
                p->goidcache = runtime_xadd64(&runtime_sched->goidgen, GoidCacheBatch);
                p->goidcacheend = p->goidcache + GoidCacheBatch;
        }
        newg->goid = p->goidcache++;

        makeGContext(newg, sp, (uintptr)spsize);

        runqput(p, newg);

        if(runtime_atomicload(&runtime_sched->npidle) != 0 && runtime_atomicload(&runtime_sched->nmspinning) == 0 && fn != runtime_main)  // TODO: fast atomic
                wakep();
        g->m->locks--;
        return newg;
}

// Put on gfree list.
// If local list is too long, transfer a batch to the global list.
static void
gfput(P *p, G *gp)
{
        gp->schedlink = (uintptr)p->gfree;
        p->gfree = gp;
        p->gfreecnt++;
        if(p->gfreecnt >= 64) {
                runtime_lock(&runtime_sched->gflock);
                while(p->gfreecnt >= 32) {
                        p->gfreecnt--;
                        gp = p->gfree;
                        p->gfree = (G*)gp->schedlink;
                        gp->schedlink = (uintptr)runtime_sched->gfree;
                        runtime_sched->gfree = gp;
                }
                runtime_unlock(&runtime_sched->gflock);
        }
}

// Get from gfree list.
// If local list is empty, grab a batch from global list.
static G*
gfget(P *p)
{
        G *gp;

retry:
        gp = p->gfree;
        if(gp == nil && runtime_sched->gfree) {
                runtime_lock(&runtime_sched->gflock);
                while(p->gfreecnt < 32 && runtime_sched->gfree) {
                        p->gfreecnt++;
                        gp = runtime_sched->gfree;
                        runtime_sched->gfree = (G*)gp->schedlink;
                        gp->schedlink = (uintptr)p->gfree;
                        p->gfree = gp;
                }
                runtime_unlock(&runtime_sched->gflock);
                goto retry;
        }
        if(gp) {
                p->gfree = (G*)gp->schedlink;
                p->gfreecnt--;
        }
        return gp;
}

// Purge all cached G's from gfree list to the global list.
static void
gfpurge(P *p)
{
        G *gp;

        runtime_lock(&runtime_sched->gflock);
        while(p->gfreecnt) {
                p->gfreecnt--;
                gp = p->gfree;
                p->gfree = (G*)gp->schedlink;
                gp->schedlink = (uintptr)runtime_sched->gfree;
                runtime_sched->gfree = gp;
        }
        runtime_unlock(&runtime_sched->gflock);
}

void
runtime_Breakpoint(void)
{
        runtime_breakpoint();
}

void runtime_Gosched (void) __asm__ (GOSYM_PREFIX "runtime.Gosched");

void
runtime_Gosched(void)
{
        runtime_gosched();
}

// Implementation of runtime.GOMAXPROCS.
// delete when scheduler is even stronger

intgo runtime_GOMAXPROCS(intgo)
  __asm__(GOSYM_PREFIX "runtime.GOMAXPROCS");

intgo
runtime_GOMAXPROCS(intgo n)
{
        intgo ret;

        if(n > _MaxGomaxprocs)
                n = _MaxGomaxprocs;
        runtime_lock(&runtime_sched->lock);
        ret = (intgo)runtime_gomaxprocs;
        if(n <= 0 || n == ret) {
                runtime_unlock(&runtime_sched->lock);
                return ret;
        }
        runtime_unlock(&runtime_sched->lock);

        runtime_acquireWorldsema();
        g->m->gcing = 1;
        runtime_stopTheWorldWithSema();
        newprocs = (int32)n;
        g->m->gcing = 0;
        runtime_releaseWorldsema();
        runtime_startTheWorldWithSema();

        return ret;
}

// lockOSThread is called by runtime.LockOSThread and runtime.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.
static void
lockOSThread(void)
{
        g->m->lockedg = g;
        g->lockedm = g->m;
}

void    runtime_LockOSThread(void) __asm__ (GOSYM_PREFIX "runtime.LockOSThread");
void
runtime_LockOSThread(void)
{
        g->m->locked |= _LockExternal;
        lockOSThread();
}

void
runtime_lockOSThread(void)
{
        g->m->locked += _LockInternal;
        lockOSThread();
}


// unlockOSThread is called by runtime.UnlockOSThread and runtime.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.
static void
unlockOSThread(void)
{
        if(g->m->locked != 0)
                return;
        g->m->lockedg = nil;
        g->lockedm = nil;
}

void    runtime_UnlockOSThread(void) __asm__ (GOSYM_PREFIX "runtime.UnlockOSThread");

void
runtime_UnlockOSThread(void)
{
        g->m->locked &= ~_LockExternal;
        unlockOSThread();
}

void
runtime_unlockOSThread(void)
{
        if(g->m->locked < _LockInternal)
                runtime_throw("runtime: internal error: misuse of lockOSThread/unlockOSThread");
        g->m->locked -= _LockInternal;
        unlockOSThread();
}

bool
runtime_lockedOSThread(void)
{
        return g->lockedm != nil && g->m->lockedg != nil;
}

int32
runtime_mcount(void)
{
        return runtime_sched->mcount;
}

static struct {
        uint32 lock;
        int32 hz;
} prof;

static void System(void) {}
static void GC(void) {}

// Called if we receive a SIGPROF signal.
void
runtime_sigprof()
{
        M *mp = g->m;
        int32 n, i;
        bool traceback;
        uintptr pcbuf[TracebackMaxFrames];
        Location locbuf[TracebackMaxFrames];
        Slice stk;

        if(prof.hz == 0)
                return;

        if(mp == nil)
                return;

        // Profiling runs concurrently with GC, so it must not allocate.
        mp->mallocing++;

        traceback = true;

        if(mp->mcache == nil)
                traceback = false;

        n = 0;

        if(runtime_atomicload(&runtime_in_callers) > 0) {
                // 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 recursive in the past.
                traceback = false;
        }

        if(traceback) {
                n = runtime_callers(0, locbuf, nelem(locbuf), false);
                for(i = 0; i < n; i++)
                        pcbuf[i] = locbuf[i].pc;
        }
        if(!traceback || n <= 0) {
                n = 2;
                pcbuf[0] = (uintptr)runtime_getcallerpc(&n);
                if(mp->gcing || mp->helpgc)
                        pcbuf[1] = (uintptr)GC;
                else
                        pcbuf[1] = (uintptr)System;
        }

        if (prof.hz != 0) {
                stk.__values = &pcbuf[0];
                stk.__count = n;
                stk.__capacity = n;

                // Simple cas-lock to coordinate with setcpuprofilerate.
                while (!runtime_cas(&prof.lock, 0, 1)) {
                        runtime_osyield();
                }
                if (prof.hz != 0) {
                        runtime_cpuprofAdd(stk);
                }
                runtime_atomicstore(&prof.lock, 0);
        }

        mp->mallocing--;
}

// Arrange to call fn with a traceback hz times a second.
void
runtime_setcpuprofilerate_m(int32 hz)
{
        // Force sane arguments.
        if(hz < 0)
                hz = 0;

        // Disable preemption, otherwise we can be rescheduled to another thread
        // that has profiling enabled.
        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.
        runtime_resetcpuprofiler(0);

        while (!runtime_cas(&prof.lock, 0, 1)) {
                runtime_osyield();
        }
        prof.hz = hz;
        runtime_atomicstore(&prof.lock, 0);

        runtime_lock(&runtime_sched->lock);
        runtime_sched->profilehz = hz;
        runtime_unlock(&runtime_sched->lock);

        if(hz != 0)
                runtime_resetcpuprofiler(hz);

        g->m->locks--;
}

// Change number of processors.  The world is stopped, sched is locked.
static void
procresize(int32 new)
{
        int32 i, old;
        bool pempty;
        G *gp;
        P *p;
        intgo j;

        old = runtime_gomaxprocs;
        if(old < 0 || old > _MaxGomaxprocs || new <= 0 || new >_MaxGomaxprocs)
                runtime_throw("procresize: invalid arg");
        // initialize new P's
        for(i = 0; i < new; i++) {
                p = runtime_allp[i];
                if(p == nil) {
                        p = (P*)runtime_mallocgc(sizeof(*p), 0, FlagNoInvokeGC);
                        p->id = i;
                        p->status = _Pgcstop;
                        p->deferpool.__values = &p->deferpoolbuf[0];
                        p->deferpool.__count = 0;
                        p->deferpool.__capacity = nelem(p->deferpoolbuf);
                        runtime_atomicstorep(&runtime_allp[i], p);
                }
                if(p->mcache == nil) {
                        if(old==0 && i==0)
                                p->mcache = g->m->mcache;  // bootstrap
                        else
                                p->mcache = runtime_allocmcache();
                }
        }

        // redistribute runnable G's evenly
        // collect all runnable goroutines in global queue preserving FIFO order
        // FIFO order is required to ensure fairness even during frequent GCs
        // see http://golang.org/issue/7126
        pempty = false;
        while(!pempty) {
                pempty = true;
                for(i = 0; i < old; i++) {
                        p = runtime_allp[i];
                        if(p->runqhead == p->runqtail)
                                continue;
                        pempty = false;
                        // pop from tail of local queue
                        p->runqtail--;
                        gp = (G*)p->runq[p->runqtail%nelem(p->runq)];
                        // push onto head of global queue
                        gp->schedlink = runtime_sched->runqhead;
                        runtime_sched->runqhead = (uintptr)gp;
                        if(runtime_sched->runqtail == 0)
                                runtime_sched->runqtail = (uintptr)gp;
                        runtime_sched->runqsize++;
                }
        }
        // fill local queues with at most nelem(p->runq)/2 goroutines
        // start at 1 because current M already executes some G and will acquire allp[0] below,
        // so if we have a spare G we want to put it into allp[1].
        for(i = 1; (uint32)i < (uint32)new * nelem(p->runq)/2 && runtime_sched->runqsize > 0; i++) {
                gp = (G*)runtime_sched->runqhead;
                runtime_sched->runqhead = gp->schedlink;
                if(runtime_sched->runqhead == 0)
                        runtime_sched->runqtail = 0;
                runtime_sched->runqsize--;
                runqput(runtime_allp[i%new], gp);
        }

        // free unused P's
        for(i = new; i < old; i++) {
                p = runtime_allp[i];
                for(j = 0; j < p->deferpool.__count; j++) {
                        ((struct _defer**)p->deferpool.__values)[j] = nil;
                }
                p->deferpool.__count = 0;
                runtime_freemcache(p->mcache);
                p->mcache = nil;
                gfpurge(p);
                p->status = _Pdead;
                // can't free P itself because it can be referenced by an M in syscall
        }

        if(g->m->p)
                ((P*)g->m->p)->m = 0;
        g->m->p = 0;
        g->m->mcache = nil;
        p = runtime_allp[0];
        p->m = 0;
        p->status = _Pidle;
        acquirep(p);
        for(i = new-1; i > 0; i--) {
                p = runtime_allp[i];
                p->status = _Pidle;
                pidleput(p);
        }
        runtime_atomicstore((uint32*)&runtime_gomaxprocs, new);
}

// Associate p and the current m.
static void
acquirep(P *p)
{
        M *m;

        m = g->m;
        if(m->p || m->mcache)
                runtime_throw("acquirep: already in go");
        if(p->m || p->status != _Pidle) {
                runtime_printf("acquirep: p->m=%p(%d) p->status=%d\n", p->m, p->m ? ((M*)p->m)->id : 0, p->status);
                runtime_throw("acquirep: invalid p state");
        }
        m->mcache = p->mcache;
        m->p = (uintptr)p;
        p->m = (uintptr)m;
        p->status = _Prunning;
}

// Disassociate p and the current m.
static P*
releasep(void)
{
        M *m;
        P *p;

        m = g->m;
        if(m->p == 0 || m->mcache == nil)
                runtime_throw("releasep: invalid arg");
        p = (P*)m->p;
        if((M*)p->m != m || p->mcache != m->mcache || p->status != _Prunning) {
                runtime_printf("releasep: m=%p m->p=%p p->m=%p m->mcache=%p p->mcache=%p p->status=%d\n",
                        m, m->p, p->m, m->mcache, p->mcache, p->status);
                runtime_throw("releasep: invalid p state");
        }
        m->p = 0;
        m->mcache = nil;
        p->m = 0;
        p->status = _Pidle;
        return p;
}

static void
incidlelocked(int32 v)
{
        runtime_lock(&runtime_sched->lock);
        runtime_sched->nmidlelocked += v;
        if(v > 0)
                checkdead();
        runtime_unlock(&runtime_sched->lock);
}

static void
sysmon(void)
{
        uint32 idle, delay;
        int64 now, lastpoll, lasttrace;
        G *gp;

        lasttrace = 0;
        idle = 0;  // how many cycles in succession we had not wokeup somebody
        delay = 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;
                runtime_usleep(delay);
                if(runtime_debug.schedtrace <= 0 &&
                        (runtime_sched->gcwaiting || runtime_atomicload(&runtime_sched->npidle) == (uint32)runtime_gomaxprocs)) {  // TODO: fast atomic
                        runtime_lock(&runtime_sched->lock);
                        if(runtime_atomicload(&runtime_sched->gcwaiting) || runtime_atomicload(&runtime_sched->npidle) == (uint32)runtime_gomaxprocs) {
                                runtime_atomicstore(&runtime_sched->sysmonwait, 1);
                                runtime_unlock(&runtime_sched->lock);
                                runtime_notesleep(&runtime_sched->sysmonnote);
                                runtime_noteclear(&runtime_sched->sysmonnote);
                                idle = 0;
                                delay = 20;
                        } else
                                runtime_unlock(&runtime_sched->lock);
                }
                // poll network if not polled for more than 10ms
                lastpoll = runtime_atomicload64(&runtime_sched->lastpoll);
                now = runtime_nanotime();
                if(lastpoll != 0 && lastpoll + 10*1000*1000 < now) {
                        runtime_cas64(&runtime_sched->lastpoll, lastpoll, now);
                        gp = runtime_netpoll(false);  // non-blocking
                        if(gp) {
                                // 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(gp);
                                incidlelocked(1);
                        }
                }
                // retake P's blocked in syscalls
                // and preempt long running G's
                if(retake(now))
                        idle = 0;
                else
                        idle++;

                if(runtime_debug.schedtrace > 0 && lasttrace + runtime_debug.schedtrace*1000000ll <= now) {
                        lasttrace = now;
                        runtime_schedtrace(runtime_debug.scheddetail);
                }
        }
}

typedef struct Pdesc Pdesc;
struct Pdesc
{
        uint32  schedtick;
        int64   schedwhen;
        uint32  syscalltick;
        int64   syscallwhen;
};
static Pdesc pdesc[_MaxGomaxprocs];

static uint32
retake(int64 now)
{
        uint32 i, s, n;
        int64 t;
        P *p;
        Pdesc *pd;

        n = 0;
        for(i = 0; i < (uint32)runtime_gomaxprocs; i++) {
                p = runtime_allp[i];
                if(p==nil)
                        continue;
                pd = &pdesc[i];
                s = p->status;
                if(s == _Psyscall) {
                        // Retake P from syscall if it's there for more than 1 sysmon tick (at least 20us).
                        t = p->syscalltick;
                        if(pd->syscalltick != t) {
                                pd->syscalltick = 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(p->runqhead == p->runqtail &&
                                runtime_atomicload(&runtime_sched->nmspinning) + runtime_atomicload(&runtime_sched->npidle) > 0 &&
                                pd->syscallwhen + 10*1000*1000 > now)
                                continue;
                        // 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(runtime_cas(&p->status, s, _Pidle)) {
                                n++;
                                handoffp(p);
                        }
                        incidlelocked(1);
                } else if(s == _Prunning) {
                        // Preempt G if it's running for more than 10ms.
                        t = p->schedtick;
                        if(pd->schedtick != t) {
                                pd->schedtick = t;
                                pd->schedwhen = now;
                                continue;
                        }
                        if(pd->schedwhen + 10*1000*1000 > now)
                                continue;
                        // preemptone(p);
                }
        }
        return 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.
static bool
preemptall(void)
{
        return false;
}

// Put mp on midle list.
// Sched must be locked.
static void
mput(M *mp)
{
        mp->schedlink = runtime_sched->midle;
        runtime_sched->midle = (uintptr)mp;
        runtime_sched->nmidle++;
        checkdead();
}

// Try to get an m from midle list.
// Sched must be locked.
static M*
mget(void)
{
        M *mp;

        if((mp = (M*)runtime_sched->midle) != nil){
                runtime_sched->midle = mp->schedlink;
                runtime_sched->nmidle--;
        }
        return mp;
}

// Put gp on the global runnable queue.
// Sched must be locked.
static void
globrunqput(G *gp)
{
        gp->schedlink = 0;
        if(runtime_sched->runqtail)
                ((G*)runtime_sched->runqtail)->schedlink = (uintptr)gp;
        else
                runtime_sched->runqhead = (uintptr)gp;
        runtime_sched->runqtail = (uintptr)gp;
        runtime_sched->runqsize++;
}

// Put a batch of runnable goroutines on the global runnable queue.
// Sched must be locked.
static void
globrunqputbatch(G *ghead, G *gtail, int32 n)
{
        gtail->schedlink = 0;
        if(runtime_sched->runqtail)
                ((G*)runtime_sched->runqtail)->schedlink = (uintptr)ghead;
        else
                runtime_sched->runqhead = (uintptr)ghead;
        runtime_sched->runqtail = (uintptr)gtail;
        runtime_sched->runqsize += n;
}

// Try get a batch of G's from the global runnable queue.
// Sched must be locked.
static G*
globrunqget(P *p, int32 max)
{
        G *gp, *gp1;
        int32 n;

        if(runtime_sched->runqsize == 0)
                return nil;
        n = runtime_sched->runqsize/runtime_gomaxprocs+1;
        if(n > runtime_sched->runqsize)
                n = runtime_sched->runqsize;
        if(max > 0 && n > max)
                n = max;
        if((uint32)n > nelem(p->runq)/2)
                n = nelem(p->runq)/2;
        runtime_sched->runqsize -= n;
        if(runtime_sched->runqsize == 0)
                runtime_sched->runqtail = 0;
        gp = (G*)runtime_sched->runqhead;
        runtime_sched->runqhead = gp->schedlink;
        n--;
        while(n--) {
                gp1 = (G*)runtime_sched->runqhead;
                runtime_sched->runqhead = gp1->schedlink;
                runqput(p, gp1);
        }
        return gp;
}

// Put p to on pidle list.
// Sched must be locked.
static void
pidleput(P *p)
{
        p->link = runtime_sched->pidle;
        runtime_sched->pidle = (uintptr)p;
        runtime_xadd(&runtime_sched->npidle, 1);  // TODO: fast atomic
}

// Try get a p from pidle list.
// Sched must be locked.
static P*
pidleget(void)
{
        P *p;

        p = (P*)runtime_sched->pidle;
        if(p) {
                runtime_sched->pidle = p->link;
                runtime_xadd(&runtime_sched->npidle, -1);  // TODO: fast atomic
        }
        return p;
}

// Try to put g on local runnable queue.
// If it's full, put onto global queue.
// Executed only by the owner P.
static void
runqput(P *p, G *gp)
{
        uint32 h, t;

retry:
        h = runtime_atomicload(&p->runqhead);  // load-acquire, synchronize with consumers
        t = p->runqtail;
        if(t - h < nelem(p->runq)) {
                p->runq[t%nelem(p->runq)] = (uintptr)gp;
                runtime_atomicstore(&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 suceed
        goto retry;
}

// Put g and a batch of work from local runnable queue on global queue.
// Executed only by the owner P.
static bool
runqputslow(P *p, G *gp, uint32 h, uint32 t)
{
        G *batch[nelem(p->runq)/2+1];
        uint32 n, i;

        // First, grab a batch from local queue.
        n = t-h;
        n = n/2;
        if(n != nelem(p->runq)/2)
                runtime_throw("runqputslow: queue is not full");
        for(i=0; i<n; i++)
                batch[i] = (G*)p->runq[(h+i)%nelem(p->runq)];
        if(!runtime_cas(&p->runqhead, h, h+n))  // cas-release, commits consume
                return false;
        batch[n] = gp;
        // Link the goroutines.
        for(i=0; i<n; i++)
                batch[i]->schedlink = (uintptr)batch[i+1];
        // Now put the batch on global queue.
        runtime_lock(&runtime_sched->lock);
        globrunqputbatch(batch[0], batch[n], n+1);
        runtime_unlock(&runtime_sched->lock);
        return true;
}

// Get g from local runnable queue.
// Executed only by the owner P.
static G*
runqget(P *p)
{
        G *gp;
        uint32 t, h;

        for(;;) {
                h = runtime_atomicload(&p->runqhead);  // load-acquire, synchronize with other consumers
                t = p->runqtail;
                if(t == h)
                        return nil;
                gp = (G*)p->runq[h%nelem(p->runq)];
                if(runtime_cas(&p->runqhead, h, h+1))  // cas-release, commits consume
                        return gp;
        }
}

// Grabs a batch of goroutines from local runnable queue.
// batch array must be of size nelem(p->runq)/2. Returns number of grabbed goroutines.
// Can be executed by any P.
static uint32
runqgrab(P *p, G **batch)
{
        uint32 t, h, n, i;

        for(;;) {
                h = runtime_atomicload(&p->runqhead);  // load-acquire, synchronize with other consumers
                t = runtime_atomicload(&p->runqtail);  // load-acquire, synchronize with the producer
                n = t-h;
                n = n - n/2;
                if(n == 0)
                        break;
                if(n > nelem(p->runq)/2)  // read inconsistent h and t
                        continue;
                for(i=0; i<n; i++)
                        batch[i] = (G*)p->runq[(h+i)%nelem(p->runq)];
                if(runtime_cas(&p->runqhead, h, h+n))  // cas-release, commits consume
                        break;
        }
        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).
static G*
runqsteal(P *p, P *p2)
{
        G *gp;
        G *batch[nelem(p->runq)/2];
        uint32 t, h, n, i;

        n = runqgrab(p2, batch);
        if(n == 0)
                return nil;
        n--;
        gp = batch[n];
        if(n == 0)
                return gp;
        h = runtime_atomicload(&p->runqhead);  // load-acquire, synchronize with consumers
        t = p->runqtail;
        if(t - h + n >= nelem(p->runq))
                runtime_throw("runqsteal: runq overflow");
        for(i=0; i<n; i++, t++)
                p->runq[t%nelem(p->runq)] = (uintptr)batch[i];
        runtime_atomicstore(&p->runqtail, t);  // store-release, makes the item available for consumption
        return gp;
}

void runtime_testSchedLocalQueue(void)
  __asm__("runtime.testSchedLocalQueue");

void
runtime_testSchedLocalQueue(void)
{
        P p;
        G gs[nelem(p.runq)];
        int32 i, j;

        runtime_memclr((byte*)&p, sizeof(p));

        for(i = 0; i < (int32)nelem(gs); i++) {
                if(runqget(&p) != nil)
                        runtime_throw("runq is not empty initially");
                for(j = 0; j < i; j++)
                        runqput(&p, &gs[i]);
                for(j = 0; j < i; j++) {
                        if(runqget(&p) != &gs[i]) {
                                runtime_printf("bad element at iter %d/%d\n", i, j);
                                runtime_throw("bad element");
                        }
                }
                if(runqget(&p) != nil)
                        runtime_throw("runq is not empty afterwards");
        }
}

void runtime_testSchedLocalQueueSteal(void)
  __asm__("runtime.testSchedLocalQueueSteal");

void
runtime_testSchedLocalQueueSteal(void)
{
        P p1, p2;
        G gs[nelem(p1.runq)], *gp;
        int32 i, j, s;

        runtime_memclr((byte*)&p1, sizeof(p1));
        runtime_memclr((byte*)&p2, sizeof(p2));

        for(i = 0; i < (int32)nelem(gs); i++) {
                for(j = 0; j < i; j++) {
                        gs[j].sig = 0;
                        runqput(&p1, &gs[j]);
                }
                gp = runqsteal(&p2, &p1);
                s = 0;
                if(gp) {
                        s++;
                        gp->sig++;
                }
                while((gp = runqget(&p2)) != nil) {
                        s++;
                        gp->sig++;
                }
                while((gp = runqget(&p1)) != nil)
                        gp->sig++;
                for(j = 0; j < i; j++) {
                        if(gs[j].sig != 1) {
                                runtime_printf("bad element %d(%d) at iter %d\n", j, gs[j].sig, i);
                                runtime_throw("bad element");
                        }
                }
                if(s != i/2 && s != i/2+1) {
                        runtime_printf("bad steal %d, want %d or %d, iter %d\n",
                                s, i/2, i/2+1, i);
                        runtime_throw("bad steal");
                }
        }
}

intgo
runtime_setmaxthreads(intgo in)
{
        intgo out;

        runtime_lock(&runtime_sched->lock);
        out = (intgo)runtime_sched->maxmcount;
        runtime_sched->maxmcount = (int32)in;
        checkmcount();
        runtime_unlock(&runtime_sched->lock);
        return out;
}

static intgo
procPin()
{
        M *mp;

        mp = runtime_m();
        mp->locks++;
        return (intgo)(((P*)mp->p)->id);
}

static void
procUnpin()
{
        runtime_m()->locks--;
}

intgo sync_runtime_procPin(void)
  __asm__ (GOSYM_PREFIX "sync.runtime_procPin");

intgo
sync_runtime_procPin()
{
        return procPin();
}

void sync_runtime_procUnpin(void)
  __asm__ (GOSYM_PREFIX  "sync.runtime_procUnpin");

void
sync_runtime_procUnpin()
{
        procUnpin();
}

intgo sync_atomic_runtime_procPin(void)
  __asm__ (GOSYM_PREFIX "sync_atomic.runtime_procPin");

intgo
sync_atomic_runtime_procPin()
{
        return procPin();
}

void sync_atomic_runtime_procUnpin(void)
  __asm__ (GOSYM_PREFIX  "sync_atomic.runtime_procUnpin");

void
sync_atomic_runtime_procUnpin()
{
        procUnpin();
}

// Return whether we are waiting for a GC.  This gc toolchain uses
// preemption instead.
bool
runtime_gcwaiting(void)
{
        return runtime_sched->gcwaiting;
}

// os_beforeExit is called from os.Exit(0).
//go:linkname os_beforeExit os.runtime_beforeExit

extern void os_beforeExit() __asm__ (GOSYM_PREFIX "os.runtime_beforeExit");

void
os_beforeExit()
{
}

// Active spinning for sync.Mutex.
//go:linkname sync_runtime_canSpin sync.runtime_canSpin

enum
{
        ACTIVE_SPIN = 4,
        ACTIVE_SPIN_CNT = 30,
};

extern _Bool sync_runtime_canSpin(intgo i)
  __asm__ (GOSYM_PREFIX "sync.runtime_canSpin");

_Bool
sync_runtime_canSpin(intgo i)
{
        P *p;

        // 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 on on other Ps.
        if (i >= ACTIVE_SPIN || runtime_ncpu <= 1 || runtime_gomaxprocs <= (int32)(runtime_sched->npidle+runtime_sched->nmspinning)+1) {
                return false;
        }
        p = (P*)g->m->p;
        return p != nil && p->runqhead == p->runqtail;
}

//go:linkname sync_runtime_doSpin sync.runtime_doSpin
//go:nosplit

extern void sync_runtime_doSpin(void)
  __asm__ (GOSYM_PREFIX "sync.runtime_doSpin");

void
sync_runtime_doSpin()
{
        runtime_procyield(ACTIVE_SPIN_CNT);
}

// For Go code to look at variables, until we port proc.go.

extern M** runtime_go_allm(void)
  __asm__ (GOSYM_PREFIX "runtime.allm");

M**
runtime_go_allm()
{
        return &runtime_allm;
}

intgo NumCPU(void) __asm__ (GOSYM_PREFIX "runtime.NumCPU");

intgo
NumCPU()
{
        return (intgo)(runtime_ncpu);
}