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// Copyright 2011 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.
//go:build (aix || darwin || netbsd || openbsd || plan9 || solaris || windows) && !goexperiment.spinbitmutex
package runtime
import (
"internal/runtime/atomic"
"unsafe"
)
// This implementation depends on OS-specific implementations of
//
// func semacreate(mp *m)
// Create a semaphore for mp, if it does not already have one.
//
// func semasleep(ns int64) int32
// If ns < 0, acquire m's semaphore and return 0.
// If ns >= 0, try to acquire m's semaphore for at most ns nanoseconds.
// Return 0 if the semaphore was acquired, -1 if interrupted or timed out.
//
// func semawakeup(mp *m)
// Wake up mp, which is or will soon be sleeping on its semaphore.
const (
active_spin = 4
active_spin_cnt = 30
passive_spin = 1
)
// mWaitList is part of the M struct, and holds the list of Ms that are waiting
// for a particular runtime.mutex.
//
// When an M is unable to immediately obtain a lock, it adds itself to the list
// of Ms waiting for the lock. It does that via this struct's next field,
// forming a singly-linked list with the mutex's key field pointing to the head
// of the list.
type mWaitList struct {
next muintptr // next m waiting for lock
}
func lockVerifyMSize() {}
func mutexContended(l *mutex) bool {
return atomic.Loaduintptr(&l.key) > locked
}
func lock(l *mutex) {
lockWithRank(l, getLockRank(l))
}
func lock2(l *mutex) {
gp := getg()
if gp.m.locks < 0 {
throw("runtimeĀ·lock: lock count")
}
gp.m.locks++
// Speculative grab for lock.
if atomic.Casuintptr(&l.key, 0, locked) {
return
}
semacreate(gp.m)
timer := &lockTimer{lock: l}
timer.begin()
// On uniprocessor's, no point spinning.
// On multiprocessors, spin for ACTIVE_SPIN attempts.
spin := 0
if ncpu > 1 {
spin = active_spin
}
Loop:
for i := 0; ; i++ {
v := atomic.Loaduintptr(&l.key)
if v&locked == 0 {
// Unlocked. Try to lock.
if atomic.Casuintptr(&l.key, v, v|locked) {
timer.end()
return
}
i = 0
}
if i < spin {
procyield(active_spin_cnt)
} else if i < spin+passive_spin {
osyield()
} else {
// Someone else has it.
// l.key points to a linked list of M's waiting
// for this lock, chained through m.mWaitList.next.
// Queue this M.
for {
gp.m.mWaitList.next = muintptr(v &^ locked)
if atomic.Casuintptr(&l.key, v, uintptr(unsafe.Pointer(gp.m))|locked) {
break
}
v = atomic.Loaduintptr(&l.key)
if v&locked == 0 {
continue Loop
}
}
if v&locked != 0 {
// Queued. Wait.
semasleep(-1)
i = 0
}
}
}
}
func unlock(l *mutex) {
unlockWithRank(l)
}
// We might not be holding a p in this code.
//
//go:nowritebarrier
func unlock2(l *mutex) {
gp := getg()
var mp *m
for {
v := atomic.Loaduintptr(&l.key)
if v == locked {
if atomic.Casuintptr(&l.key, locked, 0) {
break
}
} else {
// Other M's are waiting for the lock.
// Dequeue an M.
mp = muintptr(v &^ locked).ptr()
if atomic.Casuintptr(&l.key, v, uintptr(mp.mWaitList.next)) {
// Dequeued an M. Wake it.
semawakeup(mp) // no use of mp after this point; it's awake
break
}
}
}
gp.m.mLockProfile.recordUnlock(l)
gp.m.locks--
if gp.m.locks < 0 {
throw("runtimeĀ·unlock: lock count")
}
if gp.m.locks == 0 && gp.preempt { // restore the preemption request in case we've cleared it in newstack
gp.stackguard0 = stackPreempt
}
}