Source File
mutex.go
Belonging Package
sync
// 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.
// Package sync provides basic synchronization primitives such as mutual
// exclusion locks. Other than the Once and WaitGroup types, most are intended
// for use by low-level library routines. Higher-level synchronization is
// better done via channels and communication.
//
// Values containing the types defined in this package should not be copied.
package sync
import (
)
// Provided by runtime via linkname.
func (string)
func (string)
// A Mutex is a mutual exclusion lock.
// The zero value for a Mutex is an unlocked mutex.
//
// A Mutex must not be copied after first use.
//
// In the terminology of the Go memory model,
// the n'th call to Unlock “synchronizes before” the m'th call to Lock
// for any n < m.
// A successful call to TryLock is equivalent to a call to Lock.
// A failed call to TryLock does not establish any “synchronizes before”
// relation at all.
type Mutex struct {
state int32
sema uint32
}
// A Locker represents an object that can be locked and unlocked.
type Locker interface {
Lock()
Unlock()
}
const (
mutexLocked = 1 << iota // mutex is locked
mutexWoken
mutexStarving
mutexWaiterShift = iota
// Mutex fairness.
//
// Mutex can be in 2 modes of operations: normal and starvation.
// In normal mode waiters are queued in FIFO order, but a woken up waiter
// does not own the mutex and competes with new arriving goroutines over
// the ownership. New arriving goroutines have an advantage -- they are
// already running on CPU and there can be lots of them, so a woken up
// waiter has good chances of losing. In such case it is queued at front
// of the wait queue. If a waiter fails to acquire the mutex for more than 1ms,
// it switches mutex to the starvation mode.
//
// In starvation mode ownership of the mutex is directly handed off from
// the unlocking goroutine to the waiter at the front of the queue.
// New arriving goroutines don't try to acquire the mutex even if it appears
// to be unlocked, and don't try to spin. Instead they queue themselves at
// the tail of the wait queue.
//
// If a waiter receives ownership of the mutex and sees that either
// (1) it is the last waiter in the queue, or (2) it waited for less than 1 ms,
// it switches mutex back to normal operation mode.
//
// Normal mode has considerably better performance as a goroutine can acquire
// a mutex several times in a row even if there are blocked waiters.
// Starvation mode is important to prevent pathological cases of tail latency.
starvationThresholdNs = 1e6
)
// Lock locks m.
// If the lock is already in use, the calling goroutine
// blocks until the mutex is available.
func ( *Mutex) () {
// Fast path: grab unlocked mutex.
if atomic.CompareAndSwapInt32(&.state, 0, mutexLocked) {
if race.Enabled {
race.Acquire(unsafe.Pointer())
}
return
}
// Slow path (outlined so that the fast path can be inlined)
.lockSlow()
}
// TryLock tries to lock m and reports whether it succeeded.
//
// Note that while correct uses of TryLock do exist, they are rare,
// and use of TryLock is often a sign of a deeper problem
// in a particular use of mutexes.
func ( *Mutex) () bool {
:= .state
if &(mutexLocked|mutexStarving) != 0 {
return false
}
// There may be a goroutine waiting for the mutex, but we are
// running now and can try to grab the mutex before that
// goroutine wakes up.
if !atomic.CompareAndSwapInt32(&.state, , |mutexLocked) {
return false
}
if race.Enabled {
race.Acquire(unsafe.Pointer())
}
return true
}
func ( *Mutex) () {
var int64
:= false
:= false
:= 0
:= .state
for {
// Don't spin in starvation mode, ownership is handed off to waiters
// so we won't be able to acquire the mutex anyway.
if &(mutexLocked|mutexStarving) == mutexLocked && runtime_canSpin() {
// Active spinning makes sense.
// Try to set mutexWoken flag to inform Unlock
// to not wake other blocked goroutines.
if ! && &mutexWoken == 0 && >>mutexWaiterShift != 0 &&
atomic.CompareAndSwapInt32(&.state, , |mutexWoken) {
= true
}
runtime_doSpin()
++
= .state
continue
}
:=
// Don't try to acquire starving mutex, new arriving goroutines must queue.
if &mutexStarving == 0 {
|= mutexLocked
}
if &(mutexLocked|mutexStarving) != 0 {
+= 1 << mutexWaiterShift
}
// The current goroutine switches mutex to starvation mode.
// But if the mutex is currently unlocked, don't do the switch.
// Unlock expects that starving mutex has waiters, which will not
// be true in this case.
if && &mutexLocked != 0 {
|= mutexStarving
}
if {
// The goroutine has been woken from sleep,
// so we need to reset the flag in either case.
if &mutexWoken == 0 {
throw("sync: inconsistent mutex state")
}
&^= mutexWoken
}
if atomic.CompareAndSwapInt32(&.state, , ) {
if &(mutexLocked|mutexStarving) == 0 {
break // locked the mutex with CAS
}
// If we were already waiting before, queue at the front of the queue.
:= != 0
if == 0 {
= runtime_nanotime()
}
runtime_SemacquireMutex(&.sema, , 1)
= || runtime_nanotime()- > starvationThresholdNs
= .state
if &mutexStarving != 0 {
// If this goroutine was woken and mutex is in starvation mode,
// ownership was handed off to us but mutex is in somewhat
// inconsistent state: mutexLocked is not set and we are still
// accounted as waiter. Fix that.
if &(mutexLocked|mutexWoken) != 0 || >>mutexWaiterShift == 0 {
throw("sync: inconsistent mutex state")
}
:= int32(mutexLocked - 1<<mutexWaiterShift)
if ! || >>mutexWaiterShift == 1 {
// Exit starvation mode.
// Critical to do it here and consider wait time.
// Starvation mode is so inefficient, that two goroutines
// can go lock-step infinitely once they switch mutex
// to starvation mode.
-= mutexStarving
}
atomic.AddInt32(&.state, )
break
}
= true
= 0
} else {
= .state
}
}
if race.Enabled {
race.Acquire(unsafe.Pointer())
}
}
// Unlock unlocks m.
// It is a run-time error if m is not locked on entry to Unlock.
//
// A locked Mutex is not associated with a particular goroutine.
// It is allowed for one goroutine to lock a Mutex and then
// arrange for another goroutine to unlock it.
func ( *Mutex) () {
if race.Enabled {
_ = .state
race.Release(unsafe.Pointer())
}
// Fast path: drop lock bit.
:= atomic.AddInt32(&.state, -mutexLocked)
if != 0 {
// Outlined slow path to allow inlining the fast path.
// To hide unlockSlow during tracing we skip one extra frame when tracing GoUnblock.
.unlockSlow()
}
}
func ( *Mutex) ( int32) {
if (+mutexLocked)&mutexLocked == 0 {
fatal("sync: unlock of unlocked mutex")
}
if &mutexStarving == 0 {
:=
for {
// If there are no waiters or a goroutine has already
// been woken or grabbed the lock, no need to wake anyone.
// In starvation mode ownership is directly handed off from unlocking
// goroutine to the next waiter. We are not part of this chain,
// since we did not observe mutexStarving when we unlocked the mutex above.
// So get off the way.
if >>mutexWaiterShift == 0 || &(mutexLocked|mutexWoken|mutexStarving) != 0 {
return
}
// Grab the right to wake someone.
= ( - 1<<mutexWaiterShift) | mutexWoken
if atomic.CompareAndSwapInt32(&.state, , ) {
runtime_Semrelease(&.sema, false, 1)
return
}
= .state
}
} else {
// Starving mode: handoff mutex ownership to the next waiter, and yield
// our time slice so that the next waiter can start to run immediately.
// Note: mutexLocked is not set, the waiter will set it after wakeup.
// But mutex is still considered locked if mutexStarving is set,
// so new coming goroutines won't acquire it.
runtime_Semrelease(&.sema, true, 1)
}
}
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