127 lines
3.1 KiB
Go
127 lines
3.1 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package sync provides basic synchronization primitives such as mutual
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// exclusion locks. Other than the Once and WaitGroup types, most are intended
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// for use by low-level library routines. Higher-level synchronization is
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// better done via channels and communication.
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//
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// Values containing the types defined in this package should not be copied.
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package sync
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import (
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"sync/atomic"
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"unsafe"
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)
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// A Mutex is a mutual exclusion lock.
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// Mutexes can be created as part of other structures;
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// the zero value for a Mutex is an unlocked mutex.
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type Mutex struct {
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state int32
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sema uint32
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}
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// A Locker represents an object that can be locked and unlocked.
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type Locker interface {
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Lock()
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Unlock()
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}
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const (
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mutexLocked = 1 << iota // mutex is locked
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mutexWoken
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mutexWaiterShift = iota
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)
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// Lock locks m.
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// If the lock is already in use, the calling goroutine
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// blocks until the mutex is available.
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func (m *Mutex) Lock() {
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// Fast path: grab unlocked mutex.
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if atomic.CompareAndSwapInt32(&m.state, 0, mutexLocked) {
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if raceenabled {
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raceAcquire(unsafe.Pointer(m))
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}
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return
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}
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awoke := false
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iter := 0
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for {
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old := m.state
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new := old | mutexLocked
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if old&mutexLocked != 0 {
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if runtime_canSpin(iter) {
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// Active spinning makes sense.
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// Try to set mutexWoken flag to inform Unlock
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// to not wake other blocked goroutines.
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if !awoke && old&mutexWoken == 0 && old>>mutexWaiterShift != 0 &&
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atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {
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awoke = true
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}
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runtime_doSpin()
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iter++
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continue
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}
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new = old + 1<<mutexWaiterShift
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}
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if awoke {
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// The goroutine has been woken from sleep,
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// so we need to reset the flag in either case.
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if new&mutexWoken == 0 {
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panic("sync: inconsistent mutex state")
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}
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new &^= mutexWoken
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}
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if atomic.CompareAndSwapInt32(&m.state, old, new) {
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if old&mutexLocked == 0 {
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break
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}
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runtime_Semacquire(&m.sema)
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awoke = true
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iter = 0
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}
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}
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if raceenabled {
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raceAcquire(unsafe.Pointer(m))
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}
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}
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// Unlock unlocks m.
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// It is a run-time error if m is not locked on entry to Unlock.
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//
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// A locked Mutex is not associated with a particular goroutine.
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// It is allowed for one goroutine to lock a Mutex and then
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// arrange for another goroutine to unlock it.
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func (m *Mutex) Unlock() {
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if raceenabled {
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_ = m.state
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raceRelease(unsafe.Pointer(m))
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}
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// Fast path: drop lock bit.
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new := atomic.AddInt32(&m.state, -mutexLocked)
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if (new+mutexLocked)&mutexLocked == 0 {
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panic("sync: unlock of unlocked mutex")
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}
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old := new
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for {
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// If there are no waiters or a goroutine has already
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// been woken or grabbed the lock, no need to wake anyone.
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if old>>mutexWaiterShift == 0 || old&(mutexLocked|mutexWoken) != 0 {
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return
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}
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// Grab the right to wake someone.
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new = (old - 1<<mutexWaiterShift) | mutexWoken
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if atomic.CompareAndSwapInt32(&m.state, old, new) {
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runtime_Semrelease(&m.sema)
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return
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}
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old = m.state
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}
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}
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