Source File
rwmutex.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
import (
)
// There is a modified copy of this file in runtime/rwmutex.go.
// If you make any changes here, see if you should make them there.
// A RWMutex is a reader/writer mutual exclusion lock.
// The lock can be held by an arbitrary number of readers or a single writer.
// The zero value for a RWMutex is an unlocked mutex.
//
// A RWMutex must not be copied after first use.
//
// If any goroutine calls [RWMutex.Lock] while the lock is already held by
// one or more readers, concurrent calls to [RWMutex.RLock] will block until
// the writer has acquired (and released) the lock, to ensure that
// the lock eventually becomes available to the writer.
// Note that this prohibits recursive read-locking.
//
// In the terminology of [the Go memory model],
// the n'th call to [RWMutex.Unlock] “synchronizes before” the m'th call to Lock
// for any n < m, just as for [Mutex].
// For any call to RLock, there exists an n such that
// the n'th call to Unlock “synchronizes before” that call to RLock,
// and the corresponding call to [RWMutex.RUnlock] “synchronizes before”
// the n+1'th call to Lock.
//
// [the Go memory model]: https://go.dev/ref/mem
type RWMutex struct {
w Mutex // held if there are pending writers
writerSem uint32 // semaphore for writers to wait for completing readers
readerSem uint32 // semaphore for readers to wait for completing writers
readerCount atomic.Int32 // number of pending readers
readerWait atomic.Int32 // number of departing readers
}
const rwmutexMaxReaders = 1 << 30
// Happens-before relationships are indicated to the race detector via:
// - Unlock -> Lock: readerSem
// - Unlock -> RLock: readerSem
// - RUnlock -> Lock: writerSem
//
// The methods below temporarily disable handling of race synchronization
// events in order to provide the more precise model above to the race
// detector.
//
// For example, atomic.AddInt32 in RLock should not appear to provide
// acquire-release semantics, which would incorrectly synchronize racing
// readers, thus potentially missing races.
// RLock locks rw for reading.
//
// It should not be used for recursive read locking; a blocked Lock
// call excludes new readers from acquiring the lock. See the
// documentation on the [RWMutex] type.
func ( *RWMutex) () {
if race.Enabled {
_ = .w.state
race.Disable()
}
if .readerCount.Add(1) < 0 {
// A writer is pending, wait for it.
runtime_SemacquireRWMutexR(&.readerSem, false, 0)
}
if race.Enabled {
race.Enable()
race.Acquire(unsafe.Pointer(&.readerSem))
}
}
// TryRLock tries to lock rw for reading and reports whether it succeeded.
//
// Note that while correct uses of TryRLock do exist, they are rare,
// and use of TryRLock is often a sign of a deeper problem
// in a particular use of mutexes.
func ( *RWMutex) () bool {
if race.Enabled {
_ = .w.state
race.Disable()
}
for {
:= .readerCount.Load()
if < 0 {
if race.Enabled {
race.Enable()
}
return false
}
if .readerCount.CompareAndSwap(, +1) {
if race.Enabled {
race.Enable()
race.Acquire(unsafe.Pointer(&.readerSem))
}
return true
}
}
}
// RUnlock undoes a single [RWMutex.RLock] call;
// it does not affect other simultaneous readers.
// It is a run-time error if rw is not locked for reading
// on entry to RUnlock.
func ( *RWMutex) () {
if race.Enabled {
_ = .w.state
race.ReleaseMerge(unsafe.Pointer(&.writerSem))
race.Disable()
}
if := .readerCount.Add(-1); < 0 {
// Outlined slow-path to allow the fast-path to be inlined
.rUnlockSlow()
}
if race.Enabled {
race.Enable()
}
}
func ( *RWMutex) ( int32) {
if +1 == 0 || +1 == -rwmutexMaxReaders {
race.Enable()
fatal("sync: RUnlock of unlocked RWMutex")
}
// A writer is pending.
if .readerWait.Add(-1) == 0 {
// The last reader unblocks the writer.
runtime_Semrelease(&.writerSem, false, 1)
}
}
// Lock locks rw for writing.
// If the lock is already locked for reading or writing,
// Lock blocks until the lock is available.
func ( *RWMutex) () {
if race.Enabled {
_ = .w.state
race.Disable()
}
// First, resolve competition with other writers.
.w.Lock()
// Announce to readers there is a pending writer.
:= .readerCount.Add(-rwmutexMaxReaders) + rwmutexMaxReaders
// Wait for active readers.
if != 0 && .readerWait.Add() != 0 {
runtime_SemacquireRWMutex(&.writerSem, false, 0)
}
if race.Enabled {
race.Enable()
race.Acquire(unsafe.Pointer(&.readerSem))
race.Acquire(unsafe.Pointer(&.writerSem))
}
}
// TryLock tries to lock rw for writing 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 ( *RWMutex) () bool {
if race.Enabled {
_ = .w.state
race.Disable()
}
if !.w.TryLock() {
if race.Enabled {
race.Enable()
}
return false
}
if !.readerCount.CompareAndSwap(0, -rwmutexMaxReaders) {
.w.Unlock()
if race.Enabled {
race.Enable()
}
return false
}
if race.Enabled {
race.Enable()
race.Acquire(unsafe.Pointer(&.readerSem))
race.Acquire(unsafe.Pointer(&.writerSem))
}
return true
}
// Unlock unlocks rw for writing. It is a run-time error if rw is
// not locked for writing on entry to Unlock.
//
// As with Mutexes, a locked [RWMutex] is not associated with a particular
// goroutine. One goroutine may [RWMutex.RLock] ([RWMutex.Lock]) a RWMutex and then
// arrange for another goroutine to [RWMutex.RUnlock] ([RWMutex.Unlock]) it.
func ( *RWMutex) () {
if race.Enabled {
_ = .w.state
race.Release(unsafe.Pointer(&.readerSem))
race.Disable()
}
// Announce to readers there is no active writer.
:= .readerCount.Add(rwmutexMaxReaders)
if >= rwmutexMaxReaders {
race.Enable()
fatal("sync: Unlock of unlocked RWMutex")
}
// Unblock blocked readers, if any.
for := 0; < int(); ++ {
runtime_Semrelease(&.readerSem, false, 0)
}
// Allow other writers to proceed.
.w.Unlock()
if race.Enabled {
race.Enable()
}
}
// syscall_hasWaitingReaders reports whether any goroutine is waiting
// to acquire a read lock on rw. This exists because syscall.ForkLock
// is an RWMutex, and we can't change that without breaking compatibility.
// We don't need or want RWMutex semantics for ForkLock, and we use
// this private API to avoid having to change the type of ForkLock.
// For more details see the syscall package.
//
//go:linkname syscall_hasWaitingReaders syscall.hasWaitingReaders
func syscall_hasWaitingReaders( *RWMutex) bool {
:= .readerCount.Load()
return < 0 && +rwmutexMaxReaders > 0
}
// RLocker returns a [Locker] interface that implements
// the [Locker.Lock] and [Locker.Unlock] methods by calling rw.RLock and rw.RUnlock.
func ( *RWMutex) () Locker {
return (*rlocker)()
}
type rlocker RWMutex
func ( *rlocker) () { (*RWMutex)().RLock() }
func ( *rlocker) () { (*RWMutex)().RUnlock() }
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