mirror of https://go.googlesource.com/go
244 lines
7.1 KiB
Go
244 lines
7.1 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
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import (
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"internal/race"
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"sync/atomic"
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"unsafe"
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)
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// There is a modified copy of this file in runtime/rwmutex.go.
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// If you make any changes here, see if you should make them there.
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// A RWMutex is a reader/writer mutual exclusion lock.
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// The lock can be held by an arbitrary number of readers or a single writer.
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// The zero value for a RWMutex is an unlocked mutex.
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//
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// A RWMutex must not be copied after first use.
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//
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// If any goroutine calls [RWMutex.Lock] while the lock is already held by
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// one or more readers, concurrent calls to [RWMutex.RLock] will block until
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// the writer has acquired (and released) the lock, to ensure that
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// the lock eventually becomes available to the writer.
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// Note that this prohibits recursive read-locking.
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//
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// In the terminology of the Go memory model,
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// the n'th call to [RWMutex.Unlock] “synchronizes before” the m'th call to Lock
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// for any n < m, just as for [Mutex].
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// For any call to RLock, there exists an n such that
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// the n'th call to Unlock “synchronizes before” that call to RLock,
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// and the corresponding call to [RWMutex.RUnlock] “synchronizes before”
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// the n+1'th call to Lock.
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type RWMutex struct {
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w Mutex // held if there are pending writers
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writerSem uint32 // semaphore for writers to wait for completing readers
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readerSem uint32 // semaphore for readers to wait for completing writers
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readerCount atomic.Int32 // number of pending readers
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readerWait atomic.Int32 // number of departing readers
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}
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const rwmutexMaxReaders = 1 << 30
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// Happens-before relationships are indicated to the race detector via:
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// - Unlock -> Lock: readerSem
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// - Unlock -> RLock: readerSem
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// - RUnlock -> Lock: writerSem
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//
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// The methods below temporarily disable handling of race synchronization
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// events in order to provide the more precise model above to the race
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// detector.
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//
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// For example, atomic.AddInt32 in RLock should not appear to provide
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// acquire-release semantics, which would incorrectly synchronize racing
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// readers, thus potentially missing races.
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// RLock locks rw for reading.
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//
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// It should not be used for recursive read locking; a blocked Lock
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// call excludes new readers from acquiring the lock. See the
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// documentation on the [RWMutex] type.
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func (rw *RWMutex) RLock() {
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if race.Enabled {
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_ = rw.w.state
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race.Disable()
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}
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if rw.readerCount.Add(1) < 0 {
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// A writer is pending, wait for it.
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runtime_SemacquireRWMutexR(&rw.readerSem, false, 0)
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}
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if race.Enabled {
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race.Enable()
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race.Acquire(unsafe.Pointer(&rw.readerSem))
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}
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}
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// TryRLock tries to lock rw for reading and reports whether it succeeded.
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//
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// Note that while correct uses of TryRLock do exist, they are rare,
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// and use of TryRLock is often a sign of a deeper problem
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// in a particular use of mutexes.
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func (rw *RWMutex) TryRLock() bool {
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if race.Enabled {
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_ = rw.w.state
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race.Disable()
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}
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for {
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c := rw.readerCount.Load()
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if c < 0 {
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if race.Enabled {
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race.Enable()
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}
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return false
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}
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if rw.readerCount.CompareAndSwap(c, c+1) {
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if race.Enabled {
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race.Enable()
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race.Acquire(unsafe.Pointer(&rw.readerSem))
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}
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return true
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}
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}
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}
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// RUnlock undoes a single [RWMutex.RLock] call;
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// it does not affect other simultaneous readers.
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// It is a run-time error if rw is not locked for reading
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// on entry to RUnlock.
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func (rw *RWMutex) RUnlock() {
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if race.Enabled {
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_ = rw.w.state
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race.ReleaseMerge(unsafe.Pointer(&rw.writerSem))
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race.Disable()
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}
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if r := rw.readerCount.Add(-1); r < 0 {
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// Outlined slow-path to allow the fast-path to be inlined
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rw.rUnlockSlow(r)
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}
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if race.Enabled {
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race.Enable()
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}
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}
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func (rw *RWMutex) rUnlockSlow(r int32) {
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if r+1 == 0 || r+1 == -rwmutexMaxReaders {
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race.Enable()
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fatal("sync: RUnlock of unlocked RWMutex")
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}
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// A writer is pending.
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if rw.readerWait.Add(-1) == 0 {
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// The last reader unblocks the writer.
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runtime_Semrelease(&rw.writerSem, false, 1)
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}
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}
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// Lock locks rw for writing.
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// If the lock is already locked for reading or writing,
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// Lock blocks until the lock is available.
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func (rw *RWMutex) Lock() {
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if race.Enabled {
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_ = rw.w.state
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race.Disable()
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}
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// First, resolve competition with other writers.
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rw.w.Lock()
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// Announce to readers there is a pending writer.
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r := rw.readerCount.Add(-rwmutexMaxReaders) + rwmutexMaxReaders
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// Wait for active readers.
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if r != 0 && rw.readerWait.Add(r) != 0 {
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runtime_SemacquireRWMutex(&rw.writerSem, false, 0)
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}
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if race.Enabled {
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race.Enable()
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race.Acquire(unsafe.Pointer(&rw.readerSem))
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race.Acquire(unsafe.Pointer(&rw.writerSem))
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}
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}
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// TryLock tries to lock rw for writing and reports whether it succeeded.
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//
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// Note that while correct uses of TryLock do exist, they are rare,
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// and use of TryLock is often a sign of a deeper problem
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// in a particular use of mutexes.
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func (rw *RWMutex) TryLock() bool {
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if race.Enabled {
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_ = rw.w.state
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race.Disable()
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}
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if !rw.w.TryLock() {
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if race.Enabled {
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race.Enable()
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}
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return false
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}
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if !rw.readerCount.CompareAndSwap(0, -rwmutexMaxReaders) {
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rw.w.Unlock()
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if race.Enabled {
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race.Enable()
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}
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return false
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}
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if race.Enabled {
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race.Enable()
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race.Acquire(unsafe.Pointer(&rw.readerSem))
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race.Acquire(unsafe.Pointer(&rw.writerSem))
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}
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return true
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}
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// Unlock unlocks rw for writing. It is a run-time error if rw is
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// not locked for writing on entry to Unlock.
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//
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// As with Mutexes, a locked [RWMutex] is not associated with a particular
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// goroutine. One goroutine may [RWMutex.RLock] ([RWMutex.Lock]) a RWMutex and then
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// arrange for another goroutine to [RWMutex.RUnlock] ([RWMutex.Unlock]) it.
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func (rw *RWMutex) Unlock() {
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if race.Enabled {
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_ = rw.w.state
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race.Release(unsafe.Pointer(&rw.readerSem))
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race.Disable()
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}
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// Announce to readers there is no active writer.
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r := rw.readerCount.Add(rwmutexMaxReaders)
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if r >= rwmutexMaxReaders {
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race.Enable()
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fatal("sync: Unlock of unlocked RWMutex")
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}
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// Unblock blocked readers, if any.
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for i := 0; i < int(r); i++ {
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runtime_Semrelease(&rw.readerSem, false, 0)
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}
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// Allow other writers to proceed.
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rw.w.Unlock()
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if race.Enabled {
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race.Enable()
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}
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}
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// syscall_hasWaitingReaders reports whether any goroutine is waiting
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// to acquire a read lock on rw. This exists because syscall.ForkLock
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// is an RWMutex, and we can't change that without breaking compatibility.
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// We don't need or want RWMutex semantics for ForkLock, and we use
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// this private API to avoid having to change the type of ForkLock.
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// For more details see the syscall package.
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//
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//go:linkname syscall_hasWaitingReaders syscall.hasWaitingReaders
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func syscall_hasWaitingReaders(rw *RWMutex) bool {
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r := rw.readerCount.Load()
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return r < 0 && r+rwmutexMaxReaders > 0
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}
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// RLocker returns a [Locker] interface that implements
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// the [RWMutex.Lock] and [RWMutex.Unlock] methods by calling rw.RLock and rw.RUnlock.
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func (rw *RWMutex) RLocker() Locker {
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return (*rlocker)(rw)
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}
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type rlocker RWMutex
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func (r *rlocker) Lock() { (*RWMutex)(r).RLock() }
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func (r *rlocker) Unlock() { (*RWMutex)(r).RUnlock() }
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