94 lines
2.9 KiB
Go
94 lines
2.9 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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"runtime"
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"sync/atomic"
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)
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// An RWMutex is a reader/writer mutual exclusion lock.
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// The lock can be held by an arbitrary number of readers
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// or a single writer.
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// RWMutexes can be created as part of other
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// structures; the zero value for a RWMutex is
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// an unlocked mutex.
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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 int32 // number of pending readers
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readerWait int32 // number of departing readers
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}
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const rwmutexMaxReaders = 1 << 30
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// RLock locks rw for reading.
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func (rw *RWMutex) RLock() {
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if atomic.AddInt32(&rw.readerCount, 1) < 0 {
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// A writer is pending, wait for it.
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runtime.Semacquire(&rw.readerSem)
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}
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}
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// RUnlock undoes a single 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 atomic.AddInt32(&rw.readerCount, -1) < 0 {
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// A writer is pending.
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if atomic.AddInt32(&rw.readerWait, -1) == 0 {
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// The last reader unblocks the writer.
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runtime.Semrelease(&rw.writerSem)
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}
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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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// To ensure that the lock eventually becomes available,
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// a blocked Lock call excludes new readers from acquiring
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// the lock.
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func (rw *RWMutex) Lock() {
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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 := atomic.AddInt32(&rw.readerCount, -rwmutexMaxReaders) + rwmutexMaxReaders
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// Wait for active readers.
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if r != 0 && atomic.AddInt32(&rw.readerWait, r) != 0 {
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runtime.Semacquire(&rw.writerSem)
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}
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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 RLock (Lock) an RWMutex and then
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// arrange for another goroutine to RUnlock (Unlock) it.
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func (rw *RWMutex) Unlock() {
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// Announce to readers there is no active writer.
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r := atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders)
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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)
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}
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// Allow other writers to proceed.
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rw.w.Unlock()
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}
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// RLocker returns a Locker interface that implements
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// the Lock and 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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