87 lines
2.8 KiB
Go
87 lines
2.8 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 "sync/atomic"
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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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//
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// Writers take priority over Readers: no new RLocks
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// are granted while a blocked Lock call is waiting.
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type RWMutex struct {
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w Mutex // held if there are pending readers or writers
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r Mutex // held if the w is being rd
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readerCount int32 // number of pending readers
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}
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// RLock locks rw for reading.
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// If the lock is already locked for writing or there is a writer already waiting
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// to release the lock, RLock blocks until the writer has released the lock.
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func (rw *RWMutex) RLock() {
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// Use rw.r.Lock() to block granting the RLock if a goroutine
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// is waiting for its Lock. This is the prevent starvation of W in
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// this situation:
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// A: rw.RLock() // granted
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// W: rw.Lock() // waiting for rw.w().Lock()
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// B: rw.RLock() // granted
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// C: rw.RLock() // granted
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// B: rw.RUnlock()
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// ... (new readers come and go indefinitely, W is starving)
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rw.r.Lock()
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if atomic.AddInt32(&rw.readerCount, 1) == 1 {
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// The first reader locks rw.w, so writers will be blocked
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// while the readers have the RLock.
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rw.w.Lock()
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}
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rw.r.Unlock()
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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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// last reader finished, enable writers
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rw.w.Unlock()
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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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rw.r.Lock()
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rw.w.Lock()
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rw.r.Unlock()
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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() { rw.w.Unlock() }
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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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