Source file src/pkg/sync/rwmutex.go
1 // Copyright 2009 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 package sync 6 7 import "sync/atomic" 8 9 // An RWMutex is a reader/writer mutual exclusion lock. 10 // The lock can be held by an arbitrary number of readers 11 // or a single writer. 12 // RWMutexes can be created as part of other 13 // structures; the zero value for a RWMutex is 14 // an unlocked mutex. 15 type RWMutex struct { 16 w Mutex // held if there are pending writers 17 writerSem uint32 // semaphore for writers to wait for completing readers 18 readerSem uint32 // semaphore for readers to wait for completing writers 19 readerCount int32 // number of pending readers 20 readerWait int32 // number of departing readers 21 } 22 23 const rwmutexMaxReaders = 1 << 30 24 25 // RLock locks rw for reading. 26 func (rw *RWMutex) RLock() { 27 if atomic.AddInt32(&rw.readerCount, 1) < 0 { 28 // A writer is pending, wait for it. 29 runtime_Semacquire(&rw.readerSem) 30 } 31 } 32 33 // RUnlock undoes a single RLock call; 34 // it does not affect other simultaneous readers. 35 // It is a run-time error if rw is not locked for reading 36 // on entry to RUnlock. 37 func (rw *RWMutex) RUnlock() { 38 if atomic.AddInt32(&rw.readerCount, -1) < 0 { 39 // A writer is pending. 40 if atomic.AddInt32(&rw.readerWait, -1) == 0 { 41 // The last reader unblocks the writer. 42 runtime_Semrelease(&rw.writerSem) 43 } 44 } 45 } 46 47 // Lock locks rw for writing. 48 // If the lock is already locked for reading or writing, 49 // Lock blocks until the lock is available. 50 // To ensure that the lock eventually becomes available, 51 // a blocked Lock call excludes new readers from acquiring 52 // the lock. 53 func (rw *RWMutex) Lock() { 54 // First, resolve competition with other writers. 55 rw.w.Lock() 56 // Announce to readers there is a pending writer. 57 r := atomic.AddInt32(&rw.readerCount, -rwmutexMaxReaders) + rwmutexMaxReaders 58 // Wait for active readers. 59 if r != 0 && atomic.AddInt32(&rw.readerWait, r) != 0 { 60 runtime_Semacquire(&rw.writerSem) 61 } 62 } 63 64 // Unlock unlocks rw for writing. It is a run-time error if rw is 65 // not locked for writing on entry to Unlock. 66 // 67 // As with Mutexes, a locked RWMutex is not associated with a particular 68 // goroutine. One goroutine may RLock (Lock) an RWMutex and then 69 // arrange for another goroutine to RUnlock (Unlock) it. 70 func (rw *RWMutex) Unlock() { 71 // Announce to readers there is no active writer. 72 r := atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders) 73 // Unblock blocked readers, if any. 74 for i := 0; i < int(r); i++ { 75 runtime_Semrelease(&rw.readerSem) 76 } 77 // Allow other writers to proceed. 78 rw.w.Unlock() 79 } 80 81 // RLocker returns a Locker interface that implements 82 // the Lock and Unlock methods by calling rw.RLock and rw.RUnlock. 83 func (rw *RWMutex) RLocker() Locker { 84 return (*rlocker)(rw) 85 } 86 87 type rlocker RWMutex 88 89 func (r *rlocker) Lock() { (*RWMutex)(r).RLock() } 90 func (r *rlocker) Unlock() { (*RWMutex)(r).RUnlock() }