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 ( 8 "sync/atomic" 9 "unsafe" 10 ) 11 12 // An RWMutex is a reader/writer mutual exclusion lock. 13 // The lock can be held by an arbitrary number of readers 14 // or a single writer. 15 // RWMutexes can be created as part of other 16 // structures; the zero value for a RWMutex is 17 // an unlocked mutex. 18 type RWMutex struct { 19 w Mutex // held if there are pending writers 20 writerSem uint32 // semaphore for writers to wait for completing readers 21 readerSem uint32 // semaphore for readers to wait for completing writers 22 readerCount int32 // number of pending readers 23 readerWait int32 // number of departing readers 24 } 25 26 const rwmutexMaxReaders = 1 << 30 27 28 // RLock locks rw for reading. 29 func (rw *RWMutex) RLock() { 30 if raceenabled { 31 _ = rw.w.state 32 raceDisable() 33 } 34 if atomic.AddInt32(&rw.readerCount, 1) < 0 { 35 // A writer is pending, wait for it. 36 runtime_Semacquire(&rw.readerSem) 37 } 38 if raceenabled { 39 raceEnable() 40 raceAcquire(unsafe.Pointer(&rw.readerSem)) 41 } 42 } 43 44 // RUnlock undoes a single RLock call; 45 // it does not affect other simultaneous readers. 46 // It is a run-time error if rw is not locked for reading 47 // on entry to RUnlock. 48 func (rw *RWMutex) RUnlock() { 49 if raceenabled { 50 _ = rw.w.state 51 raceReleaseMerge(unsafe.Pointer(&rw.writerSem)) 52 raceDisable() 53 } 54 if r := atomic.AddInt32(&rw.readerCount, -1); r < 0 { 55 if r+1 == 0 || r+1 == -rwmutexMaxReaders { 56 raceEnable() 57 panic("sync: RUnlock of unlocked RWMutex") 58 } 59 // A writer is pending. 60 if atomic.AddInt32(&rw.readerWait, -1) == 0 { 61 // The last reader unblocks the writer. 62 runtime_Semrelease(&rw.writerSem) 63 } 64 } 65 if raceenabled { 66 raceEnable() 67 } 68 } 69 70 // Lock locks rw for writing. 71 // If the lock is already locked for reading or writing, 72 // Lock blocks until the lock is available. 73 // To ensure that the lock eventually becomes available, 74 // a blocked Lock call excludes new readers from acquiring 75 // the lock. 76 func (rw *RWMutex) Lock() { 77 if raceenabled { 78 _ = rw.w.state 79 raceDisable() 80 } 81 // First, resolve competition with other writers. 82 rw.w.Lock() 83 // Announce to readers there is a pending writer. 84 r := atomic.AddInt32(&rw.readerCount, -rwmutexMaxReaders) + rwmutexMaxReaders 85 // Wait for active readers. 86 if r != 0 && atomic.AddInt32(&rw.readerWait, r) != 0 { 87 runtime_Semacquire(&rw.writerSem) 88 } 89 if raceenabled { 90 raceEnable() 91 raceAcquire(unsafe.Pointer(&rw.readerSem)) 92 raceAcquire(unsafe.Pointer(&rw.writerSem)) 93 } 94 } 95 96 // Unlock unlocks rw for writing. It is a run-time error if rw is 97 // not locked for writing on entry to Unlock. 98 // 99 // As with Mutexes, a locked RWMutex is not associated with a particular 100 // goroutine. One goroutine may RLock (Lock) an RWMutex and then 101 // arrange for another goroutine to RUnlock (Unlock) it. 102 func (rw *RWMutex) Unlock() { 103 if raceenabled { 104 _ = rw.w.state 105 raceRelease(unsafe.Pointer(&rw.readerSem)) 106 raceRelease(unsafe.Pointer(&rw.writerSem)) 107 raceDisable() 108 } 109 110 // Announce to readers there is no active writer. 111 r := atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders) 112 if r >= rwmutexMaxReaders { 113 raceEnable() 114 panic("sync: Unlock of unlocked RWMutex") 115 } 116 // Unblock blocked readers, if any. 117 for i := 0; i < int(r); i++ { 118 runtime_Semrelease(&rw.readerSem) 119 } 120 // Allow other writers to proceed. 121 rw.w.Unlock() 122 if raceenabled { 123 raceEnable() 124 } 125 } 126 127 // RLocker returns a Locker interface that implements 128 // the Lock and Unlock methods by calling rw.RLock and rw.RUnlock. 129 func (rw *RWMutex) RLocker() Locker { 130 return (*rlocker)(rw) 131 } 132 133 type rlocker RWMutex 134 135 func (r *rlocker) Lock() { (*RWMutex)(r).RLock() } 136 func (r *rlocker) Unlock() { (*RWMutex)(r).RUnlock() } 137
