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 "internal/race" 9 "sync/atomic" 10 "unsafe" 11 ) 12 13 // There is a modified copy of this file in runtime/rwmutex.go. 14 // If you make any changes here, see if you should make them there. 15 16 // A RWMutex is a reader/writer mutual exclusion lock. 17 // The lock can be held by an arbitrary number of readers or a single writer. 18 // The zero value for a RWMutex is an unlocked mutex. 19 // 20 // A RWMutex must not be copied after first use. 21 // 22 // If any goroutine calls [RWMutex.Lock] while the lock is already held by 23 // one or more readers, concurrent calls to [RWMutex.RLock] will block until 24 // the writer has acquired (and released) the lock, to ensure that 25 // the lock eventually becomes available to the writer. 26 // Note that this prohibits recursive read-locking. 27 // 28 // In the terminology of [the Go memory model], 29 // the n'th call to [RWMutex.Unlock] “synchronizes before” the m'th call to Lock 30 // for any n < m, just as for [Mutex]. 31 // For any call to RLock, there exists an n such that 32 // the n'th call to Unlock “synchronizes before” that call to RLock, 33 // and the corresponding call to [RWMutex.RUnlock] “synchronizes before” 34 // the n+1'th call to Lock. 35 // 36 // [the Go memory model]: https://go.dev/ref/mem 37 type RWMutex struct { 38 w Mutex // held if there are pending writers 39 writerSem uint32 // semaphore for writers to wait for completing readers 40 readerSem uint32 // semaphore for readers to wait for completing writers 41 readerCount atomic.Int32 // number of pending readers 42 readerWait atomic.Int32 // number of departing readers 43 } 44 45 const rwmutexMaxReaders = 1 << 30 46 47 // Happens-before relationships are indicated to the race detector via: 48 // - Unlock -> Lock: readerSem 49 // - Unlock -> RLock: readerSem 50 // - RUnlock -> Lock: writerSem 51 // 52 // The methods below temporarily disable handling of race synchronization 53 // events in order to provide the more precise model above to the race 54 // detector. 55 // 56 // For example, atomic.AddInt32 in RLock should not appear to provide 57 // acquire-release semantics, which would incorrectly synchronize racing 58 // readers, thus potentially missing races. 59 60 // RLock locks rw for reading. 61 // 62 // It should not be used for recursive read locking; a blocked Lock 63 // call excludes new readers from acquiring the lock. See the 64 // documentation on the [RWMutex] type. 65 func (rw *RWMutex) RLock() { 66 if race.Enabled { 67 _ = rw.w.state 68 race.Disable() 69 } 70 if rw.readerCount.Add(1) < 0 { 71 // A writer is pending, wait for it. 72 runtime_SemacquireRWMutexR(&rw.readerSem, false, 0) 73 } 74 if race.Enabled { 75 race.Enable() 76 race.Acquire(unsafe.Pointer(&rw.readerSem)) 77 } 78 } 79 80 // TryRLock tries to lock rw for reading and reports whether it succeeded. 81 // 82 // Note that while correct uses of TryRLock do exist, they are rare, 83 // and use of TryRLock is often a sign of a deeper problem 84 // in a particular use of mutexes. 85 func (rw *RWMutex) TryRLock() bool { 86 if race.Enabled { 87 _ = rw.w.state 88 race.Disable() 89 } 90 for { 91 c := rw.readerCount.Load() 92 if c < 0 { 93 if race.Enabled { 94 race.Enable() 95 } 96 return false 97 } 98 if rw.readerCount.CompareAndSwap(c, c+1) { 99 if race.Enabled { 100 race.Enable() 101 race.Acquire(unsafe.Pointer(&rw.readerSem)) 102 } 103 return true 104 } 105 } 106 } 107 108 // RUnlock undoes a single [RWMutex.RLock] call; 109 // it does not affect other simultaneous readers. 110 // It is a run-time error if rw is not locked for reading 111 // on entry to RUnlock. 112 func (rw *RWMutex) RUnlock() { 113 if race.Enabled { 114 _ = rw.w.state 115 race.ReleaseMerge(unsafe.Pointer(&rw.writerSem)) 116 race.Disable() 117 } 118 if r := rw.readerCount.Add(-1); r < 0 { 119 // Outlined slow-path to allow the fast-path to be inlined 120 rw.rUnlockSlow(r) 121 } 122 if race.Enabled { 123 race.Enable() 124 } 125 } 126 127 func (rw *RWMutex) rUnlockSlow(r int32) { 128 if r+1 == 0 || r+1 == -rwmutexMaxReaders { 129 race.Enable() 130 fatal("sync: RUnlock of unlocked RWMutex") 131 } 132 // A writer is pending. 133 if rw.readerWait.Add(-1) == 0 { 134 // The last reader unblocks the writer. 135 runtime_Semrelease(&rw.writerSem, false, 1) 136 } 137 } 138 139 // Lock locks rw for writing. 140 // If the lock is already locked for reading or writing, 141 // Lock blocks until the lock is available. 142 func (rw *RWMutex) Lock() { 143 if race.Enabled { 144 _ = rw.w.state 145 race.Disable() 146 } 147 // First, resolve competition with other writers. 148 rw.w.Lock() 149 // Announce to readers there is a pending writer. 150 r := rw.readerCount.Add(-rwmutexMaxReaders) + rwmutexMaxReaders 151 // Wait for active readers. 152 if r != 0 && rw.readerWait.Add(r) != 0 { 153 runtime_SemacquireRWMutex(&rw.writerSem, false, 0) 154 } 155 if race.Enabled { 156 race.Enable() 157 race.Acquire(unsafe.Pointer(&rw.readerSem)) 158 race.Acquire(unsafe.Pointer(&rw.writerSem)) 159 } 160 } 161 162 // TryLock tries to lock rw for writing and reports whether it succeeded. 163 // 164 // Note that while correct uses of TryLock do exist, they are rare, 165 // and use of TryLock is often a sign of a deeper problem 166 // in a particular use of mutexes. 167 func (rw *RWMutex) TryLock() bool { 168 if race.Enabled { 169 _ = rw.w.state 170 race.Disable() 171 } 172 if !rw.w.TryLock() { 173 if race.Enabled { 174 race.Enable() 175 } 176 return false 177 } 178 if !rw.readerCount.CompareAndSwap(0, -rwmutexMaxReaders) { 179 rw.w.Unlock() 180 if race.Enabled { 181 race.Enable() 182 } 183 return false 184 } 185 if race.Enabled { 186 race.Enable() 187 race.Acquire(unsafe.Pointer(&rw.readerSem)) 188 race.Acquire(unsafe.Pointer(&rw.writerSem)) 189 } 190 return true 191 } 192 193 // Unlock unlocks rw for writing. It is a run-time error if rw is 194 // not locked for writing on entry to Unlock. 195 // 196 // As with Mutexes, a locked [RWMutex] is not associated with a particular 197 // goroutine. One goroutine may [RWMutex.RLock] ([RWMutex.Lock]) a RWMutex and then 198 // arrange for another goroutine to [RWMutex.RUnlock] ([RWMutex.Unlock]) it. 199 func (rw *RWMutex) Unlock() { 200 if race.Enabled { 201 _ = rw.w.state 202 race.Release(unsafe.Pointer(&rw.readerSem)) 203 race.Disable() 204 } 205 206 // Announce to readers there is no active writer. 207 r := rw.readerCount.Add(rwmutexMaxReaders) 208 if r >= rwmutexMaxReaders { 209 race.Enable() 210 fatal("sync: Unlock of unlocked RWMutex") 211 } 212 // Unblock blocked readers, if any. 213 for i := 0; i < int(r); i++ { 214 runtime_Semrelease(&rw.readerSem, false, 0) 215 } 216 // Allow other writers to proceed. 217 rw.w.Unlock() 218 if race.Enabled { 219 race.Enable() 220 } 221 } 222 223 // syscall_hasWaitingReaders reports whether any goroutine is waiting 224 // to acquire a read lock on rw. This exists because syscall.ForkLock 225 // is an RWMutex, and we can't change that without breaking compatibility. 226 // We don't need or want RWMutex semantics for ForkLock, and we use 227 // this private API to avoid having to change the type of ForkLock. 228 // For more details see the syscall package. 229 // 230 //go:linkname syscall_hasWaitingReaders syscall.hasWaitingReaders 231 func syscall_hasWaitingReaders(rw *RWMutex) bool { 232 r := rw.readerCount.Load() 233 return r < 0 && r+rwmutexMaxReaders > 0 234 } 235 236 // RLocker returns a [Locker] interface that implements 237 // the [Locker.Lock] and [Locker.Unlock] methods by calling rw.RLock and rw.RUnlock. 238 func (rw *RWMutex) RLocker() Locker { 239 return (*rlocker)(rw) 240 } 241 242 type rlocker RWMutex 243 244 func (r *rlocker) Lock() { (*RWMutex)(r).RLock() } 245 func (r *rlocker) Unlock() { (*RWMutex)(r).RUnlock() } 246