go sync包（三）读写锁（二）

读写锁 RWMutex

写锁

加锁

RWMetex 的写锁复用了 Mutex：

// Lock locks rw for writing.
// If the lock is already locked for reading or writing,
// Lock blocks until the lock is available.
func (rw *RWMutex) Lock() {if race.Enabled {_ = rw.w.staterace.Disable()}// First, resolve competition with other writers.// writer加锁rw.w.Lock()// Announce to readers there is a pending writer.// 反转 readerCount，告诉 reader 有writer竞争锁// Add(-rwmutexMaxReaders) 将 readerCount 置为负数，表示当前有 writer 竞争锁r := rw.readerCount.Add(-rwmutexMaxReaders) + rwmutexMaxReaders// Wait for active readers.// 当前有 reader 持有锁，writer 等待if r != 0 && rw.readerWait.Add(r) != 0 {runtime_SemacquireRWMutex(&rw.writerSem, false, 0)}if race.Enabled {race.Enable()race.Acquire(unsafe.Pointer(&rw.readerSem))race.Acquire(unsafe.Pointer(&rw.writerSem))}
}

解锁

解锁也是复用的 Mutex，区别在于解锁的时候会唤醒所有阻塞的 reader。

// Unlock unlocks rw for writing. It is a run-time error if rw is
// not locked for writing on entry to Unlock.
//
// As with Mutexes, a locked RWMutex is not associated with a particular
// goroutine. One goroutine may RLock (Lock) a RWMutex and then
// arrange for another goroutine to RUnlock (Unlock) it.
func (rw *RWMutex) Unlock() {if race.Enabled {_ = rw.w.staterace.Release(unsafe.Pointer(&rw.readerSem))race.Disable()}// Announce to readers there is no active writer.// 反转 readerCount，告诉 reader 没有 writer 竞争锁r := rw.readerCount.Add(rwmutexMaxReaders)if r >= rwmutexMaxReaders {race.Enable()fatal("sync: Unlock of unlocked RWMutex")}// Unblock blocked readers, if any.// 唤醒所有阻塞的 readerfor i := 0; i < int(r); i++ {runtime_Semrelease(&rw.readerSem, false, 0)}// Allow other writers to proceed.// writer 解锁rw.w.Unlock()if race.Enabled {race.Enable()}
}

读锁

加锁

// RLock locks rw for reading.
//
// It should not be used for recursive read locking; a blocked Lock
// call excludes new readers from acquiring the lock. See the
// documentation on the RWMutex type.
func (rw *RWMutex) RLock() {if race.Enabled {_ = rw.w.staterace.Disable()}// writer 加锁时，readerCount 是负数// 写锁优先级比读锁高，reader 休眠// 如果 reader+1 > 0, 当前加了读锁if rw.readerCount.Add(1) < 0 {// A writer is pending, wait for it.runtime_SemacquireRWMutexR(&rw.readerSem, false, 0)}if race.Enabled {race.Enable()race.Acquire(unsafe.Pointer(&rw.readerSem))}
}

解锁

// RUnlock undoes a single RLock call;
// it does not affect other simultaneous readers.
// It is a run-time error if rw is not locked for reading
// on entry to RUnlock.
func (rw *RWMutex) RUnlock() {if race.Enabled {_ = rw.w.staterace.ReleaseMerge(unsafe.Pointer(&rw.writerSem))race.Disable()}// 有等待的 writerif r := rw.readerCount.Add(-1); r < 0 {// Outlined slow-path to allow the fast-path to be inlinedrw.rUnlockSlow(r)}if race.Enabled {race.Enable()}
}func (rw *RWMutex) rUnlockSlow(r int32) {if r+1 == 0 || r+1 == -rwmutexMaxReaders {race.Enable()fatal("sync: RUnlock of unlocked RWMutex")}// A writer is pending.// 读锁全部解锁完毕，唤醒 writerif rw.readerWait.Add(-1) == 0 {// The last reader unblocks the writer.runtime_Semrelease(&rw.writerSem, false, 1)}
}

小结

sync.RWMutex 复用 sync.Mutex 。
写锁：
- 加锁时 writer 反转 readerCount 置为负数，如果当前有 reader 正在执行， readerCount 赋值给 readerWait，writer 休眠。
- 解锁时 writer 反转 readerCount 置为正常，并唤醒所有阻塞的 reader。
读锁：
- 加锁时 readerCount + 1，如果 readerCount < 0，reader 阻塞。
- 解锁时 readerCount - 1，如果 readerCount < 0，readerWait - 1，当 readerWait == 0 时，唤醒 writer。

sync.RWMutex 复用 sync.Mutex 。