【Go】RWMutex 源码分析

type RWMutex struct {
    w           Mutex  // held if there are pending writers
    writerSem   uint32 // semaphore for writers to wait for completing readers
    readerSem   uint32 // semaphore for readers to wait for completing writers
    readerCount int32  // number of pending readers
    readerWait  int32  // number of departing readers
}

func (rw *RWMutex) RLock() {
    if atomic.AddInt32(&rw.readerCount, 1) < 0 {
        // 阻塞读
        runtime_SemacquireMutex(&rw.readerSem, false, 0)
    }
}

func (rw *RWMutex) RUnlock() {
    if r := atomic.AddInt32(&rw.readerCount, -1); r < 0 {
        // Outlined slow-path to allow the fast-path to be inlined
        rw.rUnlockSlow(r)
    }
}

func (rw *RWMutex) rUnlockSlow(r int32) {
    if r+1 == 0 || r+1 == -rwmutexMaxReaders {
        race.Enable()
        throw("sync: RUnlock of unlocked RWMutex")
    }
    // 所有读操作结束后，触发写的写信号量
    if atomic.AddInt32(&rw.readerWait, -1) == 0 {
        // The last reader unblocks the writer.
        runtime_Semrelease(&rw.writerSem, false, 1)
    }
}

func (rw *RWMutex) Lock() {
    // 获取互斥写锁
    rw.w.Lock()
    // 阻塞读
    r := atomic.AddInt32(&rw.readerCount, -rwmutexMaxReaders) + rwmutexMaxReaders
    // 如果有人在读，需要等待读锁释放
    if r != 0 && atomic.AddInt32(&rw.readerWait, r) != 0 {
        // 阻塞等待读锁释放
        runtime_SemacquireMutex(&rw.writerSem, false, 0)
    }
}

func (rw *RWMutex) Unlock() {
    // 重新允许读锁获取
    r := atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders)
    if r >= rwmutexMaxReaders {
        race.Enable()
        throw("sync: Unlock of unlocked RWMutex")
    }
    // 触发等待中的写锁的信号量
    for i := 0; i < int(r); i++ {
        runtime_Semrelease(&rw.readerSem, false, 0)
    }
    // 互斥锁解锁
    rw.w.Unlock()
}