type RWMutex struct {
  w      Mutex // 互斥鎖
  writerSem  uint32 // 寫鎖信號量
  readerSem  uint32 // 讀鎖信號量
  readerCount int32 // 讀鎖計數器
  readerWait int32 // 獲取寫鎖時需要等待的讀鎖釋放數量
}

const rwmutexMaxReaders = 1  30  // 支持最多2^30個讀鎖

func (rw *RWMutex) Lock() {
  // 競態檢測
 if race.Enabled {
 _ = rw.w.state
 race.Disable()
 }
 // 使用Mutex鎖
 rw.w.Lock()
 // Announce to readers there is a pending writer.
 r := atomic.AddInt32(rw.readerCount, -rwmutexMaxReaders) + rwmutexMaxReaders
 // Wait for active readers.
 if r != 0  atomic.AddInt32(rw.readerWait, r) != 0 {
 runtime_Semacquire(rw.writerSem)
 }
 // 競態檢測
 if race.Enabled {
 race.Enable()
 race.Acquire(unsafe.Pointer(rw.readerSem))
 race.Acquire(unsafe.Pointer(rw.writerSem))
 }
}

func (rw *RWMutex) RLock() {
  // 競態檢測
 if race.Enabled {
 _ = rw.w.state
 race.Disable()
 }
 // 每次goroutine獲取讀鎖時，readerCount+1
  // 如果寫鎖已經被獲取，那么readerCount在-rwmutexMaxReaders與0之間，這時掛起獲取讀鎖的goroutine，
  // 如果寫鎖沒有被獲取，那么readerCount>0，獲取讀鎖,不阻塞
  // 通過readerCount判斷讀鎖與寫鎖互斥,如果有寫鎖存在就掛起goroutine,多個讀鎖可以并行
 if atomic.AddInt32(rw.readerCount, 1)  0 {
 // 將goroutine排到G隊列的后面,掛起goroutine
 runtime_Semacquire(rw.readerSem)
 }
 // 競態檢測
 if race.Enabled {
 race.Enable()
 race.Acquire(unsafe.Pointer(rw.readerSem))
 }
}

type Locker interface {
 Lock()
 Unlock()
}

func (rw *RWMutex) RLocker() Locker {
 return (*rlocker)(rw)
}