type RWMutex struct {
  w      Mutex // 互斥鎖
  writerSem  uint32 // 寫鎖信號(hào)量
  readerSem  uint32 // 讀鎖信號(hào)量
  readerCount int32 // 讀鎖計(jì)數(shù)器
  readerWait int32 // 獲取寫鎖時(shí)需要等待的讀鎖釋放數(shù)量
}

const rwmutexMaxReaders = 1 << 30  // 支持最多2^30個(gè)讀鎖

func (rw *RWMutex) Lock() {
  // 競(jìng)態(tài)檢測(cè)
 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(jìng)態(tài)檢測(cè)
 if race.Enabled {
 race.Enable()
 race.Acquire(unsafe.Pointer(&rw.readerSem))
 race.Acquire(unsafe.Pointer(&rw.writerSem))
 }
}

func (rw *RWMutex) RLock() {
  // 競(jìng)態(tài)檢測(cè)
 if race.Enabled {
 _ = rw.w.state
 race.Disable()
 }
 // 每次goroutine獲取讀鎖時(shí)，readerCount+1
  // 如果寫鎖已經(jīng)被獲取，那么readerCount在-rwmutexMaxReaders與0之間，這時(shí)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(gè)讀鎖可以并行
 if atomic.AddInt32(&rw.readerCount, 1) < 0 {
 // 將goroutine排到G隊(duì)列的后面,掛起goroutine
 runtime_Semacquire(&rw.readerSem)
 }
 // 競(jìng)態(tài)檢測(cè)
 if race.Enabled {
 race.Enable()
 race.Acquire(unsafe.Pointer(&rw.readerSem))
 }
}

type Locker interface {
 Lock()
 Unlock()
}

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