type RWMutex struct {
  w      Mutex // 互斥锁
  writerSem  uint32 // 写锁信号量
  readerSem  uint32 // 读锁信号量
  readerCount int32 // 读锁计数器
  readerWait int32 // 获取写锁时需要等待的读锁释放数量
}

const rwmutexMaxReaders = 1 << 30  // 支持最多2^30个读锁

func (rw *RWMutex) Lock() {
  // 竞态检测
 if race.Enabled {
 _ = rw.w.state
 race.Disable()
 }
 // 使用Mutex锁
 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)
 }
 // 竞态检测
 if race.Enabled {
 race.Enable()
 race.Acquire(unsafe.Pointer(&rw.readerSem))
 race.Acquire(unsafe.Pointer(&rw.writerSem))
 }
}

func (rw *RWMutex) RLock() {
  // 竞态检测
 if race.Enabled {
 _ = rw.w.state
 race.Disable()
 }
 // 每次goroutine获取读锁时，readerCount+1
  // 如果写锁已经被获取，那么readerCount在-rwmutexMaxReaders与0之间，这时挂起获取读锁的goroutine，
  // 如果写锁没有被获取，那么readerCount>0，获取读锁,不阻塞
  // 通过readerCount判断读锁与写锁互斥,如果有写锁存在就挂起goroutine,多个读锁可以并行
 if atomic.AddInt32(&rw.readerCount, 1) < 0 {
 // 将goroutine排到G队列的后面,挂起goroutine
 runtime_Semacquire(&rw.readerSem)
 }
 // 竞态检测
 if race.Enabled {
 race.Enable()
 race.Acquire(unsafe.Pointer(&rw.readerSem))
 }
}

type Locker interface {
 Lock()
 Unlock()
}

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