一文带你了解Go语言中锁的实现

// Lock locks m.
// If the lock is already in use, the calling goroutine
// blocks until the mutex is available.
func (m *Mutex) Lock() {
  // Fast path: grab unlocked mutex.
  // 上锁,成功返回
  if atomic.CompareAndSwapInt32(&m.state, 0, mutexLocked) {
    if race.Enabled {
      race.Acquire(unsafe.Pointer(m))
    }
    return
  }
  // Slow path (outlined so that the fast path can be inlined)
  //已经锁上的写成进入慢锁流程
  m.lockSlow()
}

func (m *Mutex) lockSlow() {
  var waitStartTime int64 //执行时间
  starving := false //当前请求是否是饥饿模式
  awoke := false //当前请求是否是唤醒状态
  iter := 0 //自旋次数
  old := m.state //旧state值
  for {
    // Don't spin in starvation mode, ownership is handed off to waiters
    // so we won't be able to acquire the mutex anyway.
    //旧state值已上锁,并且未进入饥饿模式,且可以自旋,进入自旋逻辑
    if old&(mutexLocked|mutexStarving) == mutexLocked && runtime_canSpin(iter) {
      // Active spinning makes sense. 
      // Try to set mutexWoken flag to inform Unlock
      // to not wake other blocked goroutines.
      // 当前协程未唤醒 
      //&& old.state 为未唤起状态,就是说没有其他被唤起的waiter
      //&& waiter数>0 
      //&& m.state标记为唤起状态成功
      if !awoke && old&mutexWoken == 0 && old>>mutexWaiterShift != 0 &&
        atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {
        //标记当前协程为唤起状态
        //r: 这是为了通知在解锁Unlock()中不要再唤醒其他的waiter了
        awoke = true
      }
      //自旋
      runtime_doSpin()
      //自旋计数器
      iter++
      old = m.state
      continue
    }
    //r: old是锁当前的状态，new是期望的状态，以期于在后面的CAS操作中更改锁的状态
    //new代表期望的state值
    new := old
    // Don't try to acquire starving mutex, new arriving goroutines must queue.
    //old不是饥饿状态,new带上上锁标志位,也就是饥饿状态不上锁
    if old&mutexStarving == 0 {
      new |= mutexLocked
    }
    //旧state值是上锁状态或饥饿状态,新state waiter数+1
    //r: 表示当前goroutine将被作为waiter置于等待队列队尾
    if old&(mutexLocked|mutexStarving) != 0 {
      new += 1 << mutexWaiterShift
    }
    // The current goroutine switches mutex to starvation mode.
    // But if the mutex is currently unlocked, don't do the switch.
    // Unlock expects that starving mutex has waiters, which will not
    // be true in this case.
    //当前协程为饥饿状态&&旧state已上锁,新state加饥饿标志位
    if starving && old&mutexLocked != 0 {
      new |= mutexStarving
    }
    //r:? 当awoke为true，则表明当前goroutine在自旋逻辑中，成功修改锁的Woken状态位为1
    if awoke {
      // The goroutine has been woken from sleep,
      // so we need to reset the flag in either case.
      if new&mutexWoken == 0 {
        throw("sync: inconsistent mutex state")
      }
      //新state关闭唤醒标志位
      //r: 因为在后续的逻辑中，当前goroutine要么是拿到锁了，要么是被挂起。
      // 如果是挂起状态，那就需要等待其他释放锁的goroutine来唤醒。
      // 假如其他goroutine在unlock的时候发现Woken的位置不是0，则就不会去唤醒，那该goroutine就无法再醒来加锁。(见unlock逻辑)
​
      new &^= mutexWoken
    }
    //r: 尝试将锁的状态更新为期望状态
    if atomic.CompareAndSwapInt32(&m.state, old, new) {
      //旧state不是锁或饥饿状态,上锁成功,返回
      if old&(mutexLocked|mutexStarving) == 0 {
        break // locked the mutex with CAS
      }
      // If we were already waiting before, queue at the front of the queue.
      //r: 如果走到这里，那就证明当前goroutine没有获取到锁
      // 这里判断waitStartTime != 0就证明当前goroutine之前已经等待过了，则需要将其放置在等待队列队头
      //进入队列是否排在最前
      queueLifo := waitStartTime != 0
      if waitStartTime == 0 {
        waitStartTime = runtime_nanotime()
      }
      //阻塞
      runtime_SemacquireMutex(&m.sema, queueLifo, 1)
      //r: 被信号量唤醒之后检查当前goroutine是否应该表示为饥饿
      // （这里表示为饥饿之后，会在下一轮循环中尝试将锁的状态更改为饥饿模式）
      // 1. 如果当前goroutine已经饥饿（在上一次循环中更改了starving为true）
      // 2. 如果当前goroutine已经等待了1ms以上
      
      //被信号量唤醒后当前协程是否进入饥饿状态
      //1. 之前是饥饿状态
      //2. 运行时间超过1ms
      starving = starving || runtime_nanotime()-waitStartTime > starvationThresholdNs
      // 再次获取锁状态
      old = m.state
      if old&mutexStarving != 0 {
        // If this goroutine was woken and mutex is in starvation mode,
        // ownership was handed off to us but mutex is in somewhat
        // inconsistent state: mutexLocked is not set and we are still
        // accounted as waiter. Fix that.
        //饥饿模式协程是在Unlock()时handoff到当前协程的
        
        //r:? 如果当前锁既不是被获取也不是被唤醒状态，或者等待队列为空
        // 这代表锁状态产生了不一致的问题
        if old&(mutexLocked|mutexWoken) != 0 || old>>mutexWaiterShift == 0 {
          throw("sync: inconsistent mutex state")
        }
        //m.state 上锁,waiter数-1
        delta := int32(mutexLocked - 1<<mutexWaiterShift)
        //当前协程不是饥饿状态或旧state的waiter数=1,则m.state饥饿标志位置0
        if !starving || old>>mutexWaiterShift == 1 {
          // Exit starvation mode.
          // Critical to do it here and consider wait time.
          // Starvation mode is so inefficient, that two goroutines
          // can go lock-step infinitely once they switch mutex
          // to starvation mode.
          delta -= mutexStarving
        }
        atomic.AddInt32(&m.state, delta)
        //拿到锁,退出.
        break
      }
      awoke = true
      iter = 0
    } else {
      //执行循环前的语句,恢复最新现场
      old = m.state
    }
  }
​
  if race.Enabled {
    race.Acquire(unsafe.Pointer(m))
  }
}

// Unlock unlocks m.
// It is a run-time error if m is not locked on entry to Unlock.
//
// A locked Mutex is not associated with a particular goroutine.
// It is allowed for one goroutine to lock a Mutex and then
// arrange for another goroutine to unlock it.
func (m *Mutex) Unlock() {
  if race.Enabled {
    _ = m.state
    race.Release(unsafe.Pointer(m))
  }
​
  // Fast path: drop lock bit.
  //m.state取消锁状态,返回值new代表修改后的新值
  //如果为0代表没有其他锁了,退出;否则进入unlockSlow()
  //锁空闲有两种情况:
  //1. 所有位为0,代表没有锁了
  //2. 标志位为0, waiter数量>0,还有协程在等待解锁
  new := atomic.AddInt32(&m.state, -mutexLocked)
  if new != 0 {
    // Outlined slow path to allow inlining the fast path.
    // To hide unlockSlow during tracing we skip one extra frame when tracing GoUnblock.
    m.unlockSlow(new)
  }
}

func (m *Mutex) unlockSlow(new int32) {
  if (new+mutexLocked)&mutexLocked == 0 {
    throw("sync: unlock of unlocked mutex")
  }
  if new&mutexStarving == 0 {
    old := new
    for {
      // If there are no waiters or a goroutine has already
      // been woken or grabbed the lock, no need to wake anyone.
      // In starvation mode ownership is directly handed off from unlocking
      // goroutine to the next waiter. We are not part of this chain,
      // since we did not observe mutexStarving when we unlocked the mutex above.
      // So get off the way.
      //解锁,结束,退出
      //1. 没有waiter了
      //2. 已上锁
      //3. 锁处于唤醒状态,表示有协程被唤醒
      //4. 饥饿模式, 所有权交给了被解锁饥饿模式的waiter
      if old>>mutexWaiterShift == 0 || old&(mutexLocked|mutexWoken|mutexStarving) != 0 {
        return
      }
      // Grab the right to wake someone.
      // 如果能走到这，那就是上面的if判断没通过
      // 说明当前锁是空闲状态，但是等待队列中有waiter，且没有goroutine被唤醒
      // 所以，这里我们想要把锁的状态设置为被唤醒，等待队列waiter数-1
      new = (old - 1<<mutexWaiterShift) | mutexWoken
      if atomic.CompareAndSwapInt32(&m.state, old, new) {
        //通过信号量唤醒某一个waiter,退出
        runtime_Semrelease(&m.sema, false, 1)
        return
      }
      //失败的话,更新old信息,进入下个循环
      old = m.state
    }
  } else {
    // Starving mode: handoff mutex ownership to the next waiter, and yield
    // our time slice so that the next waiter can start to run immediately.
    // Note: mutexLocked is not set, the waiter will set it after wakeup.
    // But mutex is still considered locked if mutexStarving is set,
    // so new coming goroutines won't acquire it.
    //饥饿模式,唤醒等待队列队头waiter
    runtime_Semrelease(&m.sema, true, 1)
  }
}

func runtime_Semrelease(s *uint32, handoff bool, skipframes int)

If handoff is true, pass count directly to the first waiter.

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 当前读锁数量
  readerWait  int32  // number of departing readers 要离开的读锁数量,暨等待写锁解锁,解锁后可以释放的读锁数量
}

// 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.state
    race.Disable()
  }
  // First, resolve competition with other writers.
  
  rw.w.Lock() //通过sync.Lock()限制多写锁进入下边的逻辑
  // Announce to readers there is a pending writer.
  //r值不变, rwmutexMaxReaders值为1<<30
  //可以理解为只要读锁的数量小于1<<30位,rw.readerCount值<0表示有写锁.
  //也可以理解为加上一个负数,将31位以上都标记为1,代表有写锁, 剩余30位记录读锁数量
  r := atomic.AddInt32(&rw.readerCount, -rwmutexMaxReaders) + rwmutexMaxReaders
  // Wait for active readers.
  //r!=0 有读锁,不能释放写锁
  //将readerCount转移到readerWait,readerWait的新值!=0 (以上可以翻译为有读锁,将读锁数转移到读等待数,然后写锁阻塞,)
  // 满足上面两个条件,写锁阻塞, 等待唤醒,不返回
  if r != 0 && atomic.AddInt32(&rw.readerWait, r) != 0 {
    runtime_SemacquireMutex(&rw.writerSem, false, 0)
  }
  if race.Enabled {
    race.Enable()
    race.Acquire(unsafe.Pointer(&rw.readerSem))
    race.Acquire(unsafe.Pointer(&rw.writerSem))
  }
}

// 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.state
    race.Release(unsafe.Pointer(&rw.readerSem))
    race.Disable()
  }
​
  // Announce to readers there is no active writer.\
  //将Lock()方法减去的值加回来,变成正数
  r := atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders)
  if r >= rwmutexMaxReaders {
    race.Enable()
    throw("sync: Unlock of unlocked RWMutex")
  }
  // Unblock blocked readers, if any.
  //唤醒在RLock()方法阻塞的读操作,数量为r
  for i := 0; i < int(r); i++ {
    runtime_Semrelease(&rw.readerSem, false, 0)
  }
  // Allow other writers to proceed.
  rw.w.Unlock()
  if race.Enabled {
    race.Enable()
  }
}

// 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.state
    race.Disable()
  }
  //<0表示已上写锁,阻塞
  if atomic.AddInt32(&rw.readerCount, 1) < 0 {
    // A writer is pending, wait for it.
    runtime_SemacquireMutex(&rw.readerSem, false, 0)
  }
  if race.Enabled {
    race.Enable()
    race.Acquire(unsafe.Pointer(&rw.readerSem))
  }
}

// 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.state
    race.ReleaseMerge(unsafe.Pointer(&rw.writerSem))
    race.Disable()
  }
   //<0表示已上写锁,慢解锁
  if r := atomic.AddInt32(&rw.readerCount, -1); r < 0 {
    // Outlined slow-path to allow the fast-path to be inlined
    rw.rUnlockSlow(r)
  }
  if race.Enabled {
    race.Enable()
  }
}
​
// 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) rUnlockSlow(r int32) {
  if r+1 == 0 || r+1 == -rwmutexMaxReaders {
    race.Enable()
    throw("sync: RUnlock of unlocked RWMutex")
  }
  // A writer is pending.
  //最后一个读等待,唤醒写锁
  if atomic.AddInt32(&rw.readerWait, -1) == 0 {
    // The last reader unblocks the writer.
    runtime_Semrelease(&rw.writerSem, false, 1)
  }
}