浅谈Golang互斥锁sync.Mutex

func (m *Mutex) Lock() {
  // 如果mutex的state没有被锁，也没有等待/唤醒的goroutine, 锁处于正常状态，那么获得锁并返回
    // 比如锁第一次被goroutine请求时，就是这种状态。或者锁处于空闲的时候，也是这种状态。
  if atomic.CompareAndSwapInt32(&m.state, 0, mutexLocked) {
    return
  }
  // 如果锁已经有goroutine占用了，则进入lockslow阻塞
  m.lockSlow()
}
func (m *Mutex) lockSlow() {
  // 记录当前goroutine的等待时间
  var waitStartTime int64
  // 当前goroutine是否已经处于饥饿状态
  starving := false
  // 当前goroutine是否已唤醒
  awoke := false
  // 自旋次数
  iter := 0
  old := m.state
  for {
    // 第一个条件：1.mutex已经被锁了；2.不处于饥饿模式(如果时饥饿状态，自旋时没有用的，锁的拥有权直接交给了等待队列的第一个。)
    // 尝试自旋的条件：参考runtime_canSpin函数，详解在前言那篇博文，这里不展开
    if old&(mutexLocked|mutexStarving) == mutexLocked && runtime_canSpin(iter) {
      // 进入这里肯定是正常模式
      // 自旋的过程中如果发现state还没有设置woken标识(别的goroutine释放了自己的woken标识)，则设置它的woken标识， 并标记自己为被唤醒。
      if !awoke && old&mutexWoken == 0 && old>>mutexWaiterShift != 0 &&
        atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {
        awoke = true
      }
      runtime_doSpin()
      iter++
      old = m.state
      continue
    }
    // 到了这一步， state的状态可能是：
        // 1. 锁还没有被释放，锁处于正常状态
        // 2. 锁还没有被释放， 锁处于饥饿状态
        // 3. 锁已经被释放， 锁处于正常状态
        // 4. 锁已经被释放， 锁处于饥饿状态
        // 并且本gorutine的 awoke可能是true, 也可能是false (其它goutine已经设置了state的woken标识)
    // new 复制 state的当前状态， 用来设置新的状态
        // old 是锁当前的状态
    new := old
    // 如果old state状态不是饥饿状态, new state 设置锁， 尝试通过CAS获取锁,
        // 如果old state状态是饥饿状态, 则不设置new state的锁，因为饥饿状态下锁直接转给等待队列的第一个.
    if old&mutexStarving == 0 {//正常模式
      new |= mutexLocked
    }
    // 将等待队列的等待者的数量加1
    if old&(mutexLocked|mutexStarving) != 0 {
      new += 1 << mutexWaiterShift
    }
    // 如果当前goroutine已经处于饥饿状态， 并且old state的已被加锁,
        // 将new state的状态标记为饥饿状态, 将锁转变为饥饿状态.
    if starving && old&mutexLocked != 0 {
      new |= mutexStarving
    }
    // 如果本goroutine已经设置为唤醒状态, 需要清除new state的唤醒标记, 因为本goroutine要么获得了锁，要么进入休眠，
        // 总之state的新状态不再是woken状态.
    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")
      }
      new &^= mutexWoken
    }
    // 通过CAS设置new state值.
        // 注意new的锁标记不一定是true, 也可能只是标记一下锁的state是饥饿状态.
    if atomic.CompareAndSwapInt32(&m.state, old, new) {
      // 如果old state的状态是未被锁状态，并且锁不处于饥饿状态,
            // 那么当前goroutine已经获取了锁的拥有权，返回
      if old&(mutexLocked|mutexStarving) == 0 {
        break // locked the mutex with CAS
      }
      // 设置并计算本goroutine的等待时间
      queueLifo := waitStartTime != 0
      if waitStartTime == 0 {
        waitStartTime = runtime_nanotime()
      }
      // 既然未能获取到锁， 那么就使用sleep原语阻塞本goroutine
            // 如果是新来的goroutine,queueLifo=false, 加入到等待队列的尾部，耐心等待
            // 如果是唤醒的goroutine, queueLifo=true, 加入到等待队列的头部
      runtime_SemacquireMutex(&m.sema, queueLifo, 1)
      // sleep之后，此goroutine被唤醒
            // 计算当前goroutine是否已经处于饥饿状态.
      starving = starving || runtime_nanotime()-waitStartTime > starvationThresholdNs
      // 得到当前的锁状态
      old = m.state
      // 如果当前的state已经是饥饿状态
            // 那么锁应该处于Unlock状态，那么应该是锁被直接交给了本goroutine
      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.
        if old&(mutexLocked|mutexWoken) != 0 || old>>mutexWaiterShift == 0 {
          throw("sync: inconsistent mutex state")
        }
        // 当前goroutine用来设置锁，并将等待的goroutine数减1.
        delta := int32(mutexLocked - 1<<mutexWaiterShift)
        // 如果本goroutine是最后一个等待者，或者它并不处于饥饿状态，
                // 那么我们需要把锁的state状态设置为正常模式.
        if !starving || old>>mutexWaiterShift == 1 {
           // 退出饥饿模式
          delta -= mutexStarving
        }
        // 设置新state, 因为已经获得了锁，退出、返回
        atomic.AddInt32(&m.state, delta)
        break
      }
      awoke = true
      iter = 0
    } else {
      old = m.state
    }
  }
}

func (m *Mutex) Unlock() {
  // Fast path: drop lock bit.
  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.
      if old>>mutexWaiterShift == 0 || old&(mutexLocked|mutexWoken|mutexStarving) != 0 {
        return
      }
      // Grab the right to wake someone.
      new = (old - 1<<mutexWaiterShift) | mutexWoken
      if atomic.CompareAndSwapInt32(&m.state, old, new) {
        runtime_Semrelease(&m.sema, false, 1)
        return
      }
      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.
    runtime_Semrelease(&m.sema, true, 1)
  }
}