go sync包（二） 互斥锁（二）

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互斥锁 Mutex

mutex 的 加解锁很简单：

	var mutex sync.Mutexmutex.Lock()defer mutex.Unlock()// 加锁期间的代码逻辑

加锁

// 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.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)m.lockSlow()
}

• 当我们调用 Lock 方法的时候，会先尝试走 Fast Path，也就是如果当前互斥锁如果处于未加锁的状态，尝试加锁，只要加锁成功就直接返回。
• 否则的话就进入 slow path。

func (m *Mutex) lockSlow() {var waitStartTime int64 // 等待时间starving := false // 是否处于饥饿状态awoke := false // 是否处于唤醒状态iter := 0 // 自旋迭代次数old := m.state for {// Don't spin in starvation mode, ownership is handed off to waiters// so we won't be able to acquire the mutex anyway.// 判断当前 Goroutine 能否进入自旋// 条件：// 当前处于普通模式 && runtime_canSpin 返回 true// runtime_canSpin 返回 true//     1. 运行在多 CPU 的机器上//     2. 自旋次数不超过 4 次//     3. 当前机器上至少存在一个正在运行的处理器 P 并且处理的运行队列为空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.// 尝试设置 mutexWoken 状态，避免唤醒其他休眠的 goroutineif !awoke && old&mutexWoken == 0 && old>>mutexWaiterShift != 0 &&atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {awoke = true}// 自旋：执行 30 次 PAUSE指令，占用CPU并消耗CPU时间runtime_doSpin()iter++old = m.statecontinue}// 计算互斥锁的最新状态new := old// Don't try to acquire starving mutex, new arriving goroutines must queue.if old&mutexStarving == 0 {new |= mutexLocked}// 饥饿模式 || 锁已经被其他goroutine获取// 加入等待队列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.if starving && old&mutexLocked != 0 {new |= mutexStarving}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}// CAS更新状态获取锁// 正常模式：这段代码会设置唤醒和饥饿标记、重置迭代次数并重新执行获取锁的循环。// 饥饿模式: 当前 Goroutine 会获得互斥锁，如果等待队列中只存在当前 Goroutine，// 互斥锁还会从饥饿模式中退出。if atomic.CompareAndSwapInt32(&m.state, old, new) {if old&(mutexLocked|mutexStarving) == 0 {break // locked the mutex with CAS}// If we were already waiting before, queue at the front of the queue.// 正在等，排在最前面queueLifo := waitStartTime != 0// 设置初始化时间，计算是否超过时间要切换到公平模式if waitStartTime == 0 {waitStartTime = runtime_nanotime()}// 阻塞runtime_SemacquireMutex(&m.sema, queueLifo, 1)// 看是否超过 1ms，是的话就切换到公平模式starving = starving || runtime_nanotime()-waitStartTime > starvationThresholdNsold = m.state// 饥饿模式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")}// 退出饥饿模式delta := int32(mutexLocked - 1<<mutexWaiterShift)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)break}awoke = trueiter = 0} else {old = m.state}}if race.Enabled {race.Acquire(unsafe.Pointer(m))}
}

1. 判断当前 goroutine 能否可以进入自旋状态，可以的话自旋争抢锁。
   进入自旋状态的条件：
   • 普通模式
   • 运行在多 CPU 的机器上
   • 自旋次数不超过 4 次
   • 当前机器上至少存在一个正在运行的处理器 P 并且处理的运行队列为空
2. 普通模式：被唤醒的 goroutine 跟新到来的 goroutine 争抢锁。
   饥饿模式：新到来的 goroutine 自动加入队列末尾，由队列第一个 goroutine 获得锁。
3. 饥饿模式：
   进入条件：如果当前 goroutine 超过 1ms 都没有获取到锁就会进饥饿模式。
   退出条件：当前 goroutine 是队列中最后一个 goroutine。

解锁

// 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.staterace.Release(unsafe.Pointer(m))}// 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)}
}

• 如果该函数返回的新状态等于 0，当前 Goroutine 就成功解锁了互斥锁。
• 如果该函数返回的新状态不等于 0，这段代码会调用 sync.Mutex.unlockSlow 开始慢速解锁。

func (m *Mutex) unlockSlow(new int32) {// 校验锁状态的合法性// 如果当前互斥锁已经被解锁过，直接抛出异常中止当前程序if (new+mutexLocked)&mutexLocked == 0 {fatal("sync: unlock of unlocked mutex")}// 普通模式if new&mutexStarving == 0 {old := newfor {// 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.// // 没有等待者 || 已经被加锁 || 已经被解锁 || 公平锁if old>>mutexWaiterShift == 0 || old&(mutexLocked|mutexWoken|mutexStarving) != 0 {return}// Grab the right to wake someone.// 唤醒一个等待者new = (old - 1<<mutexWaiterShift) | mutexWokenif 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.// 饥饿模式// 将当前锁让给下一个等待者// 这里不会解除饥饿模式，所以新来的goroutine不会获得锁runtime_Semrelease(&m.sema, true, 1)}
}// Semrelease atomically increments *s and notifies a waiting goroutine
// if one is blocked in Semacquire.
// It is intended as a simple wakeup primitive for use by the synchronization
// library and should not be used directly.
// If handoff is true, pass count directly to the first waiter.
// skipframes is the number of frames to omit during tracing, counting from
// runtime_Semrelease's caller.// hadoff：
// true: 唤醒并直接移交给第一个等待者
// false: 只是唤醒操作
func runtime_Semrelease(s *uint32, handoff bool, skipframes int)

小结

互斥锁的加锁过程比较复杂，它涉及自旋、信号量以及调度等概念：

• 如果互斥锁处于初始化状态，会通过置位 mutexLocked 加锁。
• 如果互斥锁处于 mutexLocked 状态并且在普通模式下工作，会进入自旋，执行 30 次 PAUSE 指令消耗 CPU 时间等待锁的释放。
• 如果当前 Goroutine 等待锁的时间超过了 1ms，互斥锁就会切换到饥饿模式。
• 互斥锁在正常情况下会通过 runtime.sync_runtime_SemacquireMutex 将尝试获取锁的 Goroutine 切换至休眠状态，等待锁的持有者唤醒（阻塞）。
• 如果当前 Goroutine 是互斥锁上的最后一个等待的协程，那么它会将互斥锁切换回正常模式。

互斥锁的解锁过程比较简单：

• 当互斥锁已经被解锁时，调用 sync.Mutex.Unlock 会直接抛出异常。
• 当互斥锁处于饥饿模式时，将锁的所有权交给队列中的下一个等待者，等待者会负责设置 mutexLocked 标志位。
• 当互斥锁处于普通模式时，如果没有 Goroutine 等待锁的释放或者已经有被唤醒的 Goroutine 获得了锁，会直接返回。在其他情况下会通过 sync.runtime_Semrelease 唤醒对应的 Goroutine。

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