golang 中 sync.Mutex 的实现

mutex 的实现思想

mutex 主要有两个 method: Lock() 和 Unlock()

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Lock() 可以通过一个 CAS 操作来实现

func (m *Mutex) Lock() {
	for !atomic.CompareAndSwapUint32(&m.locked, 0, 1) {
	}
}

func (m *Mutex) Unlock() {
	atomic.StoreUint32(&m.locked, 0)
}

Lock() 一直进行 CAS 操作，比较耗 CPU。因此带来了一个优化：如果协程在一段时间内抢不到锁，可以把该协程挂到一个等待队列上，Unlock() 的一方除了更新锁的状态，还需要从等待队列中唤醒一个协程。

但是这个优化会存在一个问题，如果一个协程从等待队列中唤醒后再次抢锁时，锁已经被一个新来的协程抢走了，它就只能再次被挂到等待队列中，接着再被唤醒，但又可能抢锁失败...... 这个悲催的协程可能会一直抢不到锁，由此产生饥饿 (starvation) 现象。

饥饿现象会导致尾部延迟 (Tail Latency) 特别高。什么是尾部延迟？用一句话来说就是：最慢的特别慢！

如果共有 1000 个协程，假设 999 个协程可以在 1ms 内抢到锁，虽然平均时间才 2ms，但是最慢的那个协程却需要 1s 才抢到锁，这就是尾部延迟。

golang 中 mutex 的实现思想

➜  go version
go version go1.16.5 darwin/arm64

本次阅读的 go 源码版本为 go1.16.5。

golang 标准库里的 mutex 避免了饥饿现象的发生，先大致介绍一下 golang 的加锁和解锁流程，对后面的源码阅读有帮助。

锁有两种 mode，分别是 normal mode 和 starvation mode。初始为 normal mode，当一个协程来抢锁时，依旧是做 CAS 操作，如果成功了，就直接返回，如果没有抢到锁，它会做一定次数的自旋操作，等待锁被释放，在自旋操作结束后，如果锁依旧没有被释放，那么这个协程就会被放到等待队列中。如果一个处于等待队列中的协程一直都没有抢到锁，mutex 就会从 normal mode 变成 starvation mode，在 starvation mode 下，当有协程释放锁时，这个锁会被直接交给等待队列中的协程，从而避免产生饥饿线程。

除此之外，golang 还有一点小优化，当有协程正在自旋抢锁时，Unlock() 的一方不会从等待队列中唤醒协程，因为即使唤醒了，被唤醒的协程也抢不过正在自旋的协程。

下面正式开始阅读源码。

mutex 的结构以及一些 const 常量值

type Mutex struct {
	state int32
	sema  uint32
}

const (
	mutexLocked = 1 << iota // mutex is locked
	mutexWoken				
	mutexStarving
	mutexWaiterShift = iota // 3

	// Mutex fairness.
	//
	// Mutex can be in 2 modes of operations: normal and starvation.
	// In normal mode waiters are queued in FIFO order, but a woken up waiter
	// does not own the mutex and competes with new arriving goroutines over
	// the ownership. New arriving goroutines have an advantage -- they are
	// already running on CPU and there can be lots of them, so a woken up
	// waiter has good chances of losing. In such case it is queued at front
	// of the wait queue. If a waiter fails to acquire the mutex for more than 1ms,
	// it switches mutex to the starvation mode.
	//
	// In starvation mode ownership of the mutex is directly handed off from
	// the unlocking goroutine to the waiter at the front of the queue.
	// New arriving goroutines don't try to acquire the mutex even if it appears
	// to be unlocked, and don't try to spin. Instead they queue themselves at
	// the tail of the wait queue.
	//
	// If a waiter receives ownership of the mutex and sees that either
	// (1) it is the last waiter in the queue, or (2) it waited for less than 1 ms,
	// it switches mutex back to normal operation mode.
	//
	// Normal mode has considerably better performance as a goroutine can acquire
	// a mutex several times in a row even if there are blocked waiters.
	// Starvation mode is important to prevent pathological cases of tail latency.
	starvationThresholdNs = 1e6
)

mutex 的状态是通过 state 来维护的，state 有 32 个 bit。

前面 29 个 bit 用来记录当前等待队列中有多少个协程在等待，将等待队列的协程数量记录为 waiterCount。

state >> mutexWaiterShift // mutexWaiterShift 的值为 3

第 30 个 bit 表示当前 mutex 是否处于 starvation mode，将这个 bit 记为 starvationFlag。

state & mutexStarving

第 31 个 bit 表示当前是否有协程正在 (第一次) 自旋，将这个 bit 记为 wokenFlag，woken 的意思也就是醒着，代表它不在等待队列上睡眠。

state & mutexWoken

第 32 个 bit 表示当前锁是否被锁了 (感觉有点绕口哈哈) ，将这个 bit 记为 lockFlag。

state & mutexLocked

用一个图来表示这些 bit

0 0 0 0 0 0 0 0 ... 0 0				0			0
                      |				|			|
waiterCount     starvationFlag  wokenFlag   lockFlag

sema 是一个信号量，它会被用来关联一个等待队列。

分别讨论几种 case 下，代码的执行情况。

Mutex 没有被锁住，第一个协程来拿锁

func (m *Mutex) Lock() {
	// Fast path: grab unlocked mutex.
	if atomic.CompareAndSwapInt32(&m.state, 0, mutexLocked) {
		// ...
		return
	}
	// Slow path (outlined so that the fast path can be inlined)
	m.lockSlow()
}

在 Mutex 没有被锁住时，state 的值为 0，此时第一个协程来拿锁时，由于 state 的值为 0，因此 CAS 操作会成功，CAS 操作之后的 state 的值变成 1 (lockFlag = 1) ，然后 return 掉，不会进入到 m.lockSlow() 里面。

Mutex 仅被协程 A 锁住，没有其他协程抢锁，协程 A 释放锁

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)
	}
}

紧接上面，state 的值为 1，AddInt32(m.state,-1) 之后，state 的值变成了 0 (lockFlag = 0) ，new 的值为 0，然后就返回了。

Mutex 已经被协程 A 锁住，协程 B 来拿锁

func (m *Mutex) Lock() {
	// Fast path: grab unlocked mutex.
	if atomic.CompareAndSwapInt32(&m.state, 0, mutexLocked) {
		// ...
		return
	}
	// Slow path (outlined so that the fast path can be inlined)
	m.lockSlow()
}

因为 state 的值不为 0，CompareAndSwapInt32 会返回 false，所以会进入到 lockSlow() 里面

lockSlow()

首先看一下 lockSlow() 这个方法的全貌

func (m *Mutex) lockSlow() {
	var waitStartTime int64
	starving := false
	awoke := false
	iter := 0
	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.
		if old&(mutexLocked|mutexStarving) == mutexLocked && runtime_canSpin(iter) {
			// Active spinning makes sense.
			// Try to set mutexwokenFlag to inform Unlock
			// to not wake other blocked goroutines.
			if !awoke && old&mutexWoken == 0 && old>>mutexWaiterShift != 0 &&
				atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {
				awoke = true
			}
			runtime_doSpin()
			iter++
			old = m.state
			continue
		}
		new := old
		// Don't try to acquire starving mutex, new arriving goroutines must queue.
		if old&mutexStarving == 0 {
			new |= mutexLocked
		}
		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
		}
		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.
			queueLifo := waitStartTime != 0
			if waitStartTime == 0 {
				waitStartTime = runtime_nanotime()
			}
			runtime_SemacquireMutex(&m.sema, queueLifo, 1)
			starving = starving || runtime_nanotime()-waitStartTime > starvationThresholdNs
			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.
				if old&(mutexLocked|mutexWoken) != 0 || old>>mutexWaiterShift == 0 {
					throw("sync: inconsistent mutex state")
				}
				delta := int32(mutexLocked - 1<>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 = true
			iter = 0
		} else {
			old = m.state
		}
	}

	if race.Enabled {
		race.Acquire(unsafe.Pointer(m))
	}
}

第一步: doSpin (空转)

进入 for 循环后，会执行一个判断

for {
    // Don't spin in starvation mode, ownership is handed off to waiters
    // so we won't be able to acquire the mutex anyway.
    if old&(mutexLocked|mutexStarving) == mutexLocked && runtime_canSpin(iter) {
        // Active spinning makes sense.
        // Try to set mutexwokenFlag to inform Unlock
        // to not wake other blocked goroutines.
        if !awoke && old&mutexWoken == 0 && old>>mutexWaiterShift != 0 &&
        atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {
            awoke = true
        }
        runtime_doSpin()
        iter++
        old = m.state
        continue
    }
    // ...
}

runtime_canSpin(iter) 的作用是根据 iter 的值判断自否应该自旋下去。 (这个方法的实现可以在后面看到)

最初的几次判断，由于 iter 的值为 0，runtime_canSpin(iter) 会返回 true。因此

if old&(mutexLocked|mutexStarving) == mutexLocked && runtime_canSpin(iter)

这个判断会一直通过，由于 old>>mutexWaiterShift = 0 (waiterCount = 0) ，不满足第二个判断的条件，因此不会执行 CAS 操作和 awoke = true。

接着就是执行 runtime_doSpin() 了，runtime_doSpin() 会进行一些空循环，消耗了一下 CPU 时间，然后就通过 continue 进入到下一次循环了。 (runtime_doSpin具体实现也可以在后面看到)

看到看到，这段代码不是用来抢锁的，而是用来等锁变成 unlock 状态的，它会空转一定的次数，期待在空转的过程中，锁被其他的协程释放。

runtime_doSpin()

// src/runtime/lock_sema.go
const active_spin_cnt = 30
//go:linkname sync_runtime_doSpin sync.runtime_doSpin
//go:nosplit
func sync_runtime_doSpin() {
	procyield(active_spin_cnt)
}

# /src/runtime/asm_amd64.s
TEXT runtime·procyield(SB),NOSPLIT,$0-0
	MOVL	cycles+0(FP), AX
again:
	PAUSE
	SUBL	$1, AX
	JNZ	again
	RET

procyield() 会循环执行 PAUSE 指令。

runtime_canSpin()

runtime_canSpin() 的实现在 src/runtime/proc.go 里面，里面的判断比较多，但是我们只需要关注 i >= active_spin 这一个判断就行。

const active_spin     = 4
// Active spinning for sync.Mutex.
//go:linkname sync_runtime_canSpin sync.runtime_canSpin
//go:nosplit
func sync_runtime_canSpin(i int) bool {
	// sync.Mutex is cooperative, so we are conservative with spinning.
	// Spin only few times and only if running on a multicore machine and
	// GOMAXPROCS>1 and there is at least one other running P and local runq is empty.
	// As opposed to runtime mutex we don't do passive spinning here,
	// because there can be work on global runq or on other Ps.
	if i >= active_spin || ncpu <= 1 || gomaxprocs <= int32(sched.npidle+sched.nmspinning)+1 {
		return false
	}
	if p := getg().m.p.ptr(); !runqempty(p) {
		return false
	}
	return true
}

一个小插曲

在利用断点来 debug 时，发现没办法 watch sync_runtime_canSpin() 内引用的一些全局变量，例如 active_spin,ncpu,sched.npidle 这些，所以我就大力出奇迹，强行修改源码在里面声明了几个局部变量，这下可以通过 watch 局部变量来得知全局变量的值了 (机智如我哈哈) 。

func sync_runtime_canSpin(i int) bool {
	local_active_spin := active_spin
	local_ncpu := ncpu
	local_gomaxprocs := gomaxprocs
	npidle := sched.npidle
	nmspinning := sched.nmspinning
	if i >= local_active_spin || local_ncpu <= 1 ||local_gomaxprocs <= int32(npidle+nmspinning)+1 {
		return false
	}
	if p := getg().m.p.ptr(); !runqempty(p) {
		return false
	}
	return true
}

第二步: 根据旧状态来计算新状态

new := old
// Don't try to acquire starving mutex, new arriving goroutines must queue.
if old&mutexStarving == 0 {
    new |= mutexLocked
}
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.
    // ...
    new &^= mutexWoken
}

这一段代码，是根据 old state 来计算 new state，有 4 个操作

• set lockFlag: new |= mutexLocked
• 增加 waiterCount: new += 1 << mutexWaiterShift
• set starvationFlag: new |= mutexStarving
• clear wokenFlag: new &^= mutexWoken

由于在这里我们只讨论 ”Mutex 已经被协程 A 锁住，协程 B 来拿锁“ 这种情况，可以分为两种 case

• case1: 在第一步自旋的过程中，锁已经被释放了，此时 old state = 000000...000 (所有 bit 都为 0) ，经过这四个操作的洗礼后，lockFlag 被设置成了 1。
• case2: 在第一步自旋结束后，锁还没有被释放，即 old state 此时为 00000000...001 (仅 lockFlag 为 1)，经过这四个操作的洗礼后，waiterCounter = 1，lockFlag 也为 1。

第三步: 更新 state (抢锁)

if atomic.CompareAndSwapInt32(&m.state, old, new) {
    if old&(mutexLocked|mutexStarving) == 0 {
        break // locked the mutex with CAS
    }
    // ...
} else {
    old = m.state
}

这一步会通过 CAS 操作将 mutex.state 更新为我们刚刚计算得到的 new state。如果 CAS 成功，且 old 处于未上锁的状态时，就直接利用 break 退出循环返回了 (也就是上面的 case1) 。如果 CAS 失败，将会更新 old state 的值，进行下一次循环，再重复一二三步；

如果是 case2 的话，情况会稍微复杂一点

if atomic.CompareAndSwapInt32(&m.state, old, new) {
	// ...
    // If we were already waiting before, queue at the front of the queue.
    queueLifo := waitStartTime != 0
    if waitStartTime == 0 {
        waitStartTime = runtime_nanotime()
    }
    
    runtime_SemacquireMutex(&m.sema, queueLifo, 1)
    // ...
}

主要就是通过 runtime_SemacquireMutex() 把自己放进了等待队列里面，之后 runtime 不会再调度该协程，直到协程被唤醒。

关于 runtime_SemacquireMutex() 的实现，我暂时就不追究下去了，再追究下去就没完没了啦。

Mutex 被协程 A 锁住，协程 B 来抢锁但失败被放入等待队列，此时协程 A 释放锁

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)
	}
}

紧接上回，最初 state 的值为 00000000000...0001001 (waiterCount = 1, lockFlag = 1)。执行完 AddInt32(&m.state, -mutexLocked) 后，变成了 0000...001000 (waiterCount = 1) ，new 的值也为 0000...001000，接着就进入到 unlockSlow 里面了。

unlockSlow()

看看 unlockSlow() 的全貌

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<

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<

const starvationThresholdNs = 1e6
if atomic.CompareAndSwapInt32(&m.state, old, new) {
    // ...
    runtime_SemacquireMutex(&m.sema, queueLifo, 1)
    starving = starving || runtime_nanotime()-waitStartTime > starvationThresholdNs
    old = m.state
	// ...
    iter = 0
}

starving = starving || runtime_nanotime()-waitStartTime > starvationThresholdNs

// 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
}

atomic.CompareAndSwapInt32(&m.state, old, new)

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&mutexStarving == 0 {
		// ...
		for {
			// ...
			if atomic.CompareAndSwapInt32(&m.state, old, new) {
				runtime_Semrelease(&m.sema, false, 1)
				return
			}
            // ...
		}
	} 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)
	}
}

func (m *Mutex) Lock() {
	// ...
	m.lockSlow()
}

func (m *Mutex) lockSlow() {
	// ...
	for {
		// ...
		if atomic.CompareAndSwapInt32(&m.state, old, new) {
			// ...
			runtime_SemacquireMutex(&m.sema, queueLifo, 1)
			// ...
			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.
				// ...
				delta := int32(mutexLocked - 1<>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 = true
			iter = 0
		} else {
			old = m.state
		}
	}
	// ...
}

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

func (m *Mutex) lockSlow() {
    // ...
	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.
		if old&(mutexLocked|mutexStarving) == mutexLocked && runtime_canSpin(iter) {
			// ...
			runtime_doSpin()
		}
		new := old
		// Don't try to acquire starving mutex, new arriving goroutines must queue.
		if old&mutexStarving == 0 {
			new |= mutexLocked
		}
		if old&(mutexLocked|mutexStarving) != 0 {
			new += 1 << mutexWaiterShift
		}
		// ...
		if atomic.CompareAndSwapInt32(&m.state, old, new) {
			// ..
			runtime_SemacquireMutex(&m.sema, queueLifo, 1)
			// ...
		} else {
			// ...
		}
	}
	// ...
}

func (m *Mutex) lockSlow() {
	starving := false
	awoke := false
	iter := 0
	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.
		if old&(mutexLocked|mutexStarving) == mutexLocked && runtime_canSpin(iter) {
			// Active spinning makes sense.
			// Try to set mutexwokenFlag to inform Unlock
			// to not wake other blocked goroutines.
			if !awoke && old&mutexWoken == 0 && old>>mutexWaiterShift != 0 &&
				atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {
				awoke = true
			}
			runtime_doSpin()
			iter++
			old = m.state
			continue
		}
		// ...
		if atomic.CompareAndSwapInt32(&m.state, old, new) {
			// ...
			runtime_SemacquireMutex(&m.sema, queueLifo, 1)
			// ...
			awoke = true
			iter = 0
		} 
        // ...
	}
    // ...
}

if !awoke && old&mutexWoken == 0 && old>>mutexWaiterShift != 0 &&
	atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {
    awoke = true
}

func (m *Mutex) unlockSlow(new int32) {
	// ...
	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 {
                // 当 mutexwokenFlag 被设置时，会直接 return
                // 不会去等待队列唤醒 goroutine
				return
			}
			// Grab the right to wake someone.
            // 这个地方会设置 wokenFlag 哦
			new = (old - 1<

// If we were already waiting before, queue at the front of the queue.
queueLifo := waitStartTime != 0
if waitStartTime == 0 {
    waitStartTime = runtime_nanotime()
}
runtime_SemacquireMutex(&m.sema, queueLifo, 1)