Golang互斥锁-Mutex

// A Mutex is a mutual exclusion lock.
// The zero value for a Mutex is an unlocked mutex.
//
// A Mutex must not be copied after first use.
type Mutex struct {
	state int32
	sema  uint32
}

// 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.
	// 尝试使用CAS操作将state置为mutexLocked状态
	if atomic.CompareAndSwapInt32(&m.state, 0, mutexLocked) {
		// 竞态代码可忽略
		if race.Enabled {
			race.Acquire(unsafe.Pointer(m))
		}
		return
	}
	// Slow path (outlined so that the fast path can be inlined)
	// 竞争🔐
	m.lockSlow()
}

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 mutexWoken flag 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
			// 更新old
			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)
				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 = true
			iter = 0
		} else {
			old = m.state
		}
	}

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

// 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 mutexWoken flag to inform Unlock
	// to not wake other blocked goroutines.
	// 非唤醒状态 && 唤醒位不为1 && old右移三位!=0，即等待的g数不为0 && CAS置唤醒位为1
	if !awoke && old&mutexWoken == 0 && old>>mutexWaiterShift != 0 &&
		atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {
		awoke = true
	}
	runtime_doSpin()
	iter++
	old = m.state
	continue
}

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
}

new := old
// Don't try to acquire starving mutex, new arriving goroutines must queue.
// 如果是饥饿状态，不再尝试抢锁，此时只有那些在阻塞中的g能够拿锁
if old&mutexStarving == 0 {
	// 如果不是饥饿状态，当前g抢到🔐的话，state应该是上锁状态
	new |= mutexLocked
}
// 如果old处于饥饿或者上锁状态，即不为正常解锁状态，则当前g这次循环是拿不到锁的，将等待数+1
if old&(mutexLocked|mutexStarving) != 0 {
	// mutexLocked位或者mutexStarving不为0，等待锁的g数量+1
	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 {
	// 上一次循环设置了该g的饥饿标志，并且锁仍未解开，则设置饥饿模式
	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) {
	// CAS成功的当前g会将state设置为了lock状态，如果old为正常解锁状态，意味着抢到了锁
	if old&(mutexLocked|mutexStarving) == 0 {
		break // locked the mutex with CAS
	}
	// If we were already waiting before, queue at the front of the queue.
	// 如果waitStartTime不为0，表示之前已经尝试过取锁了
	queueLifo := waitStartTime != 0
	if waitStartTime == 0 {
		waitStartTime = runtime_nanotime()
	}
	runtime_SemacquireMutex(&m.sema, queueLifo, 1)
	starving = starving || runtime_nanotime()-waitStartTime > starvationThresholdNs
	old = m.state
	// 判断是否处于饥饿模式
	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")
		}
		
		delta := int32(mutexLocked - 1<<mutexWaiterShift)
		// 解除饥饿状态的情况有两种
		// 1. starving为false，即当前g的等待时间小于1ms
		// 2. 目前只有当前g在等🔐
		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
	}
	// 非饥饿模式
	awoke = true
	iter = 0
} else {
	// 未拿到🔐，重新循环
	old = m.state
}

// 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.
	// 注意这里已经将mutexLocked位置0了
	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) {
	// 状态检查，如果new是0的话，不应该进入慢解锁
	if (new+mutexLocked)&mutexLocked == 0 {
		throw("sync: unlock of unlocked mutex")
	}
	
	if new&mutexStarving == 0 {
		// 如果不在饥饿状态，此时就是正常的竞争，进入慢速解锁可能是因为存在等待的g
		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 {
				// 如果不存在等待者，或者锁已经被拿了、或者中途进入了饥饿模式。
				return
			}
			// Grab the right to wake someone.
			// 等待人-1，唤醒一个等待的g
			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.
		// 饥饿模式，将🔐的所有权移交给下一个等待者，并让出当前g的时间片从而让下一个等待者能立马开始执行。
		// 这时并未退出饥饿状态
		// 拿到🔐的等待者，将负责设置mutexLocked
		// 注意，相比上个分支，这里没有做等待人-1的操作，这个操作会由等待者执行。
		runtime_Semrelease(&m.sema, true, 1)
	}
}