go语言中的互斥锁

// A Mutex is a mutual exclusion lock.
// Mutexes can be created as part of other structures;
// 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
}

// Fast path: grab unlocked mutex.
if atomic.CompareAndSwapInt32(&m.state, 0, mutexLocked) {
	if race.Enabled {
		race.Acquire(unsafe.Pointer(m))
	}
	return
}

awoke := false
iter := 0
for {
	old := m.state
	new := old | mutexLocked
	//步骤1
	if old&mutexLocked != 0 {
		if 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++
			continue
		}
		new = old + 1<<mutexWaiterShift
	}
	//步骤2
	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
	}
	//步骤3
	if atomic.CompareAndSwapInt32(&m.state, old, new) {
		if old&mutexLocked == 0 {
			break
		}
		runtime_SemacquireMutex(&m.sema)
		awoke = true
		iter = 0
	}
}

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

state:   |32|31|...|3|2|1|
         __________/ | |
               |      | |
               |      | mutex的占用状态（1被占用，0可用）
               |      |
               |      mutex的当前goroutine是否被唤醒
               |
               当前阻塞在mutex上的goroutine数

func semacquire(addr *uint32, profile semaProfileFlags) {
	gp := getg()
	if gp != gp.m.curg {
		throw("semacquire not on the G stack")
	}

	// Easy case.
	if cansemacquire(addr) {
		return
	}

	// Harder case:
	//	increment waiter count
	//	try cansemacquire one more time, return if succeeded
	//	enqueue itself as a waiter
	//	sleep
	//	(waiter descriptor is dequeued by signaler)
	s := acquireSudog()
	root := semroot(addr)
	t0 := int64(0)
	s.releasetime = 0
	s.acquiretime = 0
	if profile&semaBlockProfile != 0 && blockprofilerate > 0 {
		t0 = cputicks()
		s.releasetime = -1
	}
	if profile&semaMutexProfile != 0 && mutexprofilerate > 0 {
		if t0 == 0 {
			t0 = cputicks()
		}
		s.acquiretime = t0
	}
	for {
		lock(&root.lock)
		// Add ourselves to nwait to disable "easy case" in semrelease.
		atomic.Xadd(&root.nwait, 1)
		// Check cansemacquire to avoid missed wakeup.
		if cansemacquire(addr) {
			atomic.Xadd(&root.nwait, -1)
			unlock(&root.lock)
			break
		}
		// Any semrelease after the cansemacquire knows we're waiting
		// (we set nwait above), so go to sleep.
		root.queue(addr, s)
		goparkunlock(&root.lock, "semacquire", traceEvGoBlockSync, 4)
		if cansemacquire(addr) {
			break
		}
	}
	if s.releasetime > 0 {
		blockevent(s.releasetime-t0, 3)
	}
	releaseSudog(s)
}

type semaRoot struct {
	lock  mutex
	head  *sudog
	tail  *sudog
	nwait uint32 // Number of waiters. Read w/o the lock.
}

root.queue(addr, s)
goparkunlock(&root.lock, "semacquire", traceEvGoBlockSync, 4)

// 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.
	new := atomic.AddInt32(&m.state, -mutexLocked)
	if (new+mutexLocked)&mutexLocked == 0 {
		throw("sync: unlock of unlocked mutex")
	}

	old := new
	for {
		// If there are no waiters or a goroutine has already
		// been woken or grabbed the lock, no need to wake anyone.
		if old>>mutexWaiterShift == 0 || old&(mutexLocked|mutexWoken) != 0 {
			return
		}
		// Grab the right to wake someone.
		new = (old - 1<<mutexWaiterShift) | mutexWoken
		if atomic.CompareAndSwapInt32(&m.state, old, new) {
			runtime_Semrelease(&m.sema)
			return
		}
		old = m.state
	}
}