Mutex是go标准库中的互斥锁，用于处理并发场景下共享资源的访问冲突问题。
1. Mutex定义：
// 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 //表示当前锁的状态
sema uint32 //信号量专用，用于阻塞/唤醒goroutine
}
// A Locker represents an object that can be locked and unlocked.
type Locker interface {
Lock()
Unlock()
}
state字段在这里有更丰富的含义，他是一个32位的整型。他的多重含义通过不同的位来表示:
在这里，
locked：表示这个锁是否被持有。
wokend：表示是否有唤醒的goroutine线程。
starving：表示是否处于饥饿状态。
waiterCount：表示等待该锁的goroutine的数量。
2. 加锁操作
// 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.
// 如果当前处于未加锁状态，则直接加锁，并返回
// CompareAndSwapINt32() 将指定的old值 0 与指定地址 &m.state中的值相比较，如果相等，将new值 mutexLocked赋值给指定地址 &m.state。
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)
// 否则，进入slow path
m.lockSlow()
}
func (m *Mutex) lockSlow() {
// 等待时间
var waitStartTime int64
// 是否处于饥饿状态
starving := false
// 是否有唤醒的goroutine
awoke := false
// 自旋迭代次数
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.
// 如果当前锁被占有且处于非饥饿状态，就进入自旋；自旋次数达到上限，再进入下一步。需要注意的是，如果这是一个被唤醒的协程，还需要将唤醒标志位置成唤醒状态1.
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 //再次获取锁的状态，之后会检查锁是否被释放了
continue
}
new := old
// Don't try to acquire starving mutex, new arriving goroutines must queue.
// 自旋结束后，检查旧状态；如果是非饥饿状态，新的状态持有锁标志位置成 1
if old&mutexStarving == 0 {
new |= mutexLocked
}
// 检查旧状态，如果锁被占用或处于饥饿状态为1，waiter数量 +1;这个时候，协程只能进入等待队列。如果当前是饥饿状态，新来得协程什么也做不了，会直接沉睡。
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.
// 如果当前协程被唤醒后，认定是饥饿状态，并且目前锁是被占用状态；那么锁的饥饿标志位被设定为1.
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.
// 协程被唤醒，需要将唤醒标志位清0，因为他要么继续沉睡，要么获取锁
if new&mutexWoken == 0 {
throw("sync: inconsistent mutex state")
}
new &^= mutexWoken
}
if atomic.CompareAndSwapInt32(&m.state, old, new) {
// 如果当前锁的状态已经被释放，并且不是饥饿状态，那么当前协程直接占用锁，并返回
if old&(mutexLocked|mutexStarving) == 0 {
break // locked the mutex with CAS
}
// If we were already waiting before, queue at the front of the queue.
// 如果没能抢到锁，就进入等待状态；被唤醒的重新沉睡的协程会被放在队列的首位，并且这里会开始计时，用于计算是否达到饥饿状态。新加入的自旋之后的协程会放在队列的尾部。
queueLifo := waitStartTime != 0
if waitStartTime == 0 {
waitStartTime = runtime_nanotime()
}
// 阻塞等待，休眠当前goroutine等待并尝试获取信号量唤醒当前goroutine
runtime_SemacquireMutex(&m.sema, queueLifo, 1)
// 唤醒之后检查锁是否处于饥饿状态
starving = starving || runtime_nanotime()-waitStartTime > starvationThresholdNs
old = m.state
// 如果锁已经处于饥饿状态，直接抢到锁，返回
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.
// 如果当前协程被唤醒，欢迎之后检查锁是否处于饥饿状态；如果处于饥饿状态，直接抢到锁，返回
if old&(mutexLocked|mutexWoken) != 0 || old>>mutexWaiterShift == 0 {
throw("sync: inconsistent mutex state")
}
// 加锁，并将waiter数减1
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.
// 最后一个 waiter或者已经不饥饿了，清楚饥饿标记
delta -= mutexStarving
}
atomic.AddInt32(&m.state, delta)
break
}
awoke = true
iter = 0
} else {
old = m.state
}
}
if race.Enabled {
race.Acquire(unsafe.Pointer(m))
}
}
加锁操作如下:
1. 新来的goroutine直接抢占锁，抢占失败进入自旋(循环)，自旋一定次数后还是没有抢到锁进入等待队列。这一步给占有CPU时间片的goroutine更多的机会，降低整体的性能消耗。
2. 当前协程被唤醒后，判断是否处于饥饿状态；
如果是非饥饿状态，那么和新来的协程走正常抢占锁的流程；因为被唤醒的协程不占用cpu时间片，经常抢不过新来的协程；如果协程超过1ms还没有获取到锁，就进入饥饿状态。
如果是饥饿状态，将锁的饥饿标志位设置为1；锁的持有者执行完任务后，会将饥饿状态的锁交给队列中的第一个协程；新来的协程不会去抢占锁，也不会spin，而会直接进入阻塞队列。
3. 解锁操作
// 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.
// 给 m.state直接减 1， 如果等于0，表示没有其他goroutine在等待，可以直接返回。
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.
// 否则，需要通过信号量机制唤醒沉睡的goroutine
m.unlockSlow(new)
}
}
func (m *Mutex) unlockSlow(new int32) {
// 解锁一个没有锁定的mutex会报错
if (new+mutexLocked)&mutexLocked == 0 {
throw("sync: unlock of unlocked mutex")
}
// 判断是否处于饥饿状态，如果不处于饥饿状态
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.
// 如果没有等待的goroutine,或者goroutine已经被唤醒或持有了锁，直接返回
if old>>mutexWaiterShift == 0 || old&(mutexLocked|mutexWoken|mutexStarving) != 0 {
return
}
// Grab the right to wake someone.
// 唤醒等待者，并将唤醒goroutine的唤醒标志位设置为 1
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.
// 如果处于饥饿状态，则直接唤醒第一个goroutine
runtime_Semrelease(&m.sema, true, 1)
}
}