先来看一段go1.12.5中Mutex的源码：

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.// Package sync provides basic synchronization primitives such as mutual
// exclusion locks. Other than the Once and WaitGroup types, most are intended
// for use by low-level library routines. Higher-level synchronization is
// better done via channels and communication.
//
// Values containing the types defined in this package should not be copied.
package syncimport ("internal/race""sync/atomic""unsafe"
)func throw(string) // provided by runtime// 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 int32sema  uint32
}// A Locker represents an object that can be locked and unlocked.
type Locker interface {Lock()Unlock()
}const (mutexLocked = 1 << iota // mutex is lockedmutexWokenmutexStarvingmutexWaiterShift = iota// 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
)// 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}var waitStartTime int64starving := falseawoke := falseiter := 0old := m.statefor {// 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.statecontinue}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 != 0if waitStartTime == 0 {waitStartTime = runtime_nanotime()}runtime_SemacquireMutex(&m.sema, queueLifo)starving = starving || runtime_nanotime()-waitStartTime > starvationThresholdNsold = m.stateif 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 = trueiter = 0} else {old = m.state}}if race.Enabled {race.Acquire(unsafe.Pointer(m))}
}// 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+mutexLocked)&mutexLocked == 0 {throw("sync: unlock of unlocked mutex")}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.if old>>mutexWaiterShift == 0 || old&(mutexLocked|mutexWoken|mutexStarving) != 0 {return}// Grab the right to wake someone.new = (old - 1<<mutexWaiterShift) | mutexWokenif atomic.CompareAndSwapInt32(&m.state, old, new) {runtime_Semrelease(&m.sema, false)return}old = m.state}} else {// Starving mode: handoff mutex ownership to the next waiter.// 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)}
}

mutex.go

Mutex结构体

type Mutex struct {state int32      //互斥锁的状态sema  uint32    //信号量，协程阻塞等待该信号量，解锁的协程释放信号量从而唤醒等待信号量的协程。
}

查资料的时候找到一张很好的展示Mutex的内存布局的图：

Locked：表示该Mutex是否已被锁定，0代表未被锁定，1代表已被锁定。
Woken：表示是否有协程已被唤醒，0代表没有协程被唤醒，1代表已有协程被唤醒，正在加锁。
Starving：表示该Mutex是否处于饥饿状态，0代表不处于饥饿状态，1代表处于饥饿状态，即阻塞超过1ms。
Waiter：表示阻塞等待锁的协程个数，协程解锁时根据此值来判断是否需要释放信号量。

协程之间抢锁实际上是抢给Locked赋值的权利，能给Locked域置1，就说明抢锁成功。抢不到就阻塞等待Mutex.sema信号量，一旦持有锁的协程解锁，等待的协程会依次被唤醒。

（这个作者写的真好，原文链接我放到文末了～）

（有时候很恍惚，自己到底有没有写博客的必要，大部分时候都是互联网的搬运工，总有一天，我也能写出纯原创的技术博客～）

两种操作模式

go的互斥有两种操作模式：正常模式和饥饿模式

在正常模式下，等待状态的协程（等待者）按照FIFO顺序排队。一个由沉睡（sleep）醒转（就绪）来的协程不拥有互斥锁，并且它对于互斥锁的竞争并不如新到来的协程有优势。新到来的协程可能已经在CPU上运行，并且可能数量还很多。但是，如果醒转来的等待者在队列中阻塞等待互斥锁超过1毫秒，正常模式将会切换到饥饿模式。

在饥饿模式下，互斥锁的所有权直接从释放锁的协程传递到队列最前的等待者。新到来的协程即使有竞争优势，也不会去争取互斥锁，相反，它们会到队列尾部排队等候。

如果醒转来的等待者获得互斥锁的所有权并且发现（1）它是队列中的最后一个等待者，或者（2）它等待不到1ms，它便会把饥饿模式切换到正常模式。

正常模式具有相当好的性能，因为即使存在阻塞的等待者，协程也可以连续多次获取互斥锁。

而饥饿模式可以预防到达协程迟迟得不到处理（反而可能被远远排在新协程之后），说白了，饥饿模式就是为防止协程进入饥饿状态忙等而设。

两种方法

Mutex只提供了两种方法，即Lock()加锁和Unlock()解锁。

加锁

以上面那张Mutex内存布局图为例，最简单毫无阻塞的加锁就是将Locked置为1，当锁已被占用，则将Waiter++，协程进入阻塞，直到Locked值变为0后被唤醒。

解锁

仍是以上面那张Mutex内存布局图为例，没有其他协程阻塞等待加锁（即Waiter为0），则只要将Locked置为0，不需要释放信号量。若解锁时，有1个或多个协程阻塞（即Waiter>0），则需要在将Locked置为0后，释放信号量唤醒阻塞的协程。

自旋

基本概念

上面我们说到，加锁时可能被阻塞，此时，协程并不是立即进入阻塞，而是会持续检测Locked是否变为0，这个过程即为自旋（spin）过程。从源码mutex源码中runtime_canSpin()和runtime_doSpin()两个方法，它们就是用来判断是否可以自旋（即是否符合自旋条件）和执行自旋的。

自旋必须满足一下所有条件：

自旋次数要足够小，通常为4，即每个协程最多自旋4次。
CPU核数要大于1，否则自旋没有意义，因为此时不可能有其他协程释放锁。
协程调度机制中的Process数量要大于1，比如使用GOMAXPROCS()将处理器设置为1就不能启动自旋。
协程调度机制中的可运行队列必须为空，否则会延迟协程调度。

限制自旋次数很好理解，关于CPU核数，一开始我不理解为什么要限制这个，不妨来设想一下协程自旋的场景，假设一个协程主动释放了锁并释放了信号量，我们的协程在对处理器的竞争中惨败，因此进入短暂自旋，以期寻找其他门路，即看看其他处理器是不是正有协程准备解锁，试想，假如只有1核，刚刚在和我们的竞争中获取取得锁控制权的协程，怎么可能在短期内释放锁，因此只能直接进入阻塞。

至于什么是GOMAXPROCS呢？就是逻辑CPU数量，它可以被设置为如下几种数值：