mutex互斥锁经历过很多版本，我们只讨论最新的版本。当前1.24.2的版本。
相关源码
之前的版本比较简单实现，当前的的版本代码几乎不可读了，相对来说复杂了很多，但其更加高效，新来的 goroutine 也参与竞争，有可能每次都会被新来的 goroutine 抢到获取锁的机会，在极端情况下，等待中的 goroutine 可能会一直获取不到锁，这就是饥饿问题。当前版本已经解决了这个最大的问题。
2016 年 Go 1.9 中 Mutex 增加了饥饿模式，让锁变得更公平，不公平的等待时间限制在 1 毫秒，并且修复了一个大 Bug：总是把唤醒的 goroutine 放在等待队列的尾部，会导致更加不公平的等待时间。其实现在的mutex源码和之前变化不是很大了。只是增加了一些特性，但基本还是那样。
Mutex 绝不容忍一个 goroutine 被落下，永远没有机会获取锁。不抛弃不放弃是它的宗旨，而且它也尽可能地让等待较长的 goroutine 更有机会获取到锁。
相关代码
type Mutex struct {
state int32
sema uint32
}
const (
mutexLocked = 1 << iota // mutex is locked
mutexWoken
mutexStarving // 从state字段中分出一个饥饿标记
mutexWaiterShift = iota
starvationThresholdNs = 1e6
)
// Lock locks m.
//
// See package [sync.Mutex] documentation.
func (m *Mutex) Lock() {
// Fast path: 幸运之路，一下就获取到了锁
// Fast path: grab unlocked mutex.
if atomic.CompareAndSwapInt32(&m.state, 0, mutexLocked) {
if race.Enabled {
race.Acquire(unsafe.Pointer(m))
}
return
}
// Slow path：缓慢之路，尝试自旋竞争或饥饿状态下饥饿goroutine竞争
// Slow path (outlined so that the fast path can be inlined)
m.lockSlow()
}
func (m *Mutex) lockSlow() {
var waitStartTime int64
starving := false // 此goroutine的饥饿标记
awoke := false // 唤醒标记
iter := 0 // 自旋次数
old := m.state // 当前的锁的状态
for {
// 锁是非饥饿状态，锁还没被释放，尝试自旋
// 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 // 再次获取锁的状态，之后会检查是否锁被释放了
continue
}
new := old
// Don't try to acquire starving mutex, new arriving goroutines must queue.
if old&mutexStarving == 0 {
new |= mutexLocked // 非饥饿状态，加锁
}
if old&(mutexLocked|mutexStarving) != 0 {
new += 1 << mutexWaiterShift // waiter数量加1
}
// 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 // 设置饥饿状态
}
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 // 新状态清除唤醒标记
}
// 成功设置新状态
if atomic.CompareAndSwapInt32(&m.state, old, new) {
// // 原来锁的状态已释放，并且不是饥饿状态，正常请求到了锁，返回
if old&(mutexLocked|mutexStarving) == 0 {
break // locked the mutex with CAS
}
// 处理饥饿状态
// 如果以前就在队列里面，加入到队列头
// If we were already waiting before, queue at the front of the queue.
queueLifo := waitStartTime != 0
if waitStartTime == 0 {
waitStartTime = runtime_nanotime()
}
// 阻塞等待
runtime_SemacquireMutex(&m.sema, queueLifo, 2)
// 唤醒之后检查锁是否应该处于饥饿状态
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.
if old&(mutexLocked|mutexWoken) != 0 || old>>mutexWaiterShift == 0 {
throw("sync: inconsistent mutex state")
}
// 加锁并且将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.
delta -= mutexStarving // 最后一个waiter或者已经不饥饿了，清除饥饿标记
}
atomic.AddInt32(&m.state, delta)
break
}
awoke = true
iter = 0
} else {
old = m.state
}
}
if race.Enabled {
race.Acquire(unsafe.Pointer(m))
}
}
// Unlock unlocks m.
//
// See package [sync.Mutex] documentation.
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 != 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+mutexLocked)&mutexLocked == 0 {
fatal("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<<mutexWaiterShift) | mutexWoken
if atomic.CompareAndSwapInt32(&m.state, old, new) {
runtime_Semrelease(&m.sema, false, 2)
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.
runtime_Semrelease(&m.sema, true, 2)
}
}通过加入饥饿模式，可以避免把机会全都留给新来的 goroutine，保证了请求锁的 goroutine 获取锁的公平性，对于我们使用锁的业务代码来说，不会有业务一直等待锁不被处理。仔细阅读源码可以得知其设计的巧妙，但这个源码不可读性太强。
使用互斥锁的易错场景
Lock/Unlock 没有成对出现
Lock/Unlock 没有成对出现，就意味着会出现死锁的情况，或者是因为 Unlock 一个未加锁的 Mutex 而导致 panic。
缺少 Unlock 的场景，常见的有三种情况：
- 代码中有太多的 if-else 分支，可能在某个分支中漏写了 Unlock
- 在重构的时候把 Unlock 给删除了
- Unlock 误写成了 Lock
重入
当一个线程获取锁时，如果没有其它线程拥有这个锁，那么，这个线程就成功获取到这个锁。之后，如果其它线程再请求这个锁，就会处于阻塞等待的状态。但是，如果拥有这把锁的线程再请求这把锁的话，不会阻塞，而是成功返回，所以叫可重入锁（有时候也叫做递归锁）。只要你拥有这把锁，你可以可着劲儿地调用，比如通过递归实现一些算法，调用者不会阻塞或者死锁。
go的mutex不是可重入锁。mutex当中并没有那些字段记录哪个goroutine 拥有这把锁，所有无法做到重入锁的。
如下面的代码：
func foo(l sync.Locker) {
fmt.Println("in foo")
l.Lock()
bar(l)
l.Unlock()
}
func bar(l sync.Locker) {
l.Lock()
fmt.Println("in bar")
l.Unlock()
}
func main() {
l := &sync.Mutex{}
foo(l)
}实现一个可重入锁并：
代码如下，其原理就是标识持有锁的goroutine ，以达到可重入的目的：
package main
import (
"fmt"
"sync"
"sync/atomic"
)
type TokenRecursiveMutex struct {
sync.Mutex
token int64 // 当前持有锁的token
recursion int32 // 重入的次数
}
// Lock 请求锁需要传入token
func (m *TokenRecursiveMutex) Lock(token int64) {
// 如果传入的token和持有锁的token一致，此时递归加锁，重入次数+1
if atomic.LoadInt64(&m.token) == token {
m.recursion++
return
}
m.Mutex.Lock()
// 上面的条件不满足，说明不是递归调用，抢到新锁后记录此锁拥有者token值
atomic.StoreInt64(&m.token, token)
m.recursion = 1
}
// Unlock 释放锁
func (m *TokenRecursiveMutex) Unlock(token int64) {
// 如果传入token 和持有锁的token不一致，不能解锁，直接运行恐慌
if atomic.LoadInt64(&m.token) != token {
panic(fmt.Sprintf("wrong the owner(%d):%d", m.token, token))
}
// 否则的话把当前此有锁的token重入次数减去1
m.recursion--
// 如果发现当前持有锁的token重入次数不等于0，说明还有其他重入锁未解锁
if m.recursion != 0 {
return
}
// 没有其他重入锁了，释放锁 之前清楚当前锁的持有者token
atomic.StoreInt64(&m.token, 0)
// 解锁
m.Mutex.Unlock()
}
func main() {
r := &TokenRecursiveMutex{}
StartTokenLayer(r, 1024)
recursiveFunc(r, 10, 1025)
}
func recursiveFunc(lock *TokenRecursiveMutex, depth int, token int64) {
lock.Lock(token)
defer lock.Unlock(token)
if depth > 0 {
fmt.Printf("depth %d\n", depth)
recursiveFunc(lock, depth-1, token) // 递归调用需要再次获取同一把锁
}
}
func StartTokenLayer(r *TokenRecursiveMutex, token int64) {
r.Lock(token)
fmt.Println("开始")
TwoTokenLayer(r, token)
r.Unlock(token)
}
func TwoTokenLayer(r *TokenRecursiveMutex, token int64) {
r.Lock(token)
fmt.Println("进入第二层")
ThreeTokenLayer(r, token)
r.Unlock(token)
}
func ThreeTokenLayer(r *TokenRecursiveMutex, token int64) {
r.Lock(token)
fmt.Println("最后一层")
r.Unlock(token)
}死锁
死锁（Deadlock） 是指两个或多个 goroutine 因互相等待对方释放资源而永久阻塞的状态。Go 的运行时（runtime）会自动检测到这种情况并抛出 fatal error: all goroutines are asleep - deadlock! 错误。
- 资源互斥：
- 共享资源（如锁、通道）被独占，其他 goroutine 无法访问。
- 持有并等待：
- Goroutine 持有资源的同时等待其他资源。
- 不可抢占：
- 资源只能由持有者释放，无法强制剥夺。
- 循环等待：
- Goroutine 之间形成环形依赖链。