闲来无事，阅读一下golang互斥锁实现
全称
mutual exclusion lock - 彼此互斥的锁
包简介
// 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.- mutex是golang比较基本的同步原语，原语=>原始语言
- mutex相比channel是轻量级的lib
Mutex Struct
type Mutex struct {
state int32 //锁标识位， 0bit-锁标记 | 1bit-唤醒标记 | 2bit-饥饿标记 | 其他-waiter
sema uint32 //锁信号量
}Const
const (
mutexLocked = 1 << iota // mutex is locked
mutexWoken
mutexStarving
mutexWaiterShift = iota
starvationThresholdNs = 1e6
）对应struct中的state标记位，starvationThresholdNs是自旋时间阈值，自旋时间超过这个值，会进入饥饿状态。
Mutex工作模式
// 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.- 正常模式
mutex如果未被上锁，或者compete goroutine 通过自旋能够获取到锁，mutex会处于该模式下
- 饥饿模式
自旋时间超过阈值，mutex会进入饥饿模式，之后的compete goroutine 不会自旋了，直接进入sema信号量的队列尾部
- lifo or fifo
饥饿状态下，compete goroutine 会以先进先出的方式进入等待队列，参与过自旋的会以后进先出的方式进入等待队列
- 自旋+lifo && starving + 不自旋 实质是性能和公平的兼顾综合考量，这块设计的很细腻！
- woken up标识位
主要应对自旋和sema信号量释放并发的场景，当有自旋goroutine时，woken up =1，此时sema释放时，发现woken up =1时，就不会从阻塞队列释放goroutine了，让处于自旋状态的goroutines竞争
上锁代码
func (m *Mutex) Lock() {
// 使用cpu的原子操作，给mutex上锁，上锁成功直接返回
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)
m.lockSlow()
}
func (m *Mutex) lockSlow() {
var waitStartTime int64
starving := false
awoke := false
iter := 0
old := m.state
for {
//locked状态 && 非饥饿状态 && 可以自旋 才能自旋
//runtime_canSpin和诸多因素有关，cpu核数，当前processor不能有等待执行的goroutine等
if old&(mutexLocked|mutexStarving) == mutexLocked && runtime_canSpin(iter) {
//设置woken up标识位
//设置条件 非woken up状态 && 有阻塞goroutine
if !awoke && old&mutexWoken == 0 && old>>mutexWaiterShift != 0 &&
atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {
awoke = true
}
//自旋
runtime_doSpin()
iter++
old = m.state
continue
}
//自旋结束or没自旋
new := old
//自旋竞争失败要上锁
if old&mutexStarving == 0 {
new |= mutexLocked
}
//等待计数+1
if old&(mutexLocked|mutexStarving) != 0 {
new += 1 << mutexWaiterShift
}
//切换饥饿模式
if starving && old&mutexLocked != 0 {
new |= mutexStarving
}
// 竞争失败后，woken up 标识位要清除
if awoke {
if new&mutexWoken == 0 {
throw("sync: inconsistent mutex state")
}
new &^= mutexWoken
}
//更新mutex state状态
if atomic.CompareAndSwapInt32(&m.state, old, new) {
if old&(mutexLocked|mutexStarving) == 0 {
break // locked the mutex with CAS
}
//自旋过的goroutine 使用lifo方式进入信号量等待队列
queueLifo := waitStartTime != 0
if waitStartTime == 0 {
waitStartTime = runtime_nanotime()
}
//调用信号量func
runtime_SemacquireMutex(&m.sema, queueLifo, 1)
starving = starving || runtime_nanotime()-waitStartTime > starvationThresholdNs
old = m.state
//不在饥饿的情况，需要清除后面的标识位
if old&mutexStarving != 0 {
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))
}
}解锁代码
func (m *Mutex) Unlock() {
if race.Enabled {
_ = m.state
race.Release(unsafe.Pointer(m))
}
new := atomic.AddInt32(&m.state, -mutexLocked)
//发现之前有其他标识位的，需要走额外逻辑
if new != 0 {
m.unlockSlow(new)
}}
func (m *Mutex) unlockSlow(new int32) {
//重复unlock 直接panic
if (new+mutexLocked)&mutexLocked == 0 {
throw("sync: unlock of unlocked mutex")
}
if new&mutexStarving == 0 {
old := new
for {
if old>>mutexWaiterShift == 0 || old&(mutexLocked|mutexWoken|mutexStarving) != 0 {
return
}
// 设置唤醒标识位
new = (old - 1<<mutexWaiterShift) | mutexWoken
if atomic.CompareAndSwapInt32(&m.state, old, new) {
//释放信号量，lifo方式释放goroutine
runtime_Semrelease(&m.sema, false, 1)
return
}
old = m.state
}
} else {
//有饥饿，fifo方式释放信号量和goroutine
runtime_Semrelease(&m.sema, true, 1)
}}
