go中runtime包里面的mutex是什么？runtime.mutex解析

其实在看go源码的时候，发现除了sync包里有个mutex以外，runtime包里也有一个mutex，这个mutex在runtime很多地方都在用。

这个runtime包里面的mutex的结构如下：

目录: /runtime/runtime2.go

代码：

/runtime/runtime2.go

type mutex struct {
	lockRankStruct
	key uintptr
}

可以看到他有两个成员，一个是lockRankStruct，一个是key。

先看下lockRankStruct：

lockRankStruct 这个结构体是个空的结构体，在网上有大佬这样描述：

lockRankStruct提供了一种运行时的静态锁排名的机制。静态锁排名会建立文件化的锁之间的总排序顺序，如果违反总的排序，则会报错。只要锁排序是按照文档设计的顺序，锁排序死锁就不会发生。如果要做Go运行时使用这个机制，你需要设置GOEXPERIMENT=staticlockranking。 默认未开启，此时lockRanStruct是一个空结构体。lockWithRank()等效于lock()

啥意思啊？按照文档设计顺序，文档是啥，文档顺序是啥？

其实这里是一种非常机翻的说法，他其实说的是在go的配置中启动GOEXPERIMENT=staticlockranking，就会按照一定的顺序检查锁，如果锁不是按照这个顺序进行了，就会出现死锁的情况，那么就会抛出throw（在runtime包内部很多错误是throw，不是panic，throw方法并没有暴露给用户使用，用户只能用recover接panic，也就说用户无法处理throw错误，项目会直接崩掉）

那这个顺序是哪来的？

这个顺序其实是已经规定好的，当然是go语言的开发人员提交的，提交这个changes的地址是go-review.googlesource.com/c/go/+/2076...

他这个顺序其实是一个map，一个已经规定好的map，lockPartialOrder，位置是runtime包中的lockrank.go

var lockPartialOrder [][]lockRank = [][]lockRank{
	lockRankSysmon:         {},
	lockRankScavenge:       {lockRankSysmon},
	lockRankForcegc:        {lockRankSysmon},
	lockRankDefer:          {},
	lockRankSweepWaiters:   {},
	lockRankAssistQueue:    {},
	lockRankSweep:          {},
	lockRankPollDesc:       {},
	lockRankCpuprof:        {},
	lockRankSched:          {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof},
	lockRankAllg:           {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched},
	lockRankAllp:           {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched},
	lockRankTimers:         {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllp, lockRankTimers},
	lockRankNetpollInit:    {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllp, lockRankTimers},
	lockRankHchan:          {lockRankSysmon, lockRankScavenge, lockRankSweep, lockRankHchan},
	lockRankNotifyList:     {},
	lockRankSudog:          {lockRankSysmon, lockRankScavenge, lockRankSweep, lockRankHchan, lockRankNotifyList},
	lockRankRwmutexW:       {},
	lockRankRwmutexR:       {lockRankSysmon, lockRankRwmutexW},
	lockRankRoot:           {},
	lockRankItab:           {},
	lockRankReflectOffs:    {lockRankItab},
	lockRankUserArenaState: {},
	lockRankTraceBuf:       {lockRankSysmon, lockRankScavenge},
	lockRankTraceStrings:   {lockRankSysmon, lockRankScavenge, lockRankTraceBuf},
	lockRankFin:            {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllg, lockRankAllp, lockRankTimers, lockRankHchan, lockRankNotifyList, lockRankItab, lockRankReflectOffs, lockRankUserArenaState, lockRankTraceBuf, lockRankTraceStrings},
	lockRankGcBitsArenas:   {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllg, lockRankAllp, lockRankTimers, lockRankHchan, lockRankNotifyList, lockRankItab, lockRankReflectOffs, lockRankUserArenaState, lockRankTraceBuf, lockRankTraceStrings},
	lockRankMheapSpecial:   {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllg, lockRankAllp, lockRankTimers, lockRankHchan, lockRankNotifyList, lockRankItab, lockRankReflectOffs, lockRankUserArenaState, lockRankTraceBuf, lockRankTraceStrings},
	lockRankMspanSpecial:   {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllg, lockRankAllp, lockRankTimers, lockRankHchan, lockRankNotifyList, lockRankItab, lockRankReflectOffs, lockRankUserArenaState, lockRankTraceBuf, lockRankTraceStrings},
	lockRankSpanSetSpine:   {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllg, lockRankAllp, lockRankTimers, lockRankHchan, lockRankNotifyList, lockRankItab, lockRankReflectOffs, lockRankUserArenaState, lockRankTraceBuf, lockRankTraceStrings},
	lockRankProfInsert:     {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllg, lockRankAllp, lockRankTimers, lockRankHchan, lockRankNotifyList, lockRankItab, lockRankReflectOffs, lockRankUserArenaState, lockRankTraceBuf, lockRankTraceStrings},
	lockRankProfBlock:      {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllg, lockRankAllp, lockRankTimers, lockRankHchan, lockRankNotifyList, lockRankItab, lockRankReflectOffs, lockRankUserArenaState, lockRankTraceBuf, lockRankTraceStrings},
	lockRankProfMemActive:  {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllg, lockRankAllp, lockRankTimers, lockRankHchan, lockRankNotifyList, lockRankItab, lockRankReflectOffs, lockRankUserArenaState, lockRankTraceBuf, lockRankTraceStrings},
	lockRankProfMemFuture:  {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllg, lockRankAllp, lockRankTimers, lockRankHchan, lockRankNotifyList, lockRankItab, lockRankReflectOffs, lockRankUserArenaState, lockRankTraceBuf, lockRankTraceStrings, lockRankProfMemActive},
	lockRankGscan:          {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllg, lockRankAllp, lockRankTimers, lockRankNetpollInit, lockRankHchan, lockRankNotifyList, lockRankRoot, lockRankItab, lockRankReflectOffs, lockRankUserArenaState, lockRankTraceBuf, lockRankTraceStrings, lockRankFin, lockRankGcBitsArenas, lockRankSpanSetSpine, lockRankProfInsert, lockRankProfBlock, lockRankProfMemActive, lockRankProfMemFuture},
	lockRankStackpool:      {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllg, lockRankAllp, lockRankTimers, lockRankNetpollInit, lockRankHchan, lockRankNotifyList, lockRankRwmutexW, lockRankRwmutexR, lockRankRoot, lockRankItab, lockRankReflectOffs, lockRankUserArenaState, lockRankTraceBuf, lockRankTraceStrings, lockRankFin, lockRankGcBitsArenas, lockRankSpanSetSpine, lockRankProfInsert, lockRankProfBlock, lockRankProfMemActive, lockRankProfMemFuture, lockRankGscan},
	lockRankStackLarge:     {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllg, lockRankAllp, lockRankTimers, lockRankNetpollInit, lockRankHchan, lockRankNotifyList, lockRankRoot, lockRankItab, lockRankReflectOffs, lockRankUserArenaState, lockRankTraceBuf, lockRankTraceStrings, lockRankFin, lockRankGcBitsArenas, lockRankSpanSetSpine, lockRankProfInsert, lockRankProfBlock, lockRankProfMemActive, lockRankProfMemFuture, lockRankGscan},
	lockRankHchanLeaf:      {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllg, lockRankAllp, lockRankTimers, lockRankNetpollInit, lockRankHchan, lockRankNotifyList, lockRankRoot, lockRankItab, lockRankReflectOffs, lockRankUserArenaState, lockRankTraceBuf, lockRankTraceStrings, lockRankFin, lockRankGcBitsArenas, lockRankSpanSetSpine, lockRankProfInsert, lockRankProfBlock, lockRankProfMemActive, lockRankProfMemFuture, lockRankGscan, lockRankHchanLeaf},
	lockRankWbufSpans:      {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankDefer, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllg, lockRankAllp, lockRankTimers, lockRankNetpollInit, lockRankHchan, lockRankNotifyList, lockRankSudog, lockRankRoot, lockRankItab, lockRankReflectOffs, lockRankUserArenaState, lockRankTraceBuf, lockRankTraceStrings, lockRankFin, lockRankGcBitsArenas, lockRankMspanSpecial, lockRankSpanSetSpine, lockRankProfInsert, lockRankProfBlock, lockRankProfMemActive, lockRankProfMemFuture, lockRankGscan},
	lockRankMheap:          {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankDefer, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllg, lockRankAllp, lockRankTimers, lockRankNetpollInit, lockRankHchan, lockRankNotifyList, lockRankSudog, lockRankRwmutexW, lockRankRwmutexR, lockRankRoot, lockRankItab, lockRankReflectOffs, lockRankUserArenaState, lockRankTraceBuf, lockRankTraceStrings, lockRankFin, lockRankGcBitsArenas, lockRankMspanSpecial, lockRankSpanSetSpine, lockRankProfInsert, lockRankProfBlock, lockRankProfMemActive, lockRankProfMemFuture, lockRankGscan, lockRankStackpool, lockRankStackLarge, lockRankWbufSpans},
	lockRankGlobalAlloc:    {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankDefer, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllg, lockRankAllp, lockRankTimers, lockRankNetpollInit, lockRankHchan, lockRankNotifyList, lockRankSudog, lockRankRwmutexW, lockRankRwmutexR, lockRankRoot, lockRankItab, lockRankReflectOffs, lockRankUserArenaState, lockRankTraceBuf, lockRankTraceStrings, lockRankFin, lockRankGcBitsArenas, lockRankMheapSpecial, lockRankMspanSpecial, lockRankSpanSetSpine, lockRankProfInsert, lockRankProfBlock, lockRankProfMemActive, lockRankProfMemFuture, lockRankGscan, lockRankStackpool, lockRankStackLarge, lockRankWbufSpans, lockRankMheap},
	lockRankTrace:          {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankDefer, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllg, lockRankAllp, lockRankTimers, lockRankNetpollInit, lockRankHchan, lockRankNotifyList, lockRankSudog, lockRankRwmutexW, lockRankRwmutexR, lockRankRoot, lockRankItab, lockRankReflectOffs, lockRankUserArenaState, lockRankTraceBuf, lockRankTraceStrings, lockRankFin, lockRankGcBitsArenas, lockRankMspanSpecial, lockRankSpanSetSpine, lockRankProfInsert, lockRankProfBlock, lockRankProfMemActive, lockRankProfMemFuture, lockRankGscan, lockRankStackpool, lockRankStackLarge, lockRankWbufSpans, lockRankMheap},
	lockRankTraceStackTab:  {lockRankSysmon, lockRankScavenge, lockRankForcegc, lockRankDefer, lockRankSweepWaiters, lockRankAssistQueue, lockRankSweep, lockRankPollDesc, lockRankCpuprof, lockRankSched, lockRankAllg, lockRankAllp, lockRankTimers, lockRankNetpollInit, lockRankHchan, lockRankNotifyList, lockRankSudog, lockRankRwmutexW, lockRankRwmutexR, lockRankRoot, lockRankItab, lockRankReflectOffs, lockRankUserArenaState, lockRankTraceBuf, lockRankTraceStrings, lockRankFin, lockRankGcBitsArenas, lockRankMspanSpecial, lockRankSpanSetSpine, lockRankProfInsert, lockRankProfBlock, lockRankProfMemActive, lockRankProfMemFuture, lockRankGscan, lockRankStackpool, lockRankStackLarge, lockRankWbufSpans, lockRankMheap, lockRankTrace},
	lockRankPanic:          {},
	lockRankDeadlock:       {lockRankPanic, lockRankDeadlock},
}

其实这玩意称之为"文档"多少有点不太直观，但是你知道是什么意思就行，不必纠结这个命名，很多命名有一定的历史因素或者直译不够信雅达，追求这个考古结果太浪费时间

那这个顺序是怎么验证的？

其实验证这个顺序的方法也不复杂，就在runtime/lockrank_on.go文件里的checkRanks方法里

func checkRanks(gp *g, prevRank, rank lockRank) {
	rankOK := false
	if rank < prevRank {
		// If rank < prevRank, then we definitely have a rank error
		rankOK = false
	} else if rank == lockRankLeafRank {
		// If new lock is a leaf lock, then the preceding lock can
		// be anything except another leaf lock.
		rankOK = prevRank < lockRankLeafRank
	} else {
		// We've now verified the total lock ranking, but we
		// also enforce the partial ordering specified by
		// lockPartialOrder as well. Two locks with the same rank
		// can only be acquired at the same time if explicitly
		// listed in the lockPartialOrder table.
		list := lockPartialOrder[rank]
		for _, entry := range list {
			if entry == prevRank {
				rankOK = true
				break
			}
		}
	}
	if !rankOK {
		printlock()
		println(gp.m.procid, " ======")
		printHeldLocks(gp)
		throw("lock ordering problem")
	}
}

​

不知道这个有啥影响么？

没有，不开启GOEXPERIMENT=staticlockranking 都不用关心这个问题，不看源码甚至不知道这个玩意，不必太在意，看到了当个乐子就行

---

再看下key：

key的实现方式有两种

runtime/lock_futex.go 文件里的futex实现

go:build dragonfly || freebsd || linux

主要是dragonfly，FreeBSD，linux 系统；

另一个就是 runtime/lock_sema.go 文件里的 sema实现

go:build aix || darwin || netbsd || openbsd || plan9 || solaris || windows

主要是 aix ， darwin ， netbsd ， openbsd ， plan9 ， solaris ， windows系统

但是实现起来也是大差不差的。

以linux的为例，毕竟日常开发还是linux的项目比较多。

首先，锁的状态有三种

const (
	mutex_unlocked = 0
	mutex_locked   = 1
	mutex_sleeping = 2

• mutex_unlocked = 该锁没有被锁定（没有G获得锁）
• mutex_locked = 锁了
• mutex_sleeping = 表示有线程调用futexsleep阻塞了

然后看lock方法，这个是获取锁的方法

func lock2(l *mutex) {
	// 获取当前运行该方法的协程G
	gp := getg()
	// 锁都是负数了，出问题了，报错，throw扔出去
	if gp.m.locks < 0 {
		throw("runtime·lock: lock count")
	}
	// 当前协程G绑定的M上的lock计数+1，不懂的去看GMP结构
	gp.m.locks++

	// 资源比较空闲时候，第一次请求锁就成功了，直接返回，表示获取锁成功
	// Xchg直接交换值，没有CAS操作，Xchg比较简单，理论上更快，Cas还要做值对比，Xchg更适合去抢锁
	v := atomic.Xchg(key32(&l.key), mutex_locked)
	if v == mutex_unlocked {
		return
	}
	// 大部分情况下，你获取是失败的，毕竟加锁的场景就是高并发引起的冲突，不然你加个毛的锁

	// 当v是mutex_unlock和mutex_sleeping的时候，也就说加锁没有成功

	// 修改名称把状态改成wait------等待
	wait := v
	// 自旋次数，如果ncpu也就是cpu核心数大于1，说明是多核cpu，那么就自旋等待下，active_spin实际上是4，那就是自旋4次，至于为什么是4，我没深究
	spin := 0
	if ncpu > 1 {
		// spin是spinning的意思"旋转的"，你打红警的时候盖特机炮的台词就有spin
		spin = active_spin
	}
	for {
		// 尝试获取锁
		for i := 0; i < spin; i++ {
			for l.key == mutex_unlocked {
				// Cas操作，获取锁。这里的wait的值第一次是上面传入的，第二次则是从for循环末尾拿到的
				if atomic.Cas(key32(&l.key), mutex_unlocked, wait) {
					return
				}
			}
			// 这个方法其实是底层调用了CPU的PAUSE指令，意在优化自旋等待的效率，主要是针对赛扬和志强cpu的
			procyield(active_spin_cnt)
		}
		// passive_spin = 1,passive -- 被动的，被动自旋，就是说上面4次自旋都没拿到，再给你一次机会，让你再获取一次锁，所以整个获取锁的自旋次数是 4 + 1
		for i := 0; i < passive_spin; i++ {
			for l.key == mutex_unlocked {
				// 同样的CAS操作获取锁
				if atomic.Cas(key32(&l.key), mutex_unlocked, wait) {
					return
				}
			}
			// 让出cpu
			osyield()
		}
		// 切换锁的状态为sleeping，xchg返回原来的值
		v = atomic.Xchg(key32(&l.key), mutex_sleeping)
		// 如果原来的值是mutex_unlocked,也就说无锁，那么mutex_sleeping 本身就算拿到锁了
		if v == mutex_unlocked {
			return
		}
		// 重新设定wait值为mutex_sleeping为下次循环做准备
		wait = mutex_sleeping
		// futexsleep调用futex函数进入睡眠。
		futexsleep(key32(&l.key), mutex_sleeping, -1)
	}
}

最后看unlock方法，这个是解锁的方法

func unlock2(l *mutex) {
	// 设置 l.key = mutex_unlocked。v是旧值
	v := atomic.Xchg(key32(&l.key), mutex_unlocked)
	// 如果已经是unlocked 你还执行解锁，那么就抛出throw异常
	if v == mutex_unlocked {
		throw("unlock of unlocked lock")
	}
	// 如果旧状态是sleeping，说明已经有其他协程在等待这个锁，处于sleep状态，这时候解锁的时候，要把那个sleep的协程唤醒
	if v == mutex_sleeping {
		// 执行futexwakeup
		futexwakeup(key32(&l.key), 1)
	}

	// 获取当前的goroutine
	gp := getg()
	// 当前g所属的M的锁计数器减一
	gp.m.locks--
	if gp.m.locks < 0 {
		throw("runtime·unlock: lock count")
	}
	// m所有的锁已经释放且g本身的preempt位在解锁（unlock）之前就被被标记为true，说明该g可以被抢占了
	if gp.m.locks == 0 && gp.preempt { 
		// 既然需要被抢占，那么就要设置stackguard0位置为stackPreempt,手动标记为需要检查栈溢出，当调度器检查到栈溢出的时候会根据Goroutine的标记进行相应处理，这里就是减产preempt并执行抢占
		gp.stackguard0 = stackPreempt
	}
}

大致就是这样了