Go-Mutex互斥量

Posted 2021-12-10 straylesley

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 sync

import (
	"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 int32
	sema  uint32
}

// A Locker represents an object that can be locked and unlocked.
type Locker interface {
	Lock()
	Unlock()
}

const (
	mutexLocked = 1 << iota // mutex is locked
	mutexWoken
	mutexStarving
	mutexWaiterShift = 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 int64
	starving := false
	awoke := false
	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.
		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
			continue
		}
		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 != 0
			if waitStartTime == 0 {
				waitStartTime = runtime_nanotime()
			}
			runtime_SemacquireMutex(&m.sema, queueLifo)
			starving = starving || runtime_nanotime()-waitStartTime > starvationThresholdNs
			old = m.state
			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")
				}
				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))
	}
}

// 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.
	new := atomic.AddInt32(&m.state, -mutexLocked)
	if (new+mutexLocked)&mutexLocked == 0 {
		throw("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)
				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：

• <1：不修改任何数值。
• =1：单核心执行。
• >1：多核并发执行

一般情况下，可以使用runtime.NumCPU查询CPU数量，并使用runtime.GOMAXPROCS()进行设置，如：runtime.GOMAXPROCS(runtime.NumCPU)，将逻辑CPU数量设置为物理CPU数量。

现在想想，对Process进行限制，是不是显而易见的事。

至于可运行队列为什么必须为空，我的理解，就是当前只能有这一条就绪线程，也就是说同时只能有一条自旋。

自旋的好处

可以更充分的利用CPU。

自旋的坏处

如果协程通过自旋获得锁，那么之前被阻塞的协程将无法获得锁，如果加锁的协程特别多，每次都通过自旋获得锁，那么之前被阻塞的协程将很难获得锁，从而进入饥饿状态。因此，在1.8版本以后，饥饿状态（即Starving为1）下不允许自旋。

补充

自旋和模式切换是有区别的，自旋发生在阻塞之前，模式切换发生在阻塞之后。

整个互斥过程是围绕着Mutex量进行的，即争夺对Mutex内存的修改权，Mutex可以看作是处理器的钥匙，争夺到Mutex的协程可以被处理。

Woken的作用：

Woken用于加锁和解锁过程的通信，譬如，同一时刻，有两个协程，一个在加锁，一个在解锁，在加锁的协程可能在自旋，此时把Woken置为1，通知解锁协程不必释放信号量了。也就是说Woken标志着当前有就绪状态的进程，不用解锁协程去通知。

参考链接

https://my.oschina.net/renhc/blog/2876211

另外还有一篇详解mutex源码的博客：

https://blog.csdn.net/qq_31967569/article/details/80987352