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"
)
// Provided by runtime via linkname.
func throw(string)
func fatal(string)
// 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.
//
// In the terminology of [the Go memory model],
// the n'th call to [Mutex.Unlock] “synchronizes before” the m'th call to [Mutex.Lock]
// for any n < m.
// A successful call to [Mutex.TryLock] is equivalent to a call to Lock.
// A failed call to TryLock does not establish any “synchronizes before”
// relation at all.
//
// [the Go memory model]: https://go.dev/ref/mem
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 001
mutexWoken // 010
mutexStarving // 100
mutexWaiterShift = iota // 3
// 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
}
// Slow path (outlined so that the fast path can be inlined)
m.lockSlow()
}
// TryLock tries to lock m and reports whether it succeeded.
//
// Note that while correct uses of TryLock do exist, they are rare,
// and use of TryLock is often a sign of a deeper problem
// in a particular use of mutexes.
func (m *Mutex) TryLock() bool {
old := m.state
if old&(mutexLocked|mutexStarving) != 0 {
return false
}
// There may be a goroutine waiting for the mutex, but we are
// running now and can try to grab the mutex before that
// goroutine wakes up.
if !atomic.CompareAndSwapInt32(&m.state, old, old|mutexLocked) {
return false
}
if race.Enabled {
race.Acquire(unsafe.Pointer(m))
}
return true
}
func (m *Mutex) lockSlow() {
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, 1)
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 != 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, 1)
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, 1)
}
}
源码详解
mutex结构
type Mutex struct {
state int32
sema uint32
}
Sync.Mutex由两个字段构成，state用来表示当前互斥锁处于的状态，sema用于控制锁状态的信号量
互斥锁state(32bit)主要记录了如下四种状态：
- waiter_num(29bit)：记录了当前等待这个锁的goroutine数量
- starving(1bit)：当前锁是否处于饥饿状态，
0: 正常状态 1: 饥饿状态 - woken(1bit)：当前锁是否有goroutine已被唤醒。
0：没有goroutine被唤醒； 1: 有goroutine正在加锁过程
Woken 状态用于加锁和解锁过程的通信，例如，同一时刻，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过程中，此时把 Woken 标记为 1，用于通知解锁协程不必释放信号量了。 - locked(1bit)：当前锁是否被goroutine持有。
0: 未被持有 1: 已被持有
sema信号量的作用：当持有锁的gorouine释放锁后，会释放sema信号量，这个信号量会唤醒之前抢锁阻塞的gorouine来获取锁。
自旋
自旋过程是指，goroutine尝试加锁时，如果当前 Locked 位为 1，则说明该锁当前是由其他协程持有，尝试加锁的协程并不会马上转入阻塞队列，而是会持续的探测 Locked 位是否变为 0。自旋的时间很短，但如果在自旋过程中发现锁已被释放，那么协程可以立即获取锁。此时即便有协程被唤醒也无法获取锁，只能再次阻塞，这个可不是我们想要的。
自旋的好处是，当加锁失败时不必立即转入阻塞，有一定机会获取到锁，这样可以避免协程的切换。但是自旋会导致阻塞队列中的goroutine一直获取不到锁，处于饥饿状态。
自旋必须满足以下所有条件：
- 自旋的次数要足够小，通常为4，即「自旋最多为4次」
- CPU 核数要大于1，否则自旋是没有意义的，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 就不能启用自旋
- 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
为了避免协程长时间无法获取锁，自1.8版本以来增加了一个状态，即 Mutex 的 Starving 状态。这个状态下不会自旋，而是直接进入阻塞队列，一旦有协程释放锁，那么一定会唤醒一个阻塞队列中的协程并成功加锁。
常量
const (
mutexLocked = 1 << iota // mutex is locked 001
mutexWoken // 010
mutexStarving // 100
mutexWaiterShift = iota // 3
starvationThresholdNs = 1e6
)
首先，iota 是Go语言中的一个预定义标识符，用于在常量声明中生成连续的整数值。它的初始值为0，并且每次在常量声明中使用时都会自动递增。
在这段代码中，首先定义了 mutexLocked 常量，它的值是 1 << iota，即 1 左移 0 位，结果仍然是 1。因此，mutexLocked 的值是 1，代表互斥锁被锁定。
接下来，mutexWoken 的值是 1 << iota，即 1 左移 1 位，结果是 2。因此，mutexWoken 的值是 2，代表互斥锁被唤醒。
然后，mutexStarving 的值是 1 << iota，即 1 左移 2 位，结果是 4。因此，mutexStarving 的值是 4，代表互斥锁出现饥饿状态。
最后，mutexWaiterShift = iota 将 iota 的值赋给 mutexWaiterShift，此时 iota 的值为 3。因此，mutexWaiterShift 的值是 3。
综上所述，这段代码定义了互斥锁的不同状态的常量值，分别是 1（锁定）、2（唤醒）、4（饥饿），并且将 iota 的值 3 赋给了 mutexWaiterShift。
常量池中还包含这样一段注释，让我们翻译一下
// 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.
互斥公平性。
互斥可以有两种操作模式：正常和饥饿。
在正常模式下，等待者按 FIFO 顺序排队，但唤醒的等待者不拥有互斥，并与新到达的 goroutine 争夺所有权。新到达的 goroutine 具有优势——它们已经在 CPU 上运行，并且可能有很多，因此唤醒的等待者很有可能失败。在这种情况下，它会排在等待队列的前面。如果等待者超过 1 毫秒无法获取互斥，它会将互斥切换到饥饿模式。
在饥饿模式下，互斥的所有权直接从解锁的 goroutine 移交给队列前面的等待者。新到达的 goroutine 不会尝试获取互斥，即使它似乎已被解锁，也不会尝试旋转。相反，它们将自己排在等待队列的末尾。
如果等待者获得互斥锁的所有权并发现
(1) 它是队列中的最后一个等待者，或者 (2) 它等待的时间少于 1 毫秒，
它会将互斥锁切换回正常操作模式。
正常模式的性能要好得多，因为即使有阻塞的等待者，goroutine 也可以连续多次获取互斥锁。
饥饿模式对于防止尾部延迟的情况非常重要（即避免饥饿）。
old & (mutexLocked | mutexStarving)
old & (mutexLocked | mutexStarving) 使用按位与操作符（&）检查 old 是否处于 mutexLocked 或 mutexStarving 状态。这里的 mutexLocked 和 mutexStarving 都是 2 的幂，因此它们在二进制表示中只有一个位为 1。这使得我们可以在一个整数值中存储和检查多个这样的标记。
- 首先，使用按位或操作符（
|）将mutexLocked和mutexStarving进行按位或操作。结果是一个整数值，其二进制表示中的第 1 位和第 3 位都为 1（即0101，十进制为 5）。 - 接下来，使用按位与操作符（
&）将old与上一步的结果相与。这个操作会将old的二进制表示与0101进行按位与操作，即比较old中的第 1 位和第 3 位是否为 1。 - 如果第 1 位或第 3 位为 1，那么结果将不为零，表示
old具有mutexLocked或mutexStarving状态标记。只要具有其中一个标记，结果都将不为0
通过这个表达式，我们可以快速地检查整数值 old 是否同时具有 mutexLocked 和 mutexStarving 这两个状态标记，从而了解互斥锁的当前状态。这是一种高效的位操作技巧，可以节省内存空间，并提高代码执行速度。
Lock
func (m *Mutex) Lock() {
// 这里首先尝试直接上锁
// 当mutex不处在locked状态时，尝试直接上锁，也就是将locked位修改为1
if atomic.CompareAndSwapInt32(&m.state, 0, mutexLocked) {
// 这里记录了获取锁的操作，暂时不用管
if race.Enabled {
race.Acquire(unsafe.Pointer(m))
}
// 获取锁成功，直接返回
return
}
// Slow path (outlined so that the fast path can be inlined)
// 如果上面的获取过程没有成功，说明mutex已经被锁定，走slow
m.lockSlow()
}
func (m *Mutex) lockSlow() {
var waitStartTime int64
starving := false
// awoke用来跟踪当前的 goroutine 是否已经成功设置了 mutexWoken 标志，即是否已经通知 Unlock 不要唤醒其他被阻塞的 goroutine。
awoke := false
// 自旋次数
iter := 0
// 获取mutex的状态
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.
// 这里的操作很精妙，old&(mutexLocked|mutexStarving) == mutexLocked
// 这个操作要求old&(101) == 001也就是old在mutexLocked位上必须为1，在mutexStarving位上必须为0，
// 即，处在锁定但不饥饿状态，饥饿状态下不允许自旋。
// runtime_canSpin(iter)是另一些条件，包括多核、压力不大并且在一定次数内可以自旋
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.
// !awoke没有成功设置mutexWoken
// old&mutexWoken == 0，mutex锁没有设置woken位
// old>>mutexWaiterShift != 0没有等待线程
// atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken)成功设置woken位
// awoke = true当前线程已成功设置mutexWoken位
if !awoke && old&mutexWoken == 0 && old>>mutexWaiterShift != 0 &&
atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {
awoke = true
}
// 自旋
runtime_doSpin()
// 自旋次数
iter++
old = m.state
continue
}
// 1. 锁仍被锁定，处于正常状态
// 2. 锁仍被锁定，处于饥饿状态
// 3. 锁被释放，处于正常状态
// 4. 锁被释放，处于饥饿状态
new := old
// Don't try to acquire starving mutex, new arriving goroutines must queue.
// 如果old状态不是饥饿状态,设置new为锁定状态
if old&mutexStarving == 0 {
new |= mutexLocked
}
// 锁定或饥饿
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, 1)
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))
}
}
func sync_runtime_canSpin(i int) bool {
if i >= active_spin || ncpu <= 1 || gomaxprocs <= int32(sched.npidle+sched.nmspinning)+1 {
return false
}
if p := getg().m.p.ptr(); !runqempty(p) {
return false
}
return true
}