[Golang1.20 source code reading] sync/mutex.go

// 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.
// 包同步提供基本的同步原语，例如互斥锁。除了Once和WaitGroup类型之外，大多数类型都用于低级库例程。
// 通过信道和通信可以更好地实现更高级别的同步。
// Values containing the types defined in this package should not be copied.
// 不应复制包含此包中定义的类型的值。
package sync

import (
	"internal/race"
	"sync/atomic"
	"unsafe"
)

// Provided by runtime via linkname.
// 由运行时通过链接名称提供。
func throw(string)
func fatal(string)

// A Mutex is a mutual exclusion lock.
// Mutex是一种互斥锁。
// The zero value for a Mutex is an unlocked mutex.
// Mutex的零值是一个未锁定的互斥量。
// A Mutex must not be copied after first use.
// Mutex在首次使用后不得复制。
// In the terminology of the Go memory model,
// the n'th call to Unlock “synchronizes before” the m'th call to Lock
// for any n < m.
// 在Go内存模型的术语中，对于任何n＜m，第n次调用Unlock“同步于”第m次调用Lock。
// A successful call to TryLock is equivalent to a call to Lock.
// A failed call to TryLock does not establish any “synchronizes before”
// relation at all.
// 对TryLock的成功调用相当于对Lock的调用。对TryLock的失败调用根本不会建立任何“同步之前”关系。
type Mutex struct {
	state int32 // 表示锁的状态：waiter、starving、woken、locked
	sema  uint32 // 表示信号量
}

// A Locker represents an object that can be locked and unlocked.
// Locker表示可以锁定和解锁的对象。
type Locker interface {
	Lock()
	Unlock()
}

const (
	mutexLocked = 1 << iota // mutex is locked 互斥被锁定
	mutexWoken // 表示是否有协程已被唤醒，0-没有被唤醒协程 1-有被唤醒协程
	mutexStarving // 表示该Mutex是否处于饥饿状态，0-没有饥饿，1-饥饿
	mutexWaiterShift = iota // 表示阻塞等待锁的个数，协程解锁时根据次值判断是否需要释放信号量

	// Mutex fairness.
	// Mutex公平。
	// 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.
	// 互斥可以有两种操作模式：正常和饥饿。在正常模式下，服务者按先进先出顺序排队，但被唤醒的服务者不拥有互斥，并与新到达的goroutine争夺所有权。
	// 新来的goroutine有一个优势——它们已经在CPU上运行了，而且可能有很多，所以醒来的服务者很有可能会输。
	// 在这种情况下，它被排在等待队列的前面。如果服务程序在超过1ms的时间内未能获取互斥对象，则会将互斥对象切换到饥饿模式。
	// 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.
	// 在饥饿模式下，互斥体的所有权直接从解锁goroutine移交给队列前面的服务者。
	// 新到达的goroutine不会试图获取互斥体，即使它看起来已经解锁，也不会试图旋转。相反，他们把自己排在等待队列的尾部。
	// 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.
	// 如果服务对象接收到互斥对象的所有权，并且发现（1）它是队列中的最后一个服务对象，或者（2）它等待的时间小于1ms，则将互斥对象切换回正常操作模式。
	// 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.
	// 普通模式的性能要好得多，因为goroutine可以连续多次获取互斥对象，即使存在阻塞的等待程序。饥饿模式对于预防尾部潜伏的病理性病例很重要。
	starvationThresholdNs = 1e6
)

// Lock locks m. 锁定锁m。
// If the lock is already in use, the calling goroutine
// blocks until the mutex is available.
// 如果锁已经在使用中，则调用goroutine会阻塞，直到互斥对象可用。
func (m *Mutex) Lock() {
	// Fast path: grab unlocked mutex.
	// 快速路径：获取未锁定的互斥。
	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)
	// 慢速路径（勾勒出轮廓，以便可以内联快速路径）
	m.lockSlow()
}

// TryLock tries to lock m and reports whether it succeeded.
// TryLock尝试锁定m并报告是否成功。
// 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.
// 请注意，虽然TryLock的正确使用确实存在，但它们很少使用，并且TryLock通常是互斥体的特定使用中更深层次问题的标志。
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.
	// 可能有一个goroutine在等待互斥，但我们现在正在运行，可以尝试在goroutine唤醒之前获取互斥。
	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.
		// 不要在饥饿模式下旋转，所有权被交给了服务员，所以我们无论如何都无法获得互斥。
		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.
			// 主动旋转是有意义的。尝试设置mutexWoken标志来通知Unlock不要唤醒其他被阻止的goroutine。
			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.
		// 不要试图获取饥饿的互斥，新到达的goroutine必须排队。
		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.
		// 当前goroutine将互斥体切换到饥饿模式。但如果互斥体当前未锁定，则不要进行切换。Unlock预计饥饿的互斥体会有等待程序，但在这种情况下情况并非如此。
		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.
			// goroutine已经从睡眠中唤醒，所以无论哪种情况，我们都需要重置标志。
			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 使用CAS锁定互斥锁
			}
			// If we were already waiting before, queue at the front of the queue.
			// 如果我们之前已经在等了，那就排在队伍的最前面。
			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.
				// 如果这个goroutine被唤醒，并且mutex处于饥饿模式，那么所有权就交给了我们，
				// 但mutex处于某种不一致的状态：mutexLocked没有设置，我们仍然被视为服务员。解决这个问题。
				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. 退出饥饿模式。
					// 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.
					// 关键是要在这里做到这一点，并考虑等待时间。饥饿模式是如此低效，以至于两个goroutine一旦将互斥切换到饥饿模式，就可以无限锁定。
					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. 解锁解锁m。
// It is a run-time error if m is not locked on entry to Unlock.
// 如果m在进入Unlock时没有被锁定，这是一个运行时错误。
// A locked Mutex is not associated with a particular goroutine.
// 锁定的Mutex与特定的goroutine没有关联。
// It is allowed for one goroutine to lock a Mutex and then
// arrange for another goroutine to unlock it.
// 允许一个goroutine锁定Mutex，然后安排另一个gorroutine解锁它。
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. 勾勒出慢速路径以允许内联快速路径。
		// To hide unlockSlow during tracing we skip one extra frame when tracing GoUnblock.
		// 为了在追踪过程中隐藏unlockSlow，我们在追踪GoUnblock时跳过一个额外的帧。
		// 如果有等待的goroutine，唤醒等待的goroutine
		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.
			// 如果没有服务员，或者一个goroutine已经被叫醒或被锁了，就不需要叫醒任何人。在饥饿模式下，所有权直接从解锁goroutine转移到下一个服务员。
			// 我们不是这个链的一部分，因为当我们解锁上面的互斥时，我们没有观察到互斥饥饿。所以让开。
			if old>>mutexWaiterShift == 0 || old&(mutexLocked|mutexWoken|mutexStarving) != 0 {
				return
			}
			// Grab the right to wake someone.
			// 抓住权利叫醒某人。
			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.
		// 饥饿模式：将互斥的所有权移交给下一个服务员，并产生我们的时间片，以便下一个侍者可以立即开始运行。
		// 注意：没有设置互斥锁定，服务员会在唤醒后设置它。但如果设置了互斥饥饿，互斥仍然被认为是锁定的，所以新的goroutine不会获取它。
		runtime_Semrelease(&m.sema, true, 1)
	}
}