// 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 (
//0001 = 1
mutexLocked = 1 << iota // mutex is locked
//0010 = 2
mutexWoken
//0100 = 4
mutexStarving
//0011 = 3
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.
//如果 state 是 0， 没有加锁，进行加锁并返回
//如果 state 不是 0, 已经加锁，执行 m.lockSlow() 进行排队加锁等待
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
//进入 lockSlow 说明加锁失败, 该锁已经被锁定，进行挂起等待,
//但是在这个过程中会进行自旋， awoke 标记在自旋过程中, 加锁的进行是否要进行锁释放
//如果是，不用进行释放，自旋的协程直接加锁
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.
// old & (mutex | mutexStarving) = mutexLocked ; old 处于 lock 状态，但是不处于 mutexStarving 状态
//判断协程是否可以自旋, 协程自旋条件如下
// 1 互斥锁只有在普通模式才能进入自旋；
// 2 runtime.sync_runtime_canSpin 需要返回 true：
// 2.1 运行在多 CPU 的机器上；
// 2.2 当前 Goroutine 为了获取该锁进入自旋的次数小于四次；
// 2.3 当前机器上至少存在一个正在运行的处理器 P 并且处理的运行队列为空；
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.
//判断是否可以标记 mutexWoken ，并尝试进行标识，可以进行标记的条件为
//1. awoke == false
//2. old mutexWoken 状态为 0
//3. waitersCount ，锁的等待个数大于 0
if !awoke && old&mutexWoken == 0 && old>>mutexWaiterShift != 0 &&
atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {
awoke = true
}
//进行自旋
runtime_doSpin()
iter++
old = m.state
continue
}
new := old
// Don't try to acquire starving mutex, new arriving goroutines must queue.
//当 old 不是 mutexStarving 状态
if old&mutexStarving == 0 {
// new = new | mutexLocked; new mutexLocked 位置为1
new |= mutexLocked
}
// old 处于饥饿 或 锁定状态
if old&(mutexLocked|mutexStarving) != 0 {
//new 高位计数器加一
new += 1 << mutexWaiterShift
}
// 当前协程将锁置为饥饿状态，但是如果锁目前没有被锁, 不转换
// Unlock 方法希望饥饿的锁有 waiter
// 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.
// starving 为真，并且 old 处于锁定状态
// 这种情况是当前协程被唤醒，并且等待时间挺长了，
// 但是在尝试加锁过程中，又被别的协程加锁了，切换为饥饿模式，将饥饿状态为置为1
if starving && old&mutexLocked != 0 {
//new = new | mutexStarving； new mutexStarving 状态置为 1
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")
}
// &^ 清零运算符，根据右侧操作数为 1 的位, 对左侧操作数进行清零
// 将 new 的 mutexWoken 位置置为为 0
new &^= mutexWoken
}
if atomic.CompareAndSwapInt32(&m.state, old, new) {
// old mutexLocked 和 mutexStarving 均为0
// old 没有被锁定，并且没有饥饿
if old&(mutexLocked|mutexStarving) == 0 {
// 通过 CAS 函数获取了锁
// locked the mutex with CAS
//加锁成功，出现这种情况，说明在进行 atomic.CompareAndSwapInt32 的时候
//之前别的协程加的锁已经被释放, 此协程可以直接加锁
break
}
//加锁失败，将协程挂载到锁的信号量上面
// If we were already waiting before, queue at the front of the queue.
// queueLifo 是否将协程挂载到等待协程队列的头部
queueLifo := waitStartTime != 0
if waitStartTime == 0 {
waitStartTime = runtime_nanotime()
}
// 信号量挂载
runtime_SemacquireMutex(&m.sema, queueLifo, 1)
// 协程被唤醒， 继续尝试加锁
starving = starving || runtime_nanotime()-waitStartTime > starvationThresholdNs
old = m.state
//old 处于 mutexStarving 状态
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.
//old 处于 mutexLocked 或者 mutexWoken 状态, 但是等待的协程数为0，出现不一致状态
if old&(mutexLocked|mutexWoken) != 0 || old>>mutexWaiterShift == 0 {
throw("sync: inconsistent mutex state")
}
//delta = int32(1 - 8)
delta := int32(mutexLocked - 1<<mutexWaiterShift)
if !starving || old>>mutexWaiterShift == 1 {
//退出 starving 模式
// Exit starvation mode.
//关键是在这里做，并考虑等待时间。
// Critical to do it here and consider wait time.
//饥饿模式的效率非常低，以至于两个goroutine一旦将互斥切换到饥饿模式，就可以无限地锁定步进。
// Starvation mode is so inefficient, that two goroutines
// can go lock-step infinitely once they switch mutex
// to starvation mode.
//delta = delta - 4
delta -= mutexStarving
}
//修改状态，break 加锁成功
atomic.AddInt32(&m.state, delta)
break
}
//协程被唤醒，重新走循环，尝试加锁
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.
// 将标志位置为未加锁
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 {
throw("sync: unlock of unlocked mutex")
}
if new&mutexStarving == 0 { // 正常模式
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 { // 饥饿模式
// 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)
}
}
网友评论