Golang Mutex实现互斥的具体方法

type Mutex struct {
  // 互斥锁的状态，比如是否被锁定
  state int32
  // 表示信号量。堵塞的协程会等待该信号量，解锁的协程会释放该信号量
  sema int32
}

const (
  // 当前是否已经上锁
  mutexLocked = 1 << iota // 1
  // 当前是否有唤醒的goroutine
  mutexWoken // 2
  // 当前是否为饥饿状态
  mutexStarving // 4
  // state >> mutexWaiterShift 得到等待者数量
  mutexWaiterShift = iota // 3

  starvationThresholdNs = 1e6 // 判断是否要进入饥饿状态的阈值
)

func (m *Mutex) Lock() {
    // 快路径。直接获取未锁的mutex
  // 初始状态为0，所以只要状态存在其他任何状态位都是无法直接获取的
    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()
}

        // 只要原状态已锁且不处于饥饿状态，并满足自旋条件
        if old&(mutexLocked|mutexStarving) == mutexLocked && runtime_canSpin(iter) {
            // 在当前goroutine没有唤醒，且没有其他goroutine在尝试唤醒，且存在等待的情况下，cas标记存在goroutine正在尝试唤醒。若标记成功就设置当前goroutine已经唤醒了
            if !awoke && old&mutexWoken == 0 && old>>mutexWaiterShift != 0 &&
                atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {
                awoke = true
            }
      // 自旋
            runtime_doSpin()
      // 自旋次数加一
            iter++
      // 更新原状态
            old = m.state
            continue
        }

const (
    locked uintptr = 1

    active_spin     = 4
    active_spin_cnt = 30
    passive_spin    = 1
)

func sync_runtime_canSpin(i int) bool {
    // sync.Mutex is cooperative, so we are conservative with spinning.
    // Spin only few times and only if running on a multicore machine and
    // GOMAXPROCS>1 and there is at least one other running P and local runq is empty.
    // As opposed to runtime mutex we don't do passive spinning here,
    // because there can be work on global runq or on other Ps.
    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
}

# 定义了一个runtime.procyield的文本段,通过NOSPLIT不使用栈分裂,$0-0 表示该函数不需要任何输入参数和返回值
TEXT runtime·procyield(SB),NOSPLIT,$0-0
    # 从栈帧中读取cycles参赛值，并储存在寄存器R0中
    MOVWU    cycles+0(FP), R0
# 组成无限循环。在每次循环中，通过YIELD告诉CPU将当前线程置于休眠状态
# YIELD: x86上，实现为PAUSE指令，会暂停处理器执行，切换CPU到低功耗模式并等待更多数据到达。通常用于忙等待机制，避免无谓CPU占用
# ARM上，实现为WFE(Wait For Event)，用于等待中断或者其他事件发生。在某些情况下可能会导致CPU陷入死循环，因此需要特殊处理逻辑解决
again:
    YIELD
    # 将R0值减1
    SUBW    $1, R0
    # CBNZ(Compare and Branch on Non-Zero)检查剩余的时钟周期数是否为0。不为0就跳转到标签again并再次调用YIELD，否则就退出函数
    CBNZ    R0, again
    RET

const active_spin_cnt = 30

func sync_runtime_doSpin() {
    procyield(active_spin_cnt)
}

        new := old
        // Don't try to acquire starving mutex, new arriving goroutines must queue.
    // 若原状态不处于饥饿状态，就给新状态设置已加锁
        if old&mutexStarving == 0 {
            new |= mutexLocked
        }
    // 只要原状态处于已锁或者饥饿模式，就将新状态等待数量递增
        if old&(mutexLocked|mutexStarving) != 0 {
            new += 1 << mutexWaiterShift
        }
    // 若已经等待超过饥饿阈值时间且原状态已锁，就设置新状态为饥饿
    // 这也意味着如果已经不处于已锁状态，就可以切换回正常模式了
        if starving && old&mutexLocked != 0 {
            new |= mutexStarving
        }
    // 如果已经唤醒了（也就是没有其他正在抢占的goroutine），则在新状态中清除正在唤醒状态位
        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) {
      // 如果原状态既没有处于已锁状态，也没有处于饥饿模式
      // 那么就表示已经获取到锁，直接退出
            if old&(mutexLocked|mutexStarving) == 0 {
                break // locked the mutex with CAS
            }
      // 若已经在等待了，就排到队列头
            queueLifo := waitStartTime != 0
            if waitStartTime == 0 {
                waitStartTime = runtime_nanotime()
            }
      // 尝试获取信号量。此处获取一个信号量以实现互斥
      // 此处会进行堵塞
            runtime_SemacquireMutex(&m.sema, queueLifo, 1)
      // 被信号量唤醒之后，发现若等待时间超过饥饿阈值，就切换到饥饿模式
            starving = starving || runtime_nanotime()-waitStartTime > starvationThresholdNs
            old = m.state
      // 处于饥饿模式下
            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.
        // 若既没有已锁且正在尝试唤醒，或者等待队列为空，就代表产生了不一致的状态
                if old&(mutexLocked|mutexWoken) != 0 || old>>mutexWaiterShift == 0 {
                    throw("sync: inconsistent mutex state")
                }
        // 当前goroutine已经获取锁，等待队列减1；若等待者就一个，就切换正常模式。退出
                delta := int32(mutexLocked - 1<<mutexWaiterShift)
                if !starving || old>>mutexWaiterShift == 1 {
                    delta -= mutexStarving
                }
                atomic.AddInt32(&m.state, delta)
                break
            }
      // 不处于饥饿模式下，设置当前goroutine为唤醒状态，重置自璇次数，继续尝试获取锁
            awoke = true
            iter = 0
        } else {
      // 若锁被其他goroutine占用了，就更新原状态，继续尝试获取锁
            old = m.state
        }

func (m *Mutex) Unlock() {
    // 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) {
  // 若原状态根本没有已锁状态位
    if (new+mutexLocked)&mutexLocked == 0 {
        throw("sync: unlock of unlocked mutex")
    }
  // 若原状态不处于饥饿状态
    if new&mutexStarving == 0 {
        old := new
        for {
      // 若没有等待，或者存在goroutine已经被唤醒，或者已经被锁住了，就不需要唤醒任何人，返回
            if old>>mutexWaiterShift == 0 || old&(mutexLocked|mutexWoken|mutexStarving) != 0 {
                return
            }
            // Grab the right to wake someone.
      // 设置期望状态为正在获取状态位，并减去一个等待者
            new = (old - 1<<mutexWaiterShift) | mutexWoken
      // 尝试cas更新为期望新状态，若成功就释放信号量，失败就更新原状态，进行下一轮释放
      // 失败说明在这过程中又有goroutine在尝试获取，比如已经获取到了、变成饥饿了、自旋等
            if atomic.CompareAndSwapInt32(&m.state, old, new) {
                runtime_Semrelease(&m.sema, false, 1)
                return
            }
            old = m.state
        }
    } else {
    // 饥饿模式下就移交锁所有权给下一个等待者，并放弃时间片，以便该等待者可以快速开始
        runtime_Semrelease(&m.sema, true, 1)
    }
}