Go cond 源碼學習

概述

cond是go語言sync提供的條件變量，通過cond可以讓一系列的goroutine在觸發某個條件時才被喚醒。每一個cond結構體都包含一個鎖L。cond提供了三個方法：

• Signal：調用Signal之後可以喚醒單個goroutine。
• Broadcast：喚醒等待隊列中所有的goroutine。
• Wait：會把當前goroutine放入到隊列中等待獲取通知，調用此方法必須先Lock,不然方法裏會調用Unlock()報錯。

簡單使用

創建40個goroutine都wait阻塞住。調用Signal則喚醒第一個goroutine。調用Broadcast則喚醒所有等待的goroutine。

package main

import (
    "fmt"
    "sync"
    "time"
)

var locker = new(sync.Mutex)
var cond = sync.NewCond(locker)

func test(x int) {
    cond.L.Lock() //獲取鎖
    cond.Wait()   //等待通知  暫時阻塞
    fmt.Println(x)
    time.Sleep(time.Second * 1)
    cond.L.Unlock() //釋放鎖
}
func main() {
    for i := 0; i < 40; i++ {
        go test(i)
    }
    fmt.Println("start all")
    time.Sleep(time.Second * 3)
    fmt.Println("broadcast")
    cond.Signal() // 下發一個通知給已經獲取鎖的goroutine
    time.Sleep(time.Second * 3)
    cond.Signal() // 3秒之後 下發一個通知給已經獲取鎖的goroutine
    time.Sleep(time.Second * 3)
    cond.Broadcast() //3秒之後 下發廣播給所有等待的goroutine
    time.Sleep(time.Second * 60)
}

源碼分析

Cond

type Cond struct {
    noCopy noCopy

    // 鎖的具體實現，通常爲 mutex 或者rwmutex
    L Locker
    // notifyList對象，維護等待喚醒的goroutine隊列,使用鏈表實現
    notify  notifyList
    checker copyChecker
}

// 新建cond初始化cond對象
func NewCond(l Locker) *Cond {
    return &Cond{L: l}
}

type notifyList struct {
    // 等待數量
    wait uint32

    // 通知數量
    notify uint32

    // 鎖對象
    lock mutex
    // 鏈表頭
    head *sudog
    // 鏈表尾
    tail *sudog
}

Wait

// 等待函數
func (c *Cond) Wait() {
    c.checker.check()
    // 等待計數器加1 看下面具體實現
    t := runtime_notifyListAdd(&c.notify)
    c.L.Unlock()
    // 
    runtime_notifyListWait(&c.notify, t)
    c.L.Lock()
}

// 此函數在sema.go中控制計數器加1
func notifyListAdd(l *notifyList) uint32 {
    // This may be called concurrently, for example, when called from
    // sync.Cond.Wait while holding a RWMutex in read mode.
    return atomic.Xadd(&l.wait, 1) - 1
}

// 此函數在sema.go中
// 獲取當前goroutine 添加到鏈表末端，然後goparkunlock函數休眠阻塞當前goroutine
// goparkunlock函數會讓出當前處理器的使用權並等待調度器的喚醒
func notifyListWait(l *notifyList, t uint32) {
    lock(&l.lock)

    // Return right away if this ticket has already been notified.
    if less(t, l.notify) {
        unlock(&l.lock)
        return
    }

    // Enqueue itself.
    s := acquireSudog()
    s.g = getg()
    s.ticket = t
    s.releasetime = 0
    t0 := int64(0)
    if blockprofilerate > 0 {
        t0 = cputicks()
        s.releasetime = -1
    }
    if l.tail == nil {
        l.head = s
    } else {
        l.tail.next = s
    }
    l.tail = s
    goparkunlock(&l.lock, "semacquire", traceEvGoBlockCond, 3)
    if t0 != 0 {
        blockevent(s.releasetime-t0, 2)
    }
    releaseSudog(s)
}

Broadcast

喚醒鏈表中所有的阻塞中的goroutine，還是使用readyWithTime來實現這個功能

func (c *Cond) Broadcast() {
    c.checker.check()
    runtime_notifyListNotifyAll(&c.notify)
}

// 源代碼在sema.go中
func notifyListNotifyAll(l *notifyList) {
    // Fast-path: if there are no new waiters since the last notification
    // we don't need to acquire the lock.
    if atomic.Load(&l.wait) == atomic.Load(&l.notify) {
        return
    }

    // Pull the list out into a local variable, waiters will be readied
    // outside the lock.
    lock(&l.lock)
    s := l.head
    l.head = nil
    l.tail = nil

    // Update the next ticket to be notified. We can set it to the current
    // value of wait because any previous waiters are already in the list
    // or will notice that they have already been notified when trying to
    // add themselves to the list.
    atomic.Store(&l.notify, atomic.Load(&l.wait))
    unlock(&l.lock)

    // Go through the local list and ready all waiters.
    for s != nil {
        next := s.next
        s.next = nil
        readyWithTime(s, 4)
        s = next
    }
}

Signal

// 調用runtime_notifyListNotifyOne方法喚醒鏈表頭的goroutine
func (c *Cond) Signal() {
    c.checker.check()
    runtime_notifyListNotifyOne(&c.notify)
}

// runtime_notifyListNotifyOne具體實現 獲取鏈表頭部的G，然後調用readyWithTime喚醒goroutine
// 源代碼在sema.go中
func notifyListNotifyOne(l *notifyList) {
    if atomic.Load(&l.wait) == atomic.Load(&l.notify) {
        return
    }

    lock(&l.lock)

    t := l.notify
    if t == atomic.Load(&l.wait) {
        unlock(&l.lock)
        return
    }

    atomic.Store(&l.notify, t+1)
    
    for p, s := (*sudog)(nil), l.head; s != nil; p, s = s, s.next {
        if s.ticket == t {
            n := s.next
            if p != nil {
                p.next = n
            } else {
                l.head = n
            }
            if n == nil {
                l.tail = p
            }
            unlock(&l.lock)
            s.next = nil
            readyWithTime(s, 4)
            return
        }
    }
    unlock(&l.lock)
}