前言
- sync.Cond的核心实现 - 通过一个锁，封装了notify 通知的实现，包括了单个通知与广播这两种方式
- sync.Cond与channel的异同 - channel应用于一收一发的场景，sync.Cond应用于多收一发的场景
代码
package main import ( "fmt" "sync" "time" ) // 示例来自https://stackoverflow.com/questions/36857167/how-to-correctly-use-sync-cond //fatal error: all goroutines are asleep - deadlock! //goroutine 1 [sync.Cond.Wait]: 因为c.Wait()里面加锁，而外面没有解锁，造成死锁 func syncCondErr() { m := sync.Mutex{} c := sync.NewCond(&m) go func() { time.Sleep(1 * time.Second) c.Broadcast() }() m.Lock() time.Sleep(2 * time.Second) c.Wait() } //Tip:正确使用方式 func syncCondExplain() { m := sync.Mutex{} c := sync.NewCond(&m) // Tip: 主协程先获得锁 c.L.Lock() go func() { // Tip: 协程一开始无法获得锁 c.L.Lock() defer c.L.Unlock() fmt.Println("3. 该协程获得了锁") time.Sleep(2 * time.Second) // Tip: 通过notify进行广播通知，cond.Signal()用于随机一个单播 c.Broadcast() fmt.Println("4. 该协程执行完毕，即将执行defer中的解锁操作") }() fmt.Println("1. 主协程获得锁") time.Sleep(1 * time.Second) fmt.Println("2. 主协程依旧抢占着锁获得锁") // Tip: 看一下Wait的大致实现，可以了解到，它是先释放锁，阻塞等待，直到收到了notify，又进行加锁 c.Wait() // Tip: 记得释放锁 c.L.Unlock() fmt.Println("Done") } //Tip:例子 func syncCond() { lock := sync.Mutex{} cond := sync.NewCond(&lock) for i := 0; i < 5; i++ { go func(i int) { cond.L.Lock() defer cond.L.Unlock() cond.Wait() fmt.Printf("No.%d Goroutine Receive\n", i) }(i) } time.Sleep(time.Second) cond.Broadcast() //cond.Signal() time.Sleep(time.Second) }
源码分析
//Tip:wait先解锁，阻塞等待notify，得到后再加锁 func (c *Cond) Wait() { c.checker.check() t := runtime_notifyListAdd(&c.notify) c.L.Unlock() runtime_notifyListWait(&c.notify, t) c.L.Lock() } //广播 func (c *Cond) Broadcast() { c.checker.check() runtime_notifyListNotifyAll(&c.notify) } // notifyListNotifyAll notifies all entries in the list. //go:linkname notifyListNotifyAll sync.runtime_notifyListNotifyAll func notifyListNotifyAll(l *notifyList) { // Go through the local list and ready all waiters. for s != nil { next := s.next s.next = nil readyWithTime(s, 4) s = next } } //单播 func (c *Cond) Signal() { c.checker.check() runtime_notifyListNotifyOne(&c.notify) } // notifyListNotifyOne notifies one entry in the list. //go:linkname notifyListNotifyOne sync.runtime_notifyListNotifyOne func notifyListNotifyOne(l *notifyList) { 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) }
