数据结构
//sync/waitgroup.go
type WaitGroup struct {
noCopy noCopy // 不可复制,只能指针传递
// 对齐的8byte存储状态, 另外4byte存储信号。状态的高32位counter计数器,低32位waiter计数器
// counter未执行完的goroutine。waiter正在等待的goroutine。那么WaitGroup可以放多少goroutine呢?2^32
state1 [3]uint32
}
Add
//sync/waitgroup.go
func (wg *WaitGroup) Add(delta int) {
// statep状态(包含counter和waiter计数器),semap信号
statep, semap := wg.state()
// uint64(delta)<<32 获取counter计数器, 这里是counter + delta
state := atomic.AddUint64(statep, uint64(delta)<<32)
v := int32(state >> 32) // counter 计数器
w := uint32(state) // waiter 计数器
// counter 计数器 < 0 恐慌
if v < 0 {
panic("sync: negative WaitGroup counter")
}
//当 wait 和 add并发执行时,可能会发生恐慌。注意这里并不是说wait之后就不能添加Add了
if w != 0 && delta > 0 && v == int32(delta) {
panic("sync: WaitGroup misuse: Add called concurrently with Wait")
}
// add 完成
if v > 0 || w == 0 {
return
}
// This goroutine has set counter to 0 when waiters > 0.
// Now there can't be concurrent mutations of state:
// - Adds must not happen concurrently with Wait,
// - Wait does not increment waiters if it sees counter == 0.
// Still do a cheap sanity check to detect WaitGroup misuse.
if *statep != state {
panic("sync: WaitGroup misuse: Add called concurrently with Wait")
}
// Reset waiters count to 0.
*statep = 0 // 计数器置零
for ; w != 0; w-- {
runtime_Semrelease(semap, false) // 一个一个唤醒waiter
}
}
Done
//sync/waitgroup.go
func (wg *WaitGroup) Done() {
wg.Add(-1) // Done其实也是执行的Add
}
Wait
//sync/waitgroup.go
func (wg *WaitGroup) Wait() {
statep, semap := wg.state()
for {
state := atomic.LoadUint64(statep)
v := int32(state >> 32) //
w := uint32(state)
// 如果counter计数器 == 0 , 直接返回
if v == 0 {
// Counter is 0, no need to wait.
if race.Enabled {
race.Enable()
race.Acquire(unsafe.Pointer(wg))
}
return
}
// Increment waiters count.
if atomic.CompareAndSwapUint64(statep, state, state+1) { //waiter计数器+1
if race.Enabled && w == 0 {
race.Write(unsafe.Pointer(semap))
}
runtime_Semacquire(semap) //等待信号唤醒自己
if *statep != 0 {
panic("sync: WaitGroup is reused before previous Wait has returned")
}
return
}
}
}
小结
- 可以复制,只能通过指针传递
- 最多可存放2^32个未执行完的goroutine
- wait和add不要并发执行,有可能发生恐慌
- counter计数器负数时 恐慌。Add负数 或者 过多的Done都有可能恐慌
有疑问加站长微信联系(非本文作者)
![](https://static.golangjob.cn/static/img/footer.png?imageView2/2/w/280)