解析Golang sync.Once源码

数据结构

type Once struct {
    // done indicates whether the action has been performed.
    // It is first in the struct because it is used in the hot path.
    // The hot path is inlined at every call site.
    // Placing done first allows more compact instructions on some architectures (amd64/386),
    // and fewer instructions (to calculate offset) on other architectures.
    done uint32  // 初始值为0表示还未执行过，1表示已经执行过
    m    Mutex  // done 的lock
}

核心代码

func (o *Once) Do(f func()) {
    // Note: Here is an incorrect implementation of Do:
    //
    //  if atomic.CompareAndSwapUint32(&o.done, 0, 1) {
    //      f()
    //  }
    //
    // Do guarantees that when it returns, f has finished.
    // This implementation would not implement that guarantee:
    // given two simultaneous calls, the winner of the cas would
    // call f, and the second would return immediately, without
    // waiting for the first's call to f to complete.
    // This is why the slow path falls back to a mutex, and why
    // the atomic.StoreUint32 must be delayed until after f returns.

    if atomic.LoadUint32(&o.done) == 0 {  // 先检查是否没执行
        // Outlined slow-path to allow inlining of the fast-path.
        o.doSlow(f)  // 没执行就调用 doSlow
    }
}

func (o *Once) doSlow(f func()) {
    o.m.Lock()  // 加锁
    defer o.m.Unlock()
       // 采用双重检测机制 加锁判断done是否为零
    if o.done == 0 {  // 防止多个goroutine 并发进入doSlow 等待lock 后进入 再检查一次
        defer atomic.StoreUint32(&o.done, 1)  // 返回后设置已经执行
        f()  // 执行业务函数
    }
}

ref

• https://segmentfault.com/a/1190000039294152