当被阻止的goroutine的RWMutex Unlock唤醒时

Question

var mu sync.RWMutex

//goroutine 1
go func() {
    mu.Lock()
    defer mu.Unlock()
   //something else
}()
//goroutine 2
go func() {
    mu.Lock()
    defer mu.Unlock()
    //something else
}()
//goroutine 3
go func() {
    mu.RLock()
    defer mu.RUnlock()
    //something else
}()
//goroutine 4
go func() {
    mu.RLock()
    defer mu.RUnlock()
    //something else
}()

goroutine 1 now get the lock, goroutine 2,3,4 blocked. when goroutine 1 release the lock, which goroutine will be waked up first? Randomly？

Answer 1

To ensure that locks eventually becomes available to writers, a blocked Lock call excludes new readers from acquiring the lock. If goroutine 2 blocks on Lock before goroutine 3 blocks on RLock, then goroutine 2 will run before goroutine 3. The execution order is unspecified if goroutine 3 blocks on RLock before goroutine 2 blocks on Lock.