如何两次解锁互斥锁

Question

Is it safe to unlock a mutex twice? My code:

var m sync.RWMutex = sync.RWMutex{}

func Read() {
    m.RLock()
    defer m.RUnlock()

    // Do something that needs lock
    err := SomeFunction1()
    if err != nil {
        return
    }

    m.RUnlock()

    // Do something that does not need lock
    SomeFunction2()

}

I need defer m.RUnlock() for the case SomeFunction1() returns error. But when SomeFunction1() returns without error, m will be unlocked twice by m.RUnlock() and defer m.RUnlock().

Is it safe to unlock the mutex twice? If not, how should I fix my code?

Answer 1

Is it safe for unlocking a mutex twice?

Nope, you shouldn't unlock the mutex twice. It's a run-time error according to docs.

RUnlock undoes a single RLock call; it does not affect other simultaneous readers. It is a run-time error if rw is not locked for reading on entry to RUnlock.

---

If not, how should I fix my code?

I would recommend to keep the defer but only m.RUnlock() in case of error. This can easily scale in case you add more function calls between SomeFunction1() and SomeFunction2().

func Read() {
    var err error
    m.RLock()
    defer func() {
        if err != nil {
            m.RUnlock()
        }
    }()


    // Do something that needs lock
    err = SomeFunction1()
    if err != nil {
        return
    }

    m.RUnlock()

    // Do something that does not need lock
    SomeFunction2()
}

Try it on Go Playground!