调用 ResetEvent() 时是否需要互斥或锁定？

Question

I've inherited some code like this:

m_mutex.Lock();
ResetEvent( m_hSyncObject );
m_mutex.Unlock();

Same for SetEvent()

Are those Mutexes necessary in this case - do those calls behave themselves or can I get away with removing the locks? This function has already had some inc/decs of values that I made atomic earlier and now just these events are within the locks, so getting rid of them would be a big win if possible.

Answer 1

This extra mutex is almost certainly unneeded. The ResetEvent and SetEvent functions themselves are safe to call from multiple threads

Given that this code does exists it seems highly likely that the developer who wrote that code didn't understand the threading semantics they'd created. I would treat any code depending on that logic as highly suspect. It'd probably save you some time in the long run to go ahead and pre-audit that code for threading issues

Answer 2

Caveat programmer!

Manual-reset events are hard to use and may require you to hold a lock around setting and resetting the event (auto-reset events make it easier to avoid these issues).

Consider this code:

Worker() {
    WaitForSingleObject(hEvent);
    DoWork();
    ResetEvent(hEvent);
}

EventThread() {
    QueueWork();
    SetEvent(hEvent);
}

It is possible with racy interleaving for the worker to reset the event after the EventThread has signalled it, which will cause the worker to hang when it waits. To use the manual-reset event properly in this case you'd need to acquire a lock around the reset event and check the state of the queue atomically with resetting the event.

Auto-reset events let you atomically wake up and reset the event which avoids this race (you might wake up one extra time if you already were draining the queue when work came in, but you won't miss any wakes).

Answer 3

Events are atomic, so there's no need to use a mutex around a SetEvent or ResetEvent , unless there's something else along with it and the two have to be done atomically (eg, if you set one event and reset another).