Threads Waiting for Event Do Not Always Catch Event Signal

Question

I have an application wherein multiple threads wait on the same event object to signal. The problem I am seeing appears to be a type of race condition in that sometimes some threads' wait states (WaitForMultipleObjects) return as a result of the event signal and other threads' wait states apparently don't see the event signal because they don't return. These events were created using CreateEvent as manual-reset event objects.

My application handles these events such that when an event object is signaled, its "owner" thread is responsible for resetting the event object's signal state, as shown in the following code snippet. Other threads waiting on the same event do not attempt to reset its signal state.

switch ( dwObjectWaitState = ::WaitForMultipleObjects( i, pHandles, FALSE, INFINITE ) )
{
case WAIT_OBJECT_0 + BAS_MESSAGE_READY_EVT_ID:
    ::ResetEvent( pHandles[BAS_MESSAGE_READY_EVT_ID] );
    /* handles the event */
    break;
}

To put it another way, the problem I am seeing appears to be to what is described in the Remarks section for PulseEvent on the MSDN website:

If the call to PulseEvent occurs during the time when the thread has been removed from the wait state, the thread will not be released because PulseEvent releases only those threads that are waiting at the moment it is called. Therefore, PulseEvent is unreliable and should not be used by new applications. Instead, use condition variables.

If this is what is happening, the only solution I can see is for each thread to register its usage of a given event object with that object's owner thread, so that the owner thread can determine when it is safe to reset the event object's signal state.

Is there a better way to do this? Thanks.

Answer 1

Yes there is a better way:

[...] Instead, use condition variables.

http://msdn.microsoft.com/en-us/library/ms682052(v=vs.85).aspx

Look for WakeAllConditionVariable specificly

Answer 2

Why PulseEvent() is Unreliable and What to Do Without It

The auto-reset event is king!

PulseEvent did only appear in Windows NT 4.0. It did not exist in the original Windows NT 3.1. To the contrary, the reliable functions like CreateEvent, SetEvent and WaitForMultipleObjects did exist from start of the Windows NT, so consider using them.

The CreateEvent function has the bManualReset argument. If this parameter is TRUE, the function creates a manual-reset event object, which requires the use of the ResetEvent function to set the event state to non-signaled. This is not what you need. If this parameter is FALSE, the function creates an auto-reset event object, and system automatically resets the event state to non-signaled after a single waiting thread has been released.

These auto-reset events are very reliable and easy to use.

If you wait for an auto-reset event object with WaitForMultipleObjects or WaitForSingleObject, it reliably resets the event upon exit from these wait functions.

So create events the following way:

EventHandle := CreateEvent(nil, FALSE, FALSE, nil);

Wait for the event from one thread and do SetEvent from another thread. This is very simple and very reliable.

Don’t' ever call ResetEvent (since it automatically reset) or PulseEvent (since it is not reliable and deprecated). Even Microsoft has admitted that PulseEvent should not be used. See https://msdn.microsoft.com/en-us/library/windows/desktop/ms684914(v=vs.85).aspx

This function is unreliable and should not be used, because only those threads will be notified that are in the "wait" state at the moment PulseEvent is called. If they are in any other state, they will not be notified, and you may never know for sure what the thread state is. A thread waiting on a synchronization object can be momentarily removed from the wait state by a kernel-mode Asynchronous Procedure Call, and then returned to the wait state after the APC is complete. If the call to PulseEvent occurs during the time when the thread has been removed from the wait state, the thread will not be released because PulseEvent releases only those threads that are waiting at the moment it is called.

You can find out more about the kernel-mode Asynchronous Procedure Calls at the following links:

• https://msdn.microsoft.com/en-us/library/windows/desktop/ms681951(v=vs.85).aspx
• http://www.drdobbs.com/inside-nts-asynchronous-procedure-call/184416590
• http://www.osronline.com/article.cfm?id=75

We have never used PulseEvent in our applications. As about auto-reset events, we are using them since Windows NT 3.51 and they work very well.

What to Do when Multiple Threads Waiting for a Single Object

Unfortunately, your case is a little bit more complicated. You have multiple threads waiting for an event, and you have to make sure that all the threads did in fact receive the notification. There is no other reliable way other than to create own event for each thread.

You wrote theat "the only solution I can see is for each thread to register its usage of a given event object with that object's owner thread". This is correct.

You also wrote that "the owner thread can determine when it is safe to reset the event object's signal state" - this is impractical and unsafe. The best way is to use the auto-reset events, so they will reset themselves automatically.

So, you will need to have as many events as are the threads. Besides that, you will need to keep a list of registered threads. So, to notify all the threads, you will have to do SetEvent in a loop for all the event handles. This is a very fast, reliable and cheap way. Events are much cheaper than threads. So, the number of threads is an issue, not the number of events. There is virtually no limit on the kernel objects - the per-process limit on kernel handles is 2^24.

Answer 3

Use conditional variable as in PulseEvent description. The only problem is that native conditional variable on windows was implemented starting from Vista so older system like XP doesn't have it. But you can emulate conditional variable using some other synchronization objects (http://www1.cse.wustl.edu/~schmidt/win32-cv-1.html) but I think the easiest way is to use conditional variable from boost library and its notify_all method to wake up all threads (http://www.boost.org/doc/libs/1_41_0/doc/html/thread/synchronization.html#thread.synchronization.condvar_ref)

Another possibility (but not very beautiful) is to create one event for each thread and when right now you have PulseEvent you can call SetEvent for all of them. For this solution probably auto-reset events would work better.