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WaitForMultipleObjects never wakes up - Windows-bug?

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  WaitForMultipleObjects never wakes up - Windows-bug? Bonita Montero <Bonita.Montero@gmail.com> - 2016-05-22 19:19 +0200
    Re: WaitForMultipleObjects never wakes up - Windows-bug? Bonita Montero <Bonita.Montero@gmail.com> - 2016-05-23 12:55 +0200

#3354 — WaitForMultipleObjects never wakes up - Windows-bug?

I've written a very efficient condvar-implementation for Win32.
Here it is: http://pastebin.com/fdjHx9ec
I'm pretty sure it's considerably more efficient than the
condvars in Win32 since Vista and Windows Server 2008.

But I had a strange phnomenon when a thread holding the condvar
locked is singalling a waiting thread and after this is sleeping
before it is releasing the lock.
I extracted the problem in a simple programm without this kind
of efficient locking through atomic variables, but just with an
event and a semaphore. Here it is:

#include <windows.h>
#include <stdio.h>

HANDLE        hEvt,
               hSema;
bool volatile fReleased = false;

DWORD WINAPI LockAndReleaseThread( LPVOID lpvThreadParam );

int main()
{
     int const NTHREADS = 2;
     HANDLE    ahWait[2];

     ahWait[0] = ::hEvt  = CreateEvent( NULL, FALSE, TRUE, NULL );
     ahWait[1] = ::hSema = CreateSemaphore( NULL, 0, 1, NULL );

     for( int i = 0; i < NTHREADS; i++ )
         CreateThread( NULL, 0, LockAndReleaseThread, NULL, 0, NULL );

     for( ; ; )
     {
         WaitForMultipleObjects( 2, ahWait, TRUE, INFINITE );
         printf( "main thread is holding lock and received signal\n" );
         ::fReleased = false;
         SetEvent( hEvt );
     }

     return 0;
}

DWORD WINAPI LockAndReleaseThread( LPVOID lpvThreadParam )
{
     for( ; ; )
     {
         WaitForSingleObject( hEvt, INFINITE );
         printf( "spawned thread is holding lock\n" );

         if( !::fReleased )
             ReleaseSemaphore( ::hSema, 1, NULL ),
             ::fReleased = true;

         Sleep( 1000 );
         SetEvent( hEvt );
     }

     return 0;
}

The strange thing here is, that whith my Windows 10 computer,
WaitForMultipleObjects in the main-thread never awakens!
When I set NTHREADS to 1, the main-thread gets a chance, but
beyond it will freeze.
Has anyone an explanation for that?

Thanks in advance,
Bonita Montero

-- 
http://facebook.com/bonita.montero/

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#3355

I've got a solution for this problem.
In the main-thread, simply wait for the semaphore and then for the
event (in this order - or you're gonna deadlock) with WaitForSingle
Object. That's not so efficitent, but works.

int main()
{
     int const NTHREADS = 2;
     HANDLE    ahWait[2];

     ::hEvt  = CreateEvent( NULL, FALSE, TRUE, NULL );
     ::hSema = CreateSemaphore( NULL, 0, 1, NULL );
     fReleased = false;

     for( int i = 0; i < NTHREADS; i++ )
         CreateThread( NULL, 0, LockAndReleaseThread, NULL, 0, NULL );

     for( ; ; )
         WaitForSingleObject( ::hSema, INFINITE ),
         WaitForSingleObject( ::hEvt, INFINITE),
         printf( "main thread is holding lock and received signal\n" ),
         ::fReleased = false,
         SetEvent( ::hEvt );

     return 0;
}


-- 
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