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Re: starvation in pthread_once?


All,

I've redesigned pthread_once() and will shortly drop it into CVS and a
new snapshot. Please take a look over the code below and let me know of
any problems or improvements you find. The rationale is explained in the
comments.

This new version passes a new test I've added to the test suite that
exercises the routine much more intensively than previously (although it
doesn't play around with thread priorities yet).

Thanks.
Ross

  /*
   * Use a single global cond+mutex to manage access to all once_control objects.
   * Unlike a global mutex on it's own, the global cond+mutex allows faster
   * once_controls to overtake slower ones. Spurious wakeups can occur, but
   * can be tolerated.
   *
   * To maintain a separate mutex for each once_control object requires either
   * cleaning up the mutex (difficult to synchronise reliably), or leaving it
   * around forever. Since we can't make assumptions about how an application might
   * employ once_t objects, the later is considered to be unacceptable.
   *
   * Since this is being introduced as a bug fix, the global cond+mtx also avoids
   * a change in the ABI, maintaining backwards compatibility.
   *
   * The mutex should be an ERRORCHECK type to be sure we will never, in the event
   * we're cancelled before we get the lock, unlock the mutex when it's held by
   * another thread (possible with NORMAL/DEFAULT mutexes because they don't check
   * ownership).
   */

  if (!once_control->done)
    {
      if (InterlockedExchange((LPLONG) &once_control->started, (LONG) 0) == -1)
	{
	  (*init_routine) ();

	  /*
	   * Holding the mutex during the broadcast prevents threads being left
	   * behind waiting.
	   */
	  pthread_cleanup_push(pthread_mutex_unlock, (void *) &ptw32_once_control.mtx);
	  (void) pthread_mutex_lock(&ptw32_once_control.mtx);
	  once_control->done = PTW32_TRUE;
	  (void) pthread_cond_broadcast(&ptw32_once_control.cond);
	  pthread_cleanup_pop(1);
	}
      else
	{
	  pthread_cleanup_push(pthread_mutex_unlock, (void *) &ptw32_once_control.mtx);
	  (void) pthread_mutex_lock(&ptw32_once_control.mtx);
	  while (!once_control->done)
	    {
	      (void) pthread_cond_wait(&ptw32_once_control.cond, &ptw32_once_control.mtx);
	    }
	  pthread_cleanup_pop(1);
	}
    }


On Sat, 2005-03-05 at 10:18 +1100, Ross Johnson wrote:
> On Thu, 2005-03-03 at 04:32 -0500, Gottlob Frege wrote:
> > I'm concerned about the Sleep(0) in pthread_once:
> > 
> 
> Thanks. It looks like this routine needs to be redesigned.
> 
> Regards.
> Ross
> 
> >   if (!once_control->done)
> >     {
> >       if (InterlockedIncrement (&(once_control->started)) == 0)
> >         {
> >           /*
> >            * First thread to increment the started variable
> >            */
> >           (*init_routine) ();
> >           once_control->done = PTW32_TRUE;
> > 
> >         }
> >       else
> >         {
> >           /*
> >            * Block until other thread finishes executing the onceRoutine
> >            */
> >           while (!(once_control->done))
> >             {
> >               /*
> >                * The following gives up CPU cycles without pausing
> >                * unnecessarily
> >                */
> >               Sleep (0);
> >             }
> >         }
> >     }
> > 
> > IIRC, Sleep(0) does not relinquish time slices to lower priority
> > threads.  (Sleep(n) for n != 0 does, but 0 does not).  So, if a lower
> > priority thread is first in, followed closely by a higher priority
> > one, the higher priority thread will spin on Sleep(0) *forever*
> > because the lower, first thread will never get a chance to set done.
> > 
> > So even Sleep(10) should be good enough.  In theory, there could be
> > enough higher priority threads in the system that the first thread
> > still doesn't get in (ever?!), but unlikely.  And that would probably
> > mean a general design flaw of the calling code, not pthread_once.
> > 
> > ?
> > 


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