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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.
> >
> > ?
> >