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Re: starvation in pthread_once?
I was thinking of something like this: (haven't compiled it yet or anything...)
if (flag.initted) // quick and easy check
{
return;
}
if (!AtomicExchange(&flag.entry, true)) // no one was in before us?
{
if (func)
{
func();
}
AtomicExchange(&initted, true); // needs release type memory barrier
// we didn't create the event.
// it is only there if there is someone waiting
if (AtomicRead(&flag.event))
{
SetEvent(flag.event);
}
}
else
{
// wait for init.
// while waiting, create an event to wait on
AtomicIncrement(&flag.eventUsers); // mark that we have committed to
creating an event
if (!AtomicRead(&flag.event))
{
tmpEvent = CreateEvent(true, false); // manual reset (ie will never be reset)
if (AtomicCompareExchange(&flag.event, 0, tmpEvent) == 0) //
release memory barrier
{
// wasn't set, is now
}
else
{
// someone snuck in
// get rid of our attempt
CloseHandle(tmpEvent);
}
}
// check initted again in case the initting thread has finished
// and left before seeing that there was an event to trigger.
// (Now that the event IS created, if init gets finished AFTER this,
// then the event handle is guaranteed to be seen and triggered).
//
// Note also, that our first 'quick and easy' init check did NOT use
atomic read;
// this atomic read here ensures that initted is properly seen by
the CPU cache.
// And once seen as 'true' it will always stay true, so no need to
worry about the cache anymore.
// (Thus the first 'quick and easy' check might 'falsely' fail, but
it gets corrected here.
if (!AtomicRead(&flag.initted)) // needs acquire type memory barrier
{
WaitEvent(flag.event); // acquire type memory barrier
}
// last one out shut off the lights:
if (AtomicDecrement(&flag.eventUsers) == 0) // we were last
{
HANDLE tmpEvent = AtomicExchange(&flag.event, (HANDLE)-1);
if (tmpEvent && tmpEvent != (HANDLE)-1);
{
CloseHandle(tmpEvent);
}
}
}
On Tue, 08 Mar 2005 13:22:01 +1100, Ross Johnson
<ross.johnson@homemail.com.au> wrote:
> 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.
> > >
> > > ?
> > >
>
>