Re: futex: wakeup race and futex_q woken state definition

[Thomas Gleixner]

On Thu, 17 Sep 2009, Darren Hart wrote:
> > 	/*
> > 	 * !plist_node_empty() is safe here without any lock.
> > 	 * q.lock_ptr != 0 is not safe, because of ordering against wakeup.
> > 	 */
> > 	if (likely(!plist_node_empty(&q->list))) {
> > 
> > If we move set_current_state() before the queue_me() this check is
> > still an optimization to avoid the schedule call in case we have been
> > woken up already. But the comment is still wrong as the wakeup code
> > has changed:
> > 
> > The old version did:
> > 
> >      plist_del(&q->list);
> >      wake_up_all(&q->waiters);
> >      q->lock_ptr = NULL;
> > 
> > Today we do:
> > 
> >      p = q->task;
> >      get_task_struct(p);
> >      plist_del(&q->list);
> >      q->lock_ptr = NULL;
> >      wake_up_state(p);
> >      put_task_struct(p);
> > 
> > We changed this because it makes no sense to use a waitqueue for a
> > single task.
> 
> Right.
> 
> 
> However, my bigger concern still remains.  If the above is only an
> optimization, we appear to have a race with wakeup where we can see a
> non-empty list here and decide to schedule and have the wakeup code remove us
> from the list, hiding it from all future futex related wakeups (signal and
> timeout would still work).

No.

Sleeper does:

	set_current_state(TASK_INTERRUPTIBLE);
	
	if (!plist_empty())
	   schedule();

So when the list removal happened before set_current_state() we don't
schedule. If the wakeup happens _after_ set_current_state() then the
wake_up_state() call will bring us back to running.

> We have also been seeing a race with the requeue_pi code with a JVM benchmark
> where the apparent owner of the pi mutex remains blocked on the condvar - this
> can be explained by the race I'm suspecting.  Also, futex_requeue_pi() is
> using futex_wait_queue_me() which expects the waker to remove the futex_q from
> the list, which isn't how things work for PI mutexes.  In an experiment, I
> moved the spin_unlock() out of queueme() and right before the call to
> schedule() to narrow the race window, and the hang we were experiencing
> appears to have gone away.

The correct thing to do is to move set_current_state() before queue_me().

Thanks,

	tglx
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