Re: futex: wakeup race and futex_q woken state definition

[Darren Hart]

Thomas Gleixner wrote:
> 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().

Ah yes, you are correct of course.  Since PI futexes do not use 
plist_node_empty() to test for wakeup, the setting of TASK_ITNERRUPTIBLE 
after the queue_me() sets the stage to call schedule() with the wrong 
task state and lose the task forever.  I have included this in my 
current patch queue.  We are running our tests to confirm the fix and 
I'll submit the series for inclusion by tomorrow.

Thanks,

--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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