Re: [RFC][PATCH 4/4] futex: Rewrite FUTEX_UNLOCK_PI

[Peter Zijlstra]

On Sat, Oct 08, 2016 at 05:53:49PM +0200, Thomas Gleixner wrote:
> On Fri, 7 Oct 2016, Peter Zijlstra wrote:
> > Solve all that by:
> > 
> >  - using futex specific rt_mutex calls that lack the fastpath, futexes
> >    have their own fastpath anyway. This makes that
> >    rt_mutex_futex_unlock() doesn't need to drop rt_mutex::wait_lock
> >    and the unlock is guaranteed if we manage to update user state.
> > 
> >  - make futex_unlock_pi() drop hb->lock early and only use
> >    rt_mutex::wait_lock to serialize against rt_mutex waiters
> >    update the futex value and unlock.
> > 
> >  - in case futex and rt_mutex disagree on waiters, side with rt_mutex
> >    and simply clear the user value. This works because either there
> >    really are no waiters left, or futex_lock_pi() triggers the
> >    lock-steal path and fixes up the WAITERS flag.
> 
> I stared at this for a few hours and while I'm not yet done analyzing all
> possible combinations I found at least one thing which is broken:
> 
> CPU 0				CPU 1
> 
> unlock_pi(f)
>   ....
>   unlock(hb->lock)
>   *f = new_owner_tid | WAITERS;
> 
> 				lock_pi(f) 
> 				  lock(hb->lock)
> 				  uval = *f;
> 				  topwaiter = futex_top_waiter();
> 				    attach_to_pi_state(uval, topwaiter->pistate);
> 				      pid = uval & TID_MASK;
> 				      if (pid != task_pid_vnr(pistate->owner))
> 				      	 return -EINVAL;
>   ....
>   pistate->owner = newowner;
> 
> So in this case we tell the caller on CPU 1 that the futex is in
> inconsistent state, because pistate->owner still points to the unlocking
> task while the user space value alread shows the new owner. So this sanity
> check triggers and we simply fail while we should not. It's [10] in the
> state matrix above attach_to_pi_state().

Urgh, yes. I think I can cure that, by taking
pi_state->pi_mutex.wait_lock in attach_to_pi_state(), but blergh.

> I suspect that there are more issues like this, especially since I did not
> look at requeue_pi yet, but by now my brain is completely fried.

Yes, I know about fried brains :-( This stuff has far too many moving
parts. I've been staring at this stuff far too long.


Also, we need better tools to stress this stuff.