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

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Oct 09, 2016 at 01:17:50PM +0200, Thomas Gleixner wrote:
> On Fri, 7 Oct 2016, Peter Zijlstra wrote:
> >  	top_waiter = futex_top_waiter(hb, &key);
> >  	if (top_waiter) {
> > -		ret = wake_futex_pi(uaddr, uval, top_waiter, hb);
> > +		struct futex_pi_state *pi_state = top_waiter->pi_state;
> > +
> > +		ret = -EINVAL;
> > +		if (!pi_state)
> > +			goto out_unlock;
> > +
> > +		/*
> > +		 * If current does not own the pi_state then the futex is
> > +		 * inconsistent and user space fiddled with the futex value.
> > +		 */
> > +		if (pi_state->owner != current)
> > +			goto out_unlock;
> > +
> > +		/*
> > +		 * Grab a reference on the pi_state and drop hb->lock.
> > +		 *
> > +		 * The reference ensures pi_state lives, dropping the hb->lock
> > +		 * is tricky.. wake_futex_pi() will take rt_mutex::wait_lock to
> > +		 * close the races against futex_lock_pi(), but in case of
> > +		 * _any_ fail we'll abort and retry the whole deal.
> > +		 */
> > +		WARN_ON_ONCE(!atomic_inc_not_zero(&pi_state->refcount));
> > +		spin_unlock(&hb->lock);
> > +
> > +		ret = wake_futex_pi(uaddr, uval, pi_state);
> > +
> > +		put_pi_state(pi_state);
> 
> put_pi_state() requires hb->lock protection AFAICT.
> 
> CPU0	       			 CPU1
> 
>     wake_futex_pi()		 attach_to_pi_state()
>     put_pi_state()
> 	refcount--;	
> 	if (!refcount)		
> 	    free_state();	
> 	    			WARN_ON(!pi_state->refcount);
> 
> we might not see the warning, but in any case the following access to
> pi_state on cpu1 is borked.

Not sure this can happen, we do all attach_to_pi_state() with hb->lock
held, and the only way to get there is through futex_q->pi_state. And as
long as that link is stable, pi_state is too.

That is, the only way for wake_futex_pi() to drop the last reference is
if there are no futex_q's referencing it anymore, but that also means
attach_to_pi_state() cannot happen (!top_waiter).