On Wed, Nov 23, 2016 at 08:20:05PM +0100, Peter Zijlstra wrote:
> > + if (oldowner == &init_task && uval != 0) {
> > + raw_spin_lock(&pi_state->owner->pi_lock);
> > + list_del_init(&pi_state->list);
> > + raw_spin_unlock(&pi_state->owner->pi_lock);
> > + pi_state->owner = NULL;
> > + return -EAGAIN;
> > @@ -2679,6 +2690,10 @@ static int futex_lock_pi(u32 __user *uad
> >
> > out_put_key:
> > put_futex_key(&q.key);
> > +
> > + if (ret == -EAGAIN)
> > + goto retry;
> > +
>
> And this is far too clever and really needs a comment. So the crucial
> point is that this is after unqueue_me_pi(), which drops the pi_state
> and loops back to lookup the pi_state again, which, hopefully, has now
> been completely destroyed -- and therefore we hit the regular
> attach_to_pi_owner() path, fixing up our 'funny' state.
>
I'm stumped on REQUEUE_PI.. this relies on attach_to_pi_owner() and
fixup_owner() being in the same function. But this is not the case for
requeue. WAIT_REQUEUE has the fixup, as its return path finds it has
acquired the outer pi-futex (uaddr2), but the lookup_pi_state() stuff is
done by CMP_REQUEUE, which does the actual transfer of the waiters from
inner futex (uaddr1) to outer futex (uaddr2).
Maybe I can restructure things a bit, I think CMP_REQUEUE would also
know who actually acquired the outer-futex, but I have to think more on
this and the brain is pretty fried...
