Messages in this thread | | | Date | Thu, 24 Nov 2016 18:56:53 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [RFC][PATCH 4/4] futex: Rewrite FUTEX_UNLOCK_PI |
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On Thu, 24 Nov 2016, Peter Zijlstra wrote: > 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
You mean LOCK_PI, right?
> 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).
Correct. WAIT_REQUEUE puts the futex on the inner (uaddr1) and then gets moved to the outer. From there it's the same thing as LOCK_PI.
> 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...
That is irrelevant at requeue time and the owner can change up to the point where the waiter is really woken by a UNLOCK_PI.
Thanks,
tglx
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