Messages in this thread | | | Date | Fri, 25 Nov 2016 10:23:26 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC][PATCH 4/4] futex: Rewrite FUTEX_UNLOCK_PI |
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On Thu, Nov 24, 2016 at 07:58:07PM +0100, Peter Zijlstra wrote:
> OK, so clearly I'm confused. So let me try again. > > LOCK_PI, does in one function: lookup_pi_state, and fixup_owner. If > fixup_owner fails with -EAGAIN, we can redo the pi_state lookup. > > The requeue stuff, otoh, has one each. REQUEUE_WAIT has fixup_owner(), > CMP_REQUEUE has lookup_pi_state. Therefore, fixup_owner failing with > -EAGAIN leaves us dead in the water. There's nothing to go back to to > retry. > > So far, so 'good', right? > > Now, as far as I understand this requeue stuff, we have 2 futexes, an > inner futex and an outer futex. The inner futex is always 'locked' and > serves as a collection pool for waiting threads. > > The requeue crap picks one (or more) waiters from the inner futex and > sticks them on the outer futex, which gives them a chance to run. > > So WAIT_REQUEUE blocks on the inner futex, but knows that if it ever > gets woken, it will be on the outer futex, and hence needs to > fixup_owner if the futex and rt_mutex state got out of sync. > > CMP_REQUEUEUEUE picks the one (or more) waiters of the inner futex and > sticks them on the outer futex. > > So far, so 'good' ? > > The thing I'm not entire sure on is what happens with the outer futex, > do we first LOCK_PI it before doing CMP_REQUEUE, giving us waiters, and > then UNLOCK_PI to let them rip? Or do we just CMP_REQUEUE and then let > whoever wins finish with UNLOCK_PI? > > > In any case, I don't think it matters much, either way we can race > betwen the 'last' UNLOCK_PI and getting rt_mutex waiters and then hit > the &init_task funny state, such that WAIT_REQUEUE waking hits EAGAIN > and we're 'stuck'. > > Now, if we always CMP_REQUEUE to a locked outer futex, then we cannot > know, at CMP_REQUEUE time, who will win and cannot fix up.
OTOH, if we always first LOCK_PI before doing CMP_REQUEUE, I don't think we can hit the funny state, LOCK_PI will have fixed it up for us.
So the question is, do we mandate LOCK_PI before CMP_REQUEUE?
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