[Re: [PATCH 3/7] wait.[ch]: Introduce the simple waitqueue (swait)
implementation] On 18/10/2014 (Sat 23:34) Peter Zijlstra wrote:
> On Fri, Oct 17, 2014 at 08:22:58PM -0400, Paul Gortmaker wrote:
> > @@ -75,6 +123,32 @@ static void __cwake_up_common(struct cwait_head *q,
> > unsigned int mode,
> > }
> > }
> >
> > +static void __swake_up_common(struct swait_head *q, unsigned int mode,
> > + int nr_exclusive)
> > +{
> > + struct swait *curr, *next;
> > + int woken = 0;
> > +
> > + list_for_each_entry_safe(curr, next, &q->task_list, node) {
> > + if (wake_up_state(curr->task, mode)) { /* <-- calls ttwu() */
> > + __remove_swait(q, curr);
> > + curr->task = NULL;
> > + /*
> > + * The waiting task can free the waiter as
> > + * soon as curr->task = NULL is written,
> > + * without taking any locks. A memory barrier
> > + * is required here to prevent the following
> > + * store to curr->task from getting ahead of
> > + * the dequeue operation.
> > + */
> > + smp_wmb();
> > + if (++woken == nr_exclusive)
> > + break;
> > + }
> > +
> > + }
> > +}
> > +
> > /**
> > * __cwake_up - wake up threads blocked on a waitqueue.
> > * @q: the complex waitqueue
> > @@ -96,6 +170,19 @@ void __cwake_up(struct cwait_head *q, unsigned int
> > mode, int nr_exclusive,
> > }
> > EXPORT_SYMBOL(__cwake_up);
> >
> > +void __swake_up(struct swait_head *q, unsigned int mode, int nr_exclusive)
> > +{
> > + unsigned long flags;
> > +
> > + if (!swait_active(q))
> > + return;
> > +
> > + raw_spin_lock_irqsave(&q->lock, flags);
> > + __swake_up_common(q, mode, nr_exclusive);
> > + raw_spin_unlock_irqrestore(&q->lock, flags);
> > +}
> > +EXPORT_SYMBOL(__swake_up);
>
> Same comment as before, that is an unbounded loop in a non preemptible
> section and therefore violates RT design principles.
Yep, I hadn't forgot about that ; see patch 6/7 -- which has your
tentative solution from before. I didn't want to squish that into
here and lose sight of it ; same for the smp barriers - I wanted
to ensure we didn't lose visibility of things needing discussion.
>
> We actually did talk about ways of fixing that.
I'll follow up to Steve's comment on what he described.
>
> Also, I'm not entirely sure we want to do the cwait thing, it looks
> painful.
The simplewait vs. complex wait as a whole, or just the rework to
make it more aligned with the existing code? FWIW, I'm not married
to this particular implementation; so if ideas have changed since,
and the plan is different than what v2 implements, that is no problem.
P.
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