On Thu, Jun 04, 2015 at 12:55:37PM +0200, Peter Zijlstra wrote:
> On Thu, Jun 04, 2015 at 12:49:02PM +0200, Peter Zijlstra wrote:
> > Needs more comments at the very least, its fully of trickery.
> >
> > @@ -1118,10 +1125,16 @@ static void __run_hrtimer(struct hrtimer
> > enum hrtimer_restart (*fn)(struct hrtimer *);
> > int restart;
> >
> > - WARN_ON(!irqs_disabled());
> > + lockdep_assert_held(&cpu_base->lock);
> >
> > debug_deactivate(timer);
> > - base->running = timer;
> > + cpu_base->running = timer;
> > +
> > + /*
> > + * separate the ->running assignment from the ->state assignment
> > + */
> > + write_seqcount_begin(&cpu_base->seq);
>
> Maybe these need to be raw_write_seqcount_latch()..
I'm properly confusing my self, so let me write a little more detail;
I didn't think it needed the double wmb because:
[S] running = timer;
[S] seq++;
WMB
[S] state = INACTIVE
I don't think it matters if we re-order the first two stores, since at
that point ->state is still ENQUEUED and we're good.
Similar for the case below, you can flip the seq increment and the
->running clear, but at that time ->state should already be ENQUEUED
again (if indeed the timer got re-armed), otherwise its not active
anymore.
> > __remove_hrtimer(timer, base, HRTIMER_STATE_INACTIVE, 0);
> > timer_stats_account_hrtimer(timer);
> > fn = timer->function;
> > @@ -1150,8 +1163,13 @@ static void __run_hrtimer(struct hrtimer
> > !(timer->state & HRTIMER_STATE_ENQUEUED))
> > enqueue_hrtimer(timer, base);
> >
> > - WARN_ON_ONCE(base->running != timer);
> > - base->running = NULL;
> > + /*
> > + * separate the ->running assignment from the ->state assignment
> > + */
> > + write_seqcount_end(&cpu_base->seq);
> > +
> > + WARN_ON_ONCE(cpu_base->running != timer);
> > + cpu_base->running = NULL;
> > }
> >
> > static void __hrtimer_run_queues(struct hrtimer_cpu_base *cpu_base,
> > ktime_t now)
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