Re: [PATCH 08/14] hrtimer: Allow hrtimer::function() to free the timer

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jun 08, 2015 at 11:14:17AM +0200, Peter Zijlstra wrote:
> > Finally. Suppose that timer->function() returns HRTIMER_RESTART
> > and hrtimer_active() is called right after __run_hrtimer() sets
> > cpu_base->running = NULL. I can't understand why hrtimer_active()
> > can't miss ENQUEUED in this case. We have wmb() in between, yes,
> > but then hrtimer_active() should do something like
> > 
> > 	active = cpu_base->running == timer;
> > 	if (!active) {
> > 		rmb();
> > 		active = state != HRTIMER_STATE_INACTIVE;
> > 	}
> > 
> > No?
> 
> Hmm, good point. Let me think about that. It would be nice to be able to
> avoid more memory barriers.

So your scenario is:

				[R] seq
				  RMB
[S] ->state = ACTIVE
  WMB
[S] ->running = NULL
				[R] ->running (== NULL)
				[R] ->state (== INACTIVE; fail to observe
				             the ->state store due to
					     lack of order)
				  RMB
				[R] seq (== seq)
[S] seq++

Conversely, if we re-order the (first) seq++ store such that it comes
first:

[S] seq++

				[R] seq
				  RMB
				[R] ->running (== NULL)
[S] ->running = timer;
  WMB
[S] ->state = INACTIVE
				[R] ->state (== INACTIVE)
				  RMB
				[R] seq (== seq)

And we have another false negative.

And in this case we need the read order the other way around, we'd need:

	active = timer->state != HRTIMER_STATE_INACTIVE;
	if (!active) {
		smp_rmb();
		active = cpu_base->running == timer;
	}

Now I think we can fix this by either doing:

	WMB
	seq++
	WMB

On both sides of __run_hrtimer(), or do

bool hrtimer_active(const struct hrtimer *timer)
{
	struct hrtimer_cpu_base *cpu_base;
	unsigned int seq;

	do {
		cpu_base = READ_ONCE(timer->base->cpu_base);
		seq = raw_read_seqcount(&cpu_base->seq);

		if (timer->state != HRTIMER_STATE_INACTIVE)
			return true;

		smp_rmb();

		if (cpu_base->running == timer)
			return true;

		smp_rmb();

		if (timer->state != HRTIMER_STATE_INACTIVE)
			return true;

	} while (read_seqcount_retry(&cpu_base->seq, seq) ||
		 cpu_base != READ_ONCE(timer->base->cpu_base));

	return false;
}
EXPORT_SYMBOL_GPL(hrtimer_active);


And since __run_hrtimer() is the more performance critical code, I think
it would be best to reduce the amount of memory barriers there.