Date:	Tue, 9 Jun 2015 23:33:18 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	umgwanakikbuti@...il.com, mingo@...e.hu, ktkhai@...allels.com,
	rostedt@...dmis.org, tglx@...utronix.de, juri.lelli@...il.com,
	pang.xunlei@...aro.org, wanpeng.li@...ux.intel.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/14] hrtimer: Allow hrtimer::function() to free the
	timer

On 06/08, Peter Zijlstra wrote:
>
> 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;
> }

You know, I simply can't convince myself I understand why this code
correct... or not.

But contrary to what I said before, I agree that we need to recheck
timer->base. This probably needs more discussion, to me it is very
unobvious why we can trust this cpu_base != READ_ONCE() check. Yes,
we have a lot of barriers, but they do not pair with each other. Lets
ignore this for now.

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

Yes, but wmb() is cheap on x86... Perhaps we can make this code
"obviously correct" ?


How about the following..... We add cpu_base->seq as before but
limit its "write" scope so that we cam use the regular read/retry.

So,

	hrtimer_active(timer)
	{

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

			if (timer->state & ENQUEUED ||
			    base->running == timer)
				return true;

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

		return false;
	}

And we need to avoid the races with 2 transitions in __run_hrtimer().

The first race is trivial, we change __run_hrtimer() to do

	write_seqcount_begin(cpu_base->seq);
	cpu_base->running = timer;
	__remove_hrtimer(timer);	// clears ENQUEUED
	write_seqcount_end(cpu_base->seq);

and hrtimer_active() obviously can't race with this section.

Then we change enqueue_hrtimer()


	+	bool need_lock = base->cpu_base->running == timer;
	+	if (need_lock)
	+		write_seqcount_begin(cpu_base->seq);
	+
		timer->state |= HRTIMER_STATE_ENQUEUED;
	+
	+	if (need_lock)
	+		write_seqcount_end(cpu_base->seq);


Now. If the timer is re-queued by the time __run_hrtimer() clears
->running we have the following sequence:

	write_seqcount_begin(cpu_base->seq);
	timer->state |= HRTIMER_STATE_ENQUEUED;
	write_seqcount_end(cpu_base->seq);

	base->running = NULL;

and I think this should equally work, because in this case we do not
care if hrtimer_active() misses "running = NULL".

Yes, we only have this 2nd write_seqcount_begin/end if the timer re-
arms itself, but otherwise we do not race. If another thread does
hrtime_start() in between we can pretend that hrtimer_active() hits
the "inactive".

What do you think?


And. Note that we can rewrite these 2 "write" critical sections in
__run_hrtimer() and enqueue_hrtimer() as

	cpu_base->running = timer;

	write_seqcount_begin(cpu_base->seq);
	write_seqcount_end(cpu_base->seq);

	__remove_hrtimer(timer);

and

	timer->state |= HRTIMER_STATE_ENQUEUED;

	write_seqcount_begin(cpu_base->seq);
	write_seqcount_end(cpu_base->seq);

	base->running = NULL;

So we can probably use write_seqcount_barrier() except I am not sure
about the 2nd wmb...

Oleg.

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