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.
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