Messages in this thread | | | Date | Mon, 8 Jun 2015 14:42:34 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 08/14] hrtimer: Allow hrtimer::function() to free the timer |
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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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