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SubjectRe: [PATCH 08/14] hrtimer: Allow hrtimer::function() to free the timer
Not sure I read this patch correctly, it doesn't apply to Linus's tree.

And I simply can not understand the complication in hrtimer_active(),
please help!

On 06/05, Peter Zijlstra wrote:
> +bool hrtimer_active(const struct hrtimer *timer)
> +{
> + struct hrtimer_cpu_base *cpu_base;
> + unsigned int seq;
> + bool active;
> +
> + do {
> + active = false;
> + cpu_base = READ_ONCE(timer->base->cpu_base);
> + seq = raw_read_seqcount(&cpu_base->seq);
> +
> + if (timer->state != HRTIMER_STATE_INACTIVE ||
> + cpu_base->running == timer)
> + active = true;

Why we can't simply return true in this case?

Unless you lock this timer, hrtimer_active() is inherently racy anyway.
Granted, it must not wrongly return False if the timer is pending or

But "false positive" does not differ from the case when (say) the
running timer->function() finishes right after hrtimer_active() returns

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

Why do we need to re-check >cpu_base?

I think we can ignore migrate_hrtimer_list(), it doesn't clear ->state.

Otherwise the timer can change its ->base only if it is not running and
inactive, and again I think we should only eliminate the false negative

And I think there is a problem. Consider a timer TIMER which always
rearms itself using some "default" timeout.

In this case __hrtimer_start_range_ns(&TIMER, ...) must preserve
hrtimer_active(&TIMER) == T. By definition, and currently this is

After this patch this is no longer true (afaics). If the timer is
pending but not running, __hrtimer_start_range_ns()->remove_hrtimer()
will clear ENQUEUED first, then set it again in enqueue_hrtimer().

This means that hrtimer_active() returns false in between. And note
that it doesn't matter if the timer changes its ->base or not, so
that 2nd cpu_base above can't help.

I think that __hrtimer_start_range_ns() should preserve ENQUEUED
like migrate_hrtimer_list() should do (see the previous email).

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) {
active = state != HRTIMER_STATE_INACTIVE;


But I am already sleeping and probably totally confused.


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