Re: [PATCH] alarmtimer: Preserve rearmed alarms during callback expiry

[Thomas Gleixner <tglx@kernel.org>]

On Sun, Nov 01 2026 at 18:39, Avi Weiss wrote:
> An alarm can be rearmed while its previous hrtimer callback is still
> running. alarmtimer_fired() then dequeues the newly armed alarm, so
> alarmtimer_suspend() no longer sees the wakeup even though the hrtimer
> remains queued.

This fails to explain wich alarm timer user is affected. POSIX-timers
and timerfd timers are not as they cancel the timer before re-arming it.
Which leaves the other two users of alarm_start_timer() which I did not
look at.

But if that actually happens then alarm_start_timer() is subtly broken
because of this sequence:

        alarm->node.expires = expires;
        alarmtime_enqueue(base, alarm);

Setting alarm->node.expires on a queued timer temporarily "corrupts" the
rbtree. That's "harmless" because it's serialized via base->lock and
timerqueue_del() does not care as of today, but that's all but robust
and future proof.

So this needs to be fixed as well and all of this needs to be
documented.
  
>  	scoped_guard(spinlock_irqsave, &base->lock)
> -		alarmtimer_dequeue(base, alarm);
> +		if (!hrtimer_is_queued(timer))
> +			alarmtimer_dequeue(base, alarm);

Reading this without a comment just makes me scratch my head and you
will scratch yours too three month down the road.

Thanks,

        tglx