Re: [PATCH 08/14] hrtimer: Allow hrtimer::function() to free the timer

[Oleg Nesterov <oleg@redhat.com>]

On 06/08, Peter Zijlstra wrote:
>
> On Mon, Jun 08, 2015 at 12:33:17AM +0200, Oleg Nesterov wrote:
>
> static inline bool hrtimer_active(const struct timer *timer)
> {
> 	return timer->state != HRTIMER_STATE_INACTIVE ||
> 	       timer->base->cpu_base->running == timer;
> }
>
> Is that is can indeed return false falsely.

Yes sure, this is obviously buggy. But again, only because
"false falsely" is wrong. OK, I see other emails from you, lets
discuss this problem there.

> > > +	} 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?
>
> Because each CPU's cpu_base has independent seq numbers, so if the timer
> migrates, the seq numbers might align just so that we fail to detect
> change, even though there was -- extremely unlikely, but possible.

Of course. However. I am not sure, but it seems to me that a) seq numbers
can't really help exactly because we ran read the wrong base, and b) we can
rely on QUEUED check. I'll send another email, but see also below.

> > 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
> > true.
>
> I must ask for a little clarification; is this the simple:
>
> 	->function()
> 		hrtimer_forward_now(&self, timeout);
> 		return HRTIMER_RESTART;
>
> Or something that 'randomly' calls:
>
> 	hrtimer_start() on a timer?

The latter,

> Because for the latter this isn't currently true for the same reason as
> you give here:
>
> > 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().
>
> That is so even with the current code; the current code uses:
>
> 	hrtimer->state & CALLBACK

Ah, indeed, I misread this code. So the code is already buggy.

> > I think that __hrtimer_start_range_ns() should preserve ENQUEUED
> > like migrate_hrtimer_list() should do (see the previous email).
>
> I tend to agree, but I think its a pre-existing problem, not one
> introduced by my proposed patch.

Yes. If we know that the timer always rearns itself then hrtimer_active()
must be always true. A "random" hrtimer_start() on this timer must not
create an "inactive" window.

So we should probably fix this in any case. And migrate_hrtimer_list()
too. And (it seems) this can help to simplify hrtimer_inactive().

Oleg.