Re: [PATCH v2 1/3] sched/dl: Implement cancel_dl_timer() to use in switched_from_dl()

Kirill Tkhai
2014-10-23T08:39:20Z

В Ср, 22/10/2014 в 11:00 +0100, Juri Lelli пишет:
> On 21/10/14 15:21, Kirill Tkhai wrote:
> > В Вт, 21/10/2014 в 12:41 +0100, Juri Lelli пишет:
> >> On 21/10/14 11:48, Kirill Tkhai wrote:
> >>> В Вт, 21/10/2014 в 11:30 +0100, Juri Lelli пишет:
> >>>> Hi Kirill,
> >>>>
> >>>> sorry for the late reply, but I was busy doing other stuff and then
> >>>> travelling.
> >>>>
> >>>> On 02/10/14 11:05, Kirill Tkhai wrote:
> >>>>> В Чт, 02/10/2014 в 11:34 +0200, Peter Zijlstra пишет:
> >>>>>> On Wed, Oct 01, 2014 at 01:04:22AM +0400, Kirill Tkhai wrote:
> >>>>>>> From: Kirill Tkhai <ktkhai@parallels.com>
> >>>>>>>
> >>>>>>> hrtimer_try_to_cancel() may bring a suprise, its call may fail.
> >>>>>>
> >>>>>> Well, not really a surprise that, its a _try_ operation after all.
> >>>>>>
> >>>>>>> raw_spin_lock(&rq->lock)
> >>>>>>> ... dl_task_timer raw_spin_lock(&rq->lock)
> >>>>>>> ... raw_spin_lock(&rq->lock) ...
> >>>>>>> switched_from_dl() ... ...
> >>>>>>> hrtimer_try_to_cancel() ... ...
> >>>>>>> switched_to_fair() ... ...
> >>>>>>> ... ... ...
> >>>>>>> ... ... ...
> >>>>>>> raw_spin_unlock(&rq->lock) ... (asquired)
> >>>>>>> ... ... ...
> >>>>>>> ... ... ...
> >>>>>>> do_exit() ... ...
> >>>>>>> schedule() ... ...
> >>>>>>> raw_spin_lock(&rq->lock) ... raw_spin_unlock(&rq->lock)
> >>>>>>> ... ... ...
> >>>>>>> raw_spin_unlock(&rq->lock) ... raw_spin_lock(&rq->lock)
> >>>>>>> ... ... (asquired)
> >>>>>>> put_task_struct() ... ...
> >>>>>>> free_task_struct() ... ...
> >>>>>>> ... ... raw_spin_unlock(&rq->lock)
> >>>>>>> ... (asquired) ...
> >>>>>>> ... ... ...
> >>>>>>> ... Surprise!!! ...
> >>>>>>>
> >>>>>>> So, let's implement 100% guaranteed way to cancel the timer and let's
> >>>>>>> be sure we are safe even in very unlikely situations.
> >>>>>>>
> >>>>>>> We do not create any problem with rq unlocking, because it already
> >>>>>>> may happed below in pull_dl_task(). No problem with deadline tasks
> >>>>>>> balancing too.
> >>>>>>
> >>>>>> That doesn't sound right. pull_dl_task() is an entirely different
> >>>>>> callchain than switched_from(). Now it might still be fine, but you
> >>>>>> cannot compare it with pull_dl_task.
> >>>>>
> >>>>> I mean that caller of switched_from_dl() already knows about this situation,
> >>>>> and we do not limit the area of its use.
> >>>>>
> >>>>
> >>>> Not sure what you mean with "the caller already knows...". Also, can you
> >>>> detail more about the different callchains?
> >>>
> >>> We have only caller of switched_from_dl(). It's check_class_changed().
> >>> This function doesn't suppose that lock is always locked during its call.
> >>>
> >>> What other details you want?
> >>>
> >>
> >> Ok, now is more clear, thanks. I was just wondering about what Peter
> >> asked. If you can detail more about why we are still fine with it,
> >> instead that just "it already was possible in pull_dl_task() below",
> >> that would be nice to have.
> >>
> >> Also, check_class_changed() is called from several places
> >> (rt_mutex_setprio() for example), are we fine with all this callplaces
> >> as well?
> >
> > Yeah. New code in the patch is working when hrtimer_try_to_cancel() fails.
> > This means the callback is running. In this case hrtimer_cancel() is just
> > waiting till the callback is finished.
> >
> > Since we are in switched_from_dl(), new class is not dl_sched_class and
> > new prio is not less MAX_DL_PRIO. So, the callback returns early just
> > after !dl_task() check. After that hrtimer_cancel() returns back too.
> >
> > The above is:
> >
> > raw_spin_lock(rq->lock); ...
> > ... dl_task_timer()
> > ... raw_spin_lock(rq->lock);
> > switched_from_dl() ...
> > hrtimer_try_to_cancel() ...
> > raw_spin_unlock(rq->lock); ...
> > hrtimer_cancel() ...
> > ... raw_spin_unlock(rq->lock);
> > ... return HRTIMER_NORESTART;
> > ... ...
> > raw_spin_lock(rq->lock); ...
> >
> >
> > But the below is also possible:
> > dl_task_timer()
> > raw_spin_lock(rq->lock);
> > ...
> > raw_spin_unlock(rq->lock);
> > raw_spin_lock(rq->lock); ...
> > switched_from_dl() ...
> > hrtimer_try_to_cancel() ...
> > ... return HRTIMER_NORESTART;
> > raw_spin_unlock(rq->lock); ...
> > hrtimer_cancel(); ...
> > raw_spin_lock(rq->lock); ...
> >
> > In this case hrtimer_cancel() returns immediately. Very unlikely case,
> > just to mention.
> >
> >
> > Nobody can manipulate the task, because check_class_changed() is
> > always called with pi_lock locked. Nobody can force the task to
> > participate in (concurrent) priority inheritance schemes (the same reason).
> >
> > All concurrent task operations require pi_lock, which is held by us.
> > No deadlocks with dl_task_timer() are possible, because it returns
> > right after !dl_task() check (it does nothing).
> >
>
> Ok, it looks right to me. It would be nice to have what above and the
> original explanation of the bug in the changelog.

I'll send new patch with your remarks.

> >>>>
> >>>> Do you have any test for this situation? Do you experienced any crash?
> >>>> As you know, the replenishment timer is of key importance for us, and
> >>>> I'd like to be 100% sure we don't introduce any problems with this
> >>>> change :).
> >>>
> >>> No, I haven't written any tests to reproduce namely this situation.
> >>> I found it by code analyzing. The same way we fixed the problem
> >>> with rq change in dl_task_timer():
> >>>
> >>> http://www.spinics.net/lists/stable/msg49080.html
> >>>
> >>
> >> Yeah, but I did write a test for that race:
> >>
> >> "Juri Lelli reports he got this race when dl_bandwidth_enabled()
> >> was not set."
> >>
> >> And after that I felt more confident about the change :).
> >
> > Ok, good. I forgot.
> >
> >>> Are you agree the race is here? It's my fix, and if brings a problem
> >>> please clarify it.
> >>>
> >>
> >> Yeah, it seems that the race may happen. I'm just saying that it would
> >> be nice to see it happening before we fix the thing. I wish I have some
> >> time to try to setup a test. Even if I can't spot any problems with your
> >> patch, apart from small comments below, not being completely confident
> >> that this doesn't introduce regression elsewhere brought me to ask from
> >> more details.
> >
> > Sadly, I have no time to write a test for this bug. I can change the comment
> > and add the description I posted above. Or I can add more description
> > if you say what should be added else.
> >
>
> So, if you are ok with it, I'd say I can take some time to do a little
> testing anyway, as the bug is there, but nobody (except you) noticed
> that yet :).
>
> >>
> >>> I'm waiting for your reply.
> >>>
> >>> Thanks,
> >>> Kirill
> >>>
> >>>>> Does this sound better?
> >>>>>
> >>>>> [PATCH] sched/dl: Implement cancel_dl_timer() to use in switched_from_dl()
> >>>>>
> >>>>> Currently used hrtimer_try_to_cancel() is racy:
> >>>>>
> >>>>> raw_spin_lock(&rq->lock)
> >>>>> ... dl_task_timer raw_spin_lock(&rq->lock)
> >>>>> ... raw_spin_lock(&rq->lock) ...
> >>>>> switched_from_dl() ... ...
> >>>>> hrtimer_try_to_cancel() ... ...
> >>>>> switched_to_fair() ... ...
> >>>>> ... ... ...
> >>>>> ... ... ...
> >>>>> raw_spin_unlock(&rq->lock) ... (asquired)
> >>>>> ... ... ...
> >>>>> ... ... ...
> >>>>> do_exit() ... ...
> >>>>> schedule() ... ...
> >>>>> raw_spin_lock(&rq->lock) ... raw_spin_unlock(&rq->lock)
> >>>>> ... ... ...
> >>>>> raw_spin_unlock(&rq->lock) ... raw_spin_lock(&rq->lock)
> >>>>> ... ... (asquired)
> >>>>> put_task_struct() ... ...
> >>>>> free_task_struct() ... ...
> >>>>> ... ... raw_spin_unlock(&rq->lock)
> >>>>> ... (asquired) ...
> >>>>> ... ... ...
> >>>>> ... (use after free) ...
> >>>>>
> >>>>>
> >>>>> So, let's implement 100% guaranteed way to cancel the timer and let's
> >>>>> be sure we are safe even in very unlikely situations.
> >>>>>
> >>>>> rq unlocking does not limit the area of switched_from_dl() use, because
> >>>>> it already was possible in pull_dl_task() below.
> >>>>>
> >>>>> Signed-off-by: Kirill Tkhai <ktkhai@parallels.com>
> >>>>>
> >>>>> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> >>>>> index abfaf3d..63f8b4a 100644
> >>>>> --- a/kernel/sched/deadline.c
> >>>>> +++ b/kernel/sched/deadline.c
> >>>>> @@ -555,11 +555,6 @@ void init_dl_task_timer(struct sched_dl_entity *dl_se)
> >>>>> {
> >>>>> struct hrtimer *timer = &dl_se->dl_timer;
> >>>>>
> >>>>> - if (hrtimer_active(timer)) {
> >>>>> - hrtimer_try_to_cancel(timer);
> >>>>> - return;
> >>>>> - }
> >>>>> -
> >>>>> hrtimer_init(timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> >>>>> timer->function = dl_task_timer;
> >>>>> }
> >>>>> @@ -1567,10 +1562,34 @@ void init_sched_dl_class(void)
> >>>>>
> >>>>> #endif /* CONFIG_SMP */
> >>>>>
> >>>>> +/*
> >>>>> + * Surely cancel task's dl_timer. May drop rq->lock.
> >>>>> + */
> >>
> >> Maybe we can add comments explaining why we are fine releasing the lock
> >> here.
> >>
>
> Does "Ensure p's dl_timer is cancelled. May drop rq->lock." sound better?
>
> >>>>> +static void cancel_dl_timer(struct rq *rq, struct task_struct *p)
> >>>>> +{
> >>>>> + struct hrtimer *dl_timer = &p->dl.dl_timer;
> >>>>> +
> >>>>> + /* Nobody will change task's class if pi_lock is held */
> >>>>> + lockdep_assert_held(&p->pi_lock);
> >>>>> +
> >>>>> + if (hrtimer_active(dl_timer)) {
> >>>>> + int ret = hrtimer_try_to_cancel(dl_timer);
> >>>>> +
> >>>>> + if (unlikely(ret == -1)) {
> >>>>> + /*
> >>>>> + * Note, p may migrate OR new deadline tasks
> >>>>> + * may appear in rq when we are unlocking it.
> >>>>> + */
> >>
> >> Yeah, some comments also here on why this is all good?
> >>
>
> Here you say what may happen. Can you add something saying why we are
> fine with this happening? Just for future reference...

Thanks!
Kirill