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

Kirill Tkhai
2014-10-21T10:48:18Z

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?

>
> 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

Are you agree the race is here? It's my fix, and if brings a problem
please clarify it.

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.
> > + */
> > +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.
> > + */
> > + raw_spin_unlock(&rq->lock);
> > + hrtimer_cancel(dl_timer);
> > + raw_spin_lock(&rq->lock);
> > + }
> > + }
> > +}
> > +
> > static void switched_from_dl(struct rq *rq, struct task_struct *p)
> > {
> > - if (hrtimer_active(&p->dl.dl_timer) && !dl_policy(p->policy))
> > - hrtimer_try_to_cancel(&p->dl.dl_timer);
> > + cancel_dl_timer(rq, p);
> >
> > __dl_clear_params(p);
> >
> >
> >
> >
>