Messages in this thread | | | Subject | Re: [PATCH v2 1/3] sched/dl: Implement cancel_dl_timer() to use in switched_from_dl() | From | Kirill Tkhai <> | Date | Tue, 21 Oct 2014 14:48:01 +0400 |
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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); > > > > > > > > >
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