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

Kirill Tkhai
2014-10-02T10:05:22Z

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.

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);