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

Juri Lelli
2014-10-21T11:41:15Z

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?

>>
>> 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 :).

> 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.

> 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.

>>> +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?

Thanks a lot Kirill!

Best,

- Juri

>>> + 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);
>>>
>>>
>>>
>>>
>>
>
>
>