linux-kernel - Re: [PATCH] sched/fair: do not announce throttled next buddy in dequeue_task

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Message-ID: <CANRm+CzgZ42=28MzBaFU3fakhrS-JXf3bcD83fh1=UX1QWOSLw@mail.gmail.com>
Date:	Wed, 13 Jul 2016 09:50:26 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Benjamin Segall <bsegall@...gle.com>
Cc:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	xlpang@...hat.com, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	stable@...r.kernel.org
Subject: Re: [PATCH] sched/fair: do not announce throttled next buddy in dequeue_task_fair

2016-07-13 1:25 GMT+08:00  <bsegall@...gle.com>:
> Konstantin Khlebnikov <khlebnikov@...dex-team.ru> writes:
>
>> On 11.07.2016 15:12, Xunlei Pang wrote:
>>> On 2016/07/11 at 17:54, Wanpeng Li wrote:
>>>> Hi Konstantin, Xunlei,
>>>> 2016-07-11 16:42 GMT+08:00 Xunlei Pang <xpang@...hat.com>:
>>>>> On 2016/07/11 at 16:22, Xunlei Pang wrote:
>>>>>> On 2016/07/11 at 15:25, Wanpeng Li wrote:
>>>>>>> 2016-06-16 20:57 GMT+08:00 Konstantin Khlebnikov <khlebnikov@...dex-team.ru>:
>>>>>>>> Hierarchy could be already throttled at this point. Throttled next
>>>>>>>> buddy could trigger null pointer dereference in pick_next_task_fair().
>>>>>>> There is cfs_rq->next check in pick_next_entity(), so how can null
>>>>>>> pointer dereference happen?
>>>>>> I guess it's the following code leading to a NULL se returned:
>>>>> s/NULL/empty-entity cfs_rq se/
>>>>>
>>>>>> pick_next_entity():
>>>>>>      if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
>>>>              ^^^^^^^^^^^^^
>>>> I think this will return false.
>>>
>>> With the wrong throttled_hierarchy(), I think this can happen. But after we have the
>>> corrected throttled_hierarchy() patch, I can't see how it is possible.
>>>
>>> dequeue_task_fair():
>>>      if (task_sleep && parent_entity(se))
>>>          set_next_buddy(parent_entity(se));
>>>
>>> How does dequeue_task_fair() with DEQUEUE_SLEEP set(true task_sleep) happen to a throttled hierarchy?
>>> IOW, a task belongs to a throttled hierarchy is running?
>>>
>>> Maybe Konstantin knows the reason.
>>
>> This function (dequeue_task_fair) check throttling but at point it could skip several
>> levels and announce as next buddy actually throttled entry.
>> Probably this bug hadn't happened but this's really hard to prove that this is impossible.
>> ->set_curr_task(), PI-boost or some tricky migration in balancer could break this easily.
>
> sched_setscheduler can call put_prev_task, which then can cause a
> throttle outside of __schedule(), then the task blocks normally and
> deactivate_task(DEQUEUE_SLEEP) happens and you lose.

The cfs_rq_throttled() check in dequeue_task_fair() will capture the
cfs_rq which is throttled in sched_setscheduler::put_prev_task path,
so nothing lost, where I miss?

Regards,
Wanpeng Li