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

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Message-ID: <CANRm+CxTudRZWv9oU3XU+8=f+FrVRUcT-EeDVems90F9+NdEyg@mail.gmail.com>
Date:	Mon, 11 Jul 2016 17:54:16 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	xlpang@...hat.com
Cc:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	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

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.

Regards,
Wanpeng Li