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

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Message-ID: <57835735.6020906@redhat.com>
Date:	Mon, 11 Jul 2016 16:22:13 +0800
From:	Xunlei Pang <xpang@...hat.com>
To:	Wanpeng Li <kernellwp@...il.com>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:	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

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:
pick_next_entity():
    if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
        se = cfs_rq->next;

Regards,
Xunlei

> Regards,
> Wanpeng Li