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

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Message-ID: <578355AC.2090507@yandex-team.ru>
Date:	Mon, 11 Jul 2016 11:15:40 +0300
From:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:	Wanpeng Li <kernellwp@...il.com>
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 11.07.2016 10:25, 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?
>

If we nominate task from throttled hiearchy as a next buddy then at some
level in pick_next_task_fair we could pick cfs_rq which has no runnable
entities - in pick_next_entiry both "curr" and "left" are NULL.

-- 
Konstantin