linux-kernel - Re: sched: fair: NULL ptr deref in check_preempt

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Message-ID: <20140217092055.GP27965@twins.programming.kicks-ass.net>
Date:	Mon, 17 Feb 2014 10:20:55 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Michael wang <wangyun@...ux.vnet.ibm.com>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Ingo Molnar <mingo@...nel.org>, Dave Jones <davej@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: sched: fair: NULL ptr deref in check_preempt_wakeup

On Mon, Feb 17, 2014 at 04:11:09PM +0800, Michael wang wrote:
> BTW, I reproduced it by steps:
> 1. change current to RT
> 2. move to a different depth cpu-cgroup
> 3. change it back to FAIR
> 
> Seems like it was caused by that RT has no task_move_group() implemented
> which could maintain depth, and that lead to a wrong depth after switched
> back to FAIR...


> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 235cfa7..4445e56 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7317,7 +7317,11 @@ static void switched_from_fair(struct rq *rq, struct task_struct *p)
>   */
>  static void switched_to_fair(struct rq *rq, struct task_struct *p)
>  {
> -	if (!p->se.on_rq)
> +	struct sched_entity *se = &p->se;
> +#ifdef CONFIG_FAIR_GROUP_SCHED
> +	se->depth = se->parent ? se->parent->depth + 1 : 0;
> +#endif
> +	if (!se->on_rq)
>  		return;
>  
>  	/*

Yes indeed. My first idea yesterday was to put it in set_task_rq() to be
absolutely sure we catch all; but if this is sufficient its better.

Thanks!
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