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Date: Wed, 19 Feb 2014 17:16:57 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Michael wang <wangyun@linux.vnet.ibm.com>
Cc: Sasha Levin <sasha.levin@oracle.com>, Ingo Molnar <mingo@kernel.org>,
        Dave Jones <davej@redhat.com>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: sched: fair: NULL ptr deref in check_preempt_wakeup
Message-ID: <20140219161657.GO15586@twins.programming.kicks-ass.net>
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On Mon, Feb 17, 2014 at 04:11:09PM +0800, Michael wang wrote:
> > While fuzzing with trinity inside a KVM tools guest running latest -next
> > kernel, I've
> > stumbled on the following:
> 
> I've reproduced the same issue with tip/master, and below patch fixed the
> problem on my box along with some rcu stall info disappeared, would you
> like to have a try?
> 
> 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...
> 
> Regards,
> Michael Wang
> 
> 
> 
> 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;
>  
>  	/*


Michael, do you think you can send a proper patch for this?