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Date:	Mon, 10 Nov 2014 10:48:28 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Kirill Tkhai <ktkhai@...allels.com>
CC:	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Kirill Tkhai <tkhai@...dex.ru>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH v4] sched/numa: fix unsafe get_task_struct() in task_numa_assign()

On 11/10/2014 05:03 AM, Peter Zijlstra wrote:
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1268,6 +1268,13 @@ static void task_numa_compare(struct tas
>  	raw_spin_unlock_irq(&dst_rq->lock);
>  
>  	/*
> +	 * Because we have preemption enabled we can get migrated around and
> +	 * end try selecting ourselves (current == env->p) as a swap candidate.
> +	 */
> +	if (cur == env->p)
> +		goto unlock;

This is too late though, because currently the lockup happens couple of lines
above that at:

        raw_spin_lock_irq(&dst_rq->lock); <=== here
        cur = dst_rq->curr;

Which got me a bit stuck trying to use your old patch since we can't access '->curr'
without locking dst_rq, but locking dst_rq is causing a lockup.


Thanks,
Sasha
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