linux-kernel - Re: sched: spinlock recursion in migrate_swap

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Message-ID: <20140522053540.GA26857@linux.vnet.ibm.com>
Date:	Thu, 22 May 2014 11:05:40 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>,
	Dave Jones <davej@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: sched: spinlock recursion in migrate_swap_stop

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 927fa33..b5e11c7 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1154,6 +1156,7 @@ int migrate_swap(struct task_struct *cur, struct task_struct *p)
>                 goto out;
>
>         trace_sched_swap_numa(cur, arg.src_cpu, p, arg.dst_cpu);
> +       BUG_ON(cur == p);

I am not sure how this check can ever be successful because at the start
of this function migrate_swap() we have

  if (arg.src_cpu == arg.dst_cpu)
	  goto out;


if cur is actually p; then should the above condition should always be
successul right?

Or am I missing something?

>         ret = stop_two_cpus(arg.dst_cpu, arg.src_cpu, migrate_swap_stop, &arg);
>
>  out:
>
>
> Which seems to get hit. This sounds like a race with task moving to
> other cpu maybe?
>

--
Thanks and Regards
Srikar Dronamraju

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