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

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Message-ID: <20140522070029.GK30445@twins.programming.kicks-ass.net>
Date:	Thu, 22 May 2014 09:00:29 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	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

On Thu, May 22, 2014 at 11:05:40AM +0530, Srikar Dronamraju wrote:
> > 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?

Yeah, current might have migrated while we were looking for a target.

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