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Date:	Thu, 22 May 2014 08:59:15 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	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 Wed, May 21, 2014 at 10:34:44PM -0400, Sasha Levin wrote:
> On 05/21/2014 12:49 PM, Peter Zijlstra wrote:
> > On Wed, May 21, 2014 at 03:19:48PM +0200, Peter Zijlstra wrote:
> >> > On Wed, May 21, 2014 at 09:08:26AM -0400, Sasha Levin wrote:
> >>> > > +++ 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);
> >>> > >         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?
> >> > 
> >> > Oi, good call that, lemme go stare.
> > I think something simple like this should be sufficient to avoid the
> > problem of selecting oneself as a flip target.
> 
> Why would that happen in the first place?

We do all that with preemption enabled, because its big and expensive,
so its entirely possible that current (env->p) got moved around while we
were doing it, at which point we'll look at it as a possible dst, while
its already our src.



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