Re: sched: spinlock recursion in migrate_swap_stop

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 21, 2014 at 09:08:26AM -0400, Sasha Levin wrote:
> On 05/20/2014 09:03 AM, Sasha Levin wrote:
> > On 05/20/2014 07:04 AM, Peter Zijlstra wrote:
> >> > On Mon, May 19, 2014 at 10:05:31PM -0400, Sasha Levin wrote:
> >>> >> ping? It seems to be easy enough to reproduce on -next, I'd be happy to try
> >>> >> debug patches/fixes.
> >> > 
> >> > Does this fuzzing you do also include hotplug? If so, does disabling
> >> > that make this problem go away?
> >> > 
> > There were no hotplug operations going on when this happens, so it seems
> > unrelated.
> 
> I've added a small test:
> 
> 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);
>         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.