Re: sched: spinlock recursion in migrate_swap_stop

[Peter Zijlstra <peterz@infradead.org>]

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.

---
 kernel/sched/fair.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 28ccf502c63c..28ba71d815ee 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1115,6 +1115,8 @@ static void task_numa_compare(struct task_numa_env *env,
 	cur = ACCESS_ONCE(dst_rq->curr);
 	if (cur->pid == 0) /* idle */
 		cur = NULL;
+	if (cur == env->p)
+		goto unlock;
 
 	/*
 	 * "imp" is the fault differential for the source task between the