[patch 6/9] sched: fix cpu hotplug

[Greg KH <gregkh@suse.de>]

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2.6.25-stable review patch.  If anyone has any objections, please let us
know.

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From: Dmitry Adamushko <dmitry.adamushko@gmail.com>

Commit 79c537998d143b127c8c662a403c3356cb885f1c upstream

the CPU hotplug problems (crashes under high-volume unplug+replug
tests) seem to be related to migrate_dead_tasks().

Firstly I added traces to see all tasks being migrated with
migrate_live_tasks() and migrate_dead_tasks(). On my setup the problem
pops up (the one with "se == NULL" in the loop of
pick_next_task_fair()) shortly after the traces indicate that some has
been migrated with migrate_dead_tasks()). btw., I can reproduce it
much faster now with just a plain cpu down/up loop.

[disclaimer] Well, unless I'm really missing something important in
this late hour [/desclaimer] pick_next_task() is not something
appropriate for migrate_dead_tasks() :-)

the following change seems to eliminate the problem on my setup
(although, I kept it running only for a few minutes to get a few
messages indicating migrate_dead_tasks() does move tasks and the
system is still ok)

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
 kernel/sched.c |    1 +
 1 file changed, 1 insertion(+)

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -5728,6 +5728,7 @@ static void migrate_dead_tasks(unsigned 
 		next = pick_next_task(rq, rq->curr);
 		if (!next)
 			break;
+		next->sched_class->put_prev_task(rq, next);
 		migrate_dead(dead_cpu, next);
 
 	}

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