hm, while looking at this code again...
Ingo,
I think we may have a race between try_to_wake_up() and migrate_live_tasks() -> move_task_off_dead_cpu()

when the later one may end up looping endlessly.
Subject: sched: prevent a potentially endless loop in move_task_off_dead_cpu()
Interrupts are enabled on other CPUs when migration_call(CPU_DEAD, ...) is called so we may get a race

between try_to_wake_up() and migrate_live_tasks() -> move_task_off_dead_cpu(). The former one may push

a task out of a dead CPU causing the later one to loop endlessly.
Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com>
---

diff --git a/kernel/sched.c b/kernel/sched.c

index 94ead43..9397b87 100644

--- a/kernel/sched.c

+++ b/kernel/sched.c

@@ -5621,8 +5621,10 @@ static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
 	double_rq_lock(rq_src, rq_dest);

 	/* Already moved. */

-	if (task_cpu(p) != src_cpu)

+	if (task_cpu(p) != src_cpu) {

+		ret = 1;

 		goto out;

+	}

 	/* Affinity changed (again). */

 	if (!cpu_isset(dest_cpu, p->cpus_allowed))

 		goto out;
---
--

That's exactly what explains a dump I got yesterday. Thanks for fixing! :)
Will apply your patch and let you know if it fixes the problem.

--

applied to tip/sched/urgent via the commit below - lets see whether we

can still get it into v2.6.26.
	Ingo
---------------->

commit dc7fab8b3bb388c57c6c4a43ba68c8a32ca25204

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

Date:   Thu Jul 10 00:32:40 2008 +0200
    sched: fix cpu hotplug
    I think we may have a race between try_to_wake_up() and

    migrate_live_tasks() -> move_task_off_dead_cpu() when the later one

    may end up looping endlessly.
    Interrupts are enabled on other CPUs when migration_call(CPU_DEAD, ...) is

    called so we may get a race between try_to_wake_up() and

    migrate_live_tasks() -> move_task_off_dead_cpu(). The former one may push

    a task out of a dead CPU causing the later one to loop endlessly.
    Heiko Carstens observed:
    | That's exactly what explains a dump I got yesterday. Thanks for fixing! :)
    Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com>

    Cc: miaox@cn.fujitsu.com

    Cc: Lai Jiangshan <laijs@cn.fujitsu.com>

    Cc: Heiko Carstens <heiko.carstens@de.ibm.com>

    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>

    Cc: Avi Kivity <avi@qumranet.com>

    Cc: Andrew Morton <akpm@linux-foundation.org>

    Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/kernel/sched.c b/kernel/sched.c

index 94ead43..9397b87 100644

--- a/kernel/sched.c

+++ b/kernel/sched.c

@@ -5621,8 +5621,10 @@ static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
 	double_rq_lock(rq_src, rq_dest);

 	/* Already moved. */

-	if (task_cpu(p) != src_cpu)

+	if (task_cpu(p) != src_cpu) {

+		ret = 1;

 		goto out;

+	}

 	/* Affinity changed (again). */

 	if (!cpu_isset(dest_cpu, p->cpus_allowed))

 		goto out;

--