Re: [BUG] CFS vs cpu hotplug

[Ingo Molnar <mingo@elte.hu>]

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

> > 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.
> 
> That's exactly what explains a dump I got yesterday. Thanks for 
> fixing! :)

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;