On Wed, Jul 27, 2011 at 11:52:18PM +0200, Thomas Gleixner wrote:
> On Tue, 26 Jul 2011, Yong Zhang wrote:
>
> > When reading the code, I'm afraid there is a race between migrate_disable()
> > and cpu hotplug. A scenario will like below:
> >
> > CPU0 CPU1
> > _cpu_down();
> > cpu_unplug_begin();
> > wait_for_completion()
> > sync_unplug_thread();
> > complete();
> > race_window? /*
> > * migrate_disable() will
> > * not take effect since
> > * hotplug is in progress
> > */
>
> Rightfully so. The caller will just block on the cpu_hotplug.lock
> mutex until the unplug operation will be done.
Yup, just notice the mutex_lock/mutex_unlock in pin_current_cpu().
But if the caller block on mutex_lock() then waked up, it's possible
that it's been migrated to another cpu. So in the 'retry' loop, we
should reget hotplug_pcp.
Code like below?
Thanks,
Yong
---
From: Yong Zhang <yong.zhang0@xxxxxxxxx>
Subject: [PATCH] kernel/cpu.c: re-acquire hotplug_pcp when pin_current_cpu() retry
When 'retry' happen, it's possible that the task has been
migrated to other cpu, and 'hotplug_pcp' is still pointing to
the stale one.
Signed-off-by: Yong Zhang <yong.zhang0@xxxxxxxxx>
---
kernel/cpu.c | 4 +++-
1 files changed, 3 insertions(+), 1 deletions(-)
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 53dd7ad..5f2382a 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -75,9 +75,11 @@ static DEFINE_PER_CPU(struct hotplug_pcp, hotplug_pcp);
*/
void pin_current_cpu(void)
{
- struct hotplug_pcp *hp = &__get_cpu_var(hotplug_pcp);
+ struct hotplug_pcp *hp;
retry:
+ hp = &__get_cpu_var(hotplug_pcp);
+
if (!hp->unplug || hp->refcount || preempt_count() > 1 ||
hp->unplug == current || (current->flags & PF_STOMPER)) {
hp->refcount++;
--
1.7.4.1
--
Only stand for myself
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