[PATCH 1/2] cgroup: fix lockdep warning for event_control

[PATCH 1/2] cgroup: fix lockdep warning for event_control

[Greg Thelen]

The cgroup_event_wake() function is called with the wait queue head
locked and it takes cgrp->event_list_lock. However, in cgroup_rmdir()
remove_wait_queue() was being called after taking
cgrp->event_list_lock.  Correct the lock ordering by using a temporary
list to obtain the event list to remove from the wait queue.

Signed-off-by: Greg Thelen <gthelen-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Signed-off-by: Aaron Durbin <adurbin-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
---
 kernel/cgroup.c |   11 ++++++++---
 1 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index ece60d4..a0d75bb 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4276,6 +4276,7 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
 	DEFINE_WAIT(wait);
 	struct cgroup_event *event, *tmp;
 	struct cgroup_subsys *ss;
+	LIST_HEAD(tmp_list);
 
 	lockdep_assert_held(&d->d_inode->i_mutex);
 	lockdep_assert_held(&cgroup_mutex);
@@ -4330,16 +4331,20 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
 	/*
 	 * Unregister events and notify userspace.
 	 * Notify userspace about cgroup removing only after rmdir of cgroup
-	 * directory to avoid race between userspace and kernelspace
+	 * directory to avoid race between userspace and kernelspace. Use
+	 * a temporary list to avoid a deadlock with cgroup_event_wake(). Since
+	 * cgroup_event_wake() is called with the wait queue head locked,
+	 * remove_wait_queue() cannot be called while holding event_list_lock.
 	 */
 	spin_lock(&cgrp->event_list_lock);
-	list_for_each_entry_safe(event, tmp, &cgrp->event_list, list) {
+	list_splice_init(&cgrp->event_list, &tmp_list);
+	spin_unlock(&cgrp->event_list_lock);
+	list_for_each_entry_safe(event, tmp, &tmp_list, list) {
 		list_del(&event->list);
 		remove_wait_queue(event->wqh, &event->wait);
 		eventfd_signal(event->eventfd, 1);
 		schedule_work(&event->remove);
 	}
-	spin_unlock(&cgrp->event_list_lock);
 
 	return 0;
 }
-- 
1.7.7.3

[PATCH 2/2] cgroup: list_del_init() on removed events

[Greg Thelen]

Use list_del_init() rather than list_del() to remove events from
cgrp->event_list.  No functional change.  This is just defensive
coding.

Signed-off-by: Greg Thelen <gthelen-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
---
 kernel/cgroup.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index a0d75bb..44c319d 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3766,7 +3766,7 @@ static int cgroup_event_wake(wait_queue_t *wait, unsigned mode,
 	if (flags & POLLHUP) {
 		__remove_wait_queue(event->wqh, &event->wait);
 		spin_lock(&cgrp->event_list_lock);
-		list_del(&event->list);
+		list_del_init(&event->list);
 		spin_unlock(&cgrp->event_list_lock);
 		/*
 		 * We are in atomic context, but cgroup_event_remove() may
@@ -4340,7 +4340,7 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
 	list_splice_init(&cgrp->event_list, &tmp_list);
 	spin_unlock(&cgrp->event_list_lock);
 	list_for_each_entry_safe(event, tmp, &tmp_list, list) {
-		list_del(&event->list);
+		list_del_init(&event->list);
 		remove_wait_queue(event->wqh, &event->wait);
 		eventfd_signal(event->eventfd, 1);
 		schedule_work(&event->remove);
-- 
1.7.7.3

Re: [PATCH 2/2] cgroup: list_del_init() on removed events

[Tejun Heo]

On Wed, Nov 28, 2012 at 01:50:45PM -0800, Greg Thelen wrote:
> Use list_del_init() rather than list_del() to remove events from
> cgrp->event_list.  No functional change.  This is just defensive
> coding.
> 
> Signed-off-by: Greg Thelen <gthelen-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>

Applied 1-2 to cgroup/for-3.8.  Thanks!

-- 
tejun