Re: [PATCH] cgroup: fix lockdep warning for event_control

On Wed, Nov 28 2012, Tejun Heo wrote:

Hello, Greg.

On Wed, Nov 28, 2012 at 12:15:42PM -0800, Greg Thelen wrote:

quoted

@@ -4276,6 +4276,7 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
 	DEFINE_WAIT(wait);
 	struct cgroup_event *event, *tmp;
 	struct cgroup_subsys *ss;
+	struct list_head tmp_list;

LIST_HEAD(tmp_list);

quoted

 
 	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_replace_init(&cgrp->event_list, &tmp_list);

list_splice_init();

would be more conventional, I think.

I used list_replace_init() because it avoided unnecessary init of
tmp_list.  But I have no objection to list_splice_init().  I'll repost
with list_splice_init().

quoted

+	spin_unlock(&cgrp->event_list_lock);
+	list_for_each_entry_safe(event, tmp, &tmp_list, list) {
 		list_del(&event->list);

Maybe convert this to list_del_init() while at it?

I assume this isn't technically required, but defensive.  Seems like we
should also convert the list_del(&event->list) in cgroup_event_wake()
for consistency.  I'll send these in separate patch.

quoted

 		remove_wait_queue(event->wqh, &event->wait);
 		eventfd_signal(event->eventfd, 1);
 		schedule_work(&event->remove);
 	}
-	spin_unlock(&cgrp->event_list_lock);

Thanks.