cgroup: fix lockdep warning for event_control

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@google.com>
Signed-off-by: Aaron Durbin <adurbin@google.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 8 insertions, 3 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index c02b05560d10..589433f7a74b 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4277,6 +4277,7 @@ 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);
@@ -4331,16 +4332,20 @@ 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;
 }