[PATCH v4 03/16] fsnotify: fix wrong lockdep annotations

[Amir Goldstein <amir73il@gmail.com>]

Commit 6960b0d909cd ("fsnotify: change locking order") changed some
of the mark_mutex locks in direct reclaim path to use:
  mutex_lock_nested(&group->mark_mutex, SINGLE_DEPTH_NESTING);

This change is explained:
 "...It uses nested locking to avoid deadlock in case we do the final
  iput() on an inode which still holds marks and thus would take the
  mutex again when calling fsnotify_inode_delete() in destroy_inode()."

The problem is that the mutex_lock_nested() is not a nested lock at
all. In fact, it has the opposite effect of preventing lockdep from
warning about a very possible deadlock.

Due to these wrong annotations, a deadlock that was introduced with
nfsd filecache in kernel v5.4 went unnoticed in v5.4.y for over two
years until it was reported recently by Khazhismel Kumykov, only to
find out that the deadlock was already fixed in kernel v5.5.

Fix the wrong lockdep annotations.

Cc: Khazhismel Kumykov <khazhy@google.com>
Fixes: 6960b0d909cd ("fsnotify: change locking order")
Link: https://lore.kernel.org/r/20220321112310.vpr7oxro2xkz5llh@quack3.lan/
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/notify/mark.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/notify/mark.c b/fs/notify/mark.c
index 4853184f7dde..c86982be2d50 100644
--- a/fs/notify/mark.c
+++ b/fs/notify/mark.c
@@ -452,7 +452,7 @@ void fsnotify_free_mark(struct fsnotify_mark *mark)
 void fsnotify_destroy_mark(struct fsnotify_mark *mark,
 			   struct fsnotify_group *group)
 {
-	mutex_lock_nested(&group->mark_mutex, SINGLE_DEPTH_NESTING);
+	mutex_lock(&group->mark_mutex);
 	fsnotify_detach_mark(mark);
 	mutex_unlock(&group->mark_mutex);
 	fsnotify_free_mark(mark);
@@ -770,7 +770,7 @@ void fsnotify_clear_marks_by_group(struct fsnotify_group *group,
 	 * move marks to free to to_free list in one go and then free marks in
 	 * to_free list one by one.
 	 */
-	mutex_lock_nested(&group->mark_mutex, SINGLE_DEPTH_NESTING);
+	mutex_lock(&group->mark_mutex);
 	list_for_each_entry_safe(mark, lmark, &group->marks_list, g_list) {
 		if (mark->connector->type == obj_type)
 			list_move(&mark->g_list, &to_free);
@@ -779,7 +779,7 @@ void fsnotify_clear_marks_by_group(struct fsnotify_group *group,
 
 clear:
 	while (1) {
-		mutex_lock_nested(&group->mark_mutex, SINGLE_DEPTH_NESTING);
+		mutex_lock(&group->mark_mutex);
 		if (list_empty(head)) {
 			mutex_unlock(&group->mark_mutex);
 			break;
-- 
2.35.1