[PATCH] fs/kernfs: Fix lockdep warning in kernfs_active()

[Chengming Zhou <zhouchengming@bytedance.com>]

syzbot found a lockdep warning in kernfs_find_and_get_node_by_id(),
bisected to the commit c25491747b21 ("kernfs: Add KERNFS_REMOVING flags"),
which didn't hold kernfs_rwsem before call kernfs_active(kn).

Since kernfs_find_and_get_node_by_id() doesn't have to get active count
of kn, only need to get a stable refcount of kn, so it should be enough
to just check kn has been KERNFS_ACTIVATED.

Reported-by: syzbot+2fdf66e68f5f882c1074@syzkaller.appspotmail.com
Fixes: c25491747b21 ("kernfs: Add KERNFS_REMOVING flags")
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
---
 fs/kernfs/dir.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 6acd9c3d4cff..08f0f1570cd7 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -705,7 +705,13 @@ struct kernfs_node *kernfs_find_and_get_node_by_id(struct kernfs_root *root,
 			goto err_unlock;
 	}
 
-	if (unlikely(!kernfs_active(kn) || !atomic_inc_not_zero(&kn->count)))
+	/*
+	 * ACTIVATED is protected with kernfs_mutex but it was clear when
+	 * @kn was added to idr and we just wanna see it set.  No need to
+	 * grab kernfs_mutex.
+	 */
+	if (unlikely(!(kn->flags & KERNFS_ACTIVATED) ||
+		     !atomic_inc_not_zero(&kn->count)))
 		goto err_unlock;
 
 	spin_unlock(&kernfs_idr_lock);
-- 
2.37.2