fsnotify_recalc_mask() fails to handle the return value of
__fsnotify_recalc_mask(), which may return an inode pointer that needs
to be released via fsnotify_drop_object() when the connector's HAS_IREF
flag transitions from set to cleared.

This manifests as a hung task with the following call trace:

  INFO: task umount:1234 blocked for more than 120 seconds.
  Call Trace:
   __schedule
   schedule
   fsnotify_sb_delete
   generic_shutdown_super
   kill_anon_super
   cleanup_mnt
   task_work_run
   do_exit
   do_group_exit

The race window that triggers the iref leak:

  Thread A (adding mark)              Thread B (removing mark)
  ──────────────────────              ────────────────────────
  fsnotify_add_mark_locked():
    fsnotify_add_mark_list():
      spin_lock(conn->lock)
      add mark_B(evictable) to list
      spin_unlock(conn->lock)
    return

    /* ---- gap: no lock held ---- */

                                      fsnotify_detach_mark(mark_A):
                                        spin_lock(mark_A->lock)
                                        clear ATTACHED flag on mark_A
                                        spin_unlock(mark_A->lock)

    fsnotify_recalc_mask():
      spin_lock(conn->lock)
      __fsnotify_recalc_mask():
        /* mark_A skipped: ATTACHED cleared */
        /* only mark_B(evictable) remains */
        want_iref = false
        has_iref = true  /* not yet cleared */
        -> HAS_IREF transitions true -> false
        -> returns inode pointer
      spin_unlock(conn->lock)
      /* BUG: return value discarded!
       * iput() and fsnotify_put_sb_watched_objects()
       * are never called */

Fix this by capturing the return value of __fsnotify_recalc_mask() and
passing it to fsnotify_drop_object() after releasing the spinlock, which
is the same pattern used in fsnotify_put_mark().

Fixes: c3638b5b1374 ("fsnotify: allow adding an inode mark without pinning inode")
Signed-off-by: Xin Yin <yinxin.x@bytedance.com>
---
 fs/notify/mark.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/notify/mark.c b/fs/notify/mark.c
index c2ed5b11b0fe6..cc93fcc2c5a9c 100644
--- a/fs/notify/mark.c
+++ b/fs/notify/mark.c
@@ -283,6 +283,8 @@ static void fsnotify_conn_set_children_dentry_flags(
 	fsnotify_set_children_dentry_flags(fsnotify_conn_inode(conn));
 }
 
+static void fsnotify_drop_object(unsigned int type, void *objp);
+
 /*
  * Calculate mask of events for a list of marks. The caller must make sure
  * connector and connector->obj cannot disappear under us.  Callers achieve
@@ -292,15 +294,19 @@ static void fsnotify_conn_set_children_dentry_flags(
 void fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
 {
 	bool update_children;
+	unsigned int type;
+	void *objp;
 
 	if (!conn)
 		return;
 
 	spin_lock(&conn->lock);
 	update_children = !fsnotify_conn_watches_children(conn);
-	__fsnotify_recalc_mask(conn);
+	objp = __fsnotify_recalc_mask(conn);
+	type = conn->type;
 	update_children &= fsnotify_conn_watches_children(conn);
 	spin_unlock(&conn->lock);
+	fsnotify_drop_object(type, objp);
 	/*
 	 * Set children's PARENT_WATCHED flags only if parent started watching.
 	 * When parent stops watching, we clear false positive PARENT_WATCHED
-- 
2.20.1