document rwsem_release() in sb_wait_write()

Not only we need to avoid the warning from lockdep_sys_exit(), the
caller of freeze_super() can never release this lock. Another thread
can do this, so there is another reason for rwsem_release().

Plus the comment should explain why we have to fool lockdep.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Jan Kara <jack@suse.com>

fs/super.c

@@ -1236,11 +1236,17 @@ static void sb_wait_write(struct super_block *sb, int level)
 {
 	s64 writers;
 
+	rwsem_acquire(&sb->s_writers.lock_map[level-1], 0, 0, _THIS_IP_);
 	/*
-	 * We just cycle-through lockdep here so that it does not complain
-	 * about returning with lock to userspace
+	 * We are going to return to userspace and forget about this lock, the
+	 * ownership goes to the caller of thaw_super() which does unlock.
+	 *
+	 * FIXME: we should do this before return from freeze_super() after we
+	 * called sync_filesystem(sb) and s_op->freeze_fs(sb), and thaw_super()
+	 * should re-acquire these locks before s_op->unfreeze_fs(sb). However
+	 * this leads to lockdep false-positives, so currently we do the early
+	 * release right after acquire.
 	 */
-	rwsem_acquire(&sb->s_writers.lock_map[level-1], 0, 0, _THIS_IP_);
 	rwsem_release(&sb->s_writers.lock_map[level-1], 1, _THIS_IP_);
 
 	do {