autofs4: correct offset mount expire check

When checking a directory tree in autofs_tree_busy() we can incorrectly
decide that the tree isn't busy.  This happens for the case of an active
offset mount as autofs4_follow_mount() follows past the active offset
mount, which has an open file handle used for expires, causing the file
handle not to count toward the busyness check.
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/autofs4/expire.c

@@ -56,13 +56,24 @@ static int autofs4_mount_busy(struct vfsmount *mnt, struct dentry *dentry)
 	mntget(mnt);
 	dget(dentry);
 
-	if (!autofs4_follow_mount(&mnt, &dentry))
+	if (!follow_down(&mnt, &dentry))
 		goto done;
 
+	if (is_autofs4_dentry(dentry)) {
+		struct autofs_sb_info *sbi = autofs4_sbi(dentry->d_sb);
+
 		/* This is an autofs submount, we can't expire it */
-	if (is_autofs4_dentry(dentry))
+		if (sbi->type == AUTOFS_TYPE_INDIRECT)
 			goto done;
 
+		/*
+		 * Otherwise it's an offset mount and we need to check
+		 * if we can umount its mount, if there is one.
+		 */
+		if (!d_mountpoint(dentry))
+			goto done;
+	}
+
 	/* Update the expiry counter if fs is busy */
 	if (!may_umount_tree(mnt)) {
 		struct autofs_info *ino = autofs4_dentry_ino(top);