xfs: drop s_umount over opening the log and RT devices

Just like get_tree_bdev needs to drop s_umount when opening the main
device, we need to do the same for the xfs log and RT devices to avoid a
potential lock order reversal with s_unmount for the mark_dead path.

It might be preferable to just drop s_umount over ->fill_super entirely,
but that will require a fairly massive audit first, so we'll do the easy
version here first.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Message-Id: <20230802154131.2221419-12-hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/xfs/xfs_super.c

@@ -462,17 +462,24 @@ STATIC int
 xfs_open_devices(
 	struct xfs_mount	*mp)
 {
-	struct block_device	*ddev = mp->m_super->s_bdev;
+	struct super_block	*sb = mp->m_super;
+	struct block_device	*ddev = sb->s_bdev;
 	struct block_device	*logdev = NULL, *rtdev = NULL;
 	int			error;
 
+	/*
+	 * blkdev_put() can't be called under s_umount, see the comment
+	 * in get_tree_bdev() for more details
+	 */
+	up_write(&sb->s_umount);
+
 	/*
 	 * Open real time and log devices - order is important.
 	 */
 	if (mp->m_logname) {
 		error = xfs_blkdev_get(mp, mp->m_logname, &logdev);
 		if (error)
-			return error;
+			goto out_relock;
 	}
 
 	if (mp->m_rtname) {
@@ -510,7 +517,10 @@ xfs_open_devices(
 		mp->m_logdev_targp = mp->m_ddev_targp;
 	}
 
-	return 0;
+	error = 0;
+out_relock:
+	down_write(&sb->s_umount);
+	return error;
 
  out_free_rtdev_targ:
 	if (mp->m_rtdev_targp)
@@ -523,7 +533,7 @@ xfs_open_devices(
  out_close_logdev:
 	if (logdev && logdev != ddev)
 		blkdev_put(logdev, mp);
-	return error;
+	goto out_relock;
 }
 
 /*