btrfs: don't commit the super block when unmounting a shutdown filesystem

When unmounting a filesystem we will try, among many other things, to
commit the super block. On a filesystem that was shutdown, though, this
will always fail with -EROFS as writes are forbidden on this context;
and an error will be reported.

Don't commit the super block on this situation, which should be fine as
the filesystem is frozen before shutdown and, therefore, it should be at
a consistent state.

Signed-off-by: Miquel Sabaté Solà <mssola@mssola.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: mssola

fs/btrfs/disk-io.c

@@ -4397,9 +4397,17 @@ void __cold close_ctree(struct btrfs_fs_info *fs_info)
 		 */
 		btrfs_flush_workqueue(fs_info->delayed_workers);
 
-		ret = btrfs_commit_super(fs_info);
-		if (ret)
-			btrfs_err(fs_info, "commit super ret %d", ret);
+		/*
+		 * If the filesystem is shutdown, then an attempt to commit the
+		 * super block (or any write) will just fail. Since we freeze
+		 * the filesystem before shutting it down, the filesystem is in
+		 * a consistent state and we don't need to commit super blocks.
+		 */
+		if (!btrfs_is_shutdown(fs_info)) {
+			ret = btrfs_commit_super(fs_info);
+			if (ret)
+				btrfs_err(fs_info, "commit super block returned %d", ret);
+		}
 	}
 
 	kthread_stop(fs_info->transaction_kthread);