btrfs: don't start transaction for scrub if the fs is mounted read-only

[BUG]
The following super simple script would crash btrfs at unmount time, if
CONFIG_BTRFS_ASSERT() is set.

 mkfs.btrfs -f $dev
 mount $dev $mnt
 xfs_io -f -c "pwrite 0 4k" $mnt/file
 umount $mnt
 mount -r ro $dev $mnt
 btrfs scrub start -Br $mnt
 umount $mnt

This will trigger the following ASSERT() introduced by commit
0a31daa4 ("btrfs: add assertion for empty list of transactions at
late stage of umount").

That patch is definitely not the cause, it just makes enough noise for
developers.

[CAUSE]
We will start transaction for the following call chain during scrub:

  scrub_enumerate_chunks()
  |- btrfs_inc_block_group_ro()
     |- btrfs_join_transaction()

However for RO mount, there is no running transaction at all, thus
btrfs_join_transaction() will start a new transaction.

Furthermore, since it's read-only mount, btrfs_sync_fs() will not call
btrfs_commit_super() to commit the new but empty transaction.

And leads to the ASSERT().

The bug has been there for a long time. Only the new ASSERT() makes it
noisy enough to be noticed.

[FIX]
For read-only scrub on read-only mount, there is no need to start a
transaction nor to allocate new chunks in btrfs_inc_block_group_ro().

Just do extra read-only mount check in btrfs_inc_block_group_ro(), and
if it's read-only, skip all chunk allocation and go inc_block_group_ro()
directly.

CC: stable@vger.kernel.org # 5.4+
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/block-group.c

@@ -2544,6 +2544,19 @@ int btrfs_inc_block_group_ro(struct btrfs_block_group *cache,
 	int ret;
 	bool dirty_bg_running;
 
+	/*
+	 * This can only happen when we are doing read-only scrub on read-only
+	 * mount.
+	 * In that case we should not start a new transaction on read-only fs.
+	 * Thus here we skip all chunk allocations.
+	 */
+	if (sb_rdonly(fs_info->sb)) {
+		mutex_lock(&fs_info->ro_block_group_mutex);
+		ret = inc_block_group_ro(cache, 0);
+		mutex_unlock(&fs_info->ro_block_group_mutex);
+		return ret;
+	}
+
 	do {
 		trans = btrfs_join_transaction(root);
 		if (IS_ERR(trans))