static noinline int join_transaction(struct btrfs_fs_info *fs_info,
				     unsigned int type)
{
	struct btrfs_transaction *cur_trans;

	spin_lock(&fs_info->trans_lock);
loop:
	/* The file system has been taken offline. No new transactions. */
	if (test_bit(BTRFS_FS_STATE_ERROR, &fs_info->fs_state)) {
		spin_unlock(&fs_info->trans_lock);
		return -EROFS;
	}

	cur_trans = fs_info->running_transaction;
	if (cur_trans) {
		if (TRANS_ABORTED(cur_trans)) {
			spin_unlock(&fs_info->trans_lock);
			return cur_trans->aborted;
		}
		if (btrfs_blocked_trans_types[cur_trans->state] & type) {
			spin_unlock(&fs_info->trans_lock);
			return -EBUSY;
		}
		refcount_inc(&cur_trans->use_count);
		atomic_inc(&cur_trans->num_writers);
		extwriter_counter_inc(cur_trans, type);
		spin_unlock(&fs_info->trans_lock);
		return 0;
	}
	spin_unlock(&fs_info->trans_lock);

	/*
	 * If we are ATTACH, we just want to catch the current transaction,
	 * and commit it. If there is no transaction, just return ENOENT.
	 */
	if (type == TRANS_ATTACH)
		return -ENOENT;

	/*
	 * JOIN_NOLOCK only happens during the transaction commit, so
	 * it is impossible that ->running_transaction is NULL
	 */
	BUG_ON(type == TRANS_JOIN_NOLOCK);

	cur_trans = kmalloc(sizeof(*cur_trans), GFP_NOFS);
	if (!cur_trans)
		return -ENOMEM;

	spin_lock(&fs_info->trans_lock);
	if (fs_info->running_transaction) {
		/*
		 * someone started a transaction after we unlocked.  Make sure
		 * to redo the checks above
		 */
		kfree(cur_trans);
		goto loop;
	} else if (test_bit(BTRFS_FS_STATE_ERROR, &fs_info->fs_state)) {
		spin_unlock(&fs_info->trans_lock);
		kfree(cur_trans);
		return -EROFS;
	}

	cur_trans->fs_info = fs_info;
	atomic_set(&cur_trans->pending_ordered, 0);
	init_waitqueue_head(&cur_trans->pending_wait);
	atomic_set(&cur_trans->num_writers, 1);
	extwriter_counter_init(cur_trans, type);
	init_waitqueue_head(&cur_trans->writer_wait);
	init_waitqueue_head(&cur_trans->commit_wait);
	cur_trans->state = TRANS_STATE_RUNNING;
	/*
	 * One for this trans handle, one so it will live on until we
	 * commit the transaction.
	 */
	refcount_set(&cur_trans->use_count, 2);
	cur_trans->flags = 0;
	cur_trans->start_time = ktime_get_seconds();

	memset(&cur_trans->delayed_refs, 0, sizeof(cur_trans->delayed_refs));

	cur_trans->delayed_refs.href_root = RB_ROOT_CACHED;
	cur_trans->delayed_refs.dirty_extent_root = RB_ROOT;
	atomic_set(&cur_trans->delayed_refs.num_entries, 0);

	/*
	 * although the tree mod log is per file system and not per transaction,
	 * the log must never go across transaction boundaries.
	 */
	smp_mb();
	if (!list_empty(&fs_info->tree_mod_seq_list))
		WARN(1, KERN_ERR "BTRFS: tree_mod_seq_list not empty when creating a fresh transaction\n");
	if (!RB_EMPTY_ROOT(&fs_info->tree_mod_log))
		WARN(1, KERN_ERR "BTRFS: tree_mod_log rb tree not empty when creating a fresh transaction\n");
	atomic64_set(&fs_info->tree_mod_seq, 0);

	spin_lock_init(&cur_trans->delayed_refs.lock);

	INIT_LIST_HEAD(&cur_trans->pending_snapshots);
	INIT_LIST_HEAD(&cur_trans->dev_update_list);
	INIT_LIST_HEAD(&cur_trans->switch_commits);
	INIT_LIST_HEAD(&cur_trans->dirty_bgs);
	INIT_LIST_HEAD(&cur_trans->io_bgs);
	INIT_LIST_HEAD(&cur_trans->dropped_roots);
	mutex_init(&cur_trans->cache_write_mutex);
	spin_lock_init(&cur_trans->dirty_bgs_lock);
	INIT_LIST_HEAD(&cur_trans->deleted_bgs);
	spin_lock_init(&cur_trans->dropped_roots_lock);
	INIT_LIST_HEAD(&cur_trans->releasing_ebs);
	spin_lock_init(&cur_trans->releasing_ebs_lock);
	list_add_tail(&cur_trans->list, &fs_info->trans_list);
	extent_io_tree_init(fs_info, &cur_trans->dirty_pages,
			IO_TREE_TRANS_DIRTY_PAGES, fs_info->btree_inode);
	extent_io_tree_init(fs_info, &cur_trans->pinned_extents,
			IO_TREE_FS_PINNED_EXTENTS, NULL);
	fs_info->generation++;
	cur_trans->transid = fs_info->generation;
	fs_info->running_transaction = cur_trans;
	cur_trans->aborted = 0;
	spin_unlock(&fs_info->trans_lock);

	return 0;
}