btrfs: simplify commit logic in try_flush_qgroup

It's no longer expected to call this function with an open transaction
so all the workarounds concerning this can be removed. In fact it'll
constitute a bug to call this function with a transaction already held
so WARN in this case.
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/qgroup.c

@@ -3543,37 +3543,19 @@ static int try_flush_qgroup(struct btrfs_root *root)
 {
 	struct btrfs_trans_handle *trans;
 	int ret;
-	bool can_commit = true;
 
-	/*
-	 * If current process holds a transaction, we shouldn't flush, as we
-	 * assume all space reservation happens before a transaction handle is
-	 * held.
-	 *
-	 * But there are cases like btrfs_delayed_item_reserve_metadata() where
-	 * we try to reserve space with one transction handle already held.
-	 * In that case we can't commit transaction, but at least try to end it
-	 * and hope the started data writes can free some space.
-	 */
-	if (current->journal_info &&
-	    current->journal_info != BTRFS_SEND_TRANS_STUB)
-		can_commit = false;
+	/* Can't hold an open transaction or we run the risk of deadlocking */
+	ASSERT(current->journal_info == NULL ||
+	       current->journal_info == BTRFS_SEND_TRANS_STUB);
+	if (WARN_ON(current->journal_info &&
+		    current->journal_info != BTRFS_SEND_TRANS_STUB))
+		return 0;
 
 	/*
 	 * We don't want to run flush again and again, so if there is a running
 	 * one, we won't try to start a new flush, but exit directly.
 	 */
 	if (test_and_set_bit(BTRFS_ROOT_QGROUP_FLUSHING, &root->state)) {
-		/*
-		 * We are already holding a transaction, thus we can block other
-		 * threads from flushing.  So exit right now. This increases
-		 * the chance of EDQUOT for heavy load and near limit cases.
-		 * But we can argue that if we're already near limit, EDQUOT is
-		 * unavoidable anyway.
-		 */
-		if (!can_commit)
-			return 0;
-
 		wait_event(root->qgroup_flush_wait,
 			!test_bit(BTRFS_ROOT_QGROUP_FLUSHING, &root->state));
 		return 0;
@@ -3590,10 +3572,7 @@ static int try_flush_qgroup(struct btrfs_root *root)
 		goto out;
 	}
 
-	if (can_commit)
-		ret = btrfs_commit_transaction(trans);
-	else
-		ret = btrfs_end_transaction(trans);
+	ret = btrfs_commit_transaction(trans);
 out:
 	clear_bit(BTRFS_ROOT_QGROUP_FLUSHING, &root->state);
 	wake_up(&root->qgroup_flush_wait);