btrfs: simplify sync/async submission in btrfs_submit_data_write_bio

btrfs_submit_data_write_bio special cases the reloc root because the
checksums are preloaded, but only does so for the !sync case.  The sync
case can't happen for data relocation, but just handling it more generally
significantly simplifies the logic.
Reviewed-by: NNikolay Borisov <nborisov@suse.com>
Tested-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -2664,28 +2664,25 @@ void btrfs_submit_data_write_bio(struct inode *inode, struct bio *bio, int mirro
 	}
 
 	/*
-	 * Rules for async/sync submit:
-	 *   a) write without checksum:			sync submit
-	 *   b) write with checksum:
-	 *      b-1) if bio is issued by fsync:		sync submit
-	 *           (sync_writers != 0)
-	 *      b-2) if root is reloc root:		sync submit
-	 *           (only in case of buffered IO)
-	 *      b-3) otherwise:				async submit
+	 * If we need to checksum, and the I/O is not issued by fsync and
+	 * friends, that is ->sync_writers != 0, defer the submission to a
+	 * workqueue to parallelize it.
+	 *
+	 * Csum items for reloc roots have already been cloned at this point,
+	 * so they are handled as part of the no-checksum case.
 	 */
 	if (!(bi->flags & BTRFS_INODE_NODATASUM) &&
-	    !test_bit(BTRFS_FS_STATE_NO_CSUMS, &fs_info->fs_state)) {
-		if (atomic_read(&bi->sync_writers)) {
-			ret = btrfs_csum_one_bio(bi, bio, (u64)-1, false);
-			if (ret)
-				goto out;
-		} else if (btrfs_is_data_reloc_root(bi->root)) {
-			; /* Csum items have already been cloned */
-		} else {
+	    !test_bit(BTRFS_FS_STATE_NO_CSUMS, &fs_info->fs_state) &&
+	    !btrfs_is_data_reloc_root(bi->root)) {
+		if (!atomic_read(&bi->sync_writers)) {
 			ret = btrfs_wq_submit_bio(inode, bio, mirror_num, 0,
 						  btrfs_submit_bio_start);
 			goto out;
 		}
+
+		ret = btrfs_csum_one_bio(bi, bio, (u64)-1, false);
+		if (ret)
+			goto out;
 	}
 	btrfs_submit_bio(fs_info, bio, mirror_num);
 	return;