Staging: DST: Do not allow empty barriers.

Do not allow empty barriers or generic_make_request() ->  scsi_setup_fs_cmnd()
will explode
Signed-off-by: NEvgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/staging/dst/dcore.c

@@ -100,10 +100,33 @@ static void dst_node_set_size(struct dst_node *n)
 static int dst_request(struct request_queue *q, struct bio *bio)
 {
 	struct dst_node *n = q->queuedata;
+	int err = -EIO;
+
+	if (bio_empty_barrier(bio) && !q->prepare_discard_fn) {
+		/*
+		 * This is a dirty^Wnice hack, but if we complete this
+		 * operation with -EOPNOTSUPP like intended, XFS
+		 * will stuck and freeze the machine. This may be
+		 * not particulary XFS problem though, but it is the
+		 * only FS which sends empty barrier at umount time
+		 * I worked with.
+		 *
+		 * Empty barriers are not allowed anyway, see 51fd77bd9f512
+		 * for example, although later it was changed to bio_discard()
+		 * only, which does not work in this case.
+		 */
+		//err = -EOPNOTSUPP;
+		err = 0;
+		goto end_io;
+	}
 
 	bio_get(bio);
 
 	return dst_process_bio(n, bio);
+
+end_io:
+	bio_endio(bio, err);
+	return err;
 }
 
 /*