nbd: clean up return value checking of sock_xmit()

Check if sock_xmit() return 0 is useless because it'll never return
0, comment it and remove such checkings.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20210916093350.1410403-6-yukuai3@huawei.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -495,7 +495,8 @@ static enum blk_eh_timer_return nbd_xmit_timeout(struct request *req,
 }
 
 /*
- *  Send or receive packet.
+ *  Send or receive packet. Return a positive value on success and
+ *  negtive value on failue, and never return 0.
  */
 static int sock_xmit(struct nbd_device *nbd, int index, int send,
 		     struct iov_iter *iter, int msg_flags, int *sent)
@@ -621,7 +622,7 @@ static int nbd_send_cmd(struct nbd_device *nbd, struct nbd_cmd *cmd, int index)
 	result = sock_xmit(nbd, index, 1, &from,
 			(type == NBD_CMD_WRITE) ? MSG_MORE : 0, &sent);
 	trace_nbd_header_sent(req, handle);
-	if (result <= 0) {
+	if (result < 0) {
 		if (was_interrupted(result)) {
 			/* If we havne't sent anything we can just return BUSY,
 			 * however if we have sent something we need to make
@@ -665,7 +666,7 @@ static int nbd_send_cmd(struct nbd_device *nbd, struct nbd_cmd *cmd, int index)
 				skip = 0;
 			}
 			result = sock_xmit(nbd, index, 1, &from, flags, &sent);
-			if (result <= 0) {
+			if (result < 0) {
 				if (was_interrupted(result)) {
 					/* We've already sent the header, we
 					 * have no choice but to set pending and
@@ -717,7 +718,7 @@ static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index)
 	reply.magic = 0;
 	iov_iter_kvec(&to, READ, &iov, 1, sizeof(reply));
 	result = sock_xmit(nbd, index, 0, &to, MSG_WAITALL, NULL);
-	if (result <= 0) {
+	if (result < 0) {
 		if (!nbd_disconnected(config))
 			dev_err(disk_to_dev(nbd->disk),
 				"Receive control failed (result %d)\n", result);
@@ -788,7 +789,7 @@ static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index)
 		rq_for_each_segment(bvec, req, iter) {
 			iov_iter_bvec(&to, READ, &bvec, 1, bvec.bv_len);
 			result = sock_xmit(nbd, index, 0, &to, MSG_WAITALL, NULL);
-			if (result <= 0) {
+			if (result < 0) {
 				dev_err(disk_to_dev(nbd->disk), "Receive data failed (result %d)\n",
 					result);
 				/*
@@ -1234,7 +1235,7 @@ static void send_disconnects(struct nbd_device *nbd)
 		iov_iter_kvec(&from, WRITE, &iov, 1, sizeof(request));
 		mutex_lock(&nsock->tx_lock);
 		ret = sock_xmit(nbd, i, 1, &from, 0, NULL);
-		if (ret <= 0)
+		if (ret < 0)
 			dev_err(disk_to_dev(nbd->disk),
 				"Send disconnect failed %d\n", ret);
 		mutex_unlock(&nsock->tx_lock);