libceph: separate CRC calculation from byte swapping

Calculate CRC in a separate step from rearranging the byte order
of the result, to improve clarity and readability.

Use offsetof() to determine the number of bytes to include in the
CRC calculation.

In read_partial_message(), switch which value gets byte-swapped,
since the just-computed CRC is already likely to be in a register.
Signed-off-by: Alex Elder <elder@dreamhost.com>
Signed-off-by: Sage Weil <sage@newdream.net>

net/ceph/messenger.c

@@ -521,6 +521,7 @@ static void prepare_write_message_footer(struct ceph_connection *con)
 static void prepare_write_message(struct ceph_connection *con)
 {
 	struct ceph_msg *m;
+	u32 crc;
 
 	ceph_con_out_kvec_reset(con);
 	con->out_kvec_is_msg = true;
@@ -569,17 +570,17 @@ static void prepare_write_message(struct ceph_connection *con)
 			m->middle->vec.iov_base);
 
 	/* fill in crc (except data pages), footer */
-	con->out_msg->hdr.crc =
-		cpu_to_le32(crc32c(0, &m->hdr,
-				      sizeof(m->hdr) - sizeof(m->hdr.crc)));
+	crc = crc32c(0, &m->hdr, offsetof(struct ceph_msg_header, crc));
+	con->out_msg->hdr.crc = cpu_to_le32(crc);
 	con->out_msg->footer.flags = CEPH_MSG_FOOTER_COMPLETE;
-	con->out_msg->footer.front_crc =
-		cpu_to_le32(crc32c(0, m->front.iov_base, m->front.iov_len));
-	if (m->middle)
-		con->out_msg->footer.middle_crc =
-			cpu_to_le32(crc32c(0, m->middle->vec.iov_base,
-					   m->middle->vec.iov_len));
-	else
+
+	crc = crc32c(0, m->front.iov_base, m->front.iov_len);
+	con->out_msg->footer.front_crc = cpu_to_le32(crc);
+	if (m->middle) {
+		crc = crc32c(0, m->middle->vec.iov_base,
+				m->middle->vec.iov_len);
+		con->out_msg->footer.middle_crc = cpu_to_le32(crc);
+	} else
 		con->out_msg->footer.middle_crc = 0;
 	con->out_msg->footer.data_crc = 0;
 	dout("prepare_write_message front_crc %u data_crc %u\n",
@@ -875,12 +876,13 @@ static int write_partial_msg_pages(struct ceph_connection *con)
 			    total_max_write);
 
 		if (do_crc && !con->out_msg_pos.did_page_crc) {
+			u32 crc;
 			void *base = kaddr + con->out_msg_pos.page_pos;
 			u32 tmpcrc = le32_to_cpu(con->out_msg->footer.data_crc);
 
 			BUG_ON(kaddr == NULL);
-			con->out_msg->footer.data_crc =
-				cpu_to_le32(crc32c(tmpcrc, base, len));
+			crc = crc32c(tmpcrc, base, len);
+			con->out_msg->footer.data_crc = cpu_to_le32(crc);
 			con->out_msg_pos.did_page_crc = true;
 		}
 		ret = kernel_sendpage(con->sock, page,
@@ -1650,8 +1652,9 @@ static int read_partial_message(struct ceph_connection *con)
 		con->in_base_pos += ret;
 		if (con->in_base_pos == sizeof(con->in_hdr)) {
 			u32 crc = crc32c(0, &con->in_hdr,
-				 sizeof(con->in_hdr) - sizeof(con->in_hdr.crc));
-			if (crc != le32_to_cpu(con->in_hdr.crc)) {
+					offsetof(struct ceph_msg_header, crc));
+
+			if (cpu_to_le32(crc) != con->in_hdr.crc) {
 				pr_err("read_partial_message bad hdr "
 				       " crc %u != expected %u\n",
 				       crc, con->in_hdr.crc);