usnic: fix wraparound sequence number issue

Sequence numbers will wrap around; it is not sufficient to check for
(seq-1) -- must use the SEQ_DIFF macro to properly handle the
wraparound.

This bug wasn't serious; it just meant we might retransmit one or two
extra times when retransmits were triggerd and the sequence numbers
wrapped around their sliding windows.

(cherry picked from commit open-mpi/ompi@d624e0d)

Author: jsquyres

opal/mca/btl/usnic/btl_usnic_ack.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2013-2015 Cisco Systems, Inc.  All rights reserved.
+ * Copyright (c) 2013-2016 Cisco Systems, Inc.  All rights reserved.
  * $COPYRIGHT$
  *
  * Additional copyrights may follow
@@ -207,8 +207,7 @@ opal_btl_usnic_ack_send(
     /* send the seq of the lowest item in the window that
        we've received */
     ack->ss_base.us_btl_header->ack_seq =
-        endpoint->endpoint_next_contig_seq_to_recv - 1;
-
+        SEQ_DIFF(endpoint->endpoint_next_contig_seq_to_recv, 1);
     ack->ss_len = sizeof(opal_btl_usnic_btl_header_t);
 
 #if MSGDEBUG1

opal/mca/btl/usnic/btl_usnic_ack.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2013-2015 Cisco Systems, Inc.  All rights reserved.
+ * Copyright (c) 2013-2016 Cisco Systems, Inc.  All rights reserved.
  * $COPYRIGHT$
  *
  * Additional copyrights may follow
@@ -92,7 +92,7 @@ opal_btl_usnic_piggyback_ack(
     if (endpoint->endpoint_ack_needed) {
         opal_btl_usnic_remove_from_endpoints_needing_ack(endpoint);
         sseg->ss_base.us_btl_header->ack_seq =
-            endpoint->endpoint_next_contig_seq_to_recv - 1;
+            SEQ_DIFF(endpoint->endpoint_next_contig_seq_to_recv, 1);
         sseg->ss_base.us_btl_header->ack_present = 1;
 #if MSGDEBUG1
         opal_output(0, "Piggy-backing ACK for sequence %"UDSEQ"\n",

opal/mca/btl/usnic/btl_usnic_stats.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2013-2015 Cisco Systems, Inc.  All rights reserved.
+ * Copyright (c) 2013-2016 Cisco Systems, Inc.  All rights reserved.
  * $COPYRIGHT$
  *
  * Additional copyrights may follow
@@ -145,8 +145,9 @@ void opal_btl_usnic_print_stats(
             /* Number of un-acked sends (i.e., sends for which we're
                still waiting for ACK) */
             send_unacked =
-                endpoint->endpoint_next_seq_to_send -
-                endpoint->endpoint_ack_seq_rcvd - 1;
+                SEQ_DIFF(endpoint->endpoint_next_seq_to_send,
+                         SEQ_DIFF(endpoint->endpoint_ack_seq_rcvd, 1));
+
             if (send_unacked > su_max) su_max = send_unacked;
             if (send_unacked < su_min) su_min = send_unacked;