rds: check for excessive looping in rds_send_xmit

Original commit from 2011 updated to include a change by
Yuval Shaia <yuval.shaia@oracle.com>
that adds a new statistic counter "send_stuck_rm"
to capture the messages looping exessively
in the send path.
Signed-off-by: Gerd Rausch <gerd.rausch@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/rds/rds.h

@@ -717,7 +717,7 @@ struct rds_statistics {
 	uint64_t	s_cong_send_blocked;
 	uint64_t	s_recv_bytes_added_to_socket;
 	uint64_t	s_recv_bytes_removed_from_socket;
-
+	uint64_t	s_send_stuck_rm;
 };
 
 /* af_rds.c */

net/rds/send.c

@@ -145,6 +145,7 @@ int rds_send_xmit(struct rds_conn_path *cp)
 	LIST_HEAD(to_be_dropped);
 	int batch_count;
 	unsigned long send_gen = 0;
+	int same_rm = 0;
 
 restart:
 	batch_count = 0;
@@ -200,6 +201,17 @@ int rds_send_xmit(struct rds_conn_path *cp)
 
 		rm = cp->cp_xmit_rm;
 
+		if (!rm) {
+			same_rm = 0;
+		} else {
+			same_rm++;
+			if (same_rm >= 4096) {
+				rds_stats_inc(s_send_stuck_rm);
+				ret = -EAGAIN;
+				break;
+			}
+		}
+
 		/*
 		 * If between sending messages, we can send a pending congestion
 		 * map update.

net/rds/stats.c

@@ -78,6 +78,7 @@ static const char *const rds_stat_names[] = {
 	"cong_send_blocked",
 	"recv_bytes_added_to_sock",
 	"recv_bytes_freed_fromsock",
+	"send_stuck_rm",
 };
 
 void rds_stats_info_copy(struct rds_info_iterator *iter,