==== //depot/user/rwatson/netperf/sys/netinet/tcp_subr.c#33 - /home/rwatson/p4/user/rwatson/netperf/sys/netinet/tcp_subr.c ====
@@ -233,6 +233,7 @@
 
 static uma_zone_t tcpcb_zone;
 static uma_zone_t tcptw_zone;
+static struct mtx isn_mtx;
 struct callout isn_callout;
 
 /*
@@ -253,6 +254,7 @@
 	tcp_rexmit_slop = TCPTV_CPU_VAR;
 
 	INP_INFO_LOCK_INIT(&tcbinfo, "tcp");
+	mtx_init(&isn_mtx, "isn_mtx", NULL, MTX_DEF);
 	LIST_INIT(&tcb);
 	tcbinfo.listhead = &tcb;
 	TUNABLE_INT_FETCH("net.inet.tcp.tcbhashsize", &hashsize);
@@ -1292,8 +1294,9 @@
  * as reseeding should not be necessary.
  *
  * Locking of the global variables isn_secret, isn_last_reseed, isn_offset,
- * isn_offset_old, and isn_ctx is performed using the TCP pcbinfo lock.  In
- * general, this means holding an exclusive (write) lock.
+ * isn_offset_old, and isn_ctx is performed using isn_mtx.  Originally we
+ * protected it using the TCP pcbinfo lock, but we saw too much contention
+ * with the pcbinfo lock when running the callout.
  */
 
 #define ISN_BYTES_PER_SECOND 1048576
@@ -1315,6 +1318,7 @@
 	INP_INFO_WLOCK_ASSERT(&tcbinfo);
 	INP_LOCK_ASSERT(tp->t_inpcb);
 
+	mtx_lock(&isn_mtx);
 	/* Seed if this is the first use, reseed if requested. */
 	if ((isn_last_reseed == 0) || ((tcp_isn_reseed_interval > 0) &&
 	     (((u_int)isn_last_reseed + (u_int)tcp_isn_reseed_interval*hz)
@@ -1347,6 +1351,7 @@
 	isn_offset += ISN_STATIC_INCREMENT +
 		(arc4random() & ISN_RANDOM_INCREMENT);
 	new_isn += isn_offset;
+	mtx_unlock(&isn_mtx);
 	return (new_isn);
 }
 
@@ -1361,7 +1366,7 @@
 {
 	u_int32_t projected_offset;
 
-	INP_INFO_WLOCK(&tcbinfo);
+	mtx_lock(&isn_mtx);
 	projected_offset = isn_offset_old + ISN_BYTES_PER_SECOND / 100;
 
 	if (projected_offset > isn_offset)
@@ -1369,7 +1374,7 @@
 
 	isn_offset_old = isn_offset;
 	callout_reset(&isn_callout, hz/100, tcp_isn_tick, NULL);
-	INP_INFO_WUNLOCK(&tcbinfo);
+	mtx_unlock(&isn_mtx);
 }
 
 /*