diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c
index 92956661996..3eb7fc84192 100644
--- a/sys/netinet/tcp_input.c
+++ b/sys/netinet/tcp_input.c
@@ -2640,6 +2640,8 @@ tcp_xmit_timer(tp, rtt)
 {
 	register int delta;
 
+	INP_LOCK_ASSERT(tp->t_inpcb);
+
 	tcpstat.tcps_rttupdated++;
 	tp->t_rttupdated++;
 	if (tp->t_srtt != 0) {
diff --git a/sys/netinet/tcp_reass.c b/sys/netinet/tcp_reass.c
index 92956661996..3eb7fc84192 100644
--- a/sys/netinet/tcp_reass.c
+++ b/sys/netinet/tcp_reass.c
@@ -2640,6 +2640,8 @@ tcp_xmit_timer(tp, rtt)
 {
 	register int delta;
 
+	INP_LOCK_ASSERT(tp->t_inpcb);
+
 	tcpstat.tcps_rttupdated++;
 	tp->t_rttupdated++;
 	if (tp->t_srtt != 0) {