Index: sys/net/if_loop.c
===================================================================
RCS file: /mount/openbsd/cvs/src/sys/net/if_loop.c,v
diff -u -p -u -p -r1.98 if_loop.c
--- sys/net/if_loop.c	29 Dec 2023 11:43:04 -0000	1.98
+++ sys/net/if_loop.c	25 Dec 2024 19:21:34 -0000
@@ -277,8 +277,8 @@ looutput(struct ifnet *ifp, struct mbuf 
 void
 lortrequest(struct ifnet *ifp, int cmd, struct rtentry *rt)
 {
-	if (rt && rt->rt_mtu == 0)
-		rt->rt_mtu = LOMTU;
+	if (rt != NULL)
+		atomic_cas_uint(&rt->rt_mtu, 0, LOMTU);
 }
 
 /*
Index: sys/net/route.c
===================================================================
RCS file: /mount/openbsd/cvs/src/sys/net/route.c,v
diff -u -p -u -p -r1.437 route.c
--- sys/net/route.c	20 Sep 2024 02:00:46 -0000	1.437
+++ sys/net/route.c	25 Dec 2024 19:21:34 -0000
@@ -557,8 +557,14 @@ rt_setgwroute(struct rtentry *rt, const 
 	 * If the MTU of next hop is 0, this will reset the MTU of the
 	 * route to run PMTUD again from scratch.
 	 */
-	if (!ISSET(rt->rt_locks, RTV_MTU) && (rt->rt_mtu > nhrt->rt_mtu))
-		rt->rt_mtu = nhrt->rt_mtu;
+	if (!ISSET(rt->rt_locks, RTV_MTU)) {
+		u_int mtu, nhmtu;
+
+		mtu = atomic_load_int(&rt->rt_mtu);
+		nhmtu = atomic_load_int(&nhrt->rt_mtu);
+		if (mtu > nhmtu)
+			atomic_cas_uint(&rt->rt_mtu, mtu, nhmtu);
+	}
 
 	/*
 	 * To avoid reference counting problems when writing link-layer
Index: sys/net/route.h
===================================================================
RCS file: /mount/openbsd/cvs/src/sys/net/route.h,v
diff -u -p -u -p -r1.210 route.h
--- sys/net/route.h	31 Mar 2024 15:53:12 -0000	1.210
+++ sys/net/route.h	25 Dec 2024 19:21:34 -0000
@@ -61,7 +61,7 @@ struct rt_kmetrics {
 	u_int64_t	rmx_pksent;	/* packets sent using this route */
 	int64_t		rmx_expire;	/* lifetime for route, e.g. redirect */
 	u_int		rmx_locks;	/* Kernel must leave these values */
-	u_int		rmx_mtu;	/* MTU for this path */
+	u_int		rmx_mtu;	/* [a] MTU for this path */
 };
 #endif
 
Index: sys/net/rtsock.c
===================================================================
RCS file: /mount/openbsd/cvs/src/sys/net/rtsock.c,v
diff -u -p -u -p -r1.375 rtsock.c
--- sys/net/rtsock.c	12 Jul 2024 17:20:18 -0000	1.375
+++ sys/net/rtsock.c	25 Dec 2024 19:21:34 -0000
@@ -1345,7 +1345,7 @@ route_cleargateway(struct rtentry *rt, v
 
 	if (ISSET(rt->rt_flags, RTF_GATEWAY) && rt->rt_gwroute == nhrt &&
 	    !ISSET(rt->rt_locks, RTV_MTU))
-		rt->rt_mtu = 0;
+		atomic_store_int(&rt->rt_mtu, 0);
 
 	return (0);
 }
@@ -1393,7 +1393,7 @@ rtm_setmetrics(u_long which, const struc
 	int64_t expire;
 
 	if (which & RTV_MTU)
-		out->rmx_mtu = in->rmx_mtu;
+		atomic_store_int(&out->rmx_mtu, in->rmx_mtu);
 	if (which & RTV_EXPIRE) {
 		expire = in->rmx_expire;
 		if (expire != 0) {
@@ -1421,7 +1421,7 @@ rtm_getmetrics(const struct rtentry *rt,
 
 	bzero(out, sizeof(*out));
 	out->rmx_locks = in->rmx_locks;
-	out->rmx_mtu = in->rmx_mtu;
+	out->rmx_mtu = atomic_load_int(&in->rmx_mtu);
 	out->rmx_expire = expire;
 	out->rmx_pksent = in->rmx_pksent;
 }
Index: sys/netinet/ip_icmp.c
===================================================================
RCS file: /mount/openbsd/cvs/src/sys/netinet/ip_icmp.c,v
diff -u -p -u -p -r1.197 ip_icmp.c
--- sys/netinet/ip_icmp.c	4 Dec 2024 22:24:11 -0000	1.197
+++ sys/netinet/ip_icmp.c	25 Dec 2024 19:21:34 -0000
@@ -1008,6 +1008,7 @@ icmp_mtudisc(struct icmp *icp, u_int rta
 {
 	struct rtentry *rt;
 	struct ifnet *ifp;
+	u_int rtmtu;
 	u_long mtu = ntohs(icp->icmp_nextmtu);  /* Why a long?  IPv6 */
 
 	rt = icmp_mtudisc_clone(icp->icmp_ip.ip_dst, rtableid, 0);
@@ -1020,17 +1021,18 @@ icmp_mtudisc(struct icmp *icp, u_int rta
 		return;
 	}
 
+	rtmtu = atomic_load_int(&rt->rt_mtu);
 	if (mtu == 0) {
 		int i = 0;
 
 		mtu = ntohs(icp->icmp_ip.ip_len);
 		/* Some 4.2BSD-based routers incorrectly adjust the ip_len */
-		if (mtu > rt->rt_mtu && rt->rt_mtu != 0)
+		if (mtu > rtmtu && rtmtu != 0)
 			mtu -= (icp->icmp_ip.ip_hl << 2);
 
 		/* If we still can't guess a value, try the route */
 		if (mtu == 0) {
-			mtu = rt->rt_mtu;
+			mtu = rtmtu;
 
 			/* If no route mtu, default to the interface mtu */
 
@@ -1055,8 +1057,8 @@ icmp_mtudisc(struct icmp *icp, u_int rta
 	if ((rt->rt_locks & RTV_MTU) == 0) {
 		if (mtu < 296 || mtu > ifp->if_mtu)
 			rt->rt_locks |= RTV_MTU;
-		else if (rt->rt_mtu > mtu || rt->rt_mtu == 0)
-			rt->rt_mtu = mtu;
+		else if (rtmtu > mtu || rtmtu == 0)
+			atomic_cas_uint(&rt->rt_mtu, rtmtu, mtu);
 	}
 
 	if_put(ifp);
@@ -1089,7 +1091,7 @@ icmp_mtudisc_timeout(struct rtentry *rt,
 			    rtableid, NULL);
 	} else {
 		if ((rt->rt_locks & RTV_MTU) == 0)
-			rt->rt_mtu = 0;
+			atomic_store_int(&rt->rt_mtu, 0);
 	}
 
 	if_put(ifp);
Index: sys/netinet/ip_input.c
===================================================================
RCS file: /mount/openbsd/cvs/src/sys/netinet/ip_input.c,v
diff -u -p -u -p -r1.402 ip_input.c
--- sys/netinet/ip_input.c	21 Nov 2024 20:15:44 -0000	1.402
+++ sys/netinet/ip_input.c	25 Dec 2024 19:21:34 -0000
@@ -1650,8 +1650,11 @@ ip_forward(struct mbuf *m, struct ifnet 
 		type = ICMP_UNREACH;
 		code = ICMP_UNREACH_NEEDFRAG;
 		if (rt != NULL) {
-			if (rt->rt_mtu) {
-				destmtu = rt->rt_mtu;
+			u_int rtmtu;
+
+			rtmtu = atomic_load_int(&rt->rt_mtu);
+			if (rtmtu != 0) {
+				destmtu = rtmtu;
 			} else {
 				struct ifnet *destifp;
 
Index: sys/netinet/ip_output.c
===================================================================
RCS file: /mount/openbsd/cvs/src/sys/netinet/ip_output.c,v
diff -u -p -u -p -r1.401 ip_output.c
--- sys/netinet/ip_output.c	2 Jul 2024 18:33:47 -0000	1.401
+++ sys/netinet/ip_output.c	25 Dec 2024 19:21:34 -0000
@@ -211,7 +211,8 @@ reroute:
 			error = EHOSTUNREACH;
 			goto bad;
 		}
-		if ((mtu = ro->ro_rt->rt_mtu) == 0)
+		mtu = atomic_load_int(&ro->ro_rt->rt_mtu);
+		if (mtu == 0)
 			mtu = ifp->if_mtu;
 
 		if (ro->ro_rt->rt_flags & RTF_GATEWAY)
@@ -470,9 +471,14 @@ sendit:
 		 */
 		if (rtisvalid(ro->ro_rt) &&
 		    ISSET(ro->ro_rt->rt_flags, RTF_HOST) &&
-		    !(ro->ro_rt->rt_locks & RTV_MTU) &&
-		    (ro->ro_rt->rt_mtu > ifp->if_mtu)) {
-			ro->ro_rt->rt_mtu = ifp->if_mtu;
+		    !(ro->ro_rt->rt_locks & RTV_MTU)) {
+			u_int rtmtu;
+
+			rtmtu = atomic_load_int(&ro->ro_rt->rt_mtu);
+			if (rtmtu > ifp->if_mtu) {
+				atomic_cas_uint(&ro->ro_rt->rt_mtu, rtmtu,
+				    ifp->if_mtu);
+			}
 		}
 		ipstat_inc(ips_cantfrag);
 		goto bad;
@@ -558,7 +564,7 @@ ip_output_ipsec_pmtu_update(struct tdb *
 	DPRINTF("spi %08x mtu %d rt %p cloned %d",
 	    ntohl(tdb->tdb_spi), tdb->tdb_mtu, rt, rt_mtucloned);
 	if (rt != NULL) {
-		rt->rt_mtu = tdb->tdb_mtu;
+		atomic_store_int(&rt->rt_mtu, tdb->tdb_mtu);
 		if (ro != NULL && ro->ro_rt != NULL) {
 			rtfree(ro->ro_rt);
 			ro->ro_rt = rtalloc(&ro->ro_dstsa, RT_RESOLVE,
Index: sys/netinet/tcp_input.c
===================================================================
RCS file: /mount/openbsd/cvs/src/sys/netinet/tcp_input.c,v
diff -u -p -u -p -r1.412 tcp_input.c
--- sys/netinet/tcp_input.c	24 Dec 2024 19:16:53 -0000	1.412
+++ sys/netinet/tcp_input.c	25 Dec 2024 19:21:34 -0000
@@ -2835,7 +2835,7 @@ tcp_mss(struct tcpcb *tp, int offer)
 	 * if there's an mtu associated with the route and we support
 	 * path MTU discovery for the underlying protocol family, use it.
 	 */
-	rtmtu = READ_ONCE(rt->rt_mtu);
+	rtmtu = atomic_load_int(&rt->rt_mtu);
 	if (rtmtu) {
 		/*
 		 * One may wish to lower MSS to take into account options,
Index: sys/netinet6/icmp6.c
===================================================================
RCS file: /mount/openbsd/cvs/src/sys/netinet6/icmp6.c,v
diff -u -p -u -p -r1.255 icmp6.c
--- sys/netinet6/icmp6.c	12 Aug 2024 11:25:27 -0000	1.255
+++ sys/netinet6/icmp6.c	25 Dec 2024 19:21:34 -0000
@@ -1016,16 +1016,20 @@ icmp6_mtudisc_update(struct ip6ctlparam 
 	rt = icmp6_mtudisc_clone(&sin6, m->m_pkthdr.ph_rtableid, 0);
 
 	if (rt != NULL && ISSET(rt->rt_flags, RTF_HOST) &&
-	    !(rt->rt_locks & RTV_MTU) &&
-	    (rt->rt_mtu > mtu || rt->rt_mtu == 0)) {
-		struct ifnet *ifp;
-
-		ifp = if_get(rt->rt_ifidx);
-		if (ifp != NULL && mtu < ifp->if_mtu) {
-			icmp6stat_inc(icp6s_pmtuchg);
-			rt->rt_mtu = mtu;
+	    !(rt->rt_locks & RTV_MTU)) {
+		u_int rtmtu;
+
+		rtmtu = atomic_load_int(&rt->rt_mtu);
+		if (rtmtu > mtu || rtmtu == 0) {
+			struct ifnet *ifp;
+
+			ifp = if_get(rt->rt_ifidx);
+			if (ifp != NULL && mtu < ifp->if_mtu) {
+				icmp6stat_inc(icp6s_pmtuchg);
+				atomic_cas_uint(&rt->rt_mtu, rtmtu, mtu);
+			}
+			if_put(ifp);
 		}
-		if_put(ifp);
 	}
 	rtfree(rt);
 
@@ -1848,7 +1852,7 @@ icmp6_mtudisc_timeout(struct rtentry *rt
 		rtdeletemsg(rt, ifp, rtableid);
 	} else {
 		if (!(rt->rt_locks & RTV_MTU))
-			rt->rt_mtu = 0;
+			atomic_store_int(&rt->rt_mtu, 0);
 	}
 
 	if_put(ifp);
Index: sys/netinet6/ip6_forward.c
===================================================================
RCS file: /mount/openbsd/cvs/src/sys/netinet6/ip6_forward.c,v
diff -u -p -u -p -r1.124 ip6_forward.c
--- sys/netinet6/ip6_forward.c	19 Jul 2024 16:58:32 -0000	1.124
+++ sys/netinet6/ip6_forward.c	25 Dec 2024 19:21:34 -0000
@@ -399,8 +399,11 @@ senderr:
 	case EMSGSIZE:
 		type = ICMP6_PACKET_TOO_BIG;
 		if (rt != NULL) {
-			if (rt->rt_mtu) {
-				destmtu = rt->rt_mtu;
+			u_int rtmtu;
+
+			rtmtu = atomic_load_int(&rt->rt_mtu);
+			if (rtmtu != 0) {
+				destmtu = rtmtu;
 			} else {
 				struct ifnet *destifp;
 
Index: sys/netinet6/ip6_output.c
===================================================================
RCS file: /mount/openbsd/cvs/src/sys/netinet6/ip6_output.c,v
diff -u -p -u -p -r1.292 ip6_output.c
--- sys/netinet6/ip6_output.c	4 Jul 2024 12:50:08 -0000	1.292
+++ sys/netinet6/ip6_output.c	25 Dec 2024 19:21:34 -0000
@@ -156,8 +156,8 @@ struct idgen32_ctx ip6_id_ctx;
  * The mbuf chain containing the packet will be freed.
  * The mbuf opt, if present, will not be freed.
  *
- * type of "mtu": rt_mtu is u_long, ifnet.ifr_mtu is int.
- * We use u_long to hold largest one, * which is rt_mtu.
+ * type of "mtu": rt_mtu is u_int, ifnet.ifr_mtu is int.
+ * We use u_long to hold largest one.  XXX should be u_int
  */
 int
 ip6_output(struct mbuf *m, struct ip6_pktopts *opt, struct route *ro,
@@ -1027,11 +1027,11 @@ ip6_insertfraghdr(struct mbuf *m0, struc
 int
 ip6_getpmtu(struct rtentry *rt, struct ifnet *ifp, u_long *mtup)
 {
-	u_int32_t mtu = 0;
+	u_int mtu, rtmtu;
 	int error = 0;
 
 	if (rt != NULL) {
-		mtu = rt->rt_mtu;
+		mtu = rtmtu = atomic_load_int(&rt->rt_mtu);
 		if (mtu == 0)
 			mtu = ifp->if_mtu;
 		else if (mtu < IPV6_MMTU) {
@@ -1048,7 +1048,7 @@ ip6_getpmtu(struct rtentry *rt, struct i
 			 */
 			mtu = ifp->if_mtu;
 			if (!(rt->rt_locks & RTV_MTU))
-				rt->rt_mtu = mtu;
+				atomic_cas_uint(&rt->rt_mtu, rtmtu, mtu);
 		}
 	} else {
 		mtu = ifp->if_mtu;
@@ -2812,7 +2812,7 @@ ip6_output_ipsec_pmtu_update(struct tdb 
 	DPRINTF("spi %08x mtu %d rt %p cloned %d",
 	    ntohl(tdb->tdb_spi), tdb->tdb_mtu, rt, rt_mtucloned);
 	if (rt != NULL) {
-		rt->rt_mtu = tdb->tdb_mtu;
+		atomic_store_int(&rt->rt_mtu, tdb->tdb_mtu);
 		if (ro != NULL && ro->ro_rt != NULL) {
 			rtfree(ro->ro_rt);
 			ro->ro_rt = rtalloc(&ro->ro_dstsa, RT_RESOLVE,
Index: sys/sys/atomic.h
===================================================================
RCS file: /mount/openbsd/cvs/src/sys/sys/atomic.h,v
diff -u -p -u -p -r1.9 atomic.h
--- sys/sys/atomic.h	21 Mar 2022 05:45:52 -0000	1.9
+++ sys/sys/atomic.h	25 Dec 2024 19:21:34 -0000
@@ -202,13 +202,13 @@ atomic_sub_long_nv(volatile unsigned lon
  */
 
 static inline unsigned int
-atomic_load_int(volatile unsigned int *p)
+atomic_load_int(volatile const unsigned int *p)
 {
 	return *p;
 }
 
 static inline unsigned long
-atomic_load_long(volatile unsigned long *p)
+atomic_load_long(volatile const unsigned long *p)
 {
 	return *p;
 }