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route mtu atomic
anyone?
On Thu, Dec 26, 2024 at 12:11:50PM +0100, Alexander Bluhm wrote:
> Hi,
>
> With TCP unlocking we run into the problem that path MTU discovery
> happens in parallel. To keep route mtu consistent, I want to make
> access to rt_mtu atomic.
>
> The compare-and-swap function is used to detect if another thread
> is modifying the mtu. In this case skip updating rt_mtu.
>
> ok?
>
> bluhm
>
> Index: net/if_loop.c
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/net/if_loop.c,v
> diff -u -p -r1.98 if_loop.c
> --- net/if_loop.c 29 Dec 2023 11:43:04 -0000 1.98
> +++ net/if_loop.c 25 Dec 2024 17:22:50 -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: net/route.c
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/net/route.c,v
> diff -u -p -r1.437 route.c
> --- net/route.c 20 Sep 2024 02:00:46 -0000 1.437
> +++ net/route.c 25 Dec 2024 17:35:50 -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: net/route.h
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/net/route.h,v
> diff -u -p -r1.210 route.h
> --- net/route.h 31 Mar 2024 15:53:12 -0000 1.210
> +++ net/route.h 25 Dec 2024 17:24:16 -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: net/rtsock.c
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/net/rtsock.c,v
> diff -u -p -r1.375 rtsock.c
> --- net/rtsock.c 12 Jul 2024 17:20:18 -0000 1.375
> +++ net/rtsock.c 25 Dec 2024 17:39:32 -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: netinet/ip_icmp.c
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/netinet/ip_icmp.c,v
> diff -u -p -r1.197 ip_icmp.c
> --- netinet/ip_icmp.c 4 Dec 2024 22:24:11 -0000 1.197
> +++ netinet/ip_icmp.c 25 Dec 2024 17:46:46 -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: netinet/ip_input.c
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/netinet/ip_input.c,v
> diff -u -p -r1.402 ip_input.c
> --- netinet/ip_input.c 21 Nov 2024 20:15:44 -0000 1.402
> +++ netinet/ip_input.c 25 Dec 2024 17:49:03 -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: netinet/ip_output.c
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/netinet/ip_output.c,v
> diff -u -p -r1.401 ip_output.c
> --- netinet/ip_output.c 2 Jul 2024 18:33:47 -0000 1.401
> +++ netinet/ip_output.c 25 Dec 2024 18:55:12 -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: netinet/tcp_input.c
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/netinet/tcp_input.c,v
> diff -u -p -r1.412 tcp_input.c
> --- netinet/tcp_input.c 24 Dec 2024 19:16:53 -0000 1.412
> +++ netinet/tcp_input.c 25 Dec 2024 18:56:37 -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: netinet6/icmp6.c
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/netinet6/icmp6.c,v
> diff -u -p -r1.255 icmp6.c
> --- netinet6/icmp6.c 12 Aug 2024 11:25:27 -0000 1.255
> +++ netinet6/icmp6.c 25 Dec 2024 19:00:50 -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: netinet6/ip6_forward.c
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/netinet6/ip6_forward.c,v
> diff -u -p -r1.124 ip6_forward.c
> --- netinet6/ip6_forward.c 19 Jul 2024 16:58:32 -0000 1.124
> +++ netinet6/ip6_forward.c 25 Dec 2024 19:02:20 -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: netinet6/ip6_output.c
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/netinet6/ip6_output.c,v
> diff -u -p -r1.292 ip6_output.c
> --- netinet6/ip6_output.c 4 Jul 2024 12:50:08 -0000 1.292
> +++ netinet6/ip6_output.c 25 Dec 2024 19:16:03 -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/atomic.h
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/sys/atomic.h,v
> diff -u -p -r1.9 atomic.h
> --- sys/atomic.h 21 Mar 2022 05:45:52 -0000 1.9
> +++ sys/atomic.h 25 Dec 2024 17:38:09 -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;
> }
route mtu atomic