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From:
Alexander Bluhm <bluhm@openbsd.org>
Subject:
Re: unlock igmp and mld6 fast timer
To:
tech@openbsd.org
Cc:
Vitaliy Makkoveev <mvs@openbsd.org>
Date:
Mon, 29 Dec 2025 13:20:05 +0100

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Thread
On Tue, Dec 02, 2025 at 11:01:36PM +0100, Alexander Bluhm wrote:
> Hi,
> 
> After moving around code for a while, I think a rwlock to protect
> if_maddrlist is the next step.  The malloc(M_WAITOK) in rti_fill()
> prevents us from using a mutex here.
> 
> So protect the TAILQ if_maddrlist with rwlock if_maddrlock.  Also
> struct in_multi and in6_multi use this lock for their state and
> timer.  Sleeps in malloc and IP output are possible.
> 
> This allows to run IGMP and MLD6 fast timeout with shared net lock.

I would like to proceed with MP multicast.

OK? objections?

bluhm

Index: net/if.c
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/net/if.c,v
diff -u -p -r1.760 if.c
--- net/if.c	11 Dec 2025 07:09:20 -0000	1.760
+++ net/if.c	29 Dec 2025 12:04:38 -0000
@@ -662,6 +662,7 @@ if_attach_common(struct ifnet *ifp)
 	TAILQ_INIT(&ifp->if_addrlist);
 	TAILQ_INIT(&ifp->if_maddrlist);
 	TAILQ_INIT(&ifp->if_groups);
+	rw_init(&ifp->if_maddrlock, "maddr");
 
 	if (!ISSET(ifp->if_xflags, IFXF_MPSAFE)) {
 		KASSERTMSG(ifp->if_qstart == NULL,
Index: net/if_var.h
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/net/if_var.h,v
diff -u -p -r1.145 if_var.h
--- net/if_var.h	19 Dec 2025 02:04:13 -0000	1.145
+++ net/if_var.h	29 Dec 2025 12:04:38 -0000
@@ -81,6 +81,7 @@
  *	K	kernel lock
  *	N	net lock
  *	T	if_tmplist_lock
+ *	m	interface multicast rwlock if_maddrlock
  *
  *  For SRP related structures that allow lock-free reads, the write lock
  *  is indicated below.
@@ -153,8 +154,9 @@ struct ifnet {				/* and the entries */
 	TAILQ_ENTRY(ifnet) if_list;	/* [NK] all struct ifnets are chained */
 	TAILQ_ENTRY(ifnet) if_tmplist;	/* [T] temporary list */
 	TAILQ_HEAD(, ifaddr) if_addrlist; /* [N] list of addresses per if */
-	TAILQ_HEAD(, ifmaddr) if_maddrlist; /* [N] list of multicast records */
+	TAILQ_HEAD(, ifmaddr) if_maddrlist; /* [m] list of multicast records */
 	TAILQ_HEAD(, ifg_list) if_groups; /* [N] list of groups per if */
+	struct rwlock if_maddrlock;
 	struct task_list if_addrhooks;	/* [I] address change callbacks */
 	struct task_list if_linkstatehooks; /* [I] link change callbacks*/
 	struct task_list if_detachhooks; /* [I] detach callbacks */
@@ -273,10 +275,10 @@ struct ifaddr {
  * Interface multicast address.
  */
 struct ifmaddr {
-	struct sockaddr		*ifma_addr;	/* Protocol address */
-	unsigned int		 ifma_ifidx;	/* Index of the interface */
+	TAILQ_ENTRY(ifmaddr)	 ifma_list;	/* [m] Per-interface list */
+	struct sockaddr		*ifma_addr;	/* [I] Protocol address */
 	struct refcnt		 ifma_refcnt;	/* Count of references */
-	TAILQ_ENTRY(ifmaddr)	 ifma_list;	/* Per-interface list */
+	unsigned int		 ifma_ifidx;	/* [I] Index of the interface */
 };
 
 /*
Index: netinet/igmp.c
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/netinet/igmp.c,v
diff -u -p -r1.93 igmp.c
--- netinet/igmp.c	2 Dec 2025 15:52:04 -0000	1.93
+++ netinet/igmp.c	29 Dec 2025 12:04:38 -0000
@@ -341,6 +341,7 @@ igmp_input_if(struct ifnet *ifp, struct 
 			 * except those that are already running and those
 			 * that belong to a "local" group (224.0.0.X).
 			 */
+			rw_enter_write(&ifp->if_maddrlock);
 			TAILQ_FOREACH(ifma, &ifp->if_maddrlist, ifma_list) {
 				if (ifma->ifma_addr->sa_family != AF_INET)
 					continue;
@@ -353,6 +354,7 @@ igmp_input_if(struct ifnet *ifp, struct 
 					running = 1;
 				}
 			}
+			rw_exit_write(&ifp->if_maddrlock);
 		} else {
 			if (!IN_MULTICAST(ip->ip_dst.s_addr)) {
 				igmpstat_inc(igps_rcv_badqueries);
@@ -372,6 +374,7 @@ igmp_input_if(struct ifnet *ifp, struct 
 			 * timers already running, check if they need to be
 			 * reset.
 			 */
+			rw_enter_write(&ifp->if_maddrlock);
 			TAILQ_FOREACH(ifma, &ifp->if_maddrlist, ifma_list) {
 				if (ifma->ifma_addr->sa_family != AF_INET)
 					continue;
@@ -400,6 +403,7 @@ igmp_input_if(struct ifnet *ifp, struct 
 					}
 				}
 			}
+			rw_exit_write(&ifp->if_maddrlock);
 		}
 
 		break;
@@ -436,6 +440,7 @@ igmp_input_if(struct ifnet *ifp, struct 
 		 * If we belong to the group being reported, stop
 		 * our timer for that group.
 		 */
+		rw_enter_write(&ifp->if_maddrlock);
 		inm = in_lookupmulti(&igmp->igmp_group, ifp);
 		if (inm != NULL) {
 			inm->inm_timer = 0;
@@ -456,6 +461,7 @@ igmp_input_if(struct ifnet *ifp, struct 
 				break;
 			}
 		}
+		rw_exit_write(&ifp->if_maddrlock);
 
 		break;
 
@@ -504,6 +510,7 @@ igmp_input_if(struct ifnet *ifp, struct 
 		 * If we belong to the group being reported, stop
 		 * our timer for that group.
 		 */
+		rw_enter_write(&ifp->if_maddrlock);
 		inm = in_lookupmulti(&igmp->igmp_group, ifp);
 		if (inm != NULL) {
 			inm->inm_timer = 0;
@@ -520,6 +527,7 @@ igmp_input_if(struct ifnet *ifp, struct 
 				break;
 			}
 		}
+		rw_exit_write(&ifp->if_maddrlock);
 
 		break;
 
@@ -542,6 +550,8 @@ igmp_joingroup(struct in_multi *inm, str
 {
 	int i, running = 0;
 
+	rw_assert_wrlock(&ifp->if_maddrlock);
+
 	inm->inm_state = IGMP_IDLE_MEMBER;
 
 	if (!IN_LOCAL_GROUP(inm->inm_addr.s_addr) &&
@@ -564,6 +574,8 @@ igmp_joingroup(struct in_multi *inm, str
 void
 igmp_leavegroup(struct in_multi *inm, struct ifnet *ifp)
 {
+	rw_assert_anylock(&ifp->if_maddrlock);
+
 	switch (inm->inm_state) {
 	case IGMP_DELAYING_MEMBER:
 	case IGMP_IDLE_MEMBER:
@@ -598,7 +610,7 @@ igmp_fasttimo(void)
 		return;
 	membar_consumer();
 
-	NET_LOCK();
+	NET_LOCK_SHARED();
 
 	TAILQ_FOREACH(ifp, &ifnetlist, if_list) {
 		if (igmp_checktimer(ifp))
@@ -608,7 +620,7 @@ igmp_fasttimo(void)
 	membar_producer();
 	atomic_store_int(&igmp_timers_are_running, running);
 
-	NET_UNLOCK();
+	NET_UNLOCK_SHARED();
 }
 
 int
@@ -618,8 +630,7 @@ igmp_checktimer(struct ifnet *ifp)
 	struct ifmaddr *ifma;
 	int running = 0;
 
-	NET_ASSERT_LOCKED();
-
+	rw_enter_write(&ifp->if_maddrlock);
 	TAILQ_FOREACH(ifma, &ifp->if_maddrlist, ifma_list) {
 		if (ifma->ifma_addr->sa_family != AF_INET)
 			continue;
@@ -640,6 +651,7 @@ igmp_checktimer(struct ifnet *ifp)
 			running = 1;
 		}
 	}
+	rw_exit_write(&ifp->if_maddrlock);
 
 	return (running);
 }
Index: netinet/in.c
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/netinet/in.c,v
diff -u -p -r1.190 in.c
--- netinet/in.c	2 Dec 2025 15:52:04 -0000	1.190
+++ netinet/in.c	29 Dec 2025 12:04:38 -0000
@@ -853,7 +853,7 @@ in_lookupmulti(const struct in_addr *add
 	struct in_multi *inm = NULL;
 	struct ifmaddr *ifma;
 
-	NET_ASSERT_LOCKED();
+	rw_assert_anylock(&ifp->if_maddrlock);
 
 	TAILQ_FOREACH(ifma, &ifp->if_maddrlist, ifma_list) {
 		if (ifma->ifma_addr->sa_family == AF_INET &&
@@ -871,54 +871,67 @@ in_lookupmulti(const struct in_addr *add
 struct in_multi *
 in_addmulti(const struct in_addr *addr, struct ifnet *ifp)
 {
-	struct in_multi *inm;
+	struct in_multi *inm, *new_inm = NULL;
 	struct ifreq ifr;
 
 	/*
 	 * See if address already in list.
 	 */
+	rw_enter_write(&ifp->if_maddrlock);
 	inm = in_lookupmulti(addr, ifp);
-	if (inm != NULL) {
-		/*
-		 * Found it; just increment the reference count.
-		 */
-		refcnt_take(&inm->inm_refcnt);
-	} else {
-		/*
-		 * New address; allocate a new multicast record
-		 * and link it into the interface's multicast list.
-		 */
-		inm = malloc(sizeof(*inm), M_IPMADDR, M_WAITOK | M_ZERO);
-		inm->inm_sin.sin_len = sizeof(struct sockaddr_in);
-		inm->inm_sin.sin_family = AF_INET;
-		inm->inm_sin.sin_addr = *addr;
-		refcnt_init_trace(&inm->inm_refcnt, DT_REFCNT_IDX_IFMADDR);
-		inm->inm_ifidx = ifp->if_index;
-		inm->inm_ifma.ifma_addr = sintosa(&inm->inm_sin);
+	if (inm != NULL)
+		goto found;
+	rw_exit_write(&ifp->if_maddrlock);
 
-		/*
-		 * Ask the network driver to update its multicast reception
-		 * filter appropriately for the new address.
-		 */
-		memset(&ifr, 0, sizeof(ifr));
-		memcpy(&ifr.ifr_addr, &inm->inm_sin, sizeof(inm->inm_sin));
-		KERNEL_LOCK();
-		if ((*ifp->if_ioctl)(ifp, SIOCADDMULTI,(caddr_t)&ifr) != 0) {
-			KERNEL_UNLOCK();
-			free(inm, M_IPMADDR, sizeof(*inm));
-			return (NULL);
-		}
+	/*
+	 * New address; allocate a new multicast record
+	 * and link it into the interface's multicast list.
+	 */
+	new_inm = malloc(sizeof(*inm), M_IPMADDR, M_WAITOK | M_ZERO);
+
+	/*
+	 * Ask the network driver to update its multicast reception
+	 * filter appropriately for the new address.
+	 */
+	memset(&ifr, 0, sizeof(ifr));
+	satosin(&ifr.ifr_addr)->sin_len = sizeof(struct sockaddr_in);
+	satosin(&ifr.ifr_addr)->sin_family = AF_INET;
+	satosin(&ifr.ifr_addr)->sin_addr = *addr;
+	KERNEL_LOCK();
+	if ((*ifp->if_ioctl)(ifp, SIOCADDMULTI,(caddr_t)&ifr) != 0) {
 		KERNEL_UNLOCK();
+		goto out;
+	}
+	KERNEL_UNLOCK();
 
-		TAILQ_INSERT_HEAD(&ifp->if_maddrlist, &inm->inm_ifma,
-		    ifma_list);
+	rw_enter_write(&ifp->if_maddrlock);
+	/* check again after unlock and lock */
+	inm = in_lookupmulti(addr, ifp);
+	if (inm != NULL)
+		goto found;
+	inm = new_inm;
+	inm->inm_sin.sin_len = sizeof(struct sockaddr_in);
+	inm->inm_sin.sin_family = AF_INET;
+	inm->inm_sin.sin_addr = *addr;
+	refcnt_init_trace(&inm->inm_refcnt, DT_REFCNT_IDX_IFMADDR);
+	inm->inm_ifidx = ifp->if_index;
+	inm->inm_ifma.ifma_addr = sintosa(&inm->inm_sin);
 
-		/*
-		 * Let IGMP know that we have joined a new IP multicast group.
-		 */
-		igmp_joingroup(inm, ifp);
-	}
+	TAILQ_INSERT_HEAD(&ifp->if_maddrlist, &inm->inm_ifma, ifma_list);
+
+	/*
+	 * Let IGMP know that we have joined a new IP multicast group.
+	 */
+	igmp_joingroup(inm, ifp);
+	rw_exit_write(&ifp->if_maddrlock);
+
+	return (inm);
 
+ found:
+	refcnt_take(&inm->inm_refcnt);
+	rw_exit_write(&ifp->if_maddrlock);
+ out:
+	free(new_inm, M_IPMADDR, sizeof(*inm));
 	return (inm);
 }
 
@@ -931,19 +944,21 @@ in_delmulti(struct in_multi *inm)
 	struct ifreq ifr;
 	struct ifnet *ifp;
 
-	NET_ASSERT_LOCKED();
-
 	if (refcnt_rele(&inm->inm_refcnt) == 0)
 		return;
 
 	ifp = if_get(inm->inm_ifidx);
 	if (ifp != NULL) {
+		rw_enter_write(&ifp->if_maddrlock);
 		/*
 		 * No remaining claims to this record; let IGMP know that
 		 * we are leaving the multicast group.
 		 */
 		igmp_leavegroup(inm, ifp);
 
+		TAILQ_REMOVE(&ifp->if_maddrlist, &inm->inm_ifma, ifma_list);
+		rw_exit_write(&ifp->if_maddrlock);
+
 		/*
 		 * Notify the network driver to update its multicast
 		 * reception filter.
@@ -956,9 +971,8 @@ in_delmulti(struct in_multi *inm)
 		(*ifp->if_ioctl)(ifp, SIOCDELMULTI, (caddr_t)&ifr);
 		KERNEL_UNLOCK();
 
-		TAILQ_REMOVE(&ifp->if_maddrlist, &inm->inm_ifma, ifma_list);
+		if_put(ifp);
 	}
-	if_put(ifp);
 
 	free(inm, M_IPMADDR, sizeof(*inm));
 }
@@ -973,8 +987,10 @@ in_hasmulti(const struct in_addr *addr, 
 	struct in_multi *inm;
 	int joined;
 
+	rw_enter_read(&ifp->if_maddrlock);
 	inm = in_lookupmulti(addr, ifp);
 	joined = (inm != NULL);
+	rw_exit_read(&ifp->if_maddrlock);
 
 	return (joined);
 }
Index: netinet/in_var.h
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/netinet/in_var.h,v
diff -u -p -r1.44 in_var.h
--- netinet/in_var.h	2 Dec 2025 15:52:04 -0000	1.44
+++ netinet/in_var.h	29 Dec 2025 12:04:38 -0000
@@ -87,11 +87,11 @@ struct in_multi {
 #define inm_refcnt		inm_ifma.ifma_refcnt
 #define inm_ifidx		inm_ifma.ifma_ifidx
 
-	struct sockaddr_in	inm_sin;   /* IPv4 multicast address */
+	struct sockaddr_in	inm_sin;   /* [I] IPv4 multicast address */
 #define inm_addr		inm_sin.sin_addr
 
-	u_int			inm_state; /* state of membership */
-	u_int			inm_timer; /* IGMP membership report timer */
+	u_int			inm_state; /* [m] state of membership */
+	u_int			inm_timer; /* [m] IGMP membership report */
 
 	struct router_info	*inm_rti;  /* router version info */
 };
Index: netinet6/in6.c
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/netinet6/in6.c,v
diff -u -p -r1.275 in6.c
--- netinet6/in6.c	13 Nov 2025 23:30:01 -0000	1.275
+++ netinet6/in6.c	29 Dec 2025 12:04:38 -0000
@@ -1007,7 +1007,7 @@ in6_lookupmulti(const struct in6_addr *a
 	struct in6_multi *in6m = NULL;
 	struct ifmaddr *ifma;
 
-	NET_ASSERT_LOCKED();
+	rw_assert_anylock(&ifp->if_maddrlock);
 
 	TAILQ_FOREACH(ifma, &ifp->if_maddrlist, ifma_list) {
 		if (ifma->ifma_addr->sa_family == AF_INET6 &&
@@ -1026,62 +1026,71 @@ in6_lookupmulti(const struct in6_addr *a
 struct in6_multi *
 in6_addmulti(const struct in6_addr *addr, struct ifnet *ifp, int *errorp)
 {
+	struct	in6_multi *in6m, *new_in6m = NULL;
 	struct	in6_ifreq ifr;
-	struct	in6_multi *in6m;
-
-	NET_ASSERT_LOCKED();
 
 	*errorp = 0;
 	/*
 	 * See if address already in list.
 	 */
+	rw_enter_write(&ifp->if_maddrlock);
 	in6m = in6_lookupmulti(addr, ifp);
-	if (in6m != NULL) {
-		/*
-		 * Found it; just increment the reference count.
-		 */
-		refcnt_take(&in6m->in6m_refcnt);
-	} else {
-		/*
-		 * New address; allocate a new multicast record
-		 * and link it into the interface's multicast list.
-		 */
-		in6m = malloc(sizeof(*in6m), M_IPMADDR, M_NOWAIT | M_ZERO);
-		if (in6m == NULL) {
-			*errorp = ENOBUFS;
-			return (NULL);
-		}
+	if (in6m != NULL)
+		goto found;
+	rw_exit_write(&ifp->if_maddrlock);
 
-		in6m->in6m_sin.sin6_len = sizeof(struct sockaddr_in6);
-		in6m->in6m_sin.sin6_family = AF_INET6;
-		in6m->in6m_sin.sin6_addr = *addr;
-		refcnt_init_trace(&in6m->in6m_refcnt, DT_REFCNT_IDX_IFMADDR);
-		in6m->in6m_ifidx = ifp->if_index;
-		in6m->in6m_ifma.ifma_addr = sin6tosa(&in6m->in6m_sin);
+	/*
+	 * New address; allocate a new multicast record
+	 * and link it into the interface's multicast list.
+	 */
+	new_in6m = malloc(sizeof(*in6m), M_IPMADDR, M_NOWAIT | M_ZERO);
+	if (new_in6m == NULL) {
+		*errorp = ENOBUFS;
+		return (NULL);
+	}
 
-		/*
-		 * Ask the network driver to update its multicast reception
-		 * filter appropriately for the new address.
-		 */
-		memcpy(&ifr.ifr_addr, &in6m->in6m_sin, sizeof(in6m->in6m_sin));
-		KERNEL_LOCK();
-		*errorp = (*ifp->if_ioctl)(ifp, SIOCADDMULTI, (caddr_t)&ifr);
-		KERNEL_UNLOCK();
-		if (*errorp) {
-			free(in6m, M_IPMADDR, sizeof(*in6m));
-			return (NULL);
-		}
+	/*
+	 * Ask the network driver to update its multicast reception
+	 * filter appropriately for the new address.
+	 */
+	memset(&ifr, 0, sizeof(ifr));
+	ifr.ifr_addr.sin6_len = sizeof(struct sockaddr_in6);
+	ifr.ifr_addr.sin6_family = AF_INET6;
+	ifr.ifr_addr.sin6_addr = *addr;
+	KERNEL_LOCK();
+	*errorp = (*ifp->if_ioctl)(ifp, SIOCADDMULTI, (caddr_t)&ifr);
+	KERNEL_UNLOCK();
+	if (*errorp)
+		goto out;
+
+        rw_enter_write(&ifp->if_maddrlock);
+        /* check again after unlock and lock */
+        in6m = in6_lookupmulti(addr, ifp);
+        if (in6m != NULL)
+                goto found;
+        in6m = new_in6m;
+	in6m->in6m_sin.sin6_len = sizeof(struct sockaddr_in6);
+	in6m->in6m_sin.sin6_family = AF_INET6;
+	in6m->in6m_sin.sin6_addr = *addr;
+	refcnt_init_trace(&in6m->in6m_refcnt, DT_REFCNT_IDX_IFMADDR);
+	in6m->in6m_ifidx = ifp->if_index;
+	in6m->in6m_ifma.ifma_addr = sin6tosa(&in6m->in6m_sin);
+	
+	TAILQ_INSERT_HEAD(&ifp->if_maddrlist, &in6m->in6m_ifma, ifma_list);
 
-		TAILQ_INSERT_HEAD(&ifp->if_maddrlist, &in6m->in6m_ifma,
-		    ifma_list);
+	/*
+	 * Let MLD6 know that we have joined a new IP6 multicast group.
+	 */
+	mld6_start_listening(in6m, ifp);
+	rw_exit_write(&ifp->if_maddrlock);
 
-		/*
-		 * Let MLD6 know that we have joined a new IP6 multicast
-		 * group.
-		 */
-		mld6_start_listening(in6m);
-	}
+	return (in6m);
 
+ found:
+	refcnt_take(&in6m->in6m_refcnt);
+	rw_exit_write(&ifp->if_maddrlock);
+ out:
+	free(new_in6m, M_IPMADDR, sizeof(*in6m));
 	return (in6m);
 }
 
@@ -1094,36 +1103,37 @@ in6_delmulti(struct in6_multi *in6m)
 	struct	in6_ifreq ifr;
 	struct	ifnet *ifp;
 
-	NET_ASSERT_LOCKED();
+	if (refcnt_rele(&in6m->in6m_refcnt) == 0)
+		return;
 
-	if (refcnt_rele(&in6m->in6m_refcnt) != 0) {
+	ifp = if_get(in6m->in6m_ifidx);
+	if (ifp != NULL) {
+		rw_enter_write(&ifp->if_maddrlock);
 		/*
 		 * No remaining claims to this record; let MLD6 know
 		 * that we are leaving the multicast group.
 		 */
-		mld6_stop_listening(in6m);
-		ifp = if_get(in6m->in6m_ifidx);
+		mld6_stop_listening(in6m, ifp);
+
+		TAILQ_REMOVE(&ifp->if_maddrlist, &in6m->in6m_ifma, ifma_list);
+		rw_exit_write(&ifp->if_maddrlock);
 
 		/*
 		 * Notify the network driver to update its multicast
 		 * reception filter.
 		 */
-		if (ifp != NULL) {
-			bzero(&ifr.ifr_addr, sizeof(struct sockaddr_in6));
-			ifr.ifr_addr.sin6_len = sizeof(struct sockaddr_in6);
-			ifr.ifr_addr.sin6_family = AF_INET6;
-			ifr.ifr_addr.sin6_addr = in6m->in6m_addr;
-			KERNEL_LOCK();
-			(*ifp->if_ioctl)(ifp, SIOCDELMULTI, (caddr_t)&ifr);
-			KERNEL_UNLOCK();
+		memset(&ifr, 0, sizeof(ifr));
+		ifr.ifr_addr.sin6_len = sizeof(struct sockaddr_in6);
+		ifr.ifr_addr.sin6_family = AF_INET6;
+		ifr.ifr_addr.sin6_addr = in6m->in6m_addr;
+		KERNEL_LOCK();
+		(*ifp->if_ioctl)(ifp, SIOCDELMULTI, (caddr_t)&ifr);
+		KERNEL_UNLOCK();
 
-			TAILQ_REMOVE(&ifp->if_maddrlist, &in6m->in6m_ifma,
-			    ifma_list);
-		}
 		if_put(ifp);
-
-		free(in6m, M_IPMADDR, sizeof(*in6m));
 	}
+
+	free(in6m, M_IPMADDR, sizeof(*in6m));
 }
 
 /*
@@ -1136,8 +1146,10 @@ in6_hasmulti(const struct in6_addr *addr
 	struct in6_multi *in6m;
 	int joined;
 
+	rw_enter_read(&ifp->if_maddrlock);
 	in6m = in6_lookupmulti(addr, ifp);
 	joined = (in6m != NULL);
+	rw_exit_read(&ifp->if_maddrlock);
 
 	return (joined);
 }
Index: netinet6/in6_var.h
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/netinet6/in6_var.h,v
diff -u -p -r1.82 in6_var.h
--- netinet6/in6_var.h	12 Nov 2025 11:37:08 -0000	1.82
+++ netinet6/in6_var.h	29 Dec 2025 12:04:38 -0000
@@ -316,11 +316,11 @@ struct in6_multi {
 #define in6m_refcnt		in6m_ifma.ifma_refcnt
 #define in6m_ifidx		in6m_ifma.ifma_ifidx
 
-	struct sockaddr_in6	in6m_sin;   /* IPv6 multicast address */
+	struct sockaddr_in6	in6m_sin;   /* [I] IPv6 multicast address */
 #define in6m_addr		in6m_sin.sin6_addr
 
-	u_int			in6m_state; /* state of membership */
-	u_int			in6m_timer; /* MLD6 membership report timer */
+	u_int			in6m_state; /* [m] state of membership */
+	u_int			in6m_timer; /* [m] MLD6 membership report */
 };
 
 static __inline struct in6_multi *
Index: netinet6/mld6.c
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/netinet6/mld6.c,v
diff -u -p -r1.70 mld6.c
--- netinet6/mld6.c	12 Nov 2025 19:11:10 -0000	1.70
+++ netinet6/mld6.c	29 Dec 2025 12:04:38 -0000
@@ -112,12 +112,14 @@ mld6_init(void)
 }
 
 void
-mld6_start_listening(struct in6_multi *in6m)
+mld6_start_listening(struct in6_multi *in6m, struct ifnet *ifp)
 {
 	/* XXX: These are necessary for KAME's link-local hack */
 	struct in6_addr all_nodes = IN6ADDR_LINKLOCAL_ALLNODES_INIT;
 	int running = 0;
 
+	rw_assert_wrlock(&ifp->if_maddrlock);
+
 	/*
 	 * RFC2710 page 10:
 	 * The node never sends a Report or Done for the link-scope all-nodes
@@ -147,12 +149,14 @@ mld6_start_listening(struct in6_multi *i
 }
 
 void
-mld6_stop_listening(struct in6_multi *in6m)
+mld6_stop_listening(struct in6_multi *in6m, struct ifnet *ifp)
 {
 	/* XXX: These are necessary for KAME's link-local hack */
 	struct in6_addr all_nodes = IN6ADDR_LINKLOCAL_ALLNODES_INIT;
 	struct in6_addr all_routers = IN6ADDR_LINKLOCAL_ALLROUTERS_INIT;
 
+	rw_assert_anylock(&ifp->if_maddrlock);
+
 	all_nodes.s6_addr16[1] = htons(in6m->in6m_ifidx);
 	/* XXX: necessary when mrouting */
 	all_routers.s6_addr16[1] = htons(in6m->in6m_ifidx);
@@ -254,6 +258,7 @@ mld6_input(struct mbuf *m, int off)
 			timer = 1;
 		all_nodes.s6_addr16[1] = htons(ifp->if_index);
 
+		rw_enter_write(&ifp->if_maddrlock);
 		TAILQ_FOREACH(ifma, &ifp->if_maddrlist, ifma_list) {
 			if (ifma->ifma_addr->sa_family != AF_INET6)
 				continue;
@@ -281,6 +286,7 @@ mld6_input(struct mbuf *m, int off)
 				}
 			}
 		}
+		rw_exit_write(&ifp->if_maddrlock);
 
 		if (IN6_IS_ADDR_MC_LINKLOCAL(&mldh->mld_addr))
 			mldh->mld_addr.s6_addr16[1] = 0; /* XXX */
@@ -308,11 +314,13 @@ mld6_input(struct mbuf *m, int off)
 		 * If we belong to the group being reported, stop
 		 * our timer for that group.
 		 */
+		rw_enter_write(&ifp->if_maddrlock);
 		in6m = in6_lookupmulti(&mldh->mld_addr, ifp);
 		if (in6m) {
 			in6m->in6m_timer = 0; /* transit to idle state */
 			in6m->in6m_state = MLD_OTHERLISTENER; /* clear flag */
 		}
+		rw_exit_write(&ifp->if_maddrlock);
 
 		if (IN6_IS_ADDR_MC_LINKLOCAL(&mldh->mld_addr))
 			mldh->mld_addr.s6_addr16[1] = 0; /* XXX */
@@ -355,7 +363,7 @@ mld6_fasttimo(void)
 		return;
 	membar_consumer();
 
-	NET_LOCK();
+	NET_LOCK_SHARED();
 
 	TAILQ_FOREACH(ifp, &ifnetlist, if_list) {
 		if (mld6_checktimer(ifp))
@@ -365,7 +373,7 @@ mld6_fasttimo(void)
 	membar_producer();
 	atomic_store_int(&mld6_timers_are_running, running);
 
-	NET_UNLOCK();
+	NET_UNLOCK_SHARED();
 }
 
 int
@@ -375,8 +383,7 @@ mld6_checktimer(struct ifnet *ifp)
 	struct ifmaddr *ifma;
 	int running = 0;
 
-	NET_ASSERT_LOCKED();
-
+	rw_enter_write(&ifp->if_maddrlock);
 	TAILQ_FOREACH(ifma, &ifp->if_maddrlist, ifma_list) {
 		if (ifma->ifma_addr->sa_family != AF_INET6)
 			continue;
@@ -390,6 +397,7 @@ mld6_checktimer(struct ifnet *ifp)
 			running = 1;
 		}
 	}
+	rw_exit_write(&ifp->if_maddrlock);
 
 	return (running);
 }
Index: netinet6/mld6_var.h
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/netinet6/mld6_var.h,v
diff -u -p -r1.7 mld6_var.h
--- netinet6/mld6_var.h	12 Nov 2025 19:11:10 -0000	1.7
+++ netinet6/mld6_var.h	29 Dec 2025 12:04:38 -0000
@@ -45,8 +45,8 @@
 
 void	mld6_init(void);
 void	mld6_input(struct mbuf *, int);
-void	mld6_start_listening(struct in6_multi *);
-void	mld6_stop_listening(struct in6_multi *);
+void	mld6_start_listening(struct in6_multi *, struct ifnet *);
+void	mld6_stop_listening(struct in6_multi *, struct ifnet *);
 void	mld6_fasttimo(void);
 #endif /* _KERNEL */