From: Alexander Bluhm <bluhm@openbsd.org>
Subject: igmp mld6 timeout membar
To: tech@openbsd.org
Date: Wed, 25 Mar 2026 20:32:26 +0100

Hi,

We had some membars in igmp and mld6 timer code that are not
necessary.  There is no need to synchronise memory access.  The
important things run within locks that have their own barriers.

These checks exists only to avoid locking without work.
Also take a shortcut if the router list is empty.

ok?

bluhm

Index: netinet/igmp.c
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/netinet/igmp.c,v
diff -u -p -r1.97 igmp.c
--- netinet/igmp.c	22 Mar 2026 23:14:00 -0000	1.97
+++ netinet/igmp.c	25 Mar 2026 17:43:49 -0000
@@ -532,10 +532,8 @@ igmp_input_if(struct ifnet *ifp, struct 
 
 	}
 
-	if (running) {
-		membar_producer();
-		atomic_store_int(&igmp_timers_are_running, running);
-	}
+	if (running)
+		atomic_store_int(&igmp_timers_are_running, 1);
 
 	/*
 	 * Pass all valid IGMP packets up to any process(es) listening
@@ -567,10 +565,8 @@ igmp_joingroup(struct in_multi *inm, str
 	} else
 		inm->inm_timer = 0;
 
-	if (running) {
-		membar_producer();
-		atomic_store_int(&igmp_timers_are_running, running);
-	}
+	if (running)
+		atomic_store_int(&igmp_timers_are_running, 1);
 }
 
 void
@@ -614,7 +610,7 @@ igmp_fasttimo(void)
 	 */
 	if (!atomic_load_int(&igmp_timers_are_running))
 		return;
-	membar_consumer();
+	atomic_store_int(&igmp_timers_are_running, 0);
 
 	NET_LOCK_SHARED();
 
@@ -624,9 +620,6 @@ igmp_fasttimo(void)
 			running = 1;
 	}
 
-	membar_producer();
-	atomic_store_int(&igmp_timers_are_running, running);
-
 	while (!STAILQ_EMPTY(&pktlist)) {
 		struct igmp_pktinfo *pkt;
 
@@ -637,6 +630,9 @@ igmp_fasttimo(void)
 	}
 
 	NET_UNLOCK_SHARED();
+
+	if (running)
+		atomic_store_int(&igmp_timers_are_running, 1);
 }
 
 int
@@ -683,6 +679,9 @@ void
 igmp_slowtimo(void)
 {
 	struct router_info *rti;
+
+	if (LIST_EMPTY(&rti_head))
+		return;
 
 	mtx_enter(&igmp_mtx);
 	LIST_FOREACH(rti, &rti_head, rti_list) {
Index: netinet6/mld6.c
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/netinet6/mld6.c,v
diff -u -p -r1.74 mld6.c
--- netinet6/mld6.c	22 Mar 2026 23:14:00 -0000	1.74
+++ netinet6/mld6.c	25 Mar 2026 17:43:49 -0000
@@ -144,10 +144,8 @@ mld6_start_listening(struct in6_multi *i
 		running = 1;
 	}
 
-	if (running) {
-		membar_producer();
-		atomic_store_int(&mld6_timers_are_running, running);
-	}
+	if (running)
+		atomic_store_int(&mld6_timers_are_running, 1);
 }
 
 void
@@ -367,10 +365,8 @@ mld6_input(struct mbuf *m, int off)
 		break;
 	}
 
-	if (running) {
-		membar_producer();
-		atomic_store_int(&mld6_timers_are_running, running);
-	}
+	if (running)
+		atomic_store_int(&mld6_timers_are_running, 1);
 
 	if_put(ifp);
 	m_freem(m);
@@ -392,7 +388,7 @@ mld6_fasttimo(void)
 	 */
 	if (!atomic_load_int(&mld6_timers_are_running))
 		return;
-	membar_consumer();
+	atomic_store_int(&mld6_timers_are_running, 0);
 
 	NET_LOCK_SHARED();
 
@@ -402,9 +398,6 @@ mld6_fasttimo(void)
 			running = 1;
 	}
 
-	membar_producer();
-	atomic_store_int(&mld6_timers_are_running, running);
-
 	while (!STAILQ_EMPTY(&pktlist)) {
 		struct mld6_pktinfo *pkt;
 
@@ -415,6 +408,9 @@ mld6_fasttimo(void)
 	}
 
 	NET_UNLOCK_SHARED();
+
+	if (running)
+		atomic_store_int(&mld6_timers_are_running, 1);
 }
 
 int