To avoid packetloss and as it is very hard from a user's perspective to
debug multicast snooping related issues it is even more crucial to properly
switch from an active to an inactive multicast snooping state than the
other way around.

Therefore adding a few kernel warnings if any of our assertions to be in
an inactive state would fail.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
---
 net/bridge/br_multicast.c | 45 +++++++++++++++++++++++++++++++++++----
 1 file changed, 41 insertions(+), 4 deletions(-)

diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
index aa2edb63a97b..f4b6ea674988 100644
--- a/net/bridge/br_multicast.c
+++ b/net/bridge/br_multicast.c
@@ -1426,10 +1426,31 @@ static struct sk_buff *br_multicast_alloc_query(struct net_bridge_mcast *brmctx,
 	return NULL;
 }
 
+static void br_ip4_multicast_assert_inactive(struct net_bridge_mcast *brmctx)
+{
+	WARN_ON_ONCE(br_multicast_snooping_active(brmctx, htons(ETH_P_IP),
+						  NULL));
+}
+
+static void br_ip6_multicast_assert_inactive(struct net_bridge_mcast *brmctx)
+{
+	WARN_ON_ONCE(br_multicast_snooping_active(brmctx, htons(ETH_P_IPV6),
+						  NULL));
+}
+
+static void br_multicast_assert_inactive(struct net_bridge_mcast *brmctx)
+{
+	br_ip4_multicast_assert_inactive(brmctx);
+	br_ip6_multicast_assert_inactive(brmctx);
+}
+
 static void br_multicast_toggle_enabled(struct net_bridge *br, bool on)
 {
 	br_opt_toggle(br, BROPT_MULTICAST_ENABLED, on);
 	br_multicast_update_active(&br->multicast_ctx);
+
+	if (!on)
+		br_multicast_assert_inactive(&br->multicast_ctx);
 }
 
 struct net_bridge_mdb_entry *br_multicast_new_group(struct net_bridge *br,
@@ -1899,9 +1920,7 @@ static void br_multicast_querier_expired(struct net_bridge_mcast *brmctx,
 					 struct bridge_mcast_own_query *query,
 					 struct timer_list *timer)
 {
-	spin_lock(&brmctx->br->multicast_lock);
-	if (br_multicast_is_stopping(brmctx->br, timer) ||
-	    br_multicast_ctx_vlan_global_disabled(brmctx) ||
+	if (br_multicast_ctx_vlan_global_disabled(brmctx) ||
 	    !br_opt_get(brmctx->br, BROPT_MULTICAST_ENABLED))
 		goto out;
 
@@ -1912,7 +1931,6 @@ static void br_multicast_querier_expired(struct net_bridge_mcast *brmctx,
 	 * if our own querier is disabled, too
 	 */
 	br_multicast_update_active(brmctx);
-	spin_unlock(&brmctx->br->multicast_lock);
 }
 
 static void br_ip4_multicast_querier_expired(struct timer_list *t)
@@ -1920,7 +1938,16 @@ static void br_ip4_multicast_querier_expired(struct timer_list *t)
 	struct net_bridge_mcast *brmctx = timer_container_of(brmctx, t,
 							     ip4_other_query.timer);
 
+	spin_lock(&brmctx->br->multicast_lock);
+	if (br_multicast_is_stopping(brmctx->br, t))
+		goto out;
+
 	br_multicast_querier_expired(brmctx, &brmctx->ip4_own_query, t);
+
+	if (!brmctx->multicast_querier)
+		br_ip4_multicast_assert_inactive(brmctx);
+out:
+	spin_unlock(&brmctx->br->multicast_lock);
 }
 
 #if IS_ENABLED(CONFIG_IPV6)
@@ -1929,7 +1956,16 @@ static void br_ip6_multicast_querier_expired(struct timer_list *t)
 	struct net_bridge_mcast *brmctx = timer_container_of(brmctx, t,
 							     ip6_other_query.timer);
 
+	spin_lock(&brmctx->br->multicast_lock);
+	if (br_multicast_is_stopping(brmctx->br, t))
+		goto out;
+
 	br_multicast_querier_expired(brmctx, &brmctx->ip6_own_query, t);
+
+	if (!brmctx->multicast_querier)
+		br_ip6_multicast_assert_inactive(brmctx);
+out:
+	spin_unlock(&brmctx->br->multicast_lock);
 }
 #endif
 
@@ -4529,6 +4565,7 @@ static void __br_multicast_stop(struct net_bridge_mcast *brmctx)
 
 	/* bridge interface is down, set multicast state to inactive */
 	br_multicast_update_active(brmctx);
+	br_multicast_assert_inactive(brmctx);
 }
 
 void br_multicast_update_vlan_mcast_ctx(struct net_bridge_vlan *v, u8 state)
-- 
2.53.0

On Sat, Mar 07, 2026 at 05:45:48AM +0100, Linus Lüssing wrote:
> To avoid packetloss and as it is very hard from a user's perspective to
> debug multicast snooping related issues it is even more crucial to properly
> switch from an active to an inactive multicast snooping state than the
> other way around.
> 
> Therefore adding a few kernel warnings if any of our assertions to be in
> an inactive state would fail.

More assertions will be needed to cover the following cases:

1. Per-VLAN multicast snooping is turned off.
2. Multicast snooping is disabled on a specific VLAN.

__br_multicast_stop() is not called in these cases with the per-VLAN
context when the VLAN is not configured on the bridge interface.