We are updating ip{4,6}_active and check all variables their state relies
on while holding the bridge multicast spinlock. However we are going to
read it without this lock in a follow-up commit on fast/data path, too.

As these variables are only booleans this shouldn't be a problem,
ip{4,6}_active will be loaded and stored atomically. And those
read sides should converge eventually. But to allow tooling to verify
this use the READ_ONCE() and WRITE_ONCE macros.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
---
 net/bridge/br_multicast.c | 25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
index 7a32c6bed111..48552720bcea 100644
--- a/net/bridge/br_multicast.c
+++ b/net/bridge/br_multicast.c
@@ -1085,9 +1085,9 @@ static void br_ip4_multicast_update_active(struct net_bridge_mcast *brmctx,
 					   bool force_inactive)
 {
 	if (force_inactive)
-		brmctx->ip4_active = false;
+		WRITE_ONCE(brmctx->ip4_active, false);
 	else
-		brmctx->ip4_active = br_ip4_multicast_querier_exists(brmctx);
+		WRITE_ONCE(brmctx->ip4_active, br_ip4_multicast_querier_exists(brmctx));
 }
 
 static void br_ip6_multicast_update_active(struct net_bridge_mcast *brmctx,
@@ -1095,23 +1095,25 @@ static void br_ip6_multicast_update_active(struct net_bridge_mcast *brmctx,
 {
 #if IS_ENABLED(CONFIG_IPV6)
 	if (force_inactive)
-		brmctx->ip6_active = false;
+		WRITE_ONCE(brmctx->ip6_active, false);
 	else
-		brmctx->ip6_active = br_ip6_multicast_querier_exists(brmctx);
+		WRITE_ONCE(brmctx->ip6_active, br_ip6_multicast_querier_exists(brmctx));
 #endif
 }
 
 static void br_multicast_notify_active(struct net_bridge_mcast *brmctx,
 				       bool ip4_active_old, bool ip6_active_old)
 {
-	if (brmctx->ip4_active == ip4_active_old &&
-	    brmctx->ip6_active == ip6_active_old)
+	int ip4_active = READ_ONCE(brmctx->ip4_active);
+	int ip6_active = READ_ONCE(brmctx->ip6_active);
+
+	if (ip4_active == ip4_active_old &&
+	    ip6_active == ip6_active_old)
 		return;
 
 	br_info(brmctx->br, "mc_active changed, vid: %i: v4: %i->%i, v6: %i->%i\n",
 		brmctx->vlan ? brmctx->vlan->vid : -1,
-		ip4_active_old, brmctx->ip4_active,
-		ip6_active_old, brmctx->ip6_active);
+		ip4_active_old, ip4_active, ip6_active_old, ip6_active);
 }
 
 /**
@@ -1136,7 +1138,8 @@ static void br_multicast_notify_active(struct net_bridge_mcast *brmctx,
  */
 static void br_multicast_update_active(struct net_bridge_mcast *brmctx)
 {
-	bool ip4_active_old = brmctx->ip4_active, ip6_active_old = brmctx->ip6_active;
+	bool ip4_active_old = READ_ONCE(brmctx->ip4_active);
+	bool ip6_active_old = READ_ONCE(brmctx->ip6_active);
 	bool force_inactive = false;
 
 	lockdep_assert_held_once(&brmctx->br->multicast_lock);
@@ -4266,13 +4269,13 @@ void br_multicast_ctx_init(struct net_bridge *br,
 	brmctx->multicast_membership_interval = 260 * HZ;
 
 	brmctx->ip4_querier.port_ifidx = 0;
-	brmctx->ip4_active = 0;
+	WRITE_ONCE(brmctx->ip4_active, 0);
 	seqcount_spinlock_init(&brmctx->ip4_querier.seq, &br->multicast_lock);
 	brmctx->multicast_igmp_version = 2;
 #if IS_ENABLED(CONFIG_IPV6)
 	brmctx->multicast_mld_version = 1;
 	brmctx->ip6_querier.port_ifidx = 0;
-	brmctx->ip6_active = 0;
+	WRITE_ONCE(brmctx->ip6_active, 0);
 	seqcount_spinlock_init(&brmctx->ip6_querier.seq, &br->multicast_lock);
 #endif
 
-- 
2.51.0

On Fri, Feb 06, 2026 at 03:52:17AM +0100, Linus Lüssing wrote:
> @@ -1095,23 +1095,25 @@ static void br_ip6_multicast_update_active(struct net_bridge_mcast *brmctx,
>  {
>  #if IS_ENABLED(CONFIG_IPV6)
>  	if (force_inactive)
> -		brmctx->ip6_active = false;
> +		WRITE_ONCE(brmctx->ip6_active, false);
>  	else
> -		brmctx->ip6_active = br_ip6_multicast_querier_exists(brmctx);
> +		WRITE_ONCE(brmctx->ip6_active, br_ip6_multicast_querier_exists(brmctx));
>  #endif
>  }
>  
>  static void br_multicast_notify_active(struct net_bridge_mcast *brmctx,
>  				       bool ip4_active_old, bool ip6_active_old)
>  {
> -	if (brmctx->ip4_active == ip4_active_old &&
> -	    brmctx->ip6_active == ip6_active_old)
> +	int ip4_active = READ_ONCE(brmctx->ip4_active);
> +	int ip6_active = READ_ONCE(brmctx->ip6_active);

I believe that this is unnecessary since we are holding the lock and it
will confuse people reading the code. We only need the READ_ONCE() later
on in the data path

> +
> +	if (ip4_active == ip4_active_old &&
> +	    ip6_active == ip6_active_old)
>  		return;
>  
>  	br_info(brmctx->br, "mc_active changed, vid: %i: v4: %i->%i, v6: %i->%i\n",
>  		brmctx->vlan ? brmctx->vlan->vid : -1,
> -		ip4_active_old, brmctx->ip4_active,
> -		ip6_active_old, brmctx->ip6_active);
> +		ip4_active_old, ip4_active, ip6_active_old, ip6_active);
>  }
>  
>  /**
> @@ -1136,7 +1138,8 @@ static void br_multicast_notify_active(struct net_bridge_mcast *brmctx,
>   */
>  static void br_multicast_update_active(struct net_bridge_mcast *brmctx)
>  {
> -	bool ip4_active_old = brmctx->ip4_active, ip6_active_old = brmctx->ip6_active;
> +	bool ip4_active_old = READ_ONCE(brmctx->ip4_active);
> +	bool ip6_active_old = READ_ONCE(brmctx->ip6_active);

Same

>  	bool force_inactive = false;
>  
>  	lockdep_assert_held_once(&brmctx->br->multicast_lock);
> @@ -4266,13 +4269,13 @@ void br_multicast_ctx_init(struct net_bridge *br,
>  	brmctx->multicast_membership_interval = 260 * HZ;
>  
>  	brmctx->ip4_querier.port_ifidx = 0;
> -	brmctx->ip4_active = 0;
> +	WRITE_ONCE(brmctx->ip4_active, 0);
>  	seqcount_spinlock_init(&brmctx->ip4_querier.seq, &br->multicast_lock);
>  	brmctx->multicast_igmp_version = 2;
>  #if IS_ENABLED(CONFIG_IPV6)
>  	brmctx->multicast_mld_version = 1;
>  	brmctx->ip6_querier.port_ifidx = 0;
> -	brmctx->ip6_active = 0;
> +	WRITE_ONCE(brmctx->ip6_active, 0);
>  	seqcount_spinlock_init(&brmctx->ip6_querier.seq, &br->multicast_lock);
>  #endif
>  
> -- 
> 2.51.0
>