netdev - Re: [PATCH net-next v2 11/14] net: bridge: mcast: track active state, prepare for outside lock reads

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Message-ID: <20260208161126.GK154003@shredder>
Date: Sun, 8 Feb 2026 18:11:26 +0200
From: Ido Schimmel <idosch@...dia.com>
To: Linus Lüssing <linus.luessing@...3.blue>
Cc: bridge@...ts.linux.dev, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Nikolay Aleksandrov <razor@...ckwall.org>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	Simon Horman <horms@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	"David S . Miller" <davem@...emloft.net>,
	Kuniyuki Iwashima <kuniyu@...gle.com>,
	Stanislav Fomichev <sdf@...ichev.me>,
	Xiao Liang <shaw.leon@...il.com>
Subject: Re: [PATCH net-next v2 11/14] net: bridge: mcast: track active
 state, prepare for outside lock reads

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
>