Subject: Re: [PATCH v2] net: bonding: use atomic instead of rtnl_mutex, to make sure peer notify updated

From: Jay Vosburgh <[email-masked]>
Date: Mon, 03 Nov 2025 22:48:26 +0100

Tonghao Zhang <[email-masked]> wrote:

>Using atomic to protect the send_peer_notif instead of rtnl_mutex.
>This approach allows safe updates in both interrupt and process
>contexts, while avoiding code complexity.
>
>In lacp mode, the rtnl might be locked, preventing ad_cond_set_peer_notif()
>from acquiring the lock and updating send_peer_notif. This patch addresses
>the issue by using a atomic. Since updating send_peer_notif does not
>require high real-time performance, such atomic updates are acceptable.
>
>After coverting the rtnl lock for send_peer_notif to atomic, in bond_mii_monitor(),
>we should check the should_notify_peers (rtnllock required) instead of
>send_peer_notif. By the way, to avoid peer notify event loss, we check
>again whether to send peer notify, such as active-backup mode failover.
>
>Cc: Jay Vosburgh <[email-masked]>
>Cc: "David S. Miller" <[email-masked]>
>Cc: Eric Dumazet <[email-masked]>
>Cc: Jakub Kicinski <[email-masked]>
>Cc: Paolo Abeni <[email-masked]>
>Cc: Simon Horman <[email-masked]>
>Cc: Jonathan Corbet <[email-masked]>
>Cc: Andrew Lunn <[email-masked]>
>Cc: Nikolay Aleksandrov <[email-masked]>
>Cc: Hangbin Liu <[email-masked]>
>Suggested-by: Jay Vosburgh <[email-masked]>
>Signed-off-by: Tonghao Zhang <[email-masked]>
>---
>v2:
>- refine the codes
>- check bond_should_notify_peers again in bond_mii_monitor(), to avoid
>  event loss. 
>- v1 https://patchwork.kernel.org/project/netdevbpf/patch/[email-masked]/
>---
> drivers/net/bonding/bond_3ad.c  |  7 ++---
> drivers/net/bonding/bond_main.c | 46 ++++++++++++++++-----------------
> include/net/bonding.h           |  9 ++++++-
> 3 files changed, 32 insertions(+), 30 deletions(-)
>
>diff --git a/drivers/net/bonding/bond_3ad.c b/drivers/net/bonding/bond_3ad.c
>index 49717b7b82a2..05c573e45450 100644
>--- a/drivers/net/bonding/bond_3ad.c
>+++ b/drivers/net/bonding/bond_3ad.c
>@@ -999,11 +999,8 @@ static void ad_cond_set_peer_notif(struct port *port)
> {
> 	struct bonding *bond = port->slave->bond;
> 
>-	if (bond->params.broadcast_neighbor && rtnl_trylock()) {
>-		bond->send_peer_notif = bond->params.num_peer_notif *
>-			max(1, bond->params.peer_notif_delay);
>-		rtnl_unlock();
>-	}
>+	if (bond->params.broadcast_neighbor)
>+		bond_peer_notify_reset(bond);
> }
> 
> /**
>diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
>index 8e592f37c28b..ae90221838d4 100644
>--- a/drivers/net/bonding/bond_main.c
>+++ b/drivers/net/bonding/bond_main.c
>@@ -1167,10 +1167,11 @@ static bool bond_should_notify_peers(struct bonding *bond)
> {
> 	struct bond_up_slave *usable;
> 	struct slave *slave = NULL;
>+	int send_peer_notif;
> 
>-	if (!bond->send_peer_notif ||
>-	    bond->send_peer_notif %
>-	    max(1, bond->params.peer_notif_delay) != 0 ||
>+	send_peer_notif = atomic_read(&bond->send_peer_notif);
>+	if (!send_peer_notif ||
>+	    send_peer_notif % max(1, bond->params.peer_notif_delay) != 0 ||
> 	    !netif_carrier_ok(bond->dev))
> 		return false;
> 
>@@ -1270,8 +1271,6 @@ void bond_change_active_slave(struct bonding *bond, struct slave *new_active)
> 						      BOND_SLAVE_NOTIFY_NOW);
> 
> 		if (new_active) {
>-			bool should_notify_peers = false;
>-
> 			bond_set_slave_active_flags(new_active,
> 						    BOND_SLAVE_NOTIFY_NOW);
> 
>@@ -1280,19 +1279,17 @@ void bond_change_active_slave(struct bonding *bond, struct slave *new_active)
> 						      old_active);
> 
> 			if (netif_running(bond->dev)) {
>-				bond->send_peer_notif =
>-					bond->params.num_peer_notif *
>-					max(1, bond->params.peer_notif_delay);
>-				should_notify_peers =
>-					bond_should_notify_peers(bond);
>+				bond_peer_notify_reset(bond);
>+
>+				if (bond_should_notify_peers(bond)) {
>+					atomic_dec(&bond->send_peer_notif);
>+					call_netdevice_notifiers(
>+							NETDEV_NOTIFY_PEERS,
>+							bond->dev);
>+				}
> 			}
> 
> 			call_netdevice_notifiers(NETDEV_BONDING_FAILOVER, bond->dev);
>-			if (should_notify_peers) {
>-				bond->send_peer_notif--;
>-				call_netdevice_notifiers(NETDEV_NOTIFY_PEERS,
>-							 bond->dev);
>-			}
> 		}
> 	}
> 
>@@ -2801,7 +2798,7 @@ static void bond_mii_monitor(struct work_struct *work)
> 
> 	rcu_read_unlock();
> 
>-	if (commit || bond->send_peer_notif) {
>+	if (commit || should_notify_peers) {
> 		/* Race avoidance with bond_close cancel of workqueue */
> 		if (!rtnl_trylock()) {
> 			delay = 1;
>@@ -2816,16 +2813,15 @@ static void bond_mii_monitor(struct work_struct *work)
> 			bond_miimon_commit(bond);
> 		}
> 
>-		if (bond->send_peer_notif) {
>-			bond->send_peer_notif--;
>-			if (should_notify_peers)
>-				call_netdevice_notifiers(NETDEV_NOTIFY_PEERS,
>-							 bond->dev);
>-		}
>+		/* check again to avoid send_peer_notif has been changed. */
>+		if (bond_should_notify_peers(bond))
>+			call_netdevice_notifiers(NETDEV_NOTIFY_PEERS, bond->dev);

	Is the risk here that user space may have set send_peer_notify
to zero?

> 
> 		rtnl_unlock();	/* might sleep, hold no other locks */
> 	}
> 
>+	atomic_dec_if_positive(&bond->send_peer_notif);
>+

	Also, it's a bit subtle, but I think this has to be outside of
the if block, or peer_notif_delay may be unreliable.  I'm not sure it
needs a comment, but could you confirm that's why this line is where it
is?

	-J

> re_arm:
> 	if (bond->params.miimon)
> 		queue_delayed_work(bond->wq, &bond->mii_work, delay);
>@@ -3773,7 +3769,7 @@ static void bond_activebackup_arp_mon(struct bonding *bond)
> 			return;
> 
> 		if (should_notify_peers) {
>-			bond->send_peer_notif--;
>+			atomic_dec(&bond->send_peer_notif);
> 			call_netdevice_notifiers(NETDEV_NOTIFY_PEERS,
> 						 bond->dev);
> 		}
>@@ -4267,6 +4263,8 @@ static int bond_open(struct net_device *bond_dev)
> 			queue_delayed_work(bond->wq, &bond->alb_work, 0);
> 	}
> 
>+	atomic_set(&bond->send_peer_notif, 0);
>+
> 	if (bond->params.miimon)  /* link check interval, in milliseconds. */
> 		queue_delayed_work(bond->wq, &bond->mii_work, 0);
> 
>@@ -4300,7 +4298,7 @@ static int bond_close(struct net_device *bond_dev)
> 	struct slave *slave;
> 
> 	bond_work_cancel_all(bond);
>-	bond->send_peer_notif = 0;
>+	atomic_set(&bond->send_peer_notif, 0);
> 	if (bond_is_lb(bond))
> 		bond_alb_deinitialize(bond);
> 	bond->recv_probe = NULL;
>diff --git a/include/net/bonding.h b/include/net/bonding.h
>index 49edc7da0586..afdfcb5bfaf0 100644
>--- a/include/net/bonding.h
>+++ b/include/net/bonding.h
>@@ -236,7 +236,7 @@ struct bonding {
> 	 */
> 	spinlock_t mode_lock;
> 	spinlock_t stats_lock;
>-	u32	 send_peer_notif;
>+	atomic_t send_peer_notif;
> 	u8       igmp_retrans;
> #ifdef CONFIG_PROC_FS
> 	struct   proc_dir_entry *proc_entry;
>@@ -814,4 +814,11 @@ static inline netdev_tx_t bond_tx_drop(struct net_device *dev, struct sk_buff *s
> 	return NET_XMIT_DROP;
> }
> 
>+static inline void bond_peer_notify_reset(struct bonding *bond)
>+{
>+	atomic_set(&bond->send_peer_notif,
>+		bond->params.num_peer_notif *
>+		max(1, bond->params.peer_notif_delay));
>+}
>+
> #endif /* _NET_BONDING_H */
>-- 
>2.34.1
>

---
	-Jay Vosburgh, [email-masked]