From: r-vdp Commit f31c7937c254 ("bonding: start slaves with link down for ARP monitor") initialises a freshly enslaved port's last_rx to jiffies - (arp_interval + 1) so that it does not "immediately cause fake detection of 'up' state". At the time, the comparison was a plain <= arp_interval and the value was just stale enough. Commit da210f559019 ("bonding: add some slack to arp monitoring time limits"), four months later, added a +arp_interval/2 slack term to every comparison (now bond_time_in_interval()) but did not widen the init to match. Since then, bond_time_in_interval(bond, last_rx, 1) is true for the first ~arp_interval/2 after enslavement even though no packet has been received: the upper bound is last_rx + 1.5*delta and last_rx was set to jiffies - delta - 1. If the ARP monitor tick lands in that window, bond_ab_arp_inspect() proposes the slave UP. If the slave is the configured primary, bond_ab_arp_commit() sets do_failover and the still-armed force_primary in bond_choose_primary_or_current() makes it the active slave regardless of primary_reselect. ARP validation as the active slave then fails (the link has not actually received anything; on SFP+ ports the PHY is often still negotiating) and the bond falls back to the backup. With primary_reselect=failure, force_primary has now been spent and the bond stays on the backup until something else triggers a reselect. Reproducer: ip link add bond0 type bond mode active-backup arp_interval 1000 \ arp_validate all arp_ip_target 192.0.2.1 \ primary eth0 primary_reselect failure # eth0: SFP+ (slow link-up), eth1: RJ45 (fast link-up) ip link set eth0 master bond0 ip link set eth1 master bond0 ip link set bond0 up # bond0 lands on eth0 via force_primary, ARP-fails it before the # SFP+ has carrier, falls to eth1, and stays there. Initialise last_rx (and the per-target array, and last_tx) to two full intervals in the past so it is outside the slack window from the start. Fixes: da210f559019 ("bonding: add some slack to arp monitoring time limits") Signed-off-by: r-vdp --- drivers/net/bonding/bond_main.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c index 522eab060f9ed..23e1544faf39f 100644 --- a/drivers/net/bonding/bond_main.c +++ b/drivers/net/bonding/bond_main.c @@ -2123,7 +2123,7 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev, new_slave->link = BOND_LINK_DOWN; new_slave->last_rx = jiffies - - (msecs_to_jiffies(bond->params.arp_interval) + 1); + (2 * msecs_to_jiffies(bond->params.arp_interval) + 1); for (i = 0; i < BOND_MAX_ARP_TARGETS; i++) new_slave->target_last_arp_rx[i] = new_slave->last_rx; --- base-commit: 24ef02f934eeb48830cff6b739abc3c62b1d107b change-id: 20260817-bonding-last-rx-0303fa048f9d Best regards, -- r-vdp