Re: [PATCH net] selftests: bonding: fix failover race in bond_options.sh prio test

[Hangbin Liu <hangbin.liu@linux.dev>]

Hi Pin-yen,
On Wed, Nov 11, 2026 at 05:46:18PM -0800, Pin-yen Lin wrote:
> The prio test intermittently fails in the arp_ip_target and
> ns_ip6_target variants:
> 
>   TEST: prio (active-backup ns_ip6_target primary_reselect 1)   [FAIL]
>   	Current active slave is eth0 but not eth2
> 
> The ARP/NS monitor marks a slave down when it has received nothing for
> too long. With arp_interval 100 and the default arp_missed_max, that is
> about 250ms for the active slave (eth1) and 350ms for a backup slave
> (eth0, eth2). Every 100ms, eth1 sends a probe: eth1 receives the reply,
> and eth0/eth2 receive a copy forwarded by the gateway bridge. Once eth1
> is down, its probes reach no one and no slave receives anything, so
> eth1 normally times out first and the new active slave is picked by
> prio:
> 
>   t=0    probe: eth1 gets the reply, eth0/eth2 get a copy
>   t=10   test sets eth1 down
>   t=300  monitor: eth1 idle 300ms -> down
>                   eth0/eth2 idle 300ms -> up, eth2 picked by prio
> 
> But the test takes eth1 down right after bond_check_connection(), whose
> ping replies only arrive on eth1:
> 
>   t=0    probe: eth1 gets the reply, eth0/eth2 get a copy
>   t=70   ping reply on eth1 only
>   t=85   test sets eth1 down
>   t=300  monitor: eth1 idle 230ms -> still up
>   t=400  monitor: eth1 idle 330ms -> down
>                   eth2 idle 400ms -> down too
> 
> eth2 can then no longer be picked. eth0 becomes the active slave if it
> happened to receive some other packet (e.g. an MLD report), otherwise
> whichever slave comes back first does. With primary_reselect 1 or 2,
> the bond keeps eth0 even after eth2 comes back.
> 
> Before taking the active slave down, wait two monitor intervals
> (200ms), so that at least one more probe is sent after the last ping
> reply and all slaves last received that probe. The miimon variants do
> not need the wait, but the extra delay is negligible.

I'm afraid the waiting time is too strict, especially with a debug kernel
since delays can happen.

Bjoern is also fixing[1] this issue by pinging a non-existent neighbor to
keep traffic flowing. Would you like to test whether that works for you?

[1] https://lore.kernel.org/netdev/20261104142018.552135-1-doebel@amazon.de/

Thanks
Hangbin

> 
> Hit in our CI on GCE VMs (4 of 51 runs); the test only uses veth. On
> the same VMs, 7 of 30 loops of the prio tests failed without this
> change, and 20 of 20 loops passed with it.
> 
> Fixes: 481b56e0391e ("selftests: bonding: re-format bond option tests")
> Signed-off-by: Pin-yen Lin <treapking@google.com>
> ---
> 
>  .../drivers/net/bonding/bond_options.sh       | 26 ++++++++++++++++---
>  1 file changed, 22 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/testing/selftests/drivers/net/bonding/bond_options.sh b/tools/testing/selftests/drivers/net/bonding/bond_options.sh
> index 187b478d0ddf..16c1c2d97005 100755
> --- a/tools/testing/selftests/drivers/net/bonding/bond_options.sh
> +++ b/tools/testing/selftests/drivers/net/bonding/bond_options.sh
> @@ -13,6 +13,24 @@ ALL_TESTS="
>  
>  lib_dir=$(dirname "$0")
>  source ${lib_dir}/bond_topo_3d1c.sh
> +
> +# The ARP/NS monitor marks a slave down when it has received nothing for too
> +# long: ~250ms for the active slave and ~350ms for a backup slave with
> +# arp_interval 100. The backups only receive the copies of the active slave's
> +# probes that the gateway bridge forwards, but the active slave also gets the
> +# replies to the pings in bond_check_connection(). If the active slave goes
> +# down right after such a reply, it times out in the same monitor run as the
> +# backups, and the new active slave is then not picked by prio. Wait two
> +# monitor intervals, so that at least one more probe is sent and all slaves
> +# last received that probe.
> +bond_link_down()
> +{
> +	local slave=$1
> +
> +	sleep 0.2
> +	ip -n ${s_ns} link set "$slave" down
> +}
> +
>  c_maddr="33:33:ff:00:00:10"
>  g_maddr="33:33:ff:00:02:54"
>  
> @@ -97,12 +115,12 @@ prio_test()
>  	check_active_slave eth1
>  
>  	# active slave should be the higher prio slave
> -	ip -n ${s_ns} link set $active_slave down
> +	bond_link_down $active_slave
>  	check_active_slave eth2
>  	bond_check_connection "fail over"
>  
>  	# when only 1 slave is up
> -	ip -n ${s_ns} link set $active_slave down
> +	bond_link_down $active_slave
>  	check_active_slave eth0
>  	bond_check_connection "only 1 slave up"
>  
> @@ -134,7 +152,7 @@ prio_test()
>  			;;
>  		"2")
>  			check_active_slave "$pre_active_slave"
> -			ip -n ${s_ns} link set $active_slave down
> +			bond_link_down $active_slave
>  			bond_check_connection "pre_active slave down"
>  			check_active_slave "eth1"
>  			;;
> @@ -153,7 +171,7 @@ prio_test()
>  		check_err $? "eth3 prio is not -50"
>  		check_active_slave "eth1"
>  
> -		ip -n ${s_ns} link set $active_slave down
> +		bond_link_down $active_slave
>  		check_active_slave "eth0"
>  		bond_check_connection "change slave prio"
>  	fi
> -- 
> 2.56.1.703.gf848ed807c-goog
> 
> base-commit: 665b4149e91b781a71bce8637c3a4d7d64dba05f