Re: [PATCH net v3] bonding: 3ad: select a port when the TX array is empty

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

Hi Lu Yun,
On Thu, Oct 08, 2026 at 10:40:31AM +0800, Yun Lu wrote:
> From: Yun Lu <luyun@kylinos.cn>
> 
> This issue was observed on a machine in real use. During 802.3ad
> bring-up in production, packets were dropped even though the bond's
> carrier was up and LACP had enabled ports for distribution. The
> transmit path only consults usable_slaves and returns no slave when
> the array is NULL or its count is zero.
> 
> LACP port eligibility and the TX array are updated separately. The
> state machine enables ports and updates carrier before scheduling
> slave_arr_work to rebuild and publish the array. The worker requires
> RTNL and retries if rtnl_trylock() fails, so RTNL contention can
> prolong the interval in which eligible ports exist but the cached
> array is empty.
> 
> Add a fallback for 802.3ad when the usable array is empty and carrier
> is up. Walk the RCU-protected slave list and use ports whose
> aggregator is active and which pass bond_slave_can_tx(), without
> taking mode_lock or changing LACP state. In 802.3ad mode the
> per-slave active flag is set only by the mux machine for ports of the
> active aggregator, so these lockless reads identify distributable
> ports with the same criteria bond_update_slave_arr() applies when
> building the array.

`bond_update_slave_arr()` is called from `bond_enslave()`, and it should be
very fast. Could we add a check in `bond_3ad_state_machine_handler()` or
another suitable place to ensure usable slaves exist before entering the
distribution state?

The fallback path seems somewhat complex and costly. Or you can wait for
Jay's feedback.

Thanks
Hangbin