Re: [PATCH net 2/3] bonding: restructure ad_churn_machine

[Hangbin Liu <liuhangbin@gmail.com>]

On Thu, Nov 27, 2025 at 11:36:43AM +0100, Paolo Abeni wrote:
> On 11/24/25 5:33 AM, Hangbin Liu wrote:
> > The current ad_churn_machine implementation only transitions the
> > actor/partner churn state to churned or none after the churn timer expires.
> > However, IEEE 802.1AX-2014 specifies that a port should enter the none
> > state immediately once the actor’s port state enters synchronization.
> > 
> > Another issue is that if the churn timer expires while the churn machine is
> > not in the monitor state (e.g. already in churn), the state may remain
> > stuck indefinitely with no further transitions. This becomes visible in
> > multi-aggregator scenarios. For example:
> > 
> > Ports 1 and 2 are in aggregator 1 (active)
> > Ports 3 and 4 are in aggregator 2 (backup)
> > 
> > Ports 1 and 2 should be in none
> > Ports 3 and 4 should be in churned
> > 
> > If a failover occurs due to port 2 link down/up, aggregator 2 becomes active.
> > Under the current implementation, the resulting states may look like:
> > 
> > agg 1 (backup): port 1 -> none, port 2 -> churned
> > agg 2 (active): ports 3,4 keep in churned.
> > 
> > The root cause is that ad_churn_machine() only clears the
> > AD_PORT_CHURNED flag and starts a timer. When a churned port becomes active,
> > its RX state becomes AD_RX_CURRENT, preventing the churn flag from being set
> > again, leaving no way to retrigger the timer. Fixing this solely in
> > ad_rx_machine() is insufficient.
> > 
> > This patch rewrites ad_churn_machine according to IEEE 802.1AX-2014
> > (Figures 6-23 and 6-24), ensuring correct churn detection, state transitions,
> > and timer behavior. With new implementation, there is no need to set
> > AD_PORT_CHURNED in ad_rx_machine().
> 
> I think this change is too invasive at this point of the cycle. I think
> it should be moved to the next one or even to net-next.

Sure, I can move it to net-next

> > @@ -1365,39 +1361,107 @@ static void ad_rx_machine(struct lacpdu *lacpdu, struct port *port)
> >   * ad_churn_machine - handle port churn's state machine
> >   * @port: the port we're looking at
> >   *
> > + * IEEE 802.1AX-2014 Figure 6-23 - Actor Churn Detection machine state diagram
> > + *
> > + *                                                     BEGIN || (! port_enabled)
> > + *                                                               |
> > + *                                      (3)                (1)   v
> > + *   +----------------------+     ActorPort.Sync     +-------------------------+
> > + *   |    NO_ACTOR_CHURN    | <--------------------- |   ACTOR_CHURN_MONITOR   |
> > + *   |======================|                        |=========================|
> > + *   | actor_churn = FALSE; |    ! ActorPort.Sync    | actor_churn = FALSE;    |
> > + *   |                      | ---------------------> | Start actor_churn_timer |
> > + *   +----------------------+           (4)          +-------------------------+
> > + *             ^                                                 |
> > + *             |                                                 |
> > + *             |                                      actor_churn_timer expired
> > + *             |                                                 |
> > + *       ActorPort.Sync                                          |  (2)
> > + *             |              +--------------------+             |
> > + *        (3)  |              |   ACTOR_CHURN      |             |
> > + *             |              |====================|             |
> > + *             +------------- | actor_churn = True | <-----------+
> > + *                            |                    |
> > + *                            +--------------------+
> > + *
> > + * Similar for the Figure 6-24 - Partner Churn Detection machine state diagram
> >   */
> >  static void ad_churn_machine(struct port *port)
> >  {
> > -	if (port->sm_vars & AD_PORT_CHURNED) {
> > +	bool partner_synced = port->partner_oper.port_state & LACP_STATE_SYNCHRONIZATION;
> > +	bool actor_synced = port->actor_oper_port_state & LACP_STATE_SYNCHRONIZATION;
> > +	bool partner_churned = port->sm_vars & AD_PORT_PARTNER_CHURN;
> > +	bool actor_churned = port->sm_vars & AD_PORT_ACTOR_CHURN;
> > +
> > +	/* ---- 1. begin or port not enabled ---- */
> > +	if ((port->sm_vars & AD_PORT_BEGIN) || !port->is_enabled) {
> >  		port->sm_vars &= ~AD_PORT_CHURNED;
> > +
> >  		port->sm_churn_actor_state = AD_CHURN_MONITOR;
> >  		port->sm_churn_partner_state = AD_CHURN_MONITOR;
> > +
> >  		port->sm_churn_actor_timer_counter =
> >  			__ad_timer_to_ticks(AD_ACTOR_CHURN_TIMER, 0);
> >  		port->sm_churn_partner_timer_counter =
> > -			 __ad_timer_to_ticks(AD_PARTNER_CHURN_TIMER, 0);
> > +			__ad_timer_to_ticks(AD_PARTNER_CHURN_TIMER, 0);
> > +
> 
> Please avoid white-space changes only, or if you are going to target
> net-next, move them to a pre-req patch.

OK, what's pre-req patch?

> 
> >  		return;
> >  	}
> > -	if (port->sm_churn_actor_timer_counter &&
> > -	    !(--port->sm_churn_actor_timer_counter) &&
> > -	    port->sm_churn_actor_state == AD_CHURN_MONITOR) {
> > -		if (port->actor_oper_port_state & LACP_STATE_SYNCHRONIZATION) {
> > +
> > +	if (port->sm_churn_actor_timer_counter)
> > +		port->sm_churn_actor_timer_counter--;
> > +
> > +	if (port->sm_churn_partner_timer_counter)
> > +		port->sm_churn_partner_timer_counter--;
> > +
> > +	/* ---- 2. timer expired, enter CHURN ---- */
> > +	if (port->sm_churn_actor_state == AD_CHURN_MONITOR &&
> > +	    !actor_churned && !port->sm_churn_actor_timer_counter) {
> > +		port->sm_vars |= AD_PORT_ACTOR_CHURN;
> > +		port->sm_churn_actor_state = AD_CHURN;
> > +		port->churn_actor_count++;
> > +		actor_churned = true;
> > +	}
> > +
> > +	if (port->sm_churn_partner_state == AD_CHURN_MONITOR &&
> > +	    !partner_churned && !port->sm_churn_partner_timer_counter) {
> > +		port->sm_vars |= AD_PORT_PARTNER_CHURN;
> > +		port->sm_churn_partner_state = AD_CHURN;
> > +		port->churn_partner_count++;
> > +		partner_churned = true;
> > +	}
> > +
> > +	/* ---- 3. CHURN_MONITOR/CHURN + sync -> NO_CHURN ---- */
> > +	if ((port->sm_churn_actor_state == AD_CHURN_MONITOR && !actor_churned) ||
> > +	    (port->sm_churn_actor_state == AD_CHURN && actor_churned)) {
> 
> Is this                                             ^^^^^^^^^^^^^^^^
> 
> test needed ? I *think* the state machine `actor_churned == true` when
> `sm_churn_actor_state == AD_CHURN`

Yeah... We don't need this in theory.

> 
> > +		if (actor_synced) {
> > +			port->sm_vars &= ~AD_PORT_ACTOR_CHURN;
> >  			port->sm_churn_actor_state = AD_NO_CHURN;
> > -		} else {
> > -			port->churn_actor_count++;
> > -			port->sm_churn_actor_state = AD_CHURN;
> > +			actor_churned = false;
> >  		}
> 
> I think this part is not described by the state diagram above?!?

This part is about path (3), port in monitor or churn, and actor is in sync.
Then move to state no_churn.

Do you mean port->sm_vars &= ~AD_PORT_ACTOR_CHURN is not described?
Hmm, maybe we don't need this after re-organise.


> 
> >  	}
> > -	if (port->sm_churn_partner_timer_counter &&
> > -	    !(--port->sm_churn_partner_timer_counter) &&
> > -	    port->sm_churn_partner_state == AD_CHURN_MONITOR) {
> > -		if (port->partner_oper.port_state & LACP_STATE_SYNCHRONIZATION) {
> > +
> > +	if ((port->sm_churn_partner_state == AD_CHURN_MONITOR && !partner_churned) ||
> > +	    (port->sm_churn_partner_state == AD_CHURN && partner_churned)) {
> > +		if (partner_synced) {
> > +			port->sm_vars &= ~AD_PORT_PARTNER_CHURN;
> >  			port->sm_churn_partner_state = AD_NO_CHURN;
> > -		} else {
> > -			port->churn_partner_count++;
> > -			port->sm_churn_partner_state = AD_CHURN;
> > +			partner_churned = false;
> >  		}
> 
> Possibly move this `if` block in a separate helper and reuse for both
> partner and actor.

OK, let me try.

> 
> >  	}
> > +
> > +	/* ---- 4. NO_CHURN + !sync -> MONITOR ---- */
> > +	if (port->sm_churn_actor_state == AD_NO_CHURN && !actor_churned && !actor_synced) {
> > +		port->sm_churn_actor_state = AD_CHURN_MONITOR;
> > +		port->sm_churn_actor_timer_counter =
> > +			__ad_timer_to_ticks(AD_ACTOR_CHURN_TIMER, 0);
> 
> Should this clear sm_vars & AD_PORT_ACTOR_CHURN, too?

Yes, or we can just remove AD_PORT_ACTOR_CHURN as I said above.

Thanks
Hangbin