Re: [dpdk-dev] [PATCH 2/2] bond: add mode 4 support

[Neil Horman <nhorman@tuxdriver.com>]

On Thu, Sep 18, 2014 at 08:07:31AM +0000, Wodkowski, PawelX wrote:
> > > +int
> > > +bond_mode_8023ad_deactivate_slave(struct rte_eth_dev *bond_dev,
> > > +		uint8_t slave_pos)
> > > +{
> > > +	struct bond_dev_private *internals = bond_dev->data->dev_private;
> > > +	struct mode8023ad_data *data = &internals->mode4;
> > > +	struct port *port;
> > > +	uint8_t i;
> > > +
> > > +	bond_mode_8023ad_stop(bond_dev);
> > > +
> > > +	/* Exclude slave from transmit policy. If this slave is an aggregator
> > > +	 * make all aggregated slaves unselected to force sellection logic
> > > +	 * to select suitable aggregator for this port	 */
> > > +	for (i = 0; i < internals->active_slave_count; i++) {
> > > +		port = &data->port_list[slave_pos];
> > > +		if (port->used_agregator_idx == slave_pos) {
> > > +			port->selected = UNSELECTED;
> > > +			port->actor_state &= ~(STATE_SYNCHRONIZATION |
> > STATE_DISTRIBUTING |
> > > +				STATE_COLLECTING);
> > > +
> > > +			/* Use default aggregator */
> > > +			port->used_agregator_idx = i;
> > > +		}
> > > +	}
> > > +
> > > +	port = &data->port_list[slave_pos];
> > > +	timer_cancel(&port->current_while_timer);
> > > +	timer_cancel(&port->periodic_timer);
> > > +	timer_cancel(&port->wait_while_timer);
> > > +	timer_cancel(&port->tx_machine_timer);
> > > +
> > These all seem rather racy.  Alarm callbacks are executed with the alarm list
> > locks not held.  So there is every possibility that you could execute these (or
> > any timer_cancel calls in this PMD in parallel with the internal state machine
> > timer callback, and leave either with a corrupted timer list (resulting from a
> > double free between here, and the actual callback site),
> 
> I don't think so. Yes, callbacks are executed with  alarm list locks not held, but 
> this is not the issue because access to list itself is guarded by lock and 
> ap->executing variable. So list will not be trashed. Check source of 
> eal_alarm_callback(), rte_eal_alarm_set() and rte_eal_alarm_cancel().
> 
Yes, you're right, the list is probably safe wht the executing bit.

> > or a timer that is
> > actually still pending when a slave is removed.
> > 
> This is not the issue also, but problem might be similar. I assumed that alarms
> are atomic but when I looked at rte alarms closer I saw a race condition
> between and rte_eal_alarm_cancel() from  bond_mode_8023ad_stop()
> and rte_eal_alarm_set() from state machines callback. This need to be 
> reworked in some way.

Yes, this is what I was referring to:

CPU0				CPU1
rte_eal_alarm_callback		bond_8023ad_deactivate_slave
-bond_8023_ad_periodic_cb	timer_cancel
timer_set

If those timer functions operate on the same timer, the result is that you can
leave the stop/deactivate slave paths with a timer function for that slave still
pending. The bonding mode needs some internal state to serialize those
operations and determine if the timer should be reactivated.

Neil