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

["Wodkowski, PawelX" <pawelx.wodkowski@intel.com>]

> -----Original Message-----
> From: Neil Horman [mailto:nhorman@tuxdriver.com]
> Sent: Thursday, September 18, 2014 18:03
> To: Wodkowski, PawelX
> Cc: dev@dpdk.org; Jastrzebski, MichalX K; Doherty, Declan
> Subject: Re: [dpdk-dev] [PATCH 2/2] bond: add mode 4 support
> 
> 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

I did rethink the issue and problem is much simpler than it looks like. I did the 
following:
1. Change internal state machine alarms to use rte_rdtsc(). This makes all 
 mode 4 internal timer_*() function not affected by any race condition.
2. Do a busy loop when canceling main callback timer until cancel is successfull.
This should do the trick about race condition. Do you agree?

Here is part involving timers I have changed:

static void
-timer_expired_cb(void *arg)
+timer_stop(uint64_t *timer)
 {
-	enum timer_state *timer = arg;
-
-	BOND_ASSERT(*timer == TIMER_RUNNING);
-	*timer = TIMER_EXPIRED;
+	*timer = 0;
 }
 
 static void
-timer_cancel(enum timer_state *timer)
+timer_set(uint64_t *timer, uint64_t timeout_ms)
 {
-	rte_eal_alarm_cancel(&timer_expired_cb, timer);
-	*timer = TIMER_NOT_STARTED;
+	*timer = rte_rdtsc() + timeout_ms * rte_get_tsc_hz() / 1000;
 }
 
+/* Forces given timer to be in expired state. */
 static void
-timer_set(enum timer_state *timer, uint64_t timeout)
+timer_force_expired(uint64_t *timer)
 {
-	rte_eal_alarm_cancel(&timer_expired_cb, timer);
-	rte_eal_alarm_set(timeout * 1000, &timer_expired_cb, timer);
-	*timer = TIMER_RUNNING;
+	*timer = rte_rdtsc();
 }
 
 static bool
-timer_is_expired(enum timer_state *timer)
+timer_is_stopped(uint64_t *timer)
 {
-	return *timer == TIMER_EXPIRED;
+	return *timer == 0;
+}
+
+/* Timer is in running state if it is not stopped nor expired */
+static bool
+timer_is_running(uint64_t *timer)
+{
+	return *timer > 0 && *timer < rte_rdtsc();
+}
+
+
+static bool
+timer_is_expired(uint64_t *timer)
+{
+	return *timer <= rte_rdtsc();
 }
---

And part stopping mode 4 callback.

-int
+void
 bond_mode_8023ad_stop(struct rte_eth_dev *bond_dev)
 {
-	rte_eal_alarm_cancel(bond_mode_8023ad_periodic_cb, bond_dev);
-	return 0;
+	/* Loop untill we cancel pending alarm. Alarm that is executing will
+	 * not be canceled but when reshedules it self it will be canceled. */
+	while (rte_eal_alarm_cancel(&bond_mode_8023ad_periodic_cb, bond_dev) == 0)
+		rte_pause();
 }