drop_monitor: Reset per-CPU data before starting to trace

The function reset_per_cpu_data() allocates and prepares a new skb for
the summary netlink alert message ('NET_DM_CMD_ALERT'). The new skb is
stored in the per-CPU 'data' variable and the old is returned.

The function is invoked during module initialization and from the
workqueue, before an alert is sent. This means that it is possible to
receive an alert with stale data, if we stopped tracing when the
hysteresis timer ('data->send_timer') was pending.

Instead of invoking the function during module initialization, invoke it
just before we start tracing and ensure we get a fresh skb.

This also allows us to remove the calls to initialize the timer and the
work item from the module initialization path, since both could have
been triggered by the error paths of reset_per_cpu_data().
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/drop_monitor.c

@@ -252,9 +252,16 @@ static int net_dm_trace_on_set(struct netlink_ext_ack *extack)
 
 	for_each_possible_cpu(cpu) {
 		struct per_cpu_dm_data *data = &per_cpu(dm_cpu_data, cpu);
+		struct sk_buff *skb;
 
 		INIT_WORK(&data->dm_alert_work, send_dm_alert);
 		timer_setup(&data->send_timer, sched_send_work, 0);
+		/* Allocate a new per-CPU skb for the summary alert message and
+		 * free the old one which might contain stale data from
+		 * previous tracing.
+		 */
+		skb = reset_per_cpu_data(data);
+		consume_skb(skb);
 	}
 
 	rc = register_trace_kfree_skb(trace_kfree_skb_hit, NULL);
@@ -475,10 +482,7 @@ static int __init init_net_drop_monitor(void)
 
 	for_each_possible_cpu(cpu) {
 		data = &per_cpu(dm_cpu_data, cpu);
-		INIT_WORK(&data->dm_alert_work, send_dm_alert);
-		timer_setup(&data->send_timer, sched_send_work, 0);
 		spin_lock_init(&data->lock);
-		reset_per_cpu_data(data);
 	}
 
 	goto out;