Wanlong Gao [off-list ref] writes:

On 01/09/2013 07:31 AM, Rusty Russell wrote:

quoted

Wanlong Gao [off-list ref] writes:

quoted

  */
 static u16 virtnet_select_queue(struct net_device *dev, struct sk_buff *skb)
 {
-	int txq = skb_rx_queue_recorded(skb) ? skb_get_rx_queue(skb) :
-		  smp_processor_id();
+	int txq = 0;
+
+	if (skb_rx_queue_recorded(skb))
+		txq = skb_get_rx_queue(skb);
+	else if ((txq = per_cpu(vq_index, smp_processor_id())) == -1)
+		txq = 0;

You should use __get_cpu_var() instead of smp_processor_id() here, ie:

        else if ((txq = __get_cpu_var(vq_index)) == -1)

And AFAICT, no reason to initialize txq to 0 to start with.

So:

        int txq;

        if (skb_rx_queue_recorded(skb))
		txq = skb_get_rx_queue(skb);
        else {
                txq = __get_cpu_var(vq_index);
                if (txq == -1)
                        txq = 0;
        }

Got it, thank you.

quoted

Now, just to confirm, I assume this can happen even if we use vq_index,
right, because of races with virtnet_set_channels?

I still can't understand this race, could you explain more? thank you.

I assume that someone can call virtnet_set_channels() while we are
inside virtnet_select_queue(), so they reduce dev->real_num_tx_queues,
causing virtnet_set_channels to do:

	while (unlikely(txq >= dev->real_num_tx_queues))
		txq -= dev->real_num_tx_queues;

Otherwise, when is this loop called?

Thanks,
Rusty.