Subject: Re: [PATCH V3 1/2] virtio-net: fix the set affinity bug when CPU IDs are not consecutive

From: Rusty Russell <[email-masked]>
Date: Wed, 09 Jan 2013 10:01:32 +1030

Wanlong Gao <[email-masked]> writes:
>   */
>  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;
        }

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

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

Thanks,
Rusty.