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

From: Wanlong Gao <[email-masked]>
Date: Wed, 09 Jan 2013 09:54:59 +0800

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

Got it, thank you.

> 
> 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.

Regards,
Wanlong Gao

> 
>   	while (unlikely(txq >= dev->real_num_tx_queues))
>   		txq -= dev->real_num_tx_queues;
> 
> 
> Thanks,
> Rusty.
>