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

Wanlong Gao
2013-01-09T02:26:04Z

On 01/09/2013 07:31 AM, Rusty Russell wrote:
> Wanlong Gao <gaowanlong@cn.fujitsu.com> 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.
>