Messages in this thread |  | | From | Rusty Russell <> | Subject | Re: [PATCH V3 1/2] virtio-net: fix the set affinity bug when CPU IDs are not consecutive | Date | Thu, 10 Jan 2013 11:19:09 +1030 |
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Wanlong Gao <gaowanlong@cn.fujitsu.com> writes: > 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.
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.
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