netdev - Re: [PATCH] ifb: add multi-queue support

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Date:	Fri, 13 Nov 2009 09:28:45 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Stephen Hemminger <shemminger@...tta.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Tom Herbert <therbert@...gle.com>, netdev@...r.kernel.org
Subject: Re: [PATCH] ifb: add multi-queue support

On Thu, Nov 12, 2009 at 11:10 PM, Patrick McHardy <kaber@...sh.net> wrote:
> Changli Gao wrote:
>>>> +                     rcu_read_lock();
>>>> +                     skb->dev = dev_get_by_index_rcu(&init_net, skb->iif);
>>>> +                     if (!skb->dev) {
>>>> +                             rcu_read_unlock();
>>>> +                             dev_kfree_skb(skb);
>>>> +                             txq->tx_dropped++;
>>>> +                             break;
>>>> +                     }
>>>> +                     rcu_read_unlock();
>>>> +                     skb->iif = dev->ifindex;
>>> What protects the device from disappearing here and below during
>>> dev_queue_xmit() and netif_rx_ni()?
>>
>> For dev_queue_xmit(), dev is holded by skb->_dst, so there is no
>> problem. But for netif_rx_ni(), I don't know how to prevent the device
>> disappearing, and it seems that all the NIC drivers have this problem.
>> Maybe there was the assumption about the execution context of
>> netif_rx() before. Now softirq can't be executed by softirqd, so the
>> packet receiving path maybe interleaved. I don't know how to prevent
>> it happening.
>
> You can either take a reference of move the rcu_read_unlock()
> after the skb submission.

good idea!

>
>>>> +             break;
>>>> +     case __constant_htons(ETH_P_IPV6):
>>>> +             ip_proto = ipv6_hdr(skb)->nexthdr;
>>>> +             addr1 = ipv6_hdr(skb)->saddr.s6_addr32[3];
>>>> +             addr2 = ipv6_hdr(skb)->daddr.s6_addr32[3];
>>>> +             ihl = 10;
>>> Where does 10 come from?
>>
>> It should be 40, after reviewing IPV6, I found that I need to loop
>> until finding the right protocol value.
>
> There is a helper for this (ipv6_skip_exthdr).
>

It is a little late. Thanks all the same, and I'll use it.

>>>> +static int ifb_get_tx_queues(struct net *net, struct nlattr *tb[],
>>>> +                          unsigned int *num_tx_queues,
>>>> +                          unsigned int *real_num_tx_queues)
>>>> +{
>>>> +     if (tb[IFLA_NTXQ]) {
>>>> +             *num_tx_queues = nla_get_u16(tb[IFLA_NTXQ]);
>>> We currently use unsigned ints for the queue number, so please
>>> use an u32 for the attribute as well.
>>>
>>>> +             *real_num_tx_queues = *num_tx_queues;
>>>> +     } else {
>>>> +             *num_tx_queues = numtxqs;
>>>> +             *real_num_tx_queues = numtxqs;
>>>> +     }
>>>> +
>>>> +     return 0;
>>>> +}
>>>> +
>>         u16                     (*ndo_select_queue)(struct net_device *dev,
>>                                                     struct sk_buff *skb);
>> use u16 as the return value so ..., and I think u16 is big enough. If
>> you insist on this, I'll use u32 instead.
>
> Well, get_tx_queues() uses unsigned int, as does struct net_device.
> I agree that it probably won't ever be needed, but there's no harm
> in using the correct type, the attribute encoding doesn't even need
> more room.
>

So, in the last patch, I use it instead. :)

-- 
Regards，
Changli Gao(xiaosuo@...il.com)
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