netdev - Re: [PATCH net] tcp: avoid creating multiple req socks with the same tuples

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Date:   Wed, 5 Jun 2019 07:18:44 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Zhiqiang Liu <liuzhiqiang26@...wei.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Mao Wenan <maowenan@...wei.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] tcp: avoid creating multiple req socks with the same
 tuples



On 6/5/19 1:52 AM, Zhiqiang Liu wrote:
> 
> 
> 在 2019/6/4 23:24, Eric Dumazet 写道:
>> On Tue, Jun 4, 2019 at 7:47 AM Mao Wenan <maowenan@...wei.com> wrote:
>>>
>>> There is one issue about bonding mode BOND_MODE_BROADCAST, and
>>> two slaves with diffierent affinity, so packets will be handled
>>> by different cpu. These are two pre-conditions in this case.
> 
>>> Signed-off-by: Mao Wenan <maowenan@...wei.com>
>>> --
>>
>> This issue has been discussed last year.
>>
>> I am afraid your patch does not solve all races.
>>
>> The lookup you add is lockless, so this is racy.
>>
>> Really the only way to solve this is to make sure that _when_ the
>> bucket lock is held,
>> we do not insert a request socket if the 4-tuple is already in the
>> chain (probably in inet_ehash_insert())
>>
>> This needs more tricky changes than your patch.
>>
> 
> This kind case is rarely used, and the condition of the issue is strict.
> If we add the "lookup" before or in inet_ehash_insert func for each reqsk,
> overall performance will be affected.
> 
> We may solve the small probability issue with a trick in the tcp_v4_rcv.
> If the ACK is invalid checked by tcp_check_req func, the req could be dropped,
> and then goto the lookup for searching another avaliable reqsk. In this way,
> the performance will not be affected in the normal process.
> 
> The patch is given as following:
> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> index a2896944aa37..9d0491587ed2 100644
> --- a/net/ipv4/tcp_ipv4.c
> +++ b/net/ipv4/tcp_ipv4.c
> @@ -1874,8 +1874,10 @@ int tcp_v4_rcv(struct sk_buff *skb)
>                         goto discard_and_relse;
>                 }
>                 if (nsk == sk) {
> -                       reqsk_put(req);
> +                       inet_csk_reqsk_queue_drop_and_put(sk, req);
>                         tcp_v4_restore_cb(skb);
> +                       sock_put(sk);
> +                       goto lookup;
>                 } else if (tcp_child_process(sk, nsk, skb)) {
>                         tcp_v4_send_reset(nsk, skb);
>                         goto discard_and_relse;
> 

This is not solving the race.

Please read again my prior emails.

If you want to work on this issue, you have to fix it for good.

Thanks.