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Subject: Re: [PATCH net v2] tcp: avoid creating multiple req socks with the
 same tuples
To:     maowenan <maowenan@huawei.com>, Eric Dumazet <edumazet@google.com>
Cc:     David Miller <davem@davemloft.net>,
        netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
References: <20190612035715.166676-1-maowenan@huawei.com>
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From:   Eric Dumazet <eric.dumazet@gmail.com>
Message-ID: <6aa69ab5-ed81-6a7f-2b2b-214e44ff0ada@gmail.com>
Date:   Thu, 13 Jun 2019 21:28:21 -0700
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On 6/13/19 9:19 PM, maowenan wrote:
> 
> 
> @Eric, for this issue I only want to check TCP_NEW_SYN_RECV sk, is it OK like below?
>  +       if (!osk && sk->sk_state == TCP_NEW_SYN_RECV)
>  +               reqsk = __inet_lookup_established(sock_net(sk), &tcp_hashinfo,
>  +                                                       sk->sk_daddr, sk->sk_dport,
>  +                                                       sk->sk_rcv_saddr, sk->sk_num,
>  +                                                       sk->sk_bound_dev_if, sk->sk_bound_dev_if);
>  +       if (unlikely(reqsk)) {
> 

Not enough.

If we have many cpus here, there is a chance another cpu has inserted a request socket, then
replaced it by an ESTABLISH socket for the same 4-tuple.

We need to take the per bucket spinlock much sooner.

And this is fine, all what matters is that we do no longer grab the listener spinlock.