netdev - Re: [RFC PATCH v2 net-next 4/4] tcp: change SYN ACK retransmit behaviour to account for rehash

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Date:   Thu, 2 Dec 2021 09:24:10 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Akhmat Karakotov <hmukos@...dex-team.ru>
Cc:     eric.dumazet@...il.com, mitradir@...dex-team.ru,
        netdev@...r.kernel.org, tom@...bertland.com, zeil@...dex-team.ru
Subject: Re: [RFC PATCH v2 net-next 4/4] tcp: change SYN ACK retransmit
 behaviour to account for rehash

On Thu, Dec 2, 2021 at 8:41 AM Akhmat Karakotov <hmukos@...dex-team.ru> wrote:
>
> Disabling rehash behavior did not affect SYN ACK retransmits because hash
> was forcefully changed bypassing the sk_rethink_hash function. This patch
> adds a condition which checks for rehash mode before resetting hash.
>
> Signed-off-by: Akhmat Karakotov <hmukos@...dex-team.ru>
> ---
>  net/ipv4/tcp_output.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 6d72f3ea48c4..7d54bbe00cde 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -4108,7 +4108,8 @@ int tcp_rtx_synack(const struct sock *sk, struct request_sock *req)
>         struct flowi fl;
>         int res;
>
> -       tcp_rsk(req)->txhash = net_tx_rndhash();
> +       if (sk->sk_txrehash == SOCK_TXREHASH_ENABLED)

Since you add a new sk->sk_txrehash, you probably want to add
READ_ONCE()/WRITE_ONCE() over the reads/writes,
because at this point the listener socket lock is not held.

      /* Paired with WRITE_ONCE() in sock_setsockopt() */
       if (READ_ONCE(sk->sk_txrehash) == SOCK_TXREHASH_ENABLED)

sock_setsockopt() would need a similar comment
    /* Paired with READ_ONCE() in tcp_rtx_synack() */
     WRITE_ONCE(sk->sk_txrehash, val);



> +               tcp_rsk(req)->txhash = net_tx_rndhash();
>         res = af_ops->send_synack(sk, NULL, &fl, req, NULL, TCP_SYNACK_NORMAL,
>                                   NULL);
>         if (!res) {
> --
> 2.17.1
>