linux-kernel - Re: [PATCH v4 6/7] net/tcp: Store SNEs + SEQs on ao

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Message-ID: <CANn89iJcfn0yEM7Pe4RGY3P0LmOsppXO7c=eVqpwVNdOY2v3zA@mail.gmail.com>
Date:   Wed, 29 Nov 2023 19:09:15 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Dmitry Safonov <dima@...sta.com>
Cc:     David Ahern <dsahern@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Francesco Ruggeri <fruggeri05@...il.com>,
        Salam Noureddine <noureddine@...sta.com>,
        Simon Horman <horms@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH v4 6/7] net/tcp: Store SNEs + SEQs on ao_info

On Wed, Nov 29, 2023 at 5:57 PM Dmitry Safonov <dima@...sta.com> wrote:
>
> RFC 5925 (6.2):
> > TCP-AO emulates a 64-bit sequence number space by inferring when to
> > increment the high-order 32-bit portion (the SNE) based on
> > transitions in the low-order portion (the TCP sequence number).
>
> snd_sne and rcv_sne are the upper 4 bytes of extended SEQ number.
> Unfortunately, reading two 4-bytes pointers can't be performed
> atomically (without synchronization).
>
> In order to avoid locks on TCP fastpath, let's just double-account for
> SEQ changes: snd_una/rcv_nxt will be lower 4 bytes of snd_sne/rcv_sne.
>

This will not work on 32bit kernels ?

Unless ao->snd_sne and ao->rcv_sneare only read/written under the
socket lock (and in this case no READ_ONCE()/WRITE_ONCE() should be
necessary)