Re: [PATCH v4 6/7] net/tcp: Store SNEs + SEQs on ao_info

[Eric Dumazet]

On Wed, Nov 29, 2023 at 5:57 PM Dmitry Safonov <dima@xxxxxxxxxx> 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)