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

From: Simon Horman <[email-masked]>
Date: Sat, 2 Dec 2023 17:16:12 +0000

On Wed, Nov 29, 2023 at 04:57:20PM +0000, Dmitry Safonov 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.
> 
> Fixes: 64382c71a557 ("net/tcp: Add TCP-AO SNE support")
> Signed-off-by: Dmitry Safonov <[email-masked]>

...

> diff --git a/include/net/tcp_ao.h b/include/net/tcp_ao.h
> index 647781080613..b8ef25d4b632 100644
> --- a/include/net/tcp_ao.h
> +++ b/include/net/tcp_ao.h
> @@ -121,8 +121,8 @@ struct tcp_ao_info {
>  	 * - for time-wait sockets the basis is tw_rcv_nxt/tw_snd_nxt.
>  	 *   tw_snd_nxt is not expected to change, while tw_rcv_nxt may.
>  	 */
> -	u32			snd_sne;
> -	u32			rcv_sne;
> +	u64			snd_sne;
> +	u64			rcv_sne;
>  	refcount_t		refcnt;		/* Protects twsk destruction */
>  	struct rcu_head		rcu;
>  };

Hi Dmitry,

In tcp_ao.c:tcp_ao_connect_init() there is a local
variable:

        struct tcp_ao_info *ao_info;

And the following assignment occurs:

                ao_info->snd_sne = htonl(tp->write_seq);

Is this still correct in light of the change of the type of snd_sne?

Flagged by Sparse.

...