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

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Message-ID: <45d63402-bd0f-4593-8e57-042c0753f3e3@arista.com>
Date:   Mon, 4 Dec 2023 17:08:20 +0000
From:   Dmitry Safonov <dima@...sta.com>
To:     Simon Horman <horms@...nel.org>
Cc:     David Ahern <dsahern@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        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>,
        netdev@...r.kernel.org
Subject: Re: [PATCH v4 6/7] net/tcp: Store SNEs + SEQs on ao_info

Hi Simon,

On 12/2/23 17:16, Simon Horman wrote:
> 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 <dima@...sta.com>
> 
> ...
> 
>> 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?

Thanks for the report.
Yes, it's correct as lower 4-bytes are initialized as initial SEQ.
I'll add a cast for it if I'll go with v5 for this patch.

> 
> Flagged by Sparse.
> 

Thanks,
             Dmitry