netdev - Re: [PATCH V2 net-next 1/2] tcp: send in-queue bytes in cmsg upon read

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Message-ID: <CACSApvarR0Hv8gXaE=+VmjXBmu76jh1eV3bTs3DQ_ztM+-BSXg@mail.gmail.com>
Date:   Mon, 30 Apr 2018 11:59:56 -0400
From:   Soheil Hassas Yeganeh <soheil@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Yuchung Cheng <ycheng@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH V2 net-next 1/2] tcp: send in-queue bytes in cmsg upon read

On Mon, Apr 30, 2018 at 11:43 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On 04/30/2018 08:38 AM, David Miller wrote:
>> From: Soheil Hassas Yeganeh <soheil.kdev@...il.com>
>> Date: Fri, 27 Apr 2018 14:57:32 -0400
>>
>>> Since the socket lock is not held when calculating the size of
>>> receive queue, TCP_INQ is a hint.  For example, it can overestimate
>>> the queue size by one byte, if FIN is received.
>>
>> I think it is even worse than that.
>>
>> If another application comes in and does a recvmsg() in parallel with
>> these calculations, you could even report a negative value.

Thanks you David. In addition to Eric's point, for TCP specifically,
it is quite uncommon to have multiple threads calling recvmsg() for
the same socket in parallel, because the application is interested in
the streamed, in-sequence bytes. Except when the application just
wants to discard the incoming stream or has a predefined frame sizes,
this wouldn't be an issue. For such cases, the proposed INQ hint is
not going to be useful.

Could you please let me know whether you have any other example in mind?

Thanks!
Soheil