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

[Eric Dumazet <eric.dumazet@gmail.com>]

On 04/30/2018 08:38 AM, David Miller wrote:
> From: Soheil Hassas Yeganeh <soheil.kdev@gmail.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.
> 
> These READ_ONCE() make it look like some of these issues are being
> addressed but they are not.
> 
> You could freeze the values just by taking sk->sk_lock.slock, but I
> don't know if that cost is considered acceptable or not.
> 
> Another idea is to sample both values in a loop, similar to a sequence
> lock sequence:
> 
> again:
> 	tmp1 = A;
> 	tmp2 = B;
> 	barrier();
> 	tmp3 = A;
> 	if (tmp1 != tmp3)
> 		goto again;
> 
> But the current state of affairs is not going to work well.
> 

We want a hint, and max_t(int, 0, ....)  does not return a negative value ?

If the hint is wrong in 0.1 % of the cases, we really do not care, it is not meant
to replace the existing precise ( well, sort of ) mechanism.

I say sort of, because by the time we have any number, TCP might have received more packets anyway.