netdev - Re: listen(2) backlog changes in or around Linux 3.1?

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Date:	Mon, 22 Oct 2012 22:08:39 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Vijay Subramanian <subramanian.vijay@...il.com>
Cc:	enh <enh@...gle.com>,
	Venkat Venkatsubra <venkat.x.venkatsubra@...cle.com>,
	netdev@...r.kernel.org
Subject: Re: listen(2) backlog changes in or around Linux 3.1?

On Mon, 2012-10-22 at 13:00 -0700, Vijay Subramanian wrote:
> >
> > If we send a SYNACK, then receive the ACK from client, and the acceptq
> > is full, we should reset the connexion. Right now we have kind of stupid
> > situation, were we drop the ACK, and leave the REQ in the SYN_RECV
> > state, so we retransmit SYNACKS.
> >
> 
> 
> It seems the third ack is remembered in inet_rsk(req)->acked in
> tcp_check_req(). However, because of the order in which the tests are performed, 
> server stills retransmits the synack needlessly. Following patch 
> (for review) prevents this synack retransmission if third ack has been 
> received.
> 
> The request_sock will expire in around 30 seconds and will be dropped if it does
> not move into accept_queue by then.  Maybe we should also call 
> req->rsk_ops->send_reset(sk,skb); 
> when the request_sock expires and is dropped?
> 

Not sure its needed, and we are under stress.

> 
> net/ipv4/inet_connection_sock.c |    5 ++---
>   1 files changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> index d34ce29..4e8e52e 100644
> --- a/net/ipv4/inet_connection_sock.c
> +++ b/net/ipv4/inet_connection_sock.c
> @@ -598,9 +598,8 @@ void inet_csk_reqsk_queue_prune(struct sock *parent,
>                                                 &expire, &resend);
>                                  req->rsk_ops->syn_ack_timeout(parent, req);
>                                  if (!expire &&
> -                                   (!resend ||
> -                                    !req->rsk_ops->rtx_syn_ack(parent, req, NULL) ||
> -                                    inet_rsk(req)->acked)) {
> +                                   (!resend || inet_rsk(req)->acked ||
> +                                    !req->rsk_ops->rtx_syn_ack(parent, req, NULL))) {
>                                          unsigned long timeo;
> 
>                                          if (req->retrans++ == 0)

I wonder then if we dont need to retransmit the synack when req moves
into accept_queue then ?

Or else how the client can 'knows' it can send data to server ?

All these facilities sound very complex and not really usable by clients
(ie users not willing to wait more than few seconds anyway)



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