netdev - Re: TCP_DEFER_ACCEPT is missing counter update

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Date:	Thu, 15 Oct 2009 08:08:34 +0200
From:	Willy Tarreau <w@....eu>
To:	David Miller <davem@...emloft.net>
Cc:	ja@....bg, netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: TCP_DEFER_ACCEPT is missing counter update

On Wed, Oct 14, 2009 at 03:43:49PM -0700, David Miller wrote:
> For now I'm pushing Willy's change into Linus's tree.
> 
> After more discussion we can revert if necessary.
> 
> I won't submit this to -stable until the discussion is fully resolved.

Makes sense, thanks David.

BTW, I found a use case I didn't think about where current behaviour
causes trouble :

   https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/134274
   http://lkml.indiana.edu/hypermail/linux/kernel/0711.0/0461.html

In summary, when front proxies establish pools of connections to
an apache server making use of TCP_DEFER_ACCEPT, the connection
never establishes on the apache server but silently expires in
SYN_RECV state. The front proxy sees lots of SYN/ACKs and sends
many ACKs trying to complete this connection and finally believes
it got it since the server eventually becomes silent. However,
when trying to send data over such a socket, the server immediately
returns an RST.

Such a problem would not happen if we would only drop the first
X packets (X >= 1 is already fine), because the front proxy would
establish the connection, send a second ACK in response to the
second SYN/ACK and the connection would then really be established
and would not have to expire early in SYN_RECV state.

If we really want to behave as it does today, well, let's not fix
it, but obviously, I fail to see what real world use it has, except
causing random and hard to debug issues :-/

Reading the articles below clearly make it think it was designed
to help with HTTP connections by skipping the first expected and
useless ACK packet before waking up the task :

   http://httpd.apache.org/docs/1.3/misc/perf-bsd44.html
   http://articles.techrepublic.com.com/5100-10878_11-1050771.html

and people still get caught :

   http://lkml.indiana.edu/hypermail/linux/kernel/0711.0/0416.html

Maybe it was a bit over-engineered, in the end causing it to fail
to satisfy the primary goal ?

Regards,
Willy

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