netdev - Re: sendfile()? Re: SO_LINGER dead: I get an immediate RST on 2.6.24?

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Date:	Wed, 14 Jan 2009 03:05:31 -0500
From:	Bill Fink <billfink@...dspring.com>
To:	Rick Jones <rick.jones2@...com>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	ilpo.jarvinen@...sinki.fi, zbr@...emap.net,
	bert.hubert@...herlabs.nl, h.willstrand@...il.com,
	netdev@...r.kernel.org
Subject: Re: sendfile()? Re: SO_LINGER dead: I get an immediate RST on
 2.6.24?

On Mon, 12 Jan 2009, Rick Jones wrote:

> Bill Fink wrote:
> > On Tue, 13 Jan 2009, Herbert Xu wrote:
> > 
> > 
> >>Bill Fink <billfink@...dspring.com> wrote:
> >>
> >>>If I understand you correctly, to hit this corner case, just after
> >>>the final TCP write, there would have to be no packets in flight
> >>>together with a zero TCP window.  To make it more bullet-proof, I
> >>>guess after seeing a zero tcpi_unacked, an additional small delay
> >>>should be performed, and then rechecking for a zero tcpi_unacked.
> >>>I don't see anything else obvious (to me anyway) in the tcp_info
> >>>that would be particularly helpful in handling this.
> >>
> >>What's wrong with idiag_wqueue? Isn't that a much more direct
> >>way to get this?
> > 
> > 
> > I'm not familiar with idiag_wqueue, but it sounds like it has something
> > to do with INET_DIAG/INET_TCP_DIAG.  It was a long time ago, but I seem
> > to recall that using INET_DIAG had a negative impact on performance,
> > and since the main point of nuttcp is to measure TCP/UDP performance,
> > that would be contrary to its primary purpose.  Also, I don't want to
> > rely on something that's not guaranteed to be part of the running kernel.
> 
> How likely is it that the "additional small delay" above would be much 
> less than waiting for a read return of zero after a shutdown(SHUT_WR) call?

I'm not sure I understand what you're getting at.  I did consider doing
something like what you suggested, but in the end decided it was simpler
to deal with a fully ESTABLISHED connection, than worrying about possible
races with a socket being (partially or fully) closed.

						-Bill
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