Date: Wed, 05 Jul 2000 10:02:02 +0100 From: Kevin Bracey <kevin.bracey@pace.co.uk> To: freebsd-net@freebsd.org Subject: Re: Race condition in TCP connection drops? Message-ID: <2de442d949%kbracey@kbracey.cam.pace.co.uk> In-Reply-To: <200007041829.OAA22401@khavrinen.lcs.mit.edu>
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In message <200007041829.OAA22401@khavrinen.lcs.mit.edu>
Garrett Wollman <wollman@khavrinen.lcs.mit.edu> wrote:
> <<On Tue, 04 Jul 2000 13:52:47 +0100, Kevin Bracey <kevin.bracey@pace.co.uk> said:
>
> > This is totally reliable on our system, because the psignal mechanism is
> > synchronous. Are there interlocks to prevent this happening on FreeBSD, or is
> > it a race condition?
>
> On all BSD systems since time immemorial (and probably Bell Labs Unix
> before that), signal delivery is asynchronous, and the kernel is
> non-preemptible, so this is not possible.
Thanks, I suspected it was something like that. A while back I tried changing
psignal call to mark per-socket signal requirements in a bitfield, and then
send them on exit from the network stack, as I was somewhat suspicious of
this sort of problem occurring. Unfortunately that caused problems when the
system was under high load (MPEG 2 video streaming in this case) - data would
come in in spurts as fast as the stack could deal with it, so the network
stack would remain looping through the interrupt queue in the packet input
routine without sending any signals, so the socket buffer would overflow
because the MPEG software wasn't getting a chance to read it.
I'll have a go at doing something like that again, but with a few more
points where signals can be triggered (eg each time round the input loop).
Also, as far as I can see there's no need for soisdisconnected() to be
called before in_pcbdetach() - if that's right, swapping those statements
would be a very easy fix for my particular problem.
--
Kevin Bracey, Principal Software Engineer
Pace Micro Technology plc Tel: +44 (0) 1223 518566
645 Newmarket Road Fax: +44 (0) 1223 518526
Cambridge, CB5 8PB, United Kingdom WWW: http://www.acorn.co.uk/
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