On Fri, 30 Mar 2007, Julian Elischer wrote:
I have been looking at the IPFW code recently, especially with respect to
locking. There are some things that could be done to improve IPFW's
behaviour when processing packets, but some of these take a toll (there is
always a toll) on the 'updating' side of things.
For example. I can make IPFW lock-free during processing of packets (i.e.
not holding any locks while traversing the list) which would solve problems
we have with lock-order reversals when it needs to look at the socket layer
(which needs socket layer locks). Unfortunatly this would make it a lot more
expensive in the case where new rules are being added to the list. possibly
a LOT more expensive. Now, this would only matter if one was adding (or
deleting) hundreds of rules per second to the firewall, but as I've
discovered, there's always SOMEONE that is doing the very thing you imagine
that no-one would ever do.
In my imagination, most of the people who did this sort of thing don't need
to do it any more as tables obviate the need for that sort of thing.
Is there anyone out there who is adding hundreds (or even dozens) of rules
per second on a continuous basis, or who wants rule changing to be a really
efficient operation? (does it matter to you if it takes a few milliSecs to
add a rule?)
Just to make sure this hits the public thread also, as I know we've talked
about it privately: Stephan Upholf has an implementation of a mostly-read lock
in the works, which avoids any atomic operations during acquisition of a read
reference, at the cost of increasing the cost of write lock acquires. This
might provide what we need without fundamentally restructuring ipfw. It would
be useful, once that is available, to examine the costs and benefits of both
approaches side-by-side. Historically, I've been quite interested in doing
something like that you describe, but the complexity cost is high, so if we
can make a simpler solution (mrlocks) work just as well, that would be
preferable.
Robert N M Watson
Computer Laboratory
University of Cambridge
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