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From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU (v3)
Date: Thu, 16 Apr 2009 15:33:54 -0700 (PDT)
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From: Patrick McHardy <kaber@trash.net>
Date: Thu, 16 Apr 2009 15:11:31 +0200

> Linus Torvalds wrote:
>> On Wed, 15 Apr 2009, Stephen Hemminger wrote:
>>> The counters are the bigger problem, otherwise we could just free
>>> table
>>> info via rcu.  Do we really have to support: replace where the counter
>>> values coming out to user space are always exactly accurate, or is it
>>> allowed to replace a rule and maybe lose some counter ticks (worst
>>> case
>>> NCPU-1).
>> Why not just read the counters fromt he old one at RCU free time (they
>> are guaranteed to be stable at that point, since we're all done with
>> those entries), and apply them at that point to the current setup?
> 
> We need the counters immediately to copy them to userspace, so waiting
> for an asynchronous RCU free is not going to work.

It just occurred to me that since all netfilter packet handling
goes through one place, we could have a sort-of "netfilter RCU"
of sorts to solve this problem.