linux-kernel - Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU (v3)

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Message-Id: <20090415.170539.34899625.davem@davemloft.net>
Date:	Wed, 15 Apr 2009 17:05:39 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	shemminger@...tta.com
Cc:	dada1@...mosbay.com, kaber@...sh.net, jeff.chua.linux@...il.com,
	paulmck@...ux.vnet.ibm.com, paulus@...ba.org, mingo@...e.hu,
	torvalds@...ux-foundation.org, laijs@...fujitsu.com,
	jengelh@...ozas.de, r000n@...0n.net, linux-kernel@...r.kernel.org,
	netfilter-devel@...r.kernel.org, netdev@...r.kernel.org,
	benh@...nel.crashing.org
Subject: Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU (v3)

From: Stephen Hemminger <shemminger@...tta.com>
Date: Wed, 15 Apr 2009 17:01:11 -0700

> 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).

I say this case doesn't matter until someone can prove that it's
any different from the IPTABLES replace operation system call
executing a few microseconds earlier or later.

There really is no difference, and we're making complexity out of
nothing just to ensure something which isn't actually guarenteed right
now.
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