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

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Date:	Fri, 17 Apr 2009 19:22:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Miller <davem@...emloft.net>
Cc:	paulmck@...ux.vnet.ibm.com, dada1@...mosbay.com,
	shemminger@...tta.com, kaber@...sh.net,
	torvalds@...ux-foundation.org, jeff.chua.linux@...il.com,
	paulus@...ba.org, mingo@...e.hu, 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, mathieu.desnoyers@...ymtl.ca
Subject: Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU (v3)

On Fri, 2009-04-17 at 01:07 -0700, David Miller wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Date: Thu, 16 Apr 2009 22:40:32 -0700
> 
> > I completely agree that this RCU is absolutely -not- 2.6.30 material.  ;-)
> 
> I don't understand why we're writing such complicated code.
> 
> Oh I see why, it's because not every arch uses the generic SMP helpers
> yet :-)
> 
> Because if they did universally, we could solve this problem so
> simply, by merely sending a remote softirq to every online cpu.  Once
> those all complete we have enough of a quiesce period, every cpu must
> have exited any netfilter packet processing code path they were in.
> 
> And we could know they complete using an atomic counter or something.

Since its a full broadcast, we can do that _today_ using on_each_cpu().
But whatever way we turn this, this will be a very expensive operation.
Imagine doing that on your 256-way for every iptables rules change.



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