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

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Message-ID: <20090416234955.GL6924@linux.vnet.ibm.com>
Date:	Thu, 16 Apr 2009 16:49:55 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	David Miller <davem@...emloft.net>
Cc:	kaber@...sh.net, torvalds@...ux-foundation.org,
	shemminger@...tta.com, dada1@...mosbay.com,
	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
Subject: Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU (v3)

On Thu, Apr 16, 2009 at 03:33:54PM -0700, David Miller wrote:
> From: Patrick McHardy <kaber@...sh.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.

OK, I am putting one together...

It will be needed sooner or later, though I suspect per-CPU locking
would work fine in this case.

							Thanx, Paul
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