Date: Thu, 16 Apr 2009 16:49:55 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: David Miller <davem@davemloft.net>
Cc: kaber@trash.net, torvalds@linux-foundation.org, shemminger@vyatta.com,
       dada1@cosmosbay.com, jeff.chua.linux@gmail.com, paulus@samba.org,
       mingo@elte.hu, laijs@cn.fujitsu.com, jengelh@medozas.de,
       r000n@r000n.net, linux-kernel@vger.kernel.org,
       netfilter-devel@vger.kernel.org, netdev@vger.kernel.org,
       benh@kernel.crashing.org
Subject: Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU (v3)
Message-ID: <20090416234955.GL6924@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
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On Thu, Apr 16, 2009 at 03:33:54PM -0700, David Miller wrote:
> 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.

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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