linux-kernel - Re: [PATCH RFC 1/2] netfilter: conntrack: remove RCU usage in conntrack notifier

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Date:	Fri, 27 Apr 2012 16:14:59 -0400
From:	Benjamin Poirier <bpoirier@...e.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, Pablo Neira Ayuso <pablo@...filter.org>,
	Patrick McHardy <kaber@...sh.net>,
	"David S. Miller" <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Frysinger <vapier@...too.org>,
	Arun Sharma <asharma@...com>, netfilter-devel@...r.kernel.org,
	netfilter@...r.kernel.org, coreteam@...filter.org,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH RFC 1/2] netfilter: conntrack: remove RCU usage in
 conntrack notifier

On 2012/04/27 20:58, Eric Dumazet wrote:
> On Fri, 2012-04-27 at 14:28 -0400, Benjamin Poirier wrote:
> > I think that the rcu usage in this code is pointless. It should either be
> > removed or, if it was intended to protect against something, it ought to make
> > that clear.
> > 
> > 1) The code does not make use of the deferred deletion/wait for completion rcu
> > api (ie. synchronize_rcu(), call_rcu()).
> > 2) It does not benefit from the barriers implied by the rcu primitives used.
> > The code deals with callback pointers. There's no need to order writes to the
> > function code (!) before writes to the function pointers here.
> > ---
> 
> At a first glance, this seems pretty wrong.
> 
> code can disappear under you, thats for sure.
> 
> CONFIG_NF_CT_NETLINK=m
> 
> hint : module unload contains an rcu barrier.
> 

Thank you Eric. I had indeed failed to consider module load/unload
cases, which are effectively "writes to function code".
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