netdev - Re: [PATCH 6/13] bridge: Add core IGMP snooping support

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Message-ID: <20100310135733.GH6267@linux.vnet.ibm.com>
Date:	Wed, 10 Mar 2010 05:57:33 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Stephen Hemminger <shemminger@...tta.com>
Subject: Re: [PATCH 6/13] bridge: Add core IGMP snooping support

On Wed, Mar 10, 2010 at 02:30:06PM +0100, Arnd Bergmann wrote:
> On Wednesday 10 March 2010, Paul E. McKenney wrote:
> > > When rt_hash_table[h].chain gets the __rcu_bh annotation, we'd have to
> > > turn first rcu_dereference into rcu_dereference_bh in order to have a clean
> > > build with sparse. Would that change be
> > > a) correct from RCU perspective,
> > > b) desirable for code inspection, and
> > > c) lockdep-clean?
> > 
> > I have a patch queued up that will make rcu_dereference_bh() handle this
> > correctly -- current -tip and mainline would complain.  Please see below
> > for a sneak preview.
> > 
> > Thoughts?
> 
> Ok, so that would mean we can convert it all to rcu_dereference_bh().
> I guess an alternative to this would be to also change the rcu_read_lock()
> inside local_bh_disable() sections to rcu_read_lock_bh(), which is not
> necessary but also not harmful, right?

It does impose additional overhead, which the networking guys are eager
to avoid, given that network links speeds are continuing to increase.  ;-)
So moving to rcu_dereference_bh() seems better to me.

(Please note that rcu_dereference_bh() imposes checking overhead only
in the presence of both CONFIG_PROVE_LOCKING and CONFIG_PROVE_RCU.)

							Thanx, Paul
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