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

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Message-ID: <20100310131317.GA6267@linux.vnet.ibm.com>
Date:	Wed, 10 Mar 2010 05:13:18 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Arnd Bergmann <arnd@...db.de>,
	"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 06:49:07PM +0800, Herbert Xu wrote:
> On Wed, Mar 10, 2010 at 11:39:43AM +0100, Eric Dumazet wrote:
> >
> > Its really rcu_dereference_bh() that could/should be used:
> > I see no problem changing
> > 
> > 
> >         local_bh_disable();
> >         ...
> >         rcu_read_lock();
> >         rcu_dereference(rt_hash_table[h].chain);
> >         rcu_read_unlock();
> >         ...
> >         local_bh_enable();
> 
> Why don't we just ignore the bh part for rcu_dereference?
> 
> After all it's call_rcu_bh and the other primitives that we really
> care about.  For rcu_dereference bh should make no difference
> whatsoever.

If CONFIG_PROVE_RCU is set, rcu_dereference() checks for rcu_read_lock()
and rcu_dereference_bh() checks for either rcu_read_lock_bh() or BH
being disabled.  Yes, this is a bit restrictive, but there are a few too
many to check by hand these days.

							Thanx, Paul
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