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

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Date:	Wed, 10 Mar 2010 11:39:43 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	paulmck@...ux.vnet.ibm.com,
	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

Le mercredi 10 mars 2010 à 10:41 +0100, Arnd Bergmann a écrit :
> On Wednesday 10 March 2010 03:14:10 Paul E. McKenney wrote:
> > On Tue, Mar 09, 2010 at 10:12:59PM +0100, Arnd Bergmann wrote:
> >
> > > I've just tried annotating net/ipv4/route.c like this and did not get
> > > very far, because the same pointers are used for rcu and rcu_bh.
> > > Could you check if this is a false positive or an actual finding?
> > 
> > Hmmm...  I am only seeing a call_rcu_bh() here, so unless I am missing
> > something, this is a real problem in TREE_PREEMPT_RCU kernels.  The
> > call_rcu_bh() only interacts with the rcu_read_lock_bh() readers, not
> > the rcu_read_lock() readers.
> > 
> > One approach is to run freed blocks through both types of grace periods,
> > I suppose.
> 
> Well, if I introduce different __rcu and __rcu_bh address space annotations,
> sparse would still not like that, because then you can only pass the annotated
> pointers into either rcu_dereference or rcu_dereference_bh.
> 
> What the code seems to be doing here is in some places
> 
> 	local_bh_disable();
> 	...
> 	rcu_read_lock();
> 	rcu_dereference(rt_hash_table[h].chain);
> 	rcu_read_unlock();
> 	...
> 	local_bh_enable();
> 
> and in others
> 
> 	rcu_read_lock_bh();
> 	rcu_dereference_bh(rt_hash_table[h].chain);
> 	rcu_read_unlock_bh();
> 
> 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?
> 

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();


to


        local_bh_disable();
        ...
        rcu_read_lock();
        rcu_dereference_bh(rt_hash_table[h].chain);
        rcu_read_unlock();
        ...
        local_bh_enable();


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