netdev - Re: [NET] ROUTE: fix rcu_dereference() uses in /proc/net/rt

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Date:	Wed, 9 Jan 2008 20:46:37 +1100
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	davem@...emloft.net, dipankar@...ibm.com, netdev@...r.kernel.org
Subject: Re: [NET] ROUTE: fix rcu_dereference() uses in /proc/net/rt_cache

On Wed, Jan 09, 2008 at 08:38:56AM +0100, Eric Dumazet wrote:
> 
> I am not sure this is valid, since it will do this :
> 
> r = rt_hash_table[st->bucket].chain;
> if (r)
>     return rcu_dereference(r);
> 
> So compiler might be dumb enough do dereference 
> &rt_hash_table[st->bucket].chain two times.

That wouldn't be a problem at all.  The key is to add a barrier between
reading the pointer:

	r = rt_hash_table[st->bucket].chain

and dereferencing it later, e.g.,

	r->u.dst.rt_next

The barrier is there so that when we dereference r we don't read
stale cache that was there before the memory at r was initialised.
How many times you read the pointer value before the barrier is
irrelevant to the effectiveness of the barrier preceding the
dereference.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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