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

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Message-ID: <20080110235111.GF9586@linux.vnet.ibm.com>
Date:	Thu, 10 Jan 2008 15:51:11 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jarek Poplawski <jarkao2@...il.com>
Cc:	Eric Dumazet <dada1@...mosbay.com>,
	Herbert Xu <herbert@...dor.apana.org.au>, davem@...emloft.net,
	dipankar@...ibm.com, netdev@...r.kernel.org
Subject: Re: [NET] ROUTE: fix rcu_dereference() uses in /proc/net/rt_cache

On Fri, Jan 11, 2008 at 12:10:42AM +0100, Jarek Poplawski wrote:
> Eric Dumazet wrote, On 01/09/2008 11:37 AM:
> ...
> > [NET] ROUTE: fix rcu_dereference() uses in /proc/net/rt_cache
> ...
> > diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> > index d337706..28484f3 100644
> > --- a/net/ipv4/route.c
> > +++ b/net/ipv4/route.c
> > @@ -283,12 +283,12 @@ static struct rtable *rt_cache_get_first(struct seq_file *seq)
> >  			break;
> >  		rcu_read_unlock_bh();
> >  	}
> > -	return r;
> > +	return rcu_dereference(r);
> >  }
> >  
> >  static struct rtable *rt_cache_get_next(struct seq_file *seq, struct rtable *r)
> >  {
> > -	struct rt_cache_iter_state *st = rcu_dereference(seq->private);
> > +	struct rt_cache_iter_state *st = seq->private;
> >  
> >  	r = r->u.dst.rt_next;
> >  	while (!r) {
> > @@ -298,7 +298,7 @@ static struct rtable *rt_cache_get_next(struct seq_file *seq, struct rtable *r)
> >  		rcu_read_lock_bh();
> >  		r = rt_hash_table[st->bucket].chain;
> >  	}
> > -	return r;
> > +	return rcu_dereference(r);
> >  }
> 
> It seems this optimization could've a side effect: if during such a
> loop updates are done, and r is seen !NULL during while() check, but
> NULL after rcu_dereference(), the listing/counting could stop too
> soon. So, IMHO, probably the first version of this patch is more
> reliable. (Or alternatively additional check is needed before return.)

Looks to me like "r" is a local variable (argument list), so there
should not be any possibility of it being changed by some other
task, right?

							Thanx, Paul
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