Subject: Re: [NET] ROUTE: fix rcu_dereference() uses in /proc/net/rt_cache

From: "Paul E. McKenney" <[email-masked]>
Date: Thu, 10 Jan 2008 15:51:11 -0800

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