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

From: Jarek Poplawski <[email-masked]>
Date: Fri, 11 Jan 2008 15:13:38 +0100

On Thu, Jan 10, 2008 at 03:51:11PM -0800, Paul E. McKenney wrote:
> 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?

It seems words could be stronger than then logic (in some cases)...
After forgetting what's dereference usually for, it's all right!

Thanks,
Jarek P.