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.
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