Subject: Re: dn_route.c momentarily exiting RCU read-side critical section

From: "Paul E. McKenney" <[email-masked]>
Date: Tue, 30 Oct 2007 08:12:54 -0700

On Tue, Oct 30, 2007 at 01:10:36AM -0700, David Miller wrote:
> From: "Paul E. McKenney" <[email-masked]>
> Date: Mon, 29 Oct 2007 14:15:40 -0700
> 
> > net/decnet/dn_route.c in dn_rt_cache_get_next() is as follows:
> > 
> > static struct dn_route *dn_rt_cache_get_next(struct seq_file *seq, struct dn_route *rt)
> > {
> > 	struct dn_rt_cache_iter_state *s = rcu_dereference(seq->private);
> > 
> > 	rt = rt->u.dst.dn_next;
> > 	while(!rt) {
> > 		rcu_read_unlock_bh();
> > 		if (--s->bucket < 0)
> > 			break;
> > 
> > ...  But what happens if seq->private is freed up right here?
> > ...  Or what prevents this from happening?
>  ...
> > Similar code is in rt_cache_get_next().
> > 
> > So, what am I missing here?
> 
> seq->private is allocated on file open (here via seq_open_private()),
> and freed up on file close (via seq_release_private).
> 
> So it cannot be freed up in the middle of an iteration.

Thank you for the info!!!

OK, for my next stupid question: why is the rcu_dereference(seq->private)
required, as opposed to simply seq->private?

							Thanx, Paul