Subject: Re: [RFC PATCH v1 2/6] netlabel: Replace protocol/NetLabel linking with refrerence counts

From: "Paul E. McKenney" <[email-masked]>
Date: Sat, 9 Aug 2008 06:23:46 -0700

On Fri, Aug 08, 2008 at 10:11:32PM -0400, Paul Moore wrote:
> On Friday 08 August 2008 6:37:16 pm Paul E. McKenney wrote:
> > On Fri, Aug 08, 2008 at 04:53:01PM -0400, Paul Moore wrote:
> > >  struct cipso_v4_doi *cipso_v4_doi_getdef(u32 doi)
> > >  {
> > > -	return cipso_v4_doi_search(doi);
> > > +	struct cipso_v4_doi *doi_def;
> > > +
> > > +	rcu_read_lock();
> > > +	doi_def = cipso_v4_doi_search(doi);
> > > +	if (doi_def)
> >
> > Suppose that the doi_def element is removed by some other CPU at
> > this point.  The reference-count check would pass (so that the
> > deletion function would decline to error out with -EBUSY), and the
> > removal would proceed normally.  (Right?)
> >
> > So we then acquire the reference count on an element that will be
> > freed after an RCU grace period, despite the fact that the reference
> > count might still be held at that point.
> >
> > Or am I missing something?  (Wouldn't be a surprise, as it is not
> > like I am familiar with this code.)
> 
> Hi Paul,
> 
> Thanks for taking a look, your point sounds reasonable to me.
> 
> > If I am correct, the usual resolution is to combine the reference
> > count and the "valid" flag, so that a zero reference counter implies
> > "not valid", allowing the atomic_inc() below to become
> > atomic_inc_not_zero(), allowing you to simply return NULL should the
> > race with removal be detected.  There are other approaches as well...
> 
> Combining the valid and refcount fields seems reasonable to me.  I took 
> your advice and made the following changes (as well as they other 
> changes to replace the valid check with atomic_read(refcount) > 0) ...
> 
> struct cipso_v4_doi *cipso_v4_doi_getdef(u32 doi)
> {
> 	struct cipso_v4_doi *doi_def;
> 
> 	rcu_read_lock();
> 	doi_def = cipso_v4_doi_search(doi);
> 	if (doi_def == NULL)
> 		goto doi_getdef_return;
> 	if (!atomic_inc_not_zero(&doi_def->refcount))
> 		doi_def = NULL;
> 
> doi_getdef_return:
> 	rcu_read_unlock();
> 	return doi_def;
> }
> 
> int cipso_v4_doi_remove(u32 doi,
> 			struct netlbl_audit *audit_info,
> 			void (*callback) (struct rcu_head * head))
> {
> 	struct cipso_v4_doi *doi_def;
> 
> 	spin_lock(&cipso_v4_doi_list_lock);
> 	doi_def = cipso_v4_doi_search(doi);
> 	if (doi_def == NULL) {
> 		spin_unlock(&cipso_v4_doi_list_lock);
> 		return -ENOENT;
> 	}
> 	if (!atomic_dec_and_test(&doi_def->refcount)) {
> 		spin_unlock(&cipso_v4_doi_list_lock);
> 		return -EBUSY;
> 	}
> 	list_del_rcu(&doi_def->list);
> 	spin_unlock(&cipso_v4_doi_list_lock);
> 
> 	cipso_v4_cache_invalidate();
> 	call_rcu(&doi_def->rcu, callback);
> 
> 	return 0;
> }
> 
> Does that look better?

Much better!!!

Of course, any other places where you decrement ->refcount will also
need to deal with the possibility of a zero result, right?  Or is
the cipso_v4_doi_remove() case the only such decrement?

						Thanx, Paul