Re: [PATCH 4/6] net neighbour: convert to RCU

[Stephen Hemminger <shemminger@osdl.org>]

On Tue, 29 Aug 2006 19:28:16 +0400
Alexey Kuznetsov <kuznet@ms2.inr.ac.ru> wrote:

> Hello!
> 
> > @@ -346,8 +354,8 @@ struct neighbour *neigh_lookup(struct ne
> >  	
> >  	NEIGH_CACHE_STAT_INC(tbl, lookups);
> >  
> > -	read_lock_bh(&tbl->lock);
> > -	hlist_for_each_entry(n, tmp, &tbl->hash_buckets[hash_val], hlist) {
> > +	rcu_read_lock();
> > +	hlist_for_each_entry_rcu(n, tmp, &tbl->hash_buckets[hash_val], hlist) {
> >  		if (dev == n->dev && !memcmp(n->primary_key, pkey, key_len)) {
> >  			neigh_hold(n);
> >  			NEIGH_CACHE_STAT_INC(tbl, hits);
> 
> Sure? Seems, you cannot grab refcnt here, the entry can be already
> released.
> 
> Probably, you should do atomic_inc_and_test() here and restart lookup,
> if it fails.
> 
> Alexey

atomic_inc_and_test is true iff result is zero, so that won't work.
But the following should work:

	hlist_for_each_entry_rcu(n, tmp, &tbl->hash_buckets[hash_val], hlist) {
		if (dev == n->dev && !memcmp(n->primary_key, pkey, key_len)) {
			if (unlikely(&atomic_inc_return(&n->refcnt) == 1)) {
				neigh_release(n);
				continue;
			}
 			NEIGH_CACHE_STAT_INC(tbl, hits);

I'll wrap the atomic_inc_return inside neigh_hold_return()


-- 
Stephen Hemminger <shemminger@osdl.org>