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

[Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>]

Hello!

> atomic_inc_and_test is true iff result is zero, so that won't work.

I meant atomic_inc_not_zero(), as Martin noticed.


> 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);

I do not think it will work. It has exactly the same race condition.

Yes, atomic_inc_not_zero() is expensive. But it looks like it is the cheapest
variant, which works correctly without more work.

Another variant would be rework use of refcnt. It can be done like rt cache:
when release of the last reference does not mean anything.

Also, probably, it makes sense to add neigh_lookup_light(), which does
not take refcnt, but required to call
neigh_release_light() (which is just rcu_read_unlock_bh()).

Alexey