Re: [PATCH 3/3] net: Convert TCP & DCCP hash tables to use RCU / hlist_nulls

On Thu, 2008-11-13 at 14:15 +0100, Eric Dumazet wrote:

+begin:
+       sk_nulls_for_each_rcu(sk, node, &head->chain) {
                if (INET_MATCH(sk, net, hash, acookie,
+                                       saddr, daddr, ports, dif)) {
+                       if (unlikely(!atomic_inc_not_zero(&sk->sk_refcnt)))
+                               goto begintw;
+                       if (unlikely(!INET_MATCH(sk, net, hash, acookie,
+                               saddr, daddr, ports, dif))) {
+                               sock_put(sk);
+                               goto begin;
+                       }

This is the validation step that verifies the race opened by using
SLAB_DESTROY_BY_RCU, right?

Does it make sense to add a little comment to these validation steps to
keep people on their toes and aware of the trickery?

+                       goto out;
+               }
        }
+       /*
+        * if the nulls value we got at the end of this lookup is
+        * not the expected one, we must restart lookup.
+        * We probably met an item that was moved to another chain.
+        */
+       if (get_nulls_value(node) != slot)
+               goto begin;