netfilter: ipset: add resched points during set listing

When sets are extremely large we can get softlockup during ipset -L.
We could fix this by adding cond_resched_rcu() at the right location
during iteration, but this only works if RCU nesting depth is 1.

At this time entire variant->list() is called under under rcu_read_lock_bh.
This used to be a read_lock_bh() but as rcu doesn't really lock anything,
it does not appear to be needed, so remove it (ipset increments set
reference count before this, so a set deletion should not be possible).

Reported-by: Li Shuang <shuali@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/ipset/ip_set_bitmap_gen.h

@@ -227,6 +227,7 @@ mtype_list(const struct ip_set *set,
 	rcu_read_lock();
 	for (; cb->args[IPSET_CB_ARG0] < map->elements;
 	     cb->args[IPSET_CB_ARG0]++) {
+		cond_resched_rcu();
 		id = cb->args[IPSET_CB_ARG0];
 		x = get_ext(set, map, id);
 		if (!test_bit(id, map->members) ||

net/netfilter/ipset/ip_set_core.c

@@ -1388,9 +1388,7 @@ dump_last:
 				set->variant->uref(set, cb, true);
 			/* fall through */
 		default:
-			rcu_read_lock_bh();
 			ret = set->variant->list(set, skb, cb);
-			rcu_read_unlock_bh();
 			if (!cb->args[IPSET_CB_ARG0])
 				/* Set is done, proceed with next one */
 				goto next_set;

net/netfilter/ipset/ip_set_hash_gen.h

@@ -1143,6 +1143,7 @@ mtype_list(const struct ip_set *set,
 	rcu_read_lock();
 	for (; cb->args[IPSET_CB_ARG0] < jhash_size(t->htable_bits);
 	     cb->args[IPSET_CB_ARG0]++) {
+		cond_resched_rcu();
 		incomplete = skb_tail_pointer(skb);
 		n = rcu_dereference(hbucket(t, cb->args[IPSET_CB_ARG0]));
 		pr_debug("cb->arg bucket: %lu, t %p n %p\n",