rtnl: provide link dump consistency info

This patch adds a change sequence counter to each net namespace
which is bumped whenever a netdevice is added or removed from
the list. If such a change occurred while a link dump took place,
the dump will have the NLM_F_DUMP_INTR flag set in the first
message which has been interrupted and in all subsequent messages
of the same dump.

Note that links may still be modified or renamed while a dump is
taking place but we can guarantee for userspace to receive a
complete list of links and not miss any.

Testing:
I have added 500 VLAN netdevices to make sure the dump is split
over multiple messages. Then while continuously dumping links in
one process I also continuously deleted and re-added a dummy
netdevice in another process. Multiple dumps per seconds have
had the NLM_F_DUMP_INTR flag set.

I guess we can wait for Johannes patch to hit net-next via the
wireless tree.  I just wanted to give this some testing right away.

Signed-off-by: Thomas Graf <tgraf@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -1032,6 +1032,8 @@ static int rtnl_dump_ifinfo(struct sk_buff *skb, struct netlink_callback *cb)
 	s_idx = cb->args[1];
 
 	rcu_read_lock();
+	cb->seq = net->dev_base_seq;
+
 	for (h = s_h; h < NETDEV_HASHENTRIES; h++, s_idx = 0) {
 		idx = 0;
 		head = &net->dev_index_head[h];
@@ -1043,6 +1045,8 @@ static int rtnl_dump_ifinfo(struct sk_buff *skb, struct netlink_callback *cb)
 					     cb->nlh->nlmsg_seq, 0,
 					     NLM_F_MULTI) <= 0)
 				goto out;
+
+			nl_dump_check_consistent(cb, nlmsg_hdr(skb));
 cont:
 			idx++;
 		}