nfnetlink: avoid unbound loop on busy Netlink socket

I see a problem with how ctnetlink GET requests are being
processed in the kernel (2.6.32.24) under high load.

The sympton is Netlink looping around nfnetlink_rcv_msg(), which
is just because netlink_unicast() came back with EAGAIN when
trying to write the newly created Netlink skb to the SK receive
buffer in ctnetlink_get_conntrack().  In this case a (possibly)
infinit loop is entered.  Mostly infinit I think in case the
userland party trying to receive those messages may be stuck in
the sendmsg() call, being unable to read anything if being single
threaded.

I tried to reproduce several times, a few times the loop
disappeared and the box proceeded normally after some minutes.
I have no explanation for this.

The attached patch tries to solve it by simple not trying again
to netlink_unicast() the reply skb and just fail with -ENOBUFS.
The reasoning is that at the point a Netlink overrun is detected
it seems counter intuitive to insist on sending one more Netlink
message.

Signed-off-by: Holger Eitzenberger <holger@eitzenberger.org>

Index: net-next-2.6/net/netfilter/nfnetlink.c
===================================================================
--- net-next-2.6.orig/net/netfilter/nfnetlink.c	2010-12-03 14:33:32.000000000 +0100
+++ net-next-2.6/net/netfilter/nfnetlink.c	2010-12-03 14:34:21.000000000 +0100
@@ -138,7 +138,6 @@
 		return 0;
 
 	type = nlh->nlmsg_type;
-replay:
 	ss = nfnetlink_get_subsys(type);
 	if (!ss) {
 #ifdef CONFIG_MODULES
@@ -169,7 +168,7 @@
 
 		err = nc->call(net->nfnl, skb, nlh, (const struct nlattr **)cda);
 		if (err == -EAGAIN)
-			goto replay;
+			err = -ENOBUFS;
 		return err;
 	}
 }