From: Nikolay Aleksandrov <razor@blackwall.org>
Date: Fri, 28 Aug 2015 10:22:20 -0700

The problem is rcu_read_unlock_bh() which triggers a warning when
irqs are disabled.  ndo_poll_controller can run with bh enabled,
disabled or irqs disabled so check if that is the case and acquire
rcu_read_lock_bh only when not running with disabled irqs.

I would say that having hard irqs disabled is a strict requirement, as
per the debugging test in netpoll_send_skb_on_dev():

	WARN_ON_ONCE(!irqs_disabled());

If you want to add the same check to netpoll_send_udp(), that's fine.

But what isn't fine is adding all of this conditional locking, we want
->poll_controller() implementations to be able to depend upon the IRQ
environment they execute in, otherwise every single implementation
might need to have ugly conditional locking as well.