struct net_device synchronization rules
=======================================
dev->open:
	Locking: Inside rtnl_lock() semaphore.
	Sleeping: OK

dev->stop:
	Locking: Inside rtnl_lock() semaphore.
	Sleeping: OK

dev->do_ioctl:
	Locking: Inside rtnl_lock() semaphore.
	Sleeping: OK

dev->get_stats:
	Locking: Inside dev_base_lock spinlock.
	Sleeping: NO

dev->hard_start_xmit:
	Locking: Inside dev->xmit_lock spinlock.
	Sleeping: NO[1]

dev->tx_timeout:
	Locking: Inside dev->xmit_lock spinlock.
	Sleeping: NO[1]

dev->set_multicast_list:
	Locking: Inside dev->xmit_lock spinlock.
	Sleeping: NO[1]


NOTE [1]: On principle, you should not sleep when a spinlock is held.
However, since this spinlock is per-net-device, we only block ourselves
if we sleep, so the effect is mitigated.