RCU read lock vs spinlocks

[David Howells <dhowells@redhat.com>]

If a function needs both a spinlock and the RCU read lock, is there a best
order in which they should be taken?  For instance, is:

	spin_lock(&my_lock);
	rcu_read_lock();
	...
	rcu_read_unlock();
	spin_unlock(&my_lock);

better than:

	rcu_read_lock();
	spin_lock(&my_lock);
	...
	spin_unlock(&my_lock);
	rcu_read_unlock();

And does it make any different if spin_lock_irq*() is used?

In fact, should spin_lock_irq*() imply rcu_read_lock()?  Given that the
current task can't be preempted if there can be no interrupt to do the
preempting?

David