This happens in only two cases:

1.	CONFIG_PREEMPT_RCU=n kernels.  But in this case, rcu_read_unlock()
	and rcu_read_lock() are free, at least for CONFIG_PROVE_LOCKING=n
	kernels.  And if you have CONFIG_PROVE_LOCKING=y, any contribution
	from rcu_read_unlock() and rcu_read_lock() will be in the noise.

Oh argh.. I completely overlooked that rcu_read_{,un}lock() are NOPs for
PREEMPT=n kernels.

/me crawls back under his stone..