RCU rather than SRCU, because RCU is almost always faster and
easier to use than is SRCU.
+ If you need to enter your read-side critical section in a
+ hardirq or exception handler, and then exit that same read-side
+ critical section in the task that was interrupted, then you need
+ to srcu_read_lock_raw() and srcu_read_unlock_raw(), which avoid
+ the lockdep checking that would otherwise this practice illegal.
+
Also unlike other forms of RCU, explicit initialization
and cleanup is required via init_srcu_struct() and
cleanup_srcu_struct(). These are passed a "struct srcu_struct"