+13. RCU callbacks can be and are executed in parallel. In many cases,
+ the callback code simply wrappers around kfree(), so that this
+ is not an issue (or, more accurately, to the extent that it is
+ an issue, the memory-allocator locking handles it). However,
+ if the callbacks do manipulate a shared data structure, they
+ must use whatever locking or other synchronization is required
+ to safely access and/or modify that data structure.
+
+ RCU callbacks are -usually- executed on the same CPU that executed
+ the corresponding call_rcu(), call_rcu_bh(), or call_rcu_sched(),
+ but are by -no- means guaranteed to be. For example, if a given
+ CPU goes offline while having an RCU callback pending, then that
+ RCU callback will execute on some surviving CPU. (If this was
+ not the case, a self-spawning RCU callback would prevent the
+ victim CPU from ever going offline.)
+
+14. SRCU (srcu_read_lock(), srcu_read_unlock(), and synchronize_srcu())