+ This is especially important to understand in the context of the
+ QSBR flavor: a registered reader thread being "online" by
+ default should be considered as within a RCU read-side critical
+ section unless explicitly put "offline". Therefore, if
+ synchronize_rcu() is called with a mutex held, this mutex, as
+ well as any mutex which has this mutex in its dependency chain
+ should only be taken when the RCU reader thread is "offline"
+ (this can be performed by calling rcu_thread_offline()).
+