++-----------------------------------------------------------------------+
+| **Quick Quiz**: |
++-----------------------------------------------------------------------+
+| But the chain of rcu_node-structure lock acquisitions guarantees |
+| that new readers will see all of the updater's pre-grace-period |
+| accesses and also guarantees that the updater's post-grace-period |
+| accesses will see all of the old reader's accesses. So why do we |
+| need all of those calls to smp_mb__after_unlock_lock()? |
++-----------------------------------------------------------------------+
+| **Answer**: |
++-----------------------------------------------------------------------+
+| Because we must provide ordering for RCU's polling grace-period |
+| primitives, for example, get_state_synchronize_rcu() and |
+| poll_state_synchronize_rcu(). Consider this code:: |
+| |
+| CPU 0 CPU 1 |
+| ---- ---- |
+| WRITE_ONCE(X, 1) WRITE_ONCE(Y, 1) |
+| g = get_state_synchronize_rcu() smp_mb() |
+| while (!poll_state_synchronize_rcu(g)) r1 = READ_ONCE(X) |
+| continue; |
+| r0 = READ_ONCE(Y) |
+| |
+| RCU guarantees that the outcome r0 == 0 && r1 == 0 will not |
+| happen, even if CPU 1 is in an RCU extended quiescent state |
+| (idle or offline) and thus won't interact directly with the RCU |
+| core processing at all. |
++-----------------------------------------------------------------------+
+