Re: [PATCH] rcu/doc: Add a quick quiz to explain further why we need smp_mb__after_unlock_lock()

[Paul E. McKenney]

On Thu, Jun 10, 2021 at 05:50:29PM +0200, Frederic Weisbecker wrote:
> Add some missing critical pieces of explanation to understand the need
> for full memory barriers throughout the whole grace period state machine,
> thanks to Paul's explanations.
> 
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> Cc: Uladzislau Rezki <urezki@xxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>

Nice!!!  And not bad wording either, though I still could not resist the
urge to wordsmith further.  Plus I combined your two examples, in order to
provide a trivial example use of the polling interfaces, if nothing else.

Please let me know if I messed anything up.

							Thanx, Paul

------------------------------------------------------------------------

commit f21b8fbdf9a59553da825265e92cedb639b4ba3c
Author: Frederic Weisbecker <frederic@xxxxxxxxxx>
Date:   Thu Jun 10 17:50:29 2021 +0200

    rcu/doc: Add a quick quiz to explain further why we need smp_mb__after_unlock_lock()
    
    Add some missing critical pieces of explanation to understand the need
    for full memory barriers throughout the whole grace period state machine,
    thanks to Paul's explanations.
    
    Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
    Cc: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
    Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
    Cc: Uladzislau Rezki <urezki@xxxxxxxxx>
    Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

diff --git a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst
index 11cdab037bff..3cd5cb4d86e5 100644
--- a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst
+++ b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst
@@ -112,6 +112,35 @@ on PowerPC.
 The ``smp_mb__after_unlock_lock()`` invocations prevent this
 ``WARN_ON()`` from triggering.
 
++-----------------------------------------------------------------------+
+| **Quick Quiz**:                                                       |
++-----------------------------------------------------------------------+
+| But the whole chain of rcu_node-structure locking guarantees that     |
+| readers see all pre-grace-period accesses from the updater and        |
+| also guarantees that the updater to see all post-grace-period         |
+| accesses from the readers.  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().  For example:                           |
+|                                                                       |
+| 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 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.                                                    |
++-----------------------------------------------------------------------+
+
 This approach must be extended to include idle CPUs, which need
 RCU's grace-period memory ordering guarantee to extend to any
 RCU read-side critical sections preceding and following the current