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Subject[PATCH rcu 4/5] rcu/doc: Add a quick quiz to explain further why we need smp_mb__after_unlock_lock()
Date
From: Frederic Weisbecker <frederic@kernel.org>

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@kernel.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
[ paulmck: Adjust code block per Akira Yokosawa. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
.../Tree-RCU-Memory-Ordering.rst | 29 +++++++++++++++++++
1 file changed, 29 insertions(+)

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 11cdab037bff6..eeb351296df11 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 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. |
++-----------------------------------------------------------------------+
+
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
--
2.31.1.189.g2e36527f23
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