linux-kernel - [PATCH rcu 4/5] rcu/doc: Add a quick quiz to explain further why we need smp_mb__after_unlock

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Date:   Wed, 21 Jul 2021 13:08:52 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH rcu 4/5] rcu/doc: Add a quick quiz to explain further why we need smp_mb__after_unlock_lock()

From: Frederic Weisbecker <frederic@...nel.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@...nel.org>
Cc: Neeraj Upadhyay <neeraju@...eaurora.org>
Cc: Joel Fernandes <joel@...lfernandes.org>
Cc: Uladzislau Rezki <urezki@...il.com>
Cc: Boqun Feng <boqun.feng@...il.com>
[ paulmck: Adjust code block per Akira Yokosawa. ]
Signed-off-by: Paul E. McKenney <paulmck@...nel.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