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Date: Fri, 11 Jun 2021 09:28:10 +0900
From: Akira Yokosawa <akiyks@...il.com>
To: paulmck@...nel.org
Cc: boqun.feng@...il.com, frederic@...nel.org, joel@...lfernandes.org,
linux-kernel@...r.kernel.org, neeraju@...eaurora.org,
urezki@...il.com, Akira Yokosawa <akiyks@...il.com>
Subject: Re: [PATCH] rcu/doc: Add a quick quiz to explain further why we need
smp_mb__after_unlock_lock()
On Thu, 10 Jun 2021 09:57:10 -0700, Paul E. McKenney wrote:
> 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@...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>
>
> 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.
Hi Paul,
See minor tweaks below to satisfy sphinx.
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit f21b8fbdf9a59553da825265e92cedb639b4ba3c
> Author: Frederic Weisbecker <frederic@...nel.org>
> 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@...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>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
>
> 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; |
This indent causes warnings from sphinx:
Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst:135: WARNING: Unexpected indentation.
Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst:137: WARNING: Block quote ends without a blank line; unexpected unindent
> +| 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
The code block in the answer can be fixed as follows:
++-----------------------------------------------------------------------+
+| **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. |
++-----------------------------------------------------------------------+
Hint: Use of "::" and indented code block.
Thanks, Akira
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