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

[Akira Yokosawa]

On Thu, 10 Jun 2021 17:48:13 -0700, Paul E. McKenney wrote:
> On Fri, Jun 11, 2021 at 09:28:10AM +0900, Akira Yokosawa wrote:
>> 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@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.
>>
>> Hi Paul,
>>
>> See minor tweaks below to satisfy sphinx.
>>
>>>
>>> 							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;                                                     |
>>
>> 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.
> 
> Thank you!
> 
> As in with the following patch to be merged into Frederic's original,
> with attribution?

Sounds good to me!

        Thanks, Akira

> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> 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 3cd5cb4d86e5..bc884ebf88bb 100644
> --- a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst
> +++ b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst
> @@ -125,15 +125,15 @@ The ``smp_mb__after_unlock_lock()`` invocations prevent this
>  +-----------------------------------------------------------------------+
>  | 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:                           |
> +| 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)                                                     |
> +|  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     |
>