linux-kernel - Re: [PATCH RFC] srcu: Yet more detail for srcu_readers_active_idx

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Message-Id: <826527A4-42E6-48AB-80C9-FEC0F330F0BE@joelfernandes.org>
Date:   Thu, 15 Dec 2022 17:16:45 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     paulmck@...nel.org
Cc:     Frederic Weisbecker <frederic@...nel.org>, boqun.feng@...il.com,
        neeraj.iitr10@...il.com, urezki@...il.com, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] srcu: Yet more detail for srcu_readers_active_idx_check() comments



> On Dec 15, 2022, at 5:10 PM, Paul E. McKenney <paulmck@...nel.org> wrote:
> 
> On Thu, Dec 15, 2022 at 04:42:15PM -0500, Joel Fernandes wrote:
>> 
>> 
>>>> On Dec 15, 2022, at 4:39 PM, Paul E. McKenney <paulmck@...nel.org> wrote:
>>> 
>>> On Thu, Dec 15, 2022 at 03:33:39PM -0500, Joel Fernandes wrote:
>>>>> On Thu, Dec 15, 2022 at 3:03 PM Joel Fernandes <joel@...lfernandes.org> wrote:
>>>>> 
>>>>> Hi Paul,
>>>>> 
>>>>>> On Thu, Dec 15, 2022 at 2:58 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>>>>> [...]
>>>>>>> If the first read section's srcu_read_unlock() and its corresponding
>>>>>>> smp_mb()  happened before the flip, then the increment of old idx
>>>>>>> would happen only once. The next srcu_read_lock() will read the new
>>>>>>> index. If the srcu_read_unlock() and it's corresponding smp_mb()
>>>>>>> happened after the flip, the old_idx will be sampled again and can be
>>>>>>> incremented twice. So it depends on how the flip races with
>>>>>>> srcu_read_unlock().
>>>>>> 
>>>>>> I do understand that a number of people like reasoning about
>>>>>> memory-barrier ordering, courtesy of the sequentially consistent portions
>>>>>> of the C and C++ memory models, but thinking in terms of the accesses
>>>>>> surrounding the memory barriers has been far less error-prone.
>>>>> 
>>>>> Sure, but we are already talking in terms of the access to idx right?
>>>>> That's what we're saying is visible by memory barriers and we are
>>>>> trying to reason here about the ordering (flip does the write to idx
>>>>> and followed by smp_mb(), and there is corresponding read of idx on
>>>>> the srcu_read_lock() side. So we are indeed talking in terms of
>>>>> access, but let me know if I missed something.
>>>>> 
>>>>>>> Also, since this is all hard to reason about I started making some
>>>>>>> diagrams, LOL. For your amusement, here is why need to scan both idx
>>>>>>> during grace period detection: https://i.imgur.com/jz4bNKd.png
>>>>>> 
>>>>>> Nice!
>>>>>> 
>>>>>> I suggest placing a gap between GP 2 and GP 3.  That way, you can make it
>>>>>> very clear that Reader 1's critical section starts after the end of GP 2
>>>>>> (thus clearly never blocking GP 2) and before GP 3 (thus possibly having
>>>>>> a reference to some data that is going to be freed at the end of GP 3).
>>>>>> 
>>>>>> I also suggest coloring Reader 1 red and Reader 2 green, given that the
>>>>>> color red generally indicates danger.
>>>>> 
>>>>> Thanks for these suggestions! I will make the update. I am planning to
>>>>> make a number of diagrams for other scenarios as well, as it helps
>>>>> visualize. Google drawing is nice for these. I am happy to share these
>>>>> with you all if there is interest :).
>>>> 
>>>> I made these updates, please see: https://i.imgur.com/hoKLvtt.png
>>>> 
>>>> Feel free to use the image for any purpose and thanks ;-)
>>> 
>>> Very good, thank you!
>>> 
>>> Would it be possible to have an arrow marked "X" or "reference to X"
>>> from the beginning of the 'Mark "x" for GC' box to the box labeled
>>> 'Enter RSCS (access "X")'?
>> 
>> I am currently away from desk. I shared the google drawing with you. Could you check and make the change, if that’s ok with you?
>> 
>> Thank you so much,
> 
> I took a cut at it.  Thoughts?

Yes perfect now :) and handy future reference! Thanks!

- Joel

> 
>                            Thanx, Paul