linux-kernel - Re: [PATCH v2] tools/memory-model: Weaken ctrl dependency definition in explanation.txt

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Date:   Mon, 12 Sep 2022 14:38:58 +0100
From:   Paul Heidekrüger <Paul.Heidekrueger@...tum.de>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Marco Elver <elver@...gle.com>,
        Charalampos Mainas <charalampos.mainas@...il.com>,
        Pramod Bhatotia <pramod.bhatotia@...tum.de>,
        Soham Chakraborty <s.s.chakraborty@...elft.nl>,
        Martin Fink <martin.fink@...tum.de>
Subject: Re: [PATCH v2] tools/memory-model: Weaken ctrl dependency definition
 in explanation.txt

On 12. Sep 2022, at 12:38, Paul E. McKenney <paulmck@...nel.org> wrote:

> On Tue, Aug 30, 2022 at 09:08:20PM +0000, Paul Heidekrüger wrote:
>> The current informal control dependency definition in explanation.txt is
>> too broad and, as discussed, needs to be updated.
>> 
>> Consider the following example:
>> 
>>> if(READ_ONCE(x))
>>>  return 42;
>>> 
>>> WRITE_ONCE(y, 42);
>>> 
>>> return 21;
>> 
>> The read event determines whether the write event will be executed "at
>> all" - as per the current definition - but the formal LKMM does not
>> recognize this as a control dependency.
>> 
>> Introduce a new definition which includes the requirement for the second
>> memory access event to syntactically lie within the arm of a non-loop
>> conditional.
>> 
>> Link: https://lore.kernel.org/all/20220615114330.2573952-1-paul.heidekrueger@in.tum.de/
>> Cc: Marco Elver <elver@...gle.com>
>> Cc: Charalampos Mainas <charalampos.mainas@...il.com>
>> Cc: Pramod Bhatotia <pramod.bhatotia@...tum.de>
>> Cc: Soham Chakraborty <s.s.chakraborty@...elft.nl>
>> Cc: Martin Fink <martin.fink@...tum.de>
>> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@...tum.de>
>> Co-developed-by: Alan Stern <stern@...land.harvard.edu>
> 
> Hearing no objections, I reverted the old version and replaced it
> with this version.  Thank you both!
> 
> 							Thanx, Paul

Oh, wait, there was further discussion [1, 2], and we finally agreed on [3].
So [3] is the final version.

I think me sending a v2 immediately after the v1 led to this out-of-order
discussion - sorry!

Many thanks,
Paul

[1]: https://lore.kernel.org/all/663d568d-a343-d44b-d33d-29998bff8f70@joelfernandes.org/
[2]: https://lore.kernel.org/all/D7E3D42D-2ABE-4D16-9DCA-0605F0C84F7D@in.tum.de/
[3]: https://lore.kernel.org/all/20220903165718.4186763-1-paul.heidekrueger@in.tum.de/


>> ---
>> 
>> v2:
>> - Fix typos
>> - Fix indentation of code snippet
>> 
>> v1:
>> @Alan, since I got it wrong the last time, I'm adding you as a co-developer after my
>> SOB. I'm sorry if this creates extra work on your side due to you having to
>> resubmit the patch now with your SOB if I understand correctly, but since it's
>> based on your wording from the other thread, I definitely wanted to give you
>> credit.
>> 
>> tools/memory-model/Documentation/explanation.txt | 7 ++++---
>> 1 file changed, 4 insertions(+), 3 deletions(-)
>> 
>> diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt
>> index ee819a402b69..0bca50cac5f4 100644
>> --- a/tools/memory-model/Documentation/explanation.txt
>> +++ b/tools/memory-model/Documentation/explanation.txt
>> @@ -464,9 +464,10 @@ to address dependencies, since the address of a location accessed
>> through a pointer will depend on the value read earlier from that
>> pointer.
>> 
>> -Finally, a read event and another memory access event are linked by a
>> -control dependency if the value obtained by the read affects whether
>> -the second event is executed at all.  Simple example:
>> +Finally, a read event X and another memory access event Y are linked by
>> +a control dependency if Y syntactically lies within an arm of an if,
>> +else or switch statement and the condition guarding Y is either data or
>> +address-dependent on X.  Simple example:
>> 
>> 	int x, y;
>> 
>> --
>> 2.35.1