linux-kernel - Re: [PATCHv2 0/4] tools/memory-model: Define more of LKMM in tools/memory-model

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Message-ID: <e45bd166-348a-95b6-c17c-dcd2525f263c@huaweicloud.com>
Date: Mon, 29 Jul 2024 17:19:34 +0200
From: Hernan Ponce de Leon <hernan.poncedeleon@...weicloud.com>
To: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
 Alan Stern <stern@...land.harvard.edu>
Cc: paulmck@...nel.org, parri.andrea@...il.com, will@...nel.org,
 peterz@...radead.org, boqun.feng@...il.com, npiggin@...il.com,
 dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
 akiyks@...il.com, dlustig@...dia.com, joel@...lfernandes.org,
 urezki@...il.com, quic_neeraju@...cinc.com, frederic@...nel.org,
 linux-kernel@...r.kernel.org, lkmm@...ts.linux.dev
Subject: Re: [PATCHv2 0/4] tools/memory-model: Define more of LKMM in
 tools/memory-model

On 7/29/2024 4:45 PM, Jonas Oberhauser wrote:
> 
> 
> Am 7/29/2024 um 3:30 PM schrieb Hernan Ponce de Leon:
>> On 7/12/2024 10:06 AM, Hernan Ponce de Leon wrote:
>>> On 6/10/2024 10:38 AM, Hernan Ponce de Leon wrote:
>>>> On 6/8/2024 3:00 AM, Alan Stern wrote:
>>>>> On Wed, Jun 05, 2024 at 09:58:42PM +0200, Jonas Oberhauser wrote:
>>>>>>
>>>>>>
>>>>>> Am 6/4/2024 um 7:56 PM schrieb Alan Stern:
>>>>>>> Just to clarify: Your first step encompasses patches 1 - 3, and the
>>>>>>> second step is patch 4.  The first three patches can be applied 
>>>>>>> now, but
>>>>>>> the last one needs to wait until herd7 has been updated.  Is this 
>>>>>>> all
>>>>>>> correct?
>>>>>>
>>>>>> Exactly.
>>>>>
>>>>> With regard to patch 4, how much thought have you and Hernan given to
>>>>> backward compatibility?  Once herd7 is changed, old memory model files
>>>>> will no longer work correctly.
>>>>>
>>>>
>>>> Honestly, I did not think much about this (at least until Akira 
>>>> mentioned in my PR). My hope was that changes to the model could be 
>>>> back-ported to previous kernel versions. However that would not work 
>>>> for existing out-of-tree files.
>>>>
>>>> My question is: is compatibility with out-of-tree files really a 
>>>> requirement? I would argue that if people are using outdated models, 
>>>> they may get wrong results anyway. This is because some of the 
>>>> changes done to lkmm during the last few years change the expected 
>>>> result for some litmus tests.
>>>>
>>>> Hernan
>>>
>>> I pushed some new changes to the code for backward compatibility [1]. 
>>> The series also needs the patch at the bottom to properly deal with 
>>> the ordering of failing CAses and non-returning operations. With it, 
>>> all litmus tests return the correct result (the script needs to pass 
>>> option -lkmm-legacy false to herd).
>>
>> I have been playing around with an alternative to this.
>>
>> Rather than implementing this as an "option", I can implemented it as 
>> a "model variant (*)" and add this to the model
> 
> How exactly do these model variants get selected?
> 
> I was thinking that another good approach could be to have a new generic 
> C model which doesn't know anything about LKMM. I believe this would be 
> specified in the header of the .litmus files?
> 
> 
>> flag ~empty (if "lkmmlatest" then 0 else _)
>>    as new-lkmm-models-require-variant-lkmmlatest
>>
>> If the user forgets to set the variant for the new model, herd7 will 
>> flag the executions showing that something is off.
>>
>> To be fully backward compatible, we would need to backport this to old 
>> models
>>
>> flag ~empty (if "lkmmlatest" then 1 else _)
>>    as new-lkmm-models-require-variant-lkmmlatest
> 
> should this be then _ else 0  ? or what does the _ do here?

Yes, my bad.

> 
> I also don't think we can backport things to old models

IIRC I have seen (non lkmm related) patches being backported to stable 
kernel versions. Why can't we do this for lkmm if backward compatibility 
is really a requirement? Otherwise I don't see a way of preventing 
developers to use old models with the new option (since I plan to keep 
the "old variant" as default, this would have to be done on purpose, but 
still).

> 
>> If the user (wrongly) sets the variant for an old model, the the 
>> executions will be flagged.
>>
>> Any thoughts?
>>
>> Hernan
>>
>> (*) This trick seems to be used for some arm models
>>
>> https://github.com/herd/herdtools7/blob/master/herd/libdir/arm-models/mixed/ec.cat#L66C1-L67C67
>