Message-ID: <cd5a269a-9c6a-a311-d796-ce65c935887b@huaweicloud.com>
Date: Mon, 29 Jul 2024 17:53:23 +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 5:44 PM, Jonas Oberhauser wrote:
> 
> 
> Am 7/29/2024 um 5:19 PM schrieb Hernan Ponce de Leon:
>> 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 I would argue that if people are using outdated 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).
> 
> I don't think this is a problem. If the old version is the default, and 
> we define it in the .cfg file for the tree version of LKMM, then it will 
> work correctly for both the old and new versions. People playing around 
> with Memory Models should be careful enough not to intentionally break 
> the model by passing bogus options.

The same was true for my implementation using the lkmm-legacy option 
rather than the model variant, but this was still considered to break 
backward compatibility.

https://github.com/herd/herdtools7/pull/865#issuecomment-2229930493

> 
> Of course, defining a new syntax identifier and putting it in all 
> headers would be more robust. But it's more work and I would only do 
> that if we really got rid of all the LKMM specifics.
> 
> Have fun,
>    jonas

Powered by blists - more mailing lists