From: Hernan Ponce de Leon <hernan.poncedeleon@huaweicloud.com>
To: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>,
Alan Stern <stern@rowland.harvard.edu>
Cc: paulmck@kernel.org, parri.andrea@gmail.com, will@kernel.org,
peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com,
dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr,
akiyks@gmail.com, dlustig@nvidia.com, joel@joelfernandes.org,
urezki@gmail.com, quic_neeraju@quicinc.com, frederic@kernel.org,
linux-kernel@vger.kernel.org, lkmm@lists.linux.dev
Subject: Re: [PATCHv2 0/4] tools/memory-model: Define more of LKMM in tools/memory-model
Date: Mon, 29 Jul 2024 17:53:23 +0200 [thread overview]
Message-ID: <cd5a269a-9c6a-a311-d796-ce65c935887b@huaweicloud.com> (raw)
In-Reply-To: <9ac4a586-ef37-4c48-8e66-df3d02b53b6a@huaweicloud.com>
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 I would argue that if people are using outdated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
next prev parent reply other threads:[~2024-07-29 15:53 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20240604152922.495908-1-jonas.oberhauser@huaweicloud.com>
[not found] ` <88c1ebc8-4805-4d1d-868a-889043899979@rowland.harvard.edu>
[not found] ` <bbc3bd10-3bf5-4b1a-a275-dd328c42e307@huaweicloud.com>
[not found] ` <f93f140b-13bc-4d00-adee-46cc1c016480@rowland.harvard.edu>
[not found] ` <4792c9e7-2594-3600-5d82-4cb1443fe670@huaweicloud.com>
2024-07-12 8:06 ` [PATCHv2 0/4] tools/memory-model: Define more of LKMM in tools/memory-model Hernan Ponce de Leon
2024-07-29 13:30 ` Hernan Ponce de Leon
2024-07-29 14:45 ` Jonas Oberhauser
2024-07-29 15:19 ` Hernan Ponce de Leon
2024-07-29 15:44 ` Jonas Oberhauser
2024-07-29 15:53 ` Hernan Ponce de Leon [this message]
2024-07-29 16:05 ` Jonas Oberhauser
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