Re: [PATCH v13 2/6] rust: introduce module_param module

["Benno Lossin" <lossin@kernel.org>]

On Tue Jul 1, 2025 at 10:43 AM CEST, Andreas Hindborg wrote:
> "Benno Lossin" <lossin@kernel.org> writes:
>> On Mon Jun 30, 2025 at 3:15 PM CEST, Andreas Hindborg wrote:
>>> "Benno Lossin" <lossin@kernel.org> writes:
>>>> On Mon Jun 30, 2025 at 1:18 PM CEST, Andreas Hindborg wrote:
>>>>> "Benno Lossin" <lossin@kernel.org> writes:
>>>>>> (no idea if the orderings are correct, I always have to think way to
>>>>>> much about that... especially since our atomics seem to only take one
>>>>>> ordering in compare_exchange?)
>>>>>>
>>>>>>> As far as I can tell, atomics may not land in v6.17, so this series
>>>>>>> will probably not be ready for merge until v6.18 at the earliest.
>>>>>>
>>>>>> Yeah, sorry about that :(
>>>>>
>>>>> Actually, perhaps we could aim at merging this code without this
>>>>> synchronization?
>>>>
>>>> I won't remember this issue in a few weeks and I fear that it will just
>>>> get buried. In fact, I already had to re-read now what the actual issue
>>>> was...
>>>>
>>>>> The lack of synchronization is only a problem if we
>>>>> support custom parsing. This patch set does not allow custom parsing
>>>>> code, so it does not suffer this issue.
>>>>
>>>> ... In doing that, I saw my original example of UB:
>>>>
>>>>     module! {
>>>>         // ...
>>>>         params: {
>>>>             my_param: i64 {
>>>>                 default: 0,
>>>>                 description: "",
>>>>             },
>>>>         },
>>>>     }
>>>>
>>>>     static BAD: &'static i64 = module_parameters::my_param.get();
>>>>
>>>> That can happen without custom parsing, so it's still a problem...
>>>
>>> Ah, got it. Thanks.
>>
>> On second thought, we *could* just make the accessor function `unsafe`.
>> Of course with a pinky promise to make the implementation safe once
>> atomics land. But I think if it helps you get your driver faster along,
>> then we should do it.
>
> No, I am OK for now with configfs.
>
> But, progress is still great. How about if we add a copy accessor
> instead for now, I think you proposed that a few million emails ago:
>
>     pub fn get(&self) -> T;
>
> or maybe rename:
>
>     pub fn copy(&self) -> T;
>
> Then we are fine safety wise for now, right? It is even sensible for
> these `T: Copy` types.

That is better than getting a reference, but still someone could read at
the same time that a write is happening (though we need some new
abstractions AFAIK?). But I fear that we forget about this issue,
because it'll be some time until we land parameters that are `!Copy` (if
at all...)

---
Cheers,
Benno