linux-kernel - Re: [PATCH v6 1/3] rust: configfs: introduce rust support for configfs

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Message-ID: <878qnaq8ke.fsf@kernel.org>
Date: Tue, 06 May 2025 13:31:45 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Miguel Ojeda" <miguel.ojeda.sandonis@...il.com>
Cc: "Danilo Krummrich" <dakr@...nel.org>,  "Miguel Ojeda"
 <ojeda@...nel.org>,  "Alex Gaynor" <alex.gaynor@...il.com>,  "Boqun Feng"
 <boqun.feng@...il.com>,  "Gary Guo" <gary@...yguo.net>,  Björn Roy Baron
 <bjorn3_gh@...tonmail.com>,  "Benno Lossin" <benno.lossin@...ton.me>,
  "Alice Ryhl" <aliceryhl@...gle.com>,  "Trevor Gross" <tmgross@...ch.edu>,
  "Joel Becker" <jlbec@...lplan.org>,  "Peter Zijlstra"
 <peterz@...radead.org>,  "Ingo Molnar" <mingo@...hat.com>,  "Will Deacon"
 <will@...nel.org>,  "Waiman Long" <longman@...hat.com>,  "Fiona Behrens"
 <me@...enk.dev>,  "Charalampos Mitrodimas" <charmitro@...teo.net>,
  "Daniel Almeida" <daniel.almeida@...labora.com>,  "Breno Leitao"
 <leitao@...ian.org>,  <rust-for-linux@...r.kernel.org>,
  <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 1/3] rust: configfs: introduce rust support for configfs

"Miguel Ojeda" <miguel.ojeda.sandonis@...il.com> writes:

> On Mon, May 5, 2025 at 9:51 AM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>>
>> So I was thinking that because I am initializing a static with a let
>> statement, it would run in const context. But I see that it is not
>> actually guaranteed.
>
> No, that is actually guaranteed, i.e. when initializing a static. But
> you aren't initializing a static here, no? Which static are you
> referring to? If you were, then the "normal" `assert!` would work,
> because it would be a const context.
>
> The `add` calls I see are just in the `let` statement, not
> initializing any static:
>
>             {
>                 const N: usize = 0usize;
>                 unsafe { CONFIGURATION_ATTRS.add::<N, 0,
> _>(&CONFIGURATION_MESSAGE_ATTR) };
>             }
>
> So it also means this comment is wrong:
>
> +        // SAFETY: This function is only called through the `configfs_attrs`
> +        // macro. This ensures that we are evaluating the function in const
> +        // context when initializing a static. As such, the reference created
> +        // below will be exclusive.
>
> Please double-check all this... :)

Oops.

>
>> Right. Which is why I opted for `build_error`. But with the `const`
>> block solution you suggested is better.
>
> I thought you opted for that because you thought the `assert!` would
> only work if not refactored. What I tried to point out was that the
> `assert!` wouldn't have worked even before the refactoring.

I made a mistake in thinking this was in const context. I'll see if I
can fix that.



Best regards,
Andreas Hindborg