Re: [PATCH v1 0/2] Add support for print exactly once

FUJITA Tomonori
2025-11-05T23:12:43Z

On Wed, 05 Nov 2025 21:59:06 +0100
Andreas Hindborg <a.hindborg@kernel.org> wrote:

> "FUJITA Tomonori" <fujita.tomonori@gmail.com> writes:
>
>> This adds the Rust equivalent of the kernel's DO_ONCE_LITE and
>> pr_*_once macros.
>>
>> A proposal for this feature was made in the past [1], but it didn't
>> reach consensus on the implementation and wasn't merged. After reading
>> the previous discussions, I implemented it using a different approach.
>>
>> In the previous proposal, a structure equivalent to std::sync::Once
>> was implemented to realize the DO_ONCE_LITE macro. The approach tried
>> to provide Once-like semantics by using two atomic values. As pointed
>> out in the previous review comments, I think this approach tries to
>> provide more functionality than needed, making it unnecessarily
>> complex. Also, because data structures in the .data..once section can
>> be cleared at any time (via debugfs clear_warn_once), an
>> implementation using two atomics wouldn't work correctly.
>>
>> Therefore, I decided to drop the idea of emulating Once and took a
>> minimal approach to implement DO_ONCE_LITE with only one atomic
>> variable. While it would be possible to implement the feature entirely
>> as a Rust macro, the functionality that can be implemented as regular
>> functions has been extracted and implemented as the OnceLite struct
>> for better code readability.
>>
>> Of course, unlike the previous proposal, this uses LKMM atomics.
>
> Please consider if it makes sense to base this on `SetOnce`. It is in
> linux-next now, but was on list here [1].

Data placed in the .data..once section can be zero-cleared when a user
writes to debugfs clear_warn_once. In that case, would such data still
be considered a valid SetOnce value?

Even if it technically represents a valid state, using data structures
whose zero-cleared state isn't clearly valid, unlike simple integer
types, doesn't seem like a good idea.