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

Andreas Hindborg
2025-11-10T12:16:55Z

Boqun Feng <boqun.feng@gmail.com> writes:

> On Thu, Nov 06, 2025 at 08:12:31AM +0900, FUJITA Tomonori wrote:
>> 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?
>>
>
> It's still a valid value I believe. In term of data races, Rust and C
> have no difference, so if writing to debugfs could cause issues in Rust,
> it would cause issues in C as well.

@Tomo you are right, `SetOnce` would not work with someone (debugfs)
asynchronously modifying the state atomic variable. It requires
exclusive access while writing the contained value.

Best regards,
Andreas Hindborg