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

Andreas Hindborg
2025-11-05T20:59:22Z

"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].

Best regards,
Andreas Hindborg

[1] https://lore.kernel.org/r/20250924-module-params-v3-v18-1-bf512c35d910@kernel.org