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

Boqun Feng
2025-11-06T14:31:59Z

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.

Regards,
Boqun

> 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.