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

Andreas Hindborg
2025-11-11T09:03:03Z

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

> On Mon, Nov 10, 2025 at 01:16:44PM +0100, Andreas Hindborg wrote:
>> 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.
>>
>
> I mean if we were to use `SetOnce` in pr_*_once(), we should just use
> `SetOnce<()>`, and the problem you mentioned doesn't exist in this case.

At the very least it would break the type invariants and assumptions of
the `SetOnce` type. There a race condition between `SetOnce::as_ref` and
`SetOnce::populate`. I don't think we are allowed to race like this,
even if the type is `()`?

At any rate if we do this, we should update the safety invariant of
`SetOnce` to state that it is valid to revert the type back to the
initial state for zero sized types that do not have a `Drop`
implementation.

Best regards,
Andreas Hindborg