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

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Nov 13, 2025 at 08:18:44PM +0900, FUJITA Tomonori wrote:
> On Thu, 13 Nov 2025 10:07:36 +0000
> Alice Ryhl <aliceryhl@google.com> wrote:
> 
> > On Wed, Nov 05, 2025 at 02:47:29PM +0900, FUJITA Tomonori wrote:
> >> 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.
> >> 
> >> [1] https://lore.kernel.org/rust-for-linux/20241126-pr_once_macros-v4-0-410b8ca9643e@tuta.io/
> > 
> > There has been a fair bit of discussion below. Here is my take on the
> > way forward:
> > 
> > 1. OnceLite should be it's own special type, and it should be in a
> >    printing-related module, not kernel::sync. The reason for this is
> >    that do_once_lite! is hooked up to a special section that allows you
> >    to reset the once status, and if someone used it for anything not
> >    related to printing, they would end up with being incorrectly reset
> >    when someone resets pr_warn_once calls.
> 
> Do you mean that only the do_once_lite! macro, which places the data
> in .data..once section, should live in a printing-related module? Or
> should everything including OnceLite struct itself, also be moved
> there?

I think both should live in the same place, and not in kernel::sync.
Whether you call the module once_lite or print is not a big deal to me.

> > 2. I would suggest just using a relaxed load followed by store instead
> >    of xchg. This is what the C-side does, and I think there is no strong
> >    reason to deviate. It allows for a single byte of data instead of i32.
> 
> You meant best-effort try-once logic like C?
> 
> pub fn call_once<F>(&self, f: F) -> bool
> where
>     F: FnOnce(),
> {
>     if self.0.load(Relaxed) == State::Incomplete {
>         self.0.store(State::Complete, Relaxed);
>         f();
>         return true;
>     }
>     false
> }

Yes that is what I meant.

> Using u8 as atomic type instead of i32 would require a fair amount of
> work, since at the moment only i32 and i64 are supported as atomic
> types, right?

Supporting u8 atomics in general is tricky, but I think *specifically*
load/store should not be a problem.

Alice