linux-kernel - Re: [PATCH v4 1/3] rust: Add `OnceLite` for executing code once

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Message-ID: <0aa72bdd-7431-49bb-a6b4-e254787db0f1@sedlak.dev>
Date: Fri, 29 Nov 2024 09:22:32 +0100
From: Daniel Sedlak <daniel@...lak.dev>
To: jens.korinth@...a.io
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <benno.lossin@...ton.me>,
 Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>,
 Rust For Linux <rust-for-linux@...r.kernel.org>,
 FUJITA Tomonori <fujita.tomonori@...il.com>,
 Dirk Behme <dirk.behme@...il.com>,
 Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/3] rust: Add `OnceLite` for executing code once



On 11/27/24 8:46 PM, jens.korinth@...a.io wrote:
>> Have you considered it to be implemented like `AtomicU32`? I think™ that
>> one atomic variable is more than enough.
> 
> Just to clarify - you mean something like this? Nevermind the magic numbers,
> I'd replace them, of course.

Yes, that's what I meant. But as Alice pointed out, you _may_ be able to 
reduce it to the one AtomicBool.

Daniel
> 
> diff --git a/rust/kernel/once_lite.rs b/rust/kernel/once_lite.rs
> index 723c3244fc85..0622ecbfced5 100644
> --- a/rust/kernel/once_lite.rs
> +++ b/rust/kernel/once_lite.rs
> @@ -16,7 +16,7 @@
> //!
> //! Reference: <https://doc.rust-lang.org/std/sync/struct.Once.html>
>   
> -use core::sync::atomic::{AtomicBool, Ordering::Relaxed};
> +use core::sync::atomic::{AtomicU32, Ordering::Acquire, Ordering::Relaxed};
>   
> /// A low-level synchronization primitive for one-time global execution.
> ///
> @@ -44,13 +44,13 @@
> /// assert_eq!(x, 42);
> /// ```
> ///
> -pub struct OnceLite(AtomicBool, AtomicBool);
> +pub struct OnceLite(AtomicU32);
>   
> impl OnceLite {
>       /// Creates a new `OnceLite` value.
>       #[inline(always)]
>       pub const fn new() -> Self {
> -        Self(AtomicBool::new(false), AtomicBool::new(false))
> +        Self(AtomicU32::new(0))
>       }
>   
>       /// Performs an initialization routine once and only once. The given
> @@ -71,10 +71,10 @@ pub const fn new() -> Self {
>       /// [`DO_ONCE_LITE_IF`]: srctree/include/once_lite.h
>       #[inline(always)]
>       pub fn call_once<F: FnOnce()>(&self, f: F) {
> -        if !self.0.load(Relaxed) && !self.0.swap(true, Relaxed) {
> -            f()
> +        if self.0.load(Relaxed) == 0 && self.0.compare_exchange(0, 1, Acquire, Relaxed) == Ok(0) {
> +            f();
> +            self.0.store(2, Relaxed);
>           };
> -        self.1.store(true, Relaxed);
>       }
>   
>       /// Returns `true` if some `call_once` call has completed successfully.
> @@ -98,7 +98,7 @@ pub fn call_once<F: FnOnce()>(&self, f: F) {
>       /// ```
>       #[inline(always)]
>       pub fn is_completed(&self) -> bool {
> -        self.1.load(Relaxed)
> +        self.0.load(Relaxed) > 1
>       }
> }
> 
>> The `rust` part should be default value for rustdoc tests, can we please
>> omit that?
> 
> Will do!
> 
> Jens