linux-kernel - Re: [PATCH 0/5] Add READ_ONCE and WRITE

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Message-ID: <20251231151216.23446b64.gary@garyguo.net>
Date: Wed, 31 Dec 2025 15:12:16 +0000
From: Gary Guo <gary@...yguo.net>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Boqun Feng <boqun.feng@...il.com>, Will Deacon <will@...nel.org>, Peter
 Zijlstra <peterz@...radead.org>, Richard Henderson
 <richard.henderson@...aro.org>, Matt Turner <mattst88@...il.com>, Magnus
 Lindholm <linmag7@...il.com>, Catalin Marinas <catalin.marinas@....com>,
 Miguel Ojeda <ojeda@...nel.org>, "Björn Roy Baron"
 <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>, Andreas
 Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, Danilo
 Krummrich <dakr@...nel.org>, Mark Rutland <mark.rutland@....com>, FUJITA
 Tomonori <fujita.tomonori@...il.com>, Frederic Weisbecker
 <frederic@...nel.org>, Lyude Paul <lyude@...hat.com>, Thomas Gleixner
 <tglx@...utronix.de>, Anna-Maria Behnsen <anna-maria@...utronix.de>, John
 Stultz <jstultz@...gle.com>, Stephen Boyd <sboyd@...nel.org>, Alexander
 Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan
 Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
 linux-alpha@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 0/5] Add READ_ONCE and WRITE_ONCE to Rust

On Wed, 31 Dec 2025 12:22:24 +0000
Alice Ryhl <aliceryhl@...gle.com> wrote:

> There are currently a few places in the kernel where we use volatile
> reads when we really should be using `READ_ONCE`. To make it possible to
> replace these with proper `READ_ONCE` calls, introduce a Rust version of
> `READ_ONCE`.
> 
> A new config option CONFIG_ARCH_USE_CUSTOM_READ_ONCE is introduced so
> that Rust is able to use conditional compilation to implement READ_ONCE
> in terms of either a volatile read, or by calling into a C helper
> function, depending on the architecture.
> 
> This series is intended to be merged through ATOMIC INFRASTRUCTURE.

Hi Alice,

I would prefer not to expose the READ_ONCE/WRITE_ONCE functions, at
least not with their atomic semantics.

Both callsites that you have converted should be using

	Atomic::from_ptr().load(Relaxed)

Please refer to the documentation of `Atomic` about this. Fujita has a
series that expand the type to u8/u16 if you need narrower accesses.

Best,
Gary


> 
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
> Alice Ryhl (5):
>       arch: add CONFIG_ARCH_USE_CUSTOM_READ_ONCE for arm64/alpha
>       rust: sync: add READ_ONCE and WRITE_ONCE
>       rust: sync: support using bool with READ_ONCE
>       rust: hrtimer: use READ_ONCE instead of read_volatile
>       rust: fs: use READ_ONCE instead of read_volatile
> 
>  MAINTAINERS                     |   2 +
>  arch/Kconfig                    |  11 +++
>  arch/alpha/Kconfig              |   1 +
>  arch/alpha/include/asm/rwonce.h |   4 +-
>  arch/arm64/Kconfig              |   1 +
>  arch/arm64/include/asm/rwonce.h |   4 +-
>  rust/helpers/helpers.c          |   1 +
>  rust/helpers/rwonce.c           |  34 +++++++
>  rust/kernel/fs/file.rs          |   8 +-
>  rust/kernel/sync.rs             |   2 +
>  rust/kernel/sync/rwonce.rs      | 207 ++++++++++++++++++++++++++++++++++++++++
>  rust/kernel/time/hrtimer.rs     |   8 +-
>  12 files changed, 268 insertions(+), 15 deletions(-)
> ---
> base-commit: f8f9c1f4d0c7a64600e2ca312dec824a0bc2f1da
> change-id: 20251230-rwonce-1e8d2ee0bcf9
> 
> Best regards,