Re: [PATCH 0/5] Add READ_ONCE and WRITE_ONCE to Rust

[Gary Guo <gary@garyguo.net>]

On Wed, 31 Dec 2025 12:22:24 +0000
Alice Ryhl <aliceryhl@google.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@google.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,