Re: [PATCH v2 1/2] rust: sync: atomic: Clarify the need of CONFIG_ARCH_SUPPORTS_ATOMIC_RMW

[Boqun Feng <boqun.feng@gmail.com>]

On Tue, Jan 20, 2026 at 04:54:51PM +0000, Gary Guo wrote:
> On Tue Jan 20, 2026 at 2:05 PM GMT, Boqun Feng wrote:
> > Currently, since all the architectures that support Rust all have
> > CONFIG_ARCH_SUPPORTS_ATOMIC_RMW selected, the helpers of atomic
> > load/store on i8 and i16 relies on CONFIG_ARCH_SUPPORTS_ATOMIC_RMW=y.
> > It's generally fine since most of architectures support that.
> >
> > The plan for CONFIG_ARCH_SUPPORTS_ATOMIC_RMW=n architectures is adding
> > their (probably lock-based) atomic load/store for i8 and i16 as their
> > atomic_{read,set}() and atomic64_{read,set}() counterpart when they
> > plans to support Rust.
> >
> > Hence use a statis_assert!() to check this and remind the future us the
> > need of the helpers. This is more clear than the #[cfg] on impl blocks
> > of i8 and i16.
> >
> > Suggested-by: Dirk Behme <dirk.behme@gmail.com>
> > Suggested-by: Benno Lossin <lossin@kernel.org>
> > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> 
> Reviewed-by: Gary Guo <gary@garyguo.net>
> 

Thanks!

> > ---
> >  rust/kernel/sync/atomic/internal.rs | 19 +++++++++++++------
> >  1 file changed, 13 insertions(+), 6 deletions(-)
> >
> > diff --git a/rust/kernel/sync/atomic/internal.rs b/rust/kernel/sync/atomic/internal.rs
> > index 0dac58bca2b3..ef516bcb02ee 100644
> > --- a/rust/kernel/sync/atomic/internal.rs
> > +++ b/rust/kernel/sync/atomic/internal.rs
> > @@ -37,16 +37,23 @@ pub trait AtomicImpl: Sized + Send + Copy + private::Sealed {
> >      type Delta;
> >  }
> >  
> > -// The current helpers of load/store uses `{WRITE,READ}_ONCE()` hence the atomicity is only
> > -// guaranteed against read-modify-write operations if the architecture supports native atomic RmW.
> > -#[cfg(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW)]
> > +// The current helpers of load/store of atomic `i8` and `i16` use `{WRITE,READ}_ONCE()` hence the
> > +// atomicity is only guaranteed against read-modify-write operations if the architecture supports
> > +// native atomic RmW.
> > +//
> > +// In the future when a CONFIG_ARCH_SUPPORTS_ATOMIC_RMW=n architecture plans to support Rust, the
> > +// load/store helpers that guarantee atomicity against RmW operations (usually via a lock) need to
> > +// be added.
> > +crate::static_assert!(
> > +    cfg!(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW),
> > +    "The current implementation of atomic i8/i16/ptr relies on the architecure being \
> > +    ARCH_SUPPORTS_ATOMIC_RMW"
> 
> The printed string when assertion fails will have 5 spaces between "being" and
> "ARCH", although it probably doesn't matter..
> 

Are you sure? My test result shows:

ERROR:root:error[E0080]: evaluation panicked: The current implementation of atomic i8/i16/ptr relies on the architecure being ARCH_SUPPORTS_ATOMIC_RMW

similar is the following playground example:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=5dd0098247503be792bc35cda8f2630f

Regards,
Boqun

> Best,
> Gary
> 
> > +);
> > +
> >  impl AtomicImpl for i8 {
> >      type Delta = Self;
> >  }
> >  
> > -// The current helpers of load/store uses `{WRITE,READ}_ONCE()` hence the atomicity is only
> > -// guaranteed against read-modify-write operations if the architecture supports native atomic RmW.
> > -#[cfg(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW)]
> >  impl AtomicImpl for i16 {
> >      type Delta = Self;
> >  }
>