Re: [PATCH v1 3/3] rust: sync: atomic: Add i8/i16 xchg and cmpxchg support

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

On Mon, Dec 29, 2025 at 08:27:01PM +0800, Boqun Feng wrote:
> On Sun, Dec 28, 2025 at 09:05:46PM +0900, FUJITA Tomonori wrote:
> > Add atomic xchg and cmpxchg operation support for i8 and i16 types
> > with tests.
> > 
> 
> I think we also needs the following, otherwise architectures may
> accidentally enable Rust but don't have the correct atomic
> implementation for i8 and i16.
> 
> diff --git a/rust/kernel/sync/atomic/predefine.rs b/rust/kernel/sync/atomic/predefine.rs
> index 248d26555ccf..a4e5bbd45eb2 100644
> --- a/rust/kernel/sync/atomic/predefine.rs
> +++ b/rust/kernel/sync/atomic/predefine.rs
> @@ -5,14 +5,22 @@
>  use crate::static_assert;
>  use core::mem::{align_of, size_of};
> 
> +// 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.
> +//
>  // SAFETY: `i8` has the same size and alignment with itself, and is round-trip transmutable to
>  // itself.
> +#[cfg(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW)]
>  unsafe impl super::AtomicType for i8 {
>      type Repr = i8;
>  }
> 
> +// 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.
> +//
>  // SAFETY: `i16` has the same size and alignment with itself, and is round-trip transmutable to
>  // itself.
> +#[cfg(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW)]
>  unsafe impl super::AtomicType for i16 {
>      type Repr = i16;
>  }
> 
> I can fold it into your patch if that works.
> 

OK, the right place should be at AtomicImpl instead of AtomicType:

diff --git a/rust/kernel/sync/atomic/internal.rs b/rust/kernel/sync/atomic/internal.rs
index ac689ce8ee8c..f4760e3a916e 100644
--- a/rust/kernel/sync/atomic/internal.rs
+++ b/rust/kernel/sync/atomic/internal.rs
@@ -37,10 +37,16 @@ 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)]
 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;
 }

Regards,
Boqun

> Regards,
> Boqun
> 
> > Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> > ---
> >  rust/kernel/sync/atomic/internal.rs  | 2 +-
> >  rust/kernel/sync/atomic/predefine.rs | 4 ++--
> >  2 files changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/rust/kernel/sync/atomic/internal.rs b/rust/kernel/sync/atomic/internal.rs
> > index 1b2a7933bc14..55a80e22a7a0 100644
> > --- a/rust/kernel/sync/atomic/internal.rs
> > +++ b/rust/kernel/sync/atomic/internal.rs
> > @@ -274,7 +274,7 @@ fn set[release](a: &AtomicRepr<Self>, v: Self) {
> >  );
> >  
> >  declare_and_impl_atomic_methods!(
> > -    [ i32 => atomic, i64 => atomic64 ]
> > +    [ i8 => atomic_i8, i16 => atomic_i16, i32 => atomic, i64 => atomic64 ]
> >      /// Exchange and compare-and-exchange atomic operations
> >      pub trait AtomicExchangeOps {
> >          /// Atomic exchange.
> > diff --git a/rust/kernel/sync/atomic/predefine.rs b/rust/kernel/sync/atomic/predefine.rs
> > index 51e9df0cf56e..248d26555ccf 100644
> > --- a/rust/kernel/sync/atomic/predefine.rs
> > +++ b/rust/kernel/sync/atomic/predefine.rs
> > @@ -149,7 +149,7 @@ fn atomic_acquire_release_tests() {
> >  
> >      #[test]
> >      fn atomic_xchg_tests() {
> > -        for_each_type!(42 in [i32, i64, u32, u64, isize, usize] |v| {
> > +        for_each_type!(42 in [i8, i16, i32, i64, u32, u64, isize, usize] |v| {
> >              let x = Atomic::new(v);
> >  
> >              let old = v;
> > @@ -162,7 +162,7 @@ fn atomic_xchg_tests() {
> >  
> >      #[test]
> >      fn atomic_cmpxchg_tests() {
> > -        for_each_type!(42 in [i32, i64, u32, u64, isize, usize] |v| {
> > +        for_each_type!(42 in [i8, i16, i32, i64, u32, u64, isize, usize] |v| {
> >              let x = Atomic::new(v);
> >  
> >              let old = v;
> > -- 
> > 2.43.0
> >