Add generic atomic support for `usize` and `isize`. Note that instead of
mapping directly to `atomic_long_t`, the represention type
(`AllowAtomic::Repr`) is selected based on CONFIG_64BIT. This reduces
the necessity of creating `atomic_long_*` helpers, which could save
the binary size of kernel if inline helpers are not available. To do so,
an internal type `isize_atomic_repr` is defined, it's `i32` in 32bit
kernel and `i64` in 64bit kernel.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
 rust/kernel/sync/atomic.rs | 50 +++++++++++++++++++++++++++++++++++---
 1 file changed, 46 insertions(+), 4 deletions(-)

diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
index eb4a47d7e2f3..3c1bb0c4d396 100644
--- a/rust/kernel/sync/atomic.rs
+++ b/rust/kernel/sync/atomic.rs
@@ -49,6 +49,35 @@ fn rhs_into_delta(rhs: i64) -> i64 {
     }
 }
 
+// Defines an internal type that always maps to the integer type which has the same size alignment
+// as `isize` and `usize`, and `isize` and `usize` are always bi-directional transmutable to
+// `isize_atomic_repr`, which also always implements `AtomicImpl`.
+#[allow(non_camel_case_types)]
+#[cfg(not(CONFIG_64BIT))]
+type isize_atomic_repr = i32;
+#[allow(non_camel_case_types)]
+#[cfg(CONFIG_64BIT)]
+type isize_atomic_repr = i64;
+
+// Ensure size and alignment requirements are checked.
+crate::static_assert!(core::mem::size_of::<isize>() == core::mem::size_of::<isize_atomic_repr>());
+crate::static_assert!(core::mem::align_of::<isize>() == core::mem::align_of::<isize_atomic_repr>());
+crate::static_assert!(core::mem::size_of::<usize>() == core::mem::size_of::<isize_atomic_repr>());
+crate::static_assert!(core::mem::align_of::<usize>() == core::mem::align_of::<isize_atomic_repr>());
+
+// SAFETY: `isize` has the same size and alignment with `isize_atomic_repr`, and is round-trip
+// transmutable to `isize_atomic_repr`.
+unsafe impl generic::AllowAtomic for isize {
+    type Repr = isize_atomic_repr;
+}
+
+// SAFETY: The wrapping add result of two `isize_atomic_repr`s is a valid `usize`.
+unsafe impl generic::AllowAtomicAdd<isize> for isize {
+    fn rhs_into_delta(rhs: isize) -> isize_atomic_repr {
+        rhs as isize_atomic_repr
+    }
+}
+
 // SAFETY: `u32` and `i32` has the same size and alignment, and `u32` is round-trip transmutable to
 // `i32`.
 unsafe impl generic::AllowAtomic for u32 {
@@ -75,6 +104,19 @@ fn rhs_into_delta(rhs: u64) -> i64 {
     }
 }
 
+// SAFETY: `usize` has the same size and alignment with `isize_atomic_repr`, and is round-trip
+// transmutable to `isize_atomic_repr`.
+unsafe impl generic::AllowAtomic for usize {
+    type Repr = isize_atomic_repr;
+}
+
+// SAFETY: The wrapping add result of two `isize_atomic_repr`s is a valid `usize`.
+unsafe impl generic::AllowAtomicAdd<usize> for usize {
+    fn rhs_into_delta(rhs: usize) -> isize_atomic_repr {
+        rhs as isize_atomic_repr
+    }
+}
+
 use crate::macros::kunit_tests;
 
 #[kunit_tests(rust_atomics)]
@@ -94,7 +136,7 @@ macro_rules! for_each_type {
 
     #[test]
     fn atomic_basic_tests() {
-        for_each_type!(42 in [i32, i64, u32, u64] |v| {
+        for_each_type!(42 in [i32, i64, u32, u64, isize, usize] |v| {
             let x = Atomic::new(v);
 
             assert_eq!(v, x.load(Relaxed));
@@ -103,7 +145,7 @@ fn atomic_basic_tests() {
 
     #[test]
     fn atomic_xchg_tests() {
-        for_each_type!(42 in [i32, i64, u32, u64] |v| {
+        for_each_type!(42 in [i32, i64, u32, u64, isize, usize] |v| {
             let x = Atomic::new(v);
 
             let old = v;
@@ -116,7 +158,7 @@ fn atomic_xchg_tests() {
 
     #[test]
     fn atomic_cmpxchg_tests() {
-        for_each_type!(42 in [i32, i64, u32, u64] |v| {
+        for_each_type!(42 in [i32, i64, u32, u64, isize, usize] |v| {
             let x = Atomic::new(v);
 
             let old = v;
@@ -131,7 +173,7 @@ fn atomic_cmpxchg_tests() {
 
     #[test]
     fn atomic_arithmetic_tests() {
-        for_each_type!(42 in [i32, i64, u32, u64] |v| {
+        for_each_type!(42 in [i32, i64, u32, u64, isize, usize] |v| {
             let x = Atomic::new(v);
 
             assert_eq!(v, x.fetch_add(12, Full));
-- 
2.39.5 (Apple Git-154)

On Mon Jul 14, 2025 at 7:36 AM CEST, Boqun Feng wrote:
> +// Defines an internal type that always maps to the integer type which has the same size alignment
> +// as `isize` and `usize`, and `isize` and `usize` are always bi-directional transmutable to
> +// `isize_atomic_repr`, which also always implements `AtomicImpl`.
> +#[allow(non_camel_case_types)]
> +#[cfg(not(CONFIG_64BIT))]
> +type isize_atomic_repr = i32;
> +#[allow(non_camel_case_types)]
> +#[cfg(CONFIG_64BIT)]
> +type isize_atomic_repr = i64;
> +
> +// Ensure size and alignment requirements are checked.
> +crate::static_assert!(core::mem::size_of::<isize>() == core::mem::size_of::<isize_atomic_repr>());
> +crate::static_assert!(core::mem::align_of::<isize>() == core::mem::align_of::<isize_atomic_repr>());
> +crate::static_assert!(core::mem::size_of::<usize>() == core::mem::size_of::<isize_atomic_repr>());
> +crate::static_assert!(core::mem::align_of::<usize>() == core::mem::align_of::<isize_atomic_repr>());

This is fine for now, but I would prefer for this to go into an
`assumptions` module like Miguel proposed some time ago.

---
Cheers,
Benno

On Mon, Jul 14, 2025 at 01:06:08PM +0200, Benno Lossin wrote:
> On Mon Jul 14, 2025 at 7:36 AM CEST, Boqun Feng wrote:
> > +// Defines an internal type that always maps to the integer type which has the same size alignment
> > +// as `isize` and `usize`, and `isize` and `usize` are always bi-directional transmutable to
> > +// `isize_atomic_repr`, which also always implements `AtomicImpl`.
> > +#[allow(non_camel_case_types)]
> > +#[cfg(not(CONFIG_64BIT))]
> > +type isize_atomic_repr = i32;
> > +#[allow(non_camel_case_types)]
> > +#[cfg(CONFIG_64BIT)]
> > +type isize_atomic_repr = i64;
> > +
> > +// Ensure size and alignment requirements are checked.
> > +crate::static_assert!(core::mem::size_of::<isize>() == core::mem::size_of::<isize_atomic_repr>());
> > +crate::static_assert!(core::mem::align_of::<isize>() == core::mem::align_of::<isize_atomic_repr>());
> > +crate::static_assert!(core::mem::size_of::<usize>() == core::mem::size_of::<isize_atomic_repr>());
> > +crate::static_assert!(core::mem::align_of::<usize>() == core::mem::align_of::<isize_atomic_repr>());
> 
> This is fine for now, but I would prefer for this to go into an
> `assumptions` module like Miguel proposed some time ago.
> 

Well, sure. Also if `assumptions` or other core kernel mod can provide
a definition similar to `isize_atomic_repr`, I'm happy to use it.

Regards,
Boqun

> ---
> Cheers,
> Benno