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 necessarity of creating `atomic_long_*` helpers, which could save
the binary size of kernel if inline helpers are not available.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
 rust/kernel/sync/atomic.rs | 71 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 71 insertions(+)

diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
index b2e81e22c105..4166ad48604f 100644
--- a/rust/kernel/sync/atomic.rs
+++ b/rust/kernel/sync/atomic.rs
@@ -102,3 +102,74 @@ fn delta_into_repr(d: Self::Delta) -> Self::Repr {
         d as _
     }
 }
+
+// SAFETY: `usize` has the same size and the alignment as `i64` for 64bit and the same as `i32` for
+// 32bit.
+unsafe impl generic::AllowAtomic for usize {
+    #[cfg(CONFIG_64BIT)]
+    type Repr = i64;
+    #[cfg(not(CONFIG_64BIT))]
+    type Repr = i32;
+
+    fn into_repr(self) -> Self::Repr {
+        self as _
+    }
+
+    fn from_repr(repr: Self::Repr) -> Self {
+        repr as _
+    }
+}
+
+/// ```rust
+/// use kernel::sync::atomic::{Atomic, Full, Relaxed};
+///
+/// let x = Atomic::new(42usize);
+///
+/// assert_eq!(42, x.fetch_add(12, Full));
+/// assert_eq!(54, x.load(Relaxed));
+///
+/// x.add(13, Relaxed);
+///
+/// assert_eq!(67, x.load(Relaxed));
+/// ```
+impl generic::AllowAtomicArithmetic for usize {
+    type Delta = usize;
+
+    fn delta_into_repr(d: Self::Delta) -> Self::Repr {
+        d as _
+    }
+}
+
+// SAFETY: `isize` has the same size and the alignment as `i64` for 64bit and the same as `i32` for
+// 32bit.
+unsafe impl generic::AllowAtomic for isize {
+    type Repr = i64;
+
+    fn into_repr(self) -> Self::Repr {
+        self as _
+    }
+
+    fn from_repr(repr: Self::Repr) -> Self {
+        repr as _
+    }
+}
+
+/// ```rust
+/// use kernel::sync::atomic::{Atomic, Full, Relaxed};
+///
+/// let x = Atomic::new(42isize);
+///
+/// assert_eq!(42, x.fetch_add(12, Full));
+/// assert_eq!(54, x.load(Relaxed));
+///
+/// x.add(13, Relaxed);
+///
+/// assert_eq!(67, x.load(Relaxed));
+/// ```
+impl generic::AllowAtomicArithmetic for isize {
+    type Delta = isize;
+
+    fn delta_into_repr(d: Self::Delta) -> Self::Repr {
+        d as _
+    }
+}
-- 
2.45.2