linux-kernel - [PATCH v6 9/9] rust: sync: atomic: Add Atomic<{usize,isize}>

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Message-Id: <20250710060052.11955-10-boqun.feng@gmail.com>
Date: Wed,  9 Jul 2025 23:00:52 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: linux-kernel@...r.kernel.org,
	rust-for-linux@...r.kernel.org,
	lkmm@...ts.linux.dev,
	linux-arch@...r.kernel.org
Cc: "Miguel Ojeda" <ojeda@...nel.org>,
	"Alex Gaynor" <alex.gaynor@...il.com>,
	"Boqun Feng" <boqun.feng@...il.com>,
	"Gary Guo" <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	"Benno Lossin" <lossin@...nel.org>,
	"Andreas Hindborg" <a.hindborg@...nel.org>,
	"Alice Ryhl" <aliceryhl@...gle.com>,
	"Trevor Gross" <tmgross@...ch.edu>,
	"Danilo Krummrich" <dakr@...nel.org>,
	"Will Deacon" <will@...nel.org>,
	"Peter Zijlstra" <peterz@...radead.org>,
	"Mark Rutland" <mark.rutland@....com>,
	"Wedson Almeida Filho" <wedsonaf@...il.com>,
	"Viresh Kumar" <viresh.kumar@...aro.org>,
	"Lyude Paul" <lyude@...hat.com>,
	"Ingo Molnar" <mingo@...nel.org>,
	"Mitchell Levy" <levymitchell0@...il.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	Alan Stern <stern@...land.harvard.edu>
Subject: [PATCH v6 9/9] rust: sync: atomic: Add Atomic<{usize,isize}>

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.

Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
Reviewed-by: Andreas Hindborg <a.hindborg@...nel.org>
Signed-off-by: Boqun Feng <boqun.feng@...il.com>
---
 rust/kernel/sync/atomic.rs | 48 ++++++++++++++++++++++++++++++++++----
 1 file changed, 44 insertions(+), 4 deletions(-)

diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
index e676bc7d9275..e1e40757d7b5 100644
--- a/rust/kernel/sync/atomic.rs
+++ b/rust/kernel/sync/atomic.rs
@@ -53,6 +53,26 @@ fn delta_into_repr(d: Self::Delta) -> Self::Repr {
     }
 }
 
+// SAFETY: For 32bit kernel, `isize` has the same size and alignment with `i32` and is round-trip
+// transmutable to it, for 64bit kernel `isize` has the same size and alignment with `i64` and is
+// round-trip transmutable to it.
+unsafe impl generic::AllowAtomic for isize {
+    #[cfg(not(CONFIG_64BIT))]
+    type Repr = i32;
+    #[cfg(CONFIG_64BIT)]
+    type Repr = i64;
+}
+
+// SAFETY: `isize` is always sound to transmute back from `i32` or `i64` when their sizes are the
+// same.
+unsafe impl generic::AllowAtomicArithmetic for isize {
+    type Delta = Self;
+
+    fn delta_into_repr(d: Self::Delta) -> Self::Repr {
+        d as Self::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 {
@@ -83,6 +103,26 @@ fn delta_into_repr(d: Self::Delta) -> Self::Repr {
     }
 }
 
+// SAFETY: For 32bit kernel, `usize` has the same size and alignment with `i32` and is round-trip
+// transmutable to it, for 64bit kernel `usize` has the same size and alignment with `i64` and is
+// round-trip transmutable to it.
+unsafe impl generic::AllowAtomic for usize {
+    #[cfg(not(CONFIG_64BIT))]
+    type Repr = i32;
+    #[cfg(CONFIG_64BIT)]
+    type Repr = i64;
+}
+
+// SAFETY: `usize` is always sound to transmute back from `i32` or `i64` when their sizes are the
+// same.
+unsafe impl generic::AllowAtomicArithmetic for usize {
+    type Delta = Self;
+
+    fn delta_into_repr(d: Self::Delta) -> Self::Repr {
+        d as Self::Repr
+    }
+}
+
 use crate::macros::kunit_tests;
 
 #[kunit_tests(rust_atomics)]
@@ -102,7 +142,7 @@ 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));
@@ -111,7 +151,7 @@ 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;
@@ -124,7 +164,7 @@ 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;
@@ -139,7 +179,7 @@ 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)