rust: sync: atomic: Add fetch_sub()

Add `Atomic::fetch_sub()` with implementation and documentation in line
with existing `Atomic::fetch_add()` implementation.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Boqun Feng <boqun@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260220-atomic-sub-v3-1-e63cbed1d2aa@kernel.org
Link: https://patch.msgid.link/20260303201701.12204-12-boqun@kernel.org

1 files changed, 5 insertions, 0 deletions

diff --git a/rust/kernel/sync/atomic/internal.rs b/rust/kernel/sync/atomic/internal.rs
index e301db4eaf919..b762dbdf6d189 100644
--- a/rust/kernel/sync/atomic/internal.rs
+++ b/rust/kernel/sync/atomic/internal.rs
@@ -340,5 +340,10 @@ declare_and_impl_atomic_methods!(
             // SAFETY: `a.as_ptr()` is valid and properly aligned.
             unsafe { bindings::#call(v, a.as_ptr().cast()) }
         }
+
+        fn fetch_sub[acquire, release, relaxed](a: &AtomicRepr<Self>, v: Self::Delta) -> Self {
+            // SAFETY: `a.as_ptr()` guarantees the returned pointer is valid and properly aligned.
+            unsafe { bindings::#call(v, a.as_ptr().cast()) }
+        }
     }
 );