[PATCH v2 0/2] rust: atomic: add `fetch_sub` and update docs

[PATCH v2 0/2] rust: atomic: add `fetch_sub` and update docs

[Andreas Hindborg]

Add `fetch_sub` and improve documentation of `fetch_add`.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
Changes in v2:
- Update example to be more clear.
- Add a patch that updates documentation for `fetch_add`.
- Link to v1: https://lore.kernel.org/r/20260128-atomic-sub-v1-1-f8c6abcbb067@kernel.org

---
Andreas Hindborg (2):
      rust: atomic: add fetch_sub
      rust: atomic: update documentation for `fetch_add`

 rust/kernel/sync/atomic.rs          | 52 +++++++++++++++++++++++++++++++++----
 rust/kernel/sync/atomic/internal.rs |  5 ++++
 2 files changed, 52 insertions(+), 5 deletions(-)
---
base-commit: 63804fed149a6750ffd28610c5c1c98cce6bd377
change-id: 20260128-atomic-sub-2ff15962df12

Best regards,
-- 
Andreas Hindborg <a.hindborg@kernel.org>

[PATCH v2 1/2] rust: atomic: add fetch_sub

[Andreas Hindborg]

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

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/sync/atomic.rs          | 43 +++++++++++++++++++++++++++++++++++++
 rust/kernel/sync/atomic/internal.rs |  5 +++++
 2 files changed, 48 insertions(+)

diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
index 4aebeacb961a2..83e03bb7ecd24 100644
--- a/rust/kernel/sync/atomic.rs
+++ b/rust/kernel/sync/atomic.rs
@@ -559,4 +559,47 @@ pub fn fetch_add<Rhs, Ordering: ordering::Ordering>(&self, v: Rhs, _: Ordering)
         // SAFETY: `ret` comes from reading `self.0`, which is a valid `T` per type invariants.
         unsafe { from_repr(ret) }
     }
+
+    /// Atomic fetch and subtract.
+    ///
+    /// Atomically updates `*self` to `(*self).wrapping_sub(v)`, and returns the value of `*self`
+    /// before the update.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::sync::atomic::{Atomic, Acquire, Full, Relaxed};
+    ///
+    /// let x = Atomic::new(42);
+    /// assert_eq!(42, x.load(Relaxed));
+    /// assert_eq!(42, x.fetch_sub(12, Acquire));
+    /// assert_eq!(30, x.load(Relaxed));
+    ///
+    /// let x = Atomic::new(42);
+    /// assert_eq!(42, x.load(Relaxed));
+    /// assert_eq!(42, x.fetch_sub(12, Full));
+    /// assert_eq!(30, x.load(Relaxed));
+    /// ```
+    #[inline(always)]
+    pub fn fetch_sub<Rhs, Ordering: ordering::Ordering>(&self, v: Rhs, _: Ordering) -> T
+    where
+        // Types that support addition also support subtraction.
+        T: AtomicAdd<Rhs>,
+    {
+        let v = T::rhs_into_delta(v);
+
+        // INVARIANT: `self.0` is a valid `T` after `atomic_fetch_sub*()` due to safety requirement
+        // of `AtomicAdd`.
+        let ret = {
+            match Ordering::TYPE {
+                OrderingType::Full => T::Repr::atomic_fetch_sub(&self.0, v),
+                OrderingType::Acquire => T::Repr::atomic_fetch_sub_acquire(&self.0, v),
+                OrderingType::Release => T::Repr::atomic_fetch_sub_release(&self.0, v),
+                OrderingType::Relaxed => T::Repr::atomic_fetch_sub_relaxed(&self.0, v),
+            }
+        };
+
+        // SAFETY: `ret` comes from reading `self.0`, which is a valid `T` per type invariants.
+        unsafe { from_repr(ret) }
+    }
 }
diff --git a/rust/kernel/sync/atomic/internal.rs b/rust/kernel/sync/atomic/internal.rs
index 6fdd8e59f45be..7eed8c66efff8 100644
--- a/rust/kernel/sync/atomic/internal.rs
+++ b/rust/kernel/sync/atomic/internal.rs
@@ -261,5 +261,10 @@ fn fetch_add[acquire, release, relaxed](a: &AtomicRepr<Self>, v: Self::Delta) ->
             // 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()` is valid and properly aligned.
+            unsafe { bindings::#call(v, a.as_ptr().cast()) }
+        }
     }
 );

-- 
2.51.2

[PATCH v2 2/2] rust: atomic: update documentation for `fetch_add`

[Andreas Hindborg]

The documentation for `fetch_add` does not indicate that the original value
is returned by `fetch_add`. Update the documentation so this is clear.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/sync/atomic.rs | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
index 83e03bb7ecd24..ce0ebd725d697 100644
--- a/rust/kernel/sync/atomic.rs
+++ b/rust/kernel/sync/atomic.rs
@@ -527,16 +527,15 @@ pub fn add<Rhs>(&self, v: Rhs, _: ordering::Relaxed)
     /// use kernel::sync::atomic::{Atomic, Acquire, Full, Relaxed};
     ///
     /// let x = Atomic::new(42);
-    ///
     /// assert_eq!(42, x.load(Relaxed));
-    ///
-    /// assert_eq!(54, { x.fetch_add(12, Acquire); x.load(Relaxed) });
+    /// assert_eq!(42, x.fetch_add(12, Acquire));
+    /// assert_eq!(54, x.load(Relaxed));
     ///
     /// let x = Atomic::new(42);
-    ///
     /// assert_eq!(42, x.load(Relaxed));
+    /// assert_eq!(42, x.fetch_add(12, Full));
+    /// assert_eq!(54, x.load(Relaxed));
     ///
-    /// assert_eq!(54, { x.fetch_add(12, Full); x.load(Relaxed) } );
     /// ```
     #[inline(always)]
     pub fn fetch_add<Rhs, Ordering: ordering::Ordering>(&self, v: Rhs, _: Ordering) -> T

-- 
2.51.2

Re: [PATCH v2 2/2] rust: atomic: update documentation for `fetch_add`

[Alice Ryhl]

On Thu, Feb 19, 2026 at 01:20:10PM +0100, Andreas Hindborg wrote:
> The documentation for `fetch_add` does not indicate that the original value
> is returned by `fetch_add`. Update the documentation so this is clear.
> 
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

Reviewed-by: Alice Ryhl <aliceryhl@google.com>

Re: [PATCH v2 2/2] rust: atomic: update documentation for `fetch_add`

[Miguel Ojeda]

On Thu, Feb 19, 2026 at 1:20 PM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>
> +    /// assert_eq!(54, x.load(Relaxed));
>      ///
> -    /// assert_eq!(54, { x.fetch_add(12, Full); x.load(Relaxed) } );
>      /// ```

Doesn't this leave an empty line at the end of the example?

Cheers,
Miguel

Re: [PATCH v2 1/2] rust: atomic: add fetch_sub

[Miguel Ojeda]

On Thu, Feb 19, 2026 at 1:20 PM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>
> +        fn fetch_sub[acquire, release, relaxed](a: &AtomicRepr<Self>, v: Self::Delta) -> Self {
> +            // SAFETY: `a.as_ptr()` is valid and properly aligned.
> +            unsafe { bindings::#call(v, a.as_ptr().cast()) }
> +        }

[ I see `fetch_add` does the same, so it is fine here in this patch
for consistency, but I thought I would leave a note here anyway... ]

The safety comment, as written, could be read as just saying something
it is true without justifying it. Something like:

    `a.as_ptr()` guarantees the returned pointer is valid and properly aligned.

would show better why it is such, rather than simply stating it.

Cheers,
Miguel

Re: [PATCH v2 1/2] rust: atomic: add fetch_sub

[Andreas Hindborg]

"Miguel Ojeda" <miguel.ojeda.sandonis@gmail.com> writes:

> On Thu, Feb 19, 2026 at 1:20 PM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>>
>> +        fn fetch_sub[acquire, release, relaxed](a: &AtomicRepr<Self>, v: Self::Delta) -> Self {
>> +            // SAFETY: `a.as_ptr()` is valid and properly aligned.
>> +            unsafe { bindings::#call(v, a.as_ptr().cast()) }
>> +        }
>
> [ I see `fetch_add` does the same, so it is fine here in this patch
> for consistency, but I thought I would leave a note here anyway... ]
>
> The safety comment, as written, could be read as just saying something
> it is true without justifying it. Something like:
>
>     `a.as_ptr()` guarantees the returned pointer is valid and properly aligned.

I'll respin and fix this, for `fetch_add` as well. I can take the
spurious newline you mentioned on same occasion.


Best regards,
Andreas Hindborg