From nobody Fri Apr 17 11:54:54 2026 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5AE582FFDF4; Fri, 20 Feb 2026 08:06:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771574809; cv=none; b=VBDgraYTIwY4YWFRbU7A/urIaeVuiR4sZFdSVkeVWv2ZT7u/JxO0/wW77tetG0dYKT7zeJTvq60squsL242mXdNn/DDEajFd4MUjUTqjz4+/5R36aqtRy7PVPqhMrfntt7A3CgNV1dyMSiICfOzxn1D3dyz/p27xxy02ZQ9ivZU= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771574809; c=relaxed/simple; bh=wSOMhlGB0jPhpHM0oJ9fgtUI0vuVFILcd104qPNJfak=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=ErdqW9kwXZrMMyu3lyewBwH4a2Z37jqaIOYvnX0O1sbbjLw/ygK/zMuHGQ//7OW3XRRQDJXc2u0pSwU6ayZLFVXS0na7ulbO56gsCG8rPe2oj4Hb58NacWCuYhXiKJWIwqAmcSxH70i8D65eY3Rl/mlT7Ti2P38RMKdPPzSDUas= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=inQGWZC6; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="inQGWZC6" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1A216C116C6; Fri, 20 Feb 2026 08:06:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1771574808; bh=wSOMhlGB0jPhpHM0oJ9fgtUI0vuVFILcd104qPNJfak=; h=From:Date:Subject:References:In-Reply-To:To:Cc:From; b=inQGWZC6WHnw6cYEKXGlt2zyYKJ99kTt6lrus/Hx8i1uffzUbdV5hv0usvZqQcnAC YIT+tlLbAfO/93OB6P95uq6cWOYeGQtW7EH9LwZyNgXnHFt2vlpm6bxP7H82kTb9zG dA8+5yTtJhVtniZNMQ/PQtAtp7AbPkPvBdG6QnQsj8xvA7p+qzvcn6Pg/TFnXo1o5d +REBpK38mwY28KQXzQf1pBYfuHAzoQoK3LF+uAwWWAhQehcVXlGt1uETNXZdnzmoX9 QOKbDtsEpfaQ3O0uAdCNMeW4R7L/4Y9FpkIK++Wg6mLj4ga39NK3YuS37LXOdGZD1r pIwiZRh4iZdCA== From: Andreas Hindborg Date: Fri, 20 Feb 2026 09:06:20 +0100 Subject: [PATCH v3 1/3] rust: atomic: add fetch_sub Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20260220-atomic-sub-v3-1-e63cbed1d2aa@kernel.org> References: <20260220-atomic-sub-v3-0-e63cbed1d2aa@kernel.org> In-Reply-To: <20260220-atomic-sub-v3-0-e63cbed1d2aa@kernel.org> To: Will Deacon , Peter Zijlstra , Boqun Feng , Mark Rutland , Gary Guo , Miguel Ojeda , =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= , Benno Lossin , Alice Ryhl , Trevor Gross , Danilo Krummrich Cc: linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, Andreas Hindborg X-Mailer: b4 0.15-dev X-Developer-Signature: v=1; a=openpgp-sha256; l=3268; i=a.hindborg@kernel.org; h=from:subject:message-id; bh=wSOMhlGB0jPhpHM0oJ9fgtUI0vuVFILcd104qPNJfak=; b=owEBbQKS/ZANAwAKAeG4Gj55KGN3AcsmYgBpmBYAZgUDP7+AeFipDJNrVBsOq8zus4usvOXes WbaL41hWNyJAjMEAAEKAB0WIQQSwflHVr98KhXWwBLhuBo+eShjdwUCaZgWAAAKCRDhuBo+eShj d3b1D/9LzFp7PQF1pQhqVK7I0Vgpwjnpg5fsgRN0o2x4uuUJLKRogNK4DGfnNcJO+Evz6VfdHaN vGRxcSdBGczujuGj5CcS0Sa0UmAHnrCJdOqYraFcr8rXnT6PU0TZLrSKh+0R0f84hTNZFBAPVec jKgUEcvXmAAALWe1+QDYj/tiBgFFgmdJ8Dw8FEcEeY3VTXKZOKcyYlWGgfxS73uKRs/tmhIFRko /7IRWby79Uobd/Q3DIRIWqDi6V3t3M+NYvzVQss/axx83jK7c3WjoqvChOiwaJU2k2pr77S2x1Y I0G7kgBHw+53qtZp+2lVzozHIgN5GSxqwaPQRscOzC2XzleGJvNwoqN7j2FzgaalWLjjcAL3Ahv BMNBywW8GWAw8G1ibzwcVNDQO50/yEmSbBzCw3LPCQUbs1d81xHwM8T63syrWGC72FImKICf/Ns mn5UDXY9AKhnbNjVB69GUMeAtZikuBINXYtAX1iQtbcEvMg63qoX9U1Aqtng6AQxGvUqA00JCQo y3GiTVZezL4dPVNDwqDCH3bm+4kjQ6R+Qch8Fifn7mIBh2tIzF6W0F4Gjj5dsy5GGGjAeaMy0QG +mqVwWHwjayfAXV+kOSMNeDtTVR5o1LS/0np9mfQlaowtzZJykRgGgIC9vCpQYrtsJDNRBHDHJ9 cHHo0poGeApA8Aw== X-Developer-Key: i=a.hindborg@kernel.org; a=openpgp; fpr=3108C10F46872E248D1FB221376EB100563EF7A7 Add `Atomic::fetch_sub` with implementation and documentation in line with existing `Atomic::fetch_add` implementation. Reviewed-by: Alice Ryhl Signed-off-by: Andreas Hindborg --- 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(&s= elf, 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 retur= ns the value of `*self` + /// before the update. + /// + /// # Examples + /// + /// ``` + /// use kernel::sync::atomic::{Atomic, Acquire, Full, Relaxed}; + /// + /// let x =3D 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 =3D 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(&self, v: Rhs, _: = Ordering) -> T + where + // Types that support addition also support subtraction. + T: AtomicAdd, + { + let v =3D T::rhs_into_delta(v); + + // INVARIANT: `self.0` is a valid `T` after `atomic_fetch_sub*()` = due to safety requirement + // of `AtomicAdd`. + let ret =3D { + match Ordering::TYPE { + OrderingType::Full =3D> T::Repr::atomic_fetch_sub(&self.0,= v), + OrderingType::Acquire =3D> T::Repr::atomic_fetch_sub_acqui= re(&self.0, v), + OrderingType::Release =3D> T::Repr::atomic_fetch_sub_relea= se(&self.0, v), + OrderingType::Relaxed =3D> T::Repr::atomic_fetch_sub_relax= ed(&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..a094f704be0ce 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= , 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, v: S= elf::Delta) -> Self { + // SAFETY: `a.as_ptr()` guarantees the returned pointer is val= id and properly aligned. + unsafe { bindings::#call(v, a.as_ptr().cast()) } + } } ); 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a=openpgp; fpr=3108C10F46872E248D1FB221376EB100563EF7A7 The documentation for `fetch_add` does not indicate that the original value is returned by `fetch_add`. Update the documentation so this is clear. Reviewed-by: Alice Ryhl Signed-off-by: Andreas Hindborg --- rust/kernel/sync/atomic.rs | 10 ++++------ 1 file changed, 4 insertions(+), 6 deletions(-) diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs index 83e03bb7ecd24..774c0c9f1c9c2 100644 --- a/rust/kernel/sync/atomic.rs +++ b/rust/kernel/sync/atomic.rs @@ -527,16 +527,14 @@ pub fn add(&self, v: Rhs, _: ordering::Relaxed) /// use kernel::sync::atomic::{Atomic, Acquire, Full, Relaxed}; /// /// let x =3D 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 =3D Atomic::new(42); - /// /// assert_eq!(42, x.load(Relaxed)); - /// - /// assert_eq!(54, { x.fetch_add(12, Full); x.load(Relaxed) } ); + /// assert_eq!(42, x.fetch_add(12, Full)); + /// assert_eq!(54, x.load(Relaxed)); /// ``` #[inline(always)] pub fn fetch_add(&self, v: Rhs, _: = Ordering) -> T --=20 2.51.2 From nobody Fri Apr 17 11:54:54 2026 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E7A502F6927; 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a=openpgp; fpr=3108C10F46872E248D1FB221376EB100563EF7A7 The safety comment used in the implementation of `fetch_add` could be read as just saying something it is true without justifying it. Update the safety comment to include justification. Signed-off-by: Andreas Hindborg Reviewed-by: Alice Ryhl --- rust/kernel/sync/atomic/internal.rs | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/rust/kernel/sync/atomic/internal.rs b/rust/kernel/sync/atomic/= internal.rs index a094f704be0ce..67745c44e710b 100644 --- a/rust/kernel/sync/atomic/internal.rs +++ b/rust/kernel/sync/atomic/internal.rs @@ -258,7 +258,7 @@ fn add[](a: &AtomicRepr, v: Self::Delta) { /// Atomically updates `*a` to `(*a).wrapping_add(v)`, and returns= the value of `*a` /// before the update. fn fetch_add[acquire, release, relaxed](a: &AtomicRepr, v: S= elf::Delta) -> Self { - // SAFETY: `a.as_ptr()` is valid and properly aligned. + // SAFETY: `a.as_ptr()` guarantees the returned pointer is val= id and properly aligned. unsafe { bindings::#call(v, a.as_ptr().cast()) } } =20 --=20 2.51.2