From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-106113.protonmail.ch (mail-106113.protonmail.ch [79.135.106.113]) (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 531BD111BF; Sat, 6 Sep 2025 04:23:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=79.135.106.113 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757132639; cv=none; b=f0lVnfCXefnGBMCM6EfNRyFJN3hj0kS1ZciyPfeG1rqglxSoPf4/HCN0ncz4mCKGM+0anLTwsEcO6qqRdlWDmTUjKCzAEprIWfrgvvf5vjOJ/bRULlR8st4sOsNfr6sOs5EJxg9Dlt/bDZQXviOwcC/VIFePAA5G1Uc4TSm/gVk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757132639; c=relaxed/simple; bh=Y5E9MLblc0DbIKjhkL0yfk/9eRnIqLD9oHWKhX2TX+A=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=TkdrDJXtCRSOZJaMSiqffrgxUJtkgySMvkaJhBIBmPCZi1/PHiy3rLA2kgkJIUBlUWBzWrpTtzEtKykXTPOGbrPtWFgRp+JQJmU2llv6WVPbqFWLlMoj4xx+162DIUYk8Jge8xGHk7v3TyktP4i5HhESZYxLiHoj4Lx9mBenSp8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=weathered-steel.dev; spf=pass smtp.mailfrom=weathered-steel.dev; dkim=pass (2048-bit key) header.d=weathered-steel.dev header.i=@weathered-steel.dev header.b=IDu4ZBxa; arc=none smtp.client-ip=79.135.106.113 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=weathered-steel.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=weathered-steel.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=weathered-steel.dev header.i=@weathered-steel.dev header.b="IDu4ZBxa" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=weathered-steel.dev; s=protonmail3; t=1757132634; x=1757391834; bh=UKI/i5BwE5drO0Z5BnzqaNyhQEpkzUHtzUlJ0Xpbx/M=; h=Date:From:To:Cc:Subject:Message-ID:References:In-Reply-To:From:To: Cc:Date:Subject:Reply-To:Feedback-ID:Message-ID:BIMI-Selector; b=IDu4ZBxacKPp8UDV8cPG8mP3u6KPX9/UZsxHAMo9gFlrVL4g/MQL8pMIaFFy4nJ7L c0xT/B31WDJmVkkscGOHLR28S4d+cNEbArPJ62VOL6ZOyddvawPzdlyvW221gRJGFa JxYQJdp2g2ILkSsWXwUEHGFPZjveUOjXLVhcP9nY7ma1dGuJv7BpHbQuzIOz5+CjC8 KPSUfS72tXHwH6t+mKnqu1kHfTpqA1PdsaLBPSZhiPic7wj9H+1F14ahOM7AMyPFrG pCDyv3eprzvTGc3Tj9bWA5jumGVN4nPBHZjx4sYdxAsBosx4DPzL9J9fM45sxDsWBe q2FKNPqOQeO6A== X-Pm-Submission-Id: 4cJg8642m6z1DF4W Date: Sat, 6 Sep 2025 04:23:47 +0000 From: Elle Rhumsaa To: Boqun Feng Cc: rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, lkmm@lists.linux.dev, Will Deacon , Peter Zijlstra , Mark Rutland , Ingo Molnar , Thomas Gleixner , "Paul E. McKenney" , stern@rowland.harvard.edu, Miguel Ojeda , alex.gaynor@gmail.com, Gary Guo , =?iso-8859-1?Q?Bj=F6rn?= Roy Baron , Benno Lossin , Alice Ryhl , Trevor Gross , Danilo Krummrich , Andreas Hindborg Subject: Re: [PATCH 06/14] rust: sync: atomic: Add the framework of arithmetic operations Message-ID: References: <20250905044141.77868-1-boqun.feng@gmail.com> <20250905044141.77868-7-boqun.feng@gmail.com> Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250905044141.77868-7-boqun.feng@gmail.com> On Thu, Sep 04, 2025 at 09:41:33PM -0700, Boqun Feng wrote: > One important set of atomic operations is the arithmetic operations, > i.e. add(), sub(), fetch_add(), add_return(), etc. However it may not > make senses for all the types that `AtomicType` to have arithmetic > operations, for example a `Foo(u32)` may not have a reasonable add() or > sub(), plus subword types (`u8` and `u16`) currently don't have > atomic arithmetic operations even on C side and might not have them in > the future in Rust (because they are usually suboptimal on a few > architecures). Therefore the plan is to add a few subtraits of > `AtomicType` describing which types have and can do atomic arithemtic > operations. > > One trait `AtomicAdd` is added, and only add() and fetch_add() are > added. The rest will be added in the future. > > Reviewed-by: Alice Ryhl > Reviewed-by: Benno Lossin > Signed-off-by: Boqun Feng > Link: https://lore.kernel.org/all/20250719030827.61357-7-boqun.feng@gmail.com/ > --- > rust/kernel/sync/atomic.rs | 94 +++++++++++++++++++++++++++- > rust/kernel/sync/atomic/predefine.rs | 14 +++++ > 2 files changed, 106 insertions(+), 2 deletions(-) > > diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs > index 4c32d12dc61e..016a6bcaf080 100644 > --- a/rust/kernel/sync/atomic.rs > +++ b/rust/kernel/sync/atomic.rs > @@ -16,7 +16,6 @@ > //! > //! [`LKMM`]: srctree/tools/memory-model/ > > -#[allow(dead_code, unreachable_pub)] > mod internal; > pub mod ordering; > mod predefine; > @@ -25,7 +24,7 @@ > pub use ordering::{Acquire, Full, Relaxed, Release}; > > use crate::build_error; > -use internal::{AtomicBasicOps, AtomicExchangeOps, AtomicRepr}; > +use internal::{AtomicArithmeticOps, AtomicBasicOps, AtomicExchangeOps, AtomicRepr}; > use ordering::OrderingType; > > /// A memory location which can be safely modified from multiple execution contexts. > @@ -112,6 +111,19 @@ pub unsafe trait AtomicType: Sized + Send + Copy { > type Repr: AtomicImpl; > } > > +/// Types that support atomic add operations. > +/// > +/// # Safety > +/// > +// TODO: Properly defines `wrapping_add` in the following comment. > +/// `wrapping_add` any value of type `Self::Repr::Delta` obtained by [`Self::rhs_into_delta()`] to > +/// any value of type `Self::Repr` obtained through transmuting a value of type `Self` to must > +/// yield a value with a bit pattern also valid for `Self`. > +pub unsafe trait AtomicAdd: AtomicType { > + /// Converts `Rhs` into the `Delta` type of the atomic implementation. > + fn rhs_into_delta(rhs: Rhs) -> ::Delta; > +} > + > #[inline(always)] > const fn into_repr(v: T) -> T::Repr { > // SAFETY: Per the safety requirement of `AtomicType`, `T` is round-trip transmutable to > @@ -459,3 +471,81 @@ fn try_cmpxchg(&self, old: &mut T, new: T, _: Orde > ret > } > } > + > +impl Atomic > +where > + T::Repr: AtomicArithmeticOps, > +{ > + /// Atomic add. > + /// > + /// Atomically updates `*self` to `(*self).wrapping_add(v)`. > + /// > + /// # Examples > + /// > + /// ``` > + /// use kernel::sync::atomic::{Atomic, Relaxed}; > + /// > + /// let x = Atomic::new(42); > + /// > + /// assert_eq!(42, x.load(Relaxed)); > + /// > + /// x.add(12, Relaxed); > + /// > + /// assert_eq!(54, x.load(Relaxed)); > + /// ``` > + #[inline(always)] > + pub fn add(&self, v: Rhs, _: ordering::Relaxed) > + where > + T: AtomicAdd, > + { > + let v = T::rhs_into_delta(v); > + > + // INVARIANT: `self.0` is a valid `T` after `atomic_add()` due to safety requirement of > + // `AtomicAdd`. > + T::Repr::atomic_add(&self.0, v); > + } > + > + /// Atomic fetch and add. > + /// > + /// Atomically updates `*self` to `(*self).wrapping_add(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!(54, { x.fetch_add(12, Acquire); x.load(Relaxed) }); > + /// > + /// let x = Atomic::new(42); > + /// > + /// assert_eq!(42, x.load(Relaxed)); > + /// > + /// assert_eq!(54, { x.fetch_add(12, Full); x.load(Relaxed) } ); > + /// ``` > + #[inline(always)] > + pub fn fetch_add(&self, v: Rhs, _: Ordering) -> T > + where > + T: AtomicAdd, > + { > + let v = T::rhs_into_delta(v); > + > + // INVARIANT: `self.0` is a valid `T` after `atomic_fetch_add*()` due to safety requirement > + // of `AtomicAdd`. > + let ret = { > + match Ordering::TYPE { > + OrderingType::Full => T::Repr::atomic_fetch_add(&self.0, v), > + OrderingType::Acquire => T::Repr::atomic_fetch_add_acquire(&self.0, v), > + OrderingType::Release => T::Repr::atomic_fetch_add_release(&self.0, v), > + OrderingType::Relaxed => T::Repr::atomic_fetch_add_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/predefine.rs b/rust/kernel/sync/atomic/predefine.rs > index 33356deee952..a6e5883be7cb 100644 > --- a/rust/kernel/sync/atomic/predefine.rs > +++ b/rust/kernel/sync/atomic/predefine.rs > @@ -8,8 +8,22 @@ unsafe impl super::AtomicType for i32 { > type Repr = i32; > } > > +// SAFETY: The wrapping add result of two `i32`s is a valid `i32`. > +unsafe impl super::AtomicAdd for i32 { > + fn rhs_into_delta(rhs: i32) -> i32 { > + rhs > + } > +} > + > // SAFETY: `i64` has the same size and alignment with itself, and is round-trip transmutable to > // itself. > unsafe impl super::AtomicType for i64 { > type Repr = i64; > } > + > +// SAFETY: The wrapping add result of two `i64`s is a valid `i64`. > +unsafe impl super::AtomicAdd for i64 { > + fn rhs_into_delta(rhs: i64) -> i64 { > + rhs > + } > +} > -- > 2.51.0 > > Reviewed-by: Elle Rhumsaa