linux-kernel - Re: [PATCH v7 6/9] rust: sync: atomic: Add the framework of arithmetic operations

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Message-Id: <DBDNE3CNUOZP.1H168Z8BD6ZKK@kernel.org>
Date: Wed, 16 Jul 2025 19:16:02 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Boqun Feng" <boqun.feng@...il.com>
Cc: <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>,
 <lkmm@...ts.linux.dev>, <linux-arch@...r.kernel.org>, "Miguel Ojeda"
 <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, "Gary Guo"
 <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "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: Re: [PATCH v7 6/9] rust: sync: atomic: Add the framework of
 arithmetic operations

On Wed Jul 16, 2025 at 5:48 PM CEST, Boqun Feng wrote:
> On Wed, Jul 16, 2025 at 05:36:05PM +0200, Benno Lossin wrote:
> [..]
>> >
>> > I have a better solution:
>> >
>> > in ops.rs
>> >
>> >     pub struct AtomicRepr<T: AtomicImpl>(UnsafeCell<T>)
>> >
>> >     impl AtomicArithmeticOps for i32 {
>> >         // a *safe* function
>> >         fn atomic_add(a: &AtomicRepr, v: i32) {
>> > 	    ...
>> > 	}
>> >     }
>> >
>> > in generic.rs
>> >
>> >     pub struct Atomic<T>(AtoimcRepr<T::Repr>);
>> >
>> >     impl<T: AtomicAdd> Atomic<T> {
>> >         fn add(&self, v: .., ...) {
>> > 	    T::Repr::atomic_add(&self.0, ...);
>> > 	}
>> >     }
>> >
>> > see:
>> >
>> > 	https://git.kernel.org/pub/scm/linux/kernel/git/boqun/linux.git/log/?h=rust-atomic-impl
>> 
>> Hmm what does the additional indirection give you?
>> 
>
> What additional indirection you mean? You cannot make atomic_add() safe
> with only `UnsafeCell<T::Repr>`.

What is the advantage of making it safe? It just moves the safety
comments into `ops.rs` which makes it harder to read due to the macros.

---
Cheers,
Benno