linux-kernel - Re: [PATCH v5 05/10] rust: sync: atomic: Add atomic {cmp,}xchg operations

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Message-Id: <DAZZJTOR5N3G.1ZC2DWWPP2YM5@kernel.org>
Date: Mon, 30 Jun 2025 17:50:25 +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>
Subject: Re: [PATCH v5 05/10] rust: sync: atomic: Add atomic {cmp,}xchg
 operations

On Mon Jun 30, 2025 at 5:24 PM CEST, Boqun Feng wrote:
> On Sat, Jun 28, 2025 at 10:00:34AM +0200, Benno Lossin wrote:
>> On Sat Jun 28, 2025 at 9:31 AM CEST, Boqun Feng wrote:
>> > On Sat, Jun 28, 2025 at 08:12:42AM +0200, Benno Lossin wrote:
>> >> On Fri Jun 27, 2025 at 3:53 PM CEST, Boqun Feng wrote:
>> >> > As for naming, the reason I choose xchg() and cmpxchg() is because they
>> >> > are the name LKMM uses for a long time, to use another name, we have to
>> >> > have a very good reason to do so and I don't see a good reason
>> >> > that the other names are better, especially, in our memory model, we use
>> >> > xchg() and cmpxchg() a lot, and they are different than Rust version
>> >> > where you can specify orderings separately. Naming LKMM xchg()/cmpxchg()
>> >> > would cause more confusion I believe.
>> >> 
>> >> I'm just not used to the name shortening from the kernel... I think it's
>> >
>> > I guess it's a bit curse of knowledge from my side...
>> >
>> >> fine to use them especially since the ordering parameters differ from
>> >> std's atomics.
>> >> 
>> >> Can you add aliases for the Rust names?
>> >> 
>> >
>> > I can, but I also want to see a real user request ;-) As a bi-model user
>> > myself, I generally don't mind the name, as you can see C++ and Rust use
>> > different names as well, what I usually do is just "tell me what's the
>> > name of the function if I need to do this" ;-)
>> 
>> I think learning Rust in the kernel is different from learning a new
>> language. Yes you're learning a specific dialect of Rust, but that's
>> what every project does.
>> 
>> You also added aliases for the C versions, so let's also add the Rust
>> ones :)
>> 
>
> Make senses, so added:
>
> --- a/rust/kernel/sync/atomic/generic.rs
> +++ b/rust/kernel/sync/atomic/generic.rs
> @@ -310,7 +310,7 @@ impl<T: AllowAtomic> Atomic<T>
>      /// assert_eq!(42, x.xchg(52, Acquire));
>      /// assert_eq!(52, x.load(Relaxed));
>      /// ```
> -    #[doc(alias("atomic_xchg", "atomic64_xchg"))]
> +    #[doc(alias("atomic_xchg", "atomic64_xchg", "swap"))]
>      #[inline(always)]
>      pub fn xchg<Ordering: Any>(&self, v: T, _: Ordering) -> T {
>          let v = T::into_repr(v);
> @@ -382,6 +382,7 @@ pub fn xchg<Ordering: Any>(&self, v: T, _: Ordering) -> T {
>          "atomic64_cmpxchg",
>          "atomic_try_cmpxchg",
>          "atomic64_try_cmpxchg"
> +        "compare_exchange"
>      ))]
>      #[inline(always)]
>      pub fn cmpxchg<Ordering: Any>(&self, mut old: T, new: T, o: Ordering) -> Result<T, T> {
>
> Seems good?

Yeah, thanks!

---
Cheers,
Benno