linux-kernel - Re: [PATCH v6 9/9] rust: sync: atomic: Add Atomic<{usize,isize}>

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Message-ID: <aHEx85VKv4F_9S61@Mac.home>
Date: Fri, 11 Jul 2025 08:46:59 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc: Benno Lossin <lossin@...nel.org>, 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>,
	Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v6 9/9] rust: sync: atomic: Add Atomic<{usize,isize}>

On Fri, Jul 11, 2025 at 04:40:15PM +0200, Miguel Ojeda wrote:
> On Fri, Jul 11, 2025 at 4:07 PM Boqun Feng <boqun.feng@...il.com> wrote:
> >
> > Thanks Miguel.
> >
> > Maybe we can do even better: having a type alias mapping to the exact
> > i{32,64,128} based on kernel configs? Like
> >
> > (in kernel/lib.rs or ffi.rs)
> >
> > // Want to buy a better name ;-)
> > #[cfg(CONFIG_128BIT)]
> > type isize_mapping = i128;
> > #[cfg(CONFIG_64BIT)]
> > type isize_mapping = i64;
> > #[cfg(not(any(CONFIG_128BIT, CONFIG_64BIT)))]
> > type isize_mapping = i32;
> >
> > similar for usize.
> >
> > Thoughts?
> 
> Yeah, I wondered about that too, but I don't know how common it will
> be (so you may want to keep it local anyway), and I wasn't sure what

Sounds good, I will put it locally.

> to call it either, because e.g. something like `isize_mapping` sounds
> like we are talking about `c_long`.
> 
> What we want is a Rust fixed-width integer of the same size of `isize`
> -- so I think you should try to pick a word that evokes a bit that
> part. Something like `fixed_isize` or words like `underlying` or
> `repr` perhaps?
> 

I end up calling it `isize_atomic_repr`, and create the diff as below:

------------>8
diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
index 7ff87b2b49d6..bb0d3d49e3f7 100644
--- a/rust/kernel/sync/atomic.rs
+++ b/rust/kernel/sync/atomic.rs
@@ -53,14 +53,21 @@ fn delta_into_repr(d: Self::Delta) -> Self::Repr {
     }
 }

+#[allow(non_camel_case_types)]
+#[cfg(not(CONFIG_64BIT))]
+type isize_atomic_repr = i32;
+#[allow(non_camel_case_types)]
+#[cfg(CONFIG_64BIT)]
+type isize_atomic_repr = i64;
+
+crate::static_assert!(core::mem::size_of::<isize>() == core::mem::size_of::<isize_atomic_repr>());
+crate::static_assert!(core::mem::align_of::<isize>() == core::mem::align_of::<isize_atomic_repr>());
+
 // SAFETY: For 32bit kernel, `isize` has the same size and alignment with `i32` and is round-trip
 // transmutable to it, for 64bit kernel `isize` has the same size and alignment with `i64` and is
 // round-trip transmutable to it.
 unsafe impl generic::AllowAtomic for isize {
-    #[cfg(not(CONFIG_64BIT))]
-    type Repr = i32;
-    #[cfg(CONFIG_64BIT)]
-    type Repr = i64;
+    type Repr = isize_atomic_repr;
 }

 // SAFETY: `isize` is always sound to transmute back from `i32` or `i64` when their sizes are the


Seems good?

Regards,
Boqun

> Cheers,
> Miguel