linux-kernel - Re: [PATCH v6 4/9] rust: sync: atomic: Add generic atomics

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Message-ID: <aHQOPPhTRI2mcEKq@tardis-2.local>
Date: Sun, 13 Jul 2025 12:51:24 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Benno Lossin <lossin@...nel.org>
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 v6 4/9] rust: sync: atomic: Add generic atomics

On Fri, Jul 11, 2025 at 10:03:07AM +0200, Benno Lossin wrote:
[...]
> > +/// # Round-trip transmutability
[...]
> > +///
> > +/// This essentially means a valid bit pattern of `T: AllowAtomic` has to be a valid bit pattern
> > +/// of `T::Repr`. This is needed because [`Atomic<T: AllowAtomic>`] operates on `T::Repr` to
> > +/// implement atomic operations on `T`.
> > +///
> > +/// Note that this is more relaxed than bidirectional transmutability (i.e. [`transmute()`] is
> > +/// always sound between `T` and `T::Repr`) because of the support for atomic variables over
> 
> s/between `T` and `T::Repr`/from `T` to `T::Repr` and back/
> 

Hmm.. I'm going to keep the "between" form, because here we are talking
about bi-directional transmutability, "from .. to .. and back" sounds
like describing round-trip transmutability.

I also re-aranged the doc comment a bit:

/// Types that support basic atomic operations.
///
/// # Round-trip transmutability
///
/// `T` is round-trip transmutable to `U` if and only if both of these properties hold:
///
/// - Any valid bit pattern for `T` is also a valid bit pattern for `U`.
/// - Transmuting (e.g. using [`transmute()`]) a value of type `T` to `U` and then to `T` again
///   yields a value that is in all aspects equivalent to the original value.
///
/// # Safety
///
/// - [`Self`] must have the same size and alignment as [`Self::Repr`].
/// - [`Self`] must be [round-trip transmutable] to  [`Self::Repr`].
///
/// Note that this is more relaxed than requiring the bi-directional transmutability (i.e.
/// [`transmute()`] is always sound between `U` to `T`) because of the support for atomic variables
/// over unit-only enums, see [Examples].
///
/// # Limitations
///
/// ...
///
/// # Examples
///
/// A unit-only enum that implements [`AllowAtomic`]:
///
/// ```
/// use kernel::sync::atomic::{generic::AllowAtomic, Atomic, Relaxed};
///
/// #[derive(Clone, Copy, PartialEq, Eq)]
/// #[repr(i32)]
/// enum State {
///     Uninit = 0,
///     Working = 1,
///     Done = 2,
/// };
///
/// ...
/// ```
/// [`transmute()`]: core::mem::transmute
/// [round-trip transmutable]: AllowAtomic#round-trip-transmutability
/// [Examples]: AllowAtomic#examples

Thanks!

Regards,
Boqun

> > +/// unit-only enums:
> 
> What are "unit-only" enums? Do you mean enums that don't have associated
> data?
> 
> > +///
> > +/// ```
> > +/// #[repr(i32)]
> > +/// enum State { Init = 0, Working = 1, Done = 2, }
> > +/// ```
[...]
> > +pub unsafe trait AllowAtomic: Sized + Send + Copy {
[...]