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

Boqun Feng
2025-07-13T19:51:33Z

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 { …