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

["Benno Lossin" <lossin@kernel.org>]

On Fri Jul 11, 2025 at 3:58 PM CEST, Boqun Feng wrote:
> On Fri, Jul 11, 2025 at 10:03:07AM +0200, Benno Lossin wrote:
>> On Thu Jul 10, 2025 at 8:00 AM CEST, Boqun Feng wrote:
>> > diff --git a/rust/kernel/sync/atomic/generic.rs b/rust/kernel/sync/atomic/generic.rs
>> > new file mode 100644
>> > index 000000000000..e044fe21b128
>> > --- /dev/null
>> > +++ b/rust/kernel/sync/atomic/generic.rs
>> > @@ -0,0 +1,289 @@
>> > +// SPDX-License-Identifier: GPL-2.0
>> > +
>> > +//! Generic atomic primitives.
>> > +
>> > +use super::ops::*;
>> > +use super::ordering::*;
>> > +use crate::build_error;
>> > +use core::cell::UnsafeCell;
>> > +
>> > +/// A generic atomic variable.
> [...]
>> 
>> > +///
>> > +/// # Guarantees
>> > +///
>> > +/// Doing an atomic operation while holding a reference of [`Self`] won't cause a data race,
>> > +/// this is guaranteed by the safety requirement of [`Self::from_ptr()`] and the extra safety
>> > +/// requirement of the usage on pointers returned by [`Self::as_ptr()`].
>> 
>> I'd rather think we turn this into an invariant that says any operations
>> on `self.0` through a shared reference is atomic.
>> 
> [...]
>> > +/// unit-only enums:
>> 
>> What are "unit-only" enums? Do you mean enums that don't have associated
>> data?
>> 
>
> Yes, I used the term mentioned at:
>
> 	https://doc.rust-lang.org/reference/items/enumerations.html#r-items.enum.unit-only

Ahhh, never saw that before :)

>> > +///
>> > +/// ```
>> > +/// #[repr(i32)]
>> > +/// enum State { Init = 0, Working = 1, Done = 2, }
>> > +/// ```
>> > +///
>> > +/// # Safety
>> > +///
>> > +/// - [`Self`] must have the same size and alignment as [`Self::Repr`].
>> > +/// - [`Self`] and [`Self::Repr`] must have the [round-trip transmutability].
> [...]
>> > +        let a = self.as_ptr().cast::<T::Repr>();
>> > +
>> > +        // SAFETY:
>> > +        // - For calling the atomic_read*() function:
>> > +        //   - `a` is a valid pointer for the function per the CAST justification above.
>> > +        //   - Per the type guarantees, the following atomic operation won't cause data races.
>> 
>> Which type guarantees? `Self`?
>> 
>
> The above "# Guarantees" of `Atomic<T>`, but yeah I think it should be
> "# Invariants".

Yeah and if we use invariants/guarantees always mention the type that
they are of and don't assume the reader will "know" :)

---
Cheers,
Benno