Re: [PATCH v2 1/2] rust: sync: atomic: Add AtomicFlag::get_mut

[Boqun Feng <boqun@kernel.org>]

On Wed, Jan 28, 2026 at 12:32:12PM +0900, FUJITA Tomonori wrote:
[...]
> >> 
> >> I think what Fujita has is more "proper". Your approach still have the issue of
> >> requiring a specific ordering of the fields. If this is messed up, then the
> >> entire thing is broken. I.e. the safety proof of `get_mut` depends on the fields
> >> being ordered correctly in `FlagInner`.
> >> 
> >> If you want to go down this route then I would just scrap `enum Flag` all
> >> together and always define it as struct, with an internal `bool` + 3 bytes of
> >> zero padding. This way we don't even need unsafe for `get_mut`.
> >> 
> > 
> > Hmm.. so like:
> > 
> >     const PAD_SIZE: usize = <3 or 0 depending on ARCHs>
> > 
> >     /// # Invariants
> >     /// `pad` has to be all zero.
> >     struct Flag {
> >         bool_field: bool,
> > 	pad: [i8; PAD_SIZE],
> >     }
> > 
> >     impl Flag {
> >         pub const fn set() -> Flag {
> > 	    Self { true, pad: [0; PAD_SIZE] }
> > 	}
> > 
> >         pub const fn clear() -> Flag {
> > 	    Self { false, pad: [0; PAD_SIZE] }
> > 	}
> >     }
> > 
> > ?
> > 
> > Yes, I think it's better ;-)
> > 
> > Also, now given that `AtomicFlag` behaves exactly like a `Atomic<bool>`,
> > should we do:
> > 
> >     /// `AtomicFlag` documentation here.
> >     #[cfg(<Arch supports byte-wise atomic>)]
> >     type AtomicFlag = Atomic<bool>;
> >     #[cfg(<Arch doesn't supprot byte-wise atomic>)]
> >     struct AtomicFlag(Atomic<BooleanFlag>);
> > 
> >     // `Flag` doesn't even need to be public.
> >     #[cfg(<Arch doesn't supprot byte-wise atomic>)]
> >     struct BooleanFlag { ... }
> > 
> > (I renamed `Flag` -> `BooleanFlag`)
> > 
> > Thoughts? I don't think there is any extra benefit of exposing
> > `Atomic<BooleanFlag>`.
> 
> Agreed.
> 
> You prefer the cleaner git history? Then I'll send the next version
> for rust-sync-fixes, to implement BooleanFlag (or Flag) and AtomicFlag
> from scratch.
> 

Yes, that'll be great, thank you!

Regards,
Boqun