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

Boqun Feng
2025-07-11T21:25:24Z

On Fri, Jul 11, 2025 at 08:34:07PM +0200, Benno Lossin wrote:
[...]
> >
> > So all your disagreement is about the "extra safety requirement" part?
> > How about I drop that:
> >
> > /// Returns a pointer to the underlying atomic `T`.
> > pub const fn as_ptr(&self) -> *mut T {
> > self.0.get()
> > }
>
> Yes that's what I had in mind.
>
> > ? I tried to add something additional information:
> >
> > /// Note that non-atomic reads and writes via the returned pointer may
> > /// cause data races if racing with atomic reads and writes per [LKMM].
> >
> > but that seems redundant, because as you said, data races are UB anyway.
>
> Yeah... I don't think the stdlib docs are too useful on this function:
>
> Doing non-atomic reads and writes on the resulting integer can be a data
> race. This method is mostly useful for FFI, where the function signature
> may use *mut i32 instead of &AtomicI32.
>
> Returning an *mut pointer from a shared reference to this atomic is safe
> because the atomic types work with interior mutability. All
> modifications of an atomic change the value through a shared reference,
> and can do so safely as long as they use atomic operations. Any use of
> the returned raw pointer requires an unsafe block and still has to
> uphold the same restriction: operations on it must be atomic.
>
> You can mention the use of this function for FFI. People might then be
> discouraged from using it for other things where it doesn't make sense.
>

I'm going to keep it simple at the beginning (i.e. using the one-line
doc comment above). I added it in an issue so that we can revisit it
later:

https://github.com/Rust-for-Linux/linux/issues/1180

For your other feebacks on patch #4, I think they are reasonable and I'm
going to apply them, except I may need an extra review on the doc
comment of Atomic<T> when I have it. Thanks!

Regards,
Boqun

> ---
> Cheers,
> Benno