Re: [PATCH v5 04/10] rust: sync: atomic: Add generic atomics

[Andreas Hindborg]

"Boqun Feng" <boqun.feng@xxxxxxxxx> writes:

> On Thu, Jun 26, 2025 at 02:15:35PM +0200, Andreas Hindborg wrote:
>> "Boqun Feng" <boqun.feng@xxxxxxxxx> writes:
>>
>> [...]
>>
>> > +
>> > +impl<T: AllowAtomic> Atomic<T> {
>> > +    /// Creates a new atomic.
>> > +    pub const fn new(v: T) -> Self {
>> > +        Self(Opaque::new(v))
>> > +    }
>> > +
>> > +    /// Creates a reference to [`Self`] from a pointer.
>> > +    ///
>> > +    /// # Safety
>> > +    ///
>> > +    /// - `ptr` has to be a valid pointer.
>> > +    /// - `ptr` has to be valid for both reads and writes for the whole lifetime `'a`.
>> > +    /// - For the whole lifetime of '`a`, other accesses to the object cannot cause data races
>> > +    ///   (defined by [`LKMM`]) against atomic operations on the returned reference.
>>
>> I feel the wording is a bit tangled here. How about something along the
>> lines of
>>
>>   For the duration of `'a`, all accesses to the object must be atomic.
>>
>
> Well, a non-atomic read vs an atomic read is not a data race (for both
> Rust memory model and LKMM), so your proposal is overly restricted.

OK, my mistake then. I thought mixing marked and plain accesses would be
considered a race. I got hat from
`tools/memory-model/Documentation/explanation.txt`:

    A "data race"
    occurs when there are two memory accesses such that:

    1.	they access the same location,

    2.	at least one of them is a store,

    3.	at least one of them is plain,

    4.	they occur on different CPUs (or in different threads on the
      same CPU), and

    5.	they execute concurrently.

I did not study all that documentation, so I might be missing a point or
two.


Best regards,
Andreas Hindborg