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

[Andreas Hindborg <a.hindborg@kernel.org>]

"Alan Stern" <stern@rowland.harvard.edu> writes:

> On Mon, Jun 30, 2025 at 11:52:35AM +0200, Andreas Hindborg wrote:
>> "Boqun Feng" <boqun.feng@gmail.com> writes:
>> > 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.
>
> You missed point 2 above: at least one of the accesses has to be a
> store.  When you're looking at a non-atomic read vs. an atomic read,
> both of them are loads and so it isn't a data race.

Ah, right. I was missing the entire point made by Boqun. Thanks for
clarifying.

Since what constitutes a race might not be immediately clear to users
(like me), can we include the section above in the safety comment,
rather than deferring to LKMM docs?


Best regards,
Andreas Hindborg