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

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Message-ID: <875xgcxpe6.fsf@kernel.org>
Date: Tue, 01 Jul 2025 10:54:09 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Alan Stern" <stern@...land.harvard.edu>
Cc: "Boqun Feng" <boqun.feng@...il.com>,  <linux-kernel@...r.kernel.org>,
  <rust-for-linux@...r.kernel.org>,  <lkmm@...ts.linux.dev>,
  <linux-arch@...r.kernel.org>,  "Miguel Ojeda" <ojeda@...nel.org>,  "Alex
 Gaynor" <alex.gaynor@...il.com>,  "Gary Guo" <gary@...yguo.net>,
  Björn
 Roy Baron <bjorn3_gh@...tonmail.com>,  "Benno Lossin" <lossin@...nel.org>,
  "Alice Ryhl" <aliceryhl@...gle.com>,  "Trevor Gross" <tmgross@...ch.edu>,
  "Danilo Krummrich" <dakr@...nel.org>,  "Will Deacon" <will@...nel.org>,
  "Peter Zijlstra" <peterz@...radead.org>,  "Mark Rutland"
 <mark.rutland@....com>,  "Wedson Almeida Filho" <wedsonaf@...il.com>,
  "Viresh Kumar" <viresh.kumar@...aro.org>,  "Lyude Paul"
 <lyude@...hat.com>,  "Ingo Molnar" <mingo@...nel.org>,  "Mitchell Levy"
 <levymitchell0@...il.com>,  "Paul E. McKenney" <paulmck@...nel.org>,
  "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,  "Linus Torvalds"
 <torvalds@...ux-foundation.org>,  "Thomas Gleixner" <tglx@...utronix.de>
Subject: Re: [PATCH v5 04/10] rust: sync: atomic: Add generic atomics

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

> On Mon, Jun 30, 2025 at 11:52:35AM +0200, Andreas Hindborg wrote:
>> "Boqun Feng" <boqun.feng@...il.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