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

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Message-ID: <7eea6ee3-4a9e-4eb5-b412-2ece02b33c6c@rowland.harvard.edu>
Date: Mon, 30 Jun 2025 10:44:41 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: Andreas Hindborg <a.hindborg@...nel.org>
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

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.

Alan