linux-kernel - Re: [PATCH 2/2] rust: sync: atomic: Add atomic operation helpers over raw pointers

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Message-ID: <CANpmjNMq_oqvOmO9F2f-v3FTr6p0EwENo70ppvKLXDjgPbR22g@mail.gmail.com>
Date: Wed, 21 Jan 2026 13:36:04 +0100
From: Marco Elver <elver@...gle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Gary Guo <gary@...yguo.net>, Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	kasan-dev@...glegroups.com, Will Deacon <will@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Mark Rutland <mark.rutland@....com>, 
	Miguel Ojeda <ojeda@...nel.org>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, 
	Elle Rhumsaa <elle@...thered-steel.dev>, "Paul E. McKenney" <paulmck@...nel.org>, 
	FUJITA Tomonori <fujita.tomonori@...il.com>
Subject: Re: [PATCH 2/2] rust: sync: atomic: Add atomic operation helpers over
 raw pointers

On Wed, 21 Jan 2026 at 13:19, Alice Ryhl <aliceryhl@...gle.com> wrote:
[...]
> > On the Rust-side documentation we mentioned that `Relaxed` always preserve
> > dependency ordering, so yes, it is closer to `consume` in the C11 model.
>
> Like in the other thread, I still think this is a mistake. Let's be
> explicit about intent and call things that they are.
> https://lore.kernel.org/all/aXDCTvyneWOeok2L@google.com/
>
> > If the idea is to add an explicit `Consume` ordering on the Rust side to
> > document the intent clearly, then I am actually somewhat in favour.

That'd be a mistake, too, as the semantics is not equivalent to "C++
consume" either, but arguably closer to it than "C++ relaxed" (I
clearly got confused by the Linux Rust Relaxed != Normal Rust
Relaxed).
It's also known that consume or any variant of it, has been deemed
unimplementable, since the compiler would have to be able to reason
about whole-program dependency chains.

> > This way, we can for example, map it to a `READ_ONCE` in most cases, but we can
> > also provide an option to upgrade such calls to `smp_load_acquire` in certain
> > cases when needed, e.g. LTO arm64.
>
> It always maps to READ_ONCE(), no? It's just that on LTO arm64 the
> READ_ONCE() macro is implemented like smp_load_acquire().
>
> > However this will mean that Rust code will have one more ordering than the C
> > API, so I am keen on knowing how Boqun, Paul, Peter and others think about this.
>
> On that point, my suggestion would be to use the standard LKMM naming
> such as rcu_dereference() or READ_ONCE().
>
> I'm told that READ_ONCE() apparently has stronger guarantees than an
> atomic consume load, but I'm not clear on what they are.

It's also meant to enforce ordering through control-dependencies, such as:

   if (READ_ONCE(x)) WRITE_ONCE(y, 1);