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

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Message-ID: <aXDPliPQs8jU_wfz@google.com>
Date: Wed, 21 Jan 2026 13:07:34 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Marco Elver <elver@...gle.com>, Gary Guo <gary@...yguo.net>, 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, Jan 21, 2026 at 08:51:48PM +0800, Boqun Feng wrote:
> On Wed, Jan 21, 2026 at 01:36:04PM +0100, Marco Elver wrote:
> [..]
> > >
> > > > 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 don't think we should confuse Rust users that `READ_ONCE()` has
> dependency orderings but `atomc_load()` doesn't. They are the same on
> the aspect. One of the reasons that I don't want to introduce
> rcu_dereference() and READ_ONCE() on Rust side is exactly this, they are
> the same at LKMM level, so should not be treated differently.

That's okay with me - I just don't think "relaxed" is a good name for
atomic_load() if that's the case.

> > > 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);
> 
> Note that it also applies to atomic_read() and atomic_set() as well.

Just to be completely clear ... am I to understand this that READ_ONCE()
and the LKMM's atomic_load() *are* the exact same thing? Because if so,
then this was really confusing:

> my argument was not about naming, it's
> about READ_ONCE() being more powerful than atomic load (no, not because
> of address dependency, they are the same on that, it's because of the
> behaviors of them regarding a current access on the same memory
> location)
> https://lore.kernel.org/all/aWuV858wU3MeYeaX@tardis-2.local/

Are they the *exact* same thing or not? Do you mean that they are the
same under LKMM, but different under some other context?

Alice