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

[Alice Ryhl <aliceryhl@google.com>]

On Tue, Jan 20, 2026 at 04:47:00PM +0000, Gary Guo wrote:
> On Tue Jan 20, 2026 at 4:23 PM GMT, Marco Elver wrote:
> > On Tue, Jan 20, 2026 at 07:52PM +0800, Boqun Feng wrote:
> >> In order to synchronize with C or external, atomic operations over raw
> >> pointers, althought previously there is always an `Atomic::from_ptr()`
> >> to provide a `&Atomic<T>`. However it's more convenient to have helpers
> >> that directly perform atomic operations on raw pointers. Hence a few are
> >> added, which are basically a `Atomic::from_ptr().op()` wrapper.
> >> 
> >> Note: for naming, since `atomic_xchg()` and `atomic_cmpxchg()` has a
> >> conflict naming to 32bit C atomic xchg/cmpxchg, hence they are just
> >> named as `xchg()` and `cmpxchg()`. For `atomic_load()` and
> >> `atomic_store()`, their 32bit C counterparts are `atomic_read()` and
> >> `atomic_set()`, so keep the `atomic_` prefix.
> >> 
> >> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> >> ---
> >>  rust/kernel/sync/atomic.rs           | 104 +++++++++++++++++++++++++++
> >>  rust/kernel/sync/atomic/predefine.rs |  46 ++++++++++++
> >>  2 files changed, 150 insertions(+)
> >> 
> >> diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
> >> index d49ee45c6eb7..6c46335bdb8c 100644
> >> --- a/rust/kernel/sync/atomic.rs
> >> +++ b/rust/kernel/sync/atomic.rs
> >> @@ -611,3 +611,107 @@ pub fn cmpxchg<Ordering: ordering::Ordering>(
> >>          }
> >>      }
> >>  }
> >> +
> >> +/// Atomic load over raw pointers.
> >> +///
> >> +/// This function provides a short-cut of `Atomic::from_ptr().load(..)`, and can be used to work
> >> +/// with C side on synchronizations:
> >> +///
> >> +/// - `atomic_load(.., Relaxed)` maps to `READ_ONCE()` when using for inter-thread communication.
> >> +/// - `atomic_load(.., Acquire)` maps to `smp_load_acquire()`.
> >
> > I'm late to the party and may have missed some discussion, but it might
> > want restating in the documentation and/or commit log:
> >
> > READ_ONCE is meant to be a dependency-ordering primitive, i.e. be more
> > like memory_order_consume than it is memory_order_relaxed. This has, to
> > the best of my knowledge, not changed; otherwise lots of kernel code
> > would be broken.
> 
> 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.
> 
> 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.

Alice