Re: [PATCH 0/5] Add READ_ONCE and WRITE_ONCE to Rust

[Boqun Feng <boqun.feng@gmail.com>]

On Thu, Jan 01, 2026 at 12:53:39AM +0000, Alice Ryhl wrote:
> On Wed, Dec 31, 2025 at 03:12:16PM +0000, Gary Guo wrote:
> > On Wed, 31 Dec 2025 12:22:24 +0000
> > Alice Ryhl <aliceryhl@google.com> wrote:
> > 
> > > There are currently a few places in the kernel where we use volatile
> > > reads when we really should be using `READ_ONCE`. To make it possible to
> > > replace these with proper `READ_ONCE` calls, introduce a Rust version of
> > > `READ_ONCE`.
> > > 
> > > A new config option CONFIG_ARCH_USE_CUSTOM_READ_ONCE is introduced so
> > > that Rust is able to use conditional compilation to implement READ_ONCE
> > > in terms of either a volatile read, or by calling into a C helper
> > > function, depending on the architecture.
> > > 
> > > This series is intended to be merged through ATOMIC INFRASTRUCTURE.
> > 
> > Hi Alice,
> > 
> > I would prefer not to expose the READ_ONCE/WRITE_ONCE functions, at
> > least not with their atomic semantics.
> > 
> > Both callsites that you have converted should be using
> > 
> > 	Atomic::from_ptr().load(Relaxed)
> > 
> > Please refer to the documentation of `Atomic` about this. Fujita has a
> > series that expand the type to u8/u16 if you need narrower accesses.
> 
> Why? If we say that we're using the LKMM, then it seems confusing to not
> have a READ_ONCE() for cases where we interact with C code, and that C
> code documents that READ_ONCE() should be used.
> 

The problem of READ_ONCE() and WRITE_ONCE() is that the semantics is
complicated. Sometimes they are used for atomicity, sometimes they are
used for preventing data race. So yes, we are using LKMM in Rust as
well, but whenever possible, we need to clarify the intentation of the
API, using Atomic::from_ptr().load(Relaxed) helps on that front.

IMO, READ_ONCE()/WRITE_ONCE() is like a "band aid" solution to a few
problems, having it would prevent us from developing a more clear view
for concurrent programming.

Regards,
Boqun

> Alice