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

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

On Tue, Jan 06, 2026 at 01:41:33PM +0100, Andreas Hindborg wrote:
> "Boqun Feng" <boqun.feng@gmail.com> writes:
> 
[...]
> >> > 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.
> 
> What is the semantics of a non-atomic write in C code under lock racing
> with a READ_ONCE/atomic relaxed read in Rust? That is the hrtimer case.
> 

Some C code believes a plain write to a properly aligned location is
atomic (see KCSAN_ASSUME_PLAIN_WRITES_ATOMIC, and no, this doesn't mean
it's recommended to assume such), and I guess that's the case for
hrtimer, if it's not much a trouble you can replace the plain write with
WRITE_ONCE() on C side ;-)

Regards,
Boqun

> 
> Best regards,
> Andreas Hindborg
> 
> 
>