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

[Marco Elver]

On Fri, 9 Jan 2026 at 03:09, Paul E. McKenney <paulmck at kernel.org> wrote:
>
> On Tue, Jan 06, 2026 at 08:28:41PM +0100, Marco Elver wrote:
> > On Tue, 6 Jan 2026 at 19:18, 'Paul E. McKenney' via kasan-dev
> > <kasan-dev at googlegroups.com> wrote:
> > > On Tue, Jan 06, 2026 at 03:56:22PM +0100, Peter Zijlstra wrote:
> > > > On Tue, Jan 06, 2026 at 09:09:37PM +0800, Boqun Feng wrote:
> > > >
> > > > > 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 ;-)
> > > >
> > > > GCC used to provide this guarantee, some of the older code was written
> > > > on that. GCC no longer provides that guarantee (there are known cases
> > > > where it breaks and all that) and newer code should not rely on this.
> > > >
> > > > All such places *SHOULD* be updated to use READ_ONCE/WRITE_ONCE.
> > >
> > > Agreed!
> > >
> > > In that vein, any objections to the patch shown below?
> >
> > I'd be in favor, as that's what we did in the very initial version of
> > KCSAN (we started strict and then loosened things up).
> >
> > However, the fallout will be even more perceived "noise", despite
> > being legitimate data races. These config knobs were added after much
> > discussion in 2019/2020, somewhere around this discussion (I think
> > that's the one that spawned KCSAN_REPORT_VALUE_CHANGE_ONLY, can't find
> > the source for KCSAN_ASSUME_PLAIN_WRITES_ATOMIC):
> > https://lore.kernel.org/all/CAHk-=wgu-QXU83ai4XBnh7JJUo2NBW41XhLWf=7wrydR4=ZP0g@mail.gmail.com/
>
> Fair point!
>
> > While the situation has gotten better since 2020, we still have latent
> > data races that need some thought (given papering over things blindly
> > with *ONCE is not right either). My recommendation these days is to
> > just set CONFIG_KCSAN_STRICT=y for those who care (although I'd wish
> > everyone cared the same amount :-)).
> >
> > Should you feel the below change is appropriate for 2026, feel free to
> > carry it (consider this my Ack).
> >
> > However, I wasn't thinking of tightening the screws until the current
> > set of known data races has gotten to a manageable amount (say below
> > 50)
> > https://syzkaller.appspot.com/upstream?manager=ci2-upstream-kcsan-gce
> > Then again, on syzbot the config can remain unchanged.
>
> Is there an easy way to map from a report to the SHA-1 that the
> corresponding test ran against?  Probably me being blind, but I am not
> seeing it.  Though I do very much like the symbolic names in those
> stack traces!

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