linux-kernel - Re: [PATCH 0/5] Add READ_ONCE and WRITE

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Message-ID: <b0f3b2a6-e69c-4718-9f05-607b8c02d745@paulmck-laptop>
Date: Thu, 8 Jan 2026 18:09:36 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Marco Elver <elver@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Gary Guo <gary@...yguo.net>,
	Will Deacon <will@...nel.org>,
	Richard Henderson <richard.henderson@...aro.org>,
	Matt Turner <mattst88@...il.com>,
	Magnus Lindholm <linmag7@...il.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>, Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	FUJITA Tomonori <fujita.tomonori@...il.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Lyude Paul <lyude@...hat.com>, Thomas Gleixner <tglx@...utronix.de>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	John Stultz <jstultz@...gle.com>, Stephen Boyd <sboyd@...nel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	kasan-dev@...glegroups.com
Subject: Re: [PATCH 0/5] Add READ_ONCE and WRITE_ONCE to Rust

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@...glegroups.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!

							Thanx, Paul

> Thanks,
> -- Marco
> 
> >                                                         Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > diff --git a/lib/Kconfig.kcsan b/lib/Kconfig.kcsan
> > index 4ce4b0c0109cb..e827e24ab5d42 100644
> > --- a/lib/Kconfig.kcsan
> > +++ b/lib/Kconfig.kcsan
> > @@ -199,7 +199,7 @@ config KCSAN_WEAK_MEMORY
> >
> >  config KCSAN_REPORT_VALUE_CHANGE_ONLY
> >         bool "Only report races where watcher observed a data value change"
> > -       default y
> > +       default n
> >         depends on !KCSAN_STRICT
> >         help
> >           If enabled and a conflicting write is observed via a watchpoint, but
> > @@ -208,7 +208,7 @@ config KCSAN_REPORT_VALUE_CHANGE_ONLY
> >
> >  config KCSAN_ASSUME_PLAIN_WRITES_ATOMIC
> >         bool "Assume that plain aligned writes up to word size are atomic"
> > -       default y
> > +       default n
> >         depends on !KCSAN_STRICT
> >         help
> >           Assume that plain aligned writes up to word size are atomic by
> >