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

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Message-ID: <CANpmjNNSCNm+A=nKdeSDAkcgiKXMEdcQUeMb4PZxWoP2t-z=3A@mail.gmail.com>
Date: Fri, 9 Jan 2026 13:00:00 +0100
From: Marco Elver <elver@...gle.com>
To: paulmck@...nel.org
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 Fri, 9 Jan 2026 at 03:09, Paul E. McKenney <paulmck@...nel.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@...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!

When viewing a report page, at the bottom in the "Crashes" table it's
in the "Commit" column.