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

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Message-ID: <7fa2c07e-acf9-4f9a-b056-4d4254ea61e5@paulmck-laptop>
Date: Tue, 6 Jan 2026 10:18:35 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: 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 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?

							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