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

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Message-ID: <20260106145622.GB3707837@noisy.programming.kicks-ass.net>
Date: Tue, 6 Jan 2026 15:56:22 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Gary Guo <gary@...yguo.net>,
	Will Deacon <will@...nel.org>,
	"Paul E. McKenney" <paulmck@...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
Subject: Re: [PATCH 0/5] Add READ_ONCE and WRITE_ONCE to Rust

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.