linux-kernel - Re: [PATCH 4/5] rust: hrtimer: use READ_ONCE instead of read

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Message-ID: <aV20fQSjMN1n6rrs@tardis-2.local>
Date: Wed, 7 Jan 2026 09:18:53 +0800
From: Boqun Feng <boqun.feng@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Gary Guo <gary@...yguo.net>, Andreas Hindborg <a.hindborg@...nel.org>,
	FUJITA Tomonori <fujita.tomonori@...il.com>, lyude@...hat.com,
	will@...nel.org, peterz@...radead.org, richard.henderson@...aro.org,
	mattst88@...il.com, linmag7@...il.com, catalin.marinas@....com,
	ojeda@...nel.org, bjorn3_gh@...tonmail.com, lossin@...nel.org,
	tmgross@...ch.edu, dakr@...nel.org, mark.rutland@....com,
	frederic@...nel.org, tglx@...utronix.de, anna-maria@...utronix.de,
	jstultz@...gle.com, sboyd@...nel.org, viro@...iv.linux.org.uk,
	brauner@...nel.org, 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 4/5] rust: hrtimer: use READ_ONCE instead of read_volatile

On Tue, Jan 06, 2026 at 06:43:17PM +0000, Alice Ryhl wrote:
> On Tue, Jan 06, 2026 at 03:23:00PM +0000, Gary Guo wrote:
> > On Tue, 06 Jan 2026 13:37:34 +0100
> > Andreas Hindborg <a.hindborg@...nel.org> wrote:
> > 
> > > "FUJITA Tomonori" <fujita.tomonori@...il.com> writes:
> > > >
> > > > Sorry, of course this should be:
> > > >
> > > > +__rust_helper ktime_t rust_helper_hrtimer_get_expires(const struct hrtimer *timer)
> > > > +{
> > > > +	return hrtimer_get_expires(timer);
> > > > +}
> > > >  
> > > 
> > > This is a potentially racy read. As far as I recall, we determined that
> > > using read_once is the proper way to handle the situation.
> > > 
> > > I do not think it makes a difference that the read is done by C code.
> > 
> > If that's the case I think the C code should be fixed by inserting the
> > READ_ONCE?
> 
> I maintain my position that if this is what you recommend C code does,
> it's confusing to not make the same recommendation for Rust abstractions
> to the same thing.

The problem here is that C code should use atomic operation here, and
it can be done via READ_ONCE() in C, and in Rust, it should be done by
Atomic::from_ptr().load().

The recommendation is not "using READ_ONCE()" for C, it should be "using
reads that are atomic here", and that's why introducing READ_ONCE() in
Rust is a bad idea, because what we need here is an atomic operation not
a "magical thing that C relies on so we are fine".

> 
> After all, nothing is stopping you from calling atomic_read() in C too.
> 

Actually using atomic_read() in C should also work, it'll be technically
correct as well.

Regards,
Boqun

> Alice