linux-kernel - Re: [PATCH] hwmon: (max16065) Use READ/WRITE_ONCE to avoid compiler optimization induced race

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Message-ID: <000915fc444a6e1f840f3d4ed6493058aefe850f.camel@decadent.org.uk>
Date: Sun, 08 Feb 2026 23:33:31 +0100
From: Ben Hutchings <ben@...adent.org.uk>
To: David Laight <david.laight.linux@...il.com>
Cc: Gui-Dong Han <hanguidong02@...il.com>, linux@...ck-us.net, 
	linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org, 
	baijiaju1990@...il.com, stable@...r.kernel.org
Subject: Re: [PATCH] hwmon: (max16065) Use READ/WRITE_ONCE to avoid compiler
 optimization induced race

On Sun, 2026-02-08 at 11:48 +0000, David Laight wrote:
> On Sat, 07 Feb 2026 12:43:29 +0100
> Ben Hutchings <ben@...adent.org.uk> wrote:
> 
> > On Sat, 2026-02-07 at 10:43 +0000, David Laight wrote:
> > > On Tue,  3 Feb 2026 20:14:43 +0800
> > > Gui-Dong Han <hanguidong02@...il.com> wrote:
> > >   
> > > > Simply copying shared data to a local variable cannot prevent data
> > > > races. The compiler is allowed to optimize away the local copy and
> > > > re-read the shared memory, causing a Time-of-Check Time-of-Use (TOCTOU)
> > > > issue if the data changes between the check and the usage.  
> > > 
> > > While the compiler is allowed to do this, is there any indication
> > > that either gcc or clang have ever done it?
> > > ISTR someone saying that they never did - although I thought that
> > > was the original justification for adding ACCESS_ONCE().  
> > 
> > They do it sometimes and it's precisely why these maros were added.  It
> > makes no sense to me to look at what these compilers currrently do (for
> > some particular versions, optimisation settings, and targets) and
> > extrapolate that to the assertion that they will never optimise away a
> > copy.
> > 
> > > READ_ONCE() also includes barriers to guarantee ordering between cpu.
> > > These are empty on x86 but add code to architectures where the cpu
> > > can (IIRC) re-order writes.
> > > This is worst on alpha but affects arm and probably ppc.  
> > 
> > No, READ_ONCE() and WRITE_ONCE() don't include any CPU memory barriers.
> 
> Look at the alpha version and the arm64 LTO code.
> The latter changes the reads to have 'acquire' semantics to stop re-ordering.
> Needed for LTO, but the thought is it might be needed in other cases.
[...]

Oh, so they do.  Sorry for "correcting" you based on my old information.

Ben.

-- 
Ben Hutchings
This sentence contradicts itself - no actually it doesn't.

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