Re: [PATCH 0/5] Add READ_ONCE and WRITE_ONCE to Rust

["Paul E. McKenney" <paulmck@kernel.org>]

On Wed, Jan 07, 2026 at 09:43:22AM +0100, Peter Zijlstra wrote:
> On Tue, Jan 06, 2026 at 10:18:35AM -0800, Paul E. McKenney 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?
> 
> Not really; although it would of course be nice if that were accompanied
> with a pile of cleanup patches taking out the worst offenders or
> somesuch ;-)

Careful what you ask for.  You might get it...  ;-)

							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