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

[Peter Zijlstra <peterz@infradead.org>]

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 ;-)

> ------------------------------------------------------------------------
> 
> 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