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

[Paul E. McKenney]

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?

							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