Re: [PATCH v3 1/3] arm64: Fix non-atomic __READ_ONCE() with CONFIG_LTO=y
From: David Laight <hidden>
Date: 2026-01-30 15:07:01
Also in:
lkml, llvm, stable
On Fri, 30 Jan 2026 14:28:24 +0100 Marco Elver [off-list ref] wrote:
The implementation of __READ_ONCE() under CONFIG_LTO=y incorrectly
qualified the fallback "once" access for types larger than 8 bytes,
which are not atomic but should still happen "once" and suppress common
compiler optimizations.
The cast `volatile typeof(__x)` applied the volatile qualifier to the
pointer type itself rather than the pointee. This created a volatile
pointer to a non-volatile type, which violated __READ_ONCE() semantics.
Fix this by casting to `volatile typeof(*__x) *`.
With a defconfig + LTO + debug options build, we see the following
functions to be affected:
xen_manage_runstate_time (884 -> 944 bytes)
xen_steal_clock (248 -> 340 bytes)
^-- use __READ_ONCE() to load vcpu_runstate_info structs
Fixes: e35123d83ee3 ("arm64: lto: Strengthen READ_ONCE() to acquire when CONFIG_LTO=y")
Cc: <redacted>
Reviewed-by: Boqun Feng <boqun@kernel.org>
Signed-off-by: Marco Elver <elver@google.com>I found this in some testing (on godbolt), so: Tested-by: David Laight <redacted>
quoted hunk ↗ jump to hunk
--- arch/arm64/include/asm/rwonce.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h index 78beceec10cd..fc0fb42b0b64 100644 --- a/arch/arm64/include/asm/rwonce.h +++ b/arch/arm64/include/asm/rwonce.h@@ -58,7 +58,7 @@ default: \ atomic = 0; \ } \ - atomic ? (typeof(*__x))__u.__val : (*(volatile typeof(__x))__x);\ + atomic ? (typeof(*__x))__u.__val : (*(volatile typeof(*__x) *)__x);\ }) #endif /* !BUILD_VDSO */