[PATCH 6.18 649/752] arm64: Fix non-atomic __READ_ONCE() with CONFIG_LTO=y

Sasha Levin
2026-02-28T17:57:44Z

From: Marco Elver <elver@google.com>

[ Upstream commit bb0c99e08ab9aa6d04b40cb63c72db9950d51749 ]

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: stable@vger.kernel.org
Reviewed-by: Boqun Feng <boqun@kernel.org>
Signed-off-by: Marco Elver <elver@google.com>
Tested-by: David Laight <david.laight.linux@gmail.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 97d9256d33c97..ac370a4a01ee9 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 */
--
2.51.0