Re: [PATCH v2 1/3] arm64: Fix non-atomic __READ_ONCE() with CONFIG_LTO=y
From: Boqun Feng <boqun@kernel.org>
Date: 2026-01-29 01:21:27
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lkml, llvm, stable
On Thu, Jan 29, 2026 at 01:52:32AM +0100, Marco Elver 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) *`.
I guess a `volatile typeof(x) *` also works. Either way, good catch! Reviewed-by: Boqun Feng <boqun@kernel.org> Regards, Boqun
quoted hunk ↗ jump to hunk
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> Signed-off-by: Marco Elver <elver@google.com> --- 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 */-- 2.53.0.rc1.217.geba53bf80e-goog