linux-kernel - [PATCH 1/3] arm64: Fix non-atomic __READ_ONCE() with CONFIG

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Message-ID: <20260126002936.2676435-2-elver@google.com>
Date: Mon, 26 Jan 2026 01:25:10 +0100
From: Marco Elver <elver@...gle.com>
To: elver@...gle.com, Peter Zijlstra <peterz@...radead.org>, Will Deacon <will@...nel.org>
Cc: Ingo Molnar <mingo@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Boqun Feng <boqun.feng@...il.com>, Waiman Long <longman@...hat.com>, 
	Bart Van Assche <bvanassche@....org>, llvm@...ts.linux.dev, 
	Catalin Marinas <catalin.marinas@....com>, Arnd Bergmann <arnd@...db.de>, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	stable@...r.kernel.org
Subject: [PATCH 1/3] arm64: Fix non-atomic __READ_ONCE() with CONFIG_LTO=y

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@...r.kernel.org>
Signed-off-by: Marco Elver <elver@...gle.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 */
-- 
2.52.0.457.g6b5491de43-goog