linux-kernel - Re: [PATCH v2 1/3] arm64: Fix non-atomic __READ_ONCE() with CONFIG

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Message-ID: <aXq2FOa1va0P_zu8@tardis.local>
Date: Wed, 28 Jan 2026 17:21:24 -0800
From: Boqun Feng <boqun@...nel.org>
To: Marco Elver <elver@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Will Deacon <will@...nel.org>,
	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: Re: [PATCH v2 1/3] arm64: Fix non-atomic __READ_ONCE() with
 CONFIG_LTO=y

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@...nel.org>

Regards,
Boqun

> 
> 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 */
> -- 
> 2.53.0.rc1.217.geba53bf80e-goog
>