linux-kernel - Re: [PATCH 2/3] arm64: Optimize __READ_ONCE() with CONFIG

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Message-Id: <d3295c84-da15-4695-8110-7e04cfaf1419@app.fastmail.com>
Date: Mon, 26 Jan 2026 08:56:11 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Marco Elver" <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>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] arm64: Optimize __READ_ONCE() with CONFIG_LTO=y

On Mon, Jan 26, 2026, at 01:25, Marco Elver wrote:
> diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h
> index fc0fb42b0b64..9963948f4b44 100644
> --- a/arch/arm64/include/asm/rwonce.h
> +++ b/arch/arm64/include/asm/rwonce.h
> @@ -32,8 +32,7 @@
>  #define __READ_ONCE(x)							\
>  ({									\
>  	typeof(&(x)) __x = &(x);					\
> -	int atomic = 1;							\
> -	union { __unqual_scalar_typeof(*__x) __val; char __c[1]; } __u;	\
> +	union { TYPEOF_UNQUAL(*__x) __val; char __c[1]; } __u;		\
>  	switch (sizeof(x)) {						\
>  	case 1:								\
>  		asm volatile(__LOAD_RCPC(b, %w0, %1)			\

How does this work with CC_HAS_TYPEOF_UNQUAL=false?

As far as I can tell, TYPEOF_UNQUAL() falls back to __typeof__
on gcc-13, clang-18 and earlier, and not strip out qualifiers.

With fd69b2f7d5f4 ("compiler: Use __typeof_unqual__() for
__unqual_scalar_typeof()"), I would expect __unqual_scalar_typeof()
to do the right thing already.

     Arnd