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

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Message-Id: <7478d2cf-9636-45c8-8ffa-8e3a3ba9baf8@app.fastmail.com>
Date: Mon, 26 Jan 2026 23:24:00 +0100
From: "Arnd Bergmann" <arnd@...db.de>
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>,
 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 20:54, Marco Elver wrote:
> On Mon, Jan 26, 2026 at 08:56AM +0100, Arnd Bergmann wrote:
>> On Mon, Jan 26, 2026, at 01:25, Marco Elver wrote:
>> 
>> 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.
>
> I think we only need to worry about Clang for LTO builds. But yeah, our
> minimum supported Clang is 15, so between 15-18 it'd be broken.

Right, I missed the #ifdef CONFIG_LTO check, so indeed gcc is
fine here.

>> With fd69b2f7d5f4 ("compiler: Use __typeof_unqual__() for
>> __unqual_scalar_typeof()"), I would expect __unqual_scalar_typeof()
>> to do the right thing already.
>
> It'd still be broken for Clang 15-18, so it won't help much. We need
> this to work for more than "scalar", so even though it'll work for Clang
> 19+ given the redefinition to __typeof_unqual__, we should deprecate the
> _Generic-based __unqual_scalar_typeof() sooner than later.
>
> I was able to make this work for older compilers:
>
...
>  #define __READ_ONCE(x)							\
>  ({									\
>  	auto __x = &(x);						\
> -	auto __ret = (TYPEOF_UNQUAL(*__x) *)__x, *__retp = &__ret;	\
> -	union { TYPEOF_UNQUAL(*__x) __val; char __c[1]; } __u;		\
> +	auto __ret = (__read_once_typeof(*__x) *)__x, *__retp = &__ret;	\
> +	union { __read_once_typeof(*__x) __val; char __c[1]; } __u;	\
>  	*__retp = &__u.__val;						\

>
> Thoughts?

Looks better than __unqual_scalar_typeof() to me. Would it make
sense to do the same __read_once_typeof() in the asm-generic
version of __READ_ONCE()? I don't remember if we discussed it
in the thread leading up to dee081bf8f82 ("READ_ONCE: Drop 
pointer qualifiers when reading from scalar types").
We probably didn't have __auto_type back then.

        Arnd