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

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Message-ID: <20260126225546.30d96049@pumpkin>
Date: Mon, 26 Jan 2026 22:55:46 +0000
From: David Laight <david.laight.linux@...il.com>
To: Marco Elver <elver@...gle.com>
Cc: Arnd Bergmann <arnd@...db.de>, 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, 26 Jan 2026 20:54:24 +0100
Marco Elver <elver@...gle.com> wrote:

> On Mon, Jan 26, 2026 at 08:56AM +0100, Arnd Bergmann wrote:
> > 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.  
> 
> 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.
> 
> > 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:
> 
> diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h
> index 85b1dd7b0274..d6c808cc01be 100644
> --- a/arch/arm64/include/asm/rwonce.h
> +++ b/arch/arm64/include/asm/rwonce.h
> @@ -19,6 +19,18 @@
>  		"ldapr"	#sfx "\t" #regs,				\
>  	ARM64_HAS_LDAPR)
>  
> +#ifdef USE_TYPEOF_UNQUAL
> +#define __read_once_typeof(x) TYPEOF_UNQUAL(x)
> +#else
> +/*
> + * Fallback for older compilers to infer an unqualified type, using the fact
> + * that __auto_type is supposed to drop qualifiers. Unlike typeof_unqual(), the
> + * type must be complete (defines an unevaluated local variable). This must
> + * already be guaranteed because sizeof(x) is used in the __READ_ONCE macro.
> + */
> +#define __read_once_typeof(x) typeof(({ __auto_type ____t = (x); ____t; }))

Did you try old versions of gcc?
gcc before 11.0 don't drop qualifiers.

Even const int y=0; __auto_type x = +y; generates a 'const' x (same for - and ~).
You need to do '0+y' to lose the const.
None of them help here because you don't want the integer promotion.

Can you use:
	union { u8 u8v, u16, u16v, u32 u32v, u64 u64v } u;
Then get the asm to write to the correct sized u.unn, then finish with:
	*(typeof(*x))&u;
or does that spill to stack for 'ptr to volatile'?

It doesn't solve the problem for other sized items, but I'm not sure
how many there really are.
If a handful changing them to READ_BIG_STRUCT_ONCE() shouldn't be a
real problem.

	David

> +#endif
> +
>  /*
>   * When building with LTO, there is an increased risk of the compiler
>   * converting an address dependency headed by a READ_ONCE() invocation
> @@ -32,8 +44,8 @@
>  #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;						\
>  	switch (sizeof(x)) {						\
>  	case 1:								\
> 
> 
> Thoughts?
>