Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y

[Will Deacon]

On Mon, Jul 06, 2020 at 05:08:20PM +0100, Dave Martin wrote:
> On Tue, Jun 30, 2020 at 06:37:34PM +0100, Will Deacon wrote:
> > diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h
> > new file mode 100644
> > index 000000000000..515e360b01a1
> > --- /dev/null
> > +++ b/arch/arm64/include/asm/rwonce.h
> > @@ -0,0 +1,63 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/*
> > + * Copyright (C) 2020 Google LLC.
> > + */
> > +#ifndef __ASM_RWONCE_H
> > +#define __ASM_RWONCE_H
> > +
> > +#ifdef CONFIG_CLANG_LTO
> > +
> > +#include <linux/compiler_types.h>
> > +#include <asm/alternative-macros.h>
> > +
> > +#ifndef BUILD_VDSO
> > +
> > +#ifdef CONFIG_AS_HAS_LDAPR
> > +#define __LOAD_RCPC(sfx, regs...)					\
> > +	ALTERNATIVE(							\
> > +		"ldar"	#sfx "\t" #regs,				\
> 
> ^ Should this be here?  It seems that READ_ONCE() will actually read
> twice... even if that doesn't actually conflict with the required
> semantics of READ_ONCE(), it looks odd.

It's patched at runtime, so it's either LDAR or LDAPR.

> Making a direct link between LTO and the memory model also seems highly
> spurious (as discussed in the other subthread) so can we have a comment
> explaining the reasoning?

Sure, although like I say, this is more about helping to progress that
conversation.

Will