From mboxrd@z Thu Jan  1 00:00:00 1970
From: Will Deacon <will@kernel.org>
Subject: Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when
 CLANG_LTO=y
Date: Mon, 6 Jul 2020 19:35:11 +0100
Message-ID: <20200706183510.GA23766@willie-the-truck>
References: <20200630173734.14057-1-will@kernel.org>
 <20200630173734.14057-19-will@kernel.org>
 <20200706160820.GC10992@arm.com>
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To: Dave Martin <Dave.Martin@arm.com>
Cc: linux-kernel@vger.kernel.org, Mark Rutland <mark.rutland@arm.com>, "Michael S. Tsirkin" <mst@redhat.com>, Peter Zijlstra <peterz@infradead.org>, Catalin Marinas <catalin.marinas@arm.com>, Jason Wang <jasowang@redhat.com>, virtualization@lists.linux-foundation.org, Arnd Bergmann <arnd@arndb.de>, Alan Stern <stern@rowland.harvard.edu>, Sami Tolvanen <samitolvanen@google.com>, Matt Turner <mattst88@gmail.com>, kernel-team@android.com, Marco Elver <elver@google.com>, Kees Cook <keescook@chromium.org>, "Paul E. McKenney" <paulmck@kernel.org>, Boqun Feng <boqun.feng@gmail.com>, Josh Triplett <josh@joshtriplett.org>, Ivan Kokshaysky <ink@jurassic.park.msu.ru>, linux-arm-kernel@lists.infradead.org, Richard Henderson <rth@twiddle.net>, Nick

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