From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Martin Subject: Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y Date: Mon, 6 Jul 2020 17:08:20 +0100 Message-ID: <20200706160820.GC10992@arm.com> References: <20200630173734.14057-1-will@kernel.org> <20200630173734.14057-19-will@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20200630173734.14057-19-will@kernel.org> Sender: linux-alpha-owner@vger.kernel.org To: Will Deacon Cc: linux-kernel@vger.kernel.org, Mark Rutland , "Michael S. Tsirkin" , Peter Zijlstra , Catalin Marinas , Jason Wang , virtualization@lists.linux-foundation.org, Arnd Bergmann , Alan Stern , Sami Tolvanen , Matt Turner , kernel-team@android.com, Marco Elver , Kees Cook , "Paul E. McKenney" , Boqun Feng , Josh Triplett , Ivan Kokshaysky , linux-arm-kernel@lists.infradead.org, Richard Henderson , Nick List-Id: virtualization@lists.linuxfoundation.org On Tue, Jun 30, 2020 at 06:37:34PM +0100, Will Deacon wrote: > When building with LTO, there is an increased risk of the compiler > converting an address dependency headed by a READ_ONCE() invocation > into a control dependency and consequently allowing for harmful > reordering by the CPU. > > Ensure that such transformations are harmless by overriding the generic > READ_ONCE() definition with one that provides acquire semantics when > building with LTO. > > Signed-off-by: Will Deacon > --- > arch/arm64/include/asm/rwonce.h | 63 +++++++++++++++++++++++++++++++ > arch/arm64/kernel/vdso/Makefile | 2 +- > arch/arm64/kernel/vdso32/Makefile | 2 +- > 3 files changed, 65 insertions(+), 2 deletions(-) > create mode 100644 arch/arm64/include/asm/rwonce.h > > 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 > +#include > + > +#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. 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? > + ".arch_extension rcpc\n" \ > + "ldapr" #sfx "\t" #regs, \ > + ARM64_HAS_LDAPR) > +#else > +#define __LOAD_RCPC(sfx, regs...) "ldar" #sfx "\t" #regs > +#endif /* CONFIG_AS_HAS_LDAPR */ [...] Cheers ---Dave