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 <will@xxxxxxxxxx> > --- > 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 <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. 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
