Re: [PATCH v3 02/19] compiler.h: Split {READ, WRITE}_ONCE definitions out into rwonce.h

From: Will Deacon <will@kernel.org>
To: boqun.feng@gmail.com
Cc: Joel Fernandes <joelaf@google.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Marco Elver <elver@google.com>, Kees Cook <keescook@chromium.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Jason Wang <jasowang@redhat.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
	linux-arm-kernel@lists.infradead.org,
	Sami Tolvanen <samitolvanen@google.com>,
	linux-alpha@vger.kernel.org,
	Alan Stern <stern@rowland.harvard.edu>,
	Matt Turner <mattst88@gmail.com>,
	kernel-team@android.com, Arnd Bergmann <arnd@arndb.de>,
	Richard Henderson <rth@twiddle.net>
Subject: Re: [PATCH v3 02/19] compiler.h: Split {READ, WRITE}_ONCE definitions out into rwonce.h
Date: Mon, 20 Jul 2020 16:55:02 +0100	[thread overview]
Message-ID: <20200720155501.GA12064@willie-the-truck> (raw)
In-Reply-To: <20200713122322.GD72639@debian-boqun.qqnc3lrjykvubdpftowmye0fmh.lx.internal.cloudapp.net>

On Mon, Jul 13, 2020 at 08:23:22PM +0800, boqun.feng@gmail.com wrote:
> On Fri, Jul 10, 2020 at 05:51:46PM +0100, Will Deacon wrote:
> > diff --git a/include/asm-generic/rwonce.h b/include/asm-generic/rwonce.h
> > new file mode 100644
> > index 000000000000..92cc2f223cb3
> > --- /dev/null
> > +++ b/include/asm-generic/rwonce.h
> > @@ -0,0 +1,91 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/*
> > + * Prevent the compiler from merging or refetching reads or writes. The
> > + * compiler is also forbidden from reordering successive instances of
> > + * READ_ONCE and WRITE_ONCE, but only when the compiler is aware of some
> > + * particular ordering. One way to make the compiler aware of ordering is to
> > + * put the two invocations of READ_ONCE or WRITE_ONCE in different C
> > + * statements.
> > + *
> > + * These two macros will also work on aggregate data types like structs or
> > + * unions.
> > + *
> > + * Their two major use cases are: (1) Mediating communication between
> > + * process-level code and irq/NMI handlers, all running on the same CPU,
> > + * and (2) Ensuring that the compiler does not fold, spindle, or otherwise
> > + * mutilate accesses that either do not require ordering or that interact
> > + * with an explicit memory barrier or atomic instruction that provides the
> > + * required ordering.
> > + */
> > +#ifndef __ASM_GENERIC_RWONCE_H
> > +#define __ASM_GENERIC_RWONCE_H
> > +
> > +#ifndef __ASSEMBLY__
> > +
> > +#include <linux/compiler_types.h>
> > +#include <linux/kasan-checks.h>
> > +#include <linux/kcsan-checks.h>
> > +
> > +#include <asm/barrier.h>
> > +
> > +/*
> > + * Use __READ_ONCE() instead of READ_ONCE() if you do not require any
> > + * atomicity or dependency ordering guarantees. Note that this may result
> > + * in tears!
> > + */
> > +#define __READ_ONCE(x)	(*(const volatile __unqual_scalar_typeof(x) *)&(x))
> > +
> > +#define __READ_ONCE_SCALAR(x)						\
> > +({									\
> > +	__unqual_scalar_typeof(x) __x = __READ_ONCE(x);			\
> > +	smp_read_barrier_depends();					\
> > +	(typeof(x))__x;							\
> > +})
> > +
> > +#define READ_ONCE(x)							\
> > +({									\
> > +	compiletime_assert_rwonce_type(x);				\
> 
> Does it make sense if we also move the definition of this compile time
> assertion into rwonce.h too?

Yes, that looks straightforward enough. Thanks for the suggestion!

I'll also try to get this lot into -next this week.

Will

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