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From: Peter Zijlstra <peterz@infradead.org>
To: Will Deacon <will@kernel.org>
Cc: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	kernel-team@android.com, Michael Ellerman <mpe@ellerman.id.au>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Segher Boessenkool <segher@kernel.crashing.org>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Peter Oberparleiter <oberpar@linux.ibm.com>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	ying.huang@intel.com, kirill.shutemov@linux.intel.com
Subject: Re: [PATCH v2 05/10] READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses
Date: Wed, 29 Jan 2020 11:49:17 +0100
Message-ID: <20200129104917.GN14946@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200125082746.GT11457@worktop.programming.kicks-ass.net>

On Sat, Jan 25, 2020 at 09:27:46AM +0100, Peter Zijlstra wrote:
> On Thu, Jan 23, 2020 at 03:33:36PM +0000, Will Deacon wrote:
> > {READ,WRITE}_ONCE() cannot guarantee atomicity for arbitrary data sizes.
> > This can be surprising to callers that might incorrectly be expecting
> > atomicity for accesses to aggregate structures, although there are other
> > callers where tearing is actually permissable (e.g. if they are using
> > something akin to sequence locking to protect the access).
> > 
> > Linus sayeth:
> > 
> >   | We could also look at being stricter for the normal READ/WRITE_ONCE(),
> >   | and require that they are
> >   |
> >   | (a) regular integer types
> >   |
> >   | (b) fit in an atomic word
> >   |
> >   | We actually did (b) for a while, until we noticed that we do it on
> >   | loff_t's etc and relaxed the rules. But maybe we could have a
> >   | "non-atomic" version of READ/WRITE_ONCE() that is used for the
> >   | questionable cases?
> > 
> > The slight snag is that we also have to support 64-bit accesses on 32-bit
> > architectures, as these appear to be widespread and tend to work out ok
> > if either the architecture supports atomic 64-bit accesses (x86, armv7)
> > or if the variable being accesses represents a virtual address and
> > therefore only requires 32-bit atomicity in practice.
> > 
> > Take a step in that direction by introducing a variant of
> > 'compiletime_assert_atomic_type()' and use it to check the pointer
> > argument to {READ,WRITE}_ONCE(). Expose __{READ,WRITE_ONCE}() variants
> > which are allowed to tear and convert the two broken callers over to the
> > new macros.
> 
> The build robot is telling me we also need this for m68k; they have:
> 
>   arch/m68k/include/asm/page.h:typedef struct { unsigned long pmd[16]; } pmd_t;

Fixed that with these patches:

  https://lkml.kernel.org/r/20200129103941.304769381@infradead.org

  reply index

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-23 15:33 [PATCH v2 00/10] Rework READ_ONCE() to improve codegen Will Deacon
2020-01-23 15:33 ` [PATCH v2 01/10] compiler/gcc: Emit build-time warning for GCC prior to version 4.8 Will Deacon
2020-01-23 15:33 ` [PATCH v2 02/10] netfilter: Avoid assigning 'const' pointer to non-const pointer Will Deacon
2020-01-23 19:07   ` Nick Desaulniers
2020-01-24  8:24     ` Peter Zijlstra
2020-01-24 17:20       ` Nick Desaulniers
2020-01-27 12:04         ` David Laight
2020-01-24 17:36       ` Linus Torvalds
2020-01-24 22:00         ` Peter Zijlstra
2020-01-27 12:21         ` David Laight
2020-01-23 15:33 ` [PATCH v2 03/10] fault_inject: Don't rely on "return value" from WRITE_ONCE() Will Deacon
2020-01-23 15:33 ` [PATCH v2 04/10] READ_ONCE: Simplify implementations of {READ,WRITE}_ONCE() Will Deacon
2020-01-23 15:33 ` [PATCH v2 05/10] READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses Will Deacon
2020-01-25  8:27   ` Peter Zijlstra
2020-01-29 10:49     ` Peter Zijlstra [this message]
2020-01-23 15:33 ` [PATCH v2 06/10] READ_ONCE: Drop pointer qualifiers when reading from scalar types Will Deacon
2020-01-23 15:33 ` [PATCH v2 07/10] locking/barriers: Use '__unqual_scalar_typeof' for load-acquire macros Will Deacon
2020-01-23 15:33 ` [PATCH v2 08/10] arm64: barrier: Use '__unqual_scalar_typeof' for acquire/release macros Will Deacon
2020-01-23 15:33 ` [PATCH v2 09/10] compiler/gcc: Raise minimum GCC version for kernel builds to 4.8 Will Deacon
2020-01-23 18:36   ` Nick Desaulniers
2020-01-24  8:26     ` Peter Zijlstra
2020-01-24 17:05       ` Nick Desaulniers
2020-01-24 23:29         ` Peter Zijlstra
2020-01-25 10:34         ` Michael Ellerman
2020-01-23 15:33 ` [PATCH v2 10/10] gcov: Remove old GCC 3.4 support Will Deacon
2020-01-23 18:51   ` Nick Desaulniers
2020-01-28 14:56   ` Peter Oberparleiter
2020-01-23 17:07 ` [PATCH v2 00/10] Rework READ_ONCE() to improve codegen David Laight
2020-01-23 17:16   ` Will Deacon
2020-01-23 17:32     ` David Laight
2020-01-23 18:45       ` Nick Desaulniers
2020-01-23 19:01         ` Arvind Sankar
2020-01-24 10:11           ` David Laight
2020-01-26  1:10           ` Qais Yousef
2020-01-27  7:26             ` Arnd Bergmann
2020-01-23 17:59 ` Linus Torvalds
2020-01-24  8:33   ` Peter Zijlstra
2020-01-24 10:41     ` Peter Zijlstra
2020-02-10  9:50     ` Masahiro Yamada
2020-02-10  9:59       ` Will Deacon
2020-01-31 10:20 ` David Howells

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