linux-kernel - Re: [PATCH v2] arch: use WRITE_ONCE/READ_ONCE in smp_store_release/smp_load

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Date:	Mon, 03 Aug 2015 11:41:58 +1000
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Andrey Konovalov <andreyknvl@...gle.com>
Cc:	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	James Hogan <james.hogan@...tec.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux390@...ibm.com, "David S. Miller" <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Arnd Bergmann <arnd@...db.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Alexander Duyck <alexander.h.duyck@...hat.com>,
	Andre Przywara <andre.przywara@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Borislav Petkov <bp@...e.de>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-ia64@...r.kernel.org, linux-metag@...r.kernel.org,
	linux-mips@...ux-mips.org, linuxppc-dev@...ts.ozlabs.org,
	linux-s390@...r.kernel.org, sparclinux@...r.kernel.org,
	linux-arch@...r.kernel.org, Dmitry Vyukov <dvyukov@...gle.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v2] arch: use WRITE_ONCE/READ_ONCE in
 smp_store_release/smp_load_acquire

On Sun, 2015-08-02 at 17:11 +0200, Andrey Konovalov wrote:
> Replace ACCESS_ONCE() macro in smp_store_release() and smp_load_acquire()
> with WRITE_ONCE() and READ_ONCE() on x86, arm, arm64, ia64, metag, mips,
> powerpc, s390, sparc and asm-generic since ACCESS_ONCE does not work
> reliably on non-scalar types.

.. and there are no restrictions on the argument to smp_load_acquire(), so it
may be a non-scalar type.

Though from a quick grep it looks like no one is doing that at the moment?

> WRITE_ONCE() and READ_ONCE() were introduced in the commits 230fa253df63
> ("kernel: Provide READ_ONCE and ASSIGN_ONCE") and 43239cbe79fc ("kernel:
> Change ASSIGN_ONCE(val, x) to WRITE_ONCE(x, val)").
> 
> Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
> ---
> Changed in v2:
>   - Other archs besides x86.
> 
>  arch/powerpc/include/asm/barrier.h  | 4 ++--

> diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
> index 51ccc72..0eca6ef 100644
> --- a/arch/powerpc/include/asm/barrier.h
> +++ b/arch/powerpc/include/asm/barrier.h
> @@ -76,12 +76,12 @@
>  do {									\
>  	compiletime_assert_atomic_type(*p);				\
>  	smp_lwsync();							\
> -	ACCESS_ONCE(*p) = (v);						\
> +	WRITE_ONCE(*p, v);						\
>  } while (0)
>  
>  #define smp_load_acquire(p)						\
>  ({									\
> -	typeof(*p) ___p1 = ACCESS_ONCE(*p);				\
> +	typeof(*p) ___p1 = READ_ONCE(*p);				\
>  	compiletime_assert_atomic_type(*p);				\
>  	smp_lwsync();							\
>  	___p1;								\

Acked-by: Michael Ellerman <mpe@...erman.id.au> (powerpc)

cheers


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