linux-kernel - Re: [PATCH v2 31/32] sh: support a 2-byte smp_store

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Date:	Mon, 4 Jan 2016 15:05:09 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	virtualization@...ts.linux-foundation.org,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	David Miller <davem@...emloft.net>, linux-ia64@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
	sparclinux@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-metag@...r.kernel.org, linux-mips@...ux-mips.org,
	x86@...nel.org, user-mode-linux-devel@...ts.sourceforge.net,
	adi-buildroot-devel@...ts.sourceforge.net,
	linux-sh@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
	xen-devel@...ts.xenproject.org, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v2 31/32] sh: support a 2-byte smp_store_mb

On Thu, Dec 31, 2015 at 09:09:47PM +0200, Michael S. Tsirkin wrote:
> At the moment, xchg on sh only supports 4 and 1 byte values, so using it
> from smp_store_mb means attempts to store a 2 byte value using this
> macro fail.
> 
> And happens to be exactly what virtio drivers want to do.
> 
> Check size and fall back to a slower, but safe, WRITE_ONCE+smp_mb.
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
>  arch/sh/include/asm/barrier.h | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/sh/include/asm/barrier.h b/arch/sh/include/asm/barrier.h
> index f887c64..0cc5735 100644
> --- a/arch/sh/include/asm/barrier.h
> +++ b/arch/sh/include/asm/barrier.h
> @@ -32,7 +32,15 @@
>  #define ctrl_barrier()	__asm__ __volatile__ ("nop;nop;nop;nop;nop;nop;nop;nop")
>  #endif
>  
> -#define __smp_store_mb(var, value) do { (void)xchg(&var, value); } while (0)
> +#define __smp_store_mb(var, value) do { \
> +	if (sizeof(var) != 4 && sizeof(var) != 1) { \
> +		 WRITE_ONCE(var, value); \
> +		__smp_mb(); \
> +	} else { \
> +		(void)xchg(&var, value);  \
> +	} \
> +} while (0)

So SH is an orphaned arch, which is also why I did not comment on using
xchg() for the UP smp_store_mb() thing.

But I really think we should try fixing the xchg() implementation
instead of this duct-tape.
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