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

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Date:	Thu, 31 Dec 2015 21:09:47 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	Peter Zijlstra <peterz@...radead.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: [PATCH v2 31/32] sh: support a 2-byte smp_store_mb

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)
+
 #define smp_store_mb(var, value) __smp_store_mb(var, value)
 
 #include <asm-generic/barrier.h>
-- 
MST

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