From: Mikael Pettersson <mikpe@it.uu.se>
Date: Mon, 15 Aug 2011 10:11:50 +0000 (+0000)
Subject: sparc32: unbreak arch_write_unlock()
X-Git-Tag: v3.1-rc3~34^2~1
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=3f6aa0b113846a8628baa649af422cfc6fb1d786;p=pandora-kernel.git

sparc32: unbreak arch_write_unlock()

The sparc32 version of arch_write_unlock() is just a plain assignment.
Unfortunately this allows the compiler to schedule side-effects in a
protected region to occur after the HW-level unlock, which is broken.
E.g., the following trivial test case gets miscompiled:

	#include <linux/spinlock.h>
	rwlock_t lock;
	int counter;
	void foo(void) { write_lock(&lock); ++counter; write_unlock(&lock); }

Fixed by adding a compiler memory barrier to arch_write_unlock().  The
sparc64 version combines the barrier and assignment into a single asm(),
and implements the operation as a static inline, so that's what I did too.

Compile-tested with sparc32_defconfig + CONFIG_SMP=y.

Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: David S. Miller <davem@davemloft.net>
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