linux-kernel - [040/244] sparc32: unbreak arch_write

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Date:	Wed, 28 Sep 2011 15:00:04 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Mikael Pettersson <mikpe@...uu.se>,
	"David S. Miller" <davem@...emloft.net>
Subject: [040/244] sparc32: unbreak arch_write_unlock()

3.0-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Mikael Pettersson <mikpe@...uu.se>

commit 3f6aa0b113846a8628baa649af422cfc6fb1d786 upstream.

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@...uu.se>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 arch/sparc/include/asm/spinlock_32.h |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

--- a/arch/sparc/include/asm/spinlock_32.h
+++ b/arch/sparc/include/asm/spinlock_32.h
@@ -131,6 +131,15 @@ static inline void arch_write_lock(arch_
 	*(volatile __u32 *)&lp->lock = ~0U;
 }

+static void inline arch_write_unlock(arch_rwlock_t *lock)
+{
+	__asm__ __volatile__(
+"	st		%%g0, [%0]"
+	: /* no outputs */
+	: "r" (lock)
+	: "memory");
+}
+
 static inline int arch_write_trylock(arch_rwlock_t *rw)
 {
 	unsigned int val;
@@ -175,8 +184,6 @@ static inline int __arch_read_trylock(ar
 	res; \
 })

-#define arch_write_unlock(rw)	do { (rw)->lock = 0; } while(0)
-
 #define arch_spin_lock_flags(lock, flags) arch_spin_lock(lock)
 #define arch_read_lock_flags(rw, flags)   arch_read_lock(rw)
 #define arch_write_lock_flags(rw, flags)  arch_write_lock(rw)

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