releases/4.9.124/parisc-remove-unnecessary-barriers-from-spinlock.h.patch

From 3b885ac1dc35b87a39ee176a6c7e2af9c789d8b8 Mon Sep 17 00:00:00 2001
From: John David Anglin <dave.anglin@bell.net>
Date: Sun, 12 Aug 2018 16:31:17 -0400
Subject: parisc: Remove unnecessary barriers from spinlock.h
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From: John David Anglin <dave.anglin@bell.net>

commit 3b885ac1dc35b87a39ee176a6c7e2af9c789d8b8 upstream.

Now that mb() is an instruction barrier, it will slow performance if we issue
unnecessary barriers.

The spinlock defines have a number of unnecessary barriers.  The __ldcw()
define is both a hardware and compiler barrier.  The mb() barriers in the
routines using __ldcw() serve no purpose.

The only barrier needed is the one in arch_spin_unlock().  We need to ensure
all accesses are complete prior to releasing the lock.

Signed-off-by: John David Anglin <dave.anglin@bell.net>
Cc: stable@vger.kernel.org # 4.0+
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/parisc/include/asm/spinlock.h |    8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

--- a/arch/parisc/include/asm/spinlock.h
+++ b/arch/parisc/include/asm/spinlock.h
@@ -26,7 +26,6 @@ static inline void arch_spin_lock_flags(
 {
 	volatile unsigned int *a;
 
-	mb();
 	a = __ldcw_align(x);
 	while (__ldcw(a) == 0)
 		while (*a == 0)
@@ -36,16 +35,15 @@ static inline void arch_spin_lock_flags(
 				local_irq_disable();
 			} else
 				cpu_relax();
-	mb();
 }
 
 static inline void arch_spin_unlock(arch_spinlock_t *x)
 {
 	volatile unsigned int *a;
-	mb();
+
 	a = __ldcw_align(x);
-	*a = 1;
 	mb();
+	*a = 1;
 }
 
 static inline int arch_spin_trylock(arch_spinlock_t *x)
@@ -53,10 +51,8 @@ static inline int arch_spin_trylock(arch
 	volatile unsigned int *a;
 	int ret;
 
-	mb();
 	a = __ldcw_align(x);
         ret = __ldcw(a) != 0;
-	mb();
 
 	return ret;
 }