[PATCH 4/6] ARM: locks: prefetch the destination word for write prior to strex

[Will Deacon]

The cost of changing a cacheline from shared to exclusive state can be
significant, especially when this is triggered by an exclusive store,
since it may result in having to retry the transaction.

This patch prefixes our {spin,read,write}_[try]lock implementations with
pldw instructions (on CPUs which support them) to try and grab the line
in exclusive state from the start.

Signed-off-by: Will Deacon <will.deacon at arm.com>
---
 arch/arm/include/asm/spinlock.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/arm/include/asm/spinlock.h b/arch/arm/include/asm/spinlock.h
index 0de7bec..3e1cc9d 100644
--- a/arch/arm/include/asm/spinlock.h
+++ b/arch/arm/include/asm/spinlock.h
@@ -5,7 +5,7 @@
 #error SMP not supported on pre-ARMv6 CPUs
 #endif
 
-#include <asm/processor.h>
+#include <linux/prefetch.h>
 
 /*
  * sev and wfe are ARMv6K extensions.  Uniprocessor ARMv6 may not have the K
@@ -70,6 +70,7 @@ static inline void arch_spin_lock(arch_spinlock_t *lock)
 	u32 newval;
 	arch_spinlock_t lockval;
 
+	prefetchw((const void *)&lock->slock);
 	__asm__ __volatile__(
 "1:	ldrex	%0, [%3]\n"
 "	add	%1, %0, %4\n"
@@ -93,6 +94,7 @@ static inline int arch_spin_trylock(arch_spinlock_t *lock)
 	unsigned long contended, res;
 	u32 slock;
 
+	prefetchw((const void *)&lock->slock);
 	do {
 		__asm__ __volatile__(
 		"	ldrex	%0, [%3]\n"
@@ -145,6 +147,7 @@ static inline void arch_write_lock(arch_rwlock_t *rw)
 {
 	unsigned long tmp;
 
+	prefetchw((const void *)&rw->lock);
 	__asm__ __volatile__(
 "1:	ldrex	%0, [%1]\n"
 "	teq	%0, #0\n"
@@ -163,6 +166,7 @@ static inline int arch_write_trylock(arch_rwlock_t *rw)
 {
 	unsigned long tmp;
 
+	prefetchw((const void *)&rw->lock);
 	__asm__ __volatile__(
 "	ldrex	%0, [%1]\n"
 "	teq	%0, #0\n"
@@ -211,6 +215,7 @@ static inline void arch_read_lock(arch_rwlock_t *rw)
 {
 	unsigned long tmp, tmp2;
 
+	prefetchw((const void *)&rw->lock);
 	__asm__ __volatile__(
 "1:	ldrex	%0, [%2]\n"
 "	adds	%0, %0, #1\n"
@@ -231,6 +236,7 @@ static inline void arch_read_unlock(arch_rwlock_t *rw)
 
 	smp_mb();
 
+	prefetchw((const void *)&rw->lock);
 	__asm__ __volatile__(
 "1:	ldrex	%0, [%2]\n"
 "	sub	%0, %0, #1\n"
@@ -249,6 +255,7 @@ static inline int arch_read_trylock(arch_rwlock_t *rw)
 {
 	unsigned long tmp, tmp2 = 1;
 
+	prefetchw((const void *)&rw->lock);
 	__asm__ __volatile__(
 "	ldrex	%0, [%2]\n"
 "	adds	%0, %0, #1\n"
-- 
1.8.2.2