s390/spinlock: optimize spin_unlock code

Use a memory barrier + store sequence instead of a load + compare and swap
sequence to unlock a spinlock and an rw lock.
For the spinlock case this saves us two memory reads and a not needed cpu
serialization after the compare and swap instruction stored the new value.

The kernel size (performance_defconfig) gets reduced by ~14k.

Average execution time of a tight inlined spin_unlock loop drops from
5.8ns to 0.7ns on a zEC12 machine.

An artificial stress test case where several counters are protected with
a single spinlock and which are only incremented while holding the spinlock
shows ~30% improvement on a 4 cpu machine.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 14 insertions, 7 deletions

diff --git a/arch/s390/include/asm/spinlock.h b/arch/s390/include/asm/spinlock.h
index 96879f7ad6da..d26ad2ac7cb2 100644
--- a/arch/s390/include/asm/spinlock.h
+++ b/arch/s390/include/asm/spinlock.h
@@ -64,11 +64,6 @@ static inline int arch_spin_trylock_once(arch_spinlock_t *lp)
 		      _raw_compare_and_swap(&lp->lock, 0, SPINLOCK_LOCKVAL));
 }
 
-static inline int arch_spin_tryrelease_once(arch_spinlock_t *lp)
-{
-	return _raw_compare_and_swap(&lp->lock, SPINLOCK_LOCKVAL, 0);
-}
-
 static inline void arch_spin_lock(arch_spinlock_t *lp)
 {
 	if (!arch_spin_trylock_once(lp))
@@ -91,7 +86,13 @@ static inline int arch_spin_trylock(arch_spinlock_t *lp)
 
 static inline void arch_spin_unlock(arch_spinlock_t *lp)
 {
-	arch_spin_tryrelease_once(lp);
+	typecheck(unsigned int, lp->lock);
+	asm volatile(
+		__ASM_BARRIER
+		"st	%1,%0\n"
+		: "+Q" (lp->lock)
+		: "d" (0)
+		: "cc", "memory");
 }
 
 static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
@@ -179,7 +180,13 @@ static inline void arch_write_lock_flags(arch_rwlock_t *rw, unsigned long flags)
 
 static inline void arch_write_unlock(arch_rwlock_t *rw)
 {
-	_raw_compare_and_swap(&rw->lock, 0x80000000, 0);
+	typecheck(unsigned int, rw->lock);
+	asm volatile(
+		__ASM_BARRIER
+		"st	%1,%0\n"
+		: "+Q" (rw->lock)
+		: "d" (0)
+		: "cc", "memory");
 }
 
 static inline int arch_read_trylock(arch_rwlock_t *rw)