powerpc/qspinlock: use a half-word store to unlock to avoid larx/stcx.

The first 16 bits of the lock are only modified by the owner, and other
modifications always use atomic operations on the entire 32 bits, so
unlocks can use plain stores on the 16 bits. This is the same kind of
optimisation done by core qspinlock code.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221126095932.1234527-3-npiggin@gmail.com

diff --git a/arch/powerpc/include/asm/qspinlock.h b/arch/powerpc/include/asm/qspinlock.h
index 6946dba..713f662 100644 (file)
--- a/arch/powerpc/include/asm/qspinlock.h
+++ b/arch/powerpc/include/asm/qspinlock.h
@@ -37,11 +37,7 @@ static __always_inline void queued_spin_lock(struct qspinlock *lock)
 
 static inline void queued_spin_unlock(struct qspinlock *lock)
 {
-       for (;;) {
-               int val = atomic_read(&lock->val);
-               if (atomic_cmpxchg_release(&lock->val, val, val & ~_Q_LOCKED_VAL) == val)
-                       return;
-       }
+       smp_store_release(&lock->locked, 0);
 }
 
 #define arch_spin_is_locked(l)         queued_spin_is_locked(l)

diff --git a/arch/powerpc/include/asm/qspinlock_types.h b/arch/powerpc/include/asm/qspinlock_types.h
index 20a36df..fe87181 100644 (file)
--- a/arch/powerpc/include/asm/qspinlock_types.h
+++ b/arch/powerpc/include/asm/qspinlock_types.h
@@ -3,12 +3,27 @@
 #define _ASM_POWERPC_QSPINLOCK_TYPES_H
 
 #include <linux/types.h>
+#include <asm/byteorder.h>
 
 typedef struct qspinlock {
-       atomic_t val;
+       union {
+               atomic_t val;
+
+#ifdef __LITTLE_ENDIAN
+               struct {
+                       u16     locked;
+                       u8      reserved[2];
+               };
+#else
+               struct {
+                       u8      reserved[2];
+                       u16     locked;
+               };
+#endif
+       };
 } arch_spinlock_t;
 
-#define        __ARCH_SPIN_LOCK_UNLOCKED       { .val = ATOMIC_INIT(0) }
+#define        __ARCH_SPIN_LOCK_UNLOCKED       { { .val = ATOMIC_INIT(0) } }
 
 /*
  * Bitfields in the lock word: