PPC32: add __down_read_trylock and __down_write_trylock implementations.

include/asm-ppc/rwsem.h

@@ -72,12 +72,26 @@ static inline void init_rwsem(struct rw_semaphore *sem)
  */
 static inline void __down_read(struct rw_semaphore *sem)
 {
-	if (atomic_inc_return((atomic_t *)(&sem->count)) >= 0)
+	if (atomic_inc_return((atomic_t *)(&sem->count)) > 0)
 		smp_wmb();
 	else
 		rwsem_down_read_failed(sem);
 }
 
+static inline int __down_read_trylock(struct rw_semaphore *sem)
+{
+	int tmp;
+
+	while ((tmp = sem->count) >= 0) {
+		if (tmp == cmpxchg(&sem->count, tmp,
+				   tmp + RWSEM_ACTIVE_READ_BIAS)) {
+			smp_wmb();
+			return 1;
+		}
+	}
+	return 0;
+}
+
 /*
  * lock for writing
  */
@@ -93,6 +107,16 @@ static inline void __down_write(struct rw_semaphore *sem)
 		rwsem_down_write_failed(sem);
 }
 
+static inline int __down_write_trylock(struct rw_semaphore *sem)
+{
+	int tmp;
+
+	tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
+		      RWSEM_ACTIVE_WRITE_BIAS);
+	smp_wmb();
+	return tmp == RWSEM_UNLOCKED_VALUE;
+}
+
 /*
  * unlock after reading
  */