x86, perfcounters: add atomic64_xchg()

Complete atomic64_t support on the 32-bit side by adding atomic64_xch().

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20090406094518.445450972@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/include/asm/atomic_32.h

@@ -291,19 +291,37 @@ atomic64_cmpxchg(atomic64_t *ptr, unsigned long long old_val,
 }
 
 /**
- * atomic64_set - set atomic64 variable
+ * atomic64_xchg - xchg atomic64 variable
  * @ptr:      pointer to type atomic64_t
  * @new_val:  value to assign
+ * @old_val:  old value that was there
  *
- * Atomically sets the value of @ptr to @new_val.
+ * Atomically xchgs the value of @ptr to @new_val and returns
+ * the old value.
  */
-static inline void atomic64_set(atomic64_t *ptr, unsigned long long new_val)
+
+static inline unsigned long long
+atomic64_xchg(atomic64_t *ptr, unsigned long long new_val)
 {
 	unsigned long long old_val;
 
 	do {
 		old_val = atomic_read(ptr);
 	} while (atomic64_cmpxchg(ptr, old_val, new_val) != old_val);
+
+	return old_val;
+}
+
+/**
+ * atomic64_set - set atomic64 variable
+ * @ptr:      pointer to type atomic64_t
+ * @new_val:  value to assign
+ *
+ * Atomically sets the value of @ptr to @new_val.
+ */
+static inline void atomic64_set(atomic64_t *ptr, unsigned long long new_val)
+{
+	atomic64_xchg(ptr, new_val);
 }
 
 /**