kref: add kref_test_and_get

Add kref_test_and_get() function, which atomically add a reference only if
refcount is not zero. This prevent to add a reference to an object that is
already being removed.

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

include/linux/kref.h

@@ -23,6 +23,7 @@ struct kref {
 
 void kref_init(struct kref *kref);
 void kref_get(struct kref *kref);
+int kref_test_and_get(struct kref *kref);
 int kref_put(struct kref *kref, void (*release) (struct kref *kref));
 
 #endif /* _KREF_H_ */

lib/kref.c

@@ -36,6 +36,18 @@ void kref_get(struct kref *kref)
 	smp_mb__after_atomic_inc();
 }
 
+/**
+ * kref_test_and_get - increment refcount for object only if refcount is not
+ * zero.
+ * @kref: object.
+ *
+ * Return non-zero if the refcount was incremented, 0 otherwise
+ */
+int kref_test_and_get(struct kref *kref)
+{
+	return atomic_inc_not_zero(&kref->refcount);
+}
+
 /**
  * kref_put - decrement refcount for object.
  * @kref: object.