Re: [Qemu-devel] [PATCH 03/13] qom: add support for weak referenced object: aka UnownedObject

[Paolo Bonzini <pbonzini@redhat.com>]

On 28/01/2015 11:03, Igor Mammedov wrote:
> it adds support for weak reference model used by glib's
> GInitiallyUnowned to QEMU's QOM model.
> 
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> ---
>  include/qom/object.h | 20 ++++++++++++++++++++
>  qom/object.c         | 20 +++++++++++++++++++-
>  2 files changed, 39 insertions(+), 1 deletion(-)
> 
> diff --git a/include/qom/object.h b/include/qom/object.h
> index 89c3092..e0e4cc8 100644
> --- a/include/qom/object.h
> +++ b/include/qom/object.h
> @@ -1314,5 +1314,25 @@ int object_child_foreach(Object *obj, int (*fn)(Object *child, void *opaque),
>   */
>  Object *container_get(Object *root, const char *path);
>  
> +#define TYPE_UNOWNED_OBJECT "unowned-object"
> +#define UNOWNED_OBJECT(obj) \
> +     OBJECT_CHECK(UnownedObject, (obj), TYPE_UNOWNED_OBJECT)
> +#define UNOWNED_OBJECT_CLASS(klass) \
> +     OBJECT_CLASS_CHECK(UnownedObjectClass, (klass), TYPE_UNOWNED_OBJECT)
> +#define UNOWNED_OBJECT_GET_CLASS \
> +     OBJECT_GET_CLASS(UnownedObjectClass, (obj), TYPE_UNOWNED_OBJECT)
> +#define OBJECT_WEAK_REF (1UL << 31)
> +
> +typedef struct UnownedObjectClass
> +{
> +    /*< private >*/
> +    ObjectClass parent_class;
> +} UnownedObjectClass;
> +
> +typedef struct UnownedObject
> +{
> +    /*< private >*/
> +    Object parent_obj;
> +} UnownedObject;
>  
>  #endif
> diff --git a/qom/object.c b/qom/object.c
> index 1812c73..96842c7 100644
> --- a/qom/object.c
> +++ b/qom/object.c
> @@ -705,6 +705,10 @@ void object_ref(Object *obj)
>      if (!obj) {
>          return;
>      }
> +    if (atomic_fetch_and(&obj->ref, ~OBJECT_WEAK_REF) & OBJECT_WEAK_REF)

Please first do a non-atomic test, since this is unlikely to happen but
atomic_fetch_and is quite expensive (it compiles into a cmpxchg loop).

Paolo

> +    {
> +        return;
> +    }
>       atomic_inc(&obj->ref);
>  }
>  
> @@ -713,7 +717,7 @@ void object_unref(Object *obj)
>      if (!obj) {
>          return;
>      }
> -    g_assert(obj->ref > 0);
> +    g_assert((obj->ref & ~OBJECT_WEAK_REF) > 0);
>  
>      /* parent always holds a reference to its children */
>      if (atomic_fetch_dec(&obj->ref) == 1) {
> @@ -1709,6 +1713,12 @@ static void object_instance_init(Object *obj)
>      object_property_add_str(obj, "type", qdev_get_type, NULL, NULL);
>  }
>  
> +
> +static void unowned_object_instance_init(Object *obj)
> +{
> +    obj->ref |= OBJECT_WEAK_REF;
> +}
> +
>  static void register_types(void)
>  {
>      static TypeInfo interface_info = {
> @@ -1724,8 +1734,16 @@ static void register_types(void)
>          .abstract = true,
>      };
>  
> +    static TypeInfo unowned_object_info = {
> +        .name = TYPE_UNOWNED_OBJECT,
> +        .instance_size = sizeof(UnownedObject),
> +        .instance_init = unowned_object_instance_init,
> +        .abstract = true,
> +    };
> +
>      type_interface = type_register_internal(&interface_info);
>      type_register_internal(&object_info);
> +    type_register_internal(&unowned_object_info);
>  }
>  
>  type_init(register_types)
>