From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:47787) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fdhX2-00089H-MW for qemu-devel@nongnu.org; Thu, 12 Jul 2018 15:45:42 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fdhX1-0001My-4L for qemu-devel@nongnu.org; Thu, 12 Jul 2018 15:45:40 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48252) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fdhX0-0001MM-N9 for qemu-devel@nongnu.org; Thu, 12 Jul 2018 15:45:39 -0400 From: Eduardo Habkost Date: Thu, 12 Jul 2018 16:45:20 -0300 Message-Id: <20180712194522.31063-2-ehabkost@redhat.com> In-Reply-To: <20180712194522.31063-1-ehabkost@redhat.com> References: <20180712194522.31063-1-ehabkost@redhat.com> Subject: [Qemu-devel] [RFC 1/3] qom: Document reference count ownership rules List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: Paolo Bonzini , =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= , Marcel Apfelbaum , "Michael S. Tsirkin" , Thomas Huth , Peter Maydell , Markus Armbruster The documentation for QOM is not clear about who owns references to objects (i.e. who is responsible for calling object_unref() later). This is important considering there are a few inconsistencies in the API (e.g. callers of object_new() need to call object_unref() later, but callers of object_new_with_props() must not do it). Update the documentation so that every mention of object references also mention who exactly owns the reference. Signed-off-by: Eduardo Habkost --- include/qom/object.h | 73 +++++++++++++++++++++++++------------------- 1 file changed, 42 insertions(+), 31 deletions(-) diff --git a/include/qom/object.h b/include/qom/object.h index f3d2308d56..08a1bbba7d 100644 --- a/include/qom/object.h +++ b/include/qom/object.h @@ -376,7 +376,7 @@ typedef void (ObjectUnparent)(Object *obj); * ObjectFree: * @obj: the object being freed * - * Called when an object's last reference is removed. + * Called when an object's last reference is dropped using object_unref(). */ typedef void (ObjectFree)(void *obj); @@ -601,8 +601,8 @@ struct InterfaceClass * @typename: The name of the type of the object to instantiate. * * This function will initialize a new object using heap allocated memory. - * The returned object has a reference count of 1, and will be freed when - * the last reference is dropped. + * The returned object has a reference count of 1, and the reference will be + * owned by the caller. * * Returns: The newly allocated and instantiated object. */ @@ -617,8 +617,8 @@ Object *object_new(const char *typename); * @...: list of property names and values * * This function will initialize a new object using heap allocated memory. - * The returned object has a reference count of 1, and will be freed when - * the last reference is dropped. + * The returned object has a reference count of 1, and the reference will + * be owned by the caller. * * The @id parameter will be used when registering the object as a * child of @parent in the composition tree. @@ -652,8 +652,8 @@ Object *object_new(const char *typename); * * * - * The returned object will have one stable reference maintained - * for as long as it is present in the object hierarchy. + * The returned object will have one reference, owned by the + * parent object (not by the caller). * * Returns: The newly allocated, instantiated & initialized object. */ @@ -713,9 +713,6 @@ Object *object_new_with_propv(const char *typename, * * * - * The returned object will have one stable reference maintained - * for as long as it is present in the object hierarchy. - * * Returns: -1 on error, 0 on success */ int object_set_props(Object *obj, @@ -744,7 +741,7 @@ int object_set_propv(Object *obj, * * This function will initialize an object. The memory for the object should * have already been allocated. The returned object has a reference count of 1, - * and will be finalized when the last reference is dropped. + * and the reference will be owned by the caller. */ void object_initialize(void *obj, size_t size, const char *typename); @@ -928,8 +925,12 @@ GSList *object_class_get_list_sorted(const char *implements_type, * object_ref: * @obj: the object * - * Increase the reference count of a object. A object cannot be freed as long - * as its reference count is greater than zero. + * Increase the reference count of a object. A object won't be freed as + * long as its reference count is greater than zero. + * + * The new reference will be owned by the caller, meaning the caller is + * responsible for ensuring object_unref() will be called once the + * reference is not needed anymore. */ void object_ref(Object *obj); @@ -937,8 +938,10 @@ void object_ref(Object *obj); * object_unref: * @obj: the object * - * Decrease the reference count of a object. A object cannot be freed as long - * as its reference count is greater than zero. + * Drop a reference to a object, decreasing its reference count. This + * function should be called only if the caller owns a reference taken using + * object_ref() or returned by functions like object_initialize() and + * object_new(). */ void object_unref(Object *obj); @@ -1104,10 +1107,13 @@ char *object_property_get_str(Object *obj, const char *name, * * Writes an object's canonical path to a property. * - * If the link property was created with - * OBJ_PROP_LINK_STRONG bit, the old target object is - * unreferenced, and a reference is added to the new target object. + * If the link property was created with OBJ_PROP_LINK_STRONG + * bit, the property will take a reference to the object, and drop + * a reference to the old object. * + * If OBJ_PROP_LINK_STRONG was not set for the property, the + * caller is responsible for ensuring the object will stay alive until the + * property is modified or deleted. */ void object_property_set_link(Object *obj, Object *value, const char *name, Error **errp); @@ -1398,7 +1404,10 @@ void object_property_add_child(Object *obj, const char *name, Object *child, Error **errp); typedef enum { - /* Unref the link pointer when the property is deleted */ + /* + * Property owns a reference to the target object. Unref the link + * pointer when the property is modified or deleted. + */ OBJ_PROP_LINK_STRONG = 0x1, } ObjectPropertyLinkFlags; @@ -1433,13 +1442,14 @@ void object_property_allow_set_link(const Object *, const char *, * link being set. If @check is NULL, the property is read-only * and cannot be set. * - * Ownership of the pointer that @child points to is transferred to the - * link property. The reference count for *@child is - * managed by the property from after the function returns till the - * property is deleted with object_property_del(). If the - * @flags OBJ_PROP_LINK_STRONG bit is set, - * the reference count is decremented when the property is deleted or - * modified. + * If the OBJ_PROP_LINK_STRONG bit is set in @flags, + * ownership of the pointer that @child points to is transferred to the + * link property, and the reference will be dropped when the property is + * modified or deleted with object_property_del(). + * + * If OBJ_PROP_LINK_STRONG is not set in @flags, the caller + * is responsible for ensuring the object pointed by @child will stay alive + * until the property is modified or deleted. */ void object_property_add_link(Object *obj, const char *name, const char *type, Object **child, @@ -1607,7 +1617,8 @@ void object_class_property_add_uint64_ptr(ObjectClass *klass, const char *name, * Add an alias for a property on an object. This function will add a property * of the same type as the forwarded property. * - * The caller must ensure that @target_obj stays alive as long as + * The property won't take a reference to @target_obj, so the + * caller must ensure that @target_obj stays alive as long as * this property exists. In the case of a child object or an alias on the same * object this will be the case. For aliases to other objects the caller is * responsible for taking a reference. @@ -1626,10 +1637,10 @@ void object_property_add_alias(Object *obj, const char *name, * Add an unmodifiable link for a property on an object. This function will * add a property of type link where TYPE is the type of @target. * - * The caller must ensure that @target stays alive as long as - * this property exists. In the case @target is a child of @obj, - * this will be the case. Otherwise, the caller is responsible for - * taking a reference. + * The property won't take a reference to @target_obj, so the + * caller must ensure that @target stays alive as long as this property + * exists. In the case @target is a child of @obj, this will be the case. + * Otherwise, the caller is responsible for taking a reference. */ void object_property_add_const_link(Object *obj, const char *name, Object *target, Error **errp); -- 2.18.0.rc1.1.g3f1ff2140