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Re: [Qemu-arm] [Qemu-devel] [PATCH 1/5] qom/object: Add a new function o


From: Eduardo Habkost
Subject: Re: [Qemu-arm] [Qemu-devel] [PATCH 1/5] qom/object: Add a new function object_initialize_as_child()
Date: Thu, 12 Jul 2018 14:15:07 -0300
User-agent: Mutt/1.9.2 (2017-12-15)

On Thu, Jul 12, 2018 at 05:30:59PM +0200, Thomas Huth wrote:
> A lot of code is using the object_initialize() function followed by a call to
> object_property_add_child() to add the newly initialized object as a child of
> the current object. Both functions increase the reference counter of the new
> object, but many spots that call these two functions then forget to drop one
> of the superfluous references. So the newly created object is often not 
> cleaned
> up correctly when the parent is destroyed. In the worst case, this can cause
> crashes, e.g. because device objects are not correctly removed from their
> parent_bus.
> Since this is a common pattern between many code spots, let's introdcue a
> new function that takes care of calling all three required initialization
> functions, first object_initialize(), then object_property_add_child() and
> finally object_unref().
> 
> Signed-off-by: Thomas Huth <address@hidden>
> ---
>  include/qom/object.h | 19 +++++++++++++++++++
>  qom/object.c         | 14 ++++++++++++++
>  2 files changed, 33 insertions(+)
> 
> diff --git a/include/qom/object.h b/include/qom/object.h
> index f3d2308..c1b254c 100644
> --- a/include/qom/object.h
> +++ b/include/qom/object.h
> @@ -749,6 +749,25 @@ int object_set_propv(Object *obj,
>  void object_initialize(void *obj, size_t size, const char *typename);
>  
>  /**
> + * object_initialize_as_child:
> + * @parentobj: The parent object to add a property to
> + * @propname: The name of the property
> + * @childobj: A pointer to the memory to be used for the object.
> + * @size: The maximum size available at @obj for the object.
> + * @type: The name of the type of the object to instantiate.
> + * @errp: If an error occurs, a pointer to an area to store the area

"to store the error"

> + *
> + * This function will initialize an object. The memory for the object should
> + * have already been allocated. The object will then be added as child 
> property
> + * to a parent with object_property_add_child() function. The returned object
> + * has a reference count of 1, and will be finalized when the last reference 
> is
> + * dropped.

I think we need to document clearly who owns the reference and is
responsible for dropping it.  In this case, we need to make it
clear that the child property will own the reference, and nobody
should drop it except object_finalize_child_property().  This is
a subtle but important difference from object_initialize().


> + */
> +void object_initialize_as_child(Object *parentobj, const char *propname,
> +                                void *childobj, size_t size, const char 
> *type,
> +                                Error **errp);
> +
> +/**
>   * object_dynamic_cast:
>   * @obj: The object to cast.
>   * @typename: The @typename to cast to.
> diff --git a/qom/object.c b/qom/object.c
> index 4609e34..de2ded0 100644
> --- a/qom/object.c
> +++ b/qom/object.c
> @@ -392,6 +392,20 @@ void object_initialize(void *data, size_t size, const 
> char *typename)
>      object_initialize_with_type(data, size, type);
>  }
>  
> +void object_initialize_as_child(Object *parentobj, const char *propname,
> +                                void *childobj, size_t size, const char 
> *type,
> +                                Error **errp)
> +{
> +    object_initialize(childobj, size, type);
> +    object_property_add_child(parentobj, propname, OBJECT(childobj), errp);
> +    /*
> +     * Since object_property_add_child added a reference to the child object,
> +     * we can drop the initial reference from object_initialize now.
> +     */
> +    object_unref(OBJECT(childobj));

I suggest "drop the reference added by object_initialize(), the
child property will own the only reference to the object".

-- 
Eduardo



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