Re: [Qemu-devel] [PATCH] qom: In function object_set_link_property(), first call object_ref(), then object_unref().

[Paolo Bonzini <pbonzini@redhat.com>]

On 02/22/2012 06:13 PM, alexander_barabash@mentor.com wrote:
> From: Alexander Barabash <alexander_barabash@mentor.com>
> 
> In the old implementation, if the new value of the property links
> to the same object, as the old value, that object is first unref-ed,
> and then ref-ed. This leads to unintended deinitialization of that object.
> 
> In the new implementation, this is fixed.
> 
> Signed-off-by: Alexander Barabash <alexander_barabash@mentor.com>
> ---
>  qom/object.c |   18 +++++++++++++-----
>  1 files changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/qom/object.c b/qom/object.c
> index 941c291..d1b3ac7 100644
> --- a/qom/object.c
> +++ b/qom/object.c
> @@ -892,19 +892,19 @@ static void object_set_link_property(Object *obj, Visitor *v, void *opaque,
>                                       const char *name, Error **errp)
>  {
>      Object **child = opaque;
> +    Object *old_target;
>      bool ambiguous = false;
>      const char *type;
>      char *path;
>      gchar *target_type;
> +    bool clear_old_target = true;
>  
>      type = object_property_get_type(obj, name, NULL);
>  
>      visit_type_str(v, &path, name, errp);
>  
> -    if (*child) {
> -        object_unref(*child);
> -        *child = NULL;
> -    }
> +    old_target = *child;
> +    *child = NULL;

You can just remove the unref here...

>      if (strcmp(path, "") != 0) {
>          Object *target;
> @@ -916,7 +916,11 @@ static void object_set_link_property(Object *obj, Visitor *v, void *opaque,
>          if (ambiguous) {
>              error_set(errp, QERR_AMBIGUOUS_PATH, path);
>          } else if (target) {
> -            object_ref(target);
> +            if (target != old_target) {
> +                object_ref(target);

... leave the unconditional ref to target here...

> +            } else {
> +                clear_old_target = false;
> +            }
>              *child = target;
>          } else {
>              target = object_resolve_path(path, &ambiguous);
> @@ -930,6 +934,10 @@ static void object_set_link_property(Object *obj, Visitor *v, void *opaque,
>      }
>  
>      g_free(path);
> +
> +    if (clear_old_target && (old_target != NULL)) {
> +        object_unref(old_target);

... and leave this unref on old_target, without the need for
clear_old_target.

> +    }
>  }
>  
>  void object_property_add_link(Object *obj, const char *name,

Paolo