Re: [Qemu-devel] [PATCH v2 23/30] bus: do not unref hotplug handler

[Paolo Bonzini <pbonzini@redhat.com>]

On 21/02/2017 15:14, Marc-André Lureau wrote:
> Apparently, none of the bus owner give a reference to the hotplug
> handler property, do not unref it on bus release.
> 
> Furthermore, a bus is allowed to be its own hotplug handler, which can
> be seen in qbus_set_bus_hotplug_handler() function. However, in this
> case, the reference can't be given to the property, or this will create
> a cyclic dependency and the bus will never be free.
> 
> Each bus owner should manage the lifecycle of the hotplug handler.
> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

Almost all qbus_set_hotplug_handler callers are using it to set the
parent device (that is, the "adapter" or "bridge", whatever you want to
call it) as the hotplug handler.

The exception is piix4_update_bus_hotplug.  This one is the only case
where OBJ_PROP_LINK_UNREF_ON_RELEASE would be right.  Luckily, there
_is_ a reference to that device somewhere else to keep it alive, namely
in /machine's acpi-device prop.  So this case is a bit hacky (not your
fault) but works as well.  In addition the PIIX4_PM device is not
hot(un)pluggable.

Can you please add a comment to piix4_update_bus_hotplug explaining that
the PIIX4PMState cannot outlive the PCIBus, because /machine keeps it alive?

> 
> diff --git a/hw/core/bus.c b/hw/core/bus.c
> index cf383fc1af..4651f24486 100644
> --- a/hw/core/bus.c
> +++ b/hw/core/bus.c
> @@ -197,7 +197,7 @@ static void qbus_initfn(Object *obj)
>                               TYPE_HOTPLUG_HANDLER,
>                               (Object **)&bus->hotplug_handler,
>                               object_property_allow_set_link,
> -                             OBJ_PROP_LINK_UNREF_ON_RELEASE,
> +                             0,
>                               NULL);
>      object_property_add_bool(obj, "realized",
>                               bus_get_realized, bus_set_realized, NULL);
> -- 
> 2.11.0.295.gd7dffce1c.dirty
> 
> 
>