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

[Paolo Bonzini <pbonzini@redhat.com>]

On 22/02/2017 14:03, Marc-André Lureau wrote:
> Hi
> 
> On Wed, Feb 22, 2017 at 3:39 PM Paolo Bonzini <pbonzini@redhat.com
> <mailto:pbonzini@redhat.com>> wrote:
> 
> 
> 
>     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
>     <mailto: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.
> 
> I understand you agree with my change, correct?

Yes.

>     Can you please add a comment to piix4_update_bus_hotplug explaining that
>     the PIIX4PMState cannot outlive the PCIBus, because /machine keeps
>     it alive?
> 
> 
>  Do you mean PCIBus cannot outlive PIIX4PMState? (ie the PIIX4PMState
> will always be there since it's linked from /machine)

Yes (or just because it's not unpluggable).

Paolo