Re: [PATCH 51/55] qdev: Make qdev_realize() support bus-less devices

[Paolo Bonzini]

On 20/05/20 17:02, Markus Armbruster wrote:
>>>
>>> qdev_realize_and_unref() remains restricted, because its reference
>>> counting would become rather confusing for bus-less devices.
>> I think it would be fine, you would just rely on the reference held by
>> the QOM parent (via the child property).
> I took one look at the contract I wrote for it, and balked :)
> 
> qdev_realize()'s contract before this patch:
> 
>     /*
>      * Realize @dev.
>      * @dev must not be plugged into a bus.
>      * Plug @dev into @bus.  This takes a reference to @dev.
>      * If @dev has no QOM parent, make one up, taking another reference.
>      * On success, return true.
>      * On failure, store an error through @errp and return false.
>      */
>     bool qdev_realize(DeviceState *dev, BusState *bus, Error **errp)
> 
> Simple enough.
> 
> This patch merely adds "If @bus, " before "plug".  Still simple enough.
> 
> qdev_realize_and_unref()'s contract:
> 
>     /*
>      * Realize @dev and drop a reference.
>      * This is like qdev_realize(), except it steals a reference rather
>      * than take one to plug @dev into @bus.  On failure, it drops that
>      * reference instead.  @bus must not be null.  Intended use:
>      *     dev = qdev_new();
>      *     [...]
>      *     qdev_realize_and_unref(dev, bus, errp);
>      * Now @dev can go away without further ado.
>      */
>     bool qdev_realize_and_unref(DeviceState *dev, BusState *bus, Error **errp)
> 
> If @bus is null, who gets to hold the stolen reference?
> 
> You seem to suggest the QOM parent.  What if @dev already has a parent?

The caller would still hold the stolen reference, and it would be
dropped.  You cannot have a device that goes away at the end of
qdev_realize_and_unref, as long as dev has a QOM parent that clings onto
dev.  Since dev will have /machine/unattached as the parent if it didn't
already have one before, the function is safe even without a bus.

Or alternatively, ignore all the stolen references stuff, and merely see
qdev_realize_and_unref as a shortcut for qdev_realize+object_unref,
because it's a common idiom.

Paolo