Re: Infinite loop in bus_unparent(), qdev bug or qdev misuse?

[Markus Armbruster <armbru@redhat.com>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 04/05/20 16:38, Markus Armbruster wrote:
>> makes no progreess because OBJECT(dev)->parent is still null, and
>> therefore object_unparent() does nothing.
>> 
>> Possible culprit: qdev_try_create() calls qdev_set_parent_bus(), which
>> adds the device to the bus, but leaves ->parent null.  If this isn't
>> wrong outright, it's at least a dangerous state.
>> 
>> Work-around: call qdev_set_id(dev, NULL) right after qdev_create().
>> This sets ->parent.
>
> That's a good one, and especially a safe one, since it matches
> qdev_device_add.  It has the disadvantage of having to touch all
> qdev_create() calls.

Also, it moves onboard devices from /machine/unattached/ to
/machine/peripheral-anon/.

I really regard it as a work-around, not a proper solution.

> Even better however would be to move the bus argument (and thus
> qdev_set_parent_bus) to qdev_init, and likewise in qdev_device_add move
> qdev_set_id after qemu_opt_foreach.  I looked at the property setters
> and couldn't find anything suspicious (somewhat to my surprise), but I
> haven't honestly tried.

Thus, we satisfy bus_unparent()'s precondition "bus children have a QOM
parent"[*] by moving "add to parent bus" next to the place where we
ensure "has QOM parent" by putting orphans under /machine/unattached/.
Makes sense.

If we add to the bus first, the precondition ceases to hold until we
realize.  Ugly, but harmless unless we manage to actually call the
function then.

I suspect we can't realize first, because the realize method may want to
use the parent bus.

We could lift putting orphans under /machine/unattached from
device_set_realized() into those callers that don't already assign a QOM
parent.  Possibly hundreds of places, hmm.

We could factor it out into a helper, and call it right before we add to
the parent bus.  Orphans with a parent bus would get adopted ealier,
orphans without one would still get adopted at realize time.

I'll play with it.


[*] Confusing terminology; two separate parent-child relationships