Re: [Qemu-devel] [PATCH] qdev: free qemu-opts when the QOM path goes away

[Paolo Bonzini <pbonzini@redhat.com>]

On 05/11/2015 13:06, Andreas Färber wrote:
> Am 04.11.2015 um 19:34 schrieb Markus Armbruster:
>> Paolo Bonzini <pbonzini@redhat.com> writes:
>>
>>> Otherwise there is a race where the DEVICE_DELETED event has been sent but
>>> attempts to reuse the ID will fail.
>>>
>>> Reported-by: Michael S. Tsirkin <mst@redhat.com>
>>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>>
>> Let's see whether I understand this.
>>
>>> ---
>>>  hw/core/qdev.c | 4 +++-
>>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/hw/core/qdev.c b/hw/core/qdev.c
>>> index 4ab04aa..92bd8bb 100644
>>> --- a/hw/core/qdev.c
>>> +++ b/hw/core/qdev.c
>>> @@ -1203,7 +1203,6 @@ static void device_finalize(Object *obj)
>>>      NamedGPIOList *ngl, *next;
>>>  
>>>      DeviceState *dev = DEVICE(obj);
>>> -    qemu_opts_del(dev->opts);
>>>  
>>>      QLIST_FOREACH_SAFE(ngl, &dev->gpios, node, next) {
>>>          QLIST_REMOVE(ngl, node);
>>> @@ -1251,6 +1250,9 @@ static void device_unparent(Object *obj)
>>>          qapi_event_send_device_deleted(!!dev->id, dev->id, path, &error_abort);
>>
>> DEVICE_DELETED sent here.
>>
>>>          g_free(path);
>>>      }
>>> +
>>> +    qemu_opts_del(dev->opts);
>>> +    dev->opts = NULL;
>>>  }
>>>  
>>>  static void device_class_init(ObjectClass *class, void *data)
>>
>> object_finalize_child_property() runs during unplug:
>>
>>     static void object_finalize_child_property(Object *obj, const char *name,
>>                                                void *opaque)
>>     {
>>         Object *child = opaque;
>>
>>         if (child->class->unparent) {
>>             (child->class->unparent)(child);  <--- calls device_unparent()
>>         }
>>         child->parent = NULL;
>>         object_unref(child);                  <--- calls device_finalize()
>>     }
>>
>> device_unparent() sends DEVICE_DELETED, but dev->opts gets only deleted
>> later, in device_finalize.  If the client tries to reuse the ID in the
>> meantime, it fails.
>>
>> Two remarks:
>>
>> 1. Wouldn't it be cleaner to delete dev-opts *before* sending
>>    DEVICE_DELETED?  Like this:
>>
>>     +++ b/hw/core/qdev.c
>>     @@ -1244,6 +1244,9 @@ static void device_unparent(Object *obj)
>>              dev->parent_bus = NULL;
>>          }
>>
>>     +    qemu_opts_del(dev->opts);
>>     +    dev->opts = NULL;
>>     +
>>          /* Only send event if the device had been completely realized */
>>          if (dev->pending_deleted_event) {
>>              gchar *path = object_get_canonical_path(OBJECT(dev));
> 
> To me this proposal sounds sane, but I did not get to tracing the code
> flow here. Paolo, which approach do you prefer and why?
> 
>> 2. If the device is a block device, then unplugging it also deletes its
>>    backend (ugly wart we keep for backward compatibility; *not* for
>>    blockdev-add, though).  This backend also has a QemuOpts.  It gets
>>    deleted in drive_info_del().  Just like device_finalize(), it runs
>>    within object_unref(), i.e. after DEVICE_DELETED is sent.  Same race,
>>    different ID, or am I missing something?
>>
>>    See also https://bugzilla.redhat.com/show_bug.cgi?id=1256044
> 
> If we can leave this patch decoupled from block layer and decide soonish
> on the desired approach, I'd be happy to include it in my upcoming
> qom-devices pull.

Ping?

Paolo