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

[Andreas Färber]

Am 15.01.2016 um 18:16 schrieb Paolo Bonzini:
> On 15/01/2016 18:03, Andreas Färber wrote:
>> Am 05.11.2015 um 13:47 schrieb Markus Armbruster:
>>> Paolo Bonzini <address@hidden> writes:
>>>> On 05/11/2015 13:06, Andreas Färber wrote:
>>>>>> 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?
>>>>
>>>> It doesn't really matter, because the BQL is being held here.
>>>>
>>>> On the other hand, if the opts are deleted in finalize, there is an
>>>> arbitrary delay because finalize is typically called after a
>>>> synchronize_rcu period.
>>>>
>>>>>>> 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.
>>>>
>>>> I agree with you, the block layer bug is separate.
>>>
>>> Related, but clearly separate.  Mentioning it in the commit message
>>> would be nice, though.
>>
>> Paolo, pong: I gathered that I should queue the original patch without
>> Markus's proposed change, correct? And do you want to add some sentence
>> to the commit message as requested by Markus?
> 
> Yes, thanks:
> 
> ----
> Note that similar races exist for other QemuOpts, which this patch
> does not attempt to fix.
> 
> For example, if the device is a block device, then unplugging it also
> deletes its backend.  However, this backend's get deleted in
> drive_info_del(), which is only called when properties are
> destroyed.  Just like device_finalize(), drive_info_del() is called
> some time after DEVICE_DELETED is sent.  A separate patch series has
> been sent to plug this other bug.  Character devices also have yet to
> be fixed.
> -----

Thanks, queued on qom-next with that addition:
https://github.com/afaerber/qemu-cpu/commits/qom-next

I'll leave Markus and others time until Monday for *-by or comments, but
I really need to get out my pull with Daniel's class properties.

Regards,
Andreas

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