[PyQt] [BUG] Use of QSGThreadedRenderLoop in QML can lead to deadlock (PyQt 5.4.2)

[Vladimir Rutsky]

On Fri, Jun 19, 2015 at 6:23 PM, Phil Thompson
<phil at riverbankcomputing.com> wrote:
> On 19/06/2015 12:22 pm, Vladimir Rutsky wrote:
>>
>> On Thu, Jun 18, 2015 at 11:20 PM, Phil Thompson
>> <phil at riverbankcomputing.com> wrote:
>>>
>>> On 18/06/2015 8:06 pm, Vladimir Rutsky wrote:
>>>>
>>>>
>>>> Hello,
>>>>
>>>> I described this issue in comment to other bug report
>>>>
>>>> (http://www.riverbankcomputing.com/pipermail/pyqt/2015-June/036048.html),
>>>> and now want to open separate thread for it, since it's not just
>>>> "application hangs on exit", but real deadlock issue that can occur in
>>>> many cases of QML usage (e.g. when window is shown or hidden).
>>>>
>>>> The deadlock is reproduced when QML rendering is done using threaded
>>>> render loop (QSGThreadedRenderLoop), which is default on modern
>>>> GNU/Linux systems (Windows always use single threaded rendering in Qt
>>>> 5.4).
>>>>
>>>> As I understand from GDB stack traces, the deadlock happens in the
>>>> following way.
>>>> Sometimes QQuickView starting to wait rendering thread for some
>>>> operation (using mutex). Rendering thread can request operation that
>>>> is implemented in Python code, this operation will try to lock Python
>>>> GIL in rendering thread, but GIL is not released in main thread, and
>>>> never be, since main thread waits rendering thread in Qt.
>>>>
>>>> One of the situations in which QQuickView waits rendering thread is
>>>> when QQuickView is being destroyed: it waits while rendering thread
>>>> will release all resources.
>>>> This case can be easily reproduced on GNU/Linux in this example:
>>>> https://gist.github.com/rutsky/d6332e3354972997e938
>>>> The example also have GDB stack traces at the moment when application
>>>> hanged on exit.
>>>>
>>>> Workaround for this issue is to force Qt to use single threaded
>>>> rendering by setting environment variable QSG_RENDER_LOOP to "basic".
>>>
>>>
>>>
>>> I've added /ReleaseGIL/ to all the QQuickWindow and QQuickView methods
>>> that
>>> seem to depend on the internal mutex.
>>>
>>> Phil
>>> _______________________________________________
>>> PyQt mailing list    PyQt at riverbankcomputing.com
>>> http://www.riverbankcomputing.com/mailman/listinfo/pyqt
>>
>>
>> Thanks, Phil, this fixed deadlock in the example.
>> Still my application hangs if render loop is destroyed from
>> QApplication destructor (not from QQuickView as was in the example).
>> I have updated example that reproduces deadlock on recent nightly
>> build (97ce4458a5c1):
>> https://gist.github.com/rutsky/d6332e3354972997e938
>> (or you can just comment explicit "del view" in previous example).
>
>
> Hopefully fixed in tonight's snapshot.
>
> Thanks,
> Phil


Thanks, Phil, this issue is not reproduced anymore in my application
with a nightly build of PyQt (d5b00e76067f).

Regards,

Vladimir Rutsky