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

[Phil Thompson]

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