REG->5.14.2: Deadlock in conjunction with (cython's) nogil when calling to separate thread from QObject::event (Paint)

cdef class X:

    def nogil_noop(self):
        with nogil:
            pass

from setuptools import setup
from Cython.Build import cythonize

setup(ext_modules = cythonize("qtbug.pyx"))

from PySide2 import QtCore, QtWidgets
from PySide2.QtCore import QEvent

import qtbug


class _Thread(QtCore.QThread):

    def __init__(self):
        super().__init__()
        self._mutex = QtCore.QMutex()
        self._pyx = qtbug.X()

    def run(self):
        while True:
            with QtCore.QMutexLocker(self._mutex):
                self._pyx.nogil_noop()

    def draw(self):
        with QtCore.QMutexLocker(self._mutex):
            pass


class _Widget(QtWidgets.QWidget):

    def __init__(self):
        super().__init__()
        self._thread = _Thread()
        self._thread.start()

    def event(self, event):
        print(event)
        if event.type() == QEvent.Paint:
            self._thread.draw()
        return super().event(event)


if __name__ == '__main__':
    import sys
    app = QtWidgets.QApplication(sys.argv)
    window = _Widget()
    window.show()
    app.exec_()

# Create and enter a Python virtualenv
virtualenv venv
. venv/bin/activate

# Build the small binding
pip install cython
python setup.py build_ext --inplace

# Install PySide and run the script
pip install -U pyside2==5.14.2.1
python runme.py

pip install -U pyside2==5.14.1
python runme.py

(gdb) bt
#0  0x00007ffff7ec3f8d in syscall () from /usr/lib/libc.so.6
#1  0x00007ffff5a8d6f5 in QBasicMutex::lockInternal() () from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Core.so.5
#2  0x00007ffff5a8d746 in QMutex::lock() () from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Core.so.5
#3  0x00007ffff65c7687 in Sbk_QMutexLocker_Init () from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/QtCore.abi3.so
#4  0x00007ffff7b401aa in _PyObject_MakeTpCall () from /usr/lib/libpython3.8.so.1.0
#5  0x00007ffff7bfdc51 in _PyEval_EvalFrameDefault () from /usr/lib/libpython3.8.so.1.0
#6  0x00007ffff7beb1d6 in ?? () from /usr/lib/libpython3.8.so.1.0
#7  0x00007ffff7bfd80a in _PyEval_EvalFrameDefault () from /usr/lib/libpython3.8.so.1.0
#8  0x00007ffff7be98f4 in _PyEval_EvalCodeWithName () from /usr/lib/libpython3.8.so.1.0
#9  0x00007ffff7beac7b in _PyFunction_Vectorcall () from /usr/lib/libpython3.8.so.1.0
#10 0x00007ffff7beb4ab in ?? () from /usr/lib/libpython3.8.so.1.0
#11 0x00007ffff7b463fd in PyObject_Call () from /usr/lib/libpython3.8.so.1.0
#12 0x00007ffff2306bb2 in QWidgetWrapper::event(QEvent*) () from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/QtWidgets.abi3.so
#13 0x00007ffff15e8b0c in QApplicationPrivate::notify_helper(QObject*, QEvent*) ()
   from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Widgets.so.5
#14 0x00007ffff15efa90 in QApplication::notify(QObject*, QEvent*) () from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Widgets.so.5
#15 0x00007ffff1e77565 in QApplicationWrapper::notify(QObject*, QEvent*) () from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/QtWidgets.abi3.so
#16 0x00007ffff5c6fc48 in QCoreApplication::notifyInternal2(QObject*, QEvent*) ()
   from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Core.so.5
#17 0x00007ffff16200c5 in QWidgetPrivate::sendPaintEvent(QRegion const&) ()
   from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Widgets.so.5
#18 0x00007ffff1620bc7 in QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, QFlags<QWidgetPrivate::DrawWidgetFlag>, QPainter*, QWidgetRepaintManager*) () from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Widgets.so.5
#19 0x00007ffff15f8f7f in QWidgetRepaintManager::paintAndFlush() () from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Widgets.so.5
#20 0x00007ffff15f9648 in QWidgetRepaintManager::sync() () from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Widgets.so.5
#21 0x00007ffff162174f in QWidgetPrivate::syncBackingStore() () from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Widgets.so.5
#22 0x00007ffff16403d7 in QWidgetWindow::handleResizeEvent(QResizeEvent*) ()
   from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Widgets.so.5
#23 0x00007ffff164391b in QWidgetWindow::event(QEvent*) () from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Widgets.so.5
#24 0x00007ffff15e8b0c in QApplicationPrivate::notify_helper(QObject*, QEvent*) ()
   from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Widgets.so.5
#25 0x00007ffff15efa90 in QApplication::notify(QObject*, QEvent*) () from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Widgets.so.5
#26 0x00007ffff1e77565 in QApplicationWrapper::notify(QObject*, QEvent*) () from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/QtWidgets.abi3.so
#27 0x00007ffff5c6fc48 in QCoreApplication::notifyInternal2(QObject*, QEvent*) ()
   from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Core.so.5
#28 0x00007ffff0cd452c in QGuiApplicationPrivate::processGeometryChangeEvent(QWindowSystemInterfacePrivate::GeometryChangeEvent*) ()
   from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Gui.so.5
#29 0x00007ffff0cd8d2d in QGuiApplicationPrivate::processWindowSystemEvent(QWindowSystemInterfacePrivate::WindowSystemEvent*) ()
   from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Gui.so.5
#30 0x00007ffff0cb4f4b in QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Gui.so.5
#31 0x00007fffef9ac33a in xcbSourceDispatch(_GSource*, int (*)(void*), void*) ()
   from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/plugins/platforms/../../lib/libQt5XcbQpa.so.5
#32 0x00007ffff2cf16be in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#33 0x00007ffff2cf3531 in ?? () from /usr/lib/libglib-2.0.so.0
#34 0x00007ffff2cf3571 in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#35 0x00007ffff5cc873c in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Core.so.5
#36 0x00007ffff5c6e662 in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Core.so.5
#37 0x00007ffff5c77590 in QCoreApplication::exec() () from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/Qt/lib/libQt5Core.so.5
#38 0x00007ffff1e75038 in Sbk_QApplicationFunc_exec_ () from /home/ux/tmp/pyside-bug/venv/lib/python3.8/site-packages/PySide2/QtWidgets.abi3.so
#39 0x00007ffff7a8d587 in ?? () from /usr/lib/libpython3.8.so.1.0
#40 0x00007ffff7bfd80a in _PyEval_EvalFrameDefault () from /usr/lib/libpython3.8.so.1.0
#41 0x00007ffff7be98f4 in _PyEval_EvalCodeWithName () from /usr/lib/libpython3.8.so.1.0
#42 0x00007ffff7c70d73 in PyEval_EvalCode () from /usr/lib/libpython3.8.so.1.0
#43 0x00007ffff7c70dc8 in ?? () from /usr/lib/libpython3.8.so.1.0
#44 0x00007ffff7c75063 in ?? () from /usr/lib/libpython3.8.so.1.0
#45 0x00007ffff7b0fdf0 in PyRun_FileExFlags () from /usr/lib/libpython3.8.so.1.0
#46 0x00007ffff7b19aa4 in PyRun_SimpleFileExFlags () from /usr/lib/libpython3.8.so.1.0
#47 0x00007ffff7c8181e in Py_RunMain () from /usr/lib/libpython3.8.so.1.0
#48 0x00007ffff7c81909 in Py_BytesMain () from /usr/lib/libpython3.8.so.1.0
#49 0x00007ffff7df1023 in __libc_start_main () from /usr/lib/libc.so.6
#50 0x000055555555505e in _start ()
(gdb)