deadlock on sending gst_event_new_flush_stop

0    __lll_lock_wait    lowlevellock.S    135    0x7ffff13f5f2c
1    _L_lock_1082    /lib/x86_64-linux-gnu/libpthread.so.0        0x7ffff13f168d
2    __GI___pthread_mutex_lock    pthread_mutex_lock.c    134    0x7ffff13f1607
3    g_mutex_lock    /lib/x86_64-linux-gnu/libglib-2.0.so.0        0x7ffff71a9981
4    ??    /usr/lib/x86_64-linux-gnu/libgstbase-1.0.so.0        0x7ffff162fd17
5    gst_element_send_event    /usr/lib/x86_64-linux-gnu/libgstreamer-1.0.so.0        0x7ffff76c8621
6    GstBlackMagic::flushData    gstblackmagic.cpp    507    0x4f953f
7    SceneView::swapScreens    sceneview.cpp    324    0x4a5462
8    PlaylistPlayer::start    playlistplayer.cpp    38    0x4de814
9    MainWindow::grabberStateChanged    mainwindow.cpp    179    0x437116
10    MainWindow::qt_static_metacall    moc_mainwindow.cpp    219    0x4ffb7d
11    QMetaObject::activate(QObject*, int, int, void**)    /opt/Qt5.2.1/5.3/gcc_64/lib/libQt5Core.so.5        0x7ffff29ab681
12    AbstractGrabber::stateChanged    moc_abstractgrabber.cpp    187    0x50e167
13    GstBlackMagic::onMessage    gstblackmagic.cpp    448    0x4f9047
14    GstBlackMagic::qt_static_metacall    moc_gstblackmagic.cpp    74    0x50d465
15    QObject::event(QEvent*)    /opt/Qt5.2.1/5.3/gcc_64/lib/libQt5Core.so.5        0x7ffff29ad7c6
16    QApplicationPrivate::notify_helper(QObject*, QEvent*)    /opt/Qt5.2.1/5.3/gcc_64/lib/libQt5Widgets.so.5        0x7ffff3be8764
17    QApplication::notify(QObject*, QEvent*)    /opt/Qt5.2.1/5.3/gcc_64/lib/libQt5Widgets.so.5        0x7ffff3bebde6
18    QCoreApplication::notifyInternal(QObject*, QEvent*)    /opt/Qt5.2.1/5.3/gcc_64/lib/libQt5Core.so.5        0x7ffff2979dd4
19    QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*)    /opt/Qt5.2.1/5.3/gcc_64/lib/libQt5Core.so.5        0x7ffff297c958
20    ??    /opt/Qt5.2.1/5.3/gcc_64/lib/libQt5Core.so.5        0x7ffff29d4013