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  1. Qt
  2. QTBUG-25752

Application hangs when emit signal on Linux in g_main_context_wakeup

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    • None
    • 4.8.0
    • Core: Threads
    • None
    • Ubuntu Linux 11.04

      The event dispatcher of a thread is deleted in:

      qthread_unix.cpp QThreadPrivate::finish()
          QMutexLocker locker(&d->mutex);
    ...

    locker.relock();
    d->terminated = false;

    QAbstractEventDispatcher *eventDispatcher = d->data->eventDispatcher;
    if (eventDispatcher) {
        d->data->eventDispatcher = 0;
        locker.unlock();
        eventDispatcher->closingDown();
        delete eventDispatcher;
        locker.relock();
    }

      (similar code in qthread_win.cpp)

      But not all of its uses are thread-safe. For example

      qcoreapplication.cpp QCoreApplication::postEvent
          ...
    if (data->eventDispatcher)
        data->eventDispatcher->wakeUp();

      QCoreApplication::postEvent locks the post event mutex list, but never does it lock the thread mutex which protects the setting of the eventDispatcher variable.

      Same thing in:

      qeventloop.cpp
      void QEventLoop::wakeUp()
{
    Q_D(QEventLoop);
    if (!d->threadData->eventDispatcher)
        return;
    d->threadData->eventDispatcher->wakeUp();
}

      And in:

      qobject.cpp QObjectPrivate::setThreadData_helper
          if (eventsMoved > 0 && targetData->eventDispatcher) {
        targetData->canWait = false;
        targetData->eventDispatcher->wakeUp();
    }

      This latter function is called with the post event mutexes locked, but not the QThreadPrivate::mutex mutex.

      One could argue that the QEventLoop one isn't supposed to be called from outside the thread (it's not marked as \threadsafe), but QCoreApplication::postEvent is and QObject::moveToThread does not mention a problem with moving to a thread that is exiting.

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            thiago Thiago Macieira
            kirill2010 Kirill
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              Created:
              Updated:
              Resolved:

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