Make QObjectPrivate::threadData a proper atomic

QObjectPrivate::threadData used to be a QThreadData *, and was
read and written from multiple threads without proper synchronization.
As an example, it was read from QCoreApplication::postEvent and
written from QObject::moveToThread, therefore causing UB.

Port threadData to a proper atomic, removing the races. Fix all usage
points.

In general, QObject is documented to be simply reentrant,
not thread-safe, and certain bits (e.g. timers, moveToThread)
are not even reentrant. The reasoning therefore is that a given
QObject's threadData is not supposed to be touched by multiple
threads without some synchronization happening elsewhere, and
therefore relaxed loads should be sufficient.

As drive-by change: refactor QCoreApplication::postEvent.
It was particularly subtle, because it had a loop using a volatile
to cope with the possibility of the receiver object switching thread
while we tried to lock its thread's event queue.

However, volatile does not achieve any synchronization, so drop it,
and refactor the algorithm using better locking primitives.
Put this algorithm in a common place, and also reuse it from
removePostedEvents, which was lacking any synchronization.

Change-Id: Icc755f7eb418ff54b33db4bdd87fd8eaf4e82c7a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/src/corelib/kernel/qeventdispatcher_unix.cpp b/src/corelib/kernel/qeventdispatcher_unix.cpp
index 5bc65b7110..2492d70005 100644
--- a/src/corelib/kernel/qeventdispatcher_unix.cpp
+++ b/src/corelib/kernel/qeventdispatcher_unix.cpp
@@ -463,13 +463,15 @@ bool QEventDispatcherUNIX::processEvents(QEventLoop::ProcessEventsFlags flags)
 
     // we are awake, broadcast it
     emit awake();
-    QCoreApplicationPrivate::sendPostedEvents(0, 0, d->threadData);
+
+    auto threadData = d->threadData.loadRelaxed();
+    QCoreApplicationPrivate::sendPostedEvents(0, 0, threadData);
 
     const bool include_timers = (flags & QEventLoop::X11ExcludeTimers) == 0;
     const bool include_notifiers = (flags & QEventLoop::ExcludeSocketNotifiers) == 0;
     const bool wait_for_events = flags & QEventLoop::WaitForMoreEvents;
 
-    const bool canWait = (d->threadData->canWaitLocked()
+    const bool canWait = (threadData->canWaitLocked()
                           && !d->interrupt.loadRelaxed()
                           && wait_for_events);