The QThread documentation doesn't mention which functions are thread-safe.

However, one would expect that at least QThread::quit and QThread::exit are thread safe, to be able to terminate the thread from within the new and the old thread.

Inspecting the code (https://code.woboq.org/qt5/qtbase/src/corelib/thread/qthread.cpp.html#568), indeed reveals a mutex mechanism is precent.

Also many other members functions are guarded by a mutex: QThread::setPriority, QThread::requestInterruption, QThread::isRunning, ...