I don't see what a mutex would do in this case. Only the thread writes to the QList. The visualizer only reads it and never changes it. So the only problem can be if the visualizer tries to access excactly the same index in the QList that is written by the loader. After the loader has completed filling the QList with frames, and doesn't touch it anymore, how can it be that there are still some frames screwed up and what would a mutex do about it?