Речь идёт о QList<QPair<QString, QString>>. Думаю, с ним проблем не будет.
Читаем доку.
QList Class Reference
Note: All functions in this class are reentrant.
A thread-safe function can be called simultaneously from multiple threads, even when the invocations use shared data, because all references to the shared data are serialized.
A reentrant function can also be called simultaneously from multiple threads, but only if each invocation uses its own data.
By extension, a class is said to be reentrant if its member functions can be called safely from multiple threads, as long as each thread uses a different instance of the class. The class is thread-safe if its member functions can be called safely from multiple threads, even if all the threads use the same instance of the class.
Нет никакой гарантии что методы чтения данных QList потокобезопасны сейчас(что можно проверить посмотрев сорцы) или будут таковыми в следующих версиях. Так что защищать надо в любом случае.
