I am not intimately familiar with Qt
Ah, so Q is Qt :-) Care to reveal what A and B are, and which the callback is that caused a deadlock?
It sounded to me like Tim explained that it is possible but unimplemented.
Trying to channel Tim: All experts for this stuff have tried and failed; Mark Hammond has a sort-of solution which Tim believes to be strictly-speaking incorrect. While people may have thought they have a solution, it is troubling that nobody can remember what the solution is. Maybe the email signature just had not enough space to write it down :-) Regards, Martin