Thank you both. I feel confident now that the Boost shared_ptr is as thread-safe as I need it to be.

With Boost shared_ptr as thread-safe as they are it shouldn't be necessary to have them in TBB too, unless there is some significant advantage like they could be made much more efficient or even safer or something.

Regarding the efficiency of reference counting per se. I'm aware that this kind of memory management comes at a price so I'm careful about not overusing it and I don't use it in "algorithm style" code.