In other words, should the following code result in a deadlock? #include <iostream> #include <boost/thread.hpp> int main(int argc, char* argv[]) { boost::shared_mutex smx; boost::upgrade_lock<boost::shared_mutex> lock_one(smx); std::cout << "Locked once." << std::endl; std::cout.flush(); boost::upgrade_lock<boost::shared_mutex> lock_two(smx); std::cout << "Locked again." << std::endl; std::cout.flush(); boost::upgrade_to_unique_lock<boost::shared_mutex> lock_three(lock_two); std::cout << "Write lock." << std::endl; return 0; } This example illustrates the problem I'm having in my app -- I have a situation where a single thread may end up performing more than a single shared lock. When it does so, it hangs on the nested upgrade_lock. Is this the expected behavior?