boost::mutex - are lock attempts queued?

I have 2 threads, master thread setting a keepalive flag, child thread 
checking that flag:

master thread function to end child thread:

   boost::mutex::scoped_lock lc (mutexAccessKeepalive, true);
   if (keepAlive)
     keepAlive = 0;
   lc.unlock();


child thread main loop:

   boost::mutex::scoped_lock lc (mutexAccessKeepalive, true);
   while (keepAlive) {
     lc.unlock();
     // do some stuff
     lc.lock();
   }
   lc.unlock();

In general, this works fine.
However, if the "// do some stuff" part (in this case listening for udp 
traffic) gets cut short (failed to bind socket) and exits fast, my 
master thread fails to acquire the lock on the mutex EVER. If I put a 
"sleep (1);" there, it works fine again and the child thread ends as 
intended.

I thought that attempts to lock an object were queued in boost, in the 
order the request was made? What happens here seems to indicate that my 
child thread is unlocking the mutex, and locks it again, before the main 
thread has a chance to "snatch" the lock.

Does anyone have an explanation and a workaround? :)

Best Regards,

    Lars