dbx recognizes a multithreaded program by detecting whether it utilizes libthread.so. The program uses libthread.so either by explicitly being compiled with -lthread or -mt, or implicitly by being compiled with -lpthread.
When it detects a multithreaded program, dbx tries to load libthread_db.so, a special system library for thread debugging located in /usr/lib.
dbx is synchronous, so when any thread or lightweight process (LWP) stops, all other threads and LWPs sympathetically stop. This behavior is sometimes referred to as the “stop the world” model.