In some cases GDB UPC does not insert a collective breakpoint, even though UPC mode is enabled.  This will create a situation where only one thread (current thread when breakpoint was inserted) stops while others complete the program or continue running the UPC barrier code.

The most likely cause of this behaviour is that UPC program is compiled with NO debugging options. To successfully use all features of the GDB UPC, a program must be compiled with "-O0 -g -dwarf-2-upc" options.