Error: All CUDA devices are used for X11 and cannot be used while debugging.

Hi all… I am trying to debug a program using cuda-gdb. When i am running the executable through cuda-gdb, Its showing Error: All CUDA devices are used for X11 and cannot be used while debugging.

Can any one please explain whats the error about is ? What is X11? Please briefly explain about error and whats the solution for solving this problem?