FreeBSD gdb does not allow to control running threads

[Jin Guojun[VFF]]

I am using GNU gdb 6.1.1 [FreeBSD], and it does not allow to set 
scheduler-locking.

gdb) show scheduler-locking
Mode for locking scheduler during execution is "off".
(gdb) set scheduler-locking on
Target 'freebsd-threads' cannot support this command.

Is any reason this feature is disabled?


-Jin