script to find who is using UNDO tablespace in oracle?

SELECT TO_CHAR(s.sid)||','||TO_CHAR(s.serial#) sid_serial,
           NVL(s.username, 'None') orauser,
           s.program,
           r.name undoseg,
           t.used_ublk * TO_NUMBER(x.value)/1024||'K' "Undo"
      FROM sys.v_$rollname    r,
           sys.v_$session     s,
           sys.v_$transaction t,
           sys.v_$parameter   x
    WHERE s.taddr = t.addr
      AND r.usn   = t.xidusn(+)
      AND x.name  = 'db_block_size';