1) Task grabs row 123 with an exclusive lock.
2) Task B grabs row number 456 with an exclusive lcok.
3) Task A row requests row 456, and begins waiting for Task B to release the lock.
4) Task B now requests row 123.
Since Task A is waiting for task B to release row 345.both jobs will wait perpetually for the other task to release their lock.
5) Fortunately, Oracle will kill task B with an ORA-60 error.