A system for parallel processing tasks by allocating the use of exclusive locks to process critical sections of a task. The system includes storing update information that is updated in response to acquisition and release of an exclusive lock. When processing a task which includes a critical section containing code affecting execution of the other task, an exclusive execution unit acquires an exclusive lock prior to processing the critical section. When the section has been processed successfully, the lock is released and update information updated. Meanwhile a second task, whose critical section does not contain code affecting execution of the other task may run in parallel, without acquiring an exclusive lock, via a nonexclusive execution unit. The nonexclusive execution unit determines that the second critical section has successfully completed if the update information has not changed during processing of the second critical section.