Safety Assurance via On-Line Monitoring \Lambda

Abstract This paper proposes a new approach and new techniques for on-line monitoring of concurrent programs to ensure that some of their safety properties are not violated. The techniques modify erroneous systems which violate a certain safety property, into new systems which satisfy the safety property by adding a new layer that controls the scheduling of steps in the system. We formally characterize the relationship between the erroneous and the new system. Safety monitors for mutualexclusion, ` \Gamma exclusion, and the producer consumer tasks are presented. A proof for the mutual-exclusion task is presented to demonstate the applicability of our approach. Our results are also of significance in the context of evolving systems-- systems which are repeatedly modified due to changes in the user requirements, user specifications, or implementation. The monitoring technique proposed here ensures that safety requirements are not violated in such evolving systems, in spite of frequent changes. 1 Introduction The design of distributed algorithms is a complicated task. Resulting algorithms and their implementations are often complex. As a consequence, a programmer who writes software cannot possibly check the resulting package's correctness under every possible scenario. Erroneous software is unacceptable for critical tasks where an error may cause severe damage. This is, for example, true of software in the medical industry