Proving Non-opacity (General Track)

Guerraoui and Kapalka defined opacity as a safety criterion for transactional memory algorithms in 2008. Researchers have shown how to prove opacity, while little is known about pitfalls that can lead to non-opacity. In this paper, we identify two problems that lead to non-opacity and we prove an impossibility result. We first show that the well-known TM algorithms DSTM and McRT don’t satisfy opacity. DSTM suffers from a write-skew anomaly, while McRT suffers from a write-exposure anomaly. We then prove that for direct-update TM algorithms, opacity is incompatible with a liveness criterion called local progress, even for fault-free systems. Our result implies that if TM algorithm designers want both opacity and local progress, they should avoid direct-update algorithms.