From Causal Consistency to Sequential Consistency in Shared Memory Systems

Sequential consistency and causal consistency constitute two of the main consistency criteria used to define the semantics of accesses in the shared memory model. An execution is sequentially consistent if all processes can agree on a same legal sequential history of all the accesses; if processes perceive distinct legal sequential histories of all the accesses, the execution is only causally consistent (legality means that a read does not get an overwritten value). This paper studies synchronization constraints that, when obeyed by operations of a given causally consistent execution, make it sequentially consistent. More precisely, the paper introduces the MSC synchronization (mixed synchronization constraint) which generalizes (1) the known DRF (data race free) and CWF (concurrent write free) synchronizations and (2) a new one called CRF (concurrent read free). The MSC synchronization allows for concurrent conflicting operations on a same object, while ensuring sequential consistency; this is particularly interesting in the context of distributed systems (where objects are possibly replicated) to cope with partition failures: conflicting operations in two distinct partitions do not necessarily block processes that issue them (as it is the case of quorum based protocols). Technically, a tag (control type) is associated with each operation, and all operations endowed with the same tag obey the same synchronization constraint.