A mixed locking/abort protocol for hard real-time systems

Serializability greatly simplifies reasoning about correctness in concurrent systems, including real-time systems. Our research addresses concurrency control protocols that accommodate analytic guarantees of schedulability, can be implemented with small bounded overheads and blocking, and ensure serializable execution of entire tasks including complete read/compute/write cycles (as opposed to serializable execution only of short embedded transactions without computation.) One such protocol which combines locking and abort is described. Among its interesting properties are that transactions scheduled by locking are never aborted, tasks are aborted only due to conflict with higher priority tasks, and the cost of abortion can be bounded for the purpose of schedulability analysis. The protocol is illustrated with an avionics example. The priority ceiling protocol can ensure schedulability of 8 tasks if serializability of only short sequences of data accesses is required, but cannot schedule even the first 2 tasks if serializability is required for whole tasks. Under reasonable assumptions our protocol achieves schedulability of the first 6 tasks while guaranteeing serializability of entire tasks