Session summary: locking protocols

Ada 2005 introduced EDF scheduling across priority bands. A version of Baker’s Stack Resource Control Protocol (called the Preemption Level Control Protocol) was also introduced so that ceiling priorities could be used within an EDF context. However, the Preemption Level Control Protocol is complex and the position paper by Aldea, Burns, Gutierrez and Gonzalez Harbour entitled “Incorporating the Deadline Floor Protocol in Ada” has proposed an alternative protocol that is conceptually much simpler and easier to implement. Alan Burns introduced the protocol and explained its main motivations and features. The protocol is targeted at single processor system and the discussion was held within this context. The protocol requires each protected object to have a related deadline associated with it. This deadline is the minimum (floor) relative deadline of all the tasks that use that protected object. Proper setting of the floors ensures that each task gets only a single block and mutual exclusion is guaranteed by the protocol itself. Several issues were raised in the discussion and these are summarised below.