Networked Systems: 7th International Conference, NETYS 2019, Marrakech, Morocco, June 19–21, 2019, Revised Selected Papers

I present some local methods for both writing and verifying large concurrent programs: pairwise normal form, dynamic addition of pairwise interactions, and deadlock-freedom via subsystem checking. In pairwise normal form, a process Pi is a set of actions, where each action is a conjunction of smaller pairwise-actions, over the neighbours of Pi (the processes that Pi interacts directly with). Variables are shared among pairs. This provides for locality and modifiability in program design, and for tractability in verification. Mutex among n processes can be expressed as 2-process mutex among every pair. If 2-process mutex is enforced among some pairs only, I obtain generalized dining philosophers. If some 2-process mutexes are replaced by a version which gives priority to one process, I obtain readers-writers. Verification of pairwise safety and liveness can be carried out by model-checking each pair in isolation, thereby avoiding state-explosion. Pairs can be added dynamically, at run time. This enables an infinite-state system to be expressed as a countably infinite number of finite-state processes. I introduce the first sound and complete characterization of deadlock for concurrent programs. Most approaches to deadlock observe that a wait-for cycle is necessary for deadlock. However, a cycle is not sufficient for deadlock, since a process in the wait-for cycle can choose to interact with a process outside the cycle. This leads to high degree of incompleteness in such methods. My approach analyzes the AND-OR generalization of a wait-for cycle, which is necessary and sufficient for a deadlock. I then impose local conditions (over small subsystems) which prevent the creation of such AND-OR wait-for cycles. My methods have been implemented in the Eshmun tool. Automated Reasoning for Weak Consistency