Deadlocks

Deadlock handling Deadlock prevention Deadlock avoidance Deadlock detection Deadlock recovery

How deadlocks arise Conflicting need for resource by two or more processes All process are unable to proceed

Deadlock Example Precess P Process Q Get A Get B Release B Release A Get B Get A Release B Release A

Four conditions for deadlocks Mutual exclusion of resources Hold and wait –A process can hold a resource and wait for another No preemption –A process will release resources voluntarily Circular wait –There is a circular wait among the processes

Deadlock prevention Make one of the four conditions false aways No deadlock! Which condition? Only circular wait condition is doable

Make circular wait condition false always Order all the resources Processes can request resources that are ordered strictly higher than what they are holding Can prove that there are cycles in wait for graph

Deadlock avoidance Allows more concurrency than prevention A decision is made dynamically whether the current resource allocation request will potentially lead to a deadlock (if granted) Needs knowledge of future resource needs

Two rules of deadlock avoidance Do not start a process if its demands might lead to a deadlock Do not grant an incremental resource request to a process if doing so might lead to a deadlock

Initial denial R = (R1, R2, …, Rm) set of system resources; m types of resources A = (V1, V2, …, Vm) available reources (not allocated to any process) at any time Claim matrix; maximum requirement of each process for each resource Current allocation matrix; what each process holds currently

Relations Ri = Vi + sum(A_ki) k going from 1 to n C_kl <= R_i for all k, I A_ki <= C_ki for all k, i (Strong condition): Start a new process P_n+1 only if R_I >= C_(n+1)i+sum(C_ki) k going from 1 to n; for all i

Resource allocation denial Banker’s algorithm Safe and unsafe states Detect if a state is unsafe Go from one safe state to another safe state

Deadlock recovery Forcibly terminate some processes to recover from deadlocks. Deadlock breaking set: Set of processes, which when terminated, will lead to the other processes’ normal termination. Cost of forcibly terminating a process

Deadlock recovery Cont’d Cost of deadlock breaking set: sum of the cost of aborting processes in the deadlock breaking set. Optimal deadlock breaking set: Deadlock breaking set of least cost. Finding the optimal deadlock breaking set is an NP-hard problem.