Module 7 - Deadlocks. After studying this chapter you will be able to develop a description of deadlocks, which prevent sets of concurrent processes from completing their tasks; to present a number of different methods for preventing or avoiding deadlocks in a computer system. | Module 7: Deadlocks System Model Deadlock Characterization Methods for Handling Deadlocks Deadlock Prevention Deadlock Avoidance Deadlock Detection Recovery from Deadlock Combined Approach to Deadlock Handling Operating System Concepts The Deadlock Problem A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set. Example System has 2 tape drives. P1 and P2 each hold one tape drive and each needs another one. Example semaphores A and B, initialized to 1 P0 P1 wait (A); wait(B) wait (B); wait(A) Operating System Concepts Bridge Crossing Example Traffic only in one direction. Each section of a bridge can be viewed as a resource. If a deadlock occurs, it can be resolved if one car backs up (preempt resources and rollback). Several cars may have to be backed upif a deadlock occurs. Starvation is possible. Operating System Concepts System Model Resource types R1, R2, . . ., Rm CPU cycles, memory space, I/O devices Each resource | Module 7: Deadlocks System Model Deadlock Characterization Methods for Handling Deadlocks Deadlock Prevention Deadlock Avoidance Deadlock Detection Recovery from Deadlock Combined Approach to Deadlock Handling Operating System Concepts The Deadlock Problem A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set. Example System has 2 tape drives. P1 and P2 each hold one tape drive and each needs another one. Example semaphores A and B, initialized to 1 P0 P1 wait (A); wait(B) wait (B); wait(A) Operating System Concepts Bridge Crossing Example Traffic only in one direction. Each section of a bridge can be viewed as a resource. If a deadlock occurs, it can be resolved if one car backs up (preempt resources and rollback). Several cars may have to be backed upif a deadlock occurs. Starvation is possible. Operating System Concepts System Model Resource types R1, R2, . . ., Rm CPU cycles, memory space, I/O devices Each resource type Ri has Wi instances. Each process utilizes a resource as follows: request use release Operating System Concepts Deadlock Characterization Mutual exclusion: only one process at a time can use a resource. Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes. No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task. Circular wait: there exists a set {P0, P1, , P0} of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, , Pn–1 is waiting for a resource that is held by Pn, and P0 is waiting for a resource that is held by P0. Deadlock can arise if four conditions hold simultaneously. Operating System Concepts Resource-Allocation Graph V is partitioned into two types: P = {P1, P2, , Pn}, the set consisting of all the processes in the system. R = {R1, R2, , Rm}, the set .
