Chapter 7: Deadlocks

7.2 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 Chapter 7: Deadlocks The Deadlock Problem System Model Deadlock Characterization Methods for Handling Deadlocks Deadlock Prevention Deadlock Avoidance Deadlock Detection Recovery from Deadlock

7.3 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 What is a Deadlock? A deadlock is impossibility to change state of a waiting process because the resources it has requested are held by other waiting processes. It is very desirable to prevent deadlocks If they can occur, we need to take care of their handling through an operating system

7.4 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 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 disk drives. P 1 and P 2 each hold one disk drive and each needs another one.

7.5 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 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 up if a deadlock occurs. Starvation is possible.

7.6 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 System Model Resource types R 1, R 2,..., R m CPU cycles, memory space, I/O devices Each resource type R i has W i instances (sectors on the hard disc, pages in the main memory, multiple CPUs, etc.) The request and release of resources are system calls ( request(), release () device; open(), close (file) file; allocate(), free() memory, etc.) Each process utilizes a resource as follows: Request (followed by the allocation of the resource) use release

7.7 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 Deadlock Characterization Mutual exclusion: only one process at a time can use a resource. If another process requests that resource, the requesting process must be delayed until the resource has been released. 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 {P 0, P 1, …, P 0 } of waiting processes such that P 0 is waiting for a resource that is held by P 1, P 1 is waiting for a resource that is held by P 2, …, P n–1 is waiting for a resource that is held by P n, and P n is waiting for a resource that is held by P 0. Deadlock can arise if four conditions hold simultaneously.

7.8 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 Resource-Allocation Graph V is partitioned into two types: P = {P 1, P 2, …, P n }, the set consisting of all the processes in the system. R = {R 1, R 2, …, R m }, the set consisting of all resource types in the system. request edge – directed edge P i  R j assignment edge – directed edge R j  P i A set of vertices V and a set of edges E.

7.9 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 Resource-Allocation Graph (Cont.) Process Resource Type with 4 instances P i requests instance of R j - request edge P i is holding an instance of R j – assignment edge PiPi PiPi RjRj RjRj

7.10 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 Example of a Resource Allocation Graph

7.11 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 The Deadlock Condition It can be shown that if the graph contains no cycles, then no process in the system is deadlocked. If the graph does contain a cycle, then a deadlock may exist.

7.12 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 Resource Allocation Graph With A Deadlock Cycles: P1  R1  P2  R3  P3  R2  P1 P2  R3  P3  R2  P2 Processes P1, P2 and P3 are deadlocked: P2 is waiting for R3, which is held by P3 P3 is waiting for either P1 or P2 to release R2 P1 is waiting for P2 to release R1

7.13 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 Graph With A Cycle But No Deadlock Cycle: P1  R1  P3  R2  P1 There is no deadlock: P4 may release its instance on resource type R2. That resource can then be allocated to P3, breaking the cycle.

7.14 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 Basic Facts If graph contains no cycles  no deadlock. If graph contains a cycle  if only one instance per resource type, then deadlock. if several instances per resource type, possibility of deadlock.

7.15 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 Methods for Handling Deadlocks Ensure that the system will never enter a deadlock state. Allow the system to enter a deadlock state and then recover. Ignore the problem and pretend that deadlocks never occur in the system.

7.16 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 Methods for Handling Deadlocks If a system does not employ either a deadlock- avoidance or deadlock-prevention algorithm, then a deadlock situation may arise. If a system neither ensures that a deadlock will never occur nor provides a mechanism for deadlock detection and recovery, then we may arrive at a situation where the system is in a deadlocked state and there is no way of recognizing what has happened.

7.17 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 Deadlock Methods for Handling Deadlocks Deadlock prevention means that a system has to ensure that at least one of the four deadlock conditions cannot hold. Deadlock avoidance requires that the operating system be given in advance additional information concerning which resources a process will request and use during its lifetime. To decide whether the current request can be satisfied or must be delayed, the system must consider the resources currently available, the resources currently allocated to each process, and the future requests and releases of each process.

7.18 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 Deadlock Characterization Mutual exclusion: only one process at a time can use a resource. If another process requests that resource, the requesting process must be delayed until the resource has been released. 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 {P 0, P 1, …, P 0 } of waiting processes such that P 0 is waiting for a resource that is held by P 1, P 1 is waiting for a resource that is held by P 2, …, P n–1 is waiting for a resource that is held by P n, and P n is waiting for a resource that is held by P 0. Deadlock can arise if four conditions hold simultaneously.

7.19 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 Deadlock Prevention Mutual Exclusion – not required for sharable resources (a read-only file is a typical example of a sharable resource), a process never needs to wait for a sharable resource; must hold for nonsharable resources (a printer is a typical example of such a resource). In general, this condition can not be denied. Hold and Wait – must guarantee that whenever a process requests a resource, it does not hold any other resources. Require a process to request and be allocated all its resources before it begins execution, or allow process to request resources only when the process has none. Low resource utilization; starvation possible.

7.20 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 Deadlock Prevention (Cont.) No Preemption – If a process that is holding some resources requests another resource that cannot be immediately allocated to it, then all resources currently being held are released. Preempted resources are added to the list of resources for which the process is waiting. Process will be restarted only when it can regain its old resources, as well as the new ones that it is requesting. Circular Wait – impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration.

7.21 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts - 7 th Edition, Feb 14, 2005 Deadlock Avoidance Simplest and most useful model requires that each process declare the maximum number of resources of each type that it may need. The deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that there can never be a circular-wait condition. Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes. Requires that the system has some additional a priori information available.