1. Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, 2005 7.1 Operating System Concepts Operating Systems Lecture 28 Handling Deadlock

2. Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, 2005 7.2 Operating System Concepts 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.

3. Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, 2005 7.3 Operating System Concepts Resource-Allocation Graph (Cont.) Process Resource Type with 4 instances P i requests instance of R j P i is holding an instance of R j PiPi PiPi RjRj RjRj

4. Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, 2005 7.4 Operating System Concepts Example of a Resource Allocation Graph

5. Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, 2005 7.5 Operating System Concepts Resource Allocation Graph With A Deadlock

6. Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, 2005 7.6 Operating System Concepts Resource Allocation Graph With A Cycle But No Deadlock

7. Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, 2005 7.7 Operating System Concepts 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.

8. Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, 2005 7.8 Operating System Concepts 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; used by most operating systems, including UNIX.

9. Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, 2005 7.9 Operating System Concepts Deadlock Prevention Mutual Exclusion – not required for sharable resources; must hold for nonsharable resources. Hold and Wait – must guarantee that whenever a process requests a resource, it does not hold any other resources.  Require 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.  Disadvantages: Low resource utilization; starvation possible. Make sure at least one of the four conditions for deadlock cannot hold:

10. Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, 2005 7.10 Operating System Concepts 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.  doesn't work well with printer resources. Works well for memory resources. Circular Wait – impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration.

11. Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, 2005 7.11 Operating System Concepts 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.

12. Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, 2005 7.12 Operating System Concepts Safe State A state is safe if the system can allocate processes to each process (up to its maximum) in some order and avoid deadlock. When a process requests an available resource, system must decide if immediate allocation leaves the system in a safe state. System is in safe state if there exists a safe sequence of all processes. Sequence is safe if for each P i, the resources that Pi can still request can be satisfied by currently available resources + resources held by all the P j, with j<i.  If P i resource needs are not immediately available, then P i can wait until all P j have finished.  When P j is finished, P i can obtain needed resources, execute, return allocated resources, and terminate.  When P i terminates, P i+1 can obtain its needed resources, and so on.

13. Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, 2005 7.13 Operating System Concepts Basic Facts If a system is in safe state  no deadlocks. If a system is in unsafe state  possibility of deadlock. Avoidance  ensure that a system will never enter an unsafe state.

14. Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, 2005 7.14 Operating System Concepts Safe, Unsafe, Deadlock State

15. Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, 2005 7.15 Operating System Concepts Example Suppose a system has 12 tape drives and 3 processes. At time t 0, the system is as follows: ProcessMax needCurrent need P 0 105 P142P142 P292P292 3 tape drives are unallocated. Is the system safe? Suppose process P 2 is allocated another tape drive at time t 1. Is the system safe?

16. Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, 2005 7.16 Operating System Concepts Resource-Allocation Graph Algorithm Works for systems with only 1 instance of each resource type. Create a resource allocation graph that uses claim edges. A Claim edge P i  R j indicated that process P j may request resource R j ; represented by a dashed line. Claim edge converts to request edge when a process requests a resource. When a resource is released by a process, assignment edge reconverts to a claim edge. Resources must be claimed a priori in the system. If there are no cycles in the graph, the system is in a safe state. Must use a cycle detection algorithm to test for a safe state.

17. Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, 2005 7.17 Operating System Concepts Resource-Allocation Graph For Deadlock Avoidance

18. Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, 2005 7.18 Operating System Concepts Unsafe State In Resource-Allocation Graph