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ch7 - Chapter 7 Deadlocks Silberschatz Galvin and Gagne...

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Silberschatz, Galvin and Gagne ©2009 Chapter 7: Deadlocks
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1.2 Silberschatz, Galvin and Gagne ©2009 Chapter 7: Deadlocks The Deadlock Problem System Model Deadlock Characterization Methods for Handling Deadlocks Deadlock Prevention Deadlock Avoidance Deadlock Detection Recovery from Deadlock
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1.3 Silberschatz, Galvin and Gagne ©2009 Chapter Objectives 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
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1.4 Silberschatz, Galvin and Gagne ©2009 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 Example semaphores A and B , initialized to 1 P 0 P 1 wait (A); wait(B) wait (B); wait(A)
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1.5 Silberschatz, Galvin and Gagne ©2009 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 Note – Most OSes do not prevent or deal with deadlocks
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1.6 Silberschatz, Galvin and Gagne ©2009 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. Each process utilizes a resource as follows: request use release
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1.7 Silberschatz, Galvin and Gagne ©2009 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 { P 0 , P 1 , …, P n } 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.
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1.8 Silberschatz, Galvin and Gagne ©2009 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 .
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1.9 Silberschatz, Galvin and Gagne ©2009 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 P i P i R j R j
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1.10 Silberschatz, Galvin and Gagne ©2009 Example of a Resource Allocation Graph
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1.11 Silberschatz, Galvin and Gagne ©2009 Resource Allocation Graph With A Deadlock
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1.12 Silberschatz, Galvin and Gagne ©2009 Graph With A Cycle But No Deadlock
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1.13
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