Deadlocks

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Deadlocks
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Introduction

• A deadlock is a state of CS (computer system)
  when all processes are waiting to get out of WAIT
  state, while holding resources and by doing that
  preventing other processes to get out of their
  WAIT states

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Deadlocks
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Utilisation of Resources

• In normal conditions, process can utilise a
  resource by
• Request (if resource is used by a different
  process, if must WAIT)
• Use
• Release

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Deadlock Creation

• Deadlocks can arise if the following conditions
  are met simultaneously
• Mutual exclusion
• At lease one resource must be non-sharable, so
  that only one process can use it at any time
• Hold and wait
• There must be a process that holds at least one
  resource and waits for more resources that are
  held by other processes

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Deadlock Creation continued

• No preemption
• Resource can only be released by the process
  holding it when the process completes its task
• Circular wait
• There must exist a set of processes that hold and
  wait for more resources which form a cyclic loop
• Example (two processes P1 P2, two resources R1
  R2)
• P1 holds R2 and waits for R1
• P2 holds R1 and waits for R2

P1
P2
R2
R1
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Resource Allocation Graphs

• In order to analyse deadlocks and how they arise,
  we can draw graphs indicating resources and
  processes, and their relationship
• Processes are drawn as circles
• Resources are in boxes

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Resource Allocation Graphs
R1
R3
P1
P2
P3
R2
R4
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Resource Allocation Graphs

• Graphs components are divided into three sets
• Processes, P P1, P2, P3
• Resources, R R1, R2, R3, R4
• Edges, E P1 ? R1, P2 ? R3, R1 ? P2, R2 ? P2,
  R2 ? P1, R3 ? P3
• Where R11, R22, R31, R44 are instances of
  resources

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Deadlock
R1
R3
P1
P2
P3
R2
R4
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Handling Deadlocks

• There are three methods used for dealing with
  deadlocks
• Prevention allocate resources in such a way
  that deadlocks cannot possibly occur
• Avoidance allow deadlocks and when detected,
  recover
• Ignore allow process starvation, and forget
  that it ever happened. This approach is used by
  most operating systems including UNIX

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Deadlock Prevention

• Allocate resources such that preconditions are
  not possible
• Mutual Exclusion shared resources means
  deadlock not possible (but cannot share all
  resources, eg. printers)
• Hold and Wait make processes allocate resources
  first and then execute

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Deadlock Prevention continued

• No Preemption Preempt. Borrow resources held by
  other processes (not always possible, eg.
  printers)
• Circular Wait impose a total ordering of
  resource types and require that each process
  requests resources in an increasing order of
  enumerations

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Deadlock Avoidance

• In avoiding deadlocks we must use an algorithm to
  find out how to allocate resources
• If we have a resource allocation system with only
  one instance of each resource, we can use the
  Resource Allocation Graph
• A much better alternative is the Bankers Algorithm

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Bankers Algorithm

• Bankers algorithm is based on the idea of a
  banking system that ensures that the bank never
  allocates its available cash
• Each process must declare the maximum number of
  instances of each resource it will need

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Bankers Algorithm continued

• When a resource is requested, the system must
  make sure that after the allocation it will
  remain in a safe state
• A safe state is a state in such a deadlock is not
  possible

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Exercises

• Analyse the following system
• Processes, P P1, P2, P3 , P4
• Resources, R R1, R2
• Edges, E P1 ? R1, P3 ? R2, R1 ? P2, R1 ? P3,
  R2 ? P1 , R2 ? P4
• Where R12, R22 are instances of resources