Lecture 12: Deadlock Ch' 3'3

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Lecture 12 Deadlock (Ch. 3.3)
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Deadlocks

• Formal definition A set of processes is
  deadlocked if each process in the set is waiting
  for an event that only another process in the set
  can cause.

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Four Conditions for Deadlock

• Mutual exclusion condition
• each resource assigned to 1 process or is
  available
• Hold-and-wait condition
• process holding resources can request additional
  resources
• No preemption condition
• previously granted resources cannot forcibly
  taken away
• Circular wait condition
• must be a circular chain of 2 or more processes
• each is waiting for resource held by next member
  of the chain

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Modeling Deadlock with Graphs

• Process circle, Resource square
• resource R assigned to process A
• process B is requesting/waiting for resource S
• process C and D are in deadlock over resources T
  and U

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Using Resource Allocation Graph
A B C
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How deadlock can be avoided
Scheduling B is delayed
(o) (p)
(q)
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Strategies for Dealing with Deadlock

• Just ignore the problem altogether.
• Prevention
• negating one of the four necessary conditions.
• Detection and recovery.
• Avoidance
• careful resource allocation.

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The Ostrich Algorithm

• Reasonable if
• deadlocks occur very rarely.
• cost of prevention is high.
• UNIX and Windows take this approach.
• It is a tradeoff between
• convenience
• correctness

General-purpose OSs do not do much towards
deadlock detection, recovery, avoidance, or
prevention.
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Deadlock Prevention
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Idea invalidate one of the four conditions for
deadlock

• Mutual exclusion condition
• Hold-and-wait condition
• No preemption condition
• Circular wait condition

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Attacking the Mutual Exclusion Condition

• Some devices (such as printer) can be spooled
• only the printer daemon uses printer resource
• thus deadlock for printer eliminated
• Not all devices can be spooled
• Principle
• avoid assigning resource when not absolutely
  necessary
• as few processes as possible actually claim the
  resource

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Attacking the Hold and Wait Condition

• Require processes to request resources before
  starting
• a process never has to wait for what it needs
• Problems
• may not know required resources at start of run
• also ties up resources other processes could be
  using
• Variation before requesting a new resource,
• process must give up all resources
• then request all immediately needed

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Attacking the No Preemption Condition

• In general this is not a viable option
• Consider a process given the printer
• halfway through its job
• now forcibly take away printer
• !!??

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Attacking the Circular Wait Condition

• Every resource has a unique number
• A process must request resources in increasing
  number order

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Requesting in Order Prevents Deadlock

• 2 resources (i gt j) , 2 processes A, B
• Deadlock can only occur if
• A holds i and requests j,
• B holds j and requests i.
• But if they requested resources in order, after A
  holds i it is not allowed to request j!
• It should have asked for j earlier (and become
  blocked).

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Summary of deadlock prevention
All these options are very restrictive, and not
very practical
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Deadlock Detection and Recovery
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Recovery from Deadlock (1)

• Recovery through preemption
• take a resource from some other process
• depends on nature of the resource
• Recovery through rollback
• checkpoint a process periodically
• use this saved state
• restart the process if it is found deadlocked

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Recovery from Deadlock (2)

• Recovery through killing processes
• crudest but simplest way to break a deadlock
• kill one of the processes in the deadlock cycle
• the other processes get its resources
• choose process that can be rerun from the
  beginning
• None of the deadlock recovery options is very
  attractive.

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Deadlock Avoidance
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Deadlock Avoidance

• Can we make decisions one at a time and always
  avoid deadlock?
• Need some advance knowledge
• Each process has to declare, in advance, what
  resources, and what is the maximum number of
  resource units, it will need

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Safe and Unsafe States

• The State of the system includes
• How many resources are allocated to each process
• What is the maximum number of resources a process
  could request
• How many are available
• A state is Safe if
• It is not deadlocked
• There exists a scheduling order in which all
  processes run to completion.

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The state in (a) is safe
Only one resource, 10 copies exist
(a) (b)
(c) (d)
(e)
After B terminates we could Schedule C
We could schedule B
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The state in (b) is not safe
(a) (b)
(c)
(d)
Schedule B
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Unsafe ! Deadlocked

• If state is safe ? deadlock can be avoided.
• If state is unsafe ? not clear
• We are making worst case assumptions, processes
  could use less than their maximum

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Bankers Algorithm (Dijkstra, 1965)

• When process requests resource X,
• Assume the process gets X
• Loop
• Find unmarked process i with Ri A // i can run
• If found A ? A Ci mark
  process i as terminated. // assume i has run,
  make its resources available
• Else System is deadlocked, giving the resource
  was unsafe

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The Banker's Algorithm for a Single Resource
(a)
(b)
(c)

• Three resource allocation states
• safe
• safe
• unsafe

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Is Deadlock Avoidance Practical?

• Bankers algorithm needs information about future
  needs (max values for each resource).
• In general OS does not have this information.
• Number of processes varies.
• Resources can disappear.