Introduction to Deadlock

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Introduction to Deadlock

• Lecture 21
• Klara Nahrstedt

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CS241 Administrative

• Read Stallings Chapter 6

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Content of This Lecture

• Goals
• Understanding deadlock
• How to avoid deadlocks
• Things covered in this lecture
• Deadlock concepts
• Deadlock conditions

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Resource (1)

• A resource is a commodity needed by a process.
• Resources can be either
• serially reusable e.g., CPU, memory, disk space,
  I/O devices, files. acquire ? use ? release
• Real life analogy?
• consumable produced by a process, needed by a
  process e.g., messages, buffers of information,
  interrupts. create ? acquire ? use Resource
  ceases to exist after it has been used, so it is
  not released.
• Real life analogy?

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Resource (2)

• Resources can also be either
• preemptible e.g., CPU, central memory
• What does it mean?
• non-preemptible e.g., tape drives.
• Any other examples?
• And resources can be either
• shared among several processes or
• dedicated exclusively to a single process.

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Using Semaphore to Share Resource
Process Q() A.Down() B.Down() use
both resource B.Up() A.Up()
Process P() A.Down() B.Down() use
both resource B.Up() A.Up()
Is it OK?
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But Deadlock can Happen!
Process Q() B.Down() A.Down() use
both resources A.Up() B.Up()
Process P() A.Down() B.Down() use
both resources B.Up() A.Up()
DEADLOCK
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Deadlock
Mechanism for Deadlock Control
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Another Example - Remember Dining Philosophers?
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Dining Philosopher Challenge

• Think Eat
• N Philosophers circular table with N chopsticks
• To eat the Philosopher must first pickup two
  chopsticks
• ith Philosopher needs ith i1th chopstick
• Only put down chopstick when Philosopher has
  finished eating
• Devise a solution which satisfies mutual
  exclusion but avoids starvation and deadlock

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Seems simple enough ?

• while(true)
• think()
• sem_wait(chopsticki)
• sem_wait(chopstick(i1) N)
• eat()
• sem_post(chopstick(i1) N)
• sem_post(chopsticki)

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Deadlock (Each P. holds left fork)

• while(true)
• think()
• sem_wait(chopsticki)
• sem_wait(chopstick(i1) N)
• eat()
• sem_post(chopstick(i1) N)
• sem_post(chopsticki)

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Deadlock Definition

• What is a deadlock?
• A process is deadlocked if it is waiting for an
  event that will never occur.
• Typically, but not necessarily, more than one
  process will be involved together in a deadlock
  (the deadly embrace).
• Is deadlock the same as starvation (or
  indefinitely postponed)?
• A process is indefinitely postponed if it is
  delayed repeatedly over a long period of time
  while the attention of the system is given to
  other processes. I.e., logically the process may
  proceed but the system never gives it the CPU.

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Conditions for Deadlock

• What conditions should exist in order to lead to
  a deadlock
• Real life analogy such as
• You take the monitor, I grab the keyboard
• Cars in intersection

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Necessary and Sufficient Conditions for Deadlock

• Mutual exclusion
• Processes claim exclusive control of the
  resources they require
• Wait-for condition
• Processes hold resources already allocated to
  them while waiting for additional resources
• No preemption condition
• Resources cannot be removed from the processes
  holding them until used to completion
• Circular wait condition
• A circular chain of processes exists in which
  each process holds one or more resources that are
  requested by the next process in the chain

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Mutual Exclusion

• Processes claim exclusive control of the
  resources they require
• How to break it?

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Wait-for Condition

• Processes hold resources already allocated to
  them while waiting for additional resources
• How to break it?

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No Preemption Condition

• Resources cannot be removed from the processes
  holding them until used to completion
• How to break it?

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Its difficult to take a tape drive away from a
process that is busy writing a tape
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Circular Wait Condition

• A circular chain of processes exists in which
  each process holds one or more resources that are
  requested by the next process in the chain
• How to break it?

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Deadlock Modeling (1)

• Resource allocation graph
• Graph have two kinds of nodes
• Processes
• Resources
• Graphs arcs have two meanings
• Arc from resource node to process node process
  holds resource (i.e., resource has been assigned
  to process and process is holding it)
• Arc from process node to resource node process
  requests resource (i.e., process is currently
  blocked waiting for that resource)

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Deadlock Modeling (2)

• Modeled with directed graphs
• 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

assign
request
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Deadlock Modeling (3)
A B
C

• How deadlock occurs

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Deadlock Modeling (4)
(o) (p)
(q)

• How deadlock can be avoided

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Resource Allocation Graph(multiple resource
types)
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Deadlock Model
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Strategies

• Strategies for dealing with Deadlocks
• detection and recovery
• dynamic avoidance
• careful resource allocation
• prevention
• negating one of the four necessary conditions

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Summary

• Principles of Deadlock
• Examples of Deadlock
• Deadlock Conditions
• Deadlock Modeling