Chapter 6: Concurrency: Mutual Exclusion and Synchronization

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Chapter 6 Concurrency Mutual Exclusion and
Synchronization

• Operating System
• Spring 2007
• Chapter 6 of textbook

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Concurrency

• An OS has many concurrent processes that run in
  parallel but share common access
• Race Condition A situation where several
  processes access and manipulate the same data
  concurrently and the outcome of the execution
  depends on the particular order in which the
  access takes place.

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Example for Race condition

• Suppose a customer wants to book a seat on UAL
  56. Ticket agent will check the -of-seats. If it
  is greater than 0, he will grab a seat and
  decrement -of-seats by 1.

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Example for Race condition(cont.)
Ticket Agent 1 P1 LOAD -of-seats P2 DEC 1 P3
STORE -of-seats
Ticket Agent 2 Q1 LOAD -of-seats Q2 DEC 1 Q3
STORE -of-seats
Ticket Agent 3 R1 LOAD -of-seats R2 DEC 1 R3
STORE -of-seats
Suppose, initially, -of-seats12 Suppose
instructions are interleaved as
P1,Q1,R1,P2,Q2,R2,P3,Q3,R3 The result would be
-of-seats11, instead of 9
To solve the above problem, we must make sure
that P1,P2,P3 must be completely executed before
we execute Q1 or R1, or Q1,Q2,Q3 must be
completely executed before we execute P1 or R1,
or R1,R2,R3 must be completely executed before we
execute P1 or Q1.
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Critical Section Problem
Critical section a segment of code in which the
process may be changing common variables,
updating a table, writing a file, and so on.
Goal To program the processes so that, at any
moment of time, at most one of the processes is
in its critical section.
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Solution to Critical-Section Problem

• Any facility to provide support for mutual
  exclusion should meet the following requirements
• Mutual exclusion must be enforced Only one
  process at a time is allowed into its critical
  section
• A process that halts in its noncritical section
  must do so without interfering with other
  processes.
• A process waiting to enter its critical section
  cannot be delayed indefinitely
• When no process is in a critical section, any
  process that requests entry to its critical
  section must be permitted to enter without delay.
• No assumption are made about the relative process
  speeds or the number of processors.
• A process remains inside its critical section for
  a finite time only.

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Three Environments

1. There is no central program to coordinate the
   processes. The processes communicate with each
   other through global variable.
2. Solution Petersons algorithm
3. not covered in this class
4. Special hardware instructions
5. TS
6. Exchange(not covered in this class)
7. There is a central program to coordinate the
   processes.
8. Semaphore

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Special Machine Instructions

• Modern machines provide special atomic hardware
  instructions
• Atomic non-interruptable
• Either test memory word and set value
• Or swap contents of two memory words

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TS Test and Set
Boolean TS(i) true if i0 it will also set i
to 1 false if i1
Initially, lock0
Pi Prefixi While( TS(lock)) do
CSi Lock0 suffixi
Problem of this program It is possible that a
process may starve if 2 processes enter the
critical section arbitrarily often. Solution for
the starvation wont be covered in this class.
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Semaphores

• A variable that has an integer value upon which 3
  operations are defined.
• Three operations
• A semaphore may be initialized to a nonnegative
  value
• The wait operation decrements the semaphore
  value. If the value becomes negative, then the
  process executing the wait is blocked
• The signal operation increments the semaphore
  value. If the value is not positive, then a
  process blocked by a wait operation is unblocked.
• Other than these 3 operations, there is no way to
  inspect or manipulate semaphores.

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Wait(s) and Signal(s)

• Wait(s) is also called P(s)
• ss-1
• if (slt0) place this process in a waiting
  queue
• Signal(s) is also called V(s)
• ss1
• if(s?0) remove a process from the waiting
  queue

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Semaphore as General Synchronization Tool

• Counting semaphore integer value can range over
  an unrestricted domain
• Binary semaphore integer value can range only
  between 0 and 1 can be simpler to implement
• Also known as mutex locks
• Wait B(s) s is a binary semaphore
• if s1 then s0 else block this process
• Signal B(s)
• if there is a blocked process then unblock
  a process else s1
• Can implement a counting semaphore S as a binary
  semaphore

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Note

• The wait and signal primitives are assumed to be
  atomic they cannot be interrupted and each
  routine can be treated as an indivisible step.

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Mutual Exclusion provided by Semaphores

• Semaphore S // initialized to 1
• Pi
• prefixi
• wait (S)
• CSi
• signal (S)
• suffixi

• Signal B(s)
• if there is a blocked process
• then unblock a process
• else s1

• Wait B(s) s is a binary semaphore
• if s1
• then s0
• else block this process

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Two classical examples

• Producer and Consumer Problem
• Readers/Writers Problem

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Two classical examples

• Producer and Consumer Problem
• Readers/Writers Problem

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Producer and Consumer Problem

• Producer can only put something in when there is
  an empty buffer
• Consumer can only take something out when there
  is a full buffer
• Producer and consumer are concurrent processes

0








consumer
producer
N buffers
N-1
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Producer and Consumer Problem(cont.)

• Global Variable
• B0..N-1 an array of size N (Buffer)
• P a semaphore, initialized to N
• C a semaphore, initialized to 0
• Local Variable
• In a ptr(integer) used by the producer, in0
  initially
• Out a ptr(integer) used by the consumer, out0
  initially

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Two classical examples

• Producer and Consumer Problem
• Readers/Writers Problem

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Readers/Writers Problem

• Suppose a data object is to be shared among
  several concurrent processes. Some of these
  processes want only to read the data object,
  while others want to update (both read and write)
• Readers Processes that read only
• Writers processes that read and write
• If a reader process is using the data object,
  then other reader processes are allowed to use it
  at the same time.
• If a writer process is using the data object,
  then no other process (reader or writer) is
  allowed to use it simultaneously.

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Solve Readers/Writers Problem using wait and
signal primitives(cont.)

• Global Variable
• Wrt is a binary semaphore, initialized to 1 Wrt
  is used by both readers and writers
• For Reader Processes
• Mutex is a binary semaphore, initialized to
  1Readcount is an integer variable, initialized
  to 0
• Mutex and readcount used by readers only

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End
Thank you!