Good for critical section

1. Operating System
Concepts
Instructor Name: Muhammad Asif
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2. Quick Recap of Previous Lecture
• CPU Scheduling
• Scheduling Criteria
• Scheduling Algorithms
• FCFS (First Come First Serve)
• Example
• SJF (Shortest Job First)
• Example
• Priority Scheduling
• Round Robin Scheduling
• Example
• Multi Level Queue Scheduling
• Multi Level Feedback Queue 2

3. Today’s Outlines
• Critical Section Problem
• Condition for the Solution to the Critical Section Problem
• Race Condition
• Peterson’s Solution
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4. Critical Section
• A Critical Section is a code segment that accesses shared
variables and has to be executed as an atomic action.
• It means that in a group of cooperating processes:
• at a given point of time, only one process must be executing its
critical section.
• If any other process also wants to execute its critical section, it
must wait until the first one finishes.
• Concurrent access to shared data may result in data
inconsistency
• Maintaining data consistency requires mechanisms to ensure
the orderly execution of cooperating processes
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5. Critical Section Problem
• n processes all competing to use some shared data
• Each process has a code segment, called critical section, in
which the shared data is accessed.
• Problem:
• Ensure that when one process is executing in its critical section, no
other process is allowed to execute in its critical section.
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6. Conditions for Solution of Critical
Section Problem
• Mutual Exclusion. If process Pi is executing in its critical
section,
• then no other processes can be executing in their critical
sections.
• Progress. If no process is executing in its critical section and
there exist some processes that wish to enter their critical
section,
• then the selection of the processes that will enter the critical
section next cannot be postponed indefinitely.
• Bounded Waiting. A bound must exist on the number of times
that other processes are allowed to enter their critical sections
• After a process has made a request to enter its Critical section
and before that request is granted.
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7. Race Condition
• The situation where several processes access and manipulate
shared data concurrently.
• The final value of the shared data depends upon which process
finishes last.
• To prevent race conditions, concurrent processes must be
synchronized.
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8. Peterson’s Solution
• Two process solution
• Assume that the LOAD and STORE instructions are atomic;
that is, cannot be interrupted.
• The two processes share two variables:
• int turn;
• Boolean flag[2]
• The variable turn indicates whose turn it is to enter the critical
section.
• The flag array is used to indicate if a process is ready to enter
the critical section. flag[i] = true implies that process Pi is
ready!
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9. Peterson’sAlgorithm
while (true) {
flag[i] = TRUE;
turn = j;
while ( flag[j] && turn == j);
CRITICAL SECTION
flag[i] = FALSE;
REMAINDER SECTION
} 9

10. Peterson’s Algorithm Explanation:
• Process P0:
while (true) {
flag[0] = TRUE;
turn = 1;
while (turn == 1 &&flag[1]== T);
CRITICAL SECTION
flag[0] = FALSE;
}
// Turn =
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Process P1:
while (true) {
flag[1] = TRUE;
turn = 0;
while (turn == 0 &&flag[0]== T);
CRITICAL SECTION
flag[1] = FALSE;
}
0 1
// Flag =

11. Any Questions?
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