1. Lecture 22 Syed Mansoor Sarwar
Operating Systems
Lecture 22
Syed Mansoor Sarwar

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Agenda for Today
Review of previous lecture
Semaphores based solutions for the critical section problem
Deadlock and starvation
Binary and counting semaphores
Recap of lecture
7 December 2018
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Review of Lecture 21
The Bakery algorithm
Hardware based solutions
7 December 2018
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A Good Solution
‘key’ local; ‘lock’ and ‘waiting’ global
All variables set to false
do {
waiting[i] = true;
key = true;
while (waiting[i] && key)
key = TestAndSet(lock);
waiting[i] = false;
7 December 2018
Critical Section
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A Good Solution
j = (i+1) % n;
while ( (j != i) && !waiting[j] )
j = (j+1) % n;
if (j == i)
lock = false;
else
waiting[j] = false;
Remainder Section }
7 December 2018
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Solution with
Test-And-Set
Is the given solution good?
Yes
Mutual Exclusion: Satisfied
Progress: Satisfied
Bounded Waiting: Satisfied
7 December 2018
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Semaphores
7 December 2018
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Semaphores
Synchronization tool
Available in operating systems
Semaphore S – integer variable that can only be accessed via two indivisible (atomic) operations, called wait and signal
7 December 2018
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Semaphores
wait(S){
while S 0
; //no-op S--; }
signal(S){
S++;
7 December 2018
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10. n-Processes CS Problem
Shared data:
semaphore mutex = 1;
Structure of Pi:
do { critical section
remainder section } while (1);
wait(mutex);
signal(mutex);
7 December 2018
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Is it a Good Solution?
Mutual Exclusion: Yes
Progress: Yes
Bounded Waiting: No
7 December 2018
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Atomic Execution
Uni-Processor Environment
Inhibit interrupts before executing code for wait() and signal()
Bus-based Multi-Processor Environment
Lock the data bus
Use a software solution
7 December 2018
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Busy Waiting
Processes wait by executing CPU instructions
Problem? Wasted CPU cycles
Solution? Modify the definition of semaphore
7 December 2018
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Recap of Lecture
Synchronization hardware
Semaphores
Semaphore based solutions for the critical section problem
Busy waiting and modified definition of semaphore
7 December 2018
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15. Lecture 22 Syed Mansoor Sarwar
Operating Systems
Lecture 22
Syed Mansoor Sarwar