Process Synchronization

Race Conditions

The Critical Section

Semaphores

Classic Synchronization Problems

 

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Race conditions usually occur if two or more processes are allowed to modify the same shared variable at the same time.

To prevent race conditions, the operating system must perform process synchronization to guarantee that only one process is updating a shared variable at any one time.

 

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part of the process that contains the instruction or instructions that access a shared variable or resource

Figure 6.1 Critical Sections for Process P1 and P2

 

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Software solutions to the critical section problem:

SOLUTION 1

Figure 6.2 Critical Sections of Processes P1 and P2 for Solution 1

 

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SOLUTION 2

Figure 6.3 Critical Section P1 and P2 for Solution 2

 

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SOLUTION 3 (Peterson’s Algorithm)

Figure 6.4 Critical Sections of Processes P1 and P2 for Solution 3

 

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SOLUTION TO THE CRITICAL SECTION PROBLEM INVOLVING SEVERAL PROCESS ( Bakery Algorithm)

Figure 6.5 Critical Section of Process P1 for the Bakery Algorithm

 

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Hardware solutions to the critical section problem:

Disabling interrupts

Special hardware instructions

These instructions are executed atomically (uninterruptible)

Figure 6.6 Definition of test_and set instruction

 

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Pi

Figure 6.7 Using the test_and_set instruction to solve the critical section problem

 

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Semaphore is a tool that can easily be used to solve more complex synchronization problems and does not use busy waiting.

Pi

Figure 6.8 Using Semaphores to Solve the Critical Section Problem

 

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The Dining Philosophers Problem

Restrictions:

1. A philosopher cannot start eating unless he has both forks.

2. A philosopher cannot pick up both forks at the same time.  He has to do it one at a time.

3. He cannot get the fork that is being used by the philosopher to his right or to his left.

Figure 6.9 The Dining Philosophers Problem

 

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Figure 6.10 Solution to the Dining Philosophers Problem

 

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The Readers and Writers Problem

Figure 6.11 Using Semaphores to Implement a Writer Process

 

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Figure 6.12 Using Semaphores to Implement a Reader Process

 

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