DIZNR INTERNATIONAL

Process Synchronization Two process Solution (Algo-2) Deadlock Concept, Flag concept.

Lui Kreyszig Lui Kreyszig

Process Synchronization Two process Solution (Algo-2) Deadlock Concept , Flag concept.

https://www.gyanodhan.com/video/7B7.%20GATE%20CSEIT/Operating%20System/635.%20Process%20Synchronization%20Two%20process%20Solution%20%28Algo-2%29%20%20%20Deadlock%20ConceptFlag%20concept.mp4

Process Synchronization – Two Process Solution (Algo-2) | Deadlock & Flag Concept

 Introduction to Process Synchronization

Process Synchronization is a mechanism to ensure that multiple processes can execute in a way that avoids inconsistencies and conflicts when accessing shared resources. One of the key challenges in synchronization is deadlock.

 Two-Process Solution (Algorithm-2)

The Two-Process Solution for synchronization ensures mutual exclusion, progress, and bounded waiting. It typically uses flags and turn variables.

 Algorithm-2 (Flag Concept Based Approach)

This method relies on two flags to indicate whether a process wants to enter the critical section.

 Assumptions:

 Algorithm (Using Flags)

// Shared Variables
boolean flag[2] = {false, false}; // Flags for P1 and P2Process P1:
while (true) {
flag[0] = true; // P1 wants to enter the critical section
while (flag[1]); // Wait if P2 is in the critical section

// Critical Section (CS)

flag[0] = false; // Exit CS
// Remainder Section
}

Process P2:
while (true) {
flag[1] = true; // P2 wants to enter the critical section
while (flag[0]); // Wait if P1 is in the critical section

// Critical Section (CS)

flag[1] = false; // Exit CS
// Remainder Section
}

 Explanation:

 Deadlock Concept

A deadlock occurs when two or more processes wait indefinitely for resources held by each other.

 Can Deadlock Occur in This Algorithm?

Yes! This algorithm can lead to deadlock in the following situation:

  1. Both P1 and P2 set their flags at the same time (flag[0] = true, flag[1] = true).
  2. Both enter their respective while loops (while (flag[1]); and while (flag[0]);).
  3. Now, both are waiting for each other to release the flag, leading to a deadlock.

 Avoiding Deadlock: Peterson’s Solution

Peterson’s Algorithm improves the Two-Process Solution by introducing a turn variable along with flags.

boolean flag[2] = {false, false};
int turn = 0; // Indicates which process gets priorityProcess P1:
while (true) {
flag[0] = true;
turn = 1;
while (flag[1] && turn == 1); // Wait if P2 has priority

// Critical Section
flag[0] = false; // Exit CS
}

Process P2:
while (true) {
flag[1] = true;
turn = 0;
while (flag[0] && turn == 0); // Wait if P1 has priority

// Critical Section
flag[1] = false; // Exit CS
}

 Why is Peterson’s Solution Deadlock-Free?

Conclusion

 Would you like more details on Deadlock Prevention & Avoidance?

Process Synchronization Two process Solution (Algo-2) Deadlock Concept, Flag concept.

Chapter 6: Synchronization

You’re referring to Process Synchronization in operating systems — specifically the two-process solution (Algorithm 2) and how it relates to deadlock and the use of flags.

Let’s break it down clearly and concisely.

Background: Process Synchronization

When two or more processes access shared resources (e.g., a shared variable), there’s a need to synchronize them to avoid race conditions. Mutual exclusion ensures only one process accesses the critical section at a time.

Peterson’s Algorithm (Two-Process Solution – Algo 2)

This is a classical solution for the mutual exclusion problem for two processes (P0 and P1) using flag variables and a turn variable.

Idea:

Each process indicates its intention to enter the critical section using a flag and uses a turn variable to decide who gets to proceed.

Code (in C-style pseudocode):

c
bool flag[2]; // flag[0] for P0, flag[1] for P1
int turn; // shared variable

// Code for process Pi (i = 0 or 1)
flag[i] = true;
turn = 1 - i;
while (flag[1 - i] && turn == 1 - i)
; // busy wait (do nothing)

// ---- Critical Section ----

// Exit section
flag[i] = false;

Key Concepts

Deadlock in Algorithm 2?

No, this algorithm does not result in deadlock.

Why?

Both processes cannot be stuck indefinitely because:

This ensures:

Important Properties Satisfied:

Property Satisfied?
Mutual Exclusion Yes
Deadlock-Free Yes
Starvation-Free Yes
Busy Waiting Yes (but undesirable in real systems)

Summary:

Let me know if you want a diagram, trace table, or extension to N-process synchronization.

Process Synchronization Two process Solution (Algo-2) Deadlock Concept, Flag concept.

OPERATING SYSTEMS PROCESS SYNCHRONIZATION

Process Synchronization Race Condition: Critical Section …