Problem: Given 2 processes i and j, you need to write a program that can guarantee mutual exclusion between the two without any additional hardware support.
Solution: There can be multiple ways to solve this problem, but most of them require additional hardware support. The simplest and the most popular way to do this is by using Peterson’s Algorithm for mutual Exclusion. It was developed by Peterson in 1981 though the initial work in this direction was done by Theodorus Jozef Dekker who came up with Dekker’s algorithm in 1960, which was later refined by Peterson and came to be known as Peterson’s Algorithm.
Basically, Peterson’s algorithm provides guaranteed mutual exclusion by using only the shared memory. It uses two ideas in the algorithm:
- Willingness to acquire lock.
- Turn to acquire lock.
Prerequisite: Multithreading in C
Explanation:
The idea is that first a thread expresses its desire to acquire a lock and sets flag[self] = 1 and then gives the other thread a chance to acquire the lock. If the thread desires to acquire the lock, then, it gets the lock and passes the chance to the 1st thread. If it does not desire to get the lock then the while loop breaks and the 1st thread gets the chance.
Implementation:
C++
// Filename: peterson_spinlock.cpp
// Use below command to compile:
// g++ -pthread peterson_spinlock.cpp -o peterson_spinlock
#include <iostream>
#include <thread>
#include <mutex>
using namespace std;
int flag[2];
int turn;
const int MAX = 1e9;
int ans = 0;
void lock_init()
{
flag[0] = flag[1] = 0;
turn = 0;
}
void lock(int self)
{
flag[self] = 1;
turn = 1 - self;
while (flag[1 - self] == 1 && turn == 1 - self);
}
void unlock(int self)
{
flag[self] = 0;
}
void func(int self)
{
int i = 0;
cout << "Thread Entered: " << self << endl;
lock(self);
for (i = 0; i < MAX; i++)
ans++;
unlock(self);
}
int main()
{
thread t1(func, 0);
thread t2(func, 1);
lock_init();
t1.join();
t2.join();
cout << "Actual Count: " << ans << " | Expected Count: " << MAX * 2 << endl;
return 0;
}
// Note : To compile your code correctly, you need to link it with the pthread library.
//g++ -pthread peterson_spinlock.cpp -o peterson_spinlock
C
// Filename: peterson_spinlock.c
// Use below command to compile:
// gcc -pthread peterson_spinlock.c -o peterson_spinlock
#include <stdio.h>
#include <pthread.h>
#include"mythreads.h"
int flag[2];
int turn;
const int MAX = 1e9;
int ans = 0;
void lock_init()
{
// Initialize lock by resetting the desire of
// both the threads to acquire the locks.
// And, giving turn to one of them.
flag[0] = flag[1] = 0;
turn = 0;
}
// Executed before entering critical section
void lock(int self)
{
// Set flag[self] = 1 saying you want to acquire lock
flag[self] = 1;
// But, first give the other thread the chance to
// acquire lock
turn = 1-self;
// Wait until the other thread looses the desire
// to acquire lock or it is your turn to get the lock.
while (flag[1-self]==1 && turn==1-self) ;
}
// Executed after leaving critical section
void unlock(int self)
{
// You do not desire to acquire lock in future.
// This will allow the other thread to acquire
// the lock.
flag[self] = 0;
}
// A Sample function run by two threads created
// in main()
void* func(void *s)
{
int i = 0;
int self = (int *)s;
printf("Thread Entered: %d\n", self);
lock(self);
// Critical section (Only one thread
// can enter here at a time)
for (i=0; i<MAX; i++)
ans++;
unlock(self);
}
// Driver code
int main()
{
// Initialized the lock then fork 2 threads
pthread_t p1, p2;
lock_init();
// Create two threads (both run func)
pthread_create(&p1, NULL, func, (void*)0);
pthread_create(&p2, NULL, func, (void*)1);
// Wait for the threads to end.
pthread_join(p1, NULL);
pthread_join(p2, NULL);
printf("Actual Count: %d | Expected Count: %d\n",
ans, MAX*2);
return 0;
}
Java
// Filename: peterson_spinlock.cpp
// Use below command to compile:
// g++ -pthread peterson_spinlock.cpp -o peterson_spinlock
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class PetersonSpinlockMain {
// Shared variables for mutual exclusion
private static int[] flag = new int[2];
private static int turn;
private static final int MAX = (int) 1e9;
private static int ans = 0;
private static Lock mutex = new ReentrantLock();
// Initialize lock variables
private static void lockInit() {
flag[0] = flag[1] = 0;
turn = 0;
}
// Acquire lock
private static void lock(int self) {
flag[self] = 1;
turn = 1 - self;
// Spin until the other thread releases the lock
while (flag[1 - self] == 1 && turn == 1 - self);
}
// Release lock
private static void unlock(int self) {
flag[self] = 0;
}
// Function representing the critical section
private static void func(int self) {
int i;
System.out.println("Thread Entered: " + self);
lock(self); // Acquire the lock
for (i = 0; i < MAX; i++)
ans++;
unlock(self); // Release the lock
}
// Main method
public static void main(String[] args) throws InterruptedException {
// Create two threads
Thread t1 = new Thread(() -> func(0));
Thread t2 = new Thread(() -> func(1));
lockInit(); // Initialize lock variables
t1.start(); // Start thread 1
t2.start(); // Start thread 2
t1.join(); // Wait for thread 1 to finish
t2.join(); // Wait for thread 2 to finish
// Print the final count
System.out.println("Actual Count: " + ans + " | Expected Count: " + MAX * 2);
}
}
// This code is contributed by utkarsh
Python
import threading
flag = [0, 0]
turn = 0
MAX = 100000 # Reduced value
ans = 0
def lock_init():
global flag, turn
flag = [0, 0]
turn = 0
def lock(self):
global flag, turn
flag[self] = 1
turn = 1 - self
while flag[1 - self] == 1 and turn == 1 - self:
pass
def unlock(self):
global flag
flag[self] = 0
def func(self):
global ans
i = 0
print("Thread Entered:", self)
lock(self)
for i in range(MAX):
ans += 1
unlock(self)
def main():
t1 = threading.Thread(target=func, args=(0,))
t2 = threading.Thread(target=func, args=(1,))
lock_init()
t1.start()
t2.start()
t1.join()
t2.join()
print("Actual Count:", ans, "| Expected Count:", MAX * 2)
if __name__ == "__main__":
main()
C#
using System;
using System.Threading;
class Program
{
const int MAX = 1000000000; // Maximum count
static int ans = 0; // Shared variable to be incremented in the critical section
// Function representing the work to be done in each thread
static void Func(object obj)
{
int self = (int)obj; // Convert object to integer representing the thread index
Console.WriteLine("Thread Entered: " + self); // Print thread entry message
// Perform some work (incrementing a counter)
for (int i = 0; i < MAX; i++)
{
lock (typeof(Program)) // Acquire lock to ensure mutual exclusion
{
ans++;
}
}
}
static void Main(string[] args)
{
Thread t1 = new Thread(Func); // Create thread 1
Thread t2 = new Thread(Func); // Create thread 2
t1.Start(0); // Start thread 1
t2.Start(1); // Start thread 2
t1.Join(); // Wait for thread 1 to finish
t2.Join(); // Wait for thread 2 to finish
// Print actual count and expected count
Console.WriteLine("Actual Count: " + ans + " | Expected Count: " + (MAX * 2));
}
}
JavaScript
const flag = [0, 0];
let turn = 0;
const MAX = 1e9;
let ans = 0;
function lock_init() {
flag[0] = flag[1] = 0;
turn = 0;
}
function lock(self) {
flag[self] = 1;
turn = 1 - self;
while (flag[1 - self] === 1 && turn === 1 - self);
}
function unlock(self) {
flag[self] = 0;
}
async function func(self) {
let i = 0;
console.log("Thread Entered:", self);
lock(self);
for (i = 0; i < MAX; i++)
ans++;
unlock(self);
}
async function main() {
lock_init();
const promise1 = func(0);
const promise2 = func(1);
await Promise.all([promise1, promise2]);
console.log("Actual Count:", ans, "| Expected Count:", MAX * 2);
}
main();
//This code is contributed by Prachi
C++
#include <iostream>
#include <pthread.h>
using namespace std;
int flag[2];
int turn;
const int MAX = 1e9;
int ans = 0;
void lock_init()
{
flag[0] = flag[1] = 0;
turn = 0;
}
void lock(int self)
{
flag[self] = 1;
turn = 1 - self;
while (flag[1 - self] == 1 && turn == 1 - self);
}
void unlock(int self)
{
flag[self] = 0;
}
void* func(void* s)
{
int i = 0;
int self = (int)s;
cout << "Thread Entered: " << self << endl;
lock(self);
for (i = 0; i < MAX; i++)
ans++;
unlock(self);
return nullptr;
}
int main()
{
pthread_t p1, p2;
lock_init();
pthread_create(&p1, nullptr, func, (void*)0);
pthread_create(&p2, nullptr, func, (void*)1);
pthread_join(p1, nullptr);
pthread_join(p2, nullptr);
cout << "Actual Count: " << ans << " | Expected Count: " << MAX * 2 << endl;
return 0;
C
// mythread.h (A wrapper header file with assert
// statements)
#ifndef __MYTHREADS_h__
#define __MYTHREADS_h__
#include <pthread.h>
#include <assert.h>
#include <sched.h>
void Pthread_mutex_lock(pthread_mutex_t *m)
{
int rc = pthread_mutex_lock(m);
assert(rc == 0);
}
void Pthread_mutex_unlock(pthread_mutex_t *m)
{
int rc = pthread_mutex_unlock(m);
assert(rc == 0);
}
void Pthread_create(pthread_t *thread, const pthread_attr_t *attr,
void *(*start_routine)(void*), void *arg)
{
int rc = pthread_create(thread, attr, start_routine, arg);
assert(rc == 0);
}
void Pthread_join(pthread_t thread, void **value_ptr)
{
int rc = pthread_join(thread, value_ptr);
assert(rc == 0);
}
#endif // __MYTHREADS_h__
Java
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class PetersonSpinlockThread {
// Shared variables for mutual exclusion
private static int[] flag = new int[2];
private static int turn;
private static final int MAX = (int) 1e9;
private static int ans = 0;
private static Lock mutex = new ReentrantLock();
// Initialize lock variables
private static void lockInit() {
flag[0] = flag[1] = 0;
turn = 0;
}
// Acquire lock
private static void lock(int self) {
flag[self] = 1;
turn = 1 - self;
// Spin until the other thread releases the lock
while (flag[1 - self] == 1 && turn == 1 - self);
}
// Release lock
private static void unlock(int self) {
flag[self] = 0;
}
// Function representing the critical section
private static void func(int self) {
int i = 0;
System.out.println("Thread Entered: " + self);
lock(self); // Acquire the lock
for (i = 0; i < MAX; i++)
ans++;
unlock(self); // Release the lock
}
// Main method
public static void main(String[] args) throws InterruptedException {
// Create two threads
Thread t1 = new Thread(() -> func(0));
Thread t2 = new Thread(() -> func(1));
lockInit(); // Initialize lock variables
t1.start(); // Start thread 1
t2.start(); // Start thread 2
t1.join(); // Wait for thread 1 to finish
t2.join(); // Wait for thread 2 to finish
// Print the final count
System.out.println("Actual Count: " + ans + " | Expected Count: " + MAX * 2);
}
}
JavaScript
const flag = [0, 0];
let turn = 0;
const MAX = 1e9;
let ans = 0;
function lock_init() {
flag[0] = flag[1] = 0;
turn = 0;
}
function lock(self) {
flag[self] = 1;
turn = 1 - self;
while (flag[1 - self] === 1 && turn === 1 - self);
}
function unlock(self) {
flag[self] = 0;
}
async function func(self) {
let i = 0;
console.log("Thread Entered:", self);
lock(self);
for (i = 0; i < MAX; i++)
ans++;
unlock(self);
}
async function main() {
lock_init();
const promise1 = func(0);
const promise2 = func(1);
await Promise.all([promise1, promise2]);
console.log("Actual Count:", ans, "| Expected Count:", MAX * 2);
}
main();
//This code is contribuited by Prachi.
Python3
import threading
# Shared variables for mutual exclusion
flag = [0, 0]
turn = 0
MAX = int(1e9)
ans = 0
mutex = threading.Lock()
# Initialize lock variables
def lock_init():
global flag, turn
flag = [0, 0]
turn = 0
# Acquire lock
def lock(self):
global flag, turn
flag[self] = 1
turn = 1 - self
# Spin until the other thread releases the lock
while flag[1 - self] == 1 and turn == 1 - self:
pass
# Release lock
def unlock(self):
global flag
flag[self] = 0
# Function representing the critical section
def func(self):
global ans
i = 0
print(f"Thread Entered: {self}")
with mutex:
lock(self) # Acquire the lock
for i in range(MAX):
ans += 1
unlock(self) # Release the lock
# Main method
def main():
# Create two threads
t1 = threading.Thread(target=lambda: func(0))
t2 = threading.Thread(target=lambda: func(1))
lock_init() # Initialize lock variables
t1.start() # Start thread 1
t2.start() # Start thread 2
t1.join() # Wait for thread 1 to finish
t2.join() # Wait for thread 2 to finish
# Print the final count
print(f"Actual Count: {ans} | Expected Count: {MAX * 2}")
if __name__ == "__main__":
main()
Output:
Thread Entered: 1
Thread Entered: 0
Actual Count: 2000000000 | Expected Count: 2000000000
The produced output is 2*109 where 109 is incremented by both threads.
If you like neveropen and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the neveropen main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
Share your thoughts in the comments
