Question

Compile and run the deadlockFree.c program below and report if this program does go into deadlock or not.

Modify this deadlockFree.c program so that you change the lock/unlock order of the mutexes. Experiment with the order of locks (and maybe extend the number of loop iterations) until you find a scenario where you can demonstrate deadlock.

//Program: deadlockFree.c A simple threaded program to demonstrate the deadlock condition.Two threads are used to wait on two MUTEXES. Careful ordering of locks should prevent deadlock. Try this and see does deadlock occur.Play with lock order to see if you can achieve deadlock!

#include<stdio.h>

#include<pthread.h>

#include<stdlib.h>

//Function declarations

void *functionA() ;

void *functionB() ;

int var_A, var_B = 0 ; // The shared variables

//Declare mutex

pthread_mutex_t   mutex1 = PTHREAD_MUTEX_INITIALIZER;

pthread_mutex_t   mutex2 = PTHREAD_MUTEX_INITIALIZER;

// MAIN

int main() /*MODIFICATION by "int" type data*/

{

    pthread_t   thread1, thread2 ;

   

    // declare 2 threads

   

    int   rc1, rc2 ;

   

    // Message to the terminal

   

    printf ("\033[2J") ;

   

    // Clear the screen

   

printf ("Deadlock experiment with MUTEX. Does this program lead to deadlock?\n") ;

printf ("=============================================================\n\n") ;

// Create first thread

if ((rc1=pthread_create(&thread1, NULL, &functionA, NULL)))

    {

        printf("Error in creating %d\n",rc1) ;

       

    }

       

        // Create next thread

if ((rc2=pthread_create(&thread2, NULL, &functionB, NULL)))

    {

        printf("Error in creating %d\n",rc2) ;

           

    }

// Remember to use the thread joins

       

pthread_join( thread1, NULL) ;

pthread_join( thread2, NULL) ;

       

pthread_mutex_destroy(&mutex1) ;

pthread_mutex_destroy(&mutex2) ;

       

// main exits

exit(0) ;

   

}

// Thread 1

void *functionA()

{

int   x = 0 ;

// interation counter for the While loop

while (x < 2)

    {

        pthread_mutex_lock(&mutex1) ;

        printf ("I'm T1, just locked mutex1\n") ;

        pthread_mutex_lock(&mutex2) ;

        printf ("I'm T1, just locked mutex2\n") ;

       

        var_A++ ;

        var_B++ ;

       

             

        printf("I am Thread 1: var_A = %d/*/*, var_B = %d*/*/ \n", var_A, var_B) ;

        pthread_mutex_unlock(&mutex2) ;

        printf ("I'm T1, just freed mutex2\n") ;

        pthread_mutex_unlock(&mutex1) ;

        printf ("I'm T1, just freed mutex1\n") ;

        x++ ;

       

    }

pthread_exit (NULL) ; // Thread exits

   

}

// Thread 2

void *functionB() {

int x = 0 ;

// interation counter

while (x < 2)

    {

   

    pthread_mutex_lock(&mutex1) ;

    printf ("I'm T2, just locked mutex1\n") ;

    pthread_mutex_lock(&mutex2) ;

    printf ("I'm T2, just locked mutex2\n") ;

    var_A++ ;var_B++ ;

   printf("I am Thread 2: var_A = %d, var_B = %d \n", var_A, var_B) ;

    pthread_mutex_unlock(&mutex2) ;

    printf ("I'm T2, just freed mutex2\n") ;

    pthread_mutex_unlock(&mutex1) ;

    printf ("I'm T2, just freed mutex1\n") ;

    x++ ;

       

    }

pthread_exit (NULL) ; // Thread exits

   

}

Answer 1

The given code does not and cannot go into a deadlock because both threads lock the mutex in the same order, mutex1, followed by mutex2.

In order for a deadlock to be possible, the threads must lock the mutex in different orders. The deadlock occurs when thread1 has locked mutex1 and is waiting for mutex2, while thread2 has locked mutex2 and is waiting for mutex1. Even with the changed order, a deadlock can occur, but is not guaranteed to occur. Increasing the loop count will increase the chances of getting a deadlock.

In the case here, a loop value of 100 caused a deadlock in the 2nd attempt. The modified code is given below followed by the output snapshot when the deadlock occurred.

//Program: deadlockFree.c A simple threaded program to demonstrate the deadlock condition.Two threads are used to wait on two MUTEXES. Careful ordering of locks should prevent deadlock. Try this and see does deadlock occur.Play with lock order to see if you can achieve deadlock!

#include<stdio.h>
#include<pthread.h>
#include<stdlib.h>

//Function declarations
void *functionA() ;
void *functionB() ;

int var_A, var_B = 0 ; // The shared variables

//Declare mutex
pthread_mutex_t   mutex1 = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t   mutex2 = PTHREAD_MUTEX_INITIALIZER;

// MAIN
int main() /*MODIFICATION by "int" type data*/
{
    pthread_t   thread1, thread2 ;

    // declare 2 threads
    int   rc1, rc2 ;

    // Message to the terminal
    printf ("\033[2J") ;

    // Clear the screen
    printf ("Deadlock experiment with MUTEX. Does this program lead to deadlock?\n") ;
    printf ("=============================================================\n\n") ;
    // Create first thread
    if ((rc1=pthread_create(&thread1, NULL, &functionA, NULL)))
    {
        printf("Error in creating %d\n",rc1) ;
    }

    // Create next thread
    if ((rc2=pthread_create(&thread2, NULL, &functionB, NULL)))
    {
        printf("Error in creating %d\n",rc2) ;
    }
    // Remember to use the thread joins

    pthread_join( thread1, NULL) ;
    pthread_join( thread2, NULL) ;

    pthread_mutex_destroy(&mutex1) ;
    pthread_mutex_destroy(&mutex2) ;

    // main exits
    exit(0) ;
}

// Thread 1
void *functionA()
{
    int   x = 0 ;
    // interation counter for the While loop
    while (x < 100)
    {
        pthread_mutex_lock(&mutex1) ;
        printf ("I'm T1, just locked mutex1\n") ;
        pthread_mutex_lock(&mutex2) ;
        printf ("I'm T1, just locked mutex2\n") ;

        var_A++ ;
        var_B++ ;

        printf("I am Thread 1: var_A = %d/*/*, var_B = %d*/*/ \n", var_A, var_B) ;
        pthread_mutex_unlock(&mutex2) ;
        printf ("I'm T1, just freed mutex2\n") ;
        pthread_mutex_unlock(&mutex1) ;
        printf ("I'm T1, just freed mutex1\n") ;
        x++ ;
    }
    pthread_exit (NULL) ; // Thread exits
}

// Thread 2
void *functionB() {
    int x = 0 ;
    // interation counter
    while (x < 100)
    {
        pthread_mutex_lock(&mutex2) ;
        printf ("I'm T2, just locked mutex2\n") ;
        pthread_mutex_lock(&mutex1) ;
        printf ("I'm T2, just locked mutex1\n") ;
        var_A++ ;var_B++ ;
        printf("I am Thread 2: var_A = %d, var_B = %d \n", var_A, var_B) ;
        pthread_mutex_unlock(&mutex1) ;
        printf ("I'm T2, just freed mutex1\n") ;
        pthread_mutex_unlock(&mutex2) ;
        printf ("I'm T2, just freed mutex2\n") ;
        x++ ;
    }
    pthread_exit (NULL) ; // Thread exits
}