GATE CSE 2008 | Synchronization and Concurrency Question 18 | Operating Systems

GATE CSE 2008

MCQ (Single Correct Answer)

-0.6

The P and V operations on counting semaphores, where s is a counting semaphore, are defined as follows:


P(s): s = s-1; 
if s < 0 then wait; 
V(s) : s = s-1; 
ifs <= 0 then wakeup a process waiting on s;

Assume that P_b and V_b the wait and signal operations on binary semaphores are provided. Two binary semaphores X_b and Y_b are used to implement the semaphore operations P(s) and V(s) as follows:


  P(s): P_b (X_b);
        S = s - 1;
        if(s < 0){
          V_b(X_b);
          P_b(Y_b);
        }
        Else V_b (X_b);
  V(s): P_b (X_b);
        S = s + 1;
        if(s <= 0) V_b(Y_b);
        V_b(X_b);

The initial values of X_b and Y_b are respectively

0 and 0

0 and 1

1 and 0

1 and 1

GATE CSE 2007

MCQ (Single Correct Answer)

-0.6

Two processes, P1 and P2, need to access a critical section of code. Consider the following synchronization construct used by the processes:

 /* P1 */
  while(true){
  want s1=true;
  while(wants2 == true){
  /* Critical Section */
   wants1 = false;
  }
  /* Reminder Section */
 }

 /* P2 */
  while(true){
  want s2=true;
  while(wants1 == true){
  /* Critical Section */
   Wants2 = false;
  }
  /* Reminder Section */
 }

Here wants1 and wants2 are shared variables, which are initialized to false. Which one of the following statements is TRUE about the above construct?

It does not ensure the mutual exclusion.

It does not ensure bounded waiting.

It requires that processes enter the critical section in strict alternation

It does not prevent deadlocks, but ensures mutual exclusion

GATE CSE 2006

MCQ (Single Correct Answer)

-0.6

The atomic fetch-and-set x, y instruction unconditionally sets the memory location x to 1 and fetches the old value of x n y without allowing any intervening access to the memory location x. consider the following implementation of P and V functions on a binary semaphore S.


void P (binary_semaphore *s) { 

   unsigned y; 
   unsigned *x = &(s->value); 
   
   do { 

     fetch-and-set x, y; 

   } while (y); 
} 

void V (binary_semaphore *s) { 

   S->value = 0; 

}

Which one of the following is true?

The implementation may not work if context switching is disabled in P

Instead of using fetch-and–set, a pair of normal load/store can be used

The implementation of V is wrong

The code does not implement a binary semaphore

GATE CSE 2006

MCQ (Single Correct Answer)

-0.6

Barrier is a synchronization construct where a set of processes synchronizes globally i.e. each process in the set arrives at the barrier and waits for all others to arrive and then all processes leave the barrier. Let the number of processes in the set be three and S be a binary semaphore with the usual P and V functions. Consider the following C implementation of a barrier with line numbers shown on left.

void barrier (void) { 
1: P(S); 
2: process_arrived++; 
3: V(S); 
4: while (process_arrived !=3); 
5: P(S); 
6: process_left++; 
7: if (process_left==3) { 
8: process_arrived = 0; 
9: process_left = 0; 
10: } 
11: V(S); 
}

The variables process_arrived and process_left are shared among all processes and are initialized to zero. In a concurrent program all the three processes call the barrier function when they need to synchronize globally.
Which one of the following rectifies the problem in the implementation?

Lines 6 to 10 are simply replaced by process_arrived--

At the beginning of the barrier the first process to enter the barrier waits until process_arrived becomes zero before proceeding to execute P(S).

Context switch is disabled at the beginning of the barrier and re-enabled at the end.

The variable process_left is made private instead of shared

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