GATE - 2012 | OS | Fetch_And_Add(X,i) is an atomic Read-Modify-Write instruction that reads the

GATE - 2012 [Operating System]

Question:

Fetch_And_Add(X,i) is an atomic Read-Modify-Write instruction that reads the value of memory location X, increments it by the value i, and returns the old value of X. It is used in the pseudocode shown below to implement a busy-wait lock. L is an unsigned integer shared variable initialized to 0. The value of 0 corresponds to lock being available, while any non-zero value corresponds to the lock being not available.

  AcquireLock(L){

         while (Fetch_And_Add(L,1))

               L = 1;

   }

  ReleaseLock(L){

         L = 0;

   }

This implementation

A	fails as L can overflow
B	fails as L can take on a non-zero value when the lock is actually available
C	works correctly but may starve some processes
D	works correctly without starvation

  

Solution:

Option (B) is Correct.

Check the loop first:
while (Fetch_And_Add (L,1))
L = 1; // A waiting process can be here just after
// the lock is released, and can make L = 1.
Assume P1 executes until while condition and preempts before executing L =1. Now P2 executes all statements, hence L = 0. Then P1 without checking L it makes L = 1 by executing the statement where it was preempted.
It takes a non-zero value (L=1) when the lock is actually available (L = 0). So option B.

Thank You