Operating Systems | Set 17

1.8

Following question has been asked in GATE 2012 CS exam.

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

Answer (B)
Take closer look the below while loop.

     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.

Consider a situation where a process has just released the lock and made L = 0. Let there be one more process waiting for the lock, means executing the AcquireLock() function. Just after the L was made 0, let the waiting processes executed the line L = 1. Now, the lock is available and L = 1. Since L is 1, the waiting process (and any other future coming processes) can not come out of the while loop.

The above problem can be resolved by changing the AcuireLock() to following.

  AcquireLock(L){
         while (Fetch_And_Add(L,1))
         { // Do Nothing }
   }

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Please write comments if you find any of the answers/explanations incorrect, or you want to share more information about the topics discussed above.

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