Question

Consider a banking system that maintains an account balance with two functions: deposit (amount) and withdraw (amount). These two functions are passed the amount that is to be deposited or withdrawn from the bank account balance.
Assume that a husband and wife share a bank account. Concurrently, the husband calls withdraw ( ) function and the wife calls deposit ( ).
Describe how a race condition is possible and what might be done to prevent the race condition from occurring?

Answer 1

Race condition is possible if husband and wife uses the above functions [ withdraw(), deposit() ] concurrently.

For example:

If the balance =5000 and husband withdraws = 1000 ,then balance = 4000.

And balance =5000 and wife deposits =1000 , then balance=6000.

We can clearly see an inconsistency, which is not acceptable.To solve this problem we apply Peterson's Solution (to prevent race condition),

Here Flags can be used for husband and wife accessing the bank account.

int turn=0;

boolean flag=false;

User 1: Husband

do{

flag[i]=TRUE;

turn=(i+1)%2

while(flag[(i+1)%2] && turn=(i+1)%2);

withdraw();

flag[i]=FALSE;

.....(remainder section)

} while(TRUE);

This process can be repeated with User2 (Wife) wherein i will be replaced by j,i=0 or 1 and j=1 or 0.

• The turn indicates which user is going to access the bank account next.
• The flags make sure the events of 2 users accessing the software are mutually exclusive

Hence the Race Condition is prevented from occurring.