Question

• Code where a process is changing shared data memory P1 X = X + 2; X 5 X = X + 3;

P1 needs to add 2 to X

P2 needs to add 3 to X

P1 reads X (5)

P2 reads X (5)

P1 adds 2 to X and stores 7

P2 adds 3 to X and stores 8

Should X be 7, 8 or 10?

Answer 1

The order of the reads and writes affects the value stored.
P1 and P2 both read the same value of X(5) since the reads occur before the writes and value of X remains unchanged. P1 writes X as 7 but since P2 stores the value of X after P1, the value stored in X by P1 will be overwritten by P2 and 8 will be stored.
So, the final value in X is 8.