Race condition in multithreading: Why to use lock mechanism for shared memory ?

Race condition is race between the users to access some resource.

Example: 
Two thread with no synchronisation or no locking mechanism for variable access, can end up in race condition.

thread1:

if (x == 5) {
    y = x * 2;
}

thread2:
x = 6;

Here problem occurs when check condition in thread1 is done, and then thread2 occurs which chnage x, then the result in y can be different(12) than expected(10).

To solve this kind of problem, Memory lock mechanism, Thread synchronisation, Interprocess ommunication etc. concepts are used.