No. There are usually talking about 2 threads accessing the same code.

Let's suppose we have an int, and want to add 2, we would do:

1. int i = sharedInt

2. i = i + 2

3. sharedInt = i

(that's what the compile actually does when you write sharedInt += 2).

If the runtime decides two stop the current thread on line 2 and lets the second one run all three lines, we would have a problem if I didn't use a mutex.