race condition in concurrent programming code example

Example: race condition

A race condition occurs when two or more threads can access
shared data and they try to change it at the same time.
Because the thread scheduling algorithm can swap between 
threads at any time, you don't know the order in which the 
threads will attempt to access the shared data.
if (x == 5) // The "Check"
{
   y = x * 2; // The "Act"

   // If another thread changed x in between "if (x == 5)" and "y = x * 2" above,
   // y will not be equal to 10.
}
In order to prevent race condition,we need to obtain lock on shared variable.