Question

In some cases we need to be sure that some section of code won’t be used by one thread while another thread is in this section. Using languages that support multi-threading it’s easy to achieve with “locking” these parts. But when we have to “simulate” threads there’s not built-in thing like lock keyword in C# or Lock interface in Java. The best way I’ve found to lock sections in these cases look likes this:

if (!locked){
    locked = true;

    do some stuff

    locked = false;
} else {
    add to queue
}

What are the drawbacks of current solution? Does it worth to actively use one?

Answer 1

Maybe two threads can enter in the if statement.

Explanation

If T1 entered in the if statement and doesn’t change the locked value. Suddenly, if the CPU change T1 for T2, locked value will continue false. Then T2 will enter into if statement too.

T1 -> !locked   -> Stopped -> Waiting -> ... //locked value still false and T2
T2 ->  Waiting  ->  Waked  -> !locked -> ... //will be able to enter into if

You should take a look at Semaphores and Mutexes and If you will only use two process you can use the Peterson’s Algorithm, but it still have a busy wait to deal!