Racing adventure in OS

Hi,Master:

Question 1:

As shown in the figure below, why does the api read this variable with critical protection?

Question 2:

Suppose two tasks have write operations to shared variables. For example:

int var;
void task1(void)
{
    OS_ENTER_CRITICAL();
    var = 1;
    OS_EXIT_CRITICAL();
}

void task2(void)
{
    OS_ENTER_CRITICAL();
    var = 2;
    OS_EXIT_CRITICAL();
}

In the OS source code, you can see that a critical section must be added to the shared variable write operation. There is no read-modify-write, and it should not cause a race condition. Why do you need to add a critical section?

Question 3:

If the compiler has done level optimization, there may be a write cache for variable writes. In this case, should the DSB data synchronization barrier be increased after the variable is written in the critical section? I don’t know when to use DSB, ISB, etc. instructions

Thank you very much.