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Racing adventure in OS


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)
    var = 1;

void task2(void)
    var = 2;

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.

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