Tuesday, April 15, 2014

Scheduling method calls in sequence

Problem

Suppose we have the following code:
class Foo {
public:
    A(.....); // If A is called, a new thread will be created 
                // and the corresponding function will be executed.
    B(.....); // same as above
    C(.....); // same as above
}
Foo f;
f.A(.....);
f.B(.....);
f.C(.....);

i) Can you design a mechanism to make sure that B is executed after A, and C is executed after B?
ii) Suppose we have the following code to use class Foo We do not know how the threads will be scheduled in the OS:
Foo f;
f.A(.....);
f.B(.....);
f.C(.....);
f.A(.....);
f.B(.....);
f.C(.....);

Can you design a mechanism to make sure that all the methods will be executed in sequence?

Solution

i) Can you design a mechanism to make sure that B is executed after A, and C is executed after B?
Semaphore s_a(0);
Semaphore s_b(0);
A {
    //
    s_a.release(1);
}
B {
    s_a.acquire(1); 
    //
    s_b.release(1);
}
C {
    s_b.acquire(1);
    //
}

ii) Can you design a mechanism to make sure that all the methods will be executed in sequence?
Semaphore s_a(0);
Semaphore s_b(0);
Semaphore s_c(1);
A{
    s_c.acquire(1); 
    // 
    s_a.release(1);
}
B{
    s_a.acquire(1); 
    // 
    s_b.release(1);
}
C{
    s_b.acquire(1); 
    // 
    s_c.release(1);
}

Thanks

References

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