Many modern computer systems provide special hardware instructions that allow us

Question

Many modern computer systems provide special hardware instructions that allow us either to test and modify the content of a word or to swap the contents of two words atomically. The following shows the hardware instruction compare_and_swap()and to use it in a code to solve a critical section problem. int compare_and_swap(int *value, int expected, int new_value) { int temp = *value; if (*value == expected) *value = new_value; return temp; } int lock = 0; int main(){ ... while (compare_and_swap(&lock, 1, 0) != 0) /* Line P */ ; /* do nothing */ /* critical section */ lock = 0; /* remainder section */ } But the above code fails to solve the critical section problem properly. Explain how it fails by showing a scenario step by step

Explanation / Answer

int compare_and_swap(int *value, int expected, int new_value)
{
        int temp = *value;
        if (*value == expected)
                *value = new_value;
        return temp;
}
int lock = 0;
int main()
{
        while (compare_and_swap(&lock, 1, 0) != 0)
        /* Line P */ ;
        /* do nothing */
        /* critical section */
        lock = 0;
        /* remainder section */
}

code analysis:
step 1:
   lock = 0
       int compare_and_swap(int *value, int expected, int new_value)
       {
               int temp = *value;//temp = 0
               if (*value == expected) //*value != 1
                   *value = new_value;   //swap didn't happen
               return temp;       //return 0
       }
now 0 ! =0 results in false in while loop.
      
this code spins endlessly in the while loop probably the correct implementation would be

do { while (compare_and_swap(&lock, 0, 1) != 0)
; /* do nothing */
/* critical section */
lock = 0;
/* remainder section */
} while (true);

int compare and swap(int *value, int expected, int new_value) {
int temp = *value;
if (*value == expected)
*value = new_value;
return temp;
}

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