Synchronization

T1                  T2
a = a + 1;          b = 2 * b;
b = b + 1;          a = 2 * a;

Single correct

• A(a = 4, b = 4); (a = 3, b = 3); (a = 4, b = 3)
• B(a = 3, b = 4); (a = 4, b = 3); (a = 3, b = 3)
• C(a = 4, b = 4); (a = 4, b = 3); (a = 3, b = 4)
• D(a = 2, b = 2); (a = 2, b = 3); (a = 3, b = 4)

1.  int counter = 0;
2.  Semaphore S = init(5);
3.  void parop(void)
4.  {
5.      wait(S);
6.      wait(S);
7.      counter++;
8.      signal(S);
9.      signal(S);
10. }

One or more correct

• AThe value of counter is 5 after all the threads successfully complete the execution of parop.
• BThe value of counter is 1 after all the threads successfully complete the execution of parop.
• CThe value of counter is 0 after all the threads successfully complete the execution of parop.
• DThere is a deadlock involving all the threads.

Answer key coming soon

do {
     c[i]=1; t[i] = pmax(t[0],...,t[n-1])+1; c[i]=0;
     for every j ≠ i in {0,...,n-1} {
          while (c[j]);
          while (t[j] != 0 && t[j]<=t[i]);
     }
     Critical Section;
     t[i]=0;
     Remainder Section;
} while (true);

Single correct

• AAt most one process can be in the critical section at any time
• BThe bounded wait condition is satisfied
• CThe progress condition is satisfied
• DIt cannot cause a deadlock

Answer key coming soon

P1( ) {                 P2( ) {
    C = B - 1;              D = 2 * B;
    B = 2 * C;              B = D - 1;
}                       }

Numerical answer