Shared Variables: freelist, list-1, list-2: block

Semaphores: sem_freelist = N, mut_freelist = 1,
            sem_freelist_minus_1 = N-1,
            sem_list1 = 0,    mut_list1 = 1,
            sem_list2 = 0,    mut_list2 = 1;

Thread-1:
var b: pointer to type block;
while (1)
{

//    wait for >1 block on freelist
   wait(sem_freelist_minus_1);
//    wait for element on freelist
   wait(sem_freelist);
//    mutual exclusion for freelist
   wait(mut_freelist);

   b := unlink(freelist);
//    mutual exclusion for freelist
   signal(mut_freelist);

   produce_information_in_block(b);

//    mutual exclusion for list1
   wait(mut_list1);

   link(b, list1);
//    mutual exclusion for list1
   signal(mut_list1);
//    signal new element on list1
   signal(sem_list1);

}

Thread-2:
var x, y: pointer to type block;
while (1)
{

//    wait for element on list1
   wait(sem_list1);
//    mutual exclusion for list1
   wait(mut_list1);

   x := unlink(list-1);

//    mutual exclusion for list1
   signal(mut_list1);

//    wait for element on freelist
   wait(sem_freelist);
//    mutual exclusion for freelist
   wait(mut_freelist);

   y := unlink(freelist);

//    mutual exclusion for freelist
   signal(mut_freelist);

   use_block_x_to_produce_info_in_y(x, y);

//    mutual exclusion for freelist
   wait(mut_freelist);
//*    link(x, freelist);
   link(x, freelist);
//    mutual exclusion for freelist
   signal(mut_freelist);
//    signal new element on freelist
   signal(sem_freelist);

//    mutual exclusion for list2
   wait(mut_list2);

   link(y, list-2);

//    mutual exclusion for list2
   signal(mut_list2);
//    signal new element on list2
   signal(sem_list2);

}

Thread-3:
var c: pointer to type block;
while (1)
{

//    wait for element on list2
   wait(sem_list2);
//    mutual exclusion for list2
   wait(mut_list2);

   c := unlink(list-2);

//    mutual exclusion for list2
   signal(mut_list2);

   consume_information_in_block(c);

//    mutual exclusion for freelist
   wait(mut_freelist);

   link(c, freelist);

//    mutual exclusion for freelist
   signal(mut_freelist);
//    signal Thread-1 to continue
   signal(sem_freelist_minus_1);
//    signal new element on freelist
   signal(sem_freelist);

}