Question

I have been asked to implement a lock free queue in c using compare and exchange, however my knowledge of pointers is fairly limited.

I have been using the following code to test my (as yet incomplete) dequeue implementation, but I believe it is looping infinitely because I’m not terribly sure how to properly use pointers/address of operator.

I have been given this CAS function to use, since I know nothing of assembler.

long __cdecl compare_exchange(long *flag, long oldvalue, long newvalue)
{
    __asm
    {
        mov ecx, flag
        mov eax, oldvalue
        mov ebx, newvalue
        lock cmpxchg [ecx], ebx
        jz iftrue
    }
    return 0;
    iftrue: return 1;
}

My current (relevant) code is as follows…

typedef struct QueueItem
{
    int data;
    struct QueueItem* next;
}item;

struct Queue
{
    item *head;
    item *tail;
}*queue;

int Dequeue()
{
    item *head;

    do
    {
        head = queue->head;
        if(head == NULL)
            return NULL_ITEM;
        printf("%d, %d, %d\n", (long *)queue->head, (long)&head, (long)&head->next);
    }
    while(!compare_exchange((long *)queue->head, (long)&head, (long)&head->next)); // Infinite loop.

    return head->data;
}

int main(int argc, char *argv[])
{
    item i, j;

    queue = (struct Queue *) malloc(sizeof(struct Queue));

    // Manually enqueue some data for testing dequeue.
    i.data = 5;
    j.data = 10;
    i.next = &j;
    j.next = NULL;

    queue->head = &i;

    printf("Dequeued: %d\n", Dequeue());
    printf("Dequeued: %d\n", Dequeue());
}

Should I be using the not operator inside of the do while loop? If I don’t use that operator, I get an output of “Dequeued 5” x2, which suggests the swap isn’t happening and that I should use the not. If so, where am I going wrong? I’d put money on it being a pointer/address operator issue.

Answer 1

There is confusion on the pointers and values. Here is corrected code:

 do
 {
    head = queue->head;
    if(head == NULL)
        return 0;
    printf("%d, %d, %d %d\n", (long *)queue->head, (long)head, (long)head->next, head->data);
  }  while (!compare_exchange((long *)&queue->head,   (long)head, (long)head->next));

You were trying to write what queue->head was pointing to, not the value of queue->head itself.

In addition, to make it work properly on multicore, I believe you need to define the head as volatile.

struct Queue
{
   volatile item *head;
   item *tail;
}*queue;