The compare_and_swap() instruction (CAS) can be used to design lock-free data structures such as stacks, queues, and lists. The program example shown below presents a possible solution to a lock-free stack using CAS instructions, where the stack is represented as a linked list of Node elements with top representing the top of the stack. Is this implementation free from race conditions?

Question

typedef struct node
{
value_t data;
struct node *next;
}

Node;
Node *top; // top of stack

void
push (value_t item)
{
Node *old_node;
Node *new_node;
new_node = malloc (sizeof (Node));
new_node->data = item;

do
{
old_node = top;
new_node->next = old_node;
}

while (compare_and_swap (top, old_node, new_node) != old_node);
}

value_t
pop ()
{
Node *old_node;
Node *new_node;

do
{
old_node = top;
if (old_node == NULL)
return NULL;
new_node = old_node->next;
}

while (compare_and_swap (top, old_node, new_node) != old_node);
return old_node->data;
}

Answer 1

I don't think it is free from race conditions