source: CIVL/examples/concurrency/two_lock_queue.cvl

/* two_lock_queue.cvl: a "Two-Lock Concurrent
 * Queue Algorithm", from Michael and Scott,
 * https://www.cs.rochester.edu/research/synchronization/pseudocode/queues.html.
 * Originally from "Simple, Fast, and
 * Practical Non-Blocking and Blocking
 * Concurrent Queue Algorithms", PODC96.
 */
#include <civlc.cvh>
#include <stdbool.h>
#include <stdlib.h>
#include <assert.h>

typedef int lock_t;
#define FREE 0
#define lock(l) $when (l==0) l=1;
#define unlock(l) l=0;

typedef struct node_t {
  int value;
  struct node_t *next;
} node_t;

typedef struct queue_t {
  node_t *Head;
  node_t *Tail;
  lock_t H_lock;
  lock_t T_lock;
} queue_t;

void initialize(queue_t *Q) {
  node_t *node = (node_t*)malloc(sizeof(node_t));

  node->next = NULL;            // Make it the only node in the linked list
  Q->Head = Q->Tail = node;     // Both Head and Tail point to it
  Q->H_lock = Q->T_lock = FREE; // Locks are initially free
}
  
void enqueue(queue_t *Q, int value) {
  node_t *node = (node_t*)malloc(sizeof(node_t)); // in root scope
  
  node->value = value;          // Copy enqueued value into node
  node->next = NULL;            // Set next pointer of node to NULL
  lock(Q->T_lock);              // Acquire T_lock in order to access Tail
  Q->Tail->next = node;         // Link node at the end of the linked list
  Q->Tail = node;               // Swing Tail to node
  unlock(Q->T_lock);            // Release T_lock
}
 
_Bool dequeue(queue_t *Q, int *pvalue) {
  node_t *node, *new_head;

  lock(Q->H_lock);              // Acquire H_lock in order to access Head
  node = Q->Head;               // Read Head
  new_head = node->next;        // Read next pointer
  if (new_head == NULL) {       // Is queue empty?
    unlock(Q->H_lock);          // Release H_lock before return
    return false;               // Queue was empty
  }
  *pvalue = new_head->value;    // Queue not empty.  Read value before release
  Q->Head = new_head;           // Swing Head to next node
  unlock(Q->H_lock);            // Release H_lock
  free(node);                   // Free node
  return true;                  // Queue was not empty, dequeue succeeded
}

void test1() {
  queue_t queue;
  int x;

  initialize(&queue);
  enqueue(&queue, 99);
  dequeue(&queue, &x);
  free(queue.Head);
  assert(x==99);
}

void test2() {
  queue_t q;
  $proc t0, t1;
  void thread(int val) { enqueue(&q, val); }
  int x, y;
  _Bool result;
  
  initialize(&q);
  t0 = $spawn thread(6);
  t1 = $spawn thread(7);
  $wait(t0);
  $wait(t1);
  result = dequeue(&q, &x);
  assert(result);
  result = dequeue(&q, &y);
  assert(result);
  assert ((x==6 && y==7) || (x==7 && y==6));
  free(q.Head);
}

void main() {
  test2();
}