first commit

lock_free_queue.c

@@ -0,0 +1,84 @@
+#include <string.h>
+#include <assert.h>
+#include <stdatomic.h>
+#include "lock_free_queue.h"
+
+void
+lock_free_queue_init(struct lock_free_queue *q, 
+                     void *arrptr, int arrlen,
+                     int cell)
+{
+    assert(arrlen % cell == 0);
+
+    q->head = 0;
+    q->tail = 0;
+    q->temp = 0;
+    q->size = arrlen / cell;
+    q->base = arrptr;
+    q->cell = cell;
+}
+
+static void*
+item_addr(struct lock_free_queue *q, int index)
+{
+    assert(index >= 0 && index < q->size);
+    return q->base + q->cell * index;
+}
+
+static int
+acquire_push_location(struct lock_free_queue *q)
+{
+    int old_temp;
+    int new_temp;
+    do {
+        old_temp = q->temp;
+        new_temp = (old_temp + 1) % q->size;
+    } while (!atomic_compare_exchange_strong(&q->temp, &old_temp, new_temp));
+    return old_temp;
+}
+
+static void 
+release_push_location(struct lock_free_queue *q, int index)
+{
+    // The current thread already inserted an
+    // item at position "index", which comes
+    // after the queue's tail.
+    //
+    // It may be possible that other threads
+    // inserted items before this one, buf still
+    // after the tail
+    //
+    // Before being able to move the tail over
+    // this element, we need to wait for other
+    // threads to do it.
+    while (q->tail != index);
+
+    // At this point only this thread is allowed
+    // to push the tail forward
+
+    q->tail = (q->tail + 1) % q->size;
+}
+
+void
+lock_free_queue_push(struct lock_free_queue *q, void *src)
+{
+    int index = acquire_push_location(q);
+
+    void *dst = item_addr(q, index);
+    memcpy(dst, src, q->cell);
+
+    release_push_location(q, index);
+}
+
+void
+lock_free_queue_pop(struct lock_free_queue *q, void *dst)
+{
+    int old_head;
+    int new_head;
+    do {
+        old_head = q->head;
+        new_head = (old_head + 1) % q->size;
+        void *src = item_addr(q, old_head);
+        memcpy(dst, src, q->cell);
+    } while (!atomic_compare_exchange_strong(&q->head, &old_head, new_head));
+}