util: Add a free list structure for use with util_sparse_array

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>

src/util/sparse_array.c

@@ -192,3 +192,88 @@ util_sparse_array_validate(struct util_sparse_array *arr)
 {
    validate_node_level(arr, arr->root, arr->root->level);
 }
+
+void
+util_sparse_array_free_list_init(struct util_sparse_array_free_list *fl,
+                                 struct util_sparse_array *arr,
+                                 uint32_t sentinel,
+                                 uint32_t next_offset)
+{
+   fl->head = sentinel;
+   fl->arr = arr;
+   fl->sentinel = sentinel;
+   fl->next_offset = next_offset;
+}
+
+static uint64_t
+free_list_head(uint64_t old, uint32_t next)
+{
+   return ((old & 0xffffffff00000000ull) + 0x100000000ull) | next;
+}
+
+void
+util_sparse_array_free_list_push(struct util_sparse_array_free_list *fl,
+                                 uint32_t *items, unsigned num_items)
+{
+   assert(num_items > 0);
+   assert(items[0] != fl->sentinel);
+   void *last_elem = util_sparse_array_get(fl->arr, items[0]);
+   uint32_t *last_next = (uint32_t *)((char *)last_elem + fl->next_offset);
+   for (unsigned i = 1; i < num_items; i++) {
+      *last_next = items[i];
+      assert(items[i] != fl->sentinel);
+      last_elem = util_sparse_array_get(fl->arr, items[i]);
+      last_next = (uint32_t *)((char *)last_elem + fl->next_offset);
+   }
+
+   uint64_t current_head, old_head;
+   old_head = p_atomic_read(&fl->head);
+   do {
+      current_head = old_head;
+      *last_next = current_head; /* Index is the bottom 32 bits */
+      uint64_t new_head = free_list_head(current_head, items[0]);
+      old_head = p_atomic_cmpxchg(&fl->head, current_head, new_head);
+   } while (old_head != current_head);
+}
+
+uint32_t
+util_sparse_array_free_list_pop_idx(struct util_sparse_array_free_list *fl)
+{
+   uint64_t current_head;
+
+   current_head = p_atomic_read(&fl->head);
+   while (1) {
+      if ((uint32_t)current_head == fl->sentinel)
+         return fl->sentinel;
+
+      uint32_t head_idx = current_head; /* Index is the bottom 32 bits */
+      void *head_elem = util_sparse_array_get(fl->arr, head_idx);
+      uint32_t *head_next = (uint32_t *)((char *)head_elem + fl->next_offset);
+      uint32_t new_head = free_list_head(current_head, *head_next);
+      uint64_t old_head = p_atomic_cmpxchg(&fl->head, current_head, new_head);
+      if (old_head == current_head)
+         return head_idx;
+      current_head = old_head;
+   }
+}
+
+void *
+util_sparse_array_free_list_pop_elem(struct util_sparse_array_free_list *fl)
+{
+   uint64_t current_head;
+
+   current_head = p_atomic_read(&fl->head);
+   while (1) {
+      if ((uint32_t)current_head == fl->sentinel)
+         return NULL;
+
+      uint32_t head_idx = current_head; /* Index is the bottom 32 bits */
+      void *head_elem = util_sparse_array_get(fl->arr, head_idx);
+      uint32_t *head_next = (uint32_t *)((char *)head_elem + fl->next_offset);
+      uint32_t new_head = free_list_head(current_head, *head_next);
+      uint64_t old_head = p_atomic_cmpxchg(&fl->head, current_head, new_head);
+      if (old_head == current_head)
+         return head_elem;
+      current_head = old_head;
+   }
+}

src/util/sparse_array.h

@@ -81,6 +81,58 @@ void *util_sparse_array_get(struct util_sparse_array *arr, uint64_t idx);
 
 void util_sparse_array_validate(struct util_sparse_array *arr);
 
+/** A thread-safe free list for use with struct util_sparse_array
+ *
+ * This data structure provides an easy way to manage a singly linked list of
+ * "free" elements backed by a util_sparse_array.  The list supports only two
+ * operations: push and pop both of which are thread-safe and lock-free.  T
+ */
+struct
+#ifdef _MSC_VER
+ __declspec(align(8))
+#else
+ __attribute__((aligned(8)))
+#endif
+util_sparse_array_free_list
+{
+   /** Head of the list
+    *
+    * The bottom 64 bits of this value are the index to the next free element
+    * or the sentinel value if the list is empty.
+    *
+    * We want this element to be 8-byte aligned.  Otherwise, the performance
+    * of atomic operations on it will be aweful on 32-bit platforms.
+    */
+   uint64_t head;
+
+   /** The array backing this free list */
+   struct util_sparse_array *arr;
+
+   /** Sentinel value to indicate the end of the list
+    *
+    * This value must never be passed into util_sparse_array_free_list_push.
+    */
+   uint32_t sentinel;
+
+   /** Offset into the array element at which to find the "next" value
+    *
+    * The assumption is that there is some uint32_t "next" value embedded in
+    * the array element for use in the free list.  This is its offset.
+    */
+   uint32_t next_offset;
+};
+
+void util_sparse_array_free_list_init(struct util_sparse_array_free_list *fl,
+                                      struct util_sparse_array *arr,
+                                      uint32_t sentinel,
+                                      uint32_t next_offset);
+
+void util_sparse_array_free_list_push(struct util_sparse_array_free_list *fl,
+                                      uint32_t *items, unsigned num_items);
+
+uint32_t util_sparse_array_free_list_pop_idx(struct util_sparse_array_free_list *fl);
+void *util_sparse_array_free_list_pop_elem(struct util_sparse_array_free_list *fl);
+
 #ifdef __cplusplus
 } /* extern C */
 #endif