Import Upstream version 2.72.4

gobject/gatomicarray.c

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+/* GObject - GLib Type, Object, Parameter and Signal Library
+ * Copyright (C) 2009 Benjamin Otte <otte@gnome.org>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General
+ * Public License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "config.h"
+
+#include "../glib/gvalgrind.h"
+#include <string.h>
+
+#include "gatomicarray.h"
+
+/* A GAtomicArray is a growable, mutable array of data
+ * generally of the form of a header of a specific size and
+ * then an array of items of a fixed size.
+ *
+ * It is possible to do lock-less read transactions from the
+ * array without any protection against other reads or writes,
+ * but such read operation must be aware that the data in the
+ * atomic array can change at any time during the transaction,
+ * and only at the end can we verify if the transaction succeeded
+ * or not. Thus the reading transaction cannot for instance
+ * dereference a pointer in the array inside the transaction.
+ *
+ * The size of an array however cannot change during a read
+ * transaction.
+ *
+ * Writes to the array is done in a copy-update style, but there
+ * is no real protection against multiple writers overwriting each
+ * others updates, so writes must be protected by an external lock.
+ */
+
+G_LOCK_DEFINE_STATIC (array);
+
+typedef struct _FreeListNode FreeListNode;
+struct _FreeListNode {
+  FreeListNode *next;
+};
+
+/* This is really a list of array memory blocks, using the
+ * first item as the next pointer to chain them together.
+ * Protected by array lock */
+static FreeListNode *freelist = NULL;
+
+/* must hold array lock */
+static gpointer
+freelist_alloc (gsize size, gboolean reuse)
+{
+  gpointer mem;
+  FreeListNode *free, **prev;
+  gsize real_size;
+
+  if (reuse)
+    {
+      for (free = freelist, prev = &freelist; free != NULL; prev = &free->next, free = free->next)
+	{
+	  if (G_ATOMIC_ARRAY_DATA_SIZE (free) == size)
+	    {
+	      *prev = free->next;
+	      return (gpointer)free;
+	    }
+	}
+    }
+
+  real_size = sizeof (gsize) + MAX (size, sizeof (FreeListNode));
+  mem = g_slice_alloc (real_size);
+  mem = ((char *) mem) + sizeof (gsize);
+  G_ATOMIC_ARRAY_DATA_SIZE (mem) = size;
+
+#if ENABLE_VALGRIND
+  VALGRIND_MALLOCLIKE_BLOCK (mem, real_size - sizeof (gsize), FALSE, FALSE);
+#endif
+
+  return mem;
+}
+
+/* must hold array lock */
+static void
+freelist_free (gpointer mem)
+{
+  FreeListNode *free;
+
+  free = mem;
+  free->next = freelist;
+  freelist = free;
+}
+
+void
+_g_atomic_array_init (GAtomicArray *array)
+{
+  array->data = NULL;
+}
+
+/* Get a copy of the data (if non-NULL) that
+ * can be changed and then re-applied with
+ * g_atomic_array_update().
+ *
+ * If additional_element_size is > 0 then
+ * then the new memory chunk is that much
+ * larger, or there were no data we return
+ * a chunk of header_size + additional_element_size.
+ * This means you can use this to grow the
+ * array part and it handles the first element
+ * being added automatically.
+ *
+ * We don't support shrinking arrays, as if
+ * we then re-grow we may reuse an old pointer
+ * value and confuse the transaction check.
+ */
+gpointer
+_g_atomic_array_copy (GAtomicArray *array,
+		      gsize header_size,
+		      gsize additional_element_size)
+{
+  guint8 *new, *old;
+  gsize old_size, new_size;
+
+  G_LOCK (array);
+  old = g_atomic_pointer_get (&array->data);
+  if (old)
+    {
+      old_size = G_ATOMIC_ARRAY_DATA_SIZE (old);
+      new_size = old_size + additional_element_size;
+      /* Don't reuse if copying to same size, as this may end
+	 up reusing the same pointer for the same array thus
+	 confusing the transaction check */
+      new = freelist_alloc (new_size, additional_element_size != 0);
+      memcpy (new, old, old_size);
+    }
+  else if (additional_element_size != 0)
+    {
+      new_size = header_size + additional_element_size;
+      new = freelist_alloc (new_size, TRUE);
+    }
+  else
+    new = NULL;
+  G_UNLOCK (array);
+  return new;
+}
+
+/* Replace the data in the array with the new data,
+ * freeing the old data (for reuse). The new data may
+ * not be smaller than the current data.
+ */
+void
+_g_atomic_array_update (GAtomicArray *array,
+			gpointer new_data)
+{
+  guint8 *old;
+
+  G_LOCK (array);
+  old = g_atomic_pointer_get (&array->data);
+
+  g_assert (old == NULL || G_ATOMIC_ARRAY_DATA_SIZE (old) <= G_ATOMIC_ARRAY_DATA_SIZE (new_data));
+
+  g_atomic_pointer_set (&array->data, new_data);
+  if (old)
+    freelist_free (old);
+  G_UNLOCK (array);
+}