gobject/gatomicarray.c

   1 /* GObject - GLib Type, Object, Parameter and Signal Library
   2  * Copyright (C) 2009 Benjamin Otte <otte@gnome.org>
   3  *
   4  * This library is free software; you can redistribute it and/or
   5  * modify it under the terms of the GNU Lesser General Public
   6  * License as published by the Free Software Foundation; either
   7  * version 2 of the License, or (at your option) any later version.
   8  *
   9  * This library is distributed in the hope that it will be useful,
  10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12  * Lesser General Public License for more details.
  13  *
  14  * You should have received a copy of the GNU Lesser General
  15  * Public License along with this library; if not, write to the
  16  * Free Software Foundation, Inc., 59 Temple Place, Suite 330,
  17  * Boston, MA 02111-1307, USA.
  18  */
  19
  20 #include "config.h"
  21
  22 #include <string.h>
  23
  24 #include "gatomicarray.h"
  25
  26 /* A GAtomicArray is a growable, mutable array of data
  27  * generally of the form of a header of a specific size and
  28  * then a array of items of a fixed size.
  29  *
  30  * It is possible to do lock-less read transactions from the
  31  * array without any protection against other reads or writes,
  32  * but such read operation must be aware that the data in the
  33  * atomic array can change at any time during the transaction,
  34  * and only at the end can we verify if the transaction succeeded
  35  * or not. Thus the reading transaction cannot for instance
  36  * dereference a pointer in the array inside the transaction.
  37  *
  38  * The size of an array however cannot change during a read
  39  * transaction.
  40  *
  41  * Writes to the array is done in a copy-update style, but there
  42  * is no real protection against multiple writers overwriting each
  43  * others updates, so writes must be protected by an external lock.
  44  */
  45
  46 G_LOCK_DEFINE_STATIC (array);
  47
  48 typedef struct _FreeListNode FreeListNode;
  49 struct _FreeListNode {
  50   FreeListNode *next;
  51 };
  52
  53 /* This is really a list of array memory blocks, using the
  54  * first item as the next pointer to chain them together.
  55  * Protected by array lock */
  56 static FreeListNode *freelist = NULL;
  57
  58 /* must hold array lock */
  59 static gpointer
  60 freelist_alloc (gsize size, gboolean reuse)
  61 {
  62   gpointer mem;
  63   FreeListNode *free, **prev;
  64   gsize real_size;
  65
  66   if (reuse)
  67     {
  68       for (free = freelist, prev = &freelist; free != NULL; prev = &free->next, free = free->next)
  69         {
  70           if (G_ATOMIC_ARRAY_DATA_SIZE (free) == size)
  71             {
  72               *prev = free->next;
  73               return (gpointer)free;
  74             }
  75         }
  76     }
  77
  78   real_size = sizeof (gsize) + MAX (size, sizeof (FreeListNode));
  79   mem = g_slice_alloc (real_size);
  80   mem = ((char *) mem) + sizeof (gsize);
  81   G_ATOMIC_ARRAY_DATA_SIZE (mem) = size;
  82   return mem;
  83 }
  84
  85 /* must hold array lock */
  86 static void
  87 freelist_free (gpointer mem)
  88 {
  89   FreeListNode *free;
  90
  91   free = mem;
  92   free->next = freelist;
  93   freelist = free;
  94 }
  95
  96 void
  97 _g_atomic_array_init (GAtomicArray *array)
  98 {
  99   array->data = NULL;
 100 }
 101
 102 /* Get a copy of the data (if non-NULL) that
 103  * can be changed and then re-applied with
 104  * g_atomic_array_update().
 105  *
 106  * If additional_element_size is > 0 then
 107  * then the new memory chunk is that much
 108  * larger, or there were no data we return
 109  * a chunk of header_size + additional_element_size.
 110  * This means you can use this to grow the
 111  * array part and it handles the first element
 112  * being added automatically.
 113  *
 114  * We don't support shrinking arrays, as if
 115  * we then re-grow we may reuse an old pointer
 116  * value and confuse the transaction check.
 117  */
 118 gpointer
 119 _g_atomic_array_copy (GAtomicArray *array,
 120                       gsize header_size,
 121                       gsize additional_element_size)
 122 {
 123   guint8 *new, *old;
 124   gsize old_size, new_size;
 125
 126   G_LOCK (array);
 127   old = g_atomic_pointer_get (&array->data);
 128   if (old)
 129     {
 130       old_size = G_ATOMIC_ARRAY_DATA_SIZE (old);
 131       new_size = old_size + additional_element_size;
 132       /* Don't reuse if copying to same size, as this may end
 133          up reusing the same pointer for the same array thus
 134          confusing the transaction check */
 135       new = freelist_alloc (new_size, additional_element_size != 0);
 136       memcpy (new, old, old_size);
 137     }
 138   else if (additional_element_size != 0)
 139     {
 140       new_size = header_size + additional_element_size;
 141       new = freelist_alloc (new_size, TRUE);
 142     }
 143   else
 144     new = NULL;
 145   G_UNLOCK (array);
 146   return new;
 147 }
 148
 149 /* Replace the data in the array with the new data,
 150  * freeing the old data (for reuse). The new data may
 151  * not be smaller than the current data.
 152  */
 153 void
 154 _g_atomic_array_update (GAtomicArray *array,
 155                         gpointer new_data)
 156 {
 157   guint8 *old;
 158
 159   G_LOCK (array);
 160   old = g_atomic_pointer_get (&array->data);
 161
 162   g_assert (old == NULL || G_ATOMIC_ARRAY_DATA_SIZE (old) <= G_ATOMIC_ARRAY_DATA_SIZE (new_data));
 163
 164   g_atomic_pointer_set (&array->data, new_data);
 165   if (old)
 166     freelist_free (old);
 167   G_UNLOCK (array);
 168 }