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 2.1 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, see <http://www.gnu.org/licenses/>.
  16  */
  17
  18 #include "config.h"
  19
  20 #include <string.h>
  21
  22 #include "gatomicarray.h"
  23
  24 /* A GAtomicArray is a growable, mutable array of data
  25  * generally of the form of a header of a specific size and
  26  * then a array of items of a fixed size.
  27  *
  28  * It is possible to do lock-less read transactions from the
  29  * array without any protection against other reads or writes,
  30  * but such read operation must be aware that the data in the
  31  * atomic array can change at any time during the transaction,
  32  * and only at the end can we verify if the transaction succeeded
  33  * or not. Thus the reading transaction cannot for instance
  34  * dereference a pointer in the array inside the transaction.
  35  *
  36  * The size of an array however cannot change during a read
  37  * transaction.
  38  *
  39  * Writes to the array is done in a copy-update style, but there
  40  * is no real protection against multiple writers overwriting each
  41  * others updates, so writes must be protected by an external lock.
  42  */
  43
  44 G_LOCK_DEFINE_STATIC (array);
  45
  46 typedef struct _FreeListNode FreeListNode;
  47 struct _FreeListNode {
  48   FreeListNode *next;
  49 };
  50
  51 /* This is really a list of array memory blocks, using the
  52  * first item as the next pointer to chain them together.
  53  * Protected by array lock */
  54 static FreeListNode *freelist = NULL;
  55
  56 /* must hold array lock */
  57 static gpointer
  58 freelist_alloc (gsize size, gboolean reuse)
  59 {
  60   gpointer mem;
  61   FreeListNode *free, **prev;
  62   gsize real_size;
  63
  64   if (reuse)
  65     {
  66       for (free = freelist, prev = &freelist; free != NULL; prev = &free->next, free = free->next)
  67         {
  68           if (G_ATOMIC_ARRAY_DATA_SIZE (free) == size)
  69             {
  70               *prev = free->next;
  71               return (gpointer)free;
  72             }
  73         }
  74     }
  75
  76   real_size = sizeof (gsize) + MAX (size, sizeof (FreeListNode));
  77   mem = g_slice_alloc (real_size);
  78   mem = ((char *) mem) + sizeof (gsize);
  79   G_ATOMIC_ARRAY_DATA_SIZE (mem) = size;
  80   return mem;
  81 }
  82
  83 /* must hold array lock */
  84 static void
  85 freelist_free (gpointer mem)
  86 {
  87   FreeListNode *free;
  88
  89   free = mem;
  90   free->next = freelist;
  91   freelist = free;
  92 }
  93
  94 void
  95 _g_atomic_array_init (GAtomicArray *array)
  96 {
  97   array->data = NULL;
  98 }
  99
 100 /* Get a copy of the data (if non-NULL) that
 101  * can be changed and then re-applied with
 102  * g_atomic_array_update().
 103  *
 104  * If additional_element_size is > 0 then
 105  * then the new memory chunk is that much
 106  * larger, or there were no data we return
 107  * a chunk of header_size + additional_element_size.
 108  * This means you can use this to grow the
 109  * array part and it handles the first element
 110  * being added automatically.
 111  *
 112  * We don't support shrinking arrays, as if
 113  * we then re-grow we may reuse an old pointer
 114  * value and confuse the transaction check.
 115  */
 116 gpointer
 117 _g_atomic_array_copy (GAtomicArray *array,
 118                       gsize header_size,
 119                       gsize additional_element_size)
 120 {
 121   guint8 *new, *old;
 122   gsize old_size, new_size;
 123
 124   G_LOCK (array);
 125   old = g_atomic_pointer_get (&array->data);
 126   if (old)
 127     {
 128       old_size = G_ATOMIC_ARRAY_DATA_SIZE (old);
 129       new_size = old_size + additional_element_size;
 130       /* Don't reuse if copying to same size, as this may end
 131          up reusing the same pointer for the same array thus
 132          confusing the transaction check */
 133       new = freelist_alloc (new_size, additional_element_size != 0);
 134       memcpy (new, old, old_size);
 135     }
 136   else if (additional_element_size != 0)
 137     {
 138       new_size = header_size + additional_element_size;
 139       new = freelist_alloc (new_size, TRUE);
 140     }
 141   else
 142     new = NULL;
 143   G_UNLOCK (array);
 144   return new;
 145 }
 146
 147 /* Replace the data in the array with the new data,
 148  * freeing the old data (for reuse). The new data may
 149  * not be smaller than the current data.
 150  */
 151 void
 152 _g_atomic_array_update (GAtomicArray *array,
 153                         gpointer new_data)
 154 {
 155   guint8 *old;
 156
 157   G_LOCK (array);
 158   old = g_atomic_pointer_get (&array->data);
 159
 160   g_assert (old == NULL || G_ATOMIC_ARRAY_DATA_SIZE (old) <= G_ATOMIC_ARRAY_DATA_SIZE (new_data));
 161
 162   g_atomic_pointer_set (&array->data, new_data);
 163   if (old)
 164     freelist_free (old);
 165   G_UNLOCK (array);
 166 }