glib/gbitlock.c

   1 /*
   2  * Copyright © 2008 Ryan Lortie
   3  * Copyright © 2010 Codethink Limited
   4  *
   5  * This library is free software; you can redistribute it and/or
   6  * modify it under the terms of the GNU Lesser General Public
   7  * License as published by the Free Software Foundation; either
   8  * version 2 of the licence, or (at your option) any later version.
   9  *
  10  * This library is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13  * Lesser General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU Lesser General Public
  16  * License along with this library; if not, write to the
  17  * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  18  * Boston, MA 02111-1307, USA.
  19  *
  20  * Author: Ryan Lortie <desrt@desrt.ca>
  21  */
  22
  23 #include "gbitlock.h"
  24
  25 #include <glib/gatomic.h>
  26 #include <glib/gslist.h>
  27 #include <glib/gthread.h>
  28
  29 #include "gthreadprivate.h"
  30 #include "config.h"
  31
  32 #include "galias.h"
  33
  34 #ifdef G_BIT_LOCK_FORCE_FUTEX_EMULATION
  35 #undef HAVE_FUTEX
  36 #endif
  37
  38 #ifndef HAVE_FUTEX
  39 static GSList *g_futex_address_list = NULL;
  40 static GMutex *g_futex_mutex = NULL;
  41 #endif
  42
  43 void
  44 _g_futex_thread_init (void) {
  45 #ifndef HAVE_FUTEX
  46   g_futex_mutex = g_mutex_new ();
  47 #endif
  48 }
  49
  50 #ifdef HAVE_FUTEX
  51 /*
  52  * We have headers for futex(2) on the build machine.  This does not
  53  * imply that every system that ever runs the resulting glib will have
  54  * kernel support for futex, but you'd have to have a pretty old
  55  * kernel in order for that not to be the case.
  56  *
  57  * If anyone actually gets bit by this, please file a bug. :)
  58  */
  59 #include <linux/futex.h>
  60 #include <syscall.h>
  61 #include <unistd.h>
  62
  63 /* < private >
  64  * g_futex_wait:
  65  * @address: a pointer to an integer
  66  * @value: the value that should be at @address
  67  *
  68  * Atomically checks that the value stored at @address is equal to
  69  * @value and then blocks.  If the value stored at @address is not
  70  * equal to @value then this function returns immediately.
  71  *
  72  * To unblock, call g_futex_wake() on @address.
  73  *
  74  * This call may spuriously unblock (for example, in response to the
  75  * process receiving a signal) but this is not guaranteed.  Unlike the
  76  * Linux system call of a similar name, there is no guarantee that a
  77  * waiting process will unblock due to a g_futex_wake() call in a
  78  * separate process.
  79  */
  80 static void
  81 g_futex_wait (const volatile gint *address,
  82               gint                 value)
  83 {
  84   syscall (SYS_futex, address, (gsize) FUTEX_WAIT, (gsize) value, NULL);
  85 }
  86
  87 /* < private >
  88  * g_futex_wake:
  89  * @address: a pointer to an integer
  90  *
  91  * Nominally, wakes one thread that is blocked in g_futex_wait() on
  92  * @address (if any thread is currently waiting).
  93  *
  94  * As mentioned in the documention for g_futex_wait(), spurious
  95  * wakeups may occur.  As such, this call may result in more than one
  96  * thread being woken up.
  97  */
  98 static void
  99 g_futex_wake (const volatile gint *address)
 100 {
 101   syscall (SYS_futex, address, (gsize) FUTEX_WAKE, (gsize) 1, NULL);
 102 }
 103
 104 #else
 105
 106 /* emulate futex(2) */
 107 typedef struct
 108 {
 109   const volatile gint *address;
 110   gint                 ref_count;
 111   GCond               *wait_queue;
 112 } WaitAddress;
 113
 114 static WaitAddress *
 115 g_futex_find_address (const volatile gint *address)
 116 {
 117   GSList *node;
 118
 119   for (node = g_futex_address_list; node; node = node->next)
 120     {
 121       WaitAddress *waiter = node->data;
 122
 123       if (waiter->address == address)
 124         return waiter;
 125     }
 126
 127   return NULL;
 128 }
 129
 130 static void
 131 g_futex_wait (const volatile gint *address,
 132               gint                 value)
 133 {
 134   g_mutex_lock (g_futex_mutex);
 135   if G_LIKELY (g_atomic_int_get (address) == value)
 136     {
 137       WaitAddress *waiter;
 138
 139       if ((waiter = g_futex_find_address (address)) == NULL)
 140         {
 141           waiter = g_slice_new (WaitAddress);
 142           waiter->address = address;
 143           waiter->wait_queue = g_cond_new ();
 144           waiter->ref_count = 0;
 145           g_futex_address_list =
 146             g_slist_prepend (g_futex_address_list, waiter);
 147         }
 148
 149       waiter->ref_count++;
 150       g_cond_wait (waiter->wait_queue, g_futex_mutex);
 151
 152       if (!--waiter->ref_count)
 153         {
 154           g_futex_address_list =
 155             g_slist_remove (g_futex_address_list, waiter);
 156           g_cond_free (waiter->wait_queue);
 157           g_slice_free (WaitAddress, waiter);
 158         }
 159     }
 160   g_mutex_unlock (g_futex_mutex);
 161 }
 162
 163 static void
 164 g_futex_wake (const volatile gint *address)
 165 {
 166   WaitAddress *waiter;
 167
 168   /* need to lock here for two reasons:
 169    *   1) need to acquire/release lock to ensure waiter is not in
 170    *      the process of registering a wait
 171    *   2) need to -stay- locked until the end to ensure a wake()
 172    *      in another thread doesn't cause 'waiter' to stop existing
 173    */
 174   g_mutex_lock (g_futex_mutex);
 175   if ((waiter = g_futex_find_address (address)))
 176     g_cond_signal (waiter->wait_queue);
 177   g_mutex_unlock (g_futex_mutex);
 178 }
 179 #endif
 180
 181 #define CONTENTION_CLASSES 11
 182 static volatile gint g_bit_lock_contended[CONTENTION_CLASSES];
 183
 184 /**
 185  * g_bit_lock:
 186  * @address: a pointer to an integer
 187  * @lock_bit: a bit value between 0 and 31
 188  *
 189  * Sets the indicated @lock_bit in @address.  If the bit is already
 190  * set, this call will block until g_bit_unlock() unsets the
 191  * corresponding bit.
 192  *
 193  * Attempting to lock on two different bits within the same integer is
 194  * not supported and will very probably cause deadlocks.
 195  *
 196  * The value of the bit that is set is (1u << @bit).  If @bit is not
 197  * between 0 and 31 then the result is undefined.
 198  *
 199  * This function accesses @address atomically.  All other accesses to
 200  * @address must be atomic in order for this function to work
 201  * reliably.
 202  *
 203  * Since: 2.24
 204  **/
 205 void
 206 g_bit_lock (volatile gint *address,
 207             gint           lock_bit)
 208 {
 209   guint v;
 210
 211  retry:
 212   v = g_atomic_int_get (address);
 213   if (v & (1u << lock_bit))
 214     /* already locked */
 215     {
 216       guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
 217
 218       g_atomic_int_add (&g_bit_lock_contended[class], +1);
 219       g_futex_wait (address, v);
 220       g_atomic_int_add (&g_bit_lock_contended[class], -1);
 221
 222       goto retry;
 223     }
 224
 225   if (!g_atomic_int_compare_and_exchange (address, v, v | (1u << lock_bit)))
 226     goto retry;
 227 }
 228
 229 /**
 230  * g_bit_trylock:
 231  * @address: a pointer to an integer
 232  * @lock_bit: a bit value between 0 and 31
 233  * @returns: %TRUE if the lock was acquired
 234  *
 235  * Sets the indicated @lock_bit in @address, returning %TRUE if
 236  * successful.  If the bit is already set, returns %FALSE immediately.
 237  *
 238  * Attempting to lock on two different bits within the same integer is
 239  * not supported.
 240  *
 241  * The value of the bit that is set is (1u << @bit).  If @bit is not
 242  * between 0 and 31 then the result is undefined.
 243  *
 244  * This function accesses @address atomically.  All other accesses to
 245  * @address must be atomic in order for this function to work
 246  * reliably.
 247  *
 248  * Since: 2.24
 249  **/
 250 gboolean
 251 g_bit_trylock (volatile gint *address,
 252                gint           lock_bit)
 253 {
 254   guint v;
 255
 256  retry:
 257   v = g_atomic_int_get (address);
 258   if (v & (1u << lock_bit))
 259     /* already locked */
 260     return FALSE;
 261
 262   if (!g_atomic_int_compare_and_exchange (address, v, v | (1u << lock_bit)))
 263     goto retry;
 264
 265   return TRUE;
 266 }
 267
 268 /**
 269  * g_bit_unlock:
 270  * @address: a pointer to an integer
 271  * @lock_bit: a bit value between 0 and 31
 272  *
 273  * Clears the indicated @lock_bit in @address.  If another thread is
 274  * currently blocked in g_bit_lock() on this same bit then it will be
 275  * woken up.
 276  *
 277  * This function accesses @address atomically.  All other accesses to
 278  * @address must be atomic in order for this function to work
 279  * reliably.
 280  *
 281  * Since: 2.24
 282  **/
 283 void
 284 g_bit_unlock (volatile gint *address,
 285               gint           lock_bit)
 286 {
 287   guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
 288   guint v;
 289
 290  retry:
 291   v = g_atomic_int_get (address);
 292   if (!g_atomic_int_compare_and_exchange (address, v, v & ~(1u << lock_bit)))
 293     goto retry;
 294
 295   if (g_atomic_int_get (&g_bit_lock_contended[class]))
 296     g_futex_wake (address);
 297 }
 298
 299 #define __G_BITLOCK_C__
 300 #include "galiasdef.c"