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/gmessages.h>
  26 #include <glib/gatomic.h>
  27 #include <glib/gslist.h>
  28 #include <glib/gthread.h>
  29 #include <glib/gslice.h>
  30
  31 #include "gthreadprivate.h"
  32 #include "config.h"
  33
  34 #undef HAVE_FUTEX
  35 #ifdef G_BIT_LOCK_FORCE_FUTEX_EMULATION
  36 #endif
  37
  38 #ifndef HAVE_FUTEX
  39 static GMutex g_futex_mutex;
  40 static GSList *g_futex_address_list = NULL;
  41 #endif
  42
  43 #ifdef HAVE_FUTEX
  44 /*
  45  * We have headers for futex(2) on the build machine.  This does not
  46  * imply that every system that ever runs the resulting glib will have
  47  * kernel support for futex, but you'd have to have a pretty old
  48  * kernel in order for that not to be the case.
  49  *
  50  * If anyone actually gets bit by this, please file a bug. :)
  51  */
  52 #include <linux/futex.h>
  53 #include <sys/syscall.h>
  54 #include <unistd.h>
  55
  56 /* < private >
  57  * g_futex_wait:
  58  * @address: a pointer to an integer
  59  * @value: the value that should be at @address
  60  *
  61  * Atomically checks that the value stored at @address is equal to
  62  * @value and then blocks.  If the value stored at @address is not
  63  * equal to @value then this function returns immediately.
  64  *
  65  * To unblock, call g_futex_wake() on @address.
  66  *
  67  * This call may spuriously unblock (for example, in response to the
  68  * process receiving a signal) but this is not guaranteed.  Unlike the
  69  * Linux system call of a similar name, there is no guarantee that a
  70  * waiting process will unblock due to a g_futex_wake() call in a
  71  * separate process.
  72  */
  73 static void
  74 g_futex_wait (const volatile gint *address,
  75               gint                 value)
  76 {
  77   syscall (__NR_futex, address, (gsize) FUTEX_WAIT, (gsize) value, NULL);
  78 }
  79
  80 /* < private >
  81  * g_futex_wake:
  82  * @address: a pointer to an integer
  83  *
  84  * Nominally, wakes one thread that is blocked in g_futex_wait() on
  85  * @address (if any thread is currently waiting).
  86  *
  87  * As mentioned in the documention for g_futex_wait(), spurious
  88  * wakeups may occur.  As such, this call may result in more than one
  89  * thread being woken up.
  90  */
  91 static void
  92 g_futex_wake (const volatile gint *address)
  93 {
  94   syscall (__NR_futex, address, (gsize) FUTEX_WAKE, (gsize) 1, NULL);
  95 }
  96
  97 #else
  98
  99 /* emulate futex(2) */
 100 typedef struct
 101 {
 102   const volatile gint *address;
 103   gint                 ref_count;
 104   GCond                wait_queue;
 105 } WaitAddress;
 106
 107 static WaitAddress *
 108 g_futex_find_address (const volatile gint *address)
 109 {
 110   GSList *node;
 111
 112   for (node = g_futex_address_list; node; node = node->next)
 113     {
 114       WaitAddress *waiter = node->data;
 115
 116       if (waiter->address == address)
 117         return waiter;
 118     }
 119
 120   return NULL;
 121 }
 122
 123 static void
 124 g_futex_wait (const volatile gint *address,
 125               gint                 value)
 126 {
 127   g_mutex_lock (&g_futex_mutex);
 128   if G_LIKELY (g_atomic_int_get (address) == value)
 129     {
 130       WaitAddress *waiter;
 131
 132       if ((waiter = g_futex_find_address (address)) == NULL)
 133         {
 134           waiter = g_slice_new (WaitAddress);
 135           waiter->address = address;
 136           g_cond_init (&waiter->wait_queue);
 137           waiter->ref_count = 0;
 138           g_futex_address_list =
 139             g_slist_prepend (g_futex_address_list, waiter);
 140         }
 141
 142       waiter->ref_count++;
 143       g_cond_wait (&waiter->wait_queue, &g_futex_mutex);
 144
 145       if (!--waiter->ref_count)
 146         {
 147           g_futex_address_list =
 148             g_slist_remove (g_futex_address_list, waiter);
 149           g_cond_clear (&waiter->wait_queue);
 150           g_slice_free (WaitAddress, waiter);
 151         }
 152     }
 153   g_mutex_unlock (&g_futex_mutex);
 154 }
 155
 156 static void
 157 g_futex_wake (const volatile gint *address)
 158 {
 159   WaitAddress *waiter;
 160
 161   /* need to lock here for two reasons:
 162    *   1) need to acquire/release lock to ensure waiter is not in
 163    *      the process of registering a wait
 164    *   2) need to -stay- locked until the end to ensure a wake()
 165    *      in another thread doesn't cause 'waiter' to stop existing
 166    */
 167   g_mutex_lock (&g_futex_mutex);
 168   if ((waiter = g_futex_find_address (address)))
 169     g_cond_signal (&waiter->wait_queue);
 170   g_mutex_unlock (&g_futex_mutex);
 171 }
 172 #endif
 173
 174 #define CONTENTION_CLASSES 11
 175 static volatile gint g_bit_lock_contended[CONTENTION_CLASSES];
 176
 177 #if (defined (i386) || defined (__amd64__))
 178   #if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 5)
 179     #define USE_ASM_GOTO 1
 180   #endif
 181 #endif
 182
 183 /**
 184  * g_bit_lock:
 185  * @address: a pointer to an integer
 186  * @lock_bit: a bit value between 0 and 31
 187  *
 188  * Sets the indicated @lock_bit in @address.  If the bit is already
 189  * set, this call will block until g_bit_unlock() unsets the
 190  * corresponding bit.
 191  *
 192  * Attempting to lock on two different bits within the same integer is
 193  * not supported and will very probably cause deadlocks.
 194  *
 195  * The value of the bit that is set is (1u << @bit).  If @bit is not
 196  * between 0 and 31 then the result is undefined.
 197  *
 198  * This function accesses @address atomically.  All other accesses to
 199  * @address must be atomic in order for this function to work
 200  * reliably.
 201  *
 202  * Since: 2.24
 203  **/
 204 void
 205 g_bit_lock (volatile gint *address,
 206             gint           lock_bit)
 207 {
 208 #ifdef USE_ASM_GOTO
 209  retry:
 210   asm volatile goto ("lock bts %1, (%0)\n"
 211                      "jc %l[contended]"
 212                      : /* no output */
 213                      : "r" (address), "r" (lock_bit)
 214                      : "cc", "memory"
 215                      : contended);
 216   return;
 217
 218  contended:
 219   {
 220     guint mask = 1u << lock_bit;
 221     guint v;
 222
 223     v = g_atomic_int_get (address);
 224     if (v & mask)
 225       {
 226         guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
 227
 228         g_atomic_int_add (&g_bit_lock_contended[class], +1);
 229         g_futex_wait (address, v);
 230         g_atomic_int_add (&g_bit_lock_contended[class], -1);
 231       }
 232   }
 233   goto retry;
 234 #else
 235   guint mask = 1u << lock_bit;
 236   guint v;
 237
 238  retry:
 239   v = g_atomic_int_or (address, mask);
 240   if (v & mask)
 241     /* already locked */
 242     {
 243       guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
 244
 245       g_atomic_int_add (&g_bit_lock_contended[class], +1);
 246       g_futex_wait (address, v);
 247       g_atomic_int_add (&g_bit_lock_contended[class], -1);
 248
 249       goto retry;
 250     }
 251 #endif
 252 }
 253
 254 /**
 255  * g_bit_trylock:
 256  * @address: a pointer to an integer
 257  * @lock_bit: a bit value between 0 and 31
 258  *
 259  * Sets the indicated @lock_bit in @address, returning %TRUE if
 260  * successful.  If the bit is already set, returns %FALSE immediately.
 261  *
 262  * Attempting to lock on two different bits within the same integer is
 263  * not supported.
 264  *
 265  * The value of the bit that is set is (1u << @bit).  If @bit is not
 266  * between 0 and 31 then the result is undefined.
 267  *
 268  * This function accesses @address atomically.  All other accesses to
 269  * @address must be atomic in order for this function to work
 270  * reliably.
 271  *
 272  * Returns: %TRUE if the lock was acquired
 273  *
 274  * Since: 2.24
 275  **/
 276 gboolean
 277 g_bit_trylock (volatile gint *address,
 278                gint           lock_bit)
 279 {
 280 #ifdef USE_ASM_GOTO
 281   gboolean result;
 282
 283   asm volatile ("lock bts %2, (%1)\n"
 284                 "setnc %%al\n"
 285                 "movzx %%al, %0"
 286                 : "=r" (result)
 287                 : "r" (address), "r" (lock_bit)
 288                 : "cc", "memory");
 289
 290   return result;
 291 #else
 292   guint mask = 1u << lock_bit;
 293   guint v;
 294
 295   v = g_atomic_int_or (address, mask);
 296
 297   return ~v & mask;
 298 #endif
 299 }
 300
 301 /**
 302  * g_bit_unlock:
 303  * @address: a pointer to an integer
 304  * @lock_bit: a bit value between 0 and 31
 305  *
 306  * Clears the indicated @lock_bit in @address.  If another thread is
 307  * currently blocked in g_bit_lock() on this same bit then it will be
 308  * woken up.
 309  *
 310  * This function accesses @address atomically.  All other accesses to
 311  * @address must be atomic in order for this function to work
 312  * reliably.
 313  *
 314  * Since: 2.24
 315  **/
 316 void
 317 g_bit_unlock (volatile gint *address,
 318               gint           lock_bit)
 319 {
 320 #ifdef USE_ASM_GOTO
 321   asm volatile ("lock btr %1, (%0)"
 322                 : /* no output */
 323                 : "r" (address), "r" (lock_bit)
 324                 : "cc", "memory");
 325 #else
 326   guint mask = 1u << lock_bit;
 327
 328   g_atomic_int_and (address, ~mask);
 329 #endif
 330
 331   {
 332     guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
 333
 334     if (g_atomic_int_get (&g_bit_lock_contended[class]))
 335       g_futex_wake (address);
 336   }
 337 }
 338
 339
 340 /* We emulate pointer-sized futex(2) because the kernel API only
 341  * supports integers.
 342  *
 343  * We assume that the 'interesting' part is always the lower order bits.
 344  * This assumption holds because pointer bitlocks are restricted to
 345  * using the low order bits of the pointer as the lock.
 346  *
 347  * On 32 bits, there is nothing to do since the pointer size is equal to
 348  * the integer size.  On little endian the lower-order bits don't move,
 349  * so do nothing.  Only on 64bit big endian do we need to do a bit of
 350  * pointer arithmetic: the low order bits are shifted by 4 bytes.  We
 351  * have a helper function that always does the right thing here.
 352  *
 353  * Since we always consider the low-order bits of the integer value, a
 354  * simple cast from (gsize) to (guint) always takes care of that.
 355  *
 356  * After that, pointer-sized futex becomes as simple as:
 357  *
 358  *   g_futex_wait (g_futex_int_address (address), (guint) value);
 359  *
 360  * and
 361  *
 362  *   g_futex_wake (g_futex_int_address (int_address));
 363  */
 364 static const volatile gint *
 365 g_futex_int_address (const volatile void *address)
 366 {
 367   const volatile gint *int_address = address;
 368
 369 #if G_BYTE_ORDER == G_BIG_ENDIAN && GLIB_SIZEOF_VOID_P == 8
 370   int_address++;
 371 #endif
 372
 373   return int_address;
 374 }
 375
 376 /**
 377  * g_pointer_bit_lock:
 378  * @address: a pointer to a #gpointer-sized value
 379  * @lock_bit: a bit value between 0 and 31
 380  *
 381  * This is equivalent to g_bit_lock, but working on pointers (or other
 382  * pointer-sized values).
 383  *
 384  * For portability reasons, you may only lock on the bottom 32 bits of
 385  * the pointer.
 386  *
 387  * Since: 2.30
 388  **/
 389 void
 390 (g_pointer_bit_lock) (volatile void *address,
 391                       gint           lock_bit)
 392 {
 393   g_return_if_fail (lock_bit < 32);
 394
 395   {
 396 #ifdef USE_ASM_GOTO
 397  retry:
 398     asm volatile goto ("lock bts %1, (%0)\n"
 399                        "jc %l[contended]"
 400                        : /* no output */
 401                        : "r" (address), "r" ((gsize) lock_bit)
 402                        : "cc", "memory"
 403                        : contended);
 404     return;
 405
 406  contended:
 407     {
 408       volatile gsize *pointer_address = address;
 409       gsize mask = 1u << lock_bit;
 410       gsize v;
 411
 412       v = (gsize) g_atomic_pointer_get (pointer_address);
 413       if (v & mask)
 414         {
 415           guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
 416
 417           g_atomic_int_add (&g_bit_lock_contended[class], +1);
 418           g_futex_wait (g_futex_int_address (address), v);
 419           g_atomic_int_add (&g_bit_lock_contended[class], -1);
 420         }
 421     }
 422     goto retry;
 423 #else
 424   volatile gsize *pointer_address = address;
 425   gsize mask = 1u << lock_bit;
 426   gsize v;
 427
 428  retry:
 429   v = g_atomic_pointer_or (pointer_address, mask);
 430   if (v & mask)
 431     /* already locked */
 432     {
 433       guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
 434
 435       g_atomic_int_add (&g_bit_lock_contended[class], +1);
 436       g_futex_wait (g_futex_int_address (address), (guint) v);
 437       g_atomic_int_add (&g_bit_lock_contended[class], -1);
 438
 439       goto retry;
 440     }
 441 #endif
 442   }
 443 }
 444
 445 /**
 446  * g_pointer_bit_trylock:
 447  * @address: a pointer to a #gpointer-sized value
 448  * @lock_bit: a bit value between 0 and 31
 449  *
 450  * This is equivalent to g_bit_trylock, but working on pointers (or
 451  * other pointer-sized values).
 452  *
 453  * For portability reasons, you may only lock on the bottom 32 bits of
 454  * the pointer.
 455  *
 456  * Returns: %TRUE if the lock was acquired
 457  *
 458  * Since: 2.30
 459  **/
 460 gboolean
 461 (g_pointer_bit_trylock) (volatile void *address,
 462                          gint           lock_bit)
 463 {
 464   g_return_val_if_fail (lock_bit < 32, FALSE);
 465
 466   {
 467 #ifdef USE_ASM_GOTO
 468     gboolean result;
 469
 470     asm volatile ("lock bts %2, (%1)\n"
 471                   "setnc %%al\n"
 472                   "movzx %%al, %0"
 473                   : "=r" (result)
 474                   : "r" (address), "r" ((gsize) lock_bit)
 475                   : "cc", "memory");
 476
 477     return result;
 478 #else
 479     volatile gsize *pointer_address = address;
 480     gsize mask = 1u << lock_bit;
 481     gsize v;
 482
 483     g_return_val_if_fail (lock_bit < 32, FALSE);
 484
 485     v = g_atomic_pointer_or (pointer_address, mask);
 486
 487     return ~v & mask;
 488 #endif
 489   }
 490 }
 491
 492 /**
 493  * g_pointer_bit_unlock:
 494  * @address: a pointer to a #gpointer-sized value
 495  * @lock_bit: a bit value between 0 and 31
 496  *
 497  * This is equivalent to g_bit_unlock, but working on pointers (or other
 498  * pointer-sized values).
 499  *
 500  * For portability reasons, you may only lock on the bottom 32 bits of
 501  * the pointer.
 502  *
 503  * Since: 2.30
 504  **/
 505 void
 506 (g_pointer_bit_unlock) (volatile void *address,
 507                         gint           lock_bit)
 508 {
 509   g_return_if_fail (lock_bit < 32);
 510
 511   {
 512 #ifdef USE_ASM_GOTO
 513     asm volatile ("lock btr %1, (%0)"
 514                   : /* no output */
 515                   : "r" (address), "r" ((gsize) lock_bit)
 516                   : "cc", "memory");
 517 #else
 518     volatile gsize *pointer_address = address;
 519     gsize mask = 1u << lock_bit;
 520
 521     g_atomic_pointer_and (pointer_address, ~mask);
 522 #endif
 523
 524     {
 525       guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
 526       if (g_atomic_int_get (&g_bit_lock_contended[class]))
 527         g_futex_wake (g_futex_int_address (address));
 528     }
 529   }
 530 }