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, see <http://www.gnu.org/licenses/>.
  17  *
  18  * Author: Ryan Lortie <desrt@desrt.ca>
  19  */
  20
  21 #include "config.h"
  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
  33 #ifdef G_BIT_LOCK_FORCE_FUTEX_EMULATION
  34 #undef HAVE_FUTEX
  35 #endif
  36
  37 #ifndef HAVE_FUTEX
  38 static GMutex g_futex_mutex;
  39 static GSList *g_futex_address_list = NULL;
  40 #endif
  41
  42 #ifdef HAVE_FUTEX
  43 /*
  44  * We have headers for futex(2) on the build machine.  This does not
  45  * imply that every system that ever runs the resulting glib will have
  46  * kernel support for futex, but you'd have to have a pretty old
  47  * kernel in order for that not to be the case.
  48  *
  49  * If anyone actually gets bit by this, please file a bug. :)
  50  */
  51 #include <linux/futex.h>
  52 #include <sys/syscall.h>
  53 #include <unistd.h>
  54
  55 #ifndef FUTEX_WAIT_PRIVATE
  56 #define FUTEX_WAIT_PRIVATE FUTEX_WAIT
  57 #define FUTEX_WAKE_PRIVATE FUTEX_WAKE
  58 #endif
  59
  60 /* < private >
  61  * g_futex_wait:
  62  * @address: a pointer to an integer
  63  * @value: the value that should be at @address
  64  *
  65  * Atomically checks that the value stored at @address is equal to
  66  * @value and then blocks.  If the value stored at @address is not
  67  * equal to @value then this function returns immediately.
  68  *
  69  * To unblock, call g_futex_wake() on @address.
  70  *
  71  * This call may spuriously unblock (for example, in response to the
  72  * process receiving a signal) but this is not guaranteed.  Unlike the
  73  * Linux system call of a similar name, there is no guarantee that a
  74  * waiting process will unblock due to a g_futex_wake() call in a
  75  * separate process.
  76  */
  77 static void
  78 g_futex_wait (const volatile gint *address,
  79               gint                 value)
  80 {
  81   syscall (__NR_futex, address, (gsize) FUTEX_WAIT_PRIVATE, (gsize) value, NULL);
  82 }
  83
  84 /* < private >
  85  * g_futex_wake:
  86  * @address: a pointer to an integer
  87  *
  88  * Nominally, wakes one thread that is blocked in g_futex_wait() on
  89  * @address (if any thread is currently waiting).
  90  *
  91  * As mentioned in the documention for g_futex_wait(), spurious
  92  * wakeups may occur.  As such, this call may result in more than one
  93  * thread being woken up.
  94  */
  95 static void
  96 g_futex_wake (const volatile gint *address)
  97 {
  98   syscall (__NR_futex, address, (gsize) FUTEX_WAKE_PRIVATE, (gsize) 1, NULL);
  99 }
 100
 101 #else
 102
 103 /* emulate futex(2) */
 104 typedef struct
 105 {
 106   const volatile gint *address;
 107   gint                 ref_count;
 108   GCond                wait_queue;
 109 } WaitAddress;
 110
 111 static WaitAddress *
 112 g_futex_find_address (const volatile gint *address)
 113 {
 114   GSList *node;
 115
 116   for (node = g_futex_address_list; node; node = node->next)
 117     {
 118       WaitAddress *waiter = node->data;
 119
 120       if (waiter->address == address)
 121         return waiter;
 122     }
 123
 124   return NULL;
 125 }
 126
 127 static void
 128 g_futex_wait (const volatile gint *address,
 129               gint                 value)
 130 {
 131   g_mutex_lock (&g_futex_mutex);
 132   if G_LIKELY (g_atomic_int_get (address) == value)
 133     {
 134       WaitAddress *waiter;
 135
 136       if ((waiter = g_futex_find_address (address)) == NULL)
 137         {
 138           waiter = g_slice_new (WaitAddress);
 139           waiter->address = address;
 140           g_cond_init (&waiter->wait_queue);
 141           waiter->ref_count = 0;
 142           g_futex_address_list =
 143             g_slist_prepend (g_futex_address_list, waiter);
 144         }
 145
 146       waiter->ref_count++;
 147       g_cond_wait (&waiter->wait_queue, &g_futex_mutex);
 148
 149       if (!--waiter->ref_count)
 150         {
 151           g_futex_address_list =
 152             g_slist_remove (g_futex_address_list, waiter);
 153           g_cond_clear (&waiter->wait_queue);
 154           g_slice_free (WaitAddress, waiter);
 155         }
 156     }
 157   g_mutex_unlock (&g_futex_mutex);
 158 }
 159
 160 static void
 161 g_futex_wake (const volatile gint *address)
 162 {
 163   WaitAddress *waiter;
 164
 165   /* need to lock here for two reasons:
 166    *   1) need to acquire/release lock to ensure waiter is not in
 167    *      the process of registering a wait
 168    *   2) need to -stay- locked until the end to ensure a wake()
 169    *      in another thread doesn't cause 'waiter' to stop existing
 170    */
 171   g_mutex_lock (&g_futex_mutex);
 172   if ((waiter = g_futex_find_address (address)))
 173     g_cond_signal (&waiter->wait_queue);
 174   g_mutex_unlock (&g_futex_mutex);
 175 }
 176 #endif
 177
 178 #define CONTENTION_CLASSES 11
 179 static volatile gint g_bit_lock_contended[CONTENTION_CLASSES];
 180
 181 #if (defined (i386) || defined (__amd64__))
 182   #if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 5)
 183     #define USE_ASM_GOTO 1
 184   #endif
 185 #endif
 186
 187 /**
 188  * g_bit_lock:
 189  * @address: a pointer to an integer
 190  * @lock_bit: a bit value between 0 and 31
 191  *
 192  * Sets the indicated @lock_bit in @address.  If the bit is already
 193  * set, this call will block until g_bit_unlock() unsets the
 194  * corresponding bit.
 195  *
 196  * Attempting to lock on two different bits within the same integer is
 197  * not supported and will very probably cause deadlocks.
 198  *
 199  * The value of the bit that is set is (1u << @bit).  If @bit is not
 200  * between 0 and 31 then the result is undefined.
 201  *
 202  * This function accesses @address atomically.  All other accesses to
 203  * @address must be atomic in order for this function to work
 204  * reliably.
 205  *
 206  * Since: 2.24
 207  **/
 208 void
 209 g_bit_lock (volatile gint *address,
 210             gint           lock_bit)
 211 {
 212 #ifdef USE_ASM_GOTO
 213  retry:
 214   __asm__ volatile goto ("lock bts %1, (%0)\n"
 215                          "jc %l[contended]"
 216                          : /* no output */
 217                          : "r" (address), "r" (lock_bit)
 218                          : "cc", "memory"
 219                          : contended);
 220   return;
 221
 222  contended:
 223   {
 224     guint mask = 1u << lock_bit;
 225     guint v;
 226
 227     v = g_atomic_int_get (address);
 228     if (v & mask)
 229       {
 230         guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
 231
 232         g_atomic_int_add (&g_bit_lock_contended[class], +1);
 233         g_futex_wait (address, v);
 234         g_atomic_int_add (&g_bit_lock_contended[class], -1);
 235       }
 236   }
 237   goto retry;
 238 #else
 239   guint mask = 1u << lock_bit;
 240   guint v;
 241
 242  retry:
 243   v = g_atomic_int_or (address, mask);
 244   if (v & mask)
 245     /* already locked */
 246     {
 247       guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
 248
 249       g_atomic_int_add (&g_bit_lock_contended[class], +1);
 250       g_futex_wait (address, v);
 251       g_atomic_int_add (&g_bit_lock_contended[class], -1);
 252
 253       goto retry;
 254     }
 255 #endif
 256 }
 257
 258 /**
 259  * g_bit_trylock:
 260  * @address: a pointer to an integer
 261  * @lock_bit: a bit value between 0 and 31
 262  *
 263  * Sets the indicated @lock_bit in @address, returning %TRUE if
 264  * successful.  If the bit is already set, returns %FALSE immediately.
 265  *
 266  * Attempting to lock on two different bits within the same integer is
 267  * not supported.
 268  *
 269  * The value of the bit that is set is (1u << @bit).  If @bit is not
 270  * between 0 and 31 then the result is undefined.
 271  *
 272  * This function accesses @address atomically.  All other accesses to
 273  * @address must be atomic in order for this function to work
 274  * reliably.
 275  *
 276  * Returns: %TRUE if the lock was acquired
 277  *
 278  * Since: 2.24
 279  **/
 280 gboolean
 281 g_bit_trylock (volatile gint *address,
 282                gint           lock_bit)
 283 {
 284 #ifdef USE_ASM_GOTO
 285   gboolean result;
 286
 287   __asm__ volatile ("lock bts %2, (%1)\n"
 288                     "setnc %%al\n"
 289                     "movzx %%al, %0"
 290                     : "=r" (result)
 291                     : "r" (address), "r" (lock_bit)
 292                     : "cc", "memory");
 293
 294   return result;
 295 #else
 296   guint mask = 1u << lock_bit;
 297   guint v;
 298
 299   v = g_atomic_int_or (address, mask);
 300
 301   return ~v & mask;
 302 #endif
 303 }
 304
 305 /**
 306  * g_bit_unlock:
 307  * @address: a pointer to an integer
 308  * @lock_bit: a bit value between 0 and 31
 309  *
 310  * Clears the indicated @lock_bit in @address.  If another thread is
 311  * currently blocked in g_bit_lock() on this same bit then it will be
 312  * woken up.
 313  *
 314  * This function accesses @address atomically.  All other accesses to
 315  * @address must be atomic in order for this function to work
 316  * reliably.
 317  *
 318  * Since: 2.24
 319  **/
 320 void
 321 g_bit_unlock (volatile gint *address,
 322               gint           lock_bit)
 323 {
 324 #ifdef USE_ASM_GOTO
 325   asm volatile ("lock btr %1, (%0)"
 326                 : /* no output */
 327                 : "r" (address), "r" (lock_bit)
 328                 : "cc", "memory");
 329 #else
 330   guint mask = 1u << lock_bit;
 331
 332   g_atomic_int_and (address, ~mask);
 333 #endif
 334
 335   {
 336     guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
 337
 338     if (g_atomic_int_get (&g_bit_lock_contended[class]))
 339       g_futex_wake (address);
 340   }
 341 }
 342
 343
 344 /* We emulate pointer-sized futex(2) because the kernel API only
 345  * supports integers.
 346  *
 347  * We assume that the 'interesting' part is always the lower order bits.
 348  * This assumption holds because pointer bitlocks are restricted to
 349  * using the low order bits of the pointer as the lock.
 350  *
 351  * On 32 bits, there is nothing to do since the pointer size is equal to
 352  * the integer size.  On little endian the lower-order bits don't move,
 353  * so do nothing.  Only on 64bit big endian do we need to do a bit of
 354  * pointer arithmetic: the low order bits are shifted by 4 bytes.  We
 355  * have a helper function that always does the right thing here.
 356  *
 357  * Since we always consider the low-order bits of the integer value, a
 358  * simple cast from (gsize) to (guint) always takes care of that.
 359  *
 360  * After that, pointer-sized futex becomes as simple as:
 361  *
 362  *   g_futex_wait (g_futex_int_address (address), (guint) value);
 363  *
 364  * and
 365  *
 366  *   g_futex_wake (g_futex_int_address (int_address));
 367  */
 368 static const volatile gint *
 369 g_futex_int_address (const volatile void *address)
 370 {
 371   const volatile gint *int_address = address;
 372
 373   /* this implementation makes these (reasonable) assumptions: */
 374   G_STATIC_ASSERT (G_BYTE_ORDER == G_LITTLE_ENDIAN ||
 375       (G_BYTE_ORDER == G_BIG_ENDIAN &&
 376        sizeof (int) == 4 &&
 377        (sizeof (gpointer) == 4 || sizeof (gpointer) == 8)));
 378
 379 #if G_BYTE_ORDER == G_BIG_ENDIAN && GLIB_SIZEOF_VOID_P == 8
 380   int_address++;
 381 #endif
 382
 383   return int_address;
 384 }
 385
 386 /**
 387  * g_pointer_bit_lock:
 388  * @address: (not nullable): a pointer to a #gpointer-sized value
 389  * @lock_bit: a bit value between 0 and 31
 390  *
 391  * This is equivalent to g_bit_lock, but working on pointers (or other
 392  * pointer-sized values).
 393  *
 394  * For portability reasons, you may only lock on the bottom 32 bits of
 395  * the pointer.
 396  *
 397  * Since: 2.30
 398  **/
 399 void
 400 (g_pointer_bit_lock) (volatile void *address,
 401                       gint           lock_bit)
 402 {
 403   g_return_if_fail (lock_bit < 32);
 404
 405   {
 406 #ifdef USE_ASM_GOTO
 407  retry:
 408     asm volatile goto ("lock bts %1, (%0)\n"
 409                        "jc %l[contended]"
 410                        : /* no output */
 411                        : "r" (address), "r" ((gsize) lock_bit)
 412                        : "cc", "memory"
 413                        : contended);
 414     return;
 415
 416  contended:
 417     {
 418       volatile gsize *pointer_address = address;
 419       gsize mask = 1u << lock_bit;
 420       gsize v;
 421
 422       v = (gsize) g_atomic_pointer_get (pointer_address);
 423       if (v & mask)
 424         {
 425           guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
 426
 427           g_atomic_int_add (&g_bit_lock_contended[class], +1);
 428           g_futex_wait (g_futex_int_address (address), v);
 429           g_atomic_int_add (&g_bit_lock_contended[class], -1);
 430         }
 431     }
 432     goto retry;
 433 #else
 434   volatile gsize *pointer_address = address;
 435   gsize mask = 1u << lock_bit;
 436   gsize v;
 437
 438  retry:
 439   v = g_atomic_pointer_or (pointer_address, mask);
 440   if (v & mask)
 441     /* already locked */
 442     {
 443       guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
 444
 445       g_atomic_int_add (&g_bit_lock_contended[class], +1);
 446       g_futex_wait (g_futex_int_address (address), (guint) v);
 447       g_atomic_int_add (&g_bit_lock_contended[class], -1);
 448
 449       goto retry;
 450     }
 451 #endif
 452   }
 453 }
 454
 455 /**
 456  * g_pointer_bit_trylock:
 457  * @address: (not nullable): a pointer to a #gpointer-sized value
 458  * @lock_bit: a bit value between 0 and 31
 459  *
 460  * This is equivalent to g_bit_trylock, but working on pointers (or
 461  * other pointer-sized values).
 462  *
 463  * For portability reasons, you may only lock on the bottom 32 bits of
 464  * the pointer.
 465  *
 466  * Returns: %TRUE if the lock was acquired
 467  *
 468  * Since: 2.30
 469  **/
 470 gboolean
 471 (g_pointer_bit_trylock) (volatile void *address,
 472                          gint           lock_bit)
 473 {
 474   g_return_val_if_fail (lock_bit < 32, FALSE);
 475
 476   {
 477 #ifdef USE_ASM_GOTO
 478     gboolean result;
 479
 480     asm volatile ("lock bts %2, (%1)\n"
 481                   "setnc %%al\n"
 482                   "movzx %%al, %0"
 483                   : "=r" (result)
 484                   : "r" (address), "r" ((gsize) lock_bit)
 485                   : "cc", "memory");
 486
 487     return result;
 488 #else
 489     volatile gsize *pointer_address = address;
 490     gsize mask = 1u << lock_bit;
 491     gsize v;
 492
 493     g_return_val_if_fail (lock_bit < 32, FALSE);
 494
 495     v = g_atomic_pointer_or (pointer_address, mask);
 496
 497     return ~v & mask;
 498 #endif
 499   }
 500 }
 501
 502 /**
 503  * g_pointer_bit_unlock:
 504  * @address: (not nullable): a pointer to a #gpointer-sized value
 505  * @lock_bit: a bit value between 0 and 31
 506  *
 507  * This is equivalent to g_bit_unlock, but working on pointers (or other
 508  * pointer-sized values).
 509  *
 510  * For portability reasons, you may only lock on the bottom 32 bits of
 511  * the pointer.
 512  *
 513  * Since: 2.30
 514  **/
 515 void
 516 (g_pointer_bit_unlock) (volatile void *address,
 517                         gint           lock_bit)
 518 {
 519   g_return_if_fail (lock_bit < 32);
 520
 521   {
 522 #ifdef USE_ASM_GOTO
 523     asm volatile ("lock btr %1, (%0)"
 524                   : /* no output */
 525                   : "r" (address), "r" ((gsize) lock_bit)
 526                   : "cc", "memory");
 527 #else
 528     volatile gsize *pointer_address = address;
 529     gsize mask = 1u << lock_bit;
 530
 531     g_atomic_pointer_and (pointer_address, ~mask);
 532 #endif
 533
 534     {
 535       guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
 536       if (g_atomic_int_get (&g_bit_lock_contended[class]))
 537         g_futex_wake (g_futex_int_address (address));
 538     }
 539   }
 540 }