2 * Copyright © 2008 Ryan Lortie
3 * Copyright © 2010 Codethink Limited
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.
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.
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.
20 * Author: Ryan Lortie <desrt@desrt.ca>
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>
31 #include "gthreadprivate.h"
35 #ifdef G_BIT_LOCK_FORCE_FUTEX_EMULATION
40 static GMutex g_futex_mutex = G_MUTEX_INIT;
41 static GSList *g_futex_address_list = NULL;
46 * We have headers for futex(2) on the build machine. This does not
47 * imply that every system that ever runs the resulting glib will have
48 * kernel support for futex, but you'd have to have a pretty old
49 * kernel in order for that not to be the case.
51 * If anyone actually gets bit by this, please file a bug. :)
53 #include <linux/futex.h>
54 #include <sys/syscall.h>
59 * @address: a pointer to an integer
60 * @value: the value that should be at @address
62 * Atomically checks that the value stored at @address is equal to
63 * @value and then blocks. If the value stored at @address is not
64 * equal to @value then this function returns immediately.
66 * To unblock, call g_futex_wake() on @address.
68 * This call may spuriously unblock (for example, in response to the
69 * process receiving a signal) but this is not guaranteed. Unlike the
70 * Linux system call of a similar name, there is no guarantee that a
71 * waiting process will unblock due to a g_futex_wake() call in a
75 g_futex_wait (const volatile gint *address,
78 syscall (__NR_futex, address, (gsize) FUTEX_WAIT, (gsize) value, NULL);
83 * @address: a pointer to an integer
85 * Nominally, wakes one thread that is blocked in g_futex_wait() on
86 * @address (if any thread is currently waiting).
88 * As mentioned in the documention for g_futex_wait(), spurious
89 * wakeups may occur. As such, this call may result in more than one
90 * thread being woken up.
93 g_futex_wake (const volatile gint *address)
95 syscall (__NR_futex, address, (gsize) FUTEX_WAKE, (gsize) 1, NULL);
100 /* emulate futex(2) */
103 const volatile gint *address;
109 g_futex_find_address (const volatile gint *address)
113 for (node = g_futex_address_list; node; node = node->next)
115 WaitAddress *waiter = node->data;
117 if (waiter->address == address)
125 g_futex_wait (const volatile gint *address,
128 g_mutex_lock (&g_futex_mutex);
129 if G_LIKELY (g_atomic_int_get (address) == value)
133 if ((waiter = g_futex_find_address (address)) == NULL)
135 waiter = g_slice_new (WaitAddress);
136 waiter->address = address;
137 waiter->wait_queue = g_cond_new ();
138 waiter->ref_count = 0;
139 g_futex_address_list =
140 g_slist_prepend (g_futex_address_list, waiter);
144 g_cond_wait (waiter->wait_queue, &g_futex_mutex);
146 if (!--waiter->ref_count)
148 g_futex_address_list =
149 g_slist_remove (g_futex_address_list, waiter);
150 g_cond_free (waiter->wait_queue);
151 g_slice_free (WaitAddress, waiter);
154 g_mutex_unlock (&g_futex_mutex);
158 g_futex_wake (const volatile gint *address)
162 /* need to lock here for two reasons:
163 * 1) need to acquire/release lock to ensure waiter is not in
164 * the process of registering a wait
165 * 2) need to -stay- locked until the end to ensure a wake()
166 * in another thread doesn't cause 'waiter' to stop existing
168 g_mutex_lock (&g_futex_mutex);
169 if ((waiter = g_futex_find_address (address)))
170 g_cond_signal (waiter->wait_queue);
171 g_mutex_unlock (&g_futex_mutex);
175 #define CONTENTION_CLASSES 11
176 static volatile gint g_bit_lock_contended[CONTENTION_CLASSES];
178 #if (defined (i386) || defined (__amd64__))
179 #if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 5)
180 #define USE_ASM_GOTO 1
186 * @address: a pointer to an integer
187 * @lock_bit: a bit value between 0 and 31
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
193 * Attempting to lock on two different bits within the same integer is
194 * not supported and will very probably cause deadlocks.
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.
199 * This function accesses @address atomically. All other accesses to
200 * @address must be atomic in order for this function to work
206 g_bit_lock (volatile gint *address,
211 asm volatile goto ("lock bts %1, (%0)\n"
214 : "r" (address), "r" (lock_bit)
221 guint mask = 1u << lock_bit;
224 v = g_atomic_int_get (address);
227 guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
229 g_atomic_int_add (&g_bit_lock_contended[class], +1);
230 g_futex_wait (address, v);
231 g_atomic_int_add (&g_bit_lock_contended[class], -1);
236 guint mask = 1u << lock_bit;
240 v = g_atomic_int_or (address, mask);
244 guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
246 g_atomic_int_add (&g_bit_lock_contended[class], +1);
247 g_futex_wait (address, v);
248 g_atomic_int_add (&g_bit_lock_contended[class], -1);
257 * @address: a pointer to an integer
258 * @lock_bit: a bit value between 0 and 31
259 * @returns: %TRUE if the lock was acquired
261 * Sets the indicated @lock_bit in @address, returning %TRUE if
262 * successful. If the bit is already set, returns %FALSE immediately.
264 * Attempting to lock on two different bits within the same integer is
267 * The value of the bit that is set is (1u << @bit). If @bit is not
268 * between 0 and 31 then the result is undefined.
270 * This function accesses @address atomically. All other accesses to
271 * @address must be atomic in order for this function to work
277 g_bit_trylock (volatile gint *address,
283 asm volatile ("lock bts %2, (%1)\n"
287 : "r" (address), "r" (lock_bit)
292 guint mask = 1u << lock_bit;
295 v = g_atomic_int_or (address, mask);
303 * @address: a pointer to an integer
304 * @lock_bit: a bit value between 0 and 31
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
310 * This function accesses @address atomically. All other accesses to
311 * @address must be atomic in order for this function to work
317 g_bit_unlock (volatile gint *address,
321 asm volatile ("lock btr %1, (%0)"
323 : "r" (address), "r" (lock_bit)
326 guint mask = 1u << lock_bit;
328 g_atomic_int_and (address, ~mask);
332 guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
334 if (g_atomic_int_get (&g_bit_lock_contended[class]))
335 g_futex_wake (address);
340 /* We emulate pointer-sized futex(2) because the kernel API only
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.
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.
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.
356 * After that, pointer-sized futex becomes as simple as:
358 * g_futex_wait (g_futex_int_address (address), (guint) value);
362 * g_futex_wake (g_futex_int_address (int_address));
364 static const volatile gint *
365 g_futex_int_address (const volatile void *address)
367 const volatile gint *int_address = address;
369 #if G_BYTE_ORDER == G_BIG_ENDIAN && GLIB_SIZEOF_VOID_P == 8
377 * g_pointer_bit_lock:
378 * @address: a pointer to a #gpointer-sized value
379 * @lock_bit: a bit value between 0 and 31
381 * This is equivalent to g_bit_lock, but working on pointers (or other
382 * pointer-sized values).
384 * For portability reasons, you may only lock on the bottom 32 bits of
390 (g_pointer_bit_lock) (volatile void *address,
393 g_return_if_fail (lock_bit < 32);
398 asm volatile goto ("lock bts %1, (%0)\n"
401 : "r" (address), "r" ((gsize) lock_bit)
408 volatile gsize *pointer_address = address;
409 gsize mask = 1u << lock_bit;
412 v = (gsize) g_atomic_pointer_get (pointer_address);
415 guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
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);
424 volatile gsize *pointer_address = address;
425 gsize mask = 1u << lock_bit;
429 v = g_atomic_pointer_or (pointer_address, mask);
433 guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
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);
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 * @returns: %TRUE if the lock was acquired
451 * This is equivalent to g_bit_trylock, but working on pointers (or
452 * other pointer-sized values).
454 * For portability reasons, you may only lock on the bottom 32 bits of
460 (g_pointer_bit_trylock) (volatile void *address,
463 g_return_val_if_fail (lock_bit < 32, FALSE);
469 asm volatile ("lock bts %2, (%1)\n"
473 : "r" (address), "r" ((gsize) lock_bit)
478 volatile gsize *pointer_address = address;
479 gsize mask = 1u << lock_bit;
482 g_return_val_if_fail (lock_bit < 32, FALSE);
484 v = g_atomic_pointer_or (pointer_address, mask);
492 * g_pointer_bit_unlock:
493 * @address: a pointer to a #gpointer-sized value
494 * @lock_bit: a bit value between 0 and 31
496 * This is equivalent to g_bit_unlock, but working on pointers (or other
497 * pointer-sized values).
499 * For portability reasons, you may only lock on the bottom 32 bits of
505 (g_pointer_bit_unlock) (volatile void *address,
508 g_return_if_fail (lock_bit < 32);
512 asm volatile ("lock btr %1, (%0)"
514 : "r" (address), "r" ((gsize) lock_bit)
517 volatile gsize *pointer_address = address;
518 gsize mask = 1u << lock_bit;
520 g_atomic_pointer_and (pointer_address, ~mask);
524 guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
525 if (g_atomic_int_get (&g_bit_lock_contended[class]))
526 g_futex_wake (g_futex_int_address (address));