[lttng-tools.git] / src / common / futex.c

/*
 * Copyright (C)  2011 - David Goulet <david.goulet@polymtl.ca>
 *                       Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License, version 2 only,
 * as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */

#define _GNU_SOURCE
#define _LGPL_SOURCE
#include <limits.h>
#include <sys/syscall.h>
#include <unistd.h>
#include <urcu.h>
#include <urcu/futex.h>

#include <common/common.h>

#include "futex.h"

/*
 * This futex wait/wake scheme only works for N wakers / 1 waiters. Hence the
 * "nto1" added to all function signature.
 *
 * Please see wait_gp()/update_counter_and_wait() calls in urcu.c in the urcu
 * git tree for a detail example of this scheme being used. futex_async() is
 * the urcu wrapper over the futex() sycall.
 *
 * There is also a formal verification available in the git tree.
 *
 *   branch: formal-model
 *   commit id: 2a8044f3493046fcc8c67016902dc7beec6f026a
 *
 * Ref: git://git.lttng.org/userspace-rcu.git
 */

/*
 * Update futex according to active or not. This scheme is used to wake every
 * libust waiting on the shared memory map futex hence the INT_MAX used in the
 * futex() call. If active, we set the value and wake everyone else we indicate
 * that we are gone (cleanup() case).
 */
LTTNG_HIDDEN
void futex_wait_update(int32_t *futex, int active)
{
	if (active) {
		uatomic_set(futex, 1);
		if (futex_async(futex, FUTEX_WAKE,
				INT_MAX, NULL, NULL, 0) < 0) {
			PERROR("futex_async");
			abort();
		}
	} else {
		uatomic_set(futex, 0);
	}

	DBG("Futex wait update active %d", active);
}

/*
 * Prepare futex.
 */
LTTNG_HIDDEN
void futex_nto1_prepare(int32_t *futex)
{
	uatomic_set(futex, -1);
	cmm_smp_mb();

	DBG("Futex n to 1 prepare done");
}

/*
 * Wait futex.
 */
LTTNG_HIDDEN
void futex_nto1_wait(int32_t *futex)
{
	cmm_smp_mb();

	if (uatomic_read(futex) != -1)
		goto end;
	while (futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
		switch (errno) {
		case EWOULDBLOCK:
			/* Value already changed. */
			goto end;
		case EINTR:
			/* Retry if interrupted by signal. */
			break;	/* Get out of switch. */
		default:
			/* Unexpected error. */
			PERROR("futex_async");
			abort();
		}
	}
end:
	DBG("Futex n to 1 wait done");
}

/*
 * Wake 1 futex.
 */
LTTNG_HIDDEN
void futex_nto1_wake(int32_t *futex)
{
	if (caa_unlikely(uatomic_read(futex) != -1))
		goto end;
	uatomic_set(futex, 0);
	if (futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0) < 0) {
		PERROR("futex_async");
		abort();
	}
end:
	DBG("Futex n to 1 wake done");
}
Commit	Line	Data
	1	/*
	2	* Copyright (C) 2011 - David Goulet <david.goulet@polymtl.ca>
	3	* Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
	4	*
	5	* This program is free software; you can redistribute it and/or modify
	6	* it under the terms of the GNU General Public License, version 2 only,
	7	* as published by the Free Software Foundation.
	8	*
	9	* This program 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
	12	* GNU General Public License for more details.
	13	*
	14	* You should have received a copy of the GNU General Public License along
	15	* with this program; if not, write to the Free Software Foundation, Inc.,
	16	* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
	17	*/
	18
	19	#define _GNU_SOURCE
	20	#define _LGPL_SOURCE
	21	#include <limits.h>
	22	#include <sys/syscall.h>
	23	#include <unistd.h>
	24	#include <urcu.h>
	25	#include <urcu/futex.h>
	26
	27	#include <common/common.h>
	28
	29	#include "futex.h"
	30
	31	/*
	32	* This futex wait/wake scheme only works for N wakers / 1 waiters. Hence the
	33	* "nto1" added to all function signature.
	34	*
	35	* Please see wait_gp()/update_counter_and_wait() calls in urcu.c in the urcu
	36	* git tree for a detail example of this scheme being used. futex_async() is
	37	* the urcu wrapper over the futex() sycall.
	38	*
	39	* There is also a formal verification available in the git tree.
	40	*
	41	* branch: formal-model
	42	* commit id: 2a8044f3493046fcc8c67016902dc7beec6f026a
	43	*
	44	* Ref: git://git.lttng.org/userspace-rcu.git
	45	*/
	46
	47	/*
	48	* Update futex according to active or not. This scheme is used to wake every
	49	* libust waiting on the shared memory map futex hence the INT_MAX used in the
	50	* futex() call. If active, we set the value and wake everyone else we indicate
	51	* that we are gone (cleanup() case).
	52	*/
	53	LTTNG_HIDDEN
	54	void futex_wait_update(int32_t *futex, int active)
	55	{
	56	if (active) {
	57	uatomic_set(futex, 1);
	58	if (futex_async(futex, FUTEX_WAKE,
	59	INT_MAX, NULL, NULL, 0) < 0) {
	60	PERROR("futex_async");
	61	abort();
	62	}
	63	} else {
	64	uatomic_set(futex, 0);
	65	}
	66
	67	DBG("Futex wait update active %d", active);
	68	}
	69
	70	/*
	71	* Prepare futex.
	72	*/
	73	LTTNG_HIDDEN
	74	void futex_nto1_prepare(int32_t *futex)
	75	{
	76	uatomic_set(futex, -1);
	77	cmm_smp_mb();
	78
	79	DBG("Futex n to 1 prepare done");
	80	}
	81
	82	/*
	83	* Wait futex.
	84	*/
	85	LTTNG_HIDDEN
	86	void futex_nto1_wait(int32_t *futex)
	87	{
	88	cmm_smp_mb();
	89
	90	if (uatomic_read(futex) != -1)
	91	goto end;
	92	while (futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
	93	switch (errno) {
	94	case EWOULDBLOCK:
	95	/* Value already changed. */
	96	goto end;
	97	case EINTR:
	98	/* Retry if interrupted by signal. */
	99	break; /* Get out of switch. */
	100	default:
	101	/* Unexpected error. */
	102	PERROR("futex_async");
	103	abort();
	104	}
	105	}
	106	end:
	107	DBG("Futex n to 1 wait done");
	108	}
	109
	110	/*
	111	* Wake 1 futex.
	112	*/
	113	LTTNG_HIDDEN
	114	void futex_nto1_wake(int32_t *futex)
	115	{
	116	if (caa_unlikely(uatomic_read(futex) != -1))
	117	goto end;
	118	uatomic_set(futex, 0);
	119	if (futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0) < 0) {
	120	PERROR("futex_async");
	121	abort();
	122	}
	123	end:
	124	DBG("Futex n to 1 wake done");
	125	}