[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 _LGPL_SOURCE
#include <limits.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 _LGPL_SOURCE
	20	#include <limits.h>
	21	#include <unistd.h>
	22	#include <urcu.h>
	23	#include <urcu/futex.h>
	24
	25	#include <common/common.h>
	26
	27	#include "futex.h"
	28
	29	/*
	30	* This futex wait/wake scheme only works for N wakers / 1 waiters. Hence the
	31	* "nto1" added to all function signature.
	32	*
	33	* Please see wait_gp()/update_counter_and_wait() calls in urcu.c in the urcu
	34	* git tree for a detail example of this scheme being used. futex_async() is
	35	* the urcu wrapper over the futex() sycall.
	36	*
	37	* There is also a formal verification available in the git tree.
	38	*
	39	* branch: formal-model
	40	* commit id: 2a8044f3493046fcc8c67016902dc7beec6f026a
	41	*
	42	* Ref: git://git.lttng.org/userspace-rcu.git
	43	*/
	44
	45	/*
	46	* Update futex according to active or not. This scheme is used to wake every
	47	* libust waiting on the shared memory map futex hence the INT_MAX used in the
	48	* futex() call. If active, we set the value and wake everyone else we indicate
	49	* that we are gone (cleanup() case).
	50	*/
	51	LTTNG_HIDDEN
	52	void futex_wait_update(int32_t *futex, int active)
	53	{
	54	if (active) {
	55	uatomic_set(futex, 1);
	56	if (futex_async(futex, FUTEX_WAKE,
	57	INT_MAX, NULL, NULL, 0) < 0) {
	58	PERROR("futex_async");
	59	abort();
	60	}
	61	} else {
	62	uatomic_set(futex, 0);
	63	}
	64
	65	DBG("Futex wait update active %d", active);
	66	}
	67
	68	/*
	69	* Prepare futex.
	70	*/
	71	LTTNG_HIDDEN
	72	void futex_nto1_prepare(int32_t *futex)
	73	{
	74	uatomic_set(futex, -1);
	75	cmm_smp_mb();
	76
	77	DBG("Futex n to 1 prepare done");
	78	}
	79
	80	/*
	81	* Wait futex.
	82	*/
	83	LTTNG_HIDDEN
	84	void futex_nto1_wait(int32_t *futex)
	85	{
	86	cmm_smp_mb();
	87
	88	if (uatomic_read(futex) != -1)
	89	goto end;
	90	while (futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
	91	switch (errno) {
	92	case EWOULDBLOCK:
	93	/* Value already changed. */
	94	goto end;
	95	case EINTR:
	96	/* Retry if interrupted by signal. */
	97	break; /* Get out of switch. */
	98	default:
	99	/* Unexpected error. */
	100	PERROR("futex_async");
	101	abort();
	102	}
	103	}
	104	end:
	105	DBG("Futex n to 1 wait done");
	106	}
	107
	108	/*
	109	* Wake 1 futex.
	110	*/
	111	LTTNG_HIDDEN
	112	void futex_nto1_wake(int32_t *futex)
	113	{
	114	if (caa_unlikely(uatomic_read(futex) != -1))
	115	goto end;
	116	uatomic_set(futex, 0);
	117	if (futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0) < 0) {
	118	PERROR("futex_async");
	119	abort();
	120	}
	121	end:
	122	DBG("Futex n to 1 wake done");
	123	}