[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 <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
099e26bd DG	1	/*
	2	* Copyright (C) 2011 - David Goulet <david.goulet@polymtl.ca>
	3	* Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
	4	*
d14d33bf AM	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.
099e26bd DG	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
d14d33bf	11	* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
099e26bd DG	12	* GNU General Public License for more details.
099e26bd DG	13	*
d14d33bf AM	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.
099e26bd DG	17	*/
099e26bd DG	18
6c1c0768	19	#define _LGPL_SOURCE
0fdd1e2c	20	#include <limits.h>
099e26bd DG	21	#include <sys/syscall.h>
	22	#include <unistd.h>
	23	#include <urcu.h>
	24	#include <urcu/futex.h>
	25
90e535ef	26	#include <common/common.h>
099e26bd DG	27
	28	#include "futex.h"
	29
	30	/*
	31	* This futex wait/wake scheme only works for N wakers / 1 waiters. Hence the
	32	* "nto1" added to all function signature.
	33	*
	34	* Please see wait_gp()/update_counter_and_wait() calls in urcu.c in the urcu
	35	* git tree for a detail example of this scheme being used. futex_async() is
	36	* the urcu wrapper over the futex() sycall.
	37	*
	38	* There is also a formal verification available in the git tree.
	39	*
	40	* branch: formal-model
	41	* commit id: 2a8044f3493046fcc8c67016902dc7beec6f026a
	42	*
	43	* Ref: git://git.lttng.org/userspace-rcu.git
	44	*/
	45
0fdd1e2c DG	46	/*
	47	* Update futex according to active or not. This scheme is used to wake every
	48	* libust waiting on the shared memory map futex hence the INT_MAX used in the
	49	* futex() call. If active, we set the value and wake everyone else we indicate
	50	* that we are gone (cleanup() case).
	51	*/
90e535ef	52	LTTNG_HIDDEN
0fdd1e2c DG	53	void futex_wait_update(int32_t *futex, int active)
	54	{
	55	if (active) {
	56	uatomic_set(futex, 1);
549731b7 MD	57	if (futex_async(futex, FUTEX_WAKE,
	58	INT_MAX, NULL, NULL, 0) < 0) {
	59	PERROR("futex_async");
	60	abort();
	61	}
0fdd1e2c DG	62	} else {
	63	uatomic_set(futex, 0);
	64	}
	65
	66	DBG("Futex wait update active %d", active);
	67	}
	68
099e26bd DG	69	/*
	70	* Prepare futex.
	71	*/
90e535ef	72	LTTNG_HIDDEN
099e26bd DG	73	void futex_nto1_prepare(int32_t *futex)
	74	{
	75	uatomic_set(futex, -1);
	76	cmm_smp_mb();
	77
	78	DBG("Futex n to 1 prepare done");
	79	}
	80
	81	/*
	82	* Wait futex.
	83	*/
90e535ef	84	LTTNG_HIDDEN
099e26bd DG	85	void futex_nto1_wait(int32_t *futex)
	86	{
	87	cmm_smp_mb();
	88
549731b7 MD	89	if (uatomic_read(futex) != -1)
	90	goto end;
	91	while (futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
	92	switch (errno) {
	93	case EWOULDBLOCK:
	94	/* Value already changed. */
	95	goto end;
	96	case EINTR:
	97	/* Retry if interrupted by signal. */
	98	break; /* Get out of switch. */
	99	default:
	100	/* Unexpected error. */
	101	PERROR("futex_async");
	102	abort();
	103	}
099e26bd	104	}
549731b7	105	end:
099e26bd DG	106	DBG("Futex n to 1 wait done");
	107	}
	108
	109	/*
	110	* Wake 1 futex.
	111	*/
90e535ef	112	LTTNG_HIDDEN
099e26bd DG	113	void futex_nto1_wake(int32_t *futex)
099e26bd DG	114	{
549731b7 MD	115	if (caa_unlikely(uatomic_read(futex) != -1))
	116	goto end;
	117	uatomic_set(futex, 0);
	118	if (futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0) < 0) {
	119	PERROR("futex_async");
	120	abort();
099e26bd	121	}
549731b7	122	end:
099e26bd DG	123	DBG("Futex n to 1 wake done");
099e26bd DG	124	}