[lttng-tools.git] / src / common / waiter.cpp

/*
 * Copyright (C) 2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 * Copyright (C) 2017 Jérémie Galarneau <jeremie.galarneau@efficios.com>
 *
 * SPDX-License-Identifier: LGPL-2.1-only
 *
 */

#include "waiter.hpp"
#include <urcu/uatomic.h>
#include <urcu/futex.h>
#include "error.hpp"
#include <poll.h>

/*
 * Number of busy-loop attempts before waiting on futex.
 */
#define WAIT_ATTEMPTS 1000

enum waiter_state {
	/* WAITER_WAITING is compared directly (futex compares it). */
	WAITER_WAITING =	0,
	/* non-zero are used as masks. */
	WAITER_WOKEN_UP =	(1 << 0),
	WAITER_RUNNING =	(1 << 1),
	WAITER_TEARDOWN =	(1 << 2),
};

void lttng_waiter_init(struct lttng_waiter *waiter)
{
	cds_wfs_node_init(&waiter->wait_queue_node);
	uatomic_set(&waiter->state, WAITER_WAITING);
	cmm_smp_mb();
}

/*
 * User must init "waiter" before passing its memory to waker thread.
 */
void lttng_waiter_wait(struct lttng_waiter *waiter)
{
	unsigned int i;

	DBG("Beginning of waiter wait period");
	/* Load and test condition before read state */
	cmm_smp_rmb();
	for (i = 0; i < WAIT_ATTEMPTS; i++) {
		if (uatomic_read(&waiter->state) != WAITER_WAITING) {
			goto skip_futex_wait;
		}
		caa_cpu_relax();
	}
	while (futex_noasync(&waiter->state, FUTEX_WAIT, WAITER_WAITING,
			NULL, NULL, 0)) {
		switch (errno) {
		case EWOULDBLOCK:
			/* Value already changed. */
			goto skip_futex_wait;
		case EINTR:
			/* Retry if interrupted by signal. */
			break;	/* Get out of switch. */
		default:
			/* Unexpected error. */
			PERROR("futex_noasync");
			abort();
		}
	}
skip_futex_wait:

	/* Tell waker thread than we are running. */
	uatomic_or(&waiter->state, WAITER_RUNNING);

	/*
	 * Wait until waker thread lets us know it's ok to tear down
	 * memory allocated for struct lttng_waiter.
	 */
	for (i = 0; i < WAIT_ATTEMPTS; i++) {
		if (uatomic_read(&waiter->state) & WAITER_TEARDOWN) {
			break;
		}
		caa_cpu_relax();
	}
	while (!(uatomic_read(&waiter->state) & WAITER_TEARDOWN)) {
		poll(NULL, 0, 10);
	}
	LTTNG_ASSERT(uatomic_read(&waiter->state) & WAITER_TEARDOWN);
	DBG("End of waiter wait period");
}

/*
 * Note: lttng_waiter_wake needs waiter to stay allocated throughout its
 * execution. In this scheme, the waiter owns the node memory, and we only allow
 * it to free this memory when it sees the WAITER_TEARDOWN flag.
 */
void lttng_waiter_wake_up(struct lttng_waiter *waiter)
{
	cmm_smp_mb();
	LTTNG_ASSERT(uatomic_read(&waiter->state) == WAITER_WAITING);
	uatomic_set(&waiter->state, WAITER_WOKEN_UP);
	if (!(uatomic_read(&waiter->state) & WAITER_RUNNING)) {
		if (futex_noasync(&waiter->state, FUTEX_WAKE, 1,
				NULL, NULL, 0) < 0) {
			PERROR("futex_noasync");
			abort();
		}
	}
	/* Allow teardown of struct urcu_wait memory. */
	uatomic_or(&waiter->state, WAITER_TEARDOWN);
}
Commit	Line	Data
287a512f	1	/*
ab5be9fa MJ	2	* Copyright (C) 2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
ab5be9fa MJ	3	* Copyright (C) 2017 Jérémie Galarneau <jeremie.galarneau@efficios.com>
287a512f	4	*
ab5be9fa	5	* SPDX-License-Identifier: LGPL-2.1-only
287a512f	6	*
287a512f JG	7	*/
287a512f JG	8
c9e313bc	9	#include "waiter.hpp"
287a512f JG	10	#include <urcu/uatomic.h>
287a512f JG	11	#include <urcu/futex.h>
c9e313bc	12	#include "error.hpp"
287a512f JG	13	#include <poll.h>
	14
	15	/*
	16	* Number of busy-loop attempts before waiting on futex.
	17	*/
	18	#define WAIT_ATTEMPTS 1000
	19
	20	enum waiter_state {
	21	/* WAITER_WAITING is compared directly (futex compares it). */
	22	WAITER_WAITING = 0,
	23	/* non-zero are used as masks. */
	24	WAITER_WOKEN_UP = (1 << 0),
	25	WAITER_RUNNING = (1 << 1),
	26	WAITER_TEARDOWN = (1 << 2),
	27	};
	28
287a512f JG	29	void lttng_waiter_init(struct lttng_waiter *waiter)
	30	{
	31	cds_wfs_node_init(&waiter->wait_queue_node);
	32	uatomic_set(&waiter->state, WAITER_WAITING);
	33	cmm_smp_mb();
	34	}
	35
	36	/*
	37	* User must init "waiter" before passing its memory to waker thread.
	38	*/
287a512f JG	39	void lttng_waiter_wait(struct lttng_waiter *waiter)
	40	{
	41	unsigned int i;
	42
	43	DBG("Beginning of waiter wait period");
	44	/* Load and test condition before read state */
	45	cmm_smp_rmb();
	46	for (i = 0; i < WAIT_ATTEMPTS; i++) {
	47	if (uatomic_read(&waiter->state) != WAITER_WAITING) {
	48	goto skip_futex_wait;
	49	}
	50	caa_cpu_relax();
	51	}
	52	while (futex_noasync(&waiter->state, FUTEX_WAIT, WAITER_WAITING,
	53	NULL, NULL, 0)) {
	54	switch (errno) {
	55	case EWOULDBLOCK:
	56	/* Value already changed. */
	57	goto skip_futex_wait;
	58	case EINTR:
	59	/* Retry if interrupted by signal. */
	60	break; /* Get out of switch. */
	61	default:
	62	/* Unexpected error. */
	63	PERROR("futex_noasync");
	64	abort();
	65	}
	66	}
	67	skip_futex_wait:
	68
	69	/* Tell waker thread than we are running. */
	70	uatomic_or(&waiter->state, WAITER_RUNNING);
	71
	72	/*
	73	* Wait until waker thread lets us know it's ok to tear down
	74	* memory allocated for struct lttng_waiter.
	75	*/
	76	for (i = 0; i < WAIT_ATTEMPTS; i++) {
	77	if (uatomic_read(&waiter->state) & WAITER_TEARDOWN) {
	78	break;
	79	}
	80	caa_cpu_relax();
	81	}
	82	while (!(uatomic_read(&waiter->state) & WAITER_TEARDOWN)) {
	83	poll(NULL, 0, 10);
	84	}
a0377dfe	85	LTTNG_ASSERT(uatomic_read(&waiter->state) & WAITER_TEARDOWN);
287a512f JG	86	DBG("End of waiter wait period");
	87	}
	88
	89	/*
	90	* Note: lttng_waiter_wake needs waiter to stay allocated throughout its
	91	* execution. In this scheme, the waiter owns the node memory, and we only allow
	92	* it to free this memory when it sees the WAITER_TEARDOWN flag.
	93	*/
287a512f JG	94	void lttng_waiter_wake_up(struct lttng_waiter *waiter)
	95	{
	96	cmm_smp_mb();
a0377dfe	97	LTTNG_ASSERT(uatomic_read(&waiter->state) == WAITER_WAITING);
287a512f JG	98	uatomic_set(&waiter->state, WAITER_WOKEN_UP);
	99	if (!(uatomic_read(&waiter->state) & WAITER_RUNNING)) {
	100	if (futex_noasync(&waiter->state, FUTEX_WAKE, 1,
	101	NULL, NULL, 0) < 0) {
	102	PERROR("futex_noasync");
	103	abort();
	104	}
	105	}
	106	/* Allow teardown of struct urcu_wait memory. */
	107	uatomic_or(&waiter->state, WAITER_TEARDOWN);
	108	}