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

/*
 * Copyright (C) 2011 EfficiOS Inc.
 * Copyright (C) 2011 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 *
 * SPDX-License-Identifier: GPL-2.0-only
 *
 */

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

#include <common/common.hpp>

#include "futex.hpp"

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

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

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

	while (uatomic_read(futex) == -1) {
		if (!futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
			/*
			 * Prior queued wakeups queued by unrelated code
			 * using the same address can cause futex wait to
			 * return 0 even through the futex value is still
			 * -1 (spurious wakeups). Check the value again
			 * in user-space to validate whether it really
			 * differs from -1.
			 */
			continue;
		}
		switch (errno) {
		case EAGAIN:
			/* Value already changed. */
			goto end;
		case EINTR:
			/* Retry if interrupted by signal. */
			break;	/* Get out of switch. Check again. */
		default:
			/* Unexpected error. */
			PERROR("futex_async");
			abort();
		}
	}
end:
	DBG("Futex n to 1 wait done");
}

/*
 * Wake 1 futex.
 */
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 EfficiOS Inc.
	3	* Copyright (C) 2011 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
	4	*
	5	* SPDX-License-Identifier: GPL-2.0-only
	6	*
	7	*/
	8
	9	#define _LGPL_SOURCE
	10	#include <limits.h>
	11	#include <unistd.h>
	12	#include <urcu.h>
	13	#include <urcu/futex.h>
	14
	15	#include <common/common.hpp>
	16
	17	#include "futex.hpp"
	18
	19	/*
	20	* This futex wait/wake scheme only works for N wakers / 1 waiters. Hence the
	21	* "nto1" added to all function signature.
	22	*
	23	* Please see wait_gp()/update_counter_and_wait() calls in urcu.c in the urcu
	24	* git tree for a detail example of this scheme being used. futex_async() is
	25	* the urcu wrapper over the futex() sycall.
	26	*
	27	* There is also a formal verification available in the git tree.
	28	*
	29	* branch: formal-model
	30	* commit id: 2a8044f3493046fcc8c67016902dc7beec6f026a
	31	*
	32	* Ref: git://git.lttng.org/userspace-rcu.git
	33	*/
	34
	35	/*
	36	* Update futex according to active or not. This scheme is used to wake every
	37	* libust waiting on the shared memory map futex hence the INT_MAX used in the
	38	* futex() call. If active, we set the value and wake everyone else we indicate
	39	* that we are gone (cleanup() case).
	40	*/
	41	void futex_wait_update(int32_t *futex, int active)
	42	{
	43	if (active) {
	44	uatomic_set(futex, 1);
	45	if (futex_async(futex, FUTEX_WAKE,
	46	INT_MAX, NULL, NULL, 0) < 0) {
	47	PERROR("futex_async");
	48	abort();
	49	}
	50	} else {
	51	uatomic_set(futex, 0);
	52	}
	53
	54	DBG("Futex wait update active %d", active);
	55	}
	56
	57	/*
	58	* Prepare futex.
	59	*/
	60	void futex_nto1_prepare(int32_t *futex)
	61	{
	62	uatomic_set(futex, -1);
	63	cmm_smp_mb();
	64
	65	DBG("Futex n to 1 prepare done");
	66	}
	67
	68	/*
	69	* Wait futex.
	70	*/
	71	void futex_nto1_wait(int32_t *futex)
	72	{
	73	cmm_smp_mb();
	74
	75	while (uatomic_read(futex) == -1) {
	76	if (!futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
	77	/*
	78	* Prior queued wakeups queued by unrelated code
	79	* using the same address can cause futex wait to
	80	* return 0 even through the futex value is still
	81	* -1 (spurious wakeups). Check the value again
	82	* in user-space to validate whether it really
	83	* differs from -1.
	84	*/
	85	continue;
	86	}
	87	switch (errno) {
	88	case EAGAIN:
	89	/* Value already changed. */
	90	goto end;
	91	case EINTR:
	92	/* Retry if interrupted by signal. */
	93	break; /* Get out of switch. Check again. */
	94	default:
	95	/* Unexpected error. */
	96	PERROR("futex_async");
	97	abort();
	98	}
	99	}
	100	end:
	101	DBG("Futex n to 1 wait done");
	102	}
	103
	104	/*
	105	* Wake 1 futex.
	106	*/
	107	void futex_nto1_wake(int32_t *futex)
	108	{
	109	if (caa_unlikely(uatomic_read(futex) != -1))
	110	goto end;
	111	uatomic_set(futex, 0);
	112	if (futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0) < 0) {
	113	PERROR("futex_async");
	114	abort();
	115	}
	116	end:
	117	DBG("Futex n to 1 wake done");
	118	}