[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 "futex.hpp"

#include <common/common.hpp>

#include <limits.h>
#include <unistd.h>
#include <urcu.h>
#include <urcu/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).
 */
void futex_wait_update(int32_t *futex, int active)
{
	if (active) {
		uatomic_set(futex, 1);
		if (futex_async(futex, FUTEX_WAKE, INT_MAX, nullptr, nullptr, 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, nullptr, nullptr, 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, nullptr, nullptr, 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 "futex.hpp"
	11
	12	#include <common/common.hpp>
	13
	14	#include <limits.h>
	15	#include <unistd.h>
	16	#include <urcu.h>
	17	#include <urcu/futex.h>
	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, INT_MAX, nullptr, nullptr, 0) < 0) {
	46	PERROR("futex_async");
	47	abort();
	48	}
	49	} else {
	50	uatomic_set(futex, 0);
	51	}
	52
	53	DBG("Futex wait update active %d", active);
	54	}
	55
	56	/*
	57	* Prepare futex.
	58	*/
	59	void futex_nto1_prepare(int32_t *futex)
	60	{
	61	uatomic_set(futex, -1);
	62	cmm_smp_mb();
	63
	64	DBG("Futex n to 1 prepare done");
	65	}
	66
	67	/*
	68	* Wait futex.
	69	*/
	70	void futex_nto1_wait(int32_t *futex)
	71	{
	72	cmm_smp_mb();
	73
	74	while (uatomic_read(futex) == -1) {
	75	if (!futex_async(futex, FUTEX_WAIT, -1, nullptr, nullptr, 0)) {
	76	/*
	77	* Prior queued wakeups queued by unrelated code
	78	* using the same address can cause futex wait to
	79	* return 0 even through the futex value is still
	80	* -1 (spurious wakeups). Check the value again
	81	* in user-space to validate whether it really
	82	* differs from -1.
	83	*/
	84	continue;
	85	}
	86	switch (errno) {
	87	case EAGAIN:
	88	/* Value already changed. */
	89	goto end;
	90	case EINTR:
	91	/* Retry if interrupted by signal. */
	92	break; /* Get out of switch. Check again. */
	93	default:
	94	/* Unexpected error. */
	95	PERROR("futex_async");
	96	abort();
	97	}
	98	}
	99	end:
	100	DBG("Futex n to 1 wait done");
	101	}
	102
	103	/*
	104	* Wake 1 futex.
	105	*/
	106	void futex_nto1_wake(int32_t *futex)
	107	{
	108	if (caa_unlikely(uatomic_read(futex) != -1))
	109	goto end;
	110	uatomic_set(futex, 0);
	111	if (futex_async(futex, FUTEX_WAKE, 1, nullptr, nullptr, 0) < 0) {
	112	PERROR("futex_async");
	113	abort();
	114	}
	115	end:
	116	DBG("Futex n to 1 wake done");
	117	}