[urcu.git] / urcu.c

/*
 * urcu.c
 *
 * Userspace RCU library
 *
 * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
 * Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library 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
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 *
 * IBM's contributions to this file may be relicensed under LGPLv2 or later.
 */

#define _BSD_SOURCE
#include <stdio.h>
#include <pthread.h>
#include <signal.h>
#include <assert.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <poll.h>

#include "urcu-static.h"
/* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
#include "urcu.h"

#ifdef RCU_MEMBARRIER
static int init_done;
int has_sys_membarrier;

void __attribute__((constructor)) rcu_init(void);
#endif

#ifdef RCU_MB
void rcu_init(void)
{
}
#endif

#ifdef RCU_SIGNAL
static int init_done;

void __attribute__((constructor)) rcu_init(void);
void __attribute__((destructor)) rcu_exit(void);
#endif

static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;

int gp_futex;

/*
 * Global grace period counter.
 * Contains the current RCU_GP_CTR_PHASE.
 * Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path.
 * Written to only by writer with mutex taken. Read by both writer and readers.
 */
unsigned long rcu_gp_ctr = RCU_GP_COUNT;

/*
 * Written to only by each individual reader. Read by both the reader and the
 * writers.
 */
struct rcu_reader __thread rcu_reader;

#ifdef DEBUG_YIELD
unsigned int yield_active;
unsigned int __thread rand_yield;
#endif

static LIST_HEAD(registry);

static void mutex_lock(pthread_mutex_t *mutex)
{
	int ret;

#ifndef DISTRUST_SIGNALS_EXTREME
	ret = pthread_mutex_lock(mutex);
	if (ret) {
		perror("Error in pthread mutex lock");
		exit(-1);
	}
#else /* #ifndef DISTRUST_SIGNALS_EXTREME */
	while ((ret = pthread_mutex_trylock(mutex)) != 0) {
		if (ret != EBUSY && ret != EINTR) {
			printf("ret = %d, errno = %d\n", ret, errno);
			perror("Error in pthread mutex lock");
			exit(-1);
		}
		if (rcu_reader.need_mb) {
			smp_mb();
			rcu_reader.need_mb = 0;
			smp_mb();
		}
		poll(NULL,0,10);
	}
#endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
}

static void mutex_unlock(pthread_mutex_t *mutex)
{
	int ret;

	ret = pthread_mutex_unlock(mutex);
	if (ret) {
		perror("Error in pthread mutex unlock");
		exit(-1);
	}
}

#ifdef RCU_MEMBARRIER
static void smp_mb_master(int group)
{
	if (likely(has_sys_membarrier))
		membarrier(MEMBARRIER_EXPEDITED);
	else
		smp_mb();
}
#endif

#ifdef RCU_MB
static void smp_mb_master(int group)
{
	smp_mb();
}
#endif

#ifdef RCU_SIGNAL
static void force_mb_all_readers(void)
{
	struct rcu_reader *index;

	/*
	 * Ask for each threads to execute a smp_mb() so we can consider the
	 * compiler barriers around rcu read lock as real memory barriers.
	 */
	if (list_empty(&registry))
		return;
	/*
	 * pthread_kill has a smp_mb(). But beware, we assume it performs
	 * a cache flush on architectures with non-coherent cache. Let's play
	 * safe and don't assume anything : we use smp_mc() to make sure the
	 * cache flush is enforced.
	 */
	list_for_each_entry(index, &registry, head) {
		index->need_mb = 1;
		smp_mc();	/* write need_mb before sending the signal */
		pthread_kill(index->tid, SIGRCU);
	}
	/*
	 * Wait for sighandler (and thus mb()) to execute on every thread.
	 *
	 * Note that the pthread_kill() will never be executed on systems
	 * that correctly deliver signals in a timely manner.  However, it
	 * is not uncommon for kernels to have bugs that can result in
	 * lost or unduly delayed signals.
	 *
	 * If you are seeing the below pthread_kill() executing much at
	 * all, we suggest testing the underlying kernel and filing the
	 * relevant bug report.  For Linux kernels, we recommend getting
	 * the Linux Test Project (LTP).
	 */
	list_for_each_entry(index, &registry, head) {
		while (index->need_mb) {
			pthread_kill(index->tid, SIGRCU);
			poll(NULL, 0, 1);
		}
	}
	smp_mb();	/* read ->need_mb before ending the barrier */
}

static void smp_mb_master(int group)
{
	force_mb_all_readers();
}
#endif /* #ifdef RCU_SIGNAL */

/*
 * synchronize_rcu() waiting. Single thread.
 */
static void wait_gp(void)
{
	/* Read reader_gp before read futex */
	smp_mb_master(RCU_MB_GROUP);
	if (uatomic_read(&gp_futex) == -1)
		futex_async(&gp_futex, FUTEX_WAIT, -1,
		      NULL, NULL, 0);
}

void update_counter_and_wait(void)
{
	LIST_HEAD(qsreaders);
	int wait_loops = 0;
	struct rcu_reader *index, *tmp;

	/* Switch parity: 0 -> 1, 1 -> 0 */
	STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR_PHASE);

	/*
	 * Must commit qparity update to memory before waiting for other parity
	 * quiescent state. Failure to do so could result in the writer waiting
	 * forever while new readers are always accessing data (no progress).
	 * Ensured by STORE_SHARED and LOAD_SHARED.
	 */

	/*
	 * Adding a smp_mb() which is _not_ formally required, but makes the
	 * model easier to understand. It does not have a big performance impact
	 * anyway, given this is the write-side.
	 */
	smp_mb();

	/*
	 * Wait for each thread rcu_reader.ctr count to become 0.
	 */
	for (;;) {
		wait_loops++;
		if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
			uatomic_dec(&gp_futex);
			/* Write futex before read reader_gp */
			smp_mb_master(RCU_MB_GROUP);
		}

		list_for_each_entry_safe(index, tmp, &registry, head) {
			if (!rcu_old_gp_ongoing(&index->ctr))
				list_move(&index->head, &qsreaders);
		}

#ifndef HAS_INCOHERENT_CACHES
		if (list_empty(&registry)) {
			if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
				/* Read reader_gp before write futex */
				smp_mb_master(RCU_MB_GROUP);
				uatomic_set(&gp_futex, 0);
			}
			break;
		} else {
			if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS)
				wait_gp();
			else
				cpu_relax();
		}
#else /* #ifndef HAS_INCOHERENT_CACHES */
		/*
		 * BUSY-LOOP. Force the reader thread to commit its
		 * rcu_reader.ctr update to memory if we wait for too long.
		 */
		if (list_empty(&registry)) {
			if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
				/* Read reader_gp before write futex */
				smp_mb_master(RCU_MB_GROUP);
				uatomic_set(&gp_futex, 0);
			}
			break;
		} else {
			switch (wait_loops) {
			case RCU_QS_ACTIVE_ATTEMPTS:
				wait_gp();
				break; /* only escape switch */
			case KICK_READER_LOOPS:
				smp_mb_master(RCU_MB_GROUP);
				wait_loops = 0;
				break; /* only escape switch */
			default:
				cpu_relax();
			}
		}
#endif /* #else #ifndef HAS_INCOHERENT_CACHES */
	}
	/* put back the reader list in the registry */
	list_splice(&qsreaders, &registry);
}

void synchronize_rcu(void)
{
	mutex_lock(&rcu_gp_lock);

	if (list_empty(&registry))
		goto out;

	/* All threads should read qparity before accessing data structure
	 * where new ptr points to. Must be done within rcu_gp_lock because it
	 * iterates on reader threads.*/
	/* Write new ptr before changing the qparity */
	smp_mb_master(RCU_MB_GROUP);

	/*
	 * Wait for previous parity to be empty of readers.
	 */
	update_counter_and_wait();	/* 0 -> 1, wait readers in parity 0 */

	/*
	 * Must finish waiting for quiescent state for parity 0 before
	 * committing qparity update to memory. Failure to do so could result in
	 * the writer waiting forever while new readers are always accessing
	 * data (no progress).
	 * Ensured by STORE_SHARED and LOAD_SHARED.
	 */

	/*
	 * Adding a smp_mb() which is _not_ formally required, but makes the
	 * model easier to understand. It does not have a big performance impact
	 * anyway, given this is the write-side.
	 */
	smp_mb();

	/*
	 * Wait for previous parity to be empty of readers.
	 */
	update_counter_and_wait();	/* 1 -> 0, wait readers in parity 1 */

	/* Finish waiting for reader threads before letting the old ptr being
	 * freed. Must be done within rcu_gp_lock because it iterates on reader
	 * threads. */
	smp_mb_master(RCU_MB_GROUP);
out:
	mutex_unlock(&rcu_gp_lock);
}

/*
 * library wrappers to be used by non-LGPL compatible source code.
 */

void rcu_read_lock(void)
{
	_rcu_read_lock();
}

void rcu_read_unlock(void)
{
	_rcu_read_unlock();
}

void rcu_register_thread(void)
{
	rcu_reader.tid = pthread_self();
	assert(rcu_reader.need_mb == 0);
	assert(rcu_reader.ctr == 0);

	mutex_lock(&rcu_gp_lock);
	rcu_init();	/* In case gcc does not support constructor attribute */
	list_add(&rcu_reader.head, &registry);
	mutex_unlock(&rcu_gp_lock);
}

void rcu_unregister_thread(void)
{
	mutex_lock(&rcu_gp_lock);
	list_del(&rcu_reader.head);
	mutex_unlock(&rcu_gp_lock);
}

#ifdef RCU_MEMBARRIER
void rcu_init(void)
{
	if (init_done)
		return;
	init_done = 1;
	if (!membarrier(MEMBARRIER_EXPEDITED | MEMBARRIER_QUERY))
		has_sys_membarrier = 1;
}
#endif

#ifdef RCU_SIGNAL
static void sigrcu_handler(int signo, siginfo_t *siginfo, void *context)
{
	/*
	 * Executing this smp_mb() is the only purpose of this signal handler.
	 * It punctually promotes barrier() into smp_mb() on every thread it is
	 * executed on.
	 */
	smp_mb();
	rcu_reader.need_mb = 0;
	smp_mb();
}

/*
 * rcu_init constructor. Called when the library is linked, but also when
 * reader threads are calling rcu_register_thread().
 * Should only be called by a single thread at a given time. This is ensured by
 * holing the rcu_gp_lock from rcu_register_thread() or by running at library
 * load time, which should not be executed by multiple threads nor concurrently
 * with rcu_register_thread() anyway.
 */
void rcu_init(void)
{
	struct sigaction act;
	int ret;

	if (init_done)
		return;
	init_done = 1;

	act.sa_sigaction = sigrcu_handler;
	act.sa_flags = SA_SIGINFO | SA_RESTART;
	sigemptyset(&act.sa_mask);
	ret = sigaction(SIGRCU, &act, NULL);
	if (ret) {
		perror("Error in sigaction");
		exit(-1);
	}
}

void rcu_exit(void)
{
	struct sigaction act;
	int ret;

	ret = sigaction(SIGRCU, NULL, &act);
	if (ret) {
		perror("Error in sigaction");
		exit(-1);
	}
	assert(act.sa_sigaction == sigrcu_handler);
	assert(list_empty(&registry));
}
#endif /* #ifdef RCU_SIGNAL */
Commit	Line	Data
	1	/*
	2	* urcu.c
	3	*
	4	* Userspace RCU library
	5	*
	6	* Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
	7	* Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
	8	*
	9	* This library is free software; you can redistribute it and/or
	10	* modify it under the terms of the GNU Lesser General Public
	11	* License as published by the Free Software Foundation; either
	12	* version 2.1 of the License, or (at your option) any later version.
	13	*
	14	* This library is distributed in the hope that it will be useful,
	15	* but WITHOUT ANY WARRANTY; without even the implied warranty of
	16	* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
	17	* Lesser General Public License for more details.
	18	*
	19	* You should have received a copy of the GNU Lesser General Public
	20	* License along with this library; if not, write to the Free Software
	21	* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
	22	*
	23	* IBM's contributions to this file may be relicensed under LGPLv2 or later.
	24	*/
	25
	26	#define _BSD_SOURCE
	27	#include <stdio.h>
	28	#include <pthread.h>
	29	#include <signal.h>
	30	#include <assert.h>
	31	#include <stdlib.h>
	32	#include <string.h>
	33	#include <errno.h>
	34	#include <poll.h>
	35
	36	#include "urcu-static.h"
	37	/* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
	38	#include "urcu.h"
	39
	40	#ifdef RCU_MEMBARRIER
	41	static int init_done;
	42	int has_sys_membarrier;
	43
	44	void __attribute__((constructor)) rcu_init(void);
	45	#endif
	46
	47	#ifdef RCU_MB
	48	void rcu_init(void)
	49	{
	50	}
	51	#endif
	52
	53	#ifdef RCU_SIGNAL
	54	static int init_done;
	55
	56	void __attribute__((constructor)) rcu_init(void);
	57	void __attribute__((destructor)) rcu_exit(void);
	58	#endif
	59
	60	static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
	61
	62	int gp_futex;
	63
	64	/*
	65	* Global grace period counter.
	66	* Contains the current RCU_GP_CTR_PHASE.
	67	* Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path.
	68	* Written to only by writer with mutex taken. Read by both writer and readers.
	69	*/
	70	unsigned long rcu_gp_ctr = RCU_GP_COUNT;
	71
	72	/*
	73	* Written to only by each individual reader. Read by both the reader and the
	74	* writers.
	75	*/
	76	struct rcu_reader __thread rcu_reader;
	77
	78	#ifdef DEBUG_YIELD
	79	unsigned int yield_active;
	80	unsigned int __thread rand_yield;
	81	#endif
	82
	83	static LIST_HEAD(registry);
	84
	85	static void mutex_lock(pthread_mutex_t *mutex)
	86	{
	87	int ret;
	88
	89	#ifndef DISTRUST_SIGNALS_EXTREME
	90	ret = pthread_mutex_lock(mutex);
	91	if (ret) {
	92	perror("Error in pthread mutex lock");
	93	exit(-1);
	94	}
	95	#else /* #ifndef DISTRUST_SIGNALS_EXTREME */
	96	while ((ret = pthread_mutex_trylock(mutex)) != 0) {
	97	if (ret != EBUSY && ret != EINTR) {
	98	printf("ret = %d, errno = %d\n", ret, errno);
	99	perror("Error in pthread mutex lock");
	100	exit(-1);
	101	}
	102	if (rcu_reader.need_mb) {
	103	smp_mb();
	104	rcu_reader.need_mb = 0;
	105	smp_mb();
	106	}
	107	poll(NULL,0,10);
	108	}
	109	#endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
	110	}
	111
	112	static void mutex_unlock(pthread_mutex_t *mutex)
	113	{
	114	int ret;
	115
	116	ret = pthread_mutex_unlock(mutex);
	117	if (ret) {
	118	perror("Error in pthread mutex unlock");
	119	exit(-1);
	120	}
	121	}
	122
	123	#ifdef RCU_MEMBARRIER
	124	static void smp_mb_master(int group)
	125	{
	126	if (likely(has_sys_membarrier))
	127	membarrier(MEMBARRIER_EXPEDITED);
	128	else
	129	smp_mb();
	130	}
	131	#endif
	132
	133	#ifdef RCU_MB
	134	static void smp_mb_master(int group)
	135	{
	136	smp_mb();
	137	}
	138	#endif
	139
	140	#ifdef RCU_SIGNAL
	141	static void force_mb_all_readers(void)
	142	{
	143	struct rcu_reader *index;
	144
	145	/*
	146	* Ask for each threads to execute a smp_mb() so we can consider the
	147	* compiler barriers around rcu read lock as real memory barriers.
	148	*/
	149	if (list_empty(&registry))
	150	return;
	151	/*
	152	* pthread_kill has a smp_mb(). But beware, we assume it performs
	153	* a cache flush on architectures with non-coherent cache. Let's play
	154	* safe and don't assume anything : we use smp_mc() to make sure the
	155	* cache flush is enforced.
	156	*/
	157	list_for_each_entry(index, &registry, head) {
	158	index->need_mb = 1;
	159	smp_mc(); /* write need_mb before sending the signal */
	160	pthread_kill(index->tid, SIGRCU);
	161	}
	162	/*
	163	* Wait for sighandler (and thus mb()) to execute on every thread.
	164	*
	165	* Note that the pthread_kill() will never be executed on systems
	166	* that correctly deliver signals in a timely manner. However, it
	167	* is not uncommon for kernels to have bugs that can result in
	168	* lost or unduly delayed signals.
	169	*
	170	* If you are seeing the below pthread_kill() executing much at
	171	* all, we suggest testing the underlying kernel and filing the
	172	* relevant bug report. For Linux kernels, we recommend getting
	173	* the Linux Test Project (LTP).
	174	*/
	175	list_for_each_entry(index, &registry, head) {
	176	while (index->need_mb) {
	177	pthread_kill(index->tid, SIGRCU);
	178	poll(NULL, 0, 1);
	179	}
	180	}
	181	smp_mb(); /* read ->need_mb before ending the barrier */
	182	}
	183
	184	static void smp_mb_master(int group)
	185	{
	186	force_mb_all_readers();
	187	}
	188	#endif /* #ifdef RCU_SIGNAL */
	189
	190	/*
	191	* synchronize_rcu() waiting. Single thread.
	192	*/
	193	static void wait_gp(void)
	194	{
	195	/* Read reader_gp before read futex */
	196	smp_mb_master(RCU_MB_GROUP);
	197	if (uatomic_read(&gp_futex) == -1)
	198	futex_async(&gp_futex, FUTEX_WAIT, -1,
	199	NULL, NULL, 0);
	200	}
	201
	202	void update_counter_and_wait(void)
	203	{
	204	LIST_HEAD(qsreaders);
	205	int wait_loops = 0;
	206	struct rcu_reader index, tmp;
	207
	208	/* Switch parity: 0 -> 1, 1 -> 0 */
	209	STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR_PHASE);
	210
	211	/*
	212	* Must commit qparity update to memory before waiting for other parity
	213	* quiescent state. Failure to do so could result in the writer waiting
	214	* forever while new readers are always accessing data (no progress).
	215	* Ensured by STORE_SHARED and LOAD_SHARED.
	216	*/
	217
	218	/*
	219	* Adding a smp_mb() which is _not_ formally required, but makes the
	220	* model easier to understand. It does not have a big performance impact
	221	* anyway, given this is the write-side.
	222	*/
	223	smp_mb();
	224
	225	/*
	226	* Wait for each thread rcu_reader.ctr count to become 0.
	227	*/
	228	for (;;) {
	229	wait_loops++;
	230	if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
	231	uatomic_dec(&gp_futex);
	232	/* Write futex before read reader_gp */
	233	smp_mb_master(RCU_MB_GROUP);
	234	}
	235
	236	list_for_each_entry_safe(index, tmp, &registry, head) {
	237	if (!rcu_old_gp_ongoing(&index->ctr))
	238	list_move(&index->head, &qsreaders);
	239	}
	240
	241	#ifndef HAS_INCOHERENT_CACHES
	242	if (list_empty(&registry)) {
	243	if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
	244	/* Read reader_gp before write futex */
	245	smp_mb_master(RCU_MB_GROUP);
	246	uatomic_set(&gp_futex, 0);
	247	}
	248	break;
	249	} else {
	250	if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS)
	251	wait_gp();
	252	else
	253	cpu_relax();
	254	}
	255	#else /* #ifndef HAS_INCOHERENT_CACHES */
	256	/*
	257	* BUSY-LOOP. Force the reader thread to commit its
	258	* rcu_reader.ctr update to memory if we wait for too long.
	259	*/
	260	if (list_empty(&registry)) {
	261	if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
	262	/* Read reader_gp before write futex */
	263	smp_mb_master(RCU_MB_GROUP);
	264	uatomic_set(&gp_futex, 0);
	265	}
	266	break;
	267	} else {
	268	switch (wait_loops) {
	269	case RCU_QS_ACTIVE_ATTEMPTS:
	270	wait_gp();
	271	break; /* only escape switch */
	272	case KICK_READER_LOOPS:
	273	smp_mb_master(RCU_MB_GROUP);
	274	wait_loops = 0;
	275	break; /* only escape switch */
	276	default:
	277	cpu_relax();
	278	}
	279	}
	280	#endif /* #else #ifndef HAS_INCOHERENT_CACHES */
	281	}
	282	/* put back the reader list in the registry */
	283	list_splice(&qsreaders, &registry);
	284	}
	285
	286	void synchronize_rcu(void)
	287	{
	288	mutex_lock(&rcu_gp_lock);
	289
	290	if (list_empty(&registry))
	291	goto out;
	292
	293	/* All threads should read qparity before accessing data structure
	294	* where new ptr points to. Must be done within rcu_gp_lock because it
	295	* iterates on reader threads.*/
	296	/* Write new ptr before changing the qparity */
	297	smp_mb_master(RCU_MB_GROUP);
	298
	299	/*
	300	* Wait for previous parity to be empty of readers.
	301	*/
	302	update_counter_and_wait(); /* 0 -> 1, wait readers in parity 0 */
	303
	304	/*
	305	* Must finish waiting for quiescent state for parity 0 before
	306	* committing qparity update to memory. Failure to do so could result in
	307	* the writer waiting forever while new readers are always accessing
	308	* data (no progress).
	309	* Ensured by STORE_SHARED and LOAD_SHARED.
	310	*/
	311
	312	/*
	313	* Adding a smp_mb() which is _not_ formally required, but makes the
	314	* model easier to understand. It does not have a big performance impact
	315	* anyway, given this is the write-side.
	316	*/
	317	smp_mb();
	318
	319	/*
	320	* Wait for previous parity to be empty of readers.
	321	*/
	322	update_counter_and_wait(); /* 1 -> 0, wait readers in parity 1 */
	323
	324	/* Finish waiting for reader threads before letting the old ptr being
	325	* freed. Must be done within rcu_gp_lock because it iterates on reader
	326	* threads. */
	327	smp_mb_master(RCU_MB_GROUP);
	328	out:
	329	mutex_unlock(&rcu_gp_lock);
	330	}
	331
	332	/*
	333	* library wrappers to be used by non-LGPL compatible source code.
	334	*/
	335
	336	void rcu_read_lock(void)
	337	{
	338	_rcu_read_lock();
	339	}
	340
	341	void rcu_read_unlock(void)
	342	{
	343	_rcu_read_unlock();
	344	}
	345
	346	void rcu_register_thread(void)
	347	{
	348	rcu_reader.tid = pthread_self();
	349	assert(rcu_reader.need_mb == 0);
	350	assert(rcu_reader.ctr == 0);
	351
	352	mutex_lock(&rcu_gp_lock);
	353	rcu_init(); /* In case gcc does not support constructor attribute */
	354	list_add(&rcu_reader.head, &registry);
	355	mutex_unlock(&rcu_gp_lock);
	356	}
	357
	358	void rcu_unregister_thread(void)
	359	{
	360	mutex_lock(&rcu_gp_lock);
	361	list_del(&rcu_reader.head);
	362	mutex_unlock(&rcu_gp_lock);
	363	}
	364
	365	#ifdef RCU_MEMBARRIER
	366	void rcu_init(void)
	367	{
	368	if (init_done)
	369	return;
	370	init_done = 1;
	371	if (!membarrier(MEMBARRIER_EXPEDITED \| MEMBARRIER_QUERY))
	372	has_sys_membarrier = 1;
	373	}
	374	#endif
	375
	376	#ifdef RCU_SIGNAL
	377	static void sigrcu_handler(int signo, siginfo_t siginfo, void context)
	378	{
	379	/*
	380	* Executing this smp_mb() is the only purpose of this signal handler.
	381	* It punctually promotes barrier() into smp_mb() on every thread it is
	382	* executed on.
	383	*/
	384	smp_mb();
	385	rcu_reader.need_mb = 0;
	386	smp_mb();
	387	}
	388
	389	/*
	390	* rcu_init constructor. Called when the library is linked, but also when
	391	* reader threads are calling rcu_register_thread().
	392	* Should only be called by a single thread at a given time. This is ensured by
	393	* holing the rcu_gp_lock from rcu_register_thread() or by running at library
	394	* load time, which should not be executed by multiple threads nor concurrently
	395	* with rcu_register_thread() anyway.
	396	*/
	397	void rcu_init(void)
	398	{
	399	struct sigaction act;
	400	int ret;
	401
	402	if (init_done)
	403	return;
	404	init_done = 1;
	405
	406	act.sa_sigaction = sigrcu_handler;
	407	act.sa_flags = SA_SIGINFO \| SA_RESTART;
	408	sigemptyset(&act.sa_mask);
	409	ret = sigaction(SIGRCU, &act, NULL);
	410	if (ret) {
	411	perror("Error in sigaction");
	412	exit(-1);
	413	}
	414	}
	415
	416	void rcu_exit(void)
	417	{
	418	struct sigaction act;
	419	int ret;
	420
	421	ret = sigaction(SIGRCU, NULL, &act);
	422	if (ret) {
	423	perror("Error in sigaction");
	424	exit(-1);
	425	}
	426	assert(act.sa_sigaction == sigrcu_handler);
	427	assert(list_empty(&registry));
	428	}
	429	#endif /* #ifdef RCU_SIGNAL */