[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.
 */

#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"

#ifndef RCU_MB
static int init_done;

void __attribute__((constructor)) rcu_init(void);
void __attribute__((destructor)) rcu_exit(void);
#else
void rcu_init(void)
{
}
#endif

static pthread_mutex_t rcu_mutex = 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.
 */
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 internal_rcu_lock(void)
{
	int ret;

#ifndef DISTRUST_SIGNALS_EXTREME
	ret = pthread_mutex_lock(&rcu_mutex);
	if (ret) {
		perror("Error in pthread mutex lock");
		exit(-1);
	}
#else /* #ifndef DISTRUST_SIGNALS_EXTREME */
	while ((ret = pthread_mutex_trylock(&rcu_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 internal_rcu_unlock(void)
{
	int ret;

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

/*
 * called with rcu_mutex held.
 */
static void switch_next_rcu_qparity(void)
{
	STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR_PHASE);
}

#ifdef RCU_MB
static void smp_mb_heavy()
{
	smp_mb();
}
#else
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_heavy()
{
	force_mb_all_readers();
}
#endif /* #else #ifdef RCU_MB */

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

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

	if (list_empty(&registry))
		return;
	/*
	 * 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_heavy();
		}

		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_heavy();
				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_heavy();
				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_heavy();
				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)
{
	internal_rcu_lock();

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

	switch_next_rcu_qparity();	/* 0 -> 1 */

	/*
	 * Must commit qparity update to memory before waiting for parity
	 * 0 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 previous parity to be empty of readers.
	 */
	wait_for_quiescent_state();	/* 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();

	switch_next_rcu_qparity();	/* 1 -> 0 */

	/*
	 * Must commit qparity update to memory before waiting for parity
	 * 1 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 previous parity to be empty of readers.
	 */
	wait_for_quiescent_state();	/* Wait readers in parity 1 */

	/* Finish waiting for reader threads before letting the old ptr being
	 * freed. Must be done within internal_rcu_lock because it iterates on
	 * reader threads. */
	smp_mb_heavy();

	internal_rcu_unlock();
}

/*
 * 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);

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

void rcu_unregister_thread(void)
{
	internal_rcu_lock();
	list_del(&rcu_reader.head);
	internal_rcu_unlock();
}

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