[urcu.git] / urcu.c

/*
 * urcu.c
 *
 * Userspace RCU library
 *
 * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 * 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-map.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 CDS_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 (CMM_LOAD_SHARED(rcu_reader.need_mb)) {
			cmm_smp_mb();
			_CMM_STORE_SHARED(rcu_reader.need_mb, 0);
			cmm_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
		cmm_smp_mb();
}
#endif

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

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

	/*
	 * Ask for each threads to execute a cmm_smp_mb() so we can consider the
	 * compiler barriers around rcu read lock as real memory barriers.
	 */
	if (cds_list_empty(&registry))
		return;
	/*
	 * pthread_kill has a cmm_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 cmm_smp_mc() to make sure the
	 * cache flush is enforced.
	 */
	cds_list_for_each_entry(index, &registry, node) {
		CMM_STORE_SHARED(index->need_mb, 1);
		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).
	 */
	cds_list_for_each_entry(index, &registry, node) {
		while (CMM_LOAD_SHARED(index->need_mb)) {
			pthread_kill(index->tid, SIGRCU);
			poll(NULL, 0, 1);
		}
	}
	cmm_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)
{
	CDS_LIST_HEAD(qsreaders);
	int wait_loops = 0;
	struct rcu_reader *index, *tmp;

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

	/*
	 * Must commit rcu_gp_ctr update to memory before waiting for quiescent
	 * state. Failure to do so could result in the writer waiting forever
	 * while new readers are always accessing data (no progress). Enforce
	 * compiler-order of store to rcu_gp_ctr before load rcu_reader ctr.
	 */
	cmm_barrier();

	/*
	 *
	 * Adding a cmm_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.
	 */
	cmm_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);
		}

		cds_list_for_each_entry_safe(index, tmp, &registry, node) {
			if (!rcu_gp_ongoing(&index->ctr))
				cds_list_move(&index->node, &qsreaders);
		}

#ifndef HAS_INCOHERENT_CACHES
		if (cds_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
				caa_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 (cds_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:
				caa_cpu_relax();
			}
		}
#endif /* #else #ifndef HAS_INCOHERENT_CACHES */
	}
	/* put back the reader list in the registry */
	cds_list_splice(&qsreaders, &registry);
}

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

	if (cds_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 next rcu_gp_ctr update to memory. Failure to do so could
	 * result in the writer waiting forever while new readers are always
	 * accessing data (no progress).  Enforce compiler-order of load
	 * rcu_reader ctr before store to rcu_gp_ctr.
	 */
	cmm_barrier();

	/*
	 * Adding a cmm_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.
	 */
	cmm_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 & RCU_GP_CTR_NEST_MASK));

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

void rcu_unregister_thread(void)
{
	mutex_lock(&rcu_gp_lock);
	cds_list_del(&rcu_reader.node);
	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 cmm_smp_mb() is the only purpose of this signal handler.
	 * It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
	 * executed on.
	 */
	cmm_smp_mb();
	_CMM_STORE_SHARED(rcu_reader.need_mb, 0);
	cmm_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(cds_list_empty(&registry));
}

#endif /* #ifdef RCU_SIGNAL */

#include "urcu-call-rcu-impl.h"
#include "urcu-defer-impl.h"
Commit	Line	Data
	1	/*
	2	* urcu.c
	3	*
	4	* Userspace RCU library
	5	*
	6	* Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
	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-map.h"
	37
	38	#include "urcu-static.h"
	39	/* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
	40	#include "urcu.h"
	41
	42	#ifdef RCU_MEMBARRIER
	43	static int init_done;
	44	int has_sys_membarrier;
	45
	46	void __attribute__((constructor)) rcu_init(void);
	47	#endif
	48
	49	#ifdef RCU_MB
	50	void rcu_init(void)
	51	{
	52	}
	53	#endif
	54
	55	#ifdef RCU_SIGNAL
	56	static int init_done;
	57
	58	void __attribute__((constructor)) rcu_init(void);
	59	void __attribute__((destructor)) rcu_exit(void);
	60	#endif
	61
	62	static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
	63
	64	int gp_futex;
	65
	66	/*
	67	* Global grace period counter.
	68	* Contains the current RCU_GP_CTR_PHASE.
	69	* Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path.
	70	* Written to only by writer with mutex taken. Read by both writer and readers.
	71	*/
	72	unsigned long rcu_gp_ctr = RCU_GP_COUNT;
	73
	74	/*
	75	* Written to only by each individual reader. Read by both the reader and the
	76	* writers.
	77	*/
	78	struct rcu_reader __thread rcu_reader;
	79
	80	#ifdef DEBUG_YIELD
	81	unsigned int yield_active;
	82	unsigned int __thread rand_yield;
	83	#endif
	84
	85	static CDS_LIST_HEAD(registry);
	86
	87	static void mutex_lock(pthread_mutex_t *mutex)
	88	{
	89	int ret;
	90
	91	#ifndef DISTRUST_SIGNALS_EXTREME
	92	ret = pthread_mutex_lock(mutex);
	93	if (ret) {
	94	perror("Error in pthread mutex lock");
	95	exit(-1);
	96	}
	97	#else /* #ifndef DISTRUST_SIGNALS_EXTREME */
	98	while ((ret = pthread_mutex_trylock(mutex)) != 0) {
	99	if (ret != EBUSY && ret != EINTR) {
	100	printf("ret = %d, errno = %d\n", ret, errno);
	101	perror("Error in pthread mutex lock");
	102	exit(-1);
	103	}
	104	if (CMM_LOAD_SHARED(rcu_reader.need_mb)) {
	105	cmm_smp_mb();
	106	_CMM_STORE_SHARED(rcu_reader.need_mb, 0);
	107	cmm_smp_mb();
	108	}
	109	poll(NULL,0,10);
	110	}
	111	#endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
	112	}
	113
	114	static void mutex_unlock(pthread_mutex_t *mutex)
	115	{
	116	int ret;
	117
	118	ret = pthread_mutex_unlock(mutex);
	119	if (ret) {
	120	perror("Error in pthread mutex unlock");
	121	exit(-1);
	122	}
	123	}
	124
	125	#ifdef RCU_MEMBARRIER
	126	static void smp_mb_master(int group)
	127	{
	128	if (likely(has_sys_membarrier))
	129	membarrier(MEMBARRIER_EXPEDITED);
	130	else
	131	cmm_smp_mb();
	132	}
	133	#endif
	134
	135	#ifdef RCU_MB
	136	static void smp_mb_master(int group)
	137	{
	138	cmm_smp_mb();
	139	}
	140	#endif
	141
	142	#ifdef RCU_SIGNAL
	143	static void force_mb_all_readers(void)
	144	{
	145	struct rcu_reader *index;
	146
	147	/*
	148	* Ask for each threads to execute a cmm_smp_mb() so we can consider the
	149	* compiler barriers around rcu read lock as real memory barriers.
	150	*/
	151	if (cds_list_empty(&registry))
	152	return;
	153	/*
	154	* pthread_kill has a cmm_smp_mb(). But beware, we assume it performs
	155	* a cache flush on architectures with non-coherent cache. Let's play
	156	* safe and don't assume anything : we use cmm_smp_mc() to make sure the
	157	* cache flush is enforced.
	158	*/
	159	cds_list_for_each_entry(index, &registry, node) {
	160	CMM_STORE_SHARED(index->need_mb, 1);
	161	pthread_kill(index->tid, SIGRCU);
	162	}
	163	/*
	164	* Wait for sighandler (and thus mb()) to execute on every thread.
	165	*
	166	* Note that the pthread_kill() will never be executed on systems
	167	* that correctly deliver signals in a timely manner. However, it
	168	* is not uncommon for kernels to have bugs that can result in
	169	* lost or unduly delayed signals.
	170	*
	171	* If you are seeing the below pthread_kill() executing much at
	172	* all, we suggest testing the underlying kernel and filing the
	173	* relevant bug report. For Linux kernels, we recommend getting
	174	* the Linux Test Project (LTP).
	175	*/
	176	cds_list_for_each_entry(index, &registry, node) {
	177	while (CMM_LOAD_SHARED(index->need_mb)) {
	178	pthread_kill(index->tid, SIGRCU);
	179	poll(NULL, 0, 1);
	180	}
	181	}
	182	cmm_smp_mb(); /* read ->need_mb before ending the barrier */
	183	}
	184
	185	static void smp_mb_master(int group)
	186	{
	187	force_mb_all_readers();
	188	}
	189	#endif /* #ifdef RCU_SIGNAL */
	190
	191	/*
	192	* synchronize_rcu() waiting. Single thread.
	193	*/
	194	static void wait_gp(void)
	195	{
	196	/* Read reader_gp before read futex */
	197	smp_mb_master(RCU_MB_GROUP);
	198	if (uatomic_read(&gp_futex) == -1)
	199	futex_async(&gp_futex, FUTEX_WAIT, -1,
	200	NULL, NULL, 0);
	201	}
	202
	203	void update_counter_and_wait(void)
	204	{
	205	CDS_LIST_HEAD(qsreaders);
	206	int wait_loops = 0;
	207	struct rcu_reader index, tmp;
	208
	209	/* Switch parity: 0 -> 1, 1 -> 0 */
	210	CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR_PHASE);
	211
	212	/*
	213	* Must commit rcu_gp_ctr update to memory before waiting for quiescent
	214	* state. Failure to do so could result in the writer waiting forever
	215	* while new readers are always accessing data (no progress). Enforce
	216	* compiler-order of store to rcu_gp_ctr before load rcu_reader ctr.
	217	*/
	218	cmm_barrier();
	219
	220	/*
	221	*
	222	* Adding a cmm_smp_mb() which is _not_ formally required, but makes the
	223	* model easier to understand. It does not have a big performance impact
	224	* anyway, given this is the write-side.
	225	*/
	226	cmm_smp_mb();
	227
	228	/*
	229	* Wait for each thread rcu_reader.ctr count to become 0.
	230	*/
	231	for (;;) {
	232	wait_loops++;
	233	if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
	234	uatomic_dec(&gp_futex);
	235	/* Write futex before read reader_gp */
	236	smp_mb_master(RCU_MB_GROUP);
	237	}
	238
	239	cds_list_for_each_entry_safe(index, tmp, &registry, node) {
	240	if (!rcu_gp_ongoing(&index->ctr))
	241	cds_list_move(&index->node, &qsreaders);
	242	}
	243
	244	#ifndef HAS_INCOHERENT_CACHES
	245	if (cds_list_empty(&registry)) {
	246	if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
	247	/* Read reader_gp before write futex */
	248	smp_mb_master(RCU_MB_GROUP);
	249	uatomic_set(&gp_futex, 0);
	250	}
	251	break;
	252	} else {
	253	if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS)
	254	wait_gp();
	255	else
	256	caa_cpu_relax();
	257	}
	258	#else /* #ifndef HAS_INCOHERENT_CACHES */
	259	/*
	260	* BUSY-LOOP. Force the reader thread to commit its
	261	* rcu_reader.ctr update to memory if we wait for too long.
	262	*/
	263	if (cds_list_empty(&registry)) {
	264	if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
	265	/* Read reader_gp before write futex */
	266	smp_mb_master(RCU_MB_GROUP);
	267	uatomic_set(&gp_futex, 0);
	268	}
	269	break;
	270	} else {
	271	switch (wait_loops) {
	272	case RCU_QS_ACTIVE_ATTEMPTS:
	273	wait_gp();
	274	break; /* only escape switch */
	275	case KICK_READER_LOOPS:
	276	smp_mb_master(RCU_MB_GROUP);
	277	wait_loops = 0;
	278	break; /* only escape switch */
	279	default:
	280	caa_cpu_relax();
	281	}
	282	}
	283	#endif /* #else #ifndef HAS_INCOHERENT_CACHES */
	284	}
	285	/* put back the reader list in the registry */
	286	cds_list_splice(&qsreaders, &registry);
	287	}
	288
	289	void synchronize_rcu(void)
	290	{
	291	mutex_lock(&rcu_gp_lock);
	292
	293	if (cds_list_empty(&registry))
	294	goto out;
	295
	296	/* All threads should read qparity before accessing data structure
	297	* where new ptr points to. Must be done within rcu_gp_lock because it
	298	* iterates on reader threads.*/
	299	/* Write new ptr before changing the qparity */
	300	smp_mb_master(RCU_MB_GROUP);
	301
	302	/*
	303	* Wait for previous parity to be empty of readers.
	304	*/
	305	update_counter_and_wait(); /* 0 -> 1, wait readers in parity 0 */
	306
	307	/*
	308	* Must finish waiting for quiescent state for parity 0 before
	309	* committing next rcu_gp_ctr update to memory. Failure to do so could
	310	* result in the writer waiting forever while new readers are always
	311	* accessing data (no progress). Enforce compiler-order of load
	312	* rcu_reader ctr before store to rcu_gp_ctr.
	313	*/
	314	cmm_barrier();
	315
	316	/*
	317	* Adding a cmm_smp_mb() which is _not_ formally required, but makes the
	318	* model easier to understand. It does not have a big performance impact
	319	* anyway, given this is the write-side.
	320	*/
	321	cmm_smp_mb();
	322
	323	/*
	324	* Wait for previous parity to be empty of readers.
	325	*/
	326	update_counter_and_wait(); /* 1 -> 0, wait readers in parity 1 */
	327
	328	/* Finish waiting for reader threads before letting the old ptr being
	329	* freed. Must be done within rcu_gp_lock because it iterates on reader
	330	* threads. */
	331	smp_mb_master(RCU_MB_GROUP);
	332	out:
	333	mutex_unlock(&rcu_gp_lock);
	334	}
	335
	336	/*
	337	* library wrappers to be used by non-LGPL compatible source code.
	338	*/
	339
	340	void rcu_read_lock(void)
	341	{
	342	_rcu_read_lock();
	343	}
	344
	345	void rcu_read_unlock(void)
	346	{
	347	_rcu_read_unlock();
	348	}
	349
	350	void rcu_register_thread(void)
	351	{
	352	rcu_reader.tid = pthread_self();
	353	assert(rcu_reader.need_mb == 0);
	354	assert(!(rcu_reader.ctr & RCU_GP_CTR_NEST_MASK));
	355
	356	mutex_lock(&rcu_gp_lock);
	357	rcu_init(); /* In case gcc does not support constructor attribute */
	358	cds_list_add(&rcu_reader.node, &registry);
	359	mutex_unlock(&rcu_gp_lock);
	360	}
	361
	362	void rcu_unregister_thread(void)
	363	{
	364	mutex_lock(&rcu_gp_lock);
	365	cds_list_del(&rcu_reader.node);
	366	mutex_unlock(&rcu_gp_lock);
	367	}
	368
	369	#ifdef RCU_MEMBARRIER
	370	void rcu_init(void)
	371	{
	372	if (init_done)
	373	return;
	374	init_done = 1;
	375	if (!membarrier(MEMBARRIER_EXPEDITED \| MEMBARRIER_QUERY))
	376	has_sys_membarrier = 1;
	377	}
	378	#endif
	379
	380	#ifdef RCU_SIGNAL
	381	static void sigrcu_handler(int signo, siginfo_t siginfo, void context)
	382	{
	383	/*
	384	* Executing this cmm_smp_mb() is the only purpose of this signal handler.
	385	* It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
	386	* executed on.
	387	*/
	388	cmm_smp_mb();
	389	_CMM_STORE_SHARED(rcu_reader.need_mb, 0);
	390	cmm_smp_mb();
	391	}
	392
	393	/*
	394	* rcu_init constructor. Called when the library is linked, but also when
	395	* reader threads are calling rcu_register_thread().
	396	* Should only be called by a single thread at a given time. This is ensured by
	397	* holing the rcu_gp_lock from rcu_register_thread() or by running at library
	398	* load time, which should not be executed by multiple threads nor concurrently
	399	* with rcu_register_thread() anyway.
	400	*/
	401	void rcu_init(void)
	402	{
	403	struct sigaction act;
	404	int ret;
	405
	406	if (init_done)
	407	return;
	408	init_done = 1;
	409
	410	act.sa_sigaction = sigrcu_handler;
	411	act.sa_flags = SA_SIGINFO \| SA_RESTART;
	412	sigemptyset(&act.sa_mask);
	413	ret = sigaction(SIGRCU, &act, NULL);
	414	if (ret) {
	415	perror("Error in sigaction");
	416	exit(-1);
	417	}
	418	}
	419
	420	void rcu_exit(void)
	421	{
	422	struct sigaction act;
	423	int ret;
	424
	425	ret = sigaction(SIGRCU, NULL, &act);
	426	if (ret) {
	427	perror("Error in sigaction");
	428	exit(-1);
	429	}
	430	assert(act.sa_sigaction == sigrcu_handler);
	431	assert(cds_list_empty(&registry));
	432	}
	433
	434	#endif /* #ifdef RCU_SIGNAL */
	435
	436	#include "urcu-call-rcu-impl.h"
	437	#include "urcu-defer-impl.h"