[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
#define _GNU_SOURCE
#define _LGPL_SOURCE
#include <stdio.h>
#include <pthread.h>
#include <signal.h>
#include <assert.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <errno.h>
#include <poll.h>

#include "urcu/wfcqueue.h"
#include "urcu/map/urcu.h"
#include "urcu/static/urcu.h"
#include "urcu-pointer.h"
#include "urcu/tls-compat.h"

#include "urcu-die.h"

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

/*
 * If a reader is really non-cooperative and refuses to commit its
 * rcu_active_readers count to memory (there is no barrier in the reader
 * per-se), kick it after a few loops waiting for it.
 */
#define KICK_READER_LOOPS 10000

/*
 * Active attempts to check for reader Q.S. before calling futex().
 */
#define RCU_QS_ACTIVE_ATTEMPTS 100

#ifdef RCU_MEMBARRIER
static int init_done;
int rcu_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;

int32_t rcu_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.
 */
DEFINE_URCU_TLS(struct rcu_reader, rcu_reader);

#ifdef DEBUG_YIELD
unsigned int rcu_yield_active;
DEFINE_URCU_TLS(unsigned int, rcu_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)
		urcu_die(ret);
#else /* #ifndef DISTRUST_SIGNALS_EXTREME */
	while ((ret = pthread_mutex_trylock(mutex)) != 0) {
		if (ret != EBUSY && ret != EINTR)
			urcu_die(ret);
		if (CMM_LOAD_SHARED(URCU_TLS(rcu_reader).need_mb)) {
			cmm_smp_mb();
			_CMM_STORE_SHARED(URCU_TLS(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)
		urcu_die(ret);
}

#ifdef RCU_MEMBARRIER
static void smp_mb_master(int group)
{
	if (caa_likely(rcu_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(&rcu_gp_futex) == -1)
		futex_async(&rcu_gp_futex, FUTEX_WAIT, -1,
		      NULL, NULL, 0);
}

static void wait_for_readers(struct cds_list_head *input_readers,
			struct cds_list_head *cur_snap_readers,
			struct cds_list_head *qsreaders)
{
	int wait_loops = 0;
	struct rcu_reader *index, *tmp;

	/*
	 * Wait for each thread URCU_TLS(rcu_reader).ctr to either
	 * indicate quiescence (not nested), or observe the current
	 * rcu_gp_ctr value.
	 */
	for (;;) {
		wait_loops++;
		if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
			uatomic_dec(&rcu_gp_futex);
			/* Write futex before read reader_gp */
			smp_mb_master(RCU_MB_GROUP);
		}

		cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
			switch (rcu_reader_state(&index->ctr)) {
			case RCU_READER_ACTIVE_CURRENT:
				if (cur_snap_readers) {
					cds_list_move(&index->node,
						cur_snap_readers);
					break;
				}
				/* Fall-through */
			case RCU_READER_INACTIVE:
				cds_list_move(&index->node, qsreaders);
				break;
			case RCU_READER_ACTIVE_OLD:
				/*
				 * Old snapshot. Leaving node in
				 * input_readers will make us busy-loop
				 * until the snapshot becomes current or
				 * the reader becomes inactive.
				 */
				break;
			}
		}

#ifndef HAS_INCOHERENT_CACHES
		if (cds_list_empty(input_readers)) {
			if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
				/* Read reader_gp before write futex */
				smp_mb_master(RCU_MB_GROUP);
				uatomic_set(&rcu_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
		 * URCU_TLS(rcu_reader).ctr update to memory if we wait
		 * for too long.
		 */
		if (cds_list_empty(input_readers)) {
			if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
				/* Read reader_gp before write futex */
				smp_mb_master(RCU_MB_GROUP);
				uatomic_set(&rcu_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 */
	}
}

void synchronize_rcu(void)
{
	CDS_LIST_HEAD(cur_snap_readers);
	CDS_LIST_HEAD(qsreaders);

	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 readers to observe original parity or be quiescent.
	 */
	wait_for_readers(&registry, &cur_snap_readers, &qsreaders);

	/*
	 * Must finish waiting for quiescent state for original parity 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
	 * URCU_TLS(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();

	/* 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 readers to observe new parity or be quiescent.
	 */
	wait_for_readers(&cur_snap_readers, NULL, &qsreaders);

	/*
	 * Put quiescent reader list back into registry.
	 */
	cds_list_splice(&qsreaders, &registry);

	/* 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)
{
	URCU_TLS(rcu_reader).tid = pthread_self();
	assert(URCU_TLS(rcu_reader).need_mb == 0);
	assert(!(URCU_TLS(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(&URCU_TLS(rcu_reader).node, &registry);
	mutex_unlock(&rcu_gp_lock);
}

void rcu_unregister_thread(void)
{
	mutex_lock(&rcu_gp_lock);
	cds_list_del(&URCU_TLS(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))
		rcu_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(URCU_TLS(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)
		urcu_die(errno);
}

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

	ret = sigaction(SIGRCU, NULL, &act);
	if (ret)
		urcu_die(errno);
	assert(act.sa_sigaction == sigrcu_handler);
	assert(cds_list_empty(&registry));
}

#endif /* #ifdef RCU_SIGNAL */

DEFINE_RCU_FLAVOR(rcu_flavor);

#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	#define _GNU_SOURCE
	28	#define _LGPL_SOURCE
	29	#include <stdio.h>
	30	#include <pthread.h>
	31	#include <signal.h>
	32	#include <assert.h>
	33	#include <stdlib.h>
	34	#include <stdint.h>
	35	#include <string.h>
	36	#include <errno.h>
	37	#include <poll.h>
	38
	39	#include "urcu/wfcqueue.h"
	40	#include "urcu/map/urcu.h"
	41	#include "urcu/static/urcu.h"
	42	#include "urcu-pointer.h"
	43	#include "urcu/tls-compat.h"
	44
	45	#include "urcu-die.h"
	46
	47	/* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
	48	#undef _LGPL_SOURCE
	49	#include "urcu.h"
	50	#define _LGPL_SOURCE
	51
	52	/*
	53	* If a reader is really non-cooperative and refuses to commit its
	54	* rcu_active_readers count to memory (there is no barrier in the reader
	55	* per-se), kick it after a few loops waiting for it.
	56	*/
	57	#define KICK_READER_LOOPS 10000
	58
	59	/*
	60	* Active attempts to check for reader Q.S. before calling futex().
	61	*/
	62	#define RCU_QS_ACTIVE_ATTEMPTS 100
	63
	64	#ifdef RCU_MEMBARRIER
	65	static int init_done;
	66	int rcu_has_sys_membarrier;
	67
	68	void __attribute__((constructor)) rcu_init(void);
	69	#endif
	70
	71	#ifdef RCU_MB
	72	void rcu_init(void)
	73	{
	74	}
	75	#endif
	76
	77	#ifdef RCU_SIGNAL
	78	static int init_done;
	79
	80	void __attribute__((constructor)) rcu_init(void);
	81	void __attribute__((destructor)) rcu_exit(void);
	82	#endif
	83
	84	static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
	85
	86	int32_t rcu_gp_futex;
	87
	88	/*
	89	* Global grace period counter.
	90	* Contains the current RCU_GP_CTR_PHASE.
	91	* Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path.
	92	* Written to only by writer with mutex taken. Read by both writer and readers.
	93	*/
	94	unsigned long rcu_gp_ctr = RCU_GP_COUNT;
	95
	96	/*
	97	* Written to only by each individual reader. Read by both the reader and the
	98	* writers.
	99	*/
	100	DEFINE_URCU_TLS(struct rcu_reader, rcu_reader);
	101
	102	#ifdef DEBUG_YIELD
	103	unsigned int rcu_yield_active;
	104	DEFINE_URCU_TLS(unsigned int, rcu_rand_yield);
	105	#endif
	106
	107	static CDS_LIST_HEAD(registry);
	108
	109	static void mutex_lock(pthread_mutex_t *mutex)
	110	{
	111	int ret;
	112
	113	#ifndef DISTRUST_SIGNALS_EXTREME
	114	ret = pthread_mutex_lock(mutex);
	115	if (ret)
	116	urcu_die(ret);
	117	#else /* #ifndef DISTRUST_SIGNALS_EXTREME */
	118	while ((ret = pthread_mutex_trylock(mutex)) != 0) {
	119	if (ret != EBUSY && ret != EINTR)
	120	urcu_die(ret);
	121	if (CMM_LOAD_SHARED(URCU_TLS(rcu_reader).need_mb)) {
	122	cmm_smp_mb();
	123	_CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
	124	cmm_smp_mb();
	125	}
	126	poll(NULL,0,10);
	127	}
	128	#endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
	129	}
	130
	131	static void mutex_unlock(pthread_mutex_t *mutex)
	132	{
	133	int ret;
	134
	135	ret = pthread_mutex_unlock(mutex);
	136	if (ret)
	137	urcu_die(ret);
	138	}
	139
	140	#ifdef RCU_MEMBARRIER
	141	static void smp_mb_master(int group)
	142	{
	143	if (caa_likely(rcu_has_sys_membarrier))
	144	membarrier(MEMBARRIER_EXPEDITED);
	145	else
	146	cmm_smp_mb();
	147	}
	148	#endif
	149
	150	#ifdef RCU_MB
	151	static void smp_mb_master(int group)
	152	{
	153	cmm_smp_mb();
	154	}
	155	#endif
	156
	157	#ifdef RCU_SIGNAL
	158	static void force_mb_all_readers(void)
	159	{
	160	struct rcu_reader *index;
	161
	162	/*
	163	* Ask for each threads to execute a cmm_smp_mb() so we can consider the
	164	* compiler barriers around rcu read lock as real memory barriers.
	165	*/
	166	if (cds_list_empty(&registry))
	167	return;
	168	/*
	169	* pthread_kill has a cmm_smp_mb(). But beware, we assume it performs
	170	* a cache flush on architectures with non-coherent cache. Let's play
	171	* safe and don't assume anything : we use cmm_smp_mc() to make sure the
	172	* cache flush is enforced.
	173	*/
	174	cds_list_for_each_entry(index, &registry, node) {
	175	CMM_STORE_SHARED(index->need_mb, 1);
	176	pthread_kill(index->tid, SIGRCU);
	177	}
	178	/*
	179	* Wait for sighandler (and thus mb()) to execute on every thread.
	180	*
	181	* Note that the pthread_kill() will never be executed on systems
	182	* that correctly deliver signals in a timely manner. However, it
	183	* is not uncommon for kernels to have bugs that can result in
	184	* lost or unduly delayed signals.
	185	*
	186	* If you are seeing the below pthread_kill() executing much at
	187	* all, we suggest testing the underlying kernel and filing the
	188	* relevant bug report. For Linux kernels, we recommend getting
	189	* the Linux Test Project (LTP).
	190	*/
	191	cds_list_for_each_entry(index, &registry, node) {
	192	while (CMM_LOAD_SHARED(index->need_mb)) {
	193	pthread_kill(index->tid, SIGRCU);
	194	poll(NULL, 0, 1);
	195	}
	196	}
	197	cmm_smp_mb(); /* read ->need_mb before ending the barrier */
	198	}
	199
	200	static void smp_mb_master(int group)
	201	{
	202	force_mb_all_readers();
	203	}
	204	#endif /* #ifdef RCU_SIGNAL */
	205
	206	/*
	207	* synchronize_rcu() waiting. Single thread.
	208	*/
	209	static void wait_gp(void)
	210	{
	211	/* Read reader_gp before read futex */
	212	smp_mb_master(RCU_MB_GROUP);
	213	if (uatomic_read(&rcu_gp_futex) == -1)
	214	futex_async(&rcu_gp_futex, FUTEX_WAIT, -1,
	215	NULL, NULL, 0);
	216	}
	217
	218	static void wait_for_readers(struct cds_list_head *input_readers,
	219	struct cds_list_head *cur_snap_readers,
	220	struct cds_list_head *qsreaders)
	221	{
	222	int wait_loops = 0;
	223	struct rcu_reader index, tmp;
	224
	225	/*
	226	* Wait for each thread URCU_TLS(rcu_reader).ctr to either
	227	* indicate quiescence (not nested), or observe the current
	228	* rcu_gp_ctr value.
	229	*/
	230	for (;;) {
	231	wait_loops++;
	232	if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
	233	uatomic_dec(&rcu_gp_futex);
	234	/* Write futex before read reader_gp */
	235	smp_mb_master(RCU_MB_GROUP);
	236	}
	237
	238	cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
	239	switch (rcu_reader_state(&index->ctr)) {
	240	case RCU_READER_ACTIVE_CURRENT:
	241	if (cur_snap_readers) {
	242	cds_list_move(&index->node,
	243	cur_snap_readers);
	244	break;
	245	}
	246	/* Fall-through */
	247	case RCU_READER_INACTIVE:
	248	cds_list_move(&index->node, qsreaders);
	249	break;
	250	case RCU_READER_ACTIVE_OLD:
	251	/*
	252	* Old snapshot. Leaving node in
	253	* input_readers will make us busy-loop
	254	* until the snapshot becomes current or
	255	* the reader becomes inactive.
	256	*/
	257	break;
	258	}
	259	}
	260
	261	#ifndef HAS_INCOHERENT_CACHES
	262	if (cds_list_empty(input_readers)) {
	263	if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
	264	/* Read reader_gp before write futex */
	265	smp_mb_master(RCU_MB_GROUP);
	266	uatomic_set(&rcu_gp_futex, 0);
	267	}
	268	break;
	269	} else {
	270	if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS)
	271	wait_gp();
	272	else
	273	caa_cpu_relax();
	274	}
	275	#else /* #ifndef HAS_INCOHERENT_CACHES */
	276	/*
	277	* BUSY-LOOP. Force the reader thread to commit its
	278	* URCU_TLS(rcu_reader).ctr update to memory if we wait
	279	* for too long.
	280	*/
	281	if (cds_list_empty(input_readers)) {
	282	if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
	283	/* Read reader_gp before write futex */
	284	smp_mb_master(RCU_MB_GROUP);
	285	uatomic_set(&rcu_gp_futex, 0);
	286	}
	287	break;
	288	} else {
	289	switch (wait_loops) {
	290	case RCU_QS_ACTIVE_ATTEMPTS:
	291	wait_gp();
	292	break; /* only escape switch */
	293	case KICK_READER_LOOPS:
	294	smp_mb_master(RCU_MB_GROUP);
	295	wait_loops = 0;
	296	break; /* only escape switch */
	297	default:
	298	caa_cpu_relax();
	299	}
	300	}
	301	#endif /* #else #ifndef HAS_INCOHERENT_CACHES */
	302	}
	303	}
	304
	305	void synchronize_rcu(void)
	306	{
	307	CDS_LIST_HEAD(cur_snap_readers);
	308	CDS_LIST_HEAD(qsreaders);
	309
	310	mutex_lock(&rcu_gp_lock);
	311
	312	if (cds_list_empty(&registry))
	313	goto out;
	314
	315	/* All threads should read qparity before accessing data structure
	316	* where new ptr points to. Must be done within rcu_gp_lock because it
	317	* iterates on reader threads.*/
	318	/* Write new ptr before changing the qparity */
	319	smp_mb_master(RCU_MB_GROUP);
	320
	321	/*
	322	* Wait for readers to observe original parity or be quiescent.
	323	*/
	324	wait_for_readers(&registry, &cur_snap_readers, &qsreaders);
	325
	326	/*
	327	* Must finish waiting for quiescent state for original parity before
	328	* committing next rcu_gp_ctr update to memory. Failure to do so could
	329	* result in the writer waiting forever while new readers are always
	330	* accessing data (no progress). Enforce compiler-order of load
	331	* URCU_TLS(rcu_reader).ctr before store to rcu_gp_ctr.
	332	*/
	333	cmm_barrier();
	334
	335	/*
	336	* Adding a cmm_smp_mb() which is _not_ formally required, but makes the
	337	* model easier to understand. It does not have a big performance impact
	338	* anyway, given this is the write-side.
	339	*/
	340	cmm_smp_mb();
	341
	342	/* Switch parity: 0 -> 1, 1 -> 0 */
	343	CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR_PHASE);
	344
	345	/*
	346	* Must commit rcu_gp_ctr update to memory before waiting for quiescent
	347	* state. Failure to do so could result in the writer waiting forever
	348	* while new readers are always accessing data (no progress). Enforce
	349	* compiler-order of store to rcu_gp_ctr before load rcu_reader ctr.
	350	*/
	351	cmm_barrier();
	352
	353	/*
	354	*
	355	* Adding a cmm_smp_mb() which is _not_ formally required, but makes the
	356	* model easier to understand. It does not have a big performance impact
	357	* anyway, given this is the write-side.
	358	*/
	359	cmm_smp_mb();
	360
	361	/*
	362	* Wait for readers to observe new parity or be quiescent.
	363	*/
	364	wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
	365
	366	/*
	367	* Put quiescent reader list back into registry.
	368	*/
	369	cds_list_splice(&qsreaders, &registry);
	370
	371	/* Finish waiting for reader threads before letting the old ptr being
	372	* freed. Must be done within rcu_gp_lock because it iterates on reader
	373	* threads. */
	374	smp_mb_master(RCU_MB_GROUP);
	375	out:
	376	mutex_unlock(&rcu_gp_lock);
	377	}
	378
	379	/*
	380	* library wrappers to be used by non-LGPL compatible source code.
	381	*/
	382
	383	void rcu_read_lock(void)
	384	{
	385	_rcu_read_lock();
	386	}
	387
	388	void rcu_read_unlock(void)
	389	{
	390	_rcu_read_unlock();
	391	}
	392
	393	void rcu_register_thread(void)
	394	{
	395	URCU_TLS(rcu_reader).tid = pthread_self();
	396	assert(URCU_TLS(rcu_reader).need_mb == 0);
	397	assert(!(URCU_TLS(rcu_reader).ctr & RCU_GP_CTR_NEST_MASK));
	398
	399	mutex_lock(&rcu_gp_lock);
	400	rcu_init(); /* In case gcc does not support constructor attribute */
	401	cds_list_add(&URCU_TLS(rcu_reader).node, &registry);
	402	mutex_unlock(&rcu_gp_lock);
	403	}
	404
	405	void rcu_unregister_thread(void)
	406	{
	407	mutex_lock(&rcu_gp_lock);
	408	cds_list_del(&URCU_TLS(rcu_reader).node);
	409	mutex_unlock(&rcu_gp_lock);
	410	}
	411
	412	#ifdef RCU_MEMBARRIER
	413	void rcu_init(void)
	414	{
	415	if (init_done)
	416	return;
	417	init_done = 1;
	418	if (!membarrier(MEMBARRIER_EXPEDITED \| MEMBARRIER_QUERY))
	419	rcu_has_sys_membarrier = 1;
	420	}
	421	#endif
	422
	423	#ifdef RCU_SIGNAL
	424	static void sigrcu_handler(int signo, siginfo_t siginfo, void context)
	425	{
	426	/*
	427	* Executing this cmm_smp_mb() is the only purpose of this signal handler.
	428	* It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
	429	* executed on.
	430	*/
	431	cmm_smp_mb();
	432	_CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
	433	cmm_smp_mb();
	434	}
	435
	436	/*
	437	* rcu_init constructor. Called when the library is linked, but also when
	438	* reader threads are calling rcu_register_thread().
	439	* Should only be called by a single thread at a given time. This is ensured by
	440	* holing the rcu_gp_lock from rcu_register_thread() or by running at library
	441	* load time, which should not be executed by multiple threads nor concurrently
	442	* with rcu_register_thread() anyway.
	443	*/
	444	void rcu_init(void)
	445	{
	446	struct sigaction act;
	447	int ret;
	448
	449	if (init_done)
	450	return;
	451	init_done = 1;
	452
	453	act.sa_sigaction = sigrcu_handler;
	454	act.sa_flags = SA_SIGINFO \| SA_RESTART;
	455	sigemptyset(&act.sa_mask);
	456	ret = sigaction(SIGRCU, &act, NULL);
	457	if (ret)
	458	urcu_die(errno);
	459	}
	460
	461	void rcu_exit(void)
	462	{
	463	struct sigaction act;
	464	int ret;
	465
	466	ret = sigaction(SIGRCU, NULL, &act);
	467	if (ret)
	468	urcu_die(errno);
	469	assert(act.sa_sigaction == sigrcu_handler);
	470	assert(cds_list_empty(&registry));
	471	}
	472
	473	#endif /* #ifdef RCU_SIGNAL */
	474
	475	DEFINE_RCU_FLAVOR(rcu_flavor);
	476
	477	#include "urcu-call-rcu-impl.h"
	478	#include "urcu-defer-impl.h"