src/urcu.c

   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 URCU_NO_COMPAT_IDENTIFIERS
  27 #define _BSD_SOURCE
  28 #define _LGPL_SOURCE
  29 #define _DEFAULT_SOURCE
  30 #include <stdio.h>
  31 #include <pthread.h>
  32 #include <signal.h>
  33 #include <assert.h>
  34 #include <stdlib.h>
  35 #include <stdint.h>
  36 #include <string.h>
  37 #include <errno.h>
  38 #include <stdbool.h>
  39 #include <poll.h>
  40
  41 #include <urcu/config.h>
  42 #include <urcu/arch.h>
  43 #include <urcu/wfcqueue.h>
  44 #include <urcu/map/urcu.h>
  45 #include <urcu/static/urcu.h>
  46 #include <urcu/pointer.h>
  47 #include <urcu/tls-compat.h>
  48
  49 #include "urcu-die.h"
  50 #include "urcu-wait.h"
  51 #include "urcu-utils.h"
  52
  53 #define URCU_API_MAP
  54 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
  55 #undef _LGPL_SOURCE
  56 #include <urcu/urcu.h>
  57 #define _LGPL_SOURCE
  58
  59 /*
  60  * If a reader is really non-cooperative and refuses to commit its
  61  * rcu_active_readers count to memory (there is no barrier in the reader
  62  * per-se), kick it after 10 loops waiting for it.
  63  */
  64 #define KICK_READER_LOOPS       10
  65
  66 /*
  67  * Active attempts to check for reader Q.S. before calling futex().
  68  */
  69 #define RCU_QS_ACTIVE_ATTEMPTS 100
  70
  71 /* If the headers do not support membarrier system call, fall back on RCU_MB */
  72 #ifdef __NR_membarrier
  73 # define membarrier(...)                syscall(__NR_membarrier, __VA_ARGS__)
  74 #else
  75 # define membarrier(...)                -ENOSYS
  76 #endif
  77
  78 enum membarrier_cmd {
  79         MEMBARRIER_CMD_QUERY                            = 0,
  80         MEMBARRIER_CMD_SHARED                           = (1 << 0),
  81         /* reserved for MEMBARRIER_CMD_SHARED_EXPEDITED (1 << 1) */
  82         /* reserved for MEMBARRIER_CMD_PRIVATE (1 << 2) */
  83         MEMBARRIER_CMD_PRIVATE_EXPEDITED                = (1 << 3),
  84         MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED       = (1 << 4),
  85 };
  86
  87 #ifdef RCU_MEMBARRIER
  88 static int init_done;
  89 static int urcu_memb_has_sys_membarrier_private_expedited;
  90
  91 #ifndef CONFIG_RCU_FORCE_SYS_MEMBARRIER
  92 /*
  93  * Explicitly initialize to zero because we can't alias a non-static
  94  * uninitialized variable.
  95  */
  96 int urcu_memb_has_sys_membarrier = 0;
  97 URCU_ATTR_ALIAS("urcu_memb_has_sys_membarrier")
  98 extern int rcu_has_sys_membarrier_memb;
  99 #endif
 100
 101 void __attribute__((constructor)) rcu_init(void);
 102 #endif
 103
 104 #ifdef RCU_MB
 105 void rcu_init(void)
 106 {
 107 }
 108 URCU_ATTR_ALIAS(urcu_stringify(rcu_init))
 109 void alias_rcu_init(void);
 110 #endif
 111
 112 #ifdef RCU_SIGNAL
 113 static int init_done;
 114
 115 void __attribute__((constructor)) rcu_init(void);
 116 void __attribute__((destructor)) rcu_exit(void);
 117 #endif
 118
 119 /*
 120  * rcu_gp_lock ensures mutual exclusion between threads calling
 121  * synchronize_rcu().
 122  */
 123 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
 124 /*
 125  * rcu_registry_lock ensures mutual exclusion between threads
 126  * registering and unregistering themselves to/from the registry, and
 127  * with threads reading that registry from synchronize_rcu(). However,
 128  * this lock is not held all the way through the completion of awaiting
 129  * for the grace period. It is sporadically released between iterations
 130  * on the registry.
 131  * rcu_registry_lock may nest inside rcu_gp_lock.
 132  */
 133 static pthread_mutex_t rcu_registry_lock = PTHREAD_MUTEX_INITIALIZER;
 134 struct urcu_gp rcu_gp = { .ctr = URCU_GP_COUNT };
 135 URCU_ATTR_ALIAS(urcu_stringify(rcu_gp))
 136 extern struct urcu_gp alias_rcu_gp;
 137
 138 /*
 139  * Written to only by each individual reader. Read by both the reader and the
 140  * writers.
 141  */
 142 DEFINE_URCU_TLS(struct urcu_reader, rcu_reader);
 143 DEFINE_URCU_TLS_ALIAS(struct urcu_reader, rcu_reader, alias_rcu_reader);
 144
 145 static CDS_LIST_HEAD(registry);
 146
 147 /*
 148  * Queue keeping threads awaiting to wait for a grace period. Contains
 149  * struct gp_waiters_thread objects.
 150  */
 151 static DEFINE_URCU_WAIT_QUEUE(gp_waiters);
 152
 153 static void mutex_lock(pthread_mutex_t *mutex)
 154 {
 155         int ret;
 156
 157 #ifndef DISTRUST_SIGNALS_EXTREME
 158         ret = pthread_mutex_lock(mutex);
 159         if (ret)
 160                 urcu_die(ret);
 161 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
 162         while ((ret = pthread_mutex_trylock(mutex)) != 0) {
 163                 if (ret != EBUSY && ret != EINTR)
 164                         urcu_die(ret);
 165                 if (CMM_LOAD_SHARED(URCU_TLS(rcu_reader).need_mb)) {
 166                         cmm_smp_mb();
 167                         _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
 168                         cmm_smp_mb();
 169                 }
 170                 (void) poll(NULL, 0, 10);
 171         }
 172 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
 173 }
 174
 175 static void mutex_unlock(pthread_mutex_t *mutex)
 176 {
 177         int ret;
 178
 179         ret = pthread_mutex_unlock(mutex);
 180         if (ret)
 181                 urcu_die(ret);
 182 }
 183
 184 #ifdef RCU_MEMBARRIER
 185 static void smp_mb_master(void)
 186 {
 187         if (caa_likely(urcu_memb_has_sys_membarrier)) {
 188                 if (membarrier(urcu_memb_has_sys_membarrier_private_expedited ?
 189                                 MEMBARRIER_CMD_PRIVATE_EXPEDITED :
 190                                 MEMBARRIER_CMD_SHARED, 0))
 191                         urcu_die(errno);
 192         } else {
 193                 cmm_smp_mb();
 194         }
 195 }
 196 #endif
 197
 198 #ifdef RCU_MB
 199 static void smp_mb_master(void)
 200 {
 201         cmm_smp_mb();
 202 }
 203 #endif
 204
 205 #ifdef RCU_SIGNAL
 206 static void force_mb_all_readers(void)
 207 {
 208         struct urcu_reader *index;
 209
 210         /*
 211          * Ask for each threads to execute a cmm_smp_mb() so we can consider the
 212          * compiler barriers around rcu read lock as real memory barriers.
 213          */
 214         if (cds_list_empty(&registry))
 215                 return;
 216         /*
 217          * pthread_kill has a cmm_smp_mb(). But beware, we assume it performs
 218          * a cache flush on architectures with non-coherent cache. Let's play
 219          * safe and don't assume anything : we use cmm_smp_mc() to make sure the
 220          * cache flush is enforced.
 221          */
 222         cds_list_for_each_entry(index, &registry, node) {
 223                 CMM_STORE_SHARED(index->need_mb, 1);
 224                 pthread_kill(index->tid, SIGRCU);
 225         }
 226         /*
 227          * Wait for sighandler (and thus mb()) to execute on every thread.
 228          *
 229          * Note that the pthread_kill() will never be executed on systems
 230          * that correctly deliver signals in a timely manner.  However, it
 231          * is not uncommon for kernels to have bugs that can result in
 232          * lost or unduly delayed signals.
 233          *
 234          * If you are seeing the below pthread_kill() executing much at
 235          * all, we suggest testing the underlying kernel and filing the
 236          * relevant bug report.  For Linux kernels, we recommend getting
 237          * the Linux Test Project (LTP).
 238          */
 239         cds_list_for_each_entry(index, &registry, node) {
 240                 while (CMM_LOAD_SHARED(index->need_mb)) {
 241                         pthread_kill(index->tid, SIGRCU);
 242                         (void) poll(NULL, 0, 1);
 243                 }
 244         }
 245         cmm_smp_mb();   /* read ->need_mb before ending the barrier */
 246 }
 247
 248 static void smp_mb_master(void)
 249 {
 250         force_mb_all_readers();
 251 }
 252 #endif /* #ifdef RCU_SIGNAL */
 253
 254 /*
 255  * synchronize_rcu() waiting. Single thread.
 256  * Always called with rcu_registry lock held. Releases this lock and
 257  * grabs it again. Holds the lock when it returns.
 258  */
 259 static void wait_gp(void)
 260 {
 261         /*
 262          * Read reader_gp before read futex. smp_mb_master() needs to
 263          * be called with the rcu registry lock held in RCU_SIGNAL
 264          * flavor.
 265          */
 266         smp_mb_master();
 267         /* Temporarily unlock the registry lock. */
 268         mutex_unlock(&rcu_registry_lock);
 269         if (uatomic_read(&rcu_gp.futex) != -1)
 270                 goto end;
 271         while (futex_async(&rcu_gp.futex, FUTEX_WAIT, -1,
 272                         NULL, NULL, 0)) {
 273                 switch (errno) {
 274                 case EWOULDBLOCK:
 275                         /* Value already changed. */
 276                         goto end;
 277                 case EINTR:
 278                         /* Retry if interrupted by signal. */
 279                         break;  /* Get out of switch. */
 280                 default:
 281                         /* Unexpected error. */
 282                         urcu_die(errno);
 283                 }
 284         }
 285 end:
 286         /*
 287          * Re-lock the registry lock before the next loop.
 288          */
 289         mutex_lock(&rcu_registry_lock);
 290 }
 291
 292 /*
 293  * Always called with rcu_registry lock held. Releases this lock between
 294  * iterations and grabs it again. Holds the lock when it returns.
 295  */
 296 static void wait_for_readers(struct cds_list_head *input_readers,
 297                         struct cds_list_head *cur_snap_readers,
 298                         struct cds_list_head *qsreaders)
 299 {
 300         unsigned int wait_loops = 0;
 301         struct urcu_reader *index, *tmp;
 302 #ifdef HAS_INCOHERENT_CACHES
 303         unsigned int wait_gp_loops = 0;
 304 #endif /* HAS_INCOHERENT_CACHES */
 305
 306         /*
 307          * Wait for each thread URCU_TLS(rcu_reader).ctr to either
 308          * indicate quiescence (not nested), or observe the current
 309          * rcu_gp.ctr value.
 310          */
 311         for (;;) {
 312                 if (wait_loops < RCU_QS_ACTIVE_ATTEMPTS)
 313                         wait_loops++;
 314                 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 315                         uatomic_dec(&rcu_gp.futex);
 316                         /* Write futex before read reader_gp */
 317                         smp_mb_master();
 318                 }
 319
 320                 cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
 321                         switch (urcu_common_reader_state(&rcu_gp, &index->ctr)) {
 322                         case URCU_READER_ACTIVE_CURRENT:
 323                                 if (cur_snap_readers) {
 324                                         cds_list_move(&index->node,
 325                                                 cur_snap_readers);
 326                                         break;
 327                                 }
 328                                 /* Fall-through */
 329                         case URCU_READER_INACTIVE:
 330                                 cds_list_move(&index->node, qsreaders);
 331                                 break;
 332                         case URCU_READER_ACTIVE_OLD:
 333                                 /*
 334                                  * Old snapshot. Leaving node in
 335                                  * input_readers will make us busy-loop
 336                                  * until the snapshot becomes current or
 337                                  * the reader becomes inactive.
 338                                  */
 339                                 break;
 340                         }
 341                 }
 342
 343 #ifndef HAS_INCOHERENT_CACHES
 344                 if (cds_list_empty(input_readers)) {
 345                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 346                                 /* Read reader_gp before write futex */
 347                                 smp_mb_master();
 348                                 uatomic_set(&rcu_gp.futex, 0);
 349                         }
 350                         break;
 351                 } else {
 352                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 353                                 /* wait_gp unlocks/locks registry lock. */
 354                                 wait_gp();
 355                         } else {
 356                                 /* Temporarily unlock the registry lock. */
 357                                 mutex_unlock(&rcu_registry_lock);
 358                                 caa_cpu_relax();
 359                                 /*
 360                                  * Re-lock the registry lock before the
 361                                  * next loop.
 362                                  */
 363                                 mutex_lock(&rcu_registry_lock);
 364                         }
 365                 }
 366 #else /* #ifndef HAS_INCOHERENT_CACHES */
 367                 /*
 368                  * BUSY-LOOP. Force the reader thread to commit its
 369                  * URCU_TLS(rcu_reader).ctr update to memory if we wait
 370                  * for too long.
 371                  */
 372                 if (cds_list_empty(input_readers)) {
 373                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 374                                 /* Read reader_gp before write futex */
 375                                 smp_mb_master();
 376                                 uatomic_set(&rcu_gp.futex, 0);
 377                         }
 378                         break;
 379                 } else {
 380                         if (wait_gp_loops == KICK_READER_LOOPS) {
 381                                 smp_mb_master();
 382                                 wait_gp_loops = 0;
 383                         }
 384                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 385                                 /* wait_gp unlocks/locks registry lock. */
 386                                 wait_gp();
 387                                 wait_gp_loops++;
 388                         } else {
 389                                 /* Temporarily unlock the registry lock. */
 390                                 mutex_unlock(&rcu_registry_lock);
 391                                 caa_cpu_relax();
 392                                 /*
 393                                  * Re-lock the registry lock before the
 394                                  * next loop.
 395                                  */
 396                                 mutex_lock(&rcu_registry_lock);
 397                         }
 398                 }
 399 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 400         }
 401 }
 402
 403 void synchronize_rcu(void)
 404 {
 405         CDS_LIST_HEAD(cur_snap_readers);
 406         CDS_LIST_HEAD(qsreaders);
 407         DEFINE_URCU_WAIT_NODE(wait, URCU_WAIT_WAITING);
 408         struct urcu_waiters waiters;
 409
 410         /*
 411          * Add ourself to gp_waiters queue of threads awaiting to wait
 412          * for a grace period. Proceed to perform the grace period only
 413          * if we are the first thread added into the queue.
 414          * The implicit memory barrier before urcu_wait_add()
 415          * orders prior memory accesses of threads put into the wait
 416          * queue before their insertion into the wait queue.
 417          */
 418         if (urcu_wait_add(&gp_waiters, &wait) != 0) {
 419                 /* Not first in queue: will be awakened by another thread. */
 420                 urcu_adaptative_busy_wait(&wait);
 421                 /* Order following memory accesses after grace period. */
 422                 cmm_smp_mb();
 423                 return;
 424         }
 425         /* We won't need to wake ourself up */
 426         urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);
 427
 428         mutex_lock(&rcu_gp_lock);
 429
 430         /*
 431          * Move all waiters into our local queue.
 432          */
 433         urcu_move_waiters(&waiters, &gp_waiters);
 434
 435         mutex_lock(&rcu_registry_lock);
 436
 437         if (cds_list_empty(&registry))
 438                 goto out;
 439
 440         /*
 441          * All threads should read qparity before accessing data structure
 442          * where new ptr points to. Must be done within rcu_registry_lock
 443          * because it iterates on reader threads.
 444          */
 445         /* Write new ptr before changing the qparity */
 446         smp_mb_master();
 447
 448         /*
 449          * Wait for readers to observe original parity or be quiescent.
 450          * wait_for_readers() can release and grab again rcu_registry_lock
 451          * interally.
 452          */
 453         wait_for_readers(&registry, &cur_snap_readers, &qsreaders);
 454
 455         /*
 456          * Must finish waiting for quiescent state for original parity before
 457          * committing next rcu_gp.ctr update to memory. Failure to do so could
 458          * result in the writer waiting forever while new readers are always
 459          * accessing data (no progress).  Enforce compiler-order of load
 460          * URCU_TLS(rcu_reader).ctr before store to rcu_gp.ctr.
 461          */
 462         cmm_barrier();
 463
 464         /*
 465          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 466          * model easier to understand. It does not have a big performance impact
 467          * anyway, given this is the write-side.
 468          */
 469         cmm_smp_mb();
 470
 471         /* Switch parity: 0 -> 1, 1 -> 0 */
 472         CMM_STORE_SHARED(rcu_gp.ctr, rcu_gp.ctr ^ URCU_GP_CTR_PHASE);
 473
 474         /*
 475          * Must commit rcu_gp.ctr update to memory before waiting for quiescent
 476          * state. Failure to do so could result in the writer waiting forever
 477          * while new readers are always accessing data (no progress). Enforce
 478          * compiler-order of store to rcu_gp.ctr before load rcu_reader ctr.
 479          */
 480         cmm_barrier();
 481
 482         /*
 483          *
 484          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 485          * model easier to understand. It does not have a big performance impact
 486          * anyway, given this is the write-side.
 487          */
 488         cmm_smp_mb();
 489
 490         /*
 491          * Wait for readers to observe new parity or be quiescent.
 492          * wait_for_readers() can release and grab again rcu_registry_lock
 493          * interally.
 494          */
 495         wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
 496
 497         /*
 498          * Put quiescent reader list back into registry.
 499          */
 500         cds_list_splice(&qsreaders, &registry);
 501
 502         /*
 503          * Finish waiting for reader threads before letting the old ptr
 504          * being freed. Must be done within rcu_registry_lock because it
 505          * iterates on reader threads.
 506          */
 507         smp_mb_master();
 508 out:
 509         mutex_unlock(&rcu_registry_lock);
 510         mutex_unlock(&rcu_gp_lock);
 511
 512         /*
 513          * Wakeup waiters only after we have completed the grace period
 514          * and have ensured the memory barriers at the end of the grace
 515          * period have been issued.
 516          */
 517         urcu_wake_all_waiters(&waiters);
 518 }
 519 URCU_ATTR_ALIAS(urcu_stringify(synchronize_rcu))
 520 void alias_synchronize_rcu();
 521
 522 /*
 523  * library wrappers to be used by non-LGPL compatible source code.
 524  */
 525
 526 void rcu_read_lock(void)
 527 {
 528         _rcu_read_lock();
 529 }
 530 URCU_ATTR_ALIAS(urcu_stringify(rcu_read_lock))
 531 void alias_rcu_read_lock();
 532
 533 void rcu_read_unlock(void)
 534 {
 535         _rcu_read_unlock();
 536 }
 537 URCU_ATTR_ALIAS(urcu_stringify(rcu_read_unlock))
 538 void alias_rcu_read_unlock();
 539
 540 int rcu_read_ongoing(void)
 541 {
 542         return _rcu_read_ongoing();
 543 }
 544 URCU_ATTR_ALIAS(urcu_stringify(rcu_read_ongoing))
 545 void alias_rcu_read_ongoing();
 546
 547 void rcu_register_thread(void)
 548 {
 549         URCU_TLS(rcu_reader).tid = pthread_self();
 550         assert(URCU_TLS(rcu_reader).need_mb == 0);
 551         assert(!(URCU_TLS(rcu_reader).ctr & URCU_GP_CTR_NEST_MASK));
 552
 553         mutex_lock(&rcu_registry_lock);
 554         assert(!URCU_TLS(rcu_reader).registered);
 555         URCU_TLS(rcu_reader).registered = 1;
 556         rcu_init();     /* In case gcc does not support constructor attribute */
 557         cds_list_add(&URCU_TLS(rcu_reader).node, &registry);
 558         mutex_unlock(&rcu_registry_lock);
 559 }
 560 URCU_ATTR_ALIAS(urcu_stringify(rcu_register_thread))
 561 void alias_rcu_register_thread();
 562
 563 void rcu_unregister_thread(void)
 564 {
 565         mutex_lock(&rcu_registry_lock);
 566         assert(URCU_TLS(rcu_reader).registered);
 567         URCU_TLS(rcu_reader).registered = 0;
 568         cds_list_del(&URCU_TLS(rcu_reader).node);
 569         mutex_unlock(&rcu_registry_lock);
 570 }
 571 URCU_ATTR_ALIAS(urcu_stringify(rcu_unregister_thread))
 572 void alias_rcu_unregister_thread();
 573
 574 #ifdef RCU_MEMBARRIER
 575
 576 #ifdef CONFIG_RCU_FORCE_SYS_MEMBARRIER
 577 static
 578 void rcu_sys_membarrier_status(bool available)
 579 {
 580         if (!available)
 581                 abort();
 582 }
 583 #else
 584 static
 585 void rcu_sys_membarrier_status(bool available)
 586 {
 587         if (!available)
 588                 return;
 589         urcu_memb_has_sys_membarrier = 1;
 590 }
 591 #endif
 592
 593 static
 594 void rcu_sys_membarrier_init(void)
 595 {
 596         bool available = false;
 597         int mask;
 598
 599         mask = membarrier(MEMBARRIER_CMD_QUERY, 0);
 600         if (mask >= 0) {
 601                 if (mask & MEMBARRIER_CMD_PRIVATE_EXPEDITED) {
 602                         if (membarrier(MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED, 0))
 603                                 urcu_die(errno);
 604                         urcu_memb_has_sys_membarrier_private_expedited = 1;
 605                         available = true;
 606                 } else if (mask & MEMBARRIER_CMD_SHARED) {
 607                         available = true;
 608                 }
 609         }
 610         rcu_sys_membarrier_status(available);
 611 }
 612
 613 void rcu_init(void)
 614 {
 615         if (init_done)
 616                 return;
 617         init_done = 1;
 618         rcu_sys_membarrier_init();
 619 }
 620 URCU_ATTR_ALIAS(urcu_stringify(rcu_init))
 621 void alias_rcu_init(void);
 622 #endif
 623
 624 #ifdef RCU_SIGNAL
 625 static void sigrcu_handler(int signo __attribute__((unused)),
 626                 siginfo_t *siginfo __attribute__((unused)),
 627                 void *context __attribute__((unused)))
 628 {
 629         /*
 630          * Executing this cmm_smp_mb() is the only purpose of this signal handler.
 631          * It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
 632          * executed on.
 633          */
 634         cmm_smp_mb();
 635         _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
 636         cmm_smp_mb();
 637 }
 638
 639 /*
 640  * rcu_init constructor. Called when the library is linked, but also when
 641  * reader threads are calling rcu_register_thread().
 642  * Should only be called by a single thread at a given time. This is ensured by
 643  * holing the rcu_registry_lock from rcu_register_thread() or by running
 644  * at library load time, which should not be executed by multiple
 645  * threads nor concurrently with rcu_register_thread() anyway.
 646  */
 647 void rcu_init(void)
 648 {
 649         struct sigaction act;
 650         int ret;
 651
 652         if (init_done)
 653                 return;
 654         init_done = 1;
 655
 656         act.sa_sigaction = sigrcu_handler;
 657         act.sa_flags = SA_SIGINFO | SA_RESTART;
 658         sigemptyset(&act.sa_mask);
 659         ret = sigaction(SIGRCU, &act, NULL);
 660         if (ret)
 661                 urcu_die(errno);
 662 }
 663 URCU_ATTR_ALIAS(urcu_stringify(rcu_init))
 664 void alias_rcu_init(void);
 665
 666 void rcu_exit(void)
 667 {
 668         /*
 669          * Don't unregister the SIGRCU signal handler anymore, because
 670          * call_rcu threads could still be using it shortly before the
 671          * application exits.
 672          * Assertion disabled because call_rcu threads are now rcu
 673          * readers, and left running at exit.
 674          * assert(cds_list_empty(&registry));
 675          */
 676 }
 677 URCU_ATTR_ALIAS(urcu_stringify(rcu_exit))
 678 void alias_rcu_exit(void);
 679
 680 #endif /* #ifdef RCU_SIGNAL */
 681
 682 DEFINE_RCU_FLAVOR(rcu_flavor);
 683 DEFINE_RCU_FLAVOR_ALIAS(rcu_flavor, alias_rcu_flavor);
 684
 685 #include "urcu-call-rcu-impl.h"
 686 #include "urcu-defer-impl.h"