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