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 <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/config.h>
  41 #include <urcu/assert.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 #endif
  98
  99 void __attribute__((constructor)) rcu_init(void);
 100 #endif
 101
 102 #ifdef RCU_MB
 103 void rcu_init(void)
 104 {
 105 }
 106 #endif
 107
 108 #ifdef RCU_SIGNAL
 109 static int init_done;
 110
 111 void __attribute__((constructor)) rcu_init(void);
 112 void __attribute__((destructor)) rcu_exit(void);
 113
 114 static DEFINE_URCU_TLS(int, rcu_signal_was_blocked);
 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
 134 /*
 135  * Written to only by each individual reader. Read by both the reader and the
 136  * writers.
 137  */
 138 DEFINE_URCU_TLS(struct urcu_reader, rcu_reader);
 139
 140 static CDS_LIST_HEAD(registry);
 141
 142 /*
 143  * Queue keeping threads awaiting to wait for a grace period. Contains
 144  * struct gp_waiters_thread objects.
 145  */
 146 static DEFINE_URCU_WAIT_QUEUE(gp_waiters);
 147
 148 static void mutex_lock(pthread_mutex_t *mutex)
 149 {
 150         int ret;
 151
 152 #ifndef DISTRUST_SIGNALS_EXTREME
 153         ret = pthread_mutex_lock(mutex);
 154         if (ret)
 155                 urcu_die(ret);
 156 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
 157         while ((ret = pthread_mutex_trylock(mutex)) != 0) {
 158                 if (ret != EBUSY && ret != EINTR)
 159                         urcu_die(ret);
 160                 if (CMM_LOAD_SHARED(URCU_TLS(rcu_reader).need_mb)) {
 161                         cmm_smp_mb();
 162                         _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
 163                         cmm_smp_mb();
 164                 }
 165                 (void) poll(NULL, 0, 10);
 166         }
 167 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
 168 }
 169
 170 static void mutex_unlock(pthread_mutex_t *mutex)
 171 {
 172         int ret;
 173
 174         ret = pthread_mutex_unlock(mutex);
 175         if (ret)
 176                 urcu_die(ret);
 177 }
 178
 179 #ifdef RCU_MEMBARRIER
 180 static void smp_mb_master(void)
 181 {
 182         if (caa_likely(urcu_memb_has_sys_membarrier)) {
 183                 if (membarrier(urcu_memb_has_sys_membarrier_private_expedited ?
 184                                 MEMBARRIER_CMD_PRIVATE_EXPEDITED :
 185                                 MEMBARRIER_CMD_SHARED, 0))
 186                         urcu_die(errno);
 187         } else {
 188                 cmm_smp_mb();
 189         }
 190 }
 191 #endif
 192
 193 #ifdef RCU_MB
 194 static void smp_mb_master(void)
 195 {
 196         cmm_smp_mb();
 197 }
 198 #endif
 199
 200 #ifdef RCU_SIGNAL
 201 static void force_mb_all_readers(void)
 202 {
 203         struct urcu_reader *index;
 204
 205         /*
 206          * Ask for each threads to execute a cmm_smp_mb() so we can consider the
 207          * compiler barriers around rcu read lock as real memory barriers.
 208          */
 209         if (cds_list_empty(&registry))
 210                 return;
 211         /*
 212          * pthread_kill has a cmm_smp_mb(). But beware, we assume it performs
 213          * a cache flush on architectures with non-coherent cache. Let's play
 214          * safe and don't assume anything : we use cmm_smp_mc() to make sure the
 215          * cache flush is enforced.
 216          */
 217         cds_list_for_each_entry(index, &registry, node) {
 218                 CMM_STORE_SHARED(index->need_mb, 1);
 219                 pthread_kill(index->tid, SIGRCU);
 220         }
 221         /*
 222          * Wait for sighandler (and thus mb()) to execute on every thread.
 223          *
 224          * Note that the pthread_kill() will never be executed on systems
 225          * that correctly deliver signals in a timely manner.  However, it
 226          * is not uncommon for kernels to have bugs that can result in
 227          * lost or unduly delayed signals.
 228          *
 229          * If you are seeing the below pthread_kill() executing much at
 230          * all, we suggest testing the underlying kernel and filing the
 231          * relevant bug report.  For Linux kernels, we recommend getting
 232          * the Linux Test Project (LTP).
 233          */
 234         cds_list_for_each_entry(index, &registry, node) {
 235                 while (CMM_LOAD_SHARED(index->need_mb)) {
 236                         pthread_kill(index->tid, SIGRCU);
 237                         (void) poll(NULL, 0, 1);
 238                 }
 239         }
 240         cmm_smp_mb();   /* read ->need_mb before ending the barrier */
 241 }
 242
 243 static void smp_mb_master(void)
 244 {
 245         force_mb_all_readers();
 246 }
 247 #endif /* #ifdef RCU_SIGNAL */
 248
 249 /*
 250  * synchronize_rcu() waiting. Single thread.
 251  * Always called with rcu_registry lock held. Releases this lock and
 252  * grabs it again. Holds the lock when it returns.
 253  */
 254 static void wait_gp(void)
 255 {
 256         /*
 257          * Read reader_gp before read futex. smp_mb_master() needs to
 258          * be called with the rcu registry lock held in RCU_SIGNAL
 259          * flavor.
 260          */
 261         smp_mb_master();
 262         /* Temporarily unlock the registry lock. */
 263         mutex_unlock(&rcu_registry_lock);
 264         while (uatomic_read(&rcu_gp.futex) == -1) {
 265                 if (!futex_async(&rcu_gp.futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
 266                         /*
 267                          * Prior queued wakeups queued by unrelated code
 268                          * using the same address can cause futex wait to
 269                          * return 0 even through the futex value is still
 270                          * -1 (spurious wakeups). Check the value again
 271                          * in user-space to validate whether it really
 272                          * differs from -1.
 273                          */
 274                         continue;
 275                 }
 276                 switch (errno) {
 277                 case EAGAIN:
 278                         /* Value already changed. */
 279                         goto end;
 280                 case EINTR:
 281                         /* Retry if interrupted by signal. */
 282                         break;  /* Get out of switch. Check again. */
 283                 default:
 284                         /* Unexpected error. */
 285                         urcu_die(errno);
 286                 }
 287         }
 288 end:
 289         /*
 290          * Re-lock the registry lock before the next loop.
 291          */
 292         mutex_lock(&rcu_registry_lock);
 293 }
 294
 295 /*
 296  * Always called with rcu_registry lock held. Releases this lock between
 297  * iterations and grabs it again. Holds the lock when it returns.
 298  */
 299 static void wait_for_readers(struct cds_list_head *input_readers,
 300                         struct cds_list_head *cur_snap_readers,
 301                         struct cds_list_head *qsreaders)
 302 {
 303         unsigned int wait_loops = 0;
 304         struct urcu_reader *index, *tmp;
 305 #ifdef HAS_INCOHERENT_CACHES
 306         unsigned int wait_gp_loops = 0;
 307 #endif /* HAS_INCOHERENT_CACHES */
 308
 309         /*
 310          * Wait for each thread URCU_TLS(rcu_reader).ctr to either
 311          * indicate quiescence (not nested), or observe the current
 312          * rcu_gp.ctr value.
 313          */
 314         for (;;) {
 315                 if (wait_loops < RCU_QS_ACTIVE_ATTEMPTS)
 316                         wait_loops++;
 317                 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 318                         uatomic_dec(&rcu_gp.futex);
 319                         /* Write futex before read reader_gp */
 320                         smp_mb_master();
 321                 }
 322
 323                 cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
 324                         switch (urcu_common_reader_state(&rcu_gp, &index->ctr)) {
 325                         case URCU_READER_ACTIVE_CURRENT:
 326                                 if (cur_snap_readers) {
 327                                         cds_list_move(&index->node,
 328                                                 cur_snap_readers);
 329                                         break;
 330                                 }
 331                                 /* Fall-through */
 332                         case URCU_READER_INACTIVE:
 333                                 cds_list_move(&index->node, qsreaders);
 334                                 break;
 335                         case URCU_READER_ACTIVE_OLD:
 336                                 /*
 337                                  * Old snapshot. Leaving node in
 338                                  * input_readers will make us busy-loop
 339                                  * until the snapshot becomes current or
 340                                  * the reader becomes inactive.
 341                                  */
 342                                 break;
 343                         }
 344                 }
 345
 346 #ifndef HAS_INCOHERENT_CACHES
 347                 if (cds_list_empty(input_readers)) {
 348                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 349                                 /* Read reader_gp before write futex */
 350                                 smp_mb_master();
 351                                 uatomic_set(&rcu_gp.futex, 0);
 352                         }
 353                         break;
 354                 } else {
 355                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 356                                 /* wait_gp unlocks/locks registry lock. */
 357                                 wait_gp();
 358                         } else {
 359                                 /* Temporarily unlock the registry lock. */
 360                                 mutex_unlock(&rcu_registry_lock);
 361                                 caa_cpu_relax();
 362                                 /*
 363                                  * Re-lock the registry lock before the
 364                                  * next loop.
 365                                  */
 366                                 mutex_lock(&rcu_registry_lock);
 367                         }
 368                 }
 369 #else /* #ifndef HAS_INCOHERENT_CACHES */
 370                 /*
 371                  * BUSY-LOOP. Force the reader thread to commit its
 372                  * URCU_TLS(rcu_reader).ctr update to memory if we wait
 373                  * for too long.
 374                  */
 375                 if (cds_list_empty(input_readers)) {
 376                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 377                                 /* Read reader_gp before write futex */
 378                                 smp_mb_master();
 379                                 uatomic_set(&rcu_gp.futex, 0);
 380                         }
 381                         break;
 382                 } else {
 383                         if (wait_gp_loops == KICK_READER_LOOPS) {
 384                                 smp_mb_master();
 385                                 wait_gp_loops = 0;
 386                         }
 387                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 388                                 /* wait_gp unlocks/locks registry lock. */
 389                                 wait_gp();
 390                                 wait_gp_loops++;
 391                         } else {
 392                                 /* Temporarily unlock the registry lock. */
 393                                 mutex_unlock(&rcu_registry_lock);
 394                                 caa_cpu_relax();
 395                                 /*
 396                                  * Re-lock the registry lock before the
 397                                  * next loop.
 398                                  */
 399                                 mutex_lock(&rcu_registry_lock);
 400                         }
 401                 }
 402 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 403         }
 404 }
 405
 406 void synchronize_rcu(void)
 407 {
 408         CDS_LIST_HEAD(cur_snap_readers);
 409         CDS_LIST_HEAD(qsreaders);
 410         DEFINE_URCU_WAIT_NODE(wait, URCU_WAIT_WAITING);
 411         struct urcu_waiters waiters;
 412
 413         /*
 414          * Add ourself to gp_waiters queue of threads awaiting to wait
 415          * for a grace period. Proceed to perform the grace period only
 416          * if we are the first thread added into the queue.
 417          * The implicit memory barrier before urcu_wait_add()
 418          * orders prior memory accesses of threads put into the wait
 419          * queue before their insertion into the wait queue.
 420          */
 421         if (urcu_wait_add(&gp_waiters, &wait) != 0) {
 422                 /* Not first in queue: will be awakened by another thread. */
 423                 urcu_adaptative_busy_wait(&wait);
 424                 /* Order following memory accesses after grace period. */
 425                 cmm_smp_mb();
 426                 return;
 427         }
 428         /* We won't need to wake ourself up */
 429         urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);
 430
 431         mutex_lock(&rcu_gp_lock);
 432
 433         /*
 434          * Move all waiters into our local queue.
 435          */
 436         urcu_move_waiters(&waiters, &gp_waiters);
 437
 438         mutex_lock(&rcu_registry_lock);
 439
 440         if (cds_list_empty(&registry))
 441                 goto out;
 442
 443         /*
 444          * All threads should read qparity before accessing data structure
 445          * where new ptr points to. Must be done within rcu_registry_lock
 446          * because it iterates on reader threads.
 447          */
 448         /* Write new ptr before changing the qparity */
 449         smp_mb_master();
 450
 451         /*
 452          * Wait for readers to observe original parity or be quiescent.
 453          * wait_for_readers() can release and grab again rcu_registry_lock
 454          * internally.
 455          */
 456         wait_for_readers(&registry, &cur_snap_readers, &qsreaders);
 457
 458         /*
 459          * Must finish waiting for quiescent state for original parity before
 460          * committing next rcu_gp.ctr update to memory. Failure to do so could
 461          * result in the writer waiting forever while new readers are always
 462          * accessing data (no progress).  Enforce compiler-order of load
 463          * URCU_TLS(rcu_reader).ctr before store to rcu_gp.ctr.
 464          */
 465         cmm_barrier();
 466
 467         /*
 468          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 469          * model easier to understand. It does not have a big performance impact
 470          * anyway, given this is the write-side.
 471          */
 472         cmm_smp_mb();
 473
 474         /* Switch parity: 0 -> 1, 1 -> 0 */
 475         CMM_STORE_SHARED(rcu_gp.ctr, rcu_gp.ctr ^ URCU_GP_CTR_PHASE);
 476
 477         /*
 478          * Must commit rcu_gp.ctr update to memory before waiting for quiescent
 479          * state. Failure to do so could result in the writer waiting forever
 480          * while new readers are always accessing data (no progress). Enforce
 481          * compiler-order of store to rcu_gp.ctr before load rcu_reader ctr.
 482          */
 483         cmm_barrier();
 484
 485         /*
 486          *
 487          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 488          * model easier to understand. It does not have a big performance impact
 489          * anyway, given this is the write-side.
 490          */
 491         cmm_smp_mb();
 492
 493         /*
 494          * Wait for readers to observe new parity or be quiescent.
 495          * wait_for_readers() can release and grab again rcu_registry_lock
 496          * internally.
 497          */
 498         wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
 499
 500         /*
 501          * Put quiescent reader list back into registry.
 502          */
 503         cds_list_splice(&qsreaders, &registry);
 504
 505         /*
 506          * Finish waiting for reader threads before letting the old ptr
 507          * being freed. Must be done within rcu_registry_lock because it
 508          * iterates on reader threads.
 509          */
 510         smp_mb_master();
 511 out:
 512         mutex_unlock(&rcu_registry_lock);
 513         mutex_unlock(&rcu_gp_lock);
 514
 515         /*
 516          * Wakeup waiters only after we have completed the grace period
 517          * and have ensured the memory barriers at the end of the grace
 518          * period have been issued.
 519          */
 520         urcu_wake_all_waiters(&waiters);
 521 }
 522
 523 /*
 524  * library wrappers to be used by non-LGPL compatible source code.
 525  */
 526
 527 void rcu_read_lock(void)
 528 {
 529         _rcu_read_lock();
 530 }
 531
 532 void rcu_read_unlock(void)
 533 {
 534         _rcu_read_unlock();
 535 }
 536
 537 int rcu_read_ongoing(void)
 538 {
 539         return _rcu_read_ongoing();
 540 }
 541
 542 #ifdef RCU_SIGNAL
 543 /*
 544  * Make sure the signal used by the urcu-signal flavor is unblocked
 545  * while the thread is registered.
 546  */
 547 static
 548 void urcu_signal_unblock(void)
 549 {
 550         sigset_t mask, oldmask;
 551         int ret;
 552
 553         ret = sigemptyset(&mask);
 554         urcu_posix_assert(!ret);
 555         ret = sigaddset(&mask, SIGRCU);
 556         urcu_posix_assert(!ret);
 557         ret = pthread_sigmask(SIG_UNBLOCK, &mask, &oldmask);
 558         urcu_posix_assert(!ret);
 559         URCU_TLS(rcu_signal_was_blocked) = sigismember(&oldmask, SIGRCU);
 560 }
 561
 562 static
 563 void urcu_signal_restore(void)
 564 {
 565         sigset_t mask;
 566         int ret;
 567
 568         if (!URCU_TLS(rcu_signal_was_blocked))
 569                 return;
 570         ret = sigemptyset(&mask);
 571         urcu_posix_assert(!ret);
 572         ret = sigaddset(&mask, SIGRCU);
 573         urcu_posix_assert(!ret);
 574         ret = pthread_sigmask(SIG_BLOCK, &mask, NULL);
 575         urcu_posix_assert(!ret);
 576 }
 577 #else
 578 static
 579 void urcu_signal_unblock(void) { }
 580 static
 581 void urcu_signal_restore(void) { }
 582 #endif
 583
 584 void rcu_register_thread(void)
 585 {
 586         urcu_signal_unblock();
 587
 588         URCU_TLS(rcu_reader).tid = pthread_self();
 589         urcu_posix_assert(URCU_TLS(rcu_reader).need_mb == 0);
 590         urcu_posix_assert(!(URCU_TLS(rcu_reader).ctr & URCU_GP_CTR_NEST_MASK));
 591
 592         mutex_lock(&rcu_registry_lock);
 593         urcu_posix_assert(!URCU_TLS(rcu_reader).registered);
 594         URCU_TLS(rcu_reader).registered = 1;
 595         rcu_init();     /* In case gcc does not support constructor attribute */
 596         cds_list_add(&URCU_TLS(rcu_reader).node, &registry);
 597         mutex_unlock(&rcu_registry_lock);
 598 }
 599
 600 void rcu_unregister_thread(void)
 601 {
 602         mutex_lock(&rcu_registry_lock);
 603         urcu_posix_assert(URCU_TLS(rcu_reader).registered);
 604         URCU_TLS(rcu_reader).registered = 0;
 605         cds_list_del(&URCU_TLS(rcu_reader).node);
 606         mutex_unlock(&rcu_registry_lock);
 607
 608         urcu_signal_restore();
 609 }
 610
 611 #ifdef RCU_MEMBARRIER
 612
 613 #ifdef CONFIG_RCU_FORCE_SYS_MEMBARRIER
 614 static
 615 void rcu_sys_membarrier_status(bool available)
 616 {
 617         if (!available)
 618                 abort();
 619 }
 620 #else
 621 static
 622 void rcu_sys_membarrier_status(bool available)
 623 {
 624         if (!available)
 625                 return;
 626         urcu_memb_has_sys_membarrier = 1;
 627 }
 628 #endif
 629
 630 static
 631 void rcu_sys_membarrier_init(void)
 632 {
 633         bool available = false;
 634         int mask;
 635
 636         mask = membarrier(MEMBARRIER_CMD_QUERY, 0);
 637         if (mask >= 0) {
 638                 if (mask & MEMBARRIER_CMD_PRIVATE_EXPEDITED) {
 639                         if (membarrier(MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED, 0))
 640                                 urcu_die(errno);
 641                         urcu_memb_has_sys_membarrier_private_expedited = 1;
 642                         available = true;
 643                 } else if (mask & MEMBARRIER_CMD_SHARED) {
 644                         available = true;
 645                 }
 646         }
 647         rcu_sys_membarrier_status(available);
 648 }
 649
 650 void rcu_init(void)
 651 {
 652         if (init_done)
 653                 return;
 654         init_done = 1;
 655         rcu_sys_membarrier_init();
 656 }
 657 #endif
 658
 659 #ifdef RCU_SIGNAL
 660 static void sigrcu_handler(int signo __attribute__((unused)),
 661                 siginfo_t *siginfo __attribute__((unused)),
 662                 void *context __attribute__((unused)))
 663 {
 664         /*
 665          * Executing this cmm_smp_mb() is the only purpose of this signal handler.
 666          * It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
 667          * executed on.
 668          */
 669         cmm_smp_mb();
 670         _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
 671         cmm_smp_mb();
 672 }
 673
 674 /*
 675  * rcu_init constructor. Called when the library is linked, but also when
 676  * reader threads are calling rcu_register_thread().
 677  * Should only be called by a single thread at a given time. This is ensured by
 678  * holing the rcu_registry_lock from rcu_register_thread() or by running
 679  * at library load time, which should not be executed by multiple
 680  * threads nor concurrently with rcu_register_thread() anyway.
 681  */
 682 void rcu_init(void)
 683 {
 684         struct sigaction act;
 685         int ret;
 686
 687         if (init_done)
 688                 return;
 689         init_done = 1;
 690
 691         act.sa_sigaction = sigrcu_handler;
 692         act.sa_flags = SA_SIGINFO | SA_RESTART;
 693         sigemptyset(&act.sa_mask);
 694         ret = sigaction(SIGRCU, &act, NULL);
 695         if (ret)
 696                 urcu_die(errno);
 697 }
 698
 699 void rcu_exit(void)
 700 {
 701         /*
 702          * Don't unregister the SIGRCU signal handler anymore, because
 703          * call_rcu threads could still be using it shortly before the
 704          * application exits.
 705          * Assertion disabled because call_rcu threads are now rcu
 706          * readers, and left running at exit.
 707          * urcu_posix_assert(cds_list_empty(&registry));
 708          */
 709 }
 710
 711 #endif /* #ifdef RCU_SIGNAL */
 712
 713 DEFINE_RCU_FLAVOR(rcu_flavor);
 714
 715 #include "urcu-call-rcu-impl.h"
 716 #include "urcu-defer-impl.h"