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