src/urcu-qsbr.c

   1 /*
   2  * urcu-qsbr.c
   3  *
   4  * Userspace RCU QSBR 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 _LGPL_SOURCE
  28 #include <stdio.h>
  29 #include <pthread.h>
  30 #include <signal.h>
  31 #include <stdlib.h>
  32 #include <stdint.h>
  33 #include <string.h>
  34 #include <errno.h>
  35 #include <poll.h>
  36
  37 #include <urcu/assert.h>
  38 #include <urcu/wfcqueue.h>
  39 #include <urcu/map/urcu-qsbr.h>
  40 #define BUILD_QSBR_LIB
  41 #include <urcu/static/urcu-qsbr.h>
  42 #include <urcu/pointer.h>
  43 #include <urcu/tls-compat.h>
  44
  45 #include "urcu-die.h"
  46 #include "urcu-wait.h"
  47 #include "urcu-utils.h"
  48
  49 #define URCU_API_MAP
  50 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
  51 #undef _LGPL_SOURCE
  52 #include <urcu/urcu-qsbr.h>
  53 #define _LGPL_SOURCE
  54
  55 void __attribute__((destructor)) urcu_qsbr_exit(void);
  56
  57 /*
  58  * rcu_gp_lock ensures mutual exclusion between threads calling
  59  * synchronize_rcu().
  60  */
  61 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
  62 /*
  63  * rcu_registry_lock ensures mutual exclusion between threads
  64  * registering and unregistering themselves to/from the registry, and
  65  * with threads reading that registry from synchronize_rcu(). However,
  66  * this lock is not held all the way through the completion of awaiting
  67  * for the grace period. It is sporadically released between iterations
  68  * on the registry.
  69  * rcu_registry_lock may nest inside rcu_gp_lock.
  70  */
  71 static pthread_mutex_t rcu_registry_lock = PTHREAD_MUTEX_INITIALIZER;
  72 struct urcu_gp urcu_qsbr_gp = { .ctr = URCU_QSBR_GP_ONLINE };
  73
  74 /*
  75  * Active attempts to check for reader Q.S. before calling futex().
  76  */
  77 #define RCU_QS_ACTIVE_ATTEMPTS 100
  78
  79 /*
  80  * Written to only by each individual reader. Read by both the reader and the
  81  * writers.
  82  */
  83 DEFINE_URCU_TLS(struct urcu_qsbr_reader, urcu_qsbr_reader);
  84
  85 static CDS_LIST_HEAD(registry);
  86
  87 /*
  88  * Queue keeping threads awaiting to wait for a grace period. Contains
  89  * struct gp_waiters_thread objects.
  90  */
  91 static DEFINE_URCU_WAIT_QUEUE(gp_waiters);
  92
  93 static void mutex_lock(pthread_mutex_t *mutex)
  94 {
  95         int ret;
  96
  97 #ifndef DISTRUST_SIGNALS_EXTREME
  98         ret = pthread_mutex_lock(mutex);
  99         if (ret)
 100                 urcu_die(ret);
 101 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
 102         while ((ret = pthread_mutex_trylock(mutex)) != 0) {
 103                 if (ret != EBUSY && ret != EINTR)
 104                         urcu_die(ret);
 105                 poll(NULL,0,10);
 106         }
 107 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
 108 }
 109
 110 static void mutex_unlock(pthread_mutex_t *mutex)
 111 {
 112         int ret;
 113
 114         ret = pthread_mutex_unlock(mutex);
 115         if (ret)
 116                 urcu_die(ret);
 117 }
 118
 119 /*
 120  * synchronize_rcu() waiting. Single thread.
 121  */
 122 static void wait_gp(void)
 123 {
 124         /* Read reader_gp before read futex */
 125         cmm_smp_rmb();
 126         while (uatomic_read(&urcu_qsbr_gp.futex) == -1) {
 127                 if (!futex_noasync(&urcu_qsbr_gp.futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
 128                         /*
 129                          * Prior queued wakeups queued by unrelated code
 130                          * using the same address can cause futex wait to
 131                          * return 0 even through the futex value is still
 132                          * -1 (spurious wakeups). Check the value again
 133                          * in user-space to validate whether it really
 134                          * differs from -1.
 135                          */
 136                         continue;
 137                 }
 138                 switch (errno) {
 139                 case EAGAIN:
 140                         /* Value already changed. */
 141                         return;
 142                 case EINTR:
 143                         /* Retry if interrupted by signal. */
 144                         break;  /* Get out of switch. Check again. */
 145                 default:
 146                         /* Unexpected error. */
 147                         urcu_die(errno);
 148                 }
 149         }
 150 }
 151
 152 /*
 153  * Always called with rcu_registry lock held. Releases this lock between
 154  * iterations and grabs it again. Holds the lock when it returns.
 155  */
 156 static void wait_for_readers(struct cds_list_head *input_readers,
 157                         struct cds_list_head *cur_snap_readers,
 158                         struct cds_list_head *qsreaders)
 159 {
 160         unsigned int wait_loops = 0;
 161         struct urcu_qsbr_reader *index, *tmp;
 162
 163         /*
 164          * Wait for each thread URCU_TLS(urcu_qsbr_reader).ctr to either
 165          * indicate quiescence (offline), or for them to observe the
 166          * current urcu_qsbr_gp.ctr value.
 167          */
 168         for (;;) {
 169                 if (wait_loops < RCU_QS_ACTIVE_ATTEMPTS)
 170                         wait_loops++;
 171                 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 172                         uatomic_set(&urcu_qsbr_gp.futex, -1);
 173                         /*
 174                          * Write futex before write waiting (the other side
 175                          * reads them in the opposite order).
 176                          */
 177                         cmm_smp_wmb();
 178                         cds_list_for_each_entry(index, input_readers, node) {
 179                                 _CMM_STORE_SHARED(index->waiting, 1);
 180                         }
 181                         /* Write futex before read reader_gp */
 182                         cmm_smp_mb();
 183                 }
 184                 cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
 185                         switch (urcu_qsbr_reader_state(&index->ctr)) {
 186                         case URCU_READER_ACTIVE_CURRENT:
 187                                 if (cur_snap_readers) {
 188                                         cds_list_move(&index->node,
 189                                                 cur_snap_readers);
 190                                         break;
 191                                 }
 192                                 /* Fall-through */
 193                         case URCU_READER_INACTIVE:
 194                                 cds_list_move(&index->node, qsreaders);
 195                                 break;
 196                         case URCU_READER_ACTIVE_OLD:
 197                                 /*
 198                                  * Old snapshot. Leaving node in
 199                                  * input_readers will make us busy-loop
 200                                  * until the snapshot becomes current or
 201                                  * the reader becomes inactive.
 202                                  */
 203                                 break;
 204                         }
 205                 }
 206
 207                 if (cds_list_empty(input_readers)) {
 208                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 209                                 /* Read reader_gp before write futex */
 210                                 cmm_smp_mb();
 211                                 uatomic_set(&urcu_qsbr_gp.futex, 0);
 212                         }
 213                         break;
 214                 } else {
 215                         /* Temporarily unlock the registry lock. */
 216                         mutex_unlock(&rcu_registry_lock);
 217                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 218                                 wait_gp();
 219                         } else {
 220 #ifndef HAS_INCOHERENT_CACHES
 221                                 caa_cpu_relax();
 222 #else /* #ifndef HAS_INCOHERENT_CACHES */
 223                                 cmm_smp_mb();
 224 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 225                         }
 226                         /* Re-lock the registry lock before the next loop. */
 227                         mutex_lock(&rcu_registry_lock);
 228                 }
 229         }
 230 }
 231
 232 /*
 233  * Using a two-subphases algorithm for architectures with smaller than 64-bit
 234  * long-size to ensure we do not encounter an overflow bug.
 235  */
 236
 237 #if (CAA_BITS_PER_LONG < 64)
 238 void urcu_qsbr_synchronize_rcu(void)
 239 {
 240         CDS_LIST_HEAD(cur_snap_readers);
 241         CDS_LIST_HEAD(qsreaders);
 242         unsigned long was_online;
 243         DEFINE_URCU_WAIT_NODE(wait, URCU_WAIT_WAITING);
 244         struct urcu_waiters waiters;
 245
 246         was_online = urcu_qsbr_read_ongoing();
 247
 248         /* All threads should read qparity before accessing data structure
 249          * where new ptr points to.  In the "then" case, rcu_thread_offline
 250          * includes a memory barrier.
 251          *
 252          * Mark the writer thread offline to make sure we don't wait for
 253          * our own quiescent state. This allows using synchronize_rcu()
 254          * in threads registered as readers.
 255          */
 256         if (was_online)
 257                 urcu_qsbr_thread_offline();
 258         else
 259                 cmm_smp_mb();
 260
 261         /*
 262          * Add ourself to gp_waiters queue of threads awaiting to wait
 263          * for a grace period. Proceed to perform the grace period only
 264          * if we are the first thread added into the queue.
 265          */
 266         if (urcu_wait_add(&gp_waiters, &wait) != 0) {
 267                 /* Not first in queue: will be awakened by another thread. */
 268                 urcu_adaptative_busy_wait(&wait);
 269                 goto gp_end;
 270         }
 271         /* We won't need to wake ourself up */
 272         urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);
 273
 274         mutex_lock(&rcu_gp_lock);
 275
 276         /*
 277          * Move all waiters into our local queue.
 278          */
 279         urcu_move_waiters(&waiters, &gp_waiters);
 280
 281         mutex_lock(&rcu_registry_lock);
 282
 283         if (cds_list_empty(&registry))
 284                 goto out;
 285
 286         /*
 287          * Wait for readers to observe original parity or be quiescent.
 288          * wait_for_readers() can release and grab again rcu_registry_lock
 289          * internally.
 290          */
 291         wait_for_readers(&registry, &cur_snap_readers, &qsreaders);
 292
 293         /*
 294          * Must finish waiting for quiescent state for original parity
 295          * before committing next urcu_qsbr_gp.ctr update to memory. Failure
 296          * to do so could result in the writer waiting forever while new
 297          * readers are always accessing data (no progress).  Enforce
 298          * compiler-order of load URCU_TLS(urcu_qsbr_reader).ctr before store
 299          * to urcu_qsbr_gp.ctr.
 300          */
 301         cmm_barrier();
 302
 303         /*
 304          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 305          * model easier to understand. It does not have a big performance impact
 306          * anyway, given this is the write-side.
 307          */
 308         cmm_smp_mb();
 309
 310         /* Switch parity: 0 -> 1, 1 -> 0 */
 311         CMM_STORE_SHARED(urcu_qsbr_gp.ctr, urcu_qsbr_gp.ctr ^ URCU_QSBR_GP_CTR);
 312
 313         /*
 314          * Must commit urcu_qsbr_gp.ctr update to memory before waiting for
 315          * quiescent state. Failure to do so could result in the writer
 316          * waiting forever while new readers are always accessing data
 317          * (no progress). Enforce compiler-order of store to urcu_qsbr_gp.ctr
 318          * before load URCU_TLS(urcu_qsbr_reader).ctr.
 319          */
 320         cmm_barrier();
 321
 322         /*
 323          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 324          * model easier to understand. It does not have a big performance impact
 325          * anyway, given this is the write-side.
 326          */
 327         cmm_smp_mb();
 328
 329         /*
 330          * Wait for readers to observe new parity or be quiescent.
 331          * wait_for_readers() can release and grab again rcu_registry_lock
 332          * internally.
 333          */
 334         wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
 335
 336         /*
 337          * Put quiescent reader list back into registry.
 338          */
 339         cds_list_splice(&qsreaders, &registry);
 340 out:
 341         mutex_unlock(&rcu_registry_lock);
 342         mutex_unlock(&rcu_gp_lock);
 343         urcu_wake_all_waiters(&waiters);
 344 gp_end:
 345         /*
 346          * Finish waiting for reader threads before letting the old ptr being
 347          * freed.
 348          */
 349         if (was_online)
 350                 urcu_qsbr_thread_online();
 351         else
 352                 cmm_smp_mb();
 353 }
 354 #else /* !(CAA_BITS_PER_LONG < 64) */
 355 void urcu_qsbr_synchronize_rcu(void)
 356 {
 357         CDS_LIST_HEAD(qsreaders);
 358         unsigned long was_online;
 359         DEFINE_URCU_WAIT_NODE(wait, URCU_WAIT_WAITING);
 360         struct urcu_waiters waiters;
 361
 362         was_online = urcu_qsbr_read_ongoing();
 363
 364         /*
 365          * Mark the writer thread offline to make sure we don't wait for
 366          * our own quiescent state. This allows using synchronize_rcu()
 367          * in threads registered as readers.
 368          */
 369         if (was_online)
 370                 urcu_qsbr_thread_offline();
 371         else
 372                 cmm_smp_mb();
 373
 374         /*
 375          * Add ourself to gp_waiters queue of threads awaiting to wait
 376          * for a grace period. Proceed to perform the grace period only
 377          * if we are the first thread added into the queue.
 378          */
 379         if (urcu_wait_add(&gp_waiters, &wait) != 0) {
 380                 /* Not first in queue: will be awakened by another thread. */
 381                 urcu_adaptative_busy_wait(&wait);
 382                 goto gp_end;
 383         }
 384         /* We won't need to wake ourself up */
 385         urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);
 386
 387         mutex_lock(&rcu_gp_lock);
 388
 389         /*
 390          * Move all waiters into our local queue.
 391          */
 392         urcu_move_waiters(&waiters, &gp_waiters);
 393
 394         mutex_lock(&rcu_registry_lock);
 395
 396         if (cds_list_empty(&registry))
 397                 goto out;
 398
 399         /* Increment current G.P. */
 400         CMM_STORE_SHARED(urcu_qsbr_gp.ctr, urcu_qsbr_gp.ctr + URCU_QSBR_GP_CTR);
 401
 402         /*
 403          * Must commit urcu_qsbr_gp.ctr update to memory before waiting for
 404          * quiescent state. Failure to do so could result in the writer
 405          * waiting forever while new readers are always accessing data
 406          * (no progress). Enforce compiler-order of store to urcu_qsbr_gp.ctr
 407          * before load URCU_TLS(urcu_qsbr_reader).ctr.
 408          */
 409         cmm_barrier();
 410
 411         /*
 412          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 413          * model easier to understand. It does not have a big performance impact
 414          * anyway, given this is the write-side.
 415          */
 416         cmm_smp_mb();
 417
 418         /*
 419          * Wait for readers to observe new count of be quiescent.
 420          * wait_for_readers() can release and grab again rcu_registry_lock
 421          * internally.
 422          */
 423         wait_for_readers(&registry, NULL, &qsreaders);
 424
 425         /*
 426          * Put quiescent reader list back into registry.
 427          */
 428         cds_list_splice(&qsreaders, &registry);
 429 out:
 430         mutex_unlock(&rcu_registry_lock);
 431         mutex_unlock(&rcu_gp_lock);
 432         urcu_wake_all_waiters(&waiters);
 433 gp_end:
 434         if (was_online)
 435                 urcu_qsbr_thread_online();
 436         else
 437                 cmm_smp_mb();
 438 }
 439 #endif  /* !(CAA_BITS_PER_LONG < 64) */
 440
 441 /*
 442  * library wrappers to be used by non-LGPL compatible source code.
 443  */
 444
 445 void urcu_qsbr_read_lock(void)
 446 {
 447         _urcu_qsbr_read_lock();
 448 }
 449
 450 void urcu_qsbr_read_unlock(void)
 451 {
 452         _urcu_qsbr_read_unlock();
 453 }
 454
 455 int urcu_qsbr_read_ongoing(void)
 456 {
 457         return _urcu_qsbr_read_ongoing();
 458 }
 459 void rcu_read_ongoing_qsbr();
 460
 461 void urcu_qsbr_quiescent_state(void)
 462 {
 463         _urcu_qsbr_quiescent_state();
 464 }
 465 void rcu_quiescent_state_qsbr();
 466
 467 void urcu_qsbr_thread_offline(void)
 468 {
 469         _urcu_qsbr_thread_offline();
 470 }
 471 void rcu_thread_offline_qsbr();
 472
 473 void urcu_qsbr_thread_online(void)
 474 {
 475         _urcu_qsbr_thread_online();
 476 }
 477
 478 void urcu_qsbr_register_thread(void)
 479 {
 480         URCU_TLS(urcu_qsbr_reader).tid = pthread_self();
 481         urcu_posix_assert(URCU_TLS(urcu_qsbr_reader).ctr == 0);
 482
 483         mutex_lock(&rcu_registry_lock);
 484         urcu_posix_assert(!URCU_TLS(urcu_qsbr_reader).registered);
 485         URCU_TLS(urcu_qsbr_reader).registered = 1;
 486         cds_list_add(&URCU_TLS(urcu_qsbr_reader).node, &registry);
 487         mutex_unlock(&rcu_registry_lock);
 488         _urcu_qsbr_thread_online();
 489 }
 490
 491 void urcu_qsbr_unregister_thread(void)
 492 {
 493         /*
 494          * We have to make the thread offline otherwise we end up dealocking
 495          * with a waiting writer.
 496          */
 497         _urcu_qsbr_thread_offline();
 498         urcu_posix_assert(URCU_TLS(urcu_qsbr_reader).registered);
 499         URCU_TLS(urcu_qsbr_reader).registered = 0;
 500         mutex_lock(&rcu_registry_lock);
 501         cds_list_del(&URCU_TLS(urcu_qsbr_reader).node);
 502         mutex_unlock(&rcu_registry_lock);
 503 }
 504
 505 void urcu_qsbr_exit(void)
 506 {
 507         /*
 508          * Assertion disabled because call_rcu threads are now rcu
 509          * readers, and left running at exit.
 510          * urcu_posix_assert(cds_list_empty(&registry));
 511          */
 512 }
 513
 514 DEFINE_RCU_FLAVOR(rcu_flavor);
 515
 516 #include "urcu-call-rcu-impl.h"
 517 #include "urcu-defer-impl.h"