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