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 _GNU_SOURCE
  27 #define _LGPL_SOURCE
  28 #include <stdio.h>
  29 #include <pthread.h>
  30 #include <signal.h>
  31 #include <assert.h>
  32 #include <stdlib.h>
  33 #include <stdint.h>
  34 #include <string.h>
  35 #include <errno.h>
  36 #include <poll.h>
  37
  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
  48 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
  49 #undef _LGPL_SOURCE
  50 #include "urcu-qsbr.h"
  51 #define _LGPL_SOURCE
  52
  53 void __attribute__((destructor)) rcu_exit(void);
  54
  55 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
  56 struct rcu_gp rcu_gp = { .ctr = RCU_GP_ONLINE };
  57
  58 /*
  59  * Active attempts to check for reader Q.S. before calling futex().
  60  */
  61 #define RCU_QS_ACTIVE_ATTEMPTS 100
  62
  63 /*
  64  * Written to only by each individual reader. Read by both the reader and the
  65  * writers.
  66  */
  67 DEFINE_URCU_TLS(struct rcu_reader, rcu_reader);
  68
  69 #ifdef DEBUG_YIELD
  70 unsigned int rcu_yield_active;
  71 DEFINE_URCU_TLS(unsigned int, rcu_rand_yield);
  72 #endif
  73
  74 static CDS_LIST_HEAD(registry);
  75
  76 /*
  77  * Queue keeping threads awaiting to wait for a grace period. Contains
  78  * struct gp_waiters_thread objects.
  79  */
  80 static DEFINE_URCU_WAIT_QUEUE(gp_waiters);
  81
  82 static void mutex_lock(pthread_mutex_t *mutex)
  83 {
  84         int ret;
  85
  86 #ifndef DISTRUST_SIGNALS_EXTREME
  87         ret = pthread_mutex_lock(mutex);
  88         if (ret)
  89                 urcu_die(ret);
  90 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
  91         while ((ret = pthread_mutex_trylock(mutex)) != 0) {
  92                 if (ret != EBUSY && ret != EINTR)
  93                         urcu_die(ret);
  94                 poll(NULL,0,10);
  95         }
  96 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
  97 }
  98
  99 static void mutex_unlock(pthread_mutex_t *mutex)
 100 {
 101         int ret;
 102
 103         ret = pthread_mutex_unlock(mutex);
 104         if (ret)
 105                 urcu_die(ret);
 106 }
 107
 108 /*
 109  * synchronize_rcu() waiting. Single thread.
 110  */
 111 static void wait_gp(void)
 112 {
 113         /* Read reader_gp before read futex */
 114         cmm_smp_rmb();
 115         if (uatomic_read(&rcu_gp.futex) == -1)
 116                 futex_noasync(&rcu_gp.futex, FUTEX_WAIT, -1,
 117                       NULL, NULL, 0);
 118 }
 119
 120 static void wait_for_readers(struct cds_list_head *input_readers,
 121                         struct cds_list_head *cur_snap_readers,
 122                         struct cds_list_head *qsreaders)
 123 {
 124         unsigned int wait_loops = 0;
 125         struct rcu_reader *index, *tmp;
 126
 127         /*
 128          * Wait for each thread URCU_TLS(rcu_reader).ctr to either
 129          * indicate quiescence (offline), or for them to observe the
 130          * current rcu_gp.ctr value.
 131          */
 132         for (;;) {
 133                 if (wait_loops < RCU_QS_ACTIVE_ATTEMPTS)
 134                         wait_loops++;
 135                 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 136                         uatomic_set(&rcu_gp.futex, -1);
 137                         /*
 138                          * Write futex before write waiting (the other side
 139                          * reads them in the opposite order).
 140                          */
 141                         cmm_smp_wmb();
 142                         cds_list_for_each_entry(index, input_readers, node) {
 143                                 _CMM_STORE_SHARED(index->waiting, 1);
 144                         }
 145                         /* Write futex before read reader_gp */
 146                         cmm_smp_mb();
 147                 }
 148                 cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
 149                         switch (rcu_reader_state(&index->ctr)) {
 150                         case RCU_READER_ACTIVE_CURRENT:
 151                                 if (cur_snap_readers) {
 152                                         cds_list_move(&index->node,
 153                                                 cur_snap_readers);
 154                                         break;
 155                                 }
 156                                 /* Fall-through */
 157                         case RCU_READER_INACTIVE:
 158                                 cds_list_move(&index->node, qsreaders);
 159                                 break;
 160                         case RCU_READER_ACTIVE_OLD:
 161                                 /*
 162                                  * Old snapshot. Leaving node in
 163                                  * input_readers will make us busy-loop
 164                                  * until the snapshot becomes current or
 165                                  * the reader becomes inactive.
 166                                  */
 167                                 break;
 168                         }
 169                 }
 170
 171                 if (cds_list_empty(input_readers)) {
 172                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 173                                 /* Read reader_gp before write futex */
 174                                 cmm_smp_mb();
 175                                 uatomic_set(&rcu_gp.futex, 0);
 176                         }
 177                         break;
 178                 } else {
 179                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 180                                 wait_gp();
 181                         } else {
 182 #ifndef HAS_INCOHERENT_CACHES
 183                                 caa_cpu_relax();
 184 #else /* #ifndef HAS_INCOHERENT_CACHES */
 185                                 cmm_smp_mb();
 186 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 187                         }
 188                 }
 189         }
 190 }
 191
 192 /*
 193  * Using a two-subphases algorithm for architectures with smaller than 64-bit
 194  * long-size to ensure we do not encounter an overflow bug.
 195  */
 196
 197 #if (CAA_BITS_PER_LONG < 64)
 198 void synchronize_rcu(void)
 199 {
 200         CDS_LIST_HEAD(cur_snap_readers);
 201         CDS_LIST_HEAD(qsreaders);
 202         unsigned long was_online;
 203         DEFINE_URCU_WAIT_NODE(wait, URCU_WAIT_WAITING);
 204         struct urcu_waiters waiters;
 205
 206         was_online = rcu_read_ongoing();
 207
 208         /* All threads should read qparity before accessing data structure
 209          * where new ptr points to.  In the "then" case, rcu_thread_offline
 210          * includes a memory barrier.
 211          *
 212          * Mark the writer thread offline to make sure we don't wait for
 213          * our own quiescent state. This allows using synchronize_rcu()
 214          * in threads registered as readers.
 215          */
 216         if (was_online)
 217                 rcu_thread_offline();
 218         else
 219                 cmm_smp_mb();
 220
 221         /*
 222          * Add ourself to gp_waiters queue of threads awaiting to wait
 223          * for a grace period. Proceed to perform the grace period only
 224          * if we are the first thread added into the queue.
 225          */
 226         if (urcu_wait_add(&gp_waiters, &wait) != 0) {
 227                 /* Not first in queue: will be awakened by another thread. */
 228                 urcu_adaptative_busy_wait(&wait);
 229                 goto gp_end;
 230         }
 231         /* We won't need to wake ourself up */
 232         urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);
 233
 234         mutex_lock(&rcu_gp_lock);
 235
 236         /*
 237          * Move all waiters into our local queue.
 238          */
 239         urcu_move_waiters(&waiters, &gp_waiters);
 240
 241         if (cds_list_empty(&registry))
 242                 goto out;
 243
 244         /*
 245          * Wait for readers to observe original parity or be quiescent.
 246          */
 247         wait_for_readers(&registry, &cur_snap_readers, &qsreaders);
 248
 249         /*
 250          * Must finish waiting for quiescent state for original parity
 251          * before committing next rcu_gp.ctr update to memory. Failure
 252          * to do so could result in the writer waiting forever while new
 253          * readers are always accessing data (no progress).  Enforce
 254          * compiler-order of load URCU_TLS(rcu_reader).ctr before store
 255          * to rcu_gp.ctr.
 256          */
 257         cmm_barrier();
 258
 259         /*
 260          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 261          * model easier to understand. It does not have a big performance impact
 262          * anyway, given this is the write-side.
 263          */
 264         cmm_smp_mb();
 265
 266         /* Switch parity: 0 -> 1, 1 -> 0 */
 267         CMM_STORE_SHARED(rcu_gp.ctr, rcu_gp.ctr ^ RCU_GP_CTR);
 268
 269         /*
 270          * Must commit rcu_gp.ctr update to memory before waiting for
 271          * quiescent state. Failure to do so could result in the writer
 272          * waiting forever while new readers are always accessing data
 273          * (no progress). Enforce compiler-order of store to rcu_gp.ctr
 274          * before load URCU_TLS(rcu_reader).ctr.
 275          */
 276         cmm_barrier();
 277
 278         /*
 279          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 280          * model easier to understand. It does not have a big performance impact
 281          * anyway, given this is the write-side.
 282          */
 283         cmm_smp_mb();
 284
 285         /*
 286          * Wait for readers to observe new parity or be quiescent.
 287          */
 288         wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
 289
 290         /*
 291          * Put quiescent reader list back into registry.
 292          */
 293         cds_list_splice(&qsreaders, &registry);
 294 out:
 295         mutex_unlock(&rcu_gp_lock);
 296         urcu_wake_all_waiters(&waiters);
 297 gp_end:
 298         /*
 299          * Finish waiting for reader threads before letting the old ptr being
 300          * freed.
 301          */
 302         if (was_online)
 303                 rcu_thread_online();
 304         else
 305                 cmm_smp_mb();
 306 }
 307 #else /* !(CAA_BITS_PER_LONG < 64) */
 308 void synchronize_rcu(void)
 309 {
 310         CDS_LIST_HEAD(qsreaders);
 311         unsigned long was_online;
 312         DEFINE_URCU_WAIT_NODE(wait, URCU_WAIT_WAITING);
 313         struct urcu_waiters waiters;
 314
 315         was_online = rcu_read_ongoing();
 316
 317         /*
 318          * Mark the writer thread offline to make sure we don't wait for
 319          * our own quiescent state. This allows using synchronize_rcu()
 320          * in threads registered as readers.
 321          */
 322         if (was_online)
 323                 rcu_thread_offline();
 324         else
 325                 cmm_smp_mb();
 326
 327         /*
 328          * Add ourself to gp_waiters queue of threads awaiting to wait
 329          * for a grace period. Proceed to perform the grace period only
 330          * if we are the first thread added into the queue.
 331          */
 332         if (urcu_wait_add(&gp_waiters, &wait) != 0) {
 333                 /* Not first in queue: will be awakened by another thread. */
 334                 urcu_adaptative_busy_wait(&wait);
 335                 goto gp_end;
 336         }
 337         /* We won't need to wake ourself up */
 338         urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);
 339
 340         mutex_lock(&rcu_gp_lock);
 341
 342         /*
 343          * Move all waiters into our local queue.
 344          */
 345         urcu_move_waiters(&waiters, &gp_waiters);
 346
 347         if (cds_list_empty(&registry))
 348                 goto out;
 349
 350         /* Increment current G.P. */
 351         CMM_STORE_SHARED(rcu_gp.ctr, rcu_gp.ctr + RCU_GP_CTR);
 352
 353         /*
 354          * Must commit rcu_gp.ctr update to memory before waiting for
 355          * quiescent state. Failure to do so could result in the writer
 356          * waiting forever while new readers are always accessing data
 357          * (no progress). Enforce compiler-order of store to rcu_gp.ctr
 358          * before load URCU_TLS(rcu_reader).ctr.
 359          */
 360         cmm_barrier();
 361
 362         /*
 363          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 364          * model easier to understand. It does not have a big performance impact
 365          * anyway, given this is the write-side.
 366          */
 367         cmm_smp_mb();
 368
 369         /*
 370          * Wait for readers to observe new count of be quiescent.
 371          */
 372         wait_for_readers(&registry, NULL, &qsreaders);
 373
 374         /*
 375          * Put quiescent reader list back into registry.
 376          */
 377         cds_list_splice(&qsreaders, &registry);
 378 out:
 379         mutex_unlock(&rcu_gp_lock);
 380         urcu_wake_all_waiters(&waiters);
 381 gp_end:
 382         if (was_online)
 383                 rcu_thread_online();
 384         else
 385                 cmm_smp_mb();
 386 }
 387 #endif  /* !(CAA_BITS_PER_LONG < 64) */
 388
 389 /*
 390  * library wrappers to be used by non-LGPL compatible source code.
 391  */
 392
 393 void rcu_read_lock(void)
 394 {
 395         _rcu_read_lock();
 396 }
 397
 398 void rcu_read_unlock(void)
 399 {
 400         _rcu_read_unlock();
 401 }
 402
 403 int rcu_read_ongoing(void)
 404 {
 405         return _rcu_read_ongoing();
 406 }
 407
 408 void rcu_quiescent_state(void)
 409 {
 410         _rcu_quiescent_state();
 411 }
 412
 413 void rcu_thread_offline(void)
 414 {
 415         _rcu_thread_offline();
 416 }
 417
 418 void rcu_thread_online(void)
 419 {
 420         _rcu_thread_online();
 421 }
 422
 423 void rcu_register_thread(void)
 424 {
 425         URCU_TLS(rcu_reader).tid = pthread_self();
 426         assert(URCU_TLS(rcu_reader).ctr == 0);
 427
 428         mutex_lock(&rcu_gp_lock);
 429         cds_list_add(&URCU_TLS(rcu_reader).node, &registry);
 430         mutex_unlock(&rcu_gp_lock);
 431         _rcu_thread_online();
 432 }
 433
 434 void rcu_unregister_thread(void)
 435 {
 436         /*
 437          * We have to make the thread offline otherwise we end up dealocking
 438          * with a waiting writer.
 439          */
 440         _rcu_thread_offline();
 441         mutex_lock(&rcu_gp_lock);
 442         cds_list_del(&URCU_TLS(rcu_reader).node);
 443         mutex_unlock(&rcu_gp_lock);
 444 }
 445
 446 void rcu_exit(void)
 447 {
 448         /*
 449          * Assertion disabled because call_rcu threads are now rcu
 450          * readers, and left running at exit.
 451          * assert(cds_list_empty(&registry));
 452          */
 453 }
 454
 455 DEFINE_RCU_FLAVOR(rcu_flavor);
 456
 457 #include "urcu-call-rcu-impl.h"
 458 #include "urcu-defer-impl.h"