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
  57 int32_t rcu_gp_futex;
  58
  59 /*
  60  * Global grace period counter.
  61  */
  62 unsigned long rcu_gp_ctr = RCU_GP_ONLINE;
  63
  64 /*
  65  * Active attempts to check for reader Q.S. before calling futex().
  66  */
  67 #define RCU_QS_ACTIVE_ATTEMPTS 100
  68
  69 /*
  70  * Written to only by each individual reader. Read by both the reader and the
  71  * writers.
  72  */
  73 DEFINE_URCU_TLS(struct rcu_reader, rcu_reader);
  74
  75 #ifdef DEBUG_YIELD
  76 unsigned int rcu_yield_active;
  77 DEFINE_URCU_TLS(unsigned int, rcu_rand_yield);
  78 #endif
  79
  80 static CDS_LIST_HEAD(registry);
  81
  82 /*
  83  * Queue keeping threads awaiting to wait for a grace period. Contains
  84  * struct gp_waiters_thread objects.
  85  */
  86 static DEFINE_URCU_WAIT_QUEUE(gp_waiters);
  87
  88 static void mutex_lock(pthread_mutex_t *mutex)
  89 {
  90         int ret;
  91
  92 #ifndef DISTRUST_SIGNALS_EXTREME
  93         ret = pthread_mutex_lock(mutex);
  94         if (ret)
  95                 urcu_die(ret);
  96 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
  97         while ((ret = pthread_mutex_trylock(mutex)) != 0) {
  98                 if (ret != EBUSY && ret != EINTR)
  99                         urcu_die(ret);
 100                 poll(NULL,0,10);
 101         }
 102 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
 103 }
 104
 105 static void mutex_unlock(pthread_mutex_t *mutex)
 106 {
 107         int ret;
 108
 109         ret = pthread_mutex_unlock(mutex);
 110         if (ret)
 111                 urcu_die(ret);
 112 }
 113
 114 /*
 115  * synchronize_rcu() waiting. Single thread.
 116  */
 117 static void wait_gp(void)
 118 {
 119         /* Read reader_gp before read futex */
 120         cmm_smp_rmb();
 121         if (uatomic_read(&rcu_gp_futex) == -1)
 122                 futex_noasync(&rcu_gp_futex, FUTEX_WAIT, -1,
 123                       NULL, NULL, 0);
 124 }
 125
 126 static void wait_for_readers(struct cds_list_head *input_readers,
 127                         struct cds_list_head *cur_snap_readers,
 128                         struct cds_list_head *qsreaders)
 129 {
 130         int wait_loops = 0;
 131         struct rcu_reader *index, *tmp;
 132
 133         /*
 134          * Wait for each thread URCU_TLS(rcu_reader).ctr to either
 135          * indicate quiescence (offline), or for them to observe the
 136          * current rcu_gp_ctr value.
 137          */
 138         for (;;) {
 139                 wait_loops++;
 140                 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 141                         uatomic_set(&rcu_gp_futex, -1);
 142                         /*
 143                          * Write futex before write waiting (the other side
 144                          * reads them in the opposite order).
 145                          */
 146                         cmm_smp_wmb();
 147                         cds_list_for_each_entry(index, input_readers, node) {
 148                                 _CMM_STORE_SHARED(index->waiting, 1);
 149                         }
 150                         /* Write futex before read reader_gp */
 151                         cmm_smp_mb();
 152                 }
 153                 cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
 154                         switch (rcu_reader_state(&index->ctr)) {
 155                         case RCU_READER_ACTIVE_CURRENT:
 156                                 if (cur_snap_readers) {
 157                                         cds_list_move(&index->node,
 158                                                 cur_snap_readers);
 159                                         break;
 160                                 }
 161                                 /* Fall-through */
 162                         case RCU_READER_INACTIVE:
 163                                 cds_list_move(&index->node, qsreaders);
 164                                 break;
 165                         case RCU_READER_ACTIVE_OLD:
 166                                 /*
 167                                  * Old snapshot. Leaving node in
 168                                  * input_readers will make us busy-loop
 169                                  * until the snapshot becomes current or
 170                                  * the reader becomes inactive.
 171                                  */
 172                                 break;
 173                         }
 174                 }
 175
 176                 if (cds_list_empty(input_readers)) {
 177                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 178                                 /* Read reader_gp before write futex */
 179                                 cmm_smp_mb();
 180                                 uatomic_set(&rcu_gp_futex, 0);
 181                         }
 182                         break;
 183                 } else {
 184                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 185                                 wait_gp();
 186                         } else {
 187 #ifndef HAS_INCOHERENT_CACHES
 188                                 caa_cpu_relax();
 189 #else /* #ifndef HAS_INCOHERENT_CACHES */
 190                                 cmm_smp_mb();
 191 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 192                         }
 193                 }
 194         }
 195 }
 196
 197 /*
 198  * Using a two-subphases algorithm for architectures with smaller than 64-bit
 199  * long-size to ensure we do not encounter an overflow bug.
 200  */
 201
 202 #if (CAA_BITS_PER_LONG < 64)
 203 void synchronize_rcu(void)
 204 {
 205         CDS_LIST_HEAD(cur_snap_readers);
 206         CDS_LIST_HEAD(qsreaders);
 207         unsigned long was_online;
 208         DEFINE_URCU_WAIT_NODE(wait, URCU_WAIT_WAITING);
 209         struct urcu_waiters waiters;
 210
 211         was_online = URCU_TLS(rcu_reader).ctr;
 212
 213         /* All threads should read qparity before accessing data structure
 214          * where new ptr points to.  In the "then" case, rcu_thread_offline
 215          * includes a memory barrier.
 216          *
 217          * Mark the writer thread offline to make sure we don't wait for
 218          * our own quiescent state. This allows using synchronize_rcu()
 219          * in threads registered as readers.
 220          */
 221         if (was_online)
 222                 rcu_thread_offline();
 223         else
 224                 cmm_smp_mb();
 225
 226         /*
 227          * Add ourself to gp_waiters queue of threads awaiting to wait
 228          * for a grace period. Proceed to perform the grace period only
 229          * if we are the first thread added into the queue.
 230          */
 231         if (urcu_wait_add(&gp_waiters, &wait) != 0) {
 232                 /* Not first in queue: will be awakened by another thread. */
 233                 urcu_adaptative_busy_wait(&wait);
 234                 goto gp_end;
 235         }
 236         /* We won't need to wake ourself up */
 237         urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);
 238
 239         mutex_lock(&rcu_gp_lock);
 240
 241         /*
 242          * Move all waiters into our local queue.
 243          */
 244         urcu_move_waiters(&waiters, &gp_waiters);
 245
 246         if (cds_list_empty(&registry))
 247                 goto out;
 248
 249         /*
 250          * Wait for readers to observe original parity or be quiescent.
 251          */
 252         wait_for_readers(&registry, &cur_snap_readers, &qsreaders);
 253
 254         /*
 255          * Must finish waiting for quiescent state for original parity
 256          * before committing next rcu_gp_ctr update to memory. Failure
 257          * to do so could result in the writer waiting forever while new
 258          * readers are always accessing data (no progress).  Enforce
 259          * compiler-order of load URCU_TLS(rcu_reader).ctr before store
 260          * to rcu_gp_ctr.
 261          */
 262         cmm_barrier();
 263
 264         /*
 265          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 266          * model easier to understand. It does not have a big performance impact
 267          * anyway, given this is the write-side.
 268          */
 269         cmm_smp_mb();
 270
 271         /* Switch parity: 0 -> 1, 1 -> 0 */
 272         CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
 273
 274         /*
 275          * Must commit rcu_gp_ctr update to memory before waiting for
 276          * quiescent state. Failure to do so could result in the writer
 277          * waiting forever while new readers are always accessing data
 278          * (no progress). Enforce compiler-order of store to rcu_gp_ctr
 279          * before load URCU_TLS(rcu_reader).ctr.
 280          */
 281         cmm_barrier();
 282
 283         /*
 284          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 285          * model easier to understand. It does not have a big performance impact
 286          * anyway, given this is the write-side.
 287          */
 288         cmm_smp_mb();
 289
 290         /*
 291          * Wait for readers to observe new parity or be quiescent.
 292          */
 293         wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
 294
 295         /*
 296          * Put quiescent reader list back into registry.
 297          */
 298         cds_list_splice(&qsreaders, &registry);
 299 out:
 300         mutex_unlock(&rcu_gp_lock);
 301         urcu_wake_all_waiters(&waiters);
 302 gp_end:
 303         /*
 304          * Finish waiting for reader threads before letting the old ptr being
 305          * freed.
 306          */
 307         if (was_online)
 308                 rcu_thread_online();
 309         else
 310                 cmm_smp_mb();
 311 }
 312 #else /* !(CAA_BITS_PER_LONG < 64) */
 313 void synchronize_rcu(void)
 314 {
 315         CDS_LIST_HEAD(qsreaders);
 316         unsigned long was_online;
 317         DEFINE_URCU_WAIT_NODE(wait, URCU_WAIT_WAITING);
 318         struct urcu_waiters waiters;
 319
 320         was_online = URCU_TLS(rcu_reader).ctr;
 321
 322         /*
 323          * Mark the writer thread offline to make sure we don't wait for
 324          * our own quiescent state. This allows using synchronize_rcu()
 325          * in threads registered as readers.
 326          */
 327         if (was_online)
 328                 rcu_thread_offline();
 329         else
 330                 cmm_smp_mb();
 331
 332         /*
 333          * Add ourself to gp_waiters queue of threads awaiting to wait
 334          * for a grace period. Proceed to perform the grace period only
 335          * if we are the first thread added into the queue.
 336          */
 337         if (urcu_wait_add(&gp_waiters, &wait) != 0) {
 338                 /* Not first in queue: will be awakened by another thread. */
 339                 urcu_adaptative_busy_wait(&wait);
 340                 goto gp_end;
 341         }
 342         /* We won't need to wake ourself up */
 343         urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);
 344
 345         mutex_lock(&rcu_gp_lock);
 346
 347         /*
 348          * Move all waiters into our local queue.
 349          */
 350         urcu_move_waiters(&waiters, &gp_waiters);
 351
 352         if (cds_list_empty(&registry))
 353                 goto out;
 354
 355         /* Increment current G.P. */
 356         CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
 357
 358         /*
 359          * Must commit rcu_gp_ctr update to memory before waiting for
 360          * quiescent state. Failure to do so could result in the writer
 361          * waiting forever while new readers are always accessing data
 362          * (no progress). Enforce compiler-order of store to rcu_gp_ctr
 363          * before load URCU_TLS(rcu_reader).ctr.
 364          */
 365         cmm_barrier();
 366
 367         /*
 368          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 369          * model easier to understand. It does not have a big performance impact
 370          * anyway, given this is the write-side.
 371          */
 372         cmm_smp_mb();
 373
 374         /*
 375          * Wait for readers to observe new count of be quiescent.
 376          */
 377         wait_for_readers(&registry, NULL, &qsreaders);
 378
 379         /*
 380          * Put quiescent reader list back into registry.
 381          */
 382         cds_list_splice(&qsreaders, &registry);
 383 out:
 384         mutex_unlock(&rcu_gp_lock);
 385         urcu_wake_all_waiters(&waiters);
 386 gp_end:
 387         if (was_online)
 388                 rcu_thread_online();
 389         else
 390                 cmm_smp_mb();
 391 }
 392 #endif  /* !(CAA_BITS_PER_LONG < 64) */
 393
 394 /*
 395  * library wrappers to be used by non-LGPL compatible source code.
 396  */
 397
 398 void rcu_read_lock(void)
 399 {
 400         _rcu_read_lock();
 401 }
 402
 403 void rcu_read_unlock(void)
 404 {
 405         _rcu_read_unlock();
 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"