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
  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 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
  55
  56 int32_t rcu_gp_futex;
  57
  58 /*
  59  * Global grace period counter.
  60  */
  61 unsigned long rcu_gp_ctr = RCU_GP_ONLINE;
  62
  63 /*
  64  * Active attempts to check for reader Q.S. before calling futex().
  65  */
  66 #define RCU_QS_ACTIVE_ATTEMPTS 100
  67
  68 /*
  69  * Written to only by each individual reader. Read by both the reader and the
  70  * writers.
  71  */
  72 DEFINE_URCU_TLS(struct rcu_reader, rcu_reader);
  73
  74 #ifdef DEBUG_YIELD
  75 unsigned int rcu_yield_active;
  76 DEFINE_URCU_TLS(unsigned int, rcu_rand_yield);
  77 #endif
  78
  79 static CDS_LIST_HEAD(registry);
  80
  81 static void mutex_lock(pthread_mutex_t *mutex)
  82 {
  83         int ret;
  84
  85 #ifndef DISTRUST_SIGNALS_EXTREME
  86         ret = pthread_mutex_lock(mutex);
  87         if (ret)
  88                 urcu_die(ret);
  89 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
  90         while ((ret = pthread_mutex_trylock(mutex)) != 0) {
  91                 if (ret != EBUSY && ret != EINTR)
  92                         urcu_die(ret);
  93                 poll(NULL,0,10);
  94         }
  95 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
  96 }
  97
  98 static void mutex_unlock(pthread_mutex_t *mutex)
  99 {
 100         int ret;
 101
 102         ret = pthread_mutex_unlock(mutex);
 103         if (ret)
 104                 urcu_die(ret);
 105 }
 106
 107 /*
 108  * synchronize_rcu() waiting. Single thread.
 109  */
 110 static void wait_gp(void)
 111 {
 112         /* Read reader_gp before read futex */
 113         cmm_smp_rmb();
 114         if (uatomic_read(&rcu_gp_futex) == -1)
 115                 futex_noasync(&rcu_gp_futex, FUTEX_WAIT, -1,
 116                       NULL, NULL, 0);
 117 }
 118
 119 static void wait_for_readers(void)
 120 {
 121         CDS_LIST_HEAD(qsreaders);
 122         int wait_loops = 0;
 123         struct rcu_reader *index, *tmp;
 124
 125         /*
 126          * Wait for each thread URCU_TLS(rcu_reader).ctr to either
 127          * indicate quiescence (offline), or for them to observe the
 128          * current rcu_gp_ctr value.
 129          */
 130         for (;;) {
 131                 wait_loops++;
 132                 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 133                         uatomic_set(&rcu_gp_futex, -1);
 134                         /*
 135                          * Write futex before write waiting (the other side
 136                          * reads them in the opposite order).
 137                          */
 138                         cmm_smp_wmb();
 139                         cds_list_for_each_entry(index, &registry, node) {
 140                                 _CMM_STORE_SHARED(index->waiting, 1);
 141                         }
 142                         /* Write futex before read reader_gp */
 143                         cmm_smp_mb();
 144                 }
 145                 cds_list_for_each_entry_safe(index, tmp, &registry, node) {
 146                         if (!rcu_gp_ongoing(&index->ctr))
 147                                 cds_list_move(&index->node, &qsreaders);
 148                 }
 149
 150                 if (cds_list_empty(&registry)) {
 151                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 152                                 /* Read reader_gp before write futex */
 153                                 cmm_smp_mb();
 154                                 uatomic_set(&rcu_gp_futex, 0);
 155                         }
 156                         break;
 157                 } else {
 158                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 159                                 wait_gp();
 160                         } else {
 161 #ifndef HAS_INCOHERENT_CACHES
 162                                 caa_cpu_relax();
 163 #else /* #ifndef HAS_INCOHERENT_CACHES */
 164                                 cmm_smp_mb();
 165 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 166                         }
 167                 }
 168         }
 169         /* put back the reader list in the registry */
 170         cds_list_splice(&qsreaders, &registry);
 171 }
 172
 173 /*
 174  * Using a two-subphases algorithm for architectures with smaller than 64-bit
 175  * long-size to ensure we do not encounter an overflow bug.
 176  */
 177
 178 #if (CAA_BITS_PER_LONG < 64)
 179 void synchronize_rcu(void)
 180 {
 181         unsigned long was_online;
 182
 183         was_online = URCU_TLS(rcu_reader).ctr;
 184
 185         /* All threads should read qparity before accessing data structure
 186          * where new ptr points to.  In the "then" case, rcu_thread_offline
 187          * includes a memory barrier.
 188          *
 189          * Mark the writer thread offline to make sure we don't wait for
 190          * our own quiescent state. This allows using synchronize_rcu()
 191          * in threads registered as readers.
 192          */
 193         if (was_online)
 194                 rcu_thread_offline();
 195         else
 196                 cmm_smp_mb();
 197
 198         mutex_lock(&rcu_gp_lock);
 199
 200         if (cds_list_empty(&registry))
 201                 goto out;
 202
 203         /*
 204          * Wait for readers to observe original parity or be quiescent.
 205          */
 206         wait_for_readers();
 207
 208         /*
 209          * Must finish waiting for quiescent state for original parity
 210          * before committing next rcu_gp_ctr update to memory. Failure
 211          * to do so could result in the writer waiting forever while new
 212          * readers are always accessing data (no progress).  Enforce
 213          * compiler-order of load URCU_TLS(rcu_reader).ctr before store
 214          * to rcu_gp_ctr.
 215          */
 216         cmm_barrier();
 217
 218         /*
 219          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 220          * model easier to understand. It does not have a big performance impact
 221          * anyway, given this is the write-side.
 222          */
 223         cmm_smp_mb();
 224
 225         /* Switch parity: 0 -> 1, 1 -> 0 */
 226         CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
 227
 228         /*
 229          * Must commit rcu_gp_ctr update to memory before waiting for
 230          * quiescent state. Failure to do so could result in the writer
 231          * waiting forever while new readers are always accessing data
 232          * (no progress). Enforce compiler-order of store to rcu_gp_ctr
 233          * before load URCU_TLS(rcu_reader).ctr.
 234          */
 235         cmm_barrier();
 236
 237         /*
 238          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 239          * model easier to understand. It does not have a big performance impact
 240          * anyway, given this is the write-side.
 241          */
 242         cmm_smp_mb();
 243
 244         /*
 245          * Wait for readers to observe new parity or be quiescent.
 246          */
 247         wait_for_readers();
 248 out:
 249         mutex_unlock(&rcu_gp_lock);
 250
 251         /*
 252          * Finish waiting for reader threads before letting the old ptr being
 253          * freed.
 254          */
 255         if (was_online)
 256                 rcu_thread_online();
 257         else
 258                 cmm_smp_mb();
 259 }
 260 #else /* !(CAA_BITS_PER_LONG < 64) */
 261 void synchronize_rcu(void)
 262 {
 263         unsigned long was_online;
 264
 265         was_online = URCU_TLS(rcu_reader).ctr;
 266
 267         /*
 268          * Mark the writer thread offline to make sure we don't wait for
 269          * our own quiescent state. This allows using synchronize_rcu()
 270          * in threads registered as readers.
 271          */
 272         if (was_online)
 273                 rcu_thread_offline();
 274         else
 275                 cmm_smp_mb();
 276
 277         mutex_lock(&rcu_gp_lock);
 278         if (cds_list_empty(&registry))
 279                 goto out;
 280
 281         /* Increment current G.P. */
 282         CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
 283
 284         /*
 285          * Must commit rcu_gp_ctr update to memory before waiting for
 286          * quiescent state. Failure to do so could result in the writer
 287          * waiting forever while new readers are always accessing data
 288          * (no progress). Enforce compiler-order of store to rcu_gp_ctr
 289          * before load URCU_TLS(rcu_reader).ctr.
 290          */
 291         cmm_barrier();
 292
 293         /*
 294          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 295          * model easier to understand. It does not have a big performance impact
 296          * anyway, given this is the write-side.
 297          */
 298         cmm_smp_mb();
 299
 300         /*
 301          * Wait for readers to observe new count of be quiescent.
 302          */
 303         wait_for_readers();
 304 out:
 305         mutex_unlock(&rcu_gp_lock);
 306
 307         if (was_online)
 308                 rcu_thread_online();
 309         else
 310                 cmm_smp_mb();
 311 }
 312 #endif  /* !(CAA_BITS_PER_LONG < 64) */
 313
 314 /*
 315  * library wrappers to be used by non-LGPL compatible source code.
 316  */
 317
 318 void rcu_read_lock(void)
 319 {
 320         _rcu_read_lock();
 321 }
 322
 323 void rcu_read_unlock(void)
 324 {
 325         _rcu_read_unlock();
 326 }
 327
 328 void rcu_quiescent_state(void)
 329 {
 330         _rcu_quiescent_state();
 331 }
 332
 333 void rcu_thread_offline(void)
 334 {
 335         _rcu_thread_offline();
 336 }
 337
 338 void rcu_thread_online(void)
 339 {
 340         _rcu_thread_online();
 341 }
 342
 343 void rcu_register_thread(void)
 344 {
 345         URCU_TLS(rcu_reader).tid = pthread_self();
 346         assert(URCU_TLS(rcu_reader).ctr == 0);
 347
 348         mutex_lock(&rcu_gp_lock);
 349         cds_list_add(&URCU_TLS(rcu_reader).node, &registry);
 350         mutex_unlock(&rcu_gp_lock);
 351         _rcu_thread_online();
 352 }
 353
 354 void rcu_unregister_thread(void)
 355 {
 356         /*
 357          * We have to make the thread offline otherwise we end up dealocking
 358          * with a waiting writer.
 359          */
 360         _rcu_thread_offline();
 361         mutex_lock(&rcu_gp_lock);
 362         cds_list_del(&URCU_TLS(rcu_reader).node);
 363         mutex_unlock(&rcu_gp_lock);
 364 }
 365
 366 void rcu_exit(void)
 367 {
 368         /*
 369          * Assertion disabled because call_rcu threads are now rcu
 370          * readers, and left running at exit.
 371          * assert(cds_list_empty(&registry));
 372          */
 373 }
 374
 375 DEFINE_RCU_FLAVOR(rcu_flavor);
 376
 377 #include "urcu-call-rcu-impl.h"
 378 #include "urcu-defer-impl.h"