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/wfqueue.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
  44 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
  45 #undef _LGPL_SOURCE
  46 #include "urcu-qsbr.h"
  47 #define _LGPL_SOURCE
  48
  49 void __attribute__((destructor)) rcu_exit(void);
  50
  51 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
  52
  53 int32_t gp_futex;
  54
  55 /*
  56  * Global grace period counter.
  57  */
  58 unsigned long rcu_gp_ctr = RCU_GP_ONLINE;
  59
  60 /*
  61  * Active attempts to check for reader Q.S. before calling futex().
  62  */
  63 #define RCU_QS_ACTIVE_ATTEMPTS 100
  64
  65 /*
  66  * Written to only by each individual reader. Read by both the reader and the
  67  * writers.
  68  */
  69 struct rcu_reader __thread rcu_reader;
  70
  71 #ifdef DEBUG_YIELD
  72 unsigned int yield_active;
  73 unsigned int __thread rand_yield;
  74 #endif
  75
  76 static CDS_LIST_HEAD(registry);
  77
  78 static void mutex_lock(pthread_mutex_t *mutex)
  79 {
  80         int ret;
  81
  82 #ifndef DISTRUST_SIGNALS_EXTREME
  83         ret = pthread_mutex_lock(mutex);
  84         if (ret) {
  85                 perror("Error in pthread mutex lock");
  86                 exit(-1);
  87         }
  88 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
  89         while ((ret = pthread_mutex_trylock(mutex)) != 0) {
  90                 if (ret != EBUSY && ret != EINTR) {
  91                         printf("ret = %d, errno = %d\n", ret, errno);
  92                         perror("Error in pthread mutex lock");
  93                         exit(-1);
  94                 }
  95                 poll(NULL,0,10);
  96         }
  97 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
  98 }
  99
 100 static void mutex_unlock(pthread_mutex_t *mutex)
 101 {
 102         int ret;
 103
 104         ret = pthread_mutex_unlock(mutex);
 105         if (ret) {
 106                 perror("Error in pthread mutex unlock");
 107                 exit(-1);
 108         }
 109 }
 110
 111 /*
 112  * synchronize_rcu() waiting. Single thread.
 113  */
 114 static void wait_gp(void)
 115 {
 116         /* Read reader_gp before read futex */
 117         cmm_smp_rmb();
 118         if (uatomic_read(&gp_futex) == -1)
 119                 futex_noasync(&gp_futex, FUTEX_WAIT, -1,
 120                       NULL, NULL, 0);
 121 }
 122
 123 static void update_counter_and_wait(void)
 124 {
 125         CDS_LIST_HEAD(qsreaders);
 126         int wait_loops = 0;
 127         struct rcu_reader *index, *tmp;
 128
 129 #if (CAA_BITS_PER_LONG < 64)
 130         /* Switch parity: 0 -> 1, 1 -> 0 */
 131         CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
 132 #else   /* !(CAA_BITS_PER_LONG < 64) */
 133         /* Increment current G.P. */
 134         CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
 135 #endif  /* !(CAA_BITS_PER_LONG < 64) */
 136
 137         /*
 138          * Must commit rcu_gp_ctr update to memory before waiting for
 139          * quiescent state. Failure to do so could result in the writer
 140          * waiting forever while new readers are always accessing data
 141          * (no progress). Enforce compiler-order of store to rcu_gp_ctr
 142          * before load rcu_reader ctr.
 143          */
 144         cmm_barrier();
 145
 146         /*
 147          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 148          * model easier to understand. It does not have a big performance impact
 149          * anyway, given this is the write-side.
 150          */
 151         cmm_smp_mb();
 152
 153         /*
 154          * Wait for each thread rcu_reader_qs_gp count to become 0.
 155          */
 156         for (;;) {
 157                 wait_loops++;
 158                 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 159                         uatomic_set(&gp_futex, -1);
 160                         /*
 161                          * Write futex before write waiting (the other side
 162                          * reads them in the opposite order).
 163                          */
 164                         cmm_smp_wmb();
 165                         cds_list_for_each_entry(index, &registry, node) {
 166                                 _CMM_STORE_SHARED(index->waiting, 1);
 167                         }
 168                         /* Write futex before read reader_gp */
 169                         cmm_smp_mb();
 170                 }
 171                 cds_list_for_each_entry_safe(index, tmp, &registry, node) {
 172                         if (!rcu_gp_ongoing(&index->ctr))
 173                                 cds_list_move(&index->node, &qsreaders);
 174                 }
 175
 176                 if (cds_list_empty(&registry)) {
 177                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 178                                 /* Read reader_gp before write futex */
 179                                 cmm_smp_mb();
 180                                 uatomic_set(&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         /* put back the reader list in the registry */
 196         cds_list_splice(&qsreaders, &registry);
 197 }
 198
 199 /*
 200  * Using a two-subphases algorithm for architectures with smaller than 64-bit
 201  * long-size to ensure we do not encounter an overflow bug.
 202  */
 203
 204 #if (CAA_BITS_PER_LONG < 64)
 205 void synchronize_rcu(void)
 206 {
 207         unsigned long was_online;
 208
 209         was_online = rcu_reader.ctr;
 210
 211         /* All threads should read qparity before accessing data structure
 212          * where new ptr points to.  In the "then" case, rcu_thread_offline
 213          * includes a memory barrier.
 214          *
 215          * Mark the writer thread offline to make sure we don't wait for
 216          * our own quiescent state. This allows using synchronize_rcu()
 217          * in threads registered as readers.
 218          */
 219         if (was_online)
 220                 rcu_thread_offline();
 221         else
 222                 cmm_smp_mb();
 223
 224         mutex_lock(&rcu_gp_lock);
 225
 226         if (cds_list_empty(&registry))
 227                 goto out;
 228
 229         /*
 230          * Wait for previous parity to be empty of readers.
 231          */
 232         update_counter_and_wait();      /* 0 -> 1, wait readers in parity 0 */
 233
 234         /*
 235          * Must finish waiting for quiescent state for parity 0 before
 236          * committing next rcu_gp_ctr update to memory. Failure to
 237          * do so could result in the writer waiting forever while new
 238          * readers are always accessing data (no progress).  Enforce
 239          * compiler-order of load rcu_reader ctr before store to
 240          * rcu_gp_ctr.
 241          */
 242         cmm_barrier();
 243
 244         /*
 245          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 246          * model easier to understand. It does not have a big performance impact
 247          * anyway, given this is the write-side.
 248          */
 249         cmm_smp_mb();
 250
 251         /*
 252          * Wait for previous parity to be empty of readers.
 253          */
 254         update_counter_and_wait();      /* 1 -> 0, wait readers in parity 1 */
 255 out:
 256         mutex_unlock(&rcu_gp_lock);
 257
 258         /*
 259          * Finish waiting for reader threads before letting the old ptr being
 260          * freed.
 261          */
 262         if (was_online)
 263                 rcu_thread_online();
 264         else
 265                 cmm_smp_mb();
 266 }
 267 #else /* !(CAA_BITS_PER_LONG < 64) */
 268 void synchronize_rcu(void)
 269 {
 270         unsigned long was_online;
 271
 272         was_online = rcu_reader.ctr;
 273
 274         /*
 275          * Mark the writer thread offline to make sure we don't wait for
 276          * our own quiescent state. This allows using synchronize_rcu()
 277          * in threads registered as readers.
 278          */
 279         if (was_online)
 280                 rcu_thread_offline();
 281         else
 282                 cmm_smp_mb();
 283
 284         mutex_lock(&rcu_gp_lock);
 285         if (cds_list_empty(&registry))
 286                 goto out;
 287         update_counter_and_wait();
 288 out:
 289         mutex_unlock(&rcu_gp_lock);
 290
 291         if (was_online)
 292                 rcu_thread_online();
 293         else
 294                 cmm_smp_mb();
 295 }
 296 #endif  /* !(CAA_BITS_PER_LONG < 64) */
 297
 298 /*
 299  * library wrappers to be used by non-LGPL compatible source code.
 300  */
 301
 302 void rcu_read_lock(void)
 303 {
 304         _rcu_read_lock();
 305 }
 306
 307 void rcu_read_unlock(void)
 308 {
 309         _rcu_read_unlock();
 310 }
 311
 312 void rcu_quiescent_state(void)
 313 {
 314         _rcu_quiescent_state();
 315 }
 316
 317 void rcu_thread_offline(void)
 318 {
 319         _rcu_thread_offline();
 320 }
 321
 322 void rcu_thread_online(void)
 323 {
 324         _rcu_thread_online();
 325 }
 326
 327 void rcu_register_thread(void)
 328 {
 329         rcu_reader.tid = pthread_self();
 330         assert(rcu_reader.ctr == 0);
 331
 332         mutex_lock(&rcu_gp_lock);
 333         cds_list_add(&rcu_reader.node, &registry);
 334         mutex_unlock(&rcu_gp_lock);
 335         _rcu_thread_online();
 336 }
 337
 338 void rcu_unregister_thread(void)
 339 {
 340         /*
 341          * We have to make the thread offline otherwise we end up dealocking
 342          * with a waiting writer.
 343          */
 344         _rcu_thread_offline();
 345         mutex_lock(&rcu_gp_lock);
 346         cds_list_del(&rcu_reader.node);
 347         mutex_unlock(&rcu_gp_lock);
 348 }
 349
 350 void rcu_exit(void)
 351 {
 352         /*
 353          * Assertion disabled because call_rcu threads are now rcu
 354          * readers, and left running at exit.
 355          * assert(cds_list_empty(&registry));
 356          */
 357 }
 358
 359 DEFINE_RCU_FLAVOR(rcu_flavor);
 360
 361 #include "urcu-call-rcu-impl.h"
 362 #include "urcu-defer-impl.h"