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 #include "urcu/tls-compat.h"
  44
  45 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
  46 #undef _LGPL_SOURCE
  47 #include "urcu-qsbr.h"
  48 #define _LGPL_SOURCE
  49
  50 void __attribute__((destructor)) rcu_exit(void);
  51
  52 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
  53
  54 int32_t gp_futex;
  55
  56 /*
  57  * Global grace period counter.
  58  */
  59 unsigned long rcu_gp_ctr = RCU_GP_ONLINE;
  60
  61 /*
  62  * Active attempts to check for reader Q.S. before calling futex().
  63  */
  64 #define RCU_QS_ACTIVE_ATTEMPTS 100
  65
  66 /*
  67  * Written to only by each individual reader. Read by both the reader and the
  68  * writers.
  69  */
  70 DEFINE_URCU_TLS(struct rcu_reader, rcu_reader);
  71
  72 #ifdef DEBUG_YIELD
  73 unsigned int yield_active;
  74 DEFINE_URCU_TLS(unsigned int, rand_yield);
  75 #endif
  76
  77 static CDS_LIST_HEAD(registry);
  78
  79 static void mutex_lock(pthread_mutex_t *mutex)
  80 {
  81         int ret;
  82
  83 #ifndef DISTRUST_SIGNALS_EXTREME
  84         ret = pthread_mutex_lock(mutex);
  85         if (ret) {
  86                 perror("Error in pthread mutex lock");
  87                 exit(-1);
  88         }
  89 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
  90         while ((ret = pthread_mutex_trylock(mutex)) != 0) {
  91                 if (ret != EBUSY && ret != EINTR) {
  92                         printf("ret = %d, errno = %d\n", ret, errno);
  93                         perror("Error in pthread mutex lock");
  94                         exit(-1);
  95                 }
  96                 poll(NULL,0,10);
  97         }
  98 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
  99 }
 100
 101 static void mutex_unlock(pthread_mutex_t *mutex)
 102 {
 103         int ret;
 104
 105         ret = pthread_mutex_unlock(mutex);
 106         if (ret) {
 107                 perror("Error in pthread mutex unlock");
 108                 exit(-1);
 109         }
 110 }
 111
 112 /*
 113  * synchronize_rcu() waiting. Single thread.
 114  */
 115 static void wait_gp(void)
 116 {
 117         /* Read reader_gp before read futex */
 118         cmm_smp_rmb();
 119         if (uatomic_read(&gp_futex) == -1)
 120                 futex_noasync(&gp_futex, FUTEX_WAIT, -1,
 121                       NULL, NULL, 0);
 122 }
 123
 124 static void update_counter_and_wait(void)
 125 {
 126         CDS_LIST_HEAD(qsreaders);
 127         int wait_loops = 0;
 128         struct rcu_reader *index, *tmp;
 129
 130 #if (CAA_BITS_PER_LONG < 64)
 131         /* Switch parity: 0 -> 1, 1 -> 0 */
 132         CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
 133 #else   /* !(CAA_BITS_PER_LONG < 64) */
 134         /* Increment current G.P. */
 135         CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
 136 #endif  /* !(CAA_BITS_PER_LONG < 64) */
 137
 138         /*
 139          * Must commit rcu_gp_ctr update to memory before waiting for
 140          * quiescent state. Failure to do so could result in the writer
 141          * waiting forever while new readers are always accessing data
 142          * (no progress). Enforce compiler-order of store to rcu_gp_ctr
 143          * before load URCU_TLS(rcu_reader).ctr.
 144          */
 145         cmm_barrier();
 146
 147         /*
 148          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 149          * model easier to understand. It does not have a big performance impact
 150          * anyway, given this is the write-side.
 151          */
 152         cmm_smp_mb();
 153
 154         /*
 155          * Wait for each thread rcu_reader_qs_gp count to become 0.
 156          */
 157         for (;;) {
 158                 wait_loops++;
 159                 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 160                         uatomic_set(&gp_futex, -1);
 161                         /*
 162                          * Write futex before write waiting (the other side
 163                          * reads them in the opposite order).
 164                          */
 165                         cmm_smp_wmb();
 166                         cds_list_for_each_entry(index, &registry, node) {
 167                                 _CMM_STORE_SHARED(index->waiting, 1);
 168                         }
 169                         /* Write futex before read reader_gp */
 170                         cmm_smp_mb();
 171                 }
 172                 cds_list_for_each_entry_safe(index, tmp, &registry, node) {
 173                         if (!rcu_gp_ongoing(&index->ctr))
 174                                 cds_list_move(&index->node, &qsreaders);
 175                 }
 176
 177                 if (cds_list_empty(&registry)) {
 178                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 179                                 /* Read reader_gp before write futex */
 180                                 cmm_smp_mb();
 181                                 uatomic_set(&gp_futex, 0);
 182                         }
 183                         break;
 184                 } else {
 185                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 186                                 wait_gp();
 187                         } else {
 188 #ifndef HAS_INCOHERENT_CACHES
 189                                 caa_cpu_relax();
 190 #else /* #ifndef HAS_INCOHERENT_CACHES */
 191                                 cmm_smp_mb();
 192 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 193                         }
 194                 }
 195         }
 196         /* put back the reader list in the registry */
 197         cds_list_splice(&qsreaders, &registry);
 198 }
 199
 200 /*
 201  * Using a two-subphases algorithm for architectures with smaller than 64-bit
 202  * long-size to ensure we do not encounter an overflow bug.
 203  */
 204
 205 #if (CAA_BITS_PER_LONG < 64)
 206 void synchronize_rcu(void)
 207 {
 208         unsigned long was_online;
 209
 210         was_online = URCU_TLS(rcu_reader).ctr;
 211
 212         /* All threads should read qparity before accessing data structure
 213          * where new ptr points to.  In the "then" case, rcu_thread_offline
 214          * includes a memory barrier.
 215          *
 216          * Mark the writer thread offline to make sure we don't wait for
 217          * our own quiescent state. This allows using synchronize_rcu()
 218          * in threads registered as readers.
 219          */
 220         if (was_online)
 221                 rcu_thread_offline();
 222         else
 223                 cmm_smp_mb();
 224
 225         mutex_lock(&rcu_gp_lock);
 226
 227         if (cds_list_empty(&registry))
 228                 goto out;
 229
 230         /*
 231          * Wait for previous parity to be empty of readers.
 232          */
 233         update_counter_and_wait();      /* 0 -> 1, wait readers in parity 0 */
 234
 235         /*
 236          * Must finish waiting for quiescent state for parity 0 before
 237          * committing next rcu_gp_ctr update to memory. Failure to
 238          * do so could result in the writer waiting forever while new
 239          * readers are always accessing data (no progress).  Enforce
 240          * compiler-order of load URCU_TLS(rcu_reader).ctr before store to
 241          * rcu_gp_ctr.
 242          */
 243         cmm_barrier();
 244
 245         /*
 246          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 247          * model easier to understand. It does not have a big performance impact
 248          * anyway, given this is the write-side.
 249          */
 250         cmm_smp_mb();
 251
 252         /*
 253          * Wait for previous parity to be empty of readers.
 254          */
 255         update_counter_and_wait();      /* 1 -> 0, wait readers in parity 1 */
 256 out:
 257         mutex_unlock(&rcu_gp_lock);
 258
 259         /*
 260          * Finish waiting for reader threads before letting the old ptr being
 261          * freed.
 262          */
 263         if (was_online)
 264                 rcu_thread_online();
 265         else
 266                 cmm_smp_mb();
 267 }
 268 #else /* !(CAA_BITS_PER_LONG < 64) */
 269 void synchronize_rcu(void)
 270 {
 271         unsigned long was_online;
 272
 273         was_online = URCU_TLS(rcu_reader).ctr;
 274
 275         /*
 276          * Mark the writer thread offline to make sure we don't wait for
 277          * our own quiescent state. This allows using synchronize_rcu()
 278          * in threads registered as readers.
 279          */
 280         if (was_online)
 281                 rcu_thread_offline();
 282         else
 283                 cmm_smp_mb();
 284
 285         mutex_lock(&rcu_gp_lock);
 286         if (cds_list_empty(&registry))
 287                 goto out;
 288         update_counter_and_wait();
 289 out:
 290         mutex_unlock(&rcu_gp_lock);
 291
 292         if (was_online)
 293                 rcu_thread_online();
 294         else
 295                 cmm_smp_mb();
 296 }
 297 #endif  /* !(CAA_BITS_PER_LONG < 64) */
 298
 299 /*
 300  * library wrappers to be used by non-LGPL compatible source code.
 301  */
 302
 303 void rcu_read_lock(void)
 304 {
 305         _rcu_read_lock();
 306 }
 307
 308 void rcu_read_unlock(void)
 309 {
 310         _rcu_read_unlock();
 311 }
 312
 313 void rcu_quiescent_state(void)
 314 {
 315         _rcu_quiescent_state();
 316 }
 317
 318 void rcu_thread_offline(void)
 319 {
 320         _rcu_thread_offline();
 321 }
 322
 323 void rcu_thread_online(void)
 324 {
 325         _rcu_thread_online();
 326 }
 327
 328 void rcu_register_thread(void)
 329 {
 330         URCU_TLS(rcu_reader).tid = pthread_self();
 331         assert(URCU_TLS(rcu_reader).ctr == 0);
 332
 333         mutex_lock(&rcu_gp_lock);
 334         cds_list_add(&URCU_TLS(rcu_reader).node, &registry);
 335         mutex_unlock(&rcu_gp_lock);
 336         _rcu_thread_online();
 337 }
 338
 339 void rcu_unregister_thread(void)
 340 {
 341         /*
 342          * We have to make the thread offline otherwise we end up dealocking
 343          * with a waiting writer.
 344          */
 345         _rcu_thread_offline();
 346         mutex_lock(&rcu_gp_lock);
 347         cds_list_del(&URCU_TLS(rcu_reader).node);
 348         mutex_unlock(&rcu_gp_lock);
 349 }
 350
 351 void rcu_exit(void)
 352 {
 353         /*
 354          * Assertion disabled because call_rcu threads are now rcu
 355          * readers, and left running at exit.
 356          * assert(cds_list_empty(&registry));
 357          */
 358 }
 359
 360 DEFINE_RCU_FLAVOR(rcu_flavor);
 361
 362 #include "urcu-call-rcu-impl.h"
 363 #include "urcu-defer-impl.h"