urcu-qsbr.c

   1 /*
   2  * urcu-qsbr.c
   3  *
   4  * Userspace RCU QSBR library
   5  *
   6  * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
   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 #include <stdio.h>
  27 #include <pthread.h>
  28 #include <signal.h>
  29 #include <assert.h>
  30 #include <stdlib.h>
  31 #include <string.h>
  32 #include <errno.h>
  33 #include <poll.h>
  34 #include <unistd.h>
  35
  36 #define BUILD_QSBR_LIB
  37 #include "urcu-qsbr-static.h"
  38 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
  39 #include "urcu-qsbr.h"
  40
  41 static pthread_mutex_t urcu_mutex = PTHREAD_MUTEX_INITIALIZER;
  42
  43 /*
  44  * Global grace period counter.
  45  */
  46 unsigned long urcu_gp_ctr = RCU_GP_ONLINE;
  47
  48 /*
  49  * Written to only by each individual reader. Read by both the reader and the
  50  * writers.
  51  */
  52 struct urcu_reader_status __thread urcu_reader_status;
  53
  54 /* Thread IDs of registered readers */
  55 #define INIT_NUM_THREADS 4
  56
  57 struct reader_registry {
  58         pthread_t tid;
  59         struct urcu_reader_status *urcu_reader_status;
  60 };
  61
  62 #ifdef DEBUG_YIELD
  63 unsigned int yield_active;
  64 unsigned int __thread rand_yield;
  65 #endif
  66
  67 static struct reader_registry *registry;
  68 static int num_readers, alloc_readers;
  69
  70 static void internal_urcu_lock(void)
  71 {
  72         int ret;
  73
  74 #ifndef DISTRUST_SIGNALS_EXTREME
  75         ret = pthread_mutex_lock(&urcu_mutex);
  76         if (ret) {
  77                 perror("Error in pthread mutex lock");
  78                 exit(-1);
  79         }
  80 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
  81         while ((ret = pthread_mutex_trylock(&urcu_mutex)) != 0) {
  82                 if (ret != EBUSY && ret != EINTR) {
  83                         printf("ret = %d, errno = %d\n", ret, errno);
  84                         perror("Error in pthread mutex lock");
  85                         exit(-1);
  86                 }
  87                 poll(NULL,0,10);
  88         }
  89 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
  90 }
  91
  92 static void internal_urcu_unlock(void)
  93 {
  94         int ret;
  95
  96         ret = pthread_mutex_unlock(&urcu_mutex);
  97         if (ret) {
  98                 perror("Error in pthread mutex unlock");
  99                 exit(-1);
 100         }
 101 }
 102
 103 /*
 104  * synchronize_rcu() waiting. Single thread.
 105  */
 106 static void wait_for_quiescent_state(void)
 107 {
 108         struct reader_registry *index;
 109
 110         if (!registry)
 111                 return;
 112         /*
 113          * Wait for each thread rcu_reader qs_gp count to become 0.
 114          */
 115         for (index = registry; index < registry + num_readers; index++) {
 116                 int wait_loops = 0;
 117
 118                 if (likely(!rcu_gp_ongoing(&index->urcu_reader_status->qs_gp)))
 119                         continue;
 120
 121                 index->urcu_reader_status->gp_waiting = 1;
 122                 while (rcu_gp_ongoing(&index->urcu_reader_status->qs_gp)) {
 123                         if (wait_loops++ == RCU_QS_ACTIVE_ATTEMPTS) {
 124                                 /* adapted wait time, in us */
 125                                 usleep(LOAD_SHARED(index->urcu_reader_status->qs_time_delta_usec) / 4);
 126                                 wait_loops = 0;
 127                         } else {
 128 #ifndef HAS_INCOHERENT_CACHES
 129                                 cpu_relax();
 130 #else /* #ifndef HAS_INCOHERENT_CACHES */
 131                                 smp_mb();
 132 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 133                         }
 134                 }
 135                 index->urcu_reader_status->gp_waiting = 0;
 136         }
 137 }
 138
 139 /*
 140  * Using a two-subphases algorithm for architectures with smaller than 64-bit
 141  * long-size to ensure we do not encounter an overflow bug.
 142  */
 143
 144 #if (BITS_PER_LONG < 64)
 145 /*
 146  * called with urcu_mutex held.
 147  */
 148 static void switch_next_urcu_qparity(void)
 149 {
 150         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_CTR);
 151 }
 152
 153 void synchronize_rcu(void)
 154 {
 155         unsigned long was_online;
 156
 157         was_online = urcu_reader_status.qs_gp;
 158
 159         /* All threads should read qparity before accessing data structure
 160          * where new ptr points to.
 161          */
 162         /* Write new ptr before changing the qparity */
 163         smp_mb();
 164
 165         /*
 166          * Mark the writer thread offline to make sure we don't wait for
 167          * our own quiescent state. This allows using synchronize_rcu() in
 168          * threads registered as readers.
 169          */
 170         if (was_online)
 171                 STORE_SHARED(urcu_reader_status.qs_gp, 0);
 172
 173         internal_urcu_lock();
 174
 175         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_ONGOING);
 176
 177         switch_next_urcu_qparity();     /* 0 -> 1 */
 178
 179         /*
 180          * Must commit qparity update to memory before waiting for parity
 181          * 0 quiescent state. Failure to do so could result in the writer
 182          * waiting forever while new readers are always accessing data (no
 183          * progress).
 184          * Ensured by STORE_SHARED and LOAD_SHARED.
 185          */
 186
 187         /*
 188          * Wait for previous parity to be empty of readers.
 189          */
 190         wait_for_quiescent_state();     /* Wait readers in parity 0 */
 191
 192         /*
 193          * Must finish waiting for quiescent state for parity 0 before
 194          * committing qparity update to memory. Failure to do so could result in
 195          * the writer waiting forever while new readers are always accessing
 196          * data (no progress).
 197          * Ensured by STORE_SHARED and LOAD_SHARED.
 198          */
 199
 200         switch_next_urcu_qparity();     /* 1 -> 0 */
 201
 202         /*
 203          * Must commit qparity update to memory before waiting for parity
 204          * 1 quiescent state. Failure to do so could result in the writer
 205          * waiting forever while new readers are always accessing data (no
 206          * progress).
 207          * Ensured by STORE_SHARED and LOAD_SHARED.
 208          */
 209
 210         /*
 211          * Wait for previous parity to be empty of readers.
 212          */
 213         wait_for_quiescent_state();     /* Wait readers in parity 1 */
 214
 215         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_ONGOING);
 216
 217         internal_urcu_unlock();
 218
 219         /*
 220          * Finish waiting for reader threads before letting the old ptr being
 221          * freed.
 222          */
 223         if (was_online)
 224                 _STORE_SHARED(urcu_reader_status.qs_gp,
 225                               LOAD_SHARED(urcu_gp_ctr));
 226         smp_mb();
 227 }
 228 #else /* !(BITS_PER_LONG < 64) */
 229 void synchronize_rcu(void)
 230 {
 231         unsigned long was_online;
 232
 233         was_online = urcu_reader_status.qs_gp;
 234
 235         /*
 236          * Mark the writer thread offline to make sure we don't wait for
 237          * our own quiescent state. This allows using synchronize_rcu() in
 238          * threads registered as readers.
 239          */
 240         smp_mb();
 241         if (was_online)
 242                 STORE_SHARED(urcu_reader_status.qs_gp, 0);
 243
 244         internal_urcu_lock();
 245         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_ONGOING);
 246         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr + RCU_GP_CTR);
 247         wait_for_quiescent_state();
 248         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_ONGOING);
 249         internal_urcu_unlock();
 250
 251         if (was_online)
 252                 _STORE_SHARED(urcu_reader_status.qs_gp,
 253                               LOAD_SHARED(urcu_gp_ctr));
 254         smp_mb();
 255 }
 256 #endif  /* !(BITS_PER_LONG < 64) */
 257
 258 /*
 259  * library wrappers to be used by non-LGPL compatible source code.
 260  */
 261
 262 void rcu_read_lock(void)
 263 {
 264         _rcu_read_lock();
 265 }
 266
 267 void rcu_read_unlock(void)
 268 {
 269         _rcu_read_unlock();
 270 }
 271
 272 void *rcu_dereference(void *p)
 273 {
 274         return _rcu_dereference(p);
 275 }
 276
 277 void *rcu_assign_pointer_sym(void **p, void *v)
 278 {
 279         wmb();
 280         return STORE_SHARED(p, v);
 281 }
 282
 283 void *rcu_cmpxchg_pointer_sym(void **p, void *old, void *_new)
 284 {
 285         wmb();
 286         return cmpxchg(p, old, _new);
 287 }
 288
 289 void *rcu_xchg_pointer_sym(void **p, void *v)
 290 {
 291         wmb();
 292         return xchg(p, v);
 293 }
 294
 295 void *rcu_publish_content_sym(void **p, void *v)
 296 {
 297         void *oldptr;
 298
 299         oldptr = _rcu_xchg_pointer(p, v);
 300         synchronize_rcu();
 301         return oldptr;
 302 }
 303
 304 void rcu_quiescent_state(void)
 305 {
 306         _rcu_quiescent_state();
 307 }
 308
 309 void rcu_thread_offline(void)
 310 {
 311         _rcu_thread_offline();
 312 }
 313
 314 void rcu_thread_online(void)
 315 {
 316         _rcu_thread_online();
 317 }
 318
 319 static void rcu_add_reader(pthread_t id)
 320 {
 321         struct reader_registry *oldarray;
 322
 323         if (!registry) {
 324                 alloc_readers = INIT_NUM_THREADS;
 325                 num_readers = 0;
 326                 registry =
 327                         malloc(sizeof(struct reader_registry) * alloc_readers);
 328         }
 329         if (alloc_readers < num_readers + 1) {
 330                 oldarray = registry;
 331                 registry = malloc(sizeof(struct reader_registry)
 332                                 * (alloc_readers << 1));
 333                 memcpy(registry, oldarray,
 334                         sizeof(struct reader_registry) * alloc_readers);
 335                 alloc_readers <<= 1;
 336                 free(oldarray);
 337         }
 338         registry[num_readers].tid = id;
 339         /* reference to the TLS of _this_ reader thread. */
 340         registry[num_readers].urcu_reader_status = &urcu_reader_status;
 341         num_readers++;
 342 }
 343
 344 /*
 345  * Never shrink (implementation limitation).
 346  * This is O(nb threads). Eventually use a hash table.
 347  */
 348 static void rcu_remove_reader(pthread_t id)
 349 {
 350         struct reader_registry *index;
 351
 352         assert(registry != NULL);
 353         for (index = registry; index < registry + num_readers; index++) {
 354                 if (pthread_equal(index->tid, id)) {
 355                         memcpy(index, &registry[num_readers - 1],
 356                                 sizeof(struct reader_registry));
 357                         registry[num_readers - 1].tid = 0;
 358                         registry[num_readers - 1].urcu_reader_status = NULL;
 359                         num_readers--;
 360                         return;
 361                 }
 362         }
 363         /* Hrm not found, forgot to register ? */
 364         assert(0);
 365 }
 366
 367 void rcu_register_thread(void)
 368 {
 369         internal_urcu_lock();
 370         rcu_add_reader(pthread_self());
 371         internal_urcu_unlock();
 372         _rcu_thread_online();
 373 }
 374
 375 void rcu_unregister_thread(void)
 376 {
 377         /*
 378          * We have to make the thread offline otherwise we end up dealocking
 379          * with a waiting writer.
 380          */
 381         _rcu_thread_offline();
 382         internal_urcu_lock();
 383         rcu_remove_reader(pthread_self());
 384         internal_urcu_unlock();
 385 }