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