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                 index->urcu_reader_status->gp_waiting = 1;
 119                 while (rcu_gp_ongoing(&index->urcu_reader_status->qs_gp)) {
 120                         if (wait_loops++ == RCU_QS_ACTIVE_ATTEMPTS) {
 121                                 /* adapted wait time, in us */
 122                                 usleep(LOAD_SHARED(index->urcu_reader_status->qs_time_delta_usec) / 4);
 123                                 wait_loops = 0;
 124                         } else {
 125 #ifndef HAS_INCOHERENT_CACHES
 126                                 cpu_relax();
 127 #else /* #ifndef HAS_INCOHERENT_CACHES */
 128                                 smp_mb();
 129 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 130                         }
 131                 }
 132                 index->urcu_reader_status->gp_waiting = 0;
 133         }
 134 }
 135
 136 /*
 137  * Using a two-subphases algorithm for architectures with smaller than 64-bit
 138  * long-size to ensure we do not encounter an overflow bug.
 139  */
 140
 141 #if (BITS_PER_LONG < 64)
 142 /*
 143  * called with urcu_mutex held.
 144  */
 145 static void switch_next_urcu_qparity(void)
 146 {
 147         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_CTR);
 148 }
 149
 150 void synchronize_rcu(void)
 151 {
 152         unsigned long was_online;
 153
 154         was_online = urcu_reader_status.qs_gp;
 155
 156         /* All threads should read qparity before accessing data structure
 157          * where new ptr points to.
 158          */
 159         /* Write new ptr before changing the qparity */
 160         smp_mb();
 161
 162         /*
 163          * Mark the writer thread offline to make sure we don't wait for
 164          * our own quiescent state. This allows using synchronize_rcu() in
 165          * threads registered as readers.
 166          */
 167         if (was_online)
 168                 STORE_SHARED(urcu_reader_status.qs_gp, 0);
 169
 170         internal_urcu_lock();
 171
 172         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_ONGOING);
 173
 174         switch_next_urcu_qparity();     /* 0 -> 1 */
 175
 176         /*
 177          * Must commit qparity update to memory before waiting for parity
 178          * 0 quiescent state. Failure to do so could result in the writer
 179          * waiting forever while new readers are always accessing data (no
 180          * progress).
 181          * Ensured by STORE_SHARED and LOAD_SHARED.
 182          */
 183
 184         /*
 185          * Wait for previous parity to be empty of readers.
 186          */
 187         wait_for_quiescent_state();     /* Wait readers in parity 0 */
 188
 189         /*
 190          * Must finish waiting for quiescent state for parity 0 before
 191          * committing qparity update to memory. Failure to do so could result in
 192          * the writer waiting forever while new readers are always accessing
 193          * data (no progress).
 194          * Ensured by STORE_SHARED and LOAD_SHARED.
 195          */
 196
 197         switch_next_urcu_qparity();     /* 1 -> 0 */
 198
 199         /*
 200          * Must commit qparity update to memory before waiting for parity
 201          * 1 quiescent state. Failure to do so could result in the writer
 202          * waiting forever while new readers are always accessing data (no
 203          * progress).
 204          * Ensured by STORE_SHARED and LOAD_SHARED.
 205          */
 206
 207         /*
 208          * Wait for previous parity to be empty of readers.
 209          */
 210         wait_for_quiescent_state();     /* Wait readers in parity 1 */
 211
 212         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_ONGOING);
 213
 214         internal_urcu_unlock();
 215
 216         /*
 217          * Finish waiting for reader threads before letting the old ptr being
 218          * freed.
 219          */
 220         if (was_online)
 221                 _STORE_SHARED(urcu_reader_status.qs_gp,
 222                               LOAD_SHARED(urcu_gp_ctr));
 223         smp_mb();
 224 }
 225 #else /* !(BITS_PER_LONG < 64) */
 226 void synchronize_rcu(void)
 227 {
 228         unsigned long was_online;
 229
 230         was_online = urcu_reader_status.qs_gp;
 231
 232         /*
 233          * Mark the writer thread offline to make sure we don't wait for
 234          * our own quiescent state. This allows using synchronize_rcu() in
 235          * threads registered as readers.
 236          */
 237         smp_mb();
 238         if (was_online)
 239                 STORE_SHARED(urcu_reader_status.qs_gp, 0);
 240
 241         internal_urcu_lock();
 242         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_ONGOING);
 243         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr + RCU_GP_CTR);
 244         wait_for_quiescent_state();
 245         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_ONGOING);
 246         internal_urcu_unlock();
 247
 248         if (was_online)
 249                 _STORE_SHARED(urcu_reader_status.qs_gp,
 250                               LOAD_SHARED(urcu_gp_ctr));
 251         smp_mb();
 252 }
 253 #endif  /* !(BITS_PER_LONG < 64) */
 254
 255 /*
 256  * library wrappers to be used by non-LGPL compatible source code.
 257  */
 258
 259 void rcu_read_lock(void)
 260 {
 261         _rcu_read_lock();
 262 }
 263
 264 void rcu_read_unlock(void)
 265 {
 266         _rcu_read_unlock();
 267 }
 268
 269 void *rcu_dereference(void *p)
 270 {
 271         return _rcu_dereference(p);
 272 }
 273
 274 void *rcu_assign_pointer_sym(void **p, void *v)
 275 {
 276         wmb();
 277         return STORE_SHARED(p, v);
 278 }
 279
 280 void *rcu_cmpxchg_pointer_sym(void **p, void *old, void *_new)
 281 {
 282         wmb();
 283         return cmpxchg(p, old, _new);
 284 }
 285
 286 void *rcu_xchg_pointer_sym(void **p, void *v)
 287 {
 288         wmb();
 289         return xchg(p, v);
 290 }
 291
 292 void *rcu_publish_content_sym(void **p, void *v)
 293 {
 294         void *oldptr;
 295
 296         oldptr = _rcu_xchg_pointer(p, v);
 297         synchronize_rcu();
 298         return oldptr;
 299 }
 300
 301 void rcu_quiescent_state(void)
 302 {
 303         _rcu_quiescent_state();
 304 }
 305
 306 void rcu_thread_offline(void)
 307 {
 308         _rcu_thread_offline();
 309 }
 310
 311 void rcu_thread_online(void)
 312 {
 313         _rcu_thread_online();
 314 }
 315
 316 static void rcu_add_reader(pthread_t id)
 317 {
 318         struct reader_registry *oldarray;
 319
 320         if (!registry) {
 321                 alloc_readers = INIT_NUM_THREADS;
 322                 num_readers = 0;
 323                 registry =
 324                         malloc(sizeof(struct reader_registry) * alloc_readers);
 325         }
 326         if (alloc_readers < num_readers + 1) {
 327                 oldarray = registry;
 328                 registry = malloc(sizeof(struct reader_registry)
 329                                 * (alloc_readers << 1));
 330                 memcpy(registry, oldarray,
 331                         sizeof(struct reader_registry) * alloc_readers);
 332                 alloc_readers <<= 1;
 333                 free(oldarray);
 334         }
 335         registry[num_readers].tid = id;
 336         /* reference to the TLS of _this_ reader thread. */
 337         registry[num_readers].urcu_reader_status = &urcu_reader_status;
 338         num_readers++;
 339 }
 340
 341 /*
 342  * Never shrink (implementation limitation).
 343  * This is O(nb threads). Eventually use a hash table.
 344  */
 345 static void rcu_remove_reader(pthread_t id)
 346 {
 347         struct reader_registry *index;
 348
 349         assert(registry != NULL);
 350         for (index = registry; index < registry + num_readers; index++) {
 351                 if (pthread_equal(index->tid, id)) {
 352                         memcpy(index, &registry[num_readers - 1],
 353                                 sizeof(struct reader_registry));
 354                         registry[num_readers - 1].tid = 0;
 355                         registry[num_readers - 1].urcu_reader_status = NULL;
 356                         num_readers--;
 357                         return;
 358                 }
 359         }
 360         /* Hrm not found, forgot to register ? */
 361         assert(0);
 362 }
 363
 364 void rcu_register_thread(void)
 365 {
 366         internal_urcu_lock();
 367         rcu_add_reader(pthread_self());
 368         internal_urcu_unlock();
 369         _rcu_thread_online();
 370 }
 371
 372 void rcu_unregister_thread(void)
 373 {
 374         /*
 375          * We have to make the thread offline otherwise we end up dealocking
 376          * with a waiting writer.
 377          */
 378         _rcu_thread_offline();
 379         internal_urcu_lock();
 380         rcu_remove_reader(pthread_self());
 381         internal_urcu_unlock();
 382 }