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 int gp_futex;
  43
  44 /*
  45  * Global grace period counter.
  46  */
  47 unsigned long urcu_gp_ctr = RCU_GP_ONLINE;
  48
  49 /*
  50  * Written to only by each individual reader. Read by both the reader and the
  51  * writers.
  52  */
  53 unsigned long __thread rcu_reader_qs_gp;
  54
  55 /* Thread IDs of registered readers */
  56 #define INIT_NUM_THREADS 4
  57
  58 struct reader_registry {
  59         pthread_t tid;
  60         unsigned long *rcu_reader_qs_gp;
  61 };
  62
  63 #ifdef DEBUG_YIELD
  64 unsigned int yield_active;
  65 unsigned int __thread rand_yield;
  66 #endif
  67
  68 static struct reader_registry *registry;
  69 static int num_readers, alloc_readers;
  70
  71 static void internal_urcu_lock(void)
  72 {
  73         int ret;
  74
  75 #ifndef DISTRUST_SIGNALS_EXTREME
  76         ret = pthread_mutex_lock(&urcu_mutex);
  77         if (ret) {
  78                 perror("Error in pthread mutex lock");
  79                 exit(-1);
  80         }
  81 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
  82         while ((ret = pthread_mutex_trylock(&urcu_mutex)) != 0) {
  83                 if (ret != EBUSY && ret != EINTR) {
  84                         printf("ret = %d, errno = %d\n", ret, errno);
  85                         perror("Error in pthread mutex lock");
  86                         exit(-1);
  87                 }
  88                 poll(NULL,0,10);
  89         }
  90 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
  91 }
  92
  93 static void internal_urcu_unlock(void)
  94 {
  95         int ret;
  96
  97         ret = pthread_mutex_unlock(&urcu_mutex);
  98         if (ret) {
  99                 perror("Error in pthread mutex unlock");
 100                 exit(-1);
 101         }
 102 }
 103
 104 /*
 105  * synchronize_rcu() waiting. Single thread.
 106  */
 107 static void wait_gp(struct reader_registry *index)
 108 {
 109         atomic_dec(&gp_futex);
 110         smp_mb(); /* Write futex before read reader_gp */
 111         if (!rcu_gp_ongoing(index->rcu_reader_qs_gp)) {
 112                 /* Read reader_gp before write futex */
 113                 smp_mb();
 114                 /* Callbacks are queued, don't wait. */
 115                 atomic_set(&gp_futex, 0);
 116         } else {
 117                 /* Read reader_gp before read futex */
 118                 smp_rmb();
 119                 if (atomic_read(&gp_futex) == -1)
 120                         futex(&gp_futex, FUTEX_WAIT, -1,
 121                               NULL, NULL, 0);
 122         }
 123 }
 124
 125 static void wait_for_quiescent_state(void)
 126 {
 127         struct reader_registry *index;
 128
 129         if (!registry)
 130                 return;
 131         /*
 132          * Wait for each thread rcu_reader_qs_gp count to become 0.
 133          */
 134         for (index = registry; index < registry + num_readers; index++) {
 135                 int wait_loops = 0;
 136
 137                 while (rcu_gp_ongoing(index->rcu_reader_qs_gp)) {
 138                         if (wait_loops++ == RCU_QS_ACTIVE_ATTEMPTS) {
 139                                 wait_gp(index);
 140                         } else {
 141 #ifndef HAS_INCOHERENT_CACHES
 142                                 cpu_relax();
 143 #else /* #ifndef HAS_INCOHERENT_CACHES */
 144                                 smp_mb();
 145 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 146                         }
 147                 }
 148         }
 149 }
 150
 151 /*
 152  * Using a two-subphases algorithm for architectures with smaller than 64-bit
 153  * long-size to ensure we do not encounter an overflow bug.
 154  */
 155
 156 #if (BITS_PER_LONG < 64)
 157 /*
 158  * called with urcu_mutex held.
 159  */
 160 static void switch_next_urcu_qparity(void)
 161 {
 162         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_CTR);
 163 }
 164
 165 void synchronize_rcu(void)
 166 {
 167         unsigned long was_online;
 168
 169         was_online = rcu_reader_qs_gp;
 170
 171         /* All threads should read qparity before accessing data structure
 172          * where new ptr points to.
 173          */
 174         /* Write new ptr before changing the qparity */
 175         smp_mb();
 176
 177         /*
 178          * Mark the writer thread offline to make sure we don't wait for
 179          * our own quiescent state. This allows using synchronize_rcu() in
 180          * threads registered as readers.
 181          */
 182         if (was_online)
 183                 STORE_SHARED(rcu_reader_qs_gp, 0);
 184
 185         internal_urcu_lock();
 186
 187         switch_next_urcu_qparity();     /* 0 -> 1 */
 188
 189         /*
 190          * Must commit qparity update to memory before waiting for parity
 191          * 0 quiescent state. Failure to do so could result in the writer
 192          * waiting forever while new readers are always accessing data (no
 193          * progress).
 194          * Ensured by STORE_SHARED and LOAD_SHARED.
 195          */
 196
 197         /*
 198          * Wait for previous parity to be empty of readers.
 199          */
 200         wait_for_quiescent_state();     /* Wait readers in parity 0 */
 201
 202         /*
 203          * Must finish waiting for quiescent state for parity 0 before
 204          * committing qparity update to memory. Failure to do so could result in
 205          * the writer waiting forever while new readers are always accessing
 206          * data (no progress).
 207          * Ensured by STORE_SHARED and LOAD_SHARED.
 208          */
 209
 210         switch_next_urcu_qparity();     /* 1 -> 0 */
 211
 212         /*
 213          * Must commit qparity update to memory before waiting for parity
 214          * 1 quiescent state. Failure to do so could result in the writer
 215          * waiting forever while new readers are always accessing data (no
 216          * progress).
 217          * Ensured by STORE_SHARED and LOAD_SHARED.
 218          */
 219
 220         /*
 221          * Wait for previous parity to be empty of readers.
 222          */
 223         wait_for_quiescent_state();     /* Wait readers in parity 1 */
 224
 225         internal_urcu_unlock();
 226
 227         /*
 228          * Finish waiting for reader threads before letting the old ptr being
 229          * freed.
 230          */
 231         if (was_online)
 232                 _STORE_SHARED(rcu_reader_qs_gp, LOAD_SHARED(urcu_gp_ctr));
 233         smp_mb();
 234 }
 235 #else /* !(BITS_PER_LONG < 64) */
 236 void synchronize_rcu(void)
 237 {
 238         unsigned long was_online;
 239
 240         was_online = rcu_reader_qs_gp;
 241
 242         /*
 243          * Mark the writer thread offline to make sure we don't wait for
 244          * our own quiescent state. This allows using synchronize_rcu() in
 245          * threads registered as readers.
 246          */
 247         smp_mb();
 248         if (was_online)
 249                 STORE_SHARED(rcu_reader_qs_gp, 0);
 250
 251         internal_urcu_lock();
 252         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr + RCU_GP_CTR);
 253         wait_for_quiescent_state();
 254         internal_urcu_unlock();
 255
 256         if (was_online)
 257                 _STORE_SHARED(rcu_reader_qs_gp, LOAD_SHARED(urcu_gp_ctr));
 258         smp_mb();
 259 }
 260 #endif  /* !(BITS_PER_LONG < 64) */
 261
 262 /*
 263  * library wrappers to be used by non-LGPL compatible source code.
 264  */
 265
 266 void rcu_read_lock(void)
 267 {
 268         _rcu_read_lock();
 269 }
 270
 271 void rcu_read_unlock(void)
 272 {
 273         _rcu_read_unlock();
 274 }
 275
 276 void *rcu_dereference(void *p)
 277 {
 278         return _rcu_dereference(p);
 279 }
 280
 281 void *rcu_assign_pointer_sym(void **p, void *v)
 282 {
 283         wmb();
 284         return STORE_SHARED(p, v);
 285 }
 286
 287 void *rcu_cmpxchg_pointer_sym(void **p, void *old, void *_new)
 288 {
 289         wmb();
 290         return cmpxchg(p, old, _new);
 291 }
 292
 293 void *rcu_xchg_pointer_sym(void **p, void *v)
 294 {
 295         wmb();
 296         return xchg(p, v);
 297 }
 298
 299 void *rcu_publish_content_sym(void **p, void *v)
 300 {
 301         void *oldptr;
 302
 303         oldptr = _rcu_xchg_pointer(p, v);
 304         synchronize_rcu();
 305         return oldptr;
 306 }
 307
 308 void rcu_quiescent_state(void)
 309 {
 310         _rcu_quiescent_state();
 311 }
 312
 313 void rcu_thread_offline(void)
 314 {
 315         _rcu_thread_offline();
 316 }
 317
 318 void rcu_thread_online(void)
 319 {
 320         _rcu_thread_online();
 321 }
 322
 323 static void rcu_add_reader(pthread_t id)
 324 {
 325         struct reader_registry *oldarray;
 326
 327         if (!registry) {
 328                 alloc_readers = INIT_NUM_THREADS;
 329                 num_readers = 0;
 330                 registry =
 331                         malloc(sizeof(struct reader_registry) * alloc_readers);
 332         }
 333         if (alloc_readers < num_readers + 1) {
 334                 oldarray = registry;
 335                 registry = malloc(sizeof(struct reader_registry)
 336                                 * (alloc_readers << 1));
 337                 memcpy(registry, oldarray,
 338                         sizeof(struct reader_registry) * alloc_readers);
 339                 alloc_readers <<= 1;
 340                 free(oldarray);
 341         }
 342         registry[num_readers].tid = id;
 343         /* reference to the TLS of _this_ reader thread. */
 344         registry[num_readers].rcu_reader_qs_gp = &rcu_reader_qs_gp;
 345         num_readers++;
 346 }
 347
 348 /*
 349  * Never shrink (implementation limitation).
 350  * This is O(nb threads). Eventually use a hash table.
 351  */
 352 static void rcu_remove_reader(pthread_t id)
 353 {
 354         struct reader_registry *index;
 355
 356         assert(registry != NULL);
 357         for (index = registry; index < registry + num_readers; index++) {
 358                 if (pthread_equal(index->tid, id)) {
 359                         memcpy(index, &registry[num_readers - 1],
 360                                 sizeof(struct reader_registry));
 361                         registry[num_readers - 1].tid = 0;
 362                         registry[num_readers - 1].rcu_reader_qs_gp = NULL;
 363                         num_readers--;
 364                         return;
 365                 }
 366         }
 367         /* Hrm not found, forgot to register ? */
 368         assert(0);
 369 }
 370
 371 void rcu_register_thread(void)
 372 {
 373         internal_urcu_lock();
 374         rcu_add_reader(pthread_self());
 375         internal_urcu_unlock();
 376         _rcu_thread_online();
 377 }
 378
 379 void rcu_unregister_thread(void)
 380 {
 381         /*
 382          * We have to make the thread offline otherwise we end up dealocking
 383          * with a waiting writer.
 384          */
 385         _rcu_thread_offline();
 386         internal_urcu_lock();
 387         rcu_remove_reader(pthread_self());
 388         internal_urcu_unlock();
 389 }