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 unsigned long __thread rcu_reader_qs_gp;
  52
  53 /* Thread IDs of registered readers */
  54 #define INIT_NUM_THREADS 4
  55
  56 struct reader_registry {
  57         pthread_t tid;
  58         unsigned long *rcu_reader_qs_gp;
  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 static void wait_for_quiescent_state(void)
 103 {
 104         struct reader_registry *index;
 105
 106         if (!registry)
 107                 return;
 108         /*
 109          * Wait for each thread rcu_reader_qs_gp count to become 0.
 110          */
 111         for (index = registry; index < registry + num_readers; index++) {
 112 #ifndef HAS_INCOHERENT_CACHES
 113                 while (rcu_gp_ongoing(index->rcu_reader_qs_gp))
 114                         cpu_relax();
 115 #else /* #ifndef HAS_INCOHERENT_CACHES */
 116                 while (rcu_gp_ongoing(index->rcu_reader_qs_gp))
 117                         smp_mb();
 118 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 119         }
 120 }
 121
 122 /*
 123  * Using a two-subphases algorithm for architectures with smaller than 64-bit
 124  * long-size to ensure we do not encounter an overflow bug.
 125  */
 126
 127 #if (BITS_PER_LONG < 64)
 128 /*
 129  * called with urcu_mutex held.
 130  */
 131 static void switch_next_urcu_qparity(void)
 132 {
 133         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_CTR);
 134 }
 135
 136 void synchronize_rcu(void)
 137 {
 138         unsigned long was_online;
 139
 140         was_online = rcu_reader_qs_gp;
 141
 142         /* All threads should read qparity before accessing data structure
 143          * where new ptr points to.
 144          */
 145         /* Write new ptr before changing the qparity */
 146         smp_mb();
 147
 148         /*
 149          * Mark the writer thread offline to make sure we don't wait for
 150          * our own quiescent state. This allows using synchronize_rcu() in
 151          * threads registered as readers.
 152          */
 153         if (was_online)
 154                 STORE_SHARED(rcu_reader_qs_gp, 0);
 155
 156         internal_urcu_lock();
 157
 158         switch_next_urcu_qparity();     /* 0 -> 1 */
 159
 160         /*
 161          * Must commit qparity update to memory before waiting for parity
 162          * 0 quiescent state. Failure to do so could result in the writer
 163          * waiting forever while new readers are always accessing data (no
 164          * progress).
 165          * Ensured by STORE_SHARED and LOAD_SHARED.
 166          */
 167
 168         /*
 169          * Wait for previous parity to be empty of readers.
 170          */
 171         wait_for_quiescent_state();     /* Wait readers in parity 0 */
 172
 173         /*
 174          * Must finish waiting for quiescent state for parity 0 before
 175          * committing qparity update to memory. Failure to do so could result in
 176          * the writer waiting forever while new readers are always accessing
 177          * data (no progress).
 178          * Ensured by STORE_SHARED and LOAD_SHARED.
 179          */
 180
 181         switch_next_urcu_qparity();     /* 1 -> 0 */
 182
 183         /*
 184          * Must commit qparity update to memory before waiting for parity
 185          * 1 quiescent state. Failure to do so could result in the writer
 186          * waiting forever while new readers are always accessing data (no
 187          * progress).
 188          * Ensured by STORE_SHARED and LOAD_SHARED.
 189          */
 190
 191         /*
 192          * Wait for previous parity to be empty of readers.
 193          */
 194         wait_for_quiescent_state();     /* Wait readers in parity 1 */
 195
 196         internal_urcu_unlock();
 197
 198         /*
 199          * Finish waiting for reader threads before letting the old ptr being
 200          * freed.
 201          */
 202         if (was_online)
 203                 _STORE_SHARED(rcu_reader_qs_gp, LOAD_SHARED(urcu_gp_ctr));
 204         smp_mb();
 205 }
 206 #else /* !(BITS_PER_LONG < 64) */
 207 void synchronize_rcu(void)
 208 {
 209         unsigned long was_online;
 210
 211         was_online = rcu_reader_qs_gp;
 212
 213         /*
 214          * Mark the writer thread offline to make sure we don't wait for
 215          * our own quiescent state. This allows using synchronize_rcu() in
 216          * threads registered as readers.
 217          */
 218         smp_mb();
 219         if (was_online)
 220                 STORE_SHARED(rcu_reader_qs_gp, 0);
 221
 222         internal_urcu_lock();
 223         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr + RCU_GP_CTR);
 224         wait_for_quiescent_state();
 225         internal_urcu_unlock();
 226
 227         if (was_online)
 228                 _STORE_SHARED(rcu_reader_qs_gp, LOAD_SHARED(urcu_gp_ctr));
 229         smp_mb();
 230 }
 231 #endif  /* !(BITS_PER_LONG < 64) */
 232
 233 /*
 234  * library wrappers to be used by non-LGPL compatible source code.
 235  */
 236
 237 void rcu_read_lock(void)
 238 {
 239         _rcu_read_lock();
 240 }
 241
 242 void rcu_read_unlock(void)
 243 {
 244         _rcu_read_unlock();
 245 }
 246
 247 void *rcu_dereference(void *p)
 248 {
 249         return _rcu_dereference(p);
 250 }
 251
 252 void *rcu_assign_pointer_sym(void **p, void *v)
 253 {
 254         wmb();
 255         return STORE_SHARED(p, v);
 256 }
 257
 258 void *rcu_xchg_pointer_sym(void **p, void *v)
 259 {
 260         wmb();
 261         return xchg(p, v);
 262 }
 263
 264 void *rcu_publish_content_sym(void **p, void *v)
 265 {
 266         void *oldptr;
 267
 268         oldptr = _rcu_xchg_pointer(p, v);
 269         synchronize_rcu();
 270         return oldptr;
 271 }
 272
 273 void rcu_quiescent_state(void)
 274 {
 275         _rcu_quiescent_state();
 276 }
 277
 278 void rcu_thread_offline(void)
 279 {
 280         _rcu_thread_offline();
 281 }
 282
 283 void rcu_thread_online(void)
 284 {
 285         _rcu_thread_online();
 286 }
 287
 288 static void rcu_add_reader(pthread_t id)
 289 {
 290         struct reader_registry *oldarray;
 291
 292         if (!registry) {
 293                 alloc_readers = INIT_NUM_THREADS;
 294                 num_readers = 0;
 295                 registry =
 296                         malloc(sizeof(struct reader_registry) * alloc_readers);
 297         }
 298         if (alloc_readers < num_readers + 1) {
 299                 oldarray = registry;
 300                 registry = malloc(sizeof(struct reader_registry)
 301                                 * (alloc_readers << 1));
 302                 memcpy(registry, oldarray,
 303                         sizeof(struct reader_registry) * alloc_readers);
 304                 alloc_readers <<= 1;
 305                 free(oldarray);
 306         }
 307         registry[num_readers].tid = id;
 308         /* reference to the TLS of _this_ reader thread. */
 309         registry[num_readers].rcu_reader_qs_gp = &rcu_reader_qs_gp;
 310         num_readers++;
 311 }
 312
 313 /*
 314  * Never shrink (implementation limitation).
 315  * This is O(nb threads). Eventually use a hash table.
 316  */
 317 static void rcu_remove_reader(pthread_t id)
 318 {
 319         struct reader_registry *index;
 320
 321         assert(registry != NULL);
 322         for (index = registry; index < registry + num_readers; index++) {
 323                 if (pthread_equal(index->tid, id)) {
 324                         memcpy(index, &registry[num_readers - 1],
 325                                 sizeof(struct reader_registry));
 326                         registry[num_readers - 1].tid = 0;
 327                         registry[num_readers - 1].rcu_reader_qs_gp = NULL;
 328                         num_readers--;
 329                         return;
 330                 }
 331         }
 332         /* Hrm not found, forgot to register ? */
 333         assert(0);
 334 }
 335
 336 void rcu_register_thread(void)
 337 {
 338         internal_urcu_lock();
 339         rcu_add_reader(pthread_self());
 340         internal_urcu_unlock();
 341         _rcu_thread_online();
 342 }
 343
 344 void rcu_unregister_thread(void)
 345 {
 346         /*
 347          * We have to make the thread offline otherwise we end up dealocking
 348          * with a waiting writer.
 349          */
 350         _rcu_thread_offline();
 351         internal_urcu_lock();
 352         rcu_remove_reader(pthread_self());
 353         internal_urcu_unlock();
 354 }