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 void __attribute__((destructor)) rcu_exit(void);
  41
  42 static pthread_mutex_t urcu_mutex = PTHREAD_MUTEX_INITIALIZER;
  43
  44 int gp_futex;
  45
  46 /*
  47  * Global grace period counter.
  48  */
  49 unsigned long urcu_gp_ctr = RCU_GP_ONLINE;
  50
  51 /*
  52  * Written to only by each individual reader. Read by both the reader and the
  53  * writers.
  54  */
  55 struct urcu_reader __thread urcu_reader;
  56
  57 #ifdef DEBUG_YIELD
  58 unsigned int yield_active;
  59 unsigned int __thread rand_yield;
  60 #endif
  61
  62 static LIST_HEAD(registry);
  63
  64 static void internal_urcu_lock(void)
  65 {
  66         int ret;
  67
  68 #ifndef DISTRUST_SIGNALS_EXTREME
  69         ret = pthread_mutex_lock(&urcu_mutex);
  70         if (ret) {
  71                 perror("Error in pthread mutex lock");
  72                 exit(-1);
  73         }
  74 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
  75         while ((ret = pthread_mutex_trylock(&urcu_mutex)) != 0) {
  76                 if (ret != EBUSY && ret != EINTR) {
  77                         printf("ret = %d, errno = %d\n", ret, errno);
  78                         perror("Error in pthread mutex lock");
  79                         exit(-1);
  80                 }
  81                 poll(NULL,0,10);
  82         }
  83 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
  84 }
  85
  86 static void internal_urcu_unlock(void)
  87 {
  88         int ret;
  89
  90         ret = pthread_mutex_unlock(&urcu_mutex);
  91         if (ret) {
  92                 perror("Error in pthread mutex unlock");
  93                 exit(-1);
  94         }
  95 }
  96
  97 /*
  98  * synchronize_rcu() waiting. Single thread.
  99  */
 100 static void wait_gp(void)
 101 {
 102         /* Read reader_gp before read futex */
 103         smp_rmb();
 104         if (uatomic_read(&gp_futex) == -1)
 105                 futex(&gp_futex, FUTEX_WAIT, -1,
 106                       NULL, NULL, 0);
 107 }
 108
 109 static void wait_for_quiescent_state(void)
 110 {
 111         LIST_HEAD(qsreaders);
 112         int wait_loops = 0;
 113         struct urcu_reader *index, *tmp;
 114
 115         if (list_empty(&registry))
 116                 return;
 117         /*
 118          * Wait for each thread rcu_reader_qs_gp count to become 0.
 119          */
 120         for (;;) {
 121                 wait_loops++;
 122                 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
 123                         uatomic_dec(&gp_futex);
 124                         /* Write futex before read reader_gp */
 125                         smp_mb();
 126                 }
 127
 128                 list_for_each_entry_safe(index, tmp, &registry, head) {
 129                         if (!rcu_gp_ongoing(&index->ctr))
 130                                 list_move(&index->head, &qsreaders);
 131                 }
 132
 133                 if (list_empty(&registry)) {
 134                         if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
 135                                 /* Read reader_gp before write futex */
 136                                 smp_mb();
 137                                 uatomic_set(&gp_futex, 0);
 138                         }
 139                         break;
 140                 } else {
 141                         if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
 142                                 wait_gp();
 143                         } else {
 144 #ifndef HAS_INCOHERENT_CACHES
 145                                 cpu_relax();
 146 #else /* #ifndef HAS_INCOHERENT_CACHES */
 147                                 smp_mb();
 148 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 149                         }
 150                 }
 151         }
 152         /* put back the reader list in the registry */
 153         list_splice(&qsreaders, &registry);
 154 }
 155
 156 /*
 157  * Using a two-subphases algorithm for architectures with smaller than 64-bit
 158  * long-size to ensure we do not encounter an overflow bug.
 159  */
 160
 161 #if (BITS_PER_LONG < 64)
 162 /*
 163  * called with urcu_mutex held.
 164  */
 165 static void switch_next_urcu_qparity(void)
 166 {
 167         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_CTR);
 168 }
 169
 170 void synchronize_rcu(void)
 171 {
 172         unsigned long was_online;
 173
 174         was_online = urcu_reader.ctr;
 175
 176         /* All threads should read qparity before accessing data structure
 177          * where new ptr points to.
 178          */
 179         /* Write new ptr before changing the qparity */
 180         smp_mb();
 181
 182         /*
 183          * Mark the writer thread offline to make sure we don't wait for
 184          * our own quiescent state. This allows using synchronize_rcu() in
 185          * threads registered as readers.
 186          */
 187         if (was_online)
 188                 STORE_SHARED(urcu_reader.ctr, 0);
 189
 190         internal_urcu_lock();
 191
 192         switch_next_urcu_qparity();     /* 0 -> 1 */
 193
 194         /*
 195          * Must commit qparity update to memory before waiting for parity
 196          * 0 quiescent state. Failure to do so could result in the writer
 197          * waiting forever while new readers are always accessing data (no
 198          * progress).
 199          * Ensured by STORE_SHARED and LOAD_SHARED.
 200          */
 201
 202         /*
 203          * Wait for previous parity to be empty of readers.
 204          */
 205         wait_for_quiescent_state();     /* Wait readers in parity 0 */
 206
 207         /*
 208          * Must finish waiting for quiescent state for parity 0 before
 209          * committing qparity update to memory. Failure to do so could result in
 210          * the writer waiting forever while new readers are always accessing
 211          * data (no progress).
 212          * Ensured by STORE_SHARED and LOAD_SHARED.
 213          */
 214
 215         switch_next_urcu_qparity();     /* 1 -> 0 */
 216
 217         /*
 218          * Must commit qparity update to memory before waiting for parity
 219          * 1 quiescent state. Failure to do so could result in the writer
 220          * waiting forever while new readers are always accessing data (no
 221          * progress).
 222          * Ensured by STORE_SHARED and LOAD_SHARED.
 223          */
 224
 225         /*
 226          * Wait for previous parity to be empty of readers.
 227          */
 228         wait_for_quiescent_state();     /* Wait readers in parity 1 */
 229
 230         internal_urcu_unlock();
 231
 232         /*
 233          * Finish waiting for reader threads before letting the old ptr being
 234          * freed.
 235          */
 236         if (was_online)
 237                 _STORE_SHARED(urcu_reader.ctr, LOAD_SHARED(urcu_gp_ctr));
 238         smp_mb();
 239 }
 240 #else /* !(BITS_PER_LONG < 64) */
 241 void synchronize_rcu(void)
 242 {
 243         unsigned long was_online;
 244
 245         was_online = urcu_reader.ctr;
 246
 247         /*
 248          * Mark the writer thread offline to make sure we don't wait for
 249          * our own quiescent state. This allows using synchronize_rcu() in
 250          * threads registered as readers.
 251          */
 252         smp_mb();
 253         if (was_online)
 254                 STORE_SHARED(urcu_reader.ctr, 0);
 255
 256         internal_urcu_lock();
 257         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr + RCU_GP_CTR);
 258         wait_for_quiescent_state();
 259         internal_urcu_unlock();
 260
 261         if (was_online)
 262                 _STORE_SHARED(urcu_reader.ctr, LOAD_SHARED(urcu_gp_ctr));
 263         smp_mb();
 264 }
 265 #endif  /* !(BITS_PER_LONG < 64) */
 266
 267 /*
 268  * library wrappers to be used by non-LGPL compatible source code.
 269  */
 270
 271 void rcu_read_lock(void)
 272 {
 273         _rcu_read_lock();
 274 }
 275
 276 void rcu_read_unlock(void)
 277 {
 278         _rcu_read_unlock();
 279 }
 280
 281 void *rcu_dereference(void *p)
 282 {
 283         return _rcu_dereference(p);
 284 }
 285
 286 void *rcu_assign_pointer_sym(void **p, void *v)
 287 {
 288         wmb();
 289         return STORE_SHARED(*p, v);
 290 }
 291
 292 void *rcu_cmpxchg_pointer_sym(void **p, void *old, void *_new)
 293 {
 294         wmb();
 295         return uatomic_cmpxchg(p, old, _new);
 296 }
 297
 298 void *rcu_xchg_pointer_sym(void **p, void *v)
 299 {
 300         wmb();
 301         return uatomic_xchg(p, v);
 302 }
 303
 304 void *rcu_publish_content_sym(void **p, void *v)
 305 {
 306         void *oldptr;
 307
 308         oldptr = _rcu_xchg_pointer(p, v);
 309         synchronize_rcu();
 310         return oldptr;
 311 }
 312
 313 void rcu_quiescent_state(void)
 314 {
 315         _rcu_quiescent_state();
 316 }
 317
 318 void rcu_thread_offline(void)
 319 {
 320         _rcu_thread_offline();
 321 }
 322
 323 void rcu_thread_online(void)
 324 {
 325         _rcu_thread_online();
 326 }
 327
 328 void rcu_register_thread(void)
 329 {
 330         urcu_reader.tid = pthread_self();
 331         assert(urcu_reader.ctr == 0);
 332
 333         internal_urcu_lock();
 334         list_add(&urcu_reader.head, &registry);
 335         internal_urcu_unlock();
 336         _rcu_thread_online();
 337 }
 338
 339 void rcu_unregister_thread(void)
 340 {
 341         /*
 342          * We have to make the thread offline otherwise we end up dealocking
 343          * with a waiting writer.
 344          */
 345         _rcu_thread_offline();
 346         internal_urcu_lock();
 347         list_del(&urcu_reader.head);
 348         internal_urcu_unlock();
 349 }
 350
 351 void rcu_exit(void)
 352 {
 353         assert(list_empty(&registry));
 354 }