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