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(struct urcu_reader *index)
 101 {
 102         uatomic_dec(&gp_futex);
 103         smp_mb(); /* Write futex before read reader_gp */
 104         if (!rcu_gp_ongoing(&index->ctr)) {
 105                 /* Read reader_gp before write futex */
 106                 smp_mb();
 107                 /* Callbacks are queued, don't wait. */
 108                 uatomic_set(&gp_futex, 0);
 109         } else {
 110                 /* Read reader_gp before read futex */
 111                 smp_rmb();
 112                 if (uatomic_read(&gp_futex) == -1)
 113                         futex(&gp_futex, FUTEX_WAIT, -1,
 114                               NULL, NULL, 0);
 115         }
 116 }
 117
 118 static void wait_for_quiescent_state(void)
 119 {
 120         struct urcu_reader *index;
 121
 122         if (list_empty(&registry))
 123                 return;
 124         /*
 125          * Wait for each thread rcu_reader_qs_gp count to become 0.
 126          */
 127         list_for_each_entry(index, &registry, head) {
 128                 int wait_loops = 0;
 129
 130                 while (rcu_gp_ongoing(&index->ctr)) {
 131                         if (wait_loops++ == RCU_QS_ACTIVE_ATTEMPTS) {
 132                                 wait_gp(index);
 133                         } else {
 134 #ifndef HAS_INCOHERENT_CACHES
 135                                 cpu_relax();
 136 #else /* #ifndef HAS_INCOHERENT_CACHES */
 137                                 smp_mb();
 138 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 139                         }
 140                 }
 141         }
 142 }
 143
 144 /*
 145  * Using a two-subphases algorithm for architectures with smaller than 64-bit
 146  * long-size to ensure we do not encounter an overflow bug.
 147  */
 148
 149 #if (BITS_PER_LONG < 64)
 150 /*
 151  * called with urcu_mutex held.
 152  */
 153 static void switch_next_urcu_qparity(void)
 154 {
 155         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_CTR);
 156 }
 157
 158 void synchronize_rcu(void)
 159 {
 160         unsigned long was_online;
 161
 162         was_online = urcu_reader.ctr;
 163
 164         /* All threads should read qparity before accessing data structure
 165          * where new ptr points to.
 166          */
 167         /* Write new ptr before changing the qparity */
 168         smp_mb();
 169
 170         /*
 171          * Mark the writer thread offline to make sure we don't wait for
 172          * our own quiescent state. This allows using synchronize_rcu() in
 173          * threads registered as readers.
 174          */
 175         if (was_online)
 176                 STORE_SHARED(urcu_reader.ctr, 0);
 177
 178         internal_urcu_lock();
 179
 180         switch_next_urcu_qparity();     /* 0 -> 1 */
 181
 182         /*
 183          * Must commit qparity update to memory before waiting for parity
 184          * 0 quiescent state. Failure to do so could result in the writer
 185          * waiting forever while new readers are always accessing data (no
 186          * progress).
 187          * Ensured by STORE_SHARED and LOAD_SHARED.
 188          */
 189
 190         /*
 191          * Wait for previous parity to be empty of readers.
 192          */
 193         wait_for_quiescent_state();     /* Wait readers in parity 0 */
 194
 195         /*
 196          * Must finish waiting for quiescent state for parity 0 before
 197          * committing qparity update to memory. Failure to do so could result in
 198          * the writer waiting forever while new readers are always accessing
 199          * data (no progress).
 200          * Ensured by STORE_SHARED and LOAD_SHARED.
 201          */
 202
 203         switch_next_urcu_qparity();     /* 1 -> 0 */
 204
 205         /*
 206          * Must commit qparity update to memory before waiting for parity
 207          * 1 quiescent state. Failure to do so could result in the writer
 208          * waiting forever while new readers are always accessing data (no
 209          * progress).
 210          * Ensured by STORE_SHARED and LOAD_SHARED.
 211          */
 212
 213         /*
 214          * Wait for previous parity to be empty of readers.
 215          */
 216         wait_for_quiescent_state();     /* Wait readers in parity 1 */
 217
 218         internal_urcu_unlock();
 219
 220         /*
 221          * Finish waiting for reader threads before letting the old ptr being
 222          * freed.
 223          */
 224         if (was_online)
 225                 _STORE_SHARED(urcu_reader.ctr, LOAD_SHARED(urcu_gp_ctr));
 226         smp_mb();
 227 }
 228 #else /* !(BITS_PER_LONG < 64) */
 229 void synchronize_rcu(void)
 230 {
 231         unsigned long was_online;
 232
 233         was_online = urcu_reader.ctr;
 234
 235         /*
 236          * Mark the writer thread offline to make sure we don't wait for
 237          * our own quiescent state. This allows using synchronize_rcu() in
 238          * threads registered as readers.
 239          */
 240         smp_mb();
 241         if (was_online)
 242                 STORE_SHARED(urcu_reader.ctr, 0);
 243
 244         internal_urcu_lock();
 245         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr + RCU_GP_CTR);
 246         wait_for_quiescent_state();
 247         internal_urcu_unlock();
 248
 249         if (was_online)
 250                 _STORE_SHARED(urcu_reader.ctr, 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 uatomic_cmpxchg(p, old, _new);
 284 }
 285
 286 void *rcu_xchg_pointer_sym(void **p, void *v)
 287 {
 288         wmb();
 289         return uatomic_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 void rcu_register_thread(void)
 317 {
 318         urcu_reader.tid = pthread_self();
 319         assert(urcu_reader.ctr == 0);
 320
 321         internal_urcu_lock();
 322         list_add(&urcu_reader.head, &registry);
 323         internal_urcu_unlock();
 324         _rcu_thread_online();
 325 }
 326
 327 void rcu_unregister_thread(void)
 328 {
 329         /*
 330          * We have to make the thread offline otherwise we end up dealocking
 331          * with a waiting writer.
 332          */
 333         _rcu_thread_offline();
 334         internal_urcu_lock();
 335         list_del(&urcu_reader.head);
 336         internal_urcu_unlock();
 337 }
 338
 339 void rcu_exit(void)
 340 {
 341         assert(list_empty(&registry));
 342 }