urcu-qsbr.c

   1 /*
   2  * urcu-qsbr.c
   3  *
   4  * Userspace RCU QSBR library
   5  *
   6  * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   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 rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
  43
  44 int gp_futex;
  45
  46 /*
  47  * Global grace period counter.
  48  */
  49 unsigned long rcu_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 rcu_reader __thread rcu_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 mutex_lock(pthread_mutex_t *mutex)
  65 {
  66         int ret;
  67
  68 #ifndef DISTRUST_SIGNALS_EXTREME
  69         ret = pthread_mutex_lock(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(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 mutex_unlock(pthread_mutex_t *mutex)
  87 {
  88         int ret;
  89
  90         ret = pthread_mutex_unlock(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_noasync(&gp_futex, FUTEX_WAIT, -1,
 106                       NULL, NULL, 0);
 107 }
 108
 109 static void update_counter_and_wait(void)
 110 {
 111         LIST_HEAD(qsreaders);
 112         int wait_loops = 0;
 113         struct rcu_reader *index, *tmp;
 114
 115 #if (BITS_PER_LONG < 64)
 116         /* Switch parity: 0 -> 1, 1 -> 0 */
 117         STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
 118 #else   /* !(BITS_PER_LONG < 64) */
 119         /* Increment current G.P. */
 120         STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
 121 #endif  /* !(BITS_PER_LONG < 64) */
 122
 123         /*
 124          * Must commit rcu_gp_ctr update to memory before waiting for quiescent
 125          * state. Failure to do so could result in the writer waiting forever
 126          * while new readers are always accessing data (no progress). Enforce
 127          * compiler-order of store to rcu_gp_ctr before load rcu_reader ctr.
 128          */
 129         barrier();
 130
 131         /*
 132          * Adding a smp_mb() which is _not_ formally required, but makes the
 133          * model easier to understand. It does not have a big performance impact
 134          * anyway, given this is the write-side.
 135          */
 136         smp_mb();
 137
 138         /*
 139          * Wait for each thread rcu_reader_qs_gp count to become 0.
 140          */
 141         for (;;) {
 142                 wait_loops++;
 143                 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
 144                         uatomic_dec(&gp_futex);
 145                         /* Write futex before read reader_gp */
 146                         smp_mb();
 147                 }
 148
 149                 list_for_each_entry_safe(index, tmp, &registry, node) {
 150                         if (!rcu_gp_ongoing(&index->ctr))
 151                                 list_move(&index->node, &qsreaders);
 152                 }
 153
 154                 if (list_empty(&registry)) {
 155                         if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
 156                                 /* Read reader_gp before write futex */
 157                                 smp_mb();
 158                                 uatomic_set(&gp_futex, 0);
 159                         }
 160                         break;
 161                 } else {
 162                         if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
 163                                 wait_gp();
 164                         } else {
 165 #ifndef HAS_INCOHERENT_CACHES
 166                                 cpu_relax();
 167 #else /* #ifndef HAS_INCOHERENT_CACHES */
 168                                 smp_mb();
 169 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 170                         }
 171                 }
 172         }
 173         /* put back the reader list in the registry */
 174         list_splice(&qsreaders, &registry);
 175 }
 176
 177 /*
 178  * Using a two-subphases algorithm for architectures with smaller than 64-bit
 179  * long-size to ensure we do not encounter an overflow bug.
 180  */
 181
 182 #if (BITS_PER_LONG < 64)
 183 void synchronize_rcu(void)
 184 {
 185         unsigned long was_online;
 186
 187         was_online = rcu_reader.ctr;
 188
 189         /* All threads should read qparity before accessing data structure
 190          * where new ptr points to.
 191          */
 192         /* Write new ptr before changing the qparity */
 193         smp_mb();
 194
 195         /*
 196          * Mark the writer thread offline to make sure we don't wait for
 197          * our own quiescent state. This allows using synchronize_rcu() in
 198          * threads registered as readers.
 199          */
 200         if (was_online)
 201                 STORE_SHARED(rcu_reader.ctr, 0);
 202
 203         mutex_lock(&rcu_gp_lock);
 204
 205         if (list_empty(&registry))
 206                 goto out;
 207
 208         /*
 209          * Wait for previous parity to be empty of readers.
 210          */
 211         update_counter_and_wait();      /* 0 -> 1, wait readers in parity 0 */
 212
 213         /*
 214          * Must finish waiting for quiescent state for parity 0 before
 215          * committing next rcu_gp_ctr update to memory. Failure to do so could
 216          * result in the writer waiting forever while new readers are always
 217          * accessing data (no progress).  Enforce compiler-order of load
 218          * rcu_reader ctr before store to rcu_gp_ctr.
 219          */
 220         barrier();
 221
 222         /*
 223          * Adding a smp_mb() which is _not_ formally required, but makes the
 224          * model easier to understand. It does not have a big performance impact
 225          * anyway, given this is the write-side.
 226          */
 227         smp_mb();
 228
 229         /*
 230          * Wait for previous parity to be empty of readers.
 231          */
 232         update_counter_and_wait();      /* 1 -> 0, wait readers in parity 1 */
 233 out:
 234         mutex_unlock(&rcu_gp_lock);
 235
 236         /*
 237          * Finish waiting for reader threads before letting the old ptr being
 238          * freed.
 239          */
 240         if (was_online)
 241                 _STORE_SHARED(rcu_reader.ctr, LOAD_SHARED(rcu_gp_ctr));
 242         smp_mb();
 243 }
 244 #else /* !(BITS_PER_LONG < 64) */
 245 void synchronize_rcu(void)
 246 {
 247         unsigned long was_online;
 248
 249         was_online = rcu_reader.ctr;
 250
 251         /*
 252          * Mark the writer thread offline to make sure we don't wait for
 253          * our own quiescent state. This allows using synchronize_rcu() in
 254          * threads registered as readers.
 255          */
 256         smp_mb();
 257         if (was_online)
 258                 STORE_SHARED(rcu_reader.ctr, 0);
 259
 260         mutex_lock(&rcu_gp_lock);
 261         if (list_empty(&registry))
 262                 goto out;
 263         update_counter_and_wait();
 264 out:
 265         mutex_unlock(&rcu_gp_lock);
 266
 267         if (was_online)
 268                 _STORE_SHARED(rcu_reader.ctr, LOAD_SHARED(rcu_gp_ctr));
 269         smp_mb();
 270 }
 271 #endif  /* !(BITS_PER_LONG < 64) */
 272
 273 /*
 274  * library wrappers to be used by non-LGPL compatible source code.
 275  */
 276
 277 void rcu_read_lock(void)
 278 {
 279         _rcu_read_lock();
 280 }
 281
 282 void rcu_read_unlock(void)
 283 {
 284         _rcu_read_unlock();
 285 }
 286
 287 void rcu_quiescent_state(void)
 288 {
 289         _rcu_quiescent_state();
 290 }
 291
 292 void rcu_thread_offline(void)
 293 {
 294         _rcu_thread_offline();
 295 }
 296
 297 void rcu_thread_online(void)
 298 {
 299         _rcu_thread_online();
 300 }
 301
 302 void rcu_register_thread(void)
 303 {
 304         rcu_reader.tid = pthread_self();
 305         assert(rcu_reader.ctr == 0);
 306
 307         mutex_lock(&rcu_gp_lock);
 308         list_add(&rcu_reader.node, &registry);
 309         mutex_unlock(&rcu_gp_lock);
 310         _rcu_thread_online();
 311 }
 312
 313 void rcu_unregister_thread(void)
 314 {
 315         /*
 316          * We have to make the thread offline otherwise we end up dealocking
 317          * with a waiting writer.
 318          */
 319         _rcu_thread_offline();
 320         mutex_lock(&rcu_gp_lock);
 321         list_del(&rcu_reader.node);
 322         mutex_unlock(&rcu_gp_lock);
 323 }
 324
 325 void rcu_exit(void)
 326 {
 327         assert(list_empty(&registry));
 328 }