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 #define _GNU_SOURCE
  27 #define _LGPL_SOURCE
  28 #include <stdio.h>
  29 #include <pthread.h>
  30 #include <signal.h>
  31 #include <assert.h>
  32 #include <stdlib.h>
  33 #include <stdint.h>
  34 #include <string.h>
  35 #include <errno.h>
  36 #include <poll.h>
  37
  38 #include "urcu/wfqueue.h"
  39 #include "urcu/map/urcu-qsbr.h"
  40 #define BUILD_QSBR_LIB
  41 #include "urcu/static/urcu-qsbr.h"
  42 #include "urcu-pointer.h"
  43 #include "urcu/tls-compat.h"
  44
  45 #include "urcu-die.h"
  46
  47 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
  48 #undef _LGPL_SOURCE
  49 #include "urcu-qsbr.h"
  50 #define _LGPL_SOURCE
  51
  52 void __attribute__((destructor)) rcu_exit(void);
  53
  54 /*
  55  * rcu_gp_lock ensures mutual exclusion between threads calling
  56  * synchronize_rcu().
  57  */
  58 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
  59 /*
  60  * rcu_registry_lock ensures mutual exclusion between threads
  61  * registering and unregistering themselves to/from the registry, and
  62  * with threads reading that registry from synchronize_rcu(). However,
  63  * this lock is not held all the way through the completion of awaiting
  64  * for the grace period. It is sporadically released between iterations
  65  * on the registry.
  66  * rcu_registry_lock may nest inside rcu_gp_lock.
  67  */
  68 static pthread_mutex_t rcu_registry_lock = PTHREAD_MUTEX_INITIALIZER;
  69
  70 int32_t gp_futex;
  71
  72 /*
  73  * Global grace period counter.
  74  */
  75 unsigned long rcu_gp_ctr = RCU_GP_ONLINE;
  76
  77 /*
  78  * Active attempts to check for reader Q.S. before calling futex().
  79  */
  80 #define RCU_QS_ACTIVE_ATTEMPTS 100
  81
  82 /*
  83  * Written to only by each individual reader. Read by both the reader and the
  84  * writers.
  85  */
  86 __DEFINE_URCU_TLS_GLOBAL(struct rcu_reader, rcu_reader);
  87
  88 #ifdef DEBUG_YIELD
  89 unsigned int yield_active;
  90 __DEFINE_URCU_TLS_GLOBAL(unsigned int, rand_yield);
  91 #endif
  92
  93 static CDS_LIST_HEAD(registry);
  94
  95 static void mutex_lock(pthread_mutex_t *mutex)
  96 {
  97         int ret;
  98
  99 #ifndef DISTRUST_SIGNALS_EXTREME
 100         ret = pthread_mutex_lock(mutex);
 101         if (ret)
 102                 urcu_die(ret);
 103 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
 104         while ((ret = pthread_mutex_trylock(mutex)) != 0) {
 105                 if (ret != EBUSY && ret != EINTR)
 106                         urcu_die(ret);
 107                 poll(NULL,0,10);
 108         }
 109 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
 110 }
 111
 112 static void mutex_unlock(pthread_mutex_t *mutex)
 113 {
 114         int ret;
 115
 116         ret = pthread_mutex_unlock(mutex);
 117         if (ret)
 118                 urcu_die(ret);
 119 }
 120
 121 /*
 122  * synchronize_rcu() waiting. Single thread.
 123  */
 124 static void wait_gp(void)
 125 {
 126         /* Read reader_gp before read futex */
 127         cmm_smp_rmb();
 128         if (uatomic_read(&gp_futex) != -1)
 129                 return;
 130         while (futex_noasync(&gp_futex, FUTEX_WAIT, -1,
 131                         NULL, NULL, 0)) {
 132                 switch (errno) {
 133                 case EWOULDBLOCK:
 134                         /* Value already changed. */
 135                         return;
 136                 case EINTR:
 137                         /* Retry if interrupted by signal. */
 138                         break;  /* Get out of switch. */
 139                 default:
 140                         /* Unexpected error. */
 141                         urcu_die(errno);
 142                 }
 143         }
 144 }
 145
 146 /*
 147  * Always called with rcu_registry lock held. Releases this lock between
 148  * iterations and grabs it again. Holds the lock when it returns.
 149  */
 150 static void update_counter_and_wait(void)
 151 {
 152         CDS_LIST_HEAD(qsreaders);
 153         unsigned int wait_loops = 0;
 154         struct rcu_reader *index, *tmp;
 155
 156 #if (CAA_BITS_PER_LONG < 64)
 157         /* Switch parity: 0 -> 1, 1 -> 0 */
 158         CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
 159 #else   /* !(CAA_BITS_PER_LONG < 64) */
 160         /* Increment current G.P. */
 161         CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
 162 #endif  /* !(CAA_BITS_PER_LONG < 64) */
 163
 164         /*
 165          * Must commit rcu_gp_ctr update to memory before waiting for
 166          * quiescent state. Failure to do so could result in the writer
 167          * waiting forever while new readers are always accessing data
 168          * (no progress). Enforce compiler-order of store to rcu_gp_ctr
 169          * before load URCU_TLS(rcu_reader).ctr.
 170          */
 171         cmm_barrier();
 172
 173         /*
 174          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 175          * model easier to understand. It does not have a big performance impact
 176          * anyway, given this is the write-side.
 177          */
 178         cmm_smp_mb();
 179
 180         /*
 181          * Wait for each thread rcu_reader_qs_gp count to become 0.
 182          */
 183         for (;;) {
 184                 if (wait_loops < RCU_QS_ACTIVE_ATTEMPTS)
 185                         wait_loops++;
 186                 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 187                         uatomic_set(&gp_futex, -1);
 188                         /*
 189                          * Write futex before write waiting (the other side
 190                          * reads them in the opposite order).
 191                          */
 192                         cmm_smp_wmb();
 193                         cds_list_for_each_entry(index, &registry, node) {
 194                                 _CMM_STORE_SHARED(index->waiting, 1);
 195                         }
 196                         /* Write futex before read reader_gp */
 197                         cmm_smp_mb();
 198                 }
 199                 cds_list_for_each_entry_safe(index, tmp, &registry, node) {
 200                         if (!rcu_gp_ongoing(&index->ctr))
 201                                 cds_list_move(&index->node, &qsreaders);
 202                 }
 203
 204                 if (cds_list_empty(&registry)) {
 205                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 206                                 /* Read reader_gp before write futex */
 207                                 cmm_smp_mb();
 208                                 uatomic_set(&gp_futex, 0);
 209                         }
 210                         break;
 211                 } else {
 212                         /* Temporarily unlock the registry lock. */
 213                         mutex_unlock(&rcu_registry_lock);
 214                         if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 215                                 wait_gp();
 216                         } else {
 217 #ifndef HAS_INCOHERENT_CACHES
 218                                 caa_cpu_relax();
 219 #else /* #ifndef HAS_INCOHERENT_CACHES */
 220                                 cmm_smp_mb();
 221 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 222                         }
 223                         /* Re-lock the registry lock before the next loop. */
 224                         mutex_lock(&rcu_registry_lock);
 225                 }
 226         }
 227         /* put back the reader list in the registry */
 228         cds_list_splice(&qsreaders, &registry);
 229 }
 230
 231 /*
 232  * Using a two-subphases algorithm for architectures with smaller than 64-bit
 233  * long-size to ensure we do not encounter an overflow bug.
 234  */
 235
 236 #if (CAA_BITS_PER_LONG < 64)
 237 void synchronize_rcu(void)
 238 {
 239         unsigned long was_online;
 240
 241         was_online = URCU_TLS(rcu_reader).ctr;
 242
 243         /* All threads should read qparity before accessing data structure
 244          * where new ptr points to.  In the "then" case, rcu_thread_offline
 245          * includes a memory barrier.
 246          *
 247          * Mark the writer thread offline to make sure we don't wait for
 248          * our own quiescent state. This allows using synchronize_rcu()
 249          * in threads registered as readers.
 250          */
 251         if (was_online)
 252                 rcu_thread_offline();
 253         else
 254                 cmm_smp_mb();
 255
 256         mutex_lock(&rcu_gp_lock);
 257         mutex_lock(&rcu_registry_lock);
 258
 259         if (cds_list_empty(&registry))
 260                 goto out;
 261
 262         /*
 263          * Wait for previous parity to be empty of readers.
 264          * update_counter_and_wait() can release and grab again
 265          * rcu_registry_lock interally.
 266          */
 267         update_counter_and_wait();      /* 0 -> 1, wait readers in parity 0 */
 268
 269         /*
 270          * Must finish waiting for quiescent state for parity 0 before
 271          * committing next rcu_gp_ctr update to memory. Failure to
 272          * do so could result in the writer waiting forever while new
 273          * readers are always accessing data (no progress).  Enforce
 274          * compiler-order of load URCU_TLS(rcu_reader).ctr before store to
 275          * rcu_gp_ctr.
 276          */
 277         cmm_barrier();
 278
 279         /*
 280          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 281          * model easier to understand. It does not have a big performance impact
 282          * anyway, given this is the write-side.
 283          */
 284         cmm_smp_mb();
 285
 286         /*
 287          * Wait for previous parity to be empty of readers.
 288          * update_counter_and_wait() can release and grab again
 289          * rcu_registry_lock interally.
 290          */
 291         update_counter_and_wait();      /* 1 -> 0, wait readers in parity 1 */
 292 out:
 293         mutex_unlock(&rcu_registry_lock);
 294         mutex_unlock(&rcu_gp_lock);
 295
 296         /*
 297          * Finish waiting for reader threads before letting the old ptr being
 298          * freed.
 299          */
 300         if (was_online)
 301                 rcu_thread_online();
 302         else
 303                 cmm_smp_mb();
 304 }
 305 #else /* !(CAA_BITS_PER_LONG < 64) */
 306 void synchronize_rcu(void)
 307 {
 308         unsigned long was_online;
 309
 310         was_online = URCU_TLS(rcu_reader).ctr;
 311
 312         /*
 313          * Mark the writer thread offline to make sure we don't wait for
 314          * our own quiescent state. This allows using synchronize_rcu()
 315          * in threads registered as readers.
 316          */
 317         if (was_online)
 318                 rcu_thread_offline();
 319         else
 320                 cmm_smp_mb();
 321
 322         mutex_lock(&rcu_gp_lock);
 323         mutex_lock(&rcu_registry_lock);
 324         if (cds_list_empty(&registry))
 325                 goto out;
 326         /*
 327          * update_counter_and_wait() can release and grab again
 328          * rcu_registry_lock interally.
 329          */
 330         update_counter_and_wait();
 331 out:
 332         mutex_unlock(&rcu_registry_lock);
 333         mutex_unlock(&rcu_gp_lock);
 334
 335         if (was_online)
 336                 rcu_thread_online();
 337         else
 338                 cmm_smp_mb();
 339 }
 340 #endif  /* !(CAA_BITS_PER_LONG < 64) */
 341
 342 /*
 343  * library wrappers to be used by non-LGPL compatible source code.
 344  */
 345
 346 void rcu_read_lock(void)
 347 {
 348         _rcu_read_lock();
 349 }
 350
 351 void rcu_read_unlock(void)
 352 {
 353         _rcu_read_unlock();
 354 }
 355
 356 void rcu_quiescent_state(void)
 357 {
 358         _rcu_quiescent_state();
 359 }
 360
 361 void rcu_thread_offline(void)
 362 {
 363         _rcu_thread_offline();
 364 }
 365
 366 void rcu_thread_online(void)
 367 {
 368         _rcu_thread_online();
 369 }
 370
 371 void rcu_register_thread(void)
 372 {
 373         URCU_TLS(rcu_reader).tid = pthread_self();
 374         assert(URCU_TLS(rcu_reader).ctr == 0);
 375
 376         mutex_lock(&rcu_registry_lock);
 377         cds_list_add(&URCU_TLS(rcu_reader).node, &registry);
 378         mutex_unlock(&rcu_registry_lock);
 379         _rcu_thread_online();
 380 }
 381
 382 void rcu_unregister_thread(void)
 383 {
 384         /*
 385          * We have to make the thread offline otherwise we end up dealocking
 386          * with a waiting writer.
 387          */
 388         _rcu_thread_offline();
 389         mutex_lock(&rcu_registry_lock);
 390         cds_list_del(&URCU_TLS(rcu_reader).node);
 391         mutex_unlock(&rcu_registry_lock);
 392 }
 393
 394 void rcu_exit(void)
 395 {
 396         /*
 397          * Assertion disabled because call_rcu threads are now rcu
 398          * readers, and left running at exit.
 399          * assert(cds_list_empty(&registry));
 400          */
 401 }
 402
 403 DEFINE_RCU_FLAVOR(rcu_flavor);
 404
 405 #include "urcu-call-rcu-impl.h"
 406 #include "urcu-defer-impl.h"