urcu.c

   1 /*
   2  * urcu.c
   3  *
   4  * Userspace RCU 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 _BSD_SOURCE
  27 #define _GNU_SOURCE
  28 #define _LGPL_SOURCE
  29 #include <stdio.h>
  30 #include <pthread.h>
  31 #include <signal.h>
  32 #include <assert.h>
  33 #include <stdlib.h>
  34 #include <stdint.h>
  35 #include <string.h>
  36 #include <errno.h>
  37 #include <poll.h>
  38
  39 #include "urcu/wfcqueue.h"
  40 #include "urcu/map/urcu.h"
  41 #include "urcu/static/urcu.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.h"
  50 #define _LGPL_SOURCE
  51
  52 /*
  53  * If a reader is really non-cooperative and refuses to commit its
  54  * rcu_active_readers count to memory (there is no barrier in the reader
  55  * per-se), kick it after a few loops waiting for it.
  56  */
  57 #define KICK_READER_LOOPS 10000
  58
  59 /*
  60  * Active attempts to check for reader Q.S. before calling futex().
  61  */
  62 #define RCU_QS_ACTIVE_ATTEMPTS 100
  63
  64 #ifdef RCU_MEMBARRIER
  65 static int init_done;
  66 int rcu_has_sys_membarrier;
  67
  68 void __attribute__((constructor)) rcu_init(void);
  69 #endif
  70
  71 #ifdef RCU_MB
  72 void rcu_init(void)
  73 {
  74 }
  75 #endif
  76
  77 #ifdef RCU_SIGNAL
  78 static int init_done;
  79
  80 void __attribute__((constructor)) rcu_init(void);
  81 void __attribute__((destructor)) rcu_exit(void);
  82 #endif
  83
  84 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
  85
  86 int32_t rcu_gp_futex;
  87
  88 /*
  89  * Global grace period counter.
  90  * Contains the current RCU_GP_CTR_PHASE.
  91  * Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path.
  92  * Written to only by writer with mutex taken. Read by both writer and readers.
  93  */
  94 unsigned long rcu_gp_ctr = RCU_GP_COUNT;
  95
  96 /*
  97  * Written to only by each individual reader. Read by both the reader and the
  98  * writers.
  99  */
 100 DEFINE_URCU_TLS(struct rcu_reader, rcu_reader);
 101
 102 #ifdef DEBUG_YIELD
 103 unsigned int rcu_yield_active;
 104 DEFINE_URCU_TLS(unsigned int, rcu_rand_yield);
 105 #endif
 106
 107 static CDS_LIST_HEAD(registry);
 108
 109 static void mutex_lock(pthread_mutex_t *mutex)
 110 {
 111         int ret;
 112
 113 #ifndef DISTRUST_SIGNALS_EXTREME
 114         ret = pthread_mutex_lock(mutex);
 115         if (ret)
 116                 urcu_die(ret);
 117 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
 118         while ((ret = pthread_mutex_trylock(mutex)) != 0) {
 119                 if (ret != EBUSY && ret != EINTR)
 120                         urcu_die(ret);
 121                 if (CMM_LOAD_SHARED(URCU_TLS(rcu_reader).need_mb)) {
 122                         cmm_smp_mb();
 123                         _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
 124                         cmm_smp_mb();
 125                 }
 126                 poll(NULL,0,10);
 127         }
 128 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
 129 }
 130
 131 static void mutex_unlock(pthread_mutex_t *mutex)
 132 {
 133         int ret;
 134
 135         ret = pthread_mutex_unlock(mutex);
 136         if (ret)
 137                 urcu_die(ret);
 138 }
 139
 140 #ifdef RCU_MEMBARRIER
 141 static void smp_mb_master(int group)
 142 {
 143         if (caa_likely(rcu_has_sys_membarrier))
 144                 membarrier(MEMBARRIER_EXPEDITED);
 145         else
 146                 cmm_smp_mb();
 147 }
 148 #endif
 149
 150 #ifdef RCU_MB
 151 static void smp_mb_master(int group)
 152 {
 153         cmm_smp_mb();
 154 }
 155 #endif
 156
 157 #ifdef RCU_SIGNAL
 158 static void force_mb_all_readers(void)
 159 {
 160         struct rcu_reader *index;
 161
 162         /*
 163          * Ask for each threads to execute a cmm_smp_mb() so we can consider the
 164          * compiler barriers around rcu read lock as real memory barriers.
 165          */
 166         if (cds_list_empty(&registry))
 167                 return;
 168         /*
 169          * pthread_kill has a cmm_smp_mb(). But beware, we assume it performs
 170          * a cache flush on architectures with non-coherent cache. Let's play
 171          * safe and don't assume anything : we use cmm_smp_mc() to make sure the
 172          * cache flush is enforced.
 173          */
 174         cds_list_for_each_entry(index, &registry, node) {
 175                 CMM_STORE_SHARED(index->need_mb, 1);
 176                 pthread_kill(index->tid, SIGRCU);
 177         }
 178         /*
 179          * Wait for sighandler (and thus mb()) to execute on every thread.
 180          *
 181          * Note that the pthread_kill() will never be executed on systems
 182          * that correctly deliver signals in a timely manner.  However, it
 183          * is not uncommon for kernels to have bugs that can result in
 184          * lost or unduly delayed signals.
 185          *
 186          * If you are seeing the below pthread_kill() executing much at
 187          * all, we suggest testing the underlying kernel and filing the
 188          * relevant bug report.  For Linux kernels, we recommend getting
 189          * the Linux Test Project (LTP).
 190          */
 191         cds_list_for_each_entry(index, &registry, node) {
 192                 while (CMM_LOAD_SHARED(index->need_mb)) {
 193                         pthread_kill(index->tid, SIGRCU);
 194                         poll(NULL, 0, 1);
 195                 }
 196         }
 197         cmm_smp_mb();   /* read ->need_mb before ending the barrier */
 198 }
 199
 200 static void smp_mb_master(int group)
 201 {
 202         force_mb_all_readers();
 203 }
 204 #endif /* #ifdef RCU_SIGNAL */
 205
 206 /*
 207  * synchronize_rcu() waiting. Single thread.
 208  */
 209 static void wait_gp(void)
 210 {
 211         /* Read reader_gp before read futex */
 212         smp_mb_master(RCU_MB_GROUP);
 213         if (uatomic_read(&rcu_gp_futex) == -1)
 214                 futex_async(&rcu_gp_futex, FUTEX_WAIT, -1,
 215                       NULL, NULL, 0);
 216 }
 217
 218 static void wait_for_readers(struct cds_list_head *input_readers,
 219                         struct cds_list_head *cur_snap_readers,
 220                         struct cds_list_head *qsreaders)
 221 {
 222         int wait_loops = 0;
 223         struct rcu_reader *index, *tmp;
 224
 225         /*
 226          * Wait for each thread URCU_TLS(rcu_reader).ctr to either
 227          * indicate quiescence (not nested), or observe the current
 228          * rcu_gp_ctr value.
 229          */
 230         for (;;) {
 231                 wait_loops++;
 232                 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
 233                         uatomic_dec(&rcu_gp_futex);
 234                         /* Write futex before read reader_gp */
 235                         smp_mb_master(RCU_MB_GROUP);
 236                 }
 237
 238                 cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
 239                         switch (rcu_reader_state(&index->ctr)) {
 240                         case RCU_READER_ACTIVE_CURRENT:
 241                                 if (cur_snap_readers) {
 242                                         cds_list_move(&index->node,
 243                                                 cur_snap_readers);
 244                                         break;
 245                                 }
 246                                 /* Fall-through */
 247                         case RCU_READER_INACTIVE:
 248                                 cds_list_move(&index->node, qsreaders);
 249                                 break;
 250                         case RCU_READER_ACTIVE_OLD:
 251                                 /*
 252                                  * Old snapshot. Leaving node in
 253                                  * input_readers will make us busy-loop
 254                                  * until the snapshot becomes current or
 255                                  * the reader becomes inactive.
 256                                  */
 257                                 break;
 258                         }
 259                 }
 260
 261 #ifndef HAS_INCOHERENT_CACHES
 262                 if (cds_list_empty(input_readers)) {
 263                         if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
 264                                 /* Read reader_gp before write futex */
 265                                 smp_mb_master(RCU_MB_GROUP);
 266                                 uatomic_set(&rcu_gp_futex, 0);
 267                         }
 268                         break;
 269                 } else {
 270                         if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS)
 271                                 wait_gp();
 272                         else
 273                                 caa_cpu_relax();
 274                 }
 275 #else /* #ifndef HAS_INCOHERENT_CACHES */
 276                 /*
 277                  * BUSY-LOOP. Force the reader thread to commit its
 278                  * URCU_TLS(rcu_reader).ctr update to memory if we wait
 279                  * for too long.
 280                  */
 281                 if (cds_list_empty(input_readers)) {
 282                         if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
 283                                 /* Read reader_gp before write futex */
 284                                 smp_mb_master(RCU_MB_GROUP);
 285                                 uatomic_set(&rcu_gp_futex, 0);
 286                         }
 287                         break;
 288                 } else {
 289                         switch (wait_loops) {
 290                         case RCU_QS_ACTIVE_ATTEMPTS:
 291                                 wait_gp();
 292                                 break; /* only escape switch */
 293                         case KICK_READER_LOOPS:
 294                                 smp_mb_master(RCU_MB_GROUP);
 295                                 wait_loops = 0;
 296                                 break; /* only escape switch */
 297                         default:
 298                                 caa_cpu_relax();
 299                         }
 300                 }
 301 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 302         }
 303 }
 304
 305 void synchronize_rcu(void)
 306 {
 307         CDS_LIST_HEAD(cur_snap_readers);
 308         CDS_LIST_HEAD(qsreaders);
 309
 310         mutex_lock(&rcu_gp_lock);
 311
 312         if (cds_list_empty(&registry))
 313                 goto out;
 314
 315         /* All threads should read qparity before accessing data structure
 316          * where new ptr points to. Must be done within rcu_gp_lock because it
 317          * iterates on reader threads.*/
 318         /* Write new ptr before changing the qparity */
 319         smp_mb_master(RCU_MB_GROUP);
 320
 321         /*
 322          * Wait for readers to observe original parity or be quiescent.
 323          */
 324         wait_for_readers(&registry, &cur_snap_readers, &qsreaders);
 325
 326         /*
 327          * Must finish waiting for quiescent state for original parity before
 328          * committing next rcu_gp_ctr update to memory. Failure to do so could
 329          * result in the writer waiting forever while new readers are always
 330          * accessing data (no progress).  Enforce compiler-order of load
 331          * URCU_TLS(rcu_reader).ctr before store to rcu_gp_ctr.
 332          */
 333         cmm_barrier();
 334
 335         /*
 336          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 337          * model easier to understand. It does not have a big performance impact
 338          * anyway, given this is the write-side.
 339          */
 340         cmm_smp_mb();
 341
 342         /* Switch parity: 0 -> 1, 1 -> 0 */
 343         CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR_PHASE);
 344
 345         /*
 346          * Must commit rcu_gp_ctr update to memory before waiting for quiescent
 347          * state. Failure to do so could result in the writer waiting forever
 348          * while new readers are always accessing data (no progress). Enforce
 349          * compiler-order of store to rcu_gp_ctr before load rcu_reader ctr.
 350          */
 351         cmm_barrier();
 352
 353         /*
 354          *
 355          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 356          * model easier to understand. It does not have a big performance impact
 357          * anyway, given this is the write-side.
 358          */
 359         cmm_smp_mb();
 360
 361         /*
 362          * Wait for readers to observe new parity or be quiescent.
 363          */
 364         wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
 365
 366         /*
 367          * Put quiescent reader list back into registry.
 368          */
 369         cds_list_splice(&qsreaders, &registry);
 370
 371         /* Finish waiting for reader threads before letting the old ptr being
 372          * freed. Must be done within rcu_gp_lock because it iterates on reader
 373          * threads. */
 374         smp_mb_master(RCU_MB_GROUP);
 375 out:
 376         mutex_unlock(&rcu_gp_lock);
 377 }
 378
 379 /*
 380  * library wrappers to be used by non-LGPL compatible source code.
 381  */
 382
 383 void rcu_read_lock(void)
 384 {
 385         _rcu_read_lock();
 386 }
 387
 388 void rcu_read_unlock(void)
 389 {
 390         _rcu_read_unlock();
 391 }
 392
 393 void rcu_register_thread(void)
 394 {
 395         URCU_TLS(rcu_reader).tid = pthread_self();
 396         assert(URCU_TLS(rcu_reader).need_mb == 0);
 397         assert(!(URCU_TLS(rcu_reader).ctr & RCU_GP_CTR_NEST_MASK));
 398
 399         mutex_lock(&rcu_gp_lock);
 400         rcu_init();     /* In case gcc does not support constructor attribute */
 401         cds_list_add(&URCU_TLS(rcu_reader).node, &registry);
 402         mutex_unlock(&rcu_gp_lock);
 403 }
 404
 405 void rcu_unregister_thread(void)
 406 {
 407         mutex_lock(&rcu_gp_lock);
 408         cds_list_del(&URCU_TLS(rcu_reader).node);
 409         mutex_unlock(&rcu_gp_lock);
 410 }
 411
 412 #ifdef RCU_MEMBARRIER
 413 void rcu_init(void)
 414 {
 415         if (init_done)
 416                 return;
 417         init_done = 1;
 418         if (!membarrier(MEMBARRIER_EXPEDITED | MEMBARRIER_QUERY))
 419                 rcu_has_sys_membarrier = 1;
 420 }
 421 #endif
 422
 423 #ifdef RCU_SIGNAL
 424 static void sigrcu_handler(int signo, siginfo_t *siginfo, void *context)
 425 {
 426         /*
 427          * Executing this cmm_smp_mb() is the only purpose of this signal handler.
 428          * It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
 429          * executed on.
 430          */
 431         cmm_smp_mb();
 432         _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
 433         cmm_smp_mb();
 434 }
 435
 436 /*
 437  * rcu_init constructor. Called when the library is linked, but also when
 438  * reader threads are calling rcu_register_thread().
 439  * Should only be called by a single thread at a given time. This is ensured by
 440  * holing the rcu_gp_lock from rcu_register_thread() or by running at library
 441  * load time, which should not be executed by multiple threads nor concurrently
 442  * with rcu_register_thread() anyway.
 443  */
 444 void rcu_init(void)
 445 {
 446         struct sigaction act;
 447         int ret;
 448
 449         if (init_done)
 450                 return;
 451         init_done = 1;
 452
 453         act.sa_sigaction = sigrcu_handler;
 454         act.sa_flags = SA_SIGINFO | SA_RESTART;
 455         sigemptyset(&act.sa_mask);
 456         ret = sigaction(SIGRCU, &act, NULL);
 457         if (ret)
 458                 urcu_die(errno);
 459 }
 460
 461 void rcu_exit(void)
 462 {
 463         struct sigaction act;
 464         int ret;
 465
 466         ret = sigaction(SIGRCU, NULL, &act);
 467         if (ret)
 468                 urcu_die(errno);
 469         assert(act.sa_sigaction == sigrcu_handler);
 470         assert(cds_list_empty(&registry));
 471 }
 472
 473 #endif /* #ifdef RCU_SIGNAL */
 474
 475 DEFINE_RCU_FLAVOR(rcu_flavor);
 476
 477 #include "urcu-call-rcu-impl.h"
 478 #include "urcu-defer-impl.h"