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 #include "urcu-wait.h"
  47
  48 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
  49 #undef _LGPL_SOURCE
  50 #include "urcu.h"
  51 #define _LGPL_SOURCE
  52
  53 /*
  54  * If a reader is really non-cooperative and refuses to commit its
  55  * rcu_active_readers count to memory (there is no barrier in the reader
  56  * per-se), kick it after a few loops waiting for it.
  57  */
  58 #define KICK_READER_LOOPS 10000
  59
  60 /*
  61  * Active attempts to check for reader Q.S. before calling futex().
  62  */
  63 #define RCU_QS_ACTIVE_ATTEMPTS 100
  64
  65 /*
  66  * RCU_MEMBARRIER is only possibly available on Linux.
  67  */
  68 #if defined(RCU_MEMBARRIER) && defined(__linux__)
  69 #include <syscall.h>
  70 #endif
  71
  72 /* If the headers do not support SYS_membarrier, fall back on RCU_MB */
  73 #ifdef SYS_membarrier
  74 # define membarrier(...)                syscall(SYS_membarrier, __VA_ARGS__)
  75 #else
  76 # define membarrier(...)                -ENOSYS
  77 #endif
  78
  79 #define MEMBARRIER_EXPEDITED            (1 << 0)
  80 #define MEMBARRIER_DELAYED              (1 << 1)
  81 #define MEMBARRIER_QUERY                (1 << 16)
  82
  83 #ifdef RCU_MEMBARRIER
  84 static int init_done;
  85 int rcu_has_sys_membarrier;
  86
  87 void __attribute__((constructor)) rcu_init(void);
  88 #endif
  89
  90 #ifdef RCU_MB
  91 void rcu_init(void)
  92 {
  93 }
  94 #endif
  95
  96 #ifdef RCU_SIGNAL
  97 static int init_done;
  98
  99 void __attribute__((constructor)) rcu_init(void);
 100 void __attribute__((destructor)) rcu_exit(void);
 101 #endif
 102
 103 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
 104 struct rcu_gp rcu_gp = { .ctr = RCU_GP_COUNT };
 105
 106 /*
 107  * Written to only by each individual reader. Read by both the reader and the
 108  * writers.
 109  */
 110 __DEFINE_URCU_TLS_GLOBAL(struct rcu_reader, rcu_reader);
 111
 112 static CDS_LIST_HEAD(registry);
 113
 114 /*
 115  * Queue keeping threads awaiting to wait for a grace period. Contains
 116  * struct gp_waiters_thread objects.
 117  */
 118 static DEFINE_URCU_WAIT_QUEUE(gp_waiters);
 119
 120 static void mutex_lock(pthread_mutex_t *mutex)
 121 {
 122         int ret;
 123
 124 #ifndef DISTRUST_SIGNALS_EXTREME
 125         ret = pthread_mutex_lock(mutex);
 126         if (ret)
 127                 urcu_die(ret);
 128 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
 129         while ((ret = pthread_mutex_trylock(mutex)) != 0) {
 130                 if (ret != EBUSY && ret != EINTR)
 131                         urcu_die(ret);
 132                 if (CMM_LOAD_SHARED(URCU_TLS(rcu_reader).need_mb)) {
 133                         cmm_smp_mb();
 134                         _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
 135                         cmm_smp_mb();
 136                 }
 137                 poll(NULL,0,10);
 138         }
 139 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
 140 }
 141
 142 static void mutex_unlock(pthread_mutex_t *mutex)
 143 {
 144         int ret;
 145
 146         ret = pthread_mutex_unlock(mutex);
 147         if (ret)
 148                 urcu_die(ret);
 149 }
 150
 151 #ifdef RCU_MEMBARRIER
 152 static void smp_mb_master(int group)
 153 {
 154         if (caa_likely(rcu_has_sys_membarrier))
 155                 (void) membarrier(MEMBARRIER_EXPEDITED);
 156         else
 157                 cmm_smp_mb();
 158 }
 159 #endif
 160
 161 #ifdef RCU_MB
 162 static void smp_mb_master(int group)
 163 {
 164         cmm_smp_mb();
 165 }
 166 #endif
 167
 168 #ifdef RCU_SIGNAL
 169 static void force_mb_all_readers(void)
 170 {
 171         struct rcu_reader *index;
 172
 173         /*
 174          * Ask for each threads to execute a cmm_smp_mb() so we can consider the
 175          * compiler barriers around rcu read lock as real memory barriers.
 176          */
 177         if (cds_list_empty(&registry))
 178                 return;
 179         /*
 180          * pthread_kill has a cmm_smp_mb(). But beware, we assume it performs
 181          * a cache flush on architectures with non-coherent cache. Let's play
 182          * safe and don't assume anything : we use cmm_smp_mc() to make sure the
 183          * cache flush is enforced.
 184          */
 185         cds_list_for_each_entry(index, &registry, node) {
 186                 CMM_STORE_SHARED(index->need_mb, 1);
 187                 pthread_kill(index->tid, SIGRCU);
 188         }
 189         /*
 190          * Wait for sighandler (and thus mb()) to execute on every thread.
 191          *
 192          * Note that the pthread_kill() will never be executed on systems
 193          * that correctly deliver signals in a timely manner.  However, it
 194          * is not uncommon for kernels to have bugs that can result in
 195          * lost or unduly delayed signals.
 196          *
 197          * If you are seeing the below pthread_kill() executing much at
 198          * all, we suggest testing the underlying kernel and filing the
 199          * relevant bug report.  For Linux kernels, we recommend getting
 200          * the Linux Test Project (LTP).
 201          */
 202         cds_list_for_each_entry(index, &registry, node) {
 203                 while (CMM_LOAD_SHARED(index->need_mb)) {
 204                         pthread_kill(index->tid, SIGRCU);
 205                         poll(NULL, 0, 1);
 206                 }
 207         }
 208         cmm_smp_mb();   /* read ->need_mb before ending the barrier */
 209 }
 210
 211 static void smp_mb_master(int group)
 212 {
 213         force_mb_all_readers();
 214 }
 215 #endif /* #ifdef RCU_SIGNAL */
 216
 217 /*
 218  * synchronize_rcu() waiting. Single thread.
 219  */
 220 static void wait_gp(void)
 221 {
 222         /* Read reader_gp before read futex */
 223         smp_mb_master(RCU_MB_GROUP);
 224         if (uatomic_read(&rcu_gp.futex) == -1)
 225                 futex_async(&rcu_gp.futex, FUTEX_WAIT, -1,
 226                       NULL, NULL, 0);
 227 }
 228
 229 static void wait_for_readers(struct cds_list_head *input_readers,
 230                         struct cds_list_head *cur_snap_readers,
 231                         struct cds_list_head *qsreaders)
 232 {
 233         int wait_loops = 0;
 234         struct rcu_reader *index, *tmp;
 235
 236         /*
 237          * Wait for each thread URCU_TLS(rcu_reader).ctr to either
 238          * indicate quiescence (not nested), or observe the current
 239          * rcu_gp.ctr value.
 240          */
 241         for (;;) {
 242                 wait_loops++;
 243                 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
 244                         uatomic_dec(&rcu_gp.futex);
 245                         /* Write futex before read reader_gp */
 246                         smp_mb_master(RCU_MB_GROUP);
 247                 }
 248
 249                 cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
 250                         switch (rcu_reader_state(&index->ctr)) {
 251                         case RCU_READER_ACTIVE_CURRENT:
 252                                 if (cur_snap_readers) {
 253                                         cds_list_move(&index->node,
 254                                                 cur_snap_readers);
 255                                         break;
 256                                 }
 257                                 /* Fall-through */
 258                         case RCU_READER_INACTIVE:
 259                                 cds_list_move(&index->node, qsreaders);
 260                                 break;
 261                         case RCU_READER_ACTIVE_OLD:
 262                                 /*
 263                                  * Old snapshot. Leaving node in
 264                                  * input_readers will make us busy-loop
 265                                  * until the snapshot becomes current or
 266                                  * the reader becomes inactive.
 267                                  */
 268                                 break;
 269                         }
 270                 }
 271
 272 #ifndef HAS_INCOHERENT_CACHES
 273                 if (cds_list_empty(input_readers)) {
 274                         if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
 275                                 /* Read reader_gp before write futex */
 276                                 smp_mb_master(RCU_MB_GROUP);
 277                                 uatomic_set(&rcu_gp.futex, 0);
 278                         }
 279                         break;
 280                 } else {
 281                         if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS)
 282                                 wait_gp();
 283                         else
 284                                 caa_cpu_relax();
 285                 }
 286 #else /* #ifndef HAS_INCOHERENT_CACHES */
 287                 /*
 288                  * BUSY-LOOP. Force the reader thread to commit its
 289                  * URCU_TLS(rcu_reader).ctr update to memory if we wait
 290                  * for too long.
 291                  */
 292                 if (cds_list_empty(input_readers)) {
 293                         if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
 294                                 /* Read reader_gp before write futex */
 295                                 smp_mb_master(RCU_MB_GROUP);
 296                                 uatomic_set(&rcu_gp.futex, 0);
 297                         }
 298                         break;
 299                 } else {
 300                         switch (wait_loops) {
 301                         case RCU_QS_ACTIVE_ATTEMPTS:
 302                                 wait_gp();
 303                                 break; /* only escape switch */
 304                         case KICK_READER_LOOPS:
 305                                 smp_mb_master(RCU_MB_GROUP);
 306                                 wait_loops = 0;
 307                                 break; /* only escape switch */
 308                         default:
 309                                 caa_cpu_relax();
 310                         }
 311                 }
 312 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 313         }
 314 }
 315
 316 void synchronize_rcu(void)
 317 {
 318         CDS_LIST_HEAD(cur_snap_readers);
 319         CDS_LIST_HEAD(qsreaders);
 320         DEFINE_URCU_WAIT_NODE(wait, URCU_WAIT_WAITING);
 321         struct urcu_waiters waiters;
 322
 323         /*
 324          * Add ourself to gp_waiters queue of threads awaiting to wait
 325          * for a grace period. Proceed to perform the grace period only
 326          * if we are the first thread added into the queue.
 327          * The implicit memory barrier before urcu_wait_add()
 328          * orders prior memory accesses of threads put into the wait
 329          * queue before their insertion into the wait queue.
 330          */
 331         if (urcu_wait_add(&gp_waiters, &wait) != 0) {
 332                 /* Not first in queue: will be awakened by another thread. */
 333                 urcu_adaptative_busy_wait(&wait);
 334                 /* Order following memory accesses after grace period. */
 335                 cmm_smp_mb();
 336                 return;
 337         }
 338         /* We won't need to wake ourself up */
 339         urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);
 340
 341         mutex_lock(&rcu_gp_lock);
 342
 343         /*
 344          * Move all waiters into our local queue.
 345          */
 346         urcu_move_waiters(&waiters, &gp_waiters);
 347
 348         if (cds_list_empty(&registry))
 349                 goto out;
 350
 351         /* All threads should read qparity before accessing data structure
 352          * where new ptr points to. Must be done within rcu_gp_lock because it
 353          * iterates on reader threads.*/
 354         /* Write new ptr before changing the qparity */
 355         smp_mb_master(RCU_MB_GROUP);
 356
 357         /*
 358          * Wait for readers to observe original parity or be quiescent.
 359          */
 360         wait_for_readers(&registry, &cur_snap_readers, &qsreaders);
 361
 362         /*
 363          * Must finish waiting for quiescent state for original parity before
 364          * committing next rcu_gp.ctr update to memory. Failure to do so could
 365          * result in the writer waiting forever while new readers are always
 366          * accessing data (no progress).  Enforce compiler-order of load
 367          * URCU_TLS(rcu_reader).ctr before store to rcu_gp.ctr.
 368          */
 369         cmm_barrier();
 370
 371         /*
 372          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 373          * model easier to understand. It does not have a big performance impact
 374          * anyway, given this is the write-side.
 375          */
 376         cmm_smp_mb();
 377
 378         /* Switch parity: 0 -> 1, 1 -> 0 */
 379         CMM_STORE_SHARED(rcu_gp.ctr, rcu_gp.ctr ^ RCU_GP_CTR_PHASE);
 380
 381         /*
 382          * Must commit rcu_gp.ctr update to memory before waiting for quiescent
 383          * state. Failure to do so could result in the writer waiting forever
 384          * while new readers are always accessing data (no progress). Enforce
 385          * compiler-order of store to rcu_gp.ctr before load rcu_reader ctr.
 386          */
 387         cmm_barrier();
 388
 389         /*
 390          *
 391          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 392          * model easier to understand. It does not have a big performance impact
 393          * anyway, given this is the write-side.
 394          */
 395         cmm_smp_mb();
 396
 397         /*
 398          * Wait for readers to observe new parity or be quiescent.
 399          */
 400         wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
 401
 402         /*
 403          * Put quiescent reader list back into registry.
 404          */
 405         cds_list_splice(&qsreaders, &registry);
 406
 407         /* Finish waiting for reader threads before letting the old ptr being
 408          * freed. Must be done within rcu_gp_lock because it iterates on reader
 409          * threads. */
 410         smp_mb_master(RCU_MB_GROUP);
 411 out:
 412         mutex_unlock(&rcu_gp_lock);
 413
 414         /*
 415          * Wakeup waiters only after we have completed the grace period
 416          * and have ensured the memory barriers at the end of the grace
 417          * period have been issued.
 418          */
 419         urcu_wake_all_waiters(&waiters);
 420 }
 421
 422 /*
 423  * library wrappers to be used by non-LGPL compatible source code.
 424  */
 425
 426 void rcu_read_lock(void)
 427 {
 428         _rcu_read_lock();
 429 }
 430
 431 void rcu_read_unlock(void)
 432 {
 433         _rcu_read_unlock();
 434 }
 435
 436 int rcu_read_ongoing(void)
 437 {
 438         return _rcu_read_ongoing();
 439 }
 440
 441 void rcu_register_thread(void)
 442 {
 443         URCU_TLS(rcu_reader).tid = pthread_self();
 444         assert(URCU_TLS(rcu_reader).need_mb == 0);
 445         assert(!(URCU_TLS(rcu_reader).ctr & RCU_GP_CTR_NEST_MASK));
 446
 447         mutex_lock(&rcu_gp_lock);
 448         rcu_init();     /* In case gcc does not support constructor attribute */
 449         cds_list_add(&URCU_TLS(rcu_reader).node, &registry);
 450         mutex_unlock(&rcu_gp_lock);
 451 }
 452
 453 void rcu_unregister_thread(void)
 454 {
 455         mutex_lock(&rcu_gp_lock);
 456         cds_list_del(&URCU_TLS(rcu_reader).node);
 457         mutex_unlock(&rcu_gp_lock);
 458 }
 459
 460 #ifdef RCU_MEMBARRIER
 461 void rcu_init(void)
 462 {
 463         if (init_done)
 464                 return;
 465         init_done = 1;
 466         if (!membarrier(MEMBARRIER_EXPEDITED | MEMBARRIER_QUERY))
 467                 rcu_has_sys_membarrier = 1;
 468 }
 469 #endif
 470
 471 #ifdef RCU_SIGNAL
 472 static void sigrcu_handler(int signo, siginfo_t *siginfo, void *context)
 473 {
 474         /*
 475          * Executing this cmm_smp_mb() is the only purpose of this signal handler.
 476          * It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
 477          * executed on.
 478          */
 479         cmm_smp_mb();
 480         _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
 481         cmm_smp_mb();
 482 }
 483
 484 /*
 485  * rcu_init constructor. Called when the library is linked, but also when
 486  * reader threads are calling rcu_register_thread().
 487  * Should only be called by a single thread at a given time. This is ensured by
 488  * holing the rcu_gp_lock from rcu_register_thread() or by running at library
 489  * load time, which should not be executed by multiple threads nor concurrently
 490  * with rcu_register_thread() anyway.
 491  */
 492 void rcu_init(void)
 493 {
 494         struct sigaction act;
 495         int ret;
 496
 497         if (init_done)
 498                 return;
 499         init_done = 1;
 500
 501         act.sa_sigaction = sigrcu_handler;
 502         act.sa_flags = SA_SIGINFO | SA_RESTART;
 503         sigemptyset(&act.sa_mask);
 504         ret = sigaction(SIGRCU, &act, NULL);
 505         if (ret)
 506                 urcu_die(errno);
 507 }
 508
 509 void rcu_exit(void)
 510 {
 511         /*
 512          * Don't unregister the SIGRCU signal handler anymore, because
 513          * call_rcu threads could still be using it shortly before the
 514          * application exits.
 515          * Assertion disabled because call_rcu threads are now rcu
 516          * readers, and left running at exit.
 517          * assert(cds_list_empty(&registry));
 518          */
 519 }
 520
 521 #endif /* #ifdef RCU_SIGNAL */
 522
 523 DEFINE_RCU_FLAVOR(rcu_flavor);
 524
 525 #include "urcu-call-rcu-impl.h"
 526 #include "urcu-defer-impl.h"