urcu-call-rcu-impl.h

   1 /*
   2  * urcu-call-rcu.c
   3  *
   4  * Userspace RCU library - batch memory reclamation with kernel API
   5  *
   6  * Copyright (c) 2010 Paul E. McKenney <paulmck@linux.vnet.ibm.com>
   7  *
   8  * This library is free software; you can redistribute it and/or
   9  * modify it under the terms of the GNU Lesser General Public
  10  * License as published by the Free Software Foundation; either
  11  * version 2.1 of the License, or (at your option) any later version.
  12  *
  13  * This library is distributed in the hope that it will be useful,
  14  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  16  * Lesser General Public License for more details.
  17  *
  18  * You should have received a copy of the GNU Lesser General Public
  19  * License along with this library; if not, write to the Free Software
  20  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  21  */
  22
  23 #define _GNU_SOURCE
  24 #include <stdio.h>
  25 #include <pthread.h>
  26 #include <signal.h>
  27 #include <assert.h>
  28 #include <stdlib.h>
  29 #include <stdint.h>
  30 #include <string.h>
  31 #include <errno.h>
  32 #include <poll.h>
  33 #include <sys/time.h>
  34 #include <unistd.h>
  35 #include <sched.h>
  36
  37 #include "config.h"
  38 #include "urcu/wfqueue.h"
  39 #include "urcu-call-rcu.h"
  40 #include "urcu-pointer.h"
  41 #include "urcu/list.h"
  42 #include "urcu/futex.h"
  43
  44 /* Data structure that identifies a call_rcu thread. */
  45
  46 struct call_rcu_data {
  47         struct cds_wfq_queue cbs;
  48         unsigned long flags;
  49         int32_t futex;
  50         unsigned long qlen; /* maintained for debugging. */
  51         pthread_t tid;
  52         int cpu_affinity;
  53         struct cds_list_head list;
  54 } __attribute__((aligned(CAA_CACHE_LINE_SIZE)));
  55
  56 /*
  57  * List of all call_rcu_data structures to keep valgrind happy.
  58  * Protected by call_rcu_mutex.
  59  */
  60
  61 CDS_LIST_HEAD(call_rcu_data_list);
  62
  63 /* Link a thread using call_rcu() to its call_rcu thread. */
  64
  65 static __thread struct call_rcu_data *thread_call_rcu_data;
  66
  67 /* Guard call_rcu thread creation. */
  68
  69 static pthread_mutex_t call_rcu_mutex = PTHREAD_MUTEX_INITIALIZER;
  70
  71 /* If a given thread does not have its own call_rcu thread, this is default. */
  72
  73 static struct call_rcu_data *default_call_rcu_data;
  74
  75 /*
  76  * If the sched_getcpu() and sysconf(_SC_NPROCESSORS_CONF) calls are
  77  * available, then we can have call_rcu threads assigned to individual
  78  * CPUs rather than only to specific threads.
  79  */
  80
  81 #if defined(HAVE_SCHED_GETCPU) && defined(HAVE_SYSCONF)
  82
  83 /*
  84  * Pointer to array of pointers to per-CPU call_rcu_data structures
  85  * and # CPUs.
  86  */
  87
  88 static struct call_rcu_data **per_cpu_call_rcu_data;
  89 static long maxcpus;
  90
  91 static void call_rcu_wait(struct call_rcu_data *crdp)
  92 {
  93         /* Read call_rcu list before read futex */
  94         cmm_smp_mb();
  95         if (uatomic_read(&crdp->futex) == -1)
  96                 futex_async(&crdp->futex, FUTEX_WAIT, -1,
  97                       NULL, NULL, 0);
  98 }
  99
 100 static void call_rcu_wake_up(struct call_rcu_data *crdp)
 101 {
 102         /* Write to call_rcu list before reading/writing futex */
 103         cmm_smp_mb();
 104         if (unlikely(uatomic_read(&crdp->futex) == -1)) {
 105                 uatomic_set(&crdp->futex, 0);
 106                 futex_async(&crdp->futex, FUTEX_WAKE, 1,
 107                       NULL, NULL, 0);
 108         }
 109 }
 110
 111 /* Allocate the array if it has not already been allocated. */
 112
 113 static void alloc_cpu_call_rcu_data(void)
 114 {
 115         struct call_rcu_data **p;
 116         static int warned = 0;
 117
 118         if (maxcpus != 0)
 119                 return;
 120         maxcpus = sysconf(_SC_NPROCESSORS_CONF);
 121         if (maxcpus <= 0) {
 122                 return;
 123         }
 124         p = malloc(maxcpus * sizeof(*per_cpu_call_rcu_data));
 125         if (p != NULL) {
 126                 memset(p, '\0', maxcpus * sizeof(*per_cpu_call_rcu_data));
 127                 per_cpu_call_rcu_data = p;
 128         } else {
 129                 if (!warned) {
 130                         fprintf(stderr, "[error] liburcu: unable to allocate per-CPU pointer array\n");
 131                 }
 132                 warned = 1;
 133         }
 134 }
 135
 136 #else /* #if defined(HAVE_SCHED_GETCPU) && defined(HAVE_SYSCONF) */
 137
 138 static const struct call_rcu_data **per_cpu_call_rcu_data = NULL;
 139 static const long maxcpus = -1;
 140
 141 static void alloc_cpu_call_rcu_data(void)
 142 {
 143 }
 144
 145 static int sched_getcpu(void)
 146 {
 147         return -1;
 148 }
 149
 150 #endif /* #else #if defined(HAVE_SCHED_GETCPU) && defined(HAVE_SYSCONF) */
 151
 152 /* Acquire the specified pthread mutex. */
 153
 154 static void call_rcu_lock(pthread_mutex_t *pmp)
 155 {
 156         if (pthread_mutex_lock(pmp) != 0) {
 157                 perror("pthread_mutex_lock");
 158                 exit(-1);
 159         }
 160 }
 161
 162 /* Release the specified pthread mutex. */
 163
 164 static void call_rcu_unlock(pthread_mutex_t *pmp)
 165 {
 166         if (pthread_mutex_unlock(pmp) != 0) {
 167                 perror("pthread_mutex_unlock");
 168                 exit(-1);
 169         }
 170 }
 171
 172 #if HAVE_SCHED_SETAFFINITY
 173 static
 174 int set_thread_cpu_affinity(struct call_rcu_data *crdp)
 175 {
 176         cpu_set_t mask;
 177
 178         if (crdp->cpu_affinity < 0)
 179                 return 0;
 180
 181         CPU_ZERO(&mask);
 182         CPU_SET(crdp->cpu_affinity, &mask);
 183 #if SCHED_SETAFFINITY_ARGS == 2
 184         return sched_setaffinity(0, &mask);
 185 #else
 186         return sched_setaffinity(0, sizeof(mask), &mask);
 187 #endif
 188 }
 189 #else
 190 static
 191 int set_thread_cpu_affinity(struct call_rcu_data *crdp)
 192 {
 193         return 0;
 194 }
 195 #endif
 196
 197 /* This is the code run by each call_rcu thread. */
 198
 199 static void *call_rcu_thread(void *arg)
 200 {
 201         unsigned long cbcount;
 202         struct cds_wfq_node *cbs;
 203         struct cds_wfq_node **cbs_tail;
 204         struct call_rcu_data *crdp = (struct call_rcu_data *)arg;
 205         struct rcu_head *rhp;
 206         int rt = !!(uatomic_read(&crdp->flags) & URCU_CALL_RCU_RT);
 207
 208         if (set_thread_cpu_affinity(crdp) != 0) {
 209                 perror("pthread_setaffinity_np");
 210                 exit(-1);
 211         }
 212
 213         thread_call_rcu_data = crdp;
 214         if (!rt) {
 215                 uatomic_dec(&crdp->futex);
 216                 /* Decrement futex before reading call_rcu list */
 217                 cmm_smp_mb();
 218         }
 219         for (;;) {
 220                 if (&crdp->cbs.head != _CMM_LOAD_SHARED(crdp->cbs.tail)) {
 221                         while ((cbs = _CMM_LOAD_SHARED(crdp->cbs.head)) == NULL)
 222                                 poll(NULL, 0, 1);
 223                         _CMM_STORE_SHARED(crdp->cbs.head, NULL);
 224                         cbs_tail = (struct cds_wfq_node **)
 225                                 uatomic_xchg(&crdp->cbs.tail, &crdp->cbs.head);
 226                         synchronize_rcu();
 227                         cbcount = 0;
 228                         do {
 229                                 while (cbs->next == NULL &&
 230                                        &cbs->next != cbs_tail)
 231                                         poll(NULL, 0, 1);
 232                                 if (cbs == &crdp->cbs.dummy) {
 233                                         cbs = cbs->next;
 234                                         continue;
 235                                 }
 236                                 rhp = (struct rcu_head *)cbs;
 237                                 cbs = cbs->next;
 238                                 rhp->func(rhp);
 239                                 cbcount++;
 240                         } while (cbs != NULL);
 241                         uatomic_sub(&crdp->qlen, cbcount);
 242                 }
 243                 if (uatomic_read(&crdp->flags) & URCU_CALL_RCU_STOP)
 244                         break;
 245                 if (!rt) {
 246                         if (&crdp->cbs.head
 247                             == _CMM_LOAD_SHARED(crdp->cbs.tail)) {
 248                                 call_rcu_wait(crdp);
 249                                 poll(NULL, 0, 10);
 250                                 uatomic_dec(&crdp->futex);
 251                                 /*
 252                                  * Decrement futex before reading
 253                                  * call_rcu list.
 254                                  */
 255                                 cmm_smp_mb();
 256                         } else {
 257                                 poll(NULL, 0, 10);
 258                         }
 259                 } else {
 260                         poll(NULL, 0, 10);
 261                 }
 262         }
 263         if (!rt) {
 264                 /*
 265                  * Read call_rcu list before write futex.
 266                  */
 267                 cmm_smp_mb();
 268                 uatomic_set(&crdp->futex, 0);
 269         }
 270         uatomic_or(&crdp->flags, URCU_CALL_RCU_STOPPED);
 271         return NULL;
 272 }
 273
 274 /*
 275  * Create both a call_rcu thread and the corresponding call_rcu_data
 276  * structure, linking the structure in as specified.  Caller must hold
 277  * call_rcu_mutex.
 278  */
 279
 280 static void call_rcu_data_init(struct call_rcu_data **crdpp,
 281                                unsigned long flags,
 282                                int cpu_affinity)
 283 {
 284         struct call_rcu_data *crdp;
 285
 286         crdp = malloc(sizeof(*crdp));
 287         if (crdp == NULL) {
 288                 fprintf(stderr, "Out of memory.\n");
 289                 exit(-1);
 290         }
 291         memset(crdp, '\0', sizeof(*crdp));
 292         cds_wfq_init(&crdp->cbs);
 293         crdp->qlen = 0;
 294         crdp->futex = 0;
 295         crdp->flags = flags;
 296         cds_list_add(&crdp->list, &call_rcu_data_list);
 297         crdp->cpu_affinity = cpu_affinity;
 298         cmm_smp_mb();  /* Structure initialized before pointer is planted. */
 299         *crdpp = crdp;
 300         if (pthread_create(&crdp->tid, NULL, call_rcu_thread, crdp) != 0) {
 301                 perror("pthread_create");
 302                 exit(-1);
 303         }
 304 }
 305
 306 /*
 307  * Return a pointer to the call_rcu_data structure for the specified
 308  * CPU, returning NULL if there is none.  We cannot automatically
 309  * created it because the platform we are running on might not define
 310  * sched_getcpu().
 311  */
 312
 313 struct call_rcu_data *get_cpu_call_rcu_data(int cpu)
 314 {
 315         static int warned = 0;
 316
 317         if (per_cpu_call_rcu_data == NULL)
 318                 return NULL;
 319         if (!warned && maxcpus > 0 && (cpu < 0 || maxcpus <= cpu)) {
 320                 fprintf(stderr, "[error] liburcu: get CPU # out of range\n");
 321                 warned = 1;
 322         }
 323         if (cpu < 0 || maxcpus <= cpu)
 324                 return NULL;
 325         return per_cpu_call_rcu_data[cpu];
 326 }
 327
 328 /*
 329  * Return the tid corresponding to the call_rcu thread whose
 330  * call_rcu_data structure is specified.
 331  */
 332
 333 pthread_t get_call_rcu_thread(struct call_rcu_data *crdp)
 334 {
 335         return crdp->tid;
 336 }
 337
 338 /*
 339  * Create a call_rcu_data structure (with thread) and return a pointer.
 340  */
 341
 342 static struct call_rcu_data *__create_call_rcu_data(unsigned long flags,
 343                                                     int cpu_affinity)
 344 {
 345         struct call_rcu_data *crdp;
 346
 347         call_rcu_data_init(&crdp, flags, cpu_affinity);
 348         return crdp;
 349 }
 350
 351 struct call_rcu_data *create_call_rcu_data(unsigned long flags,
 352                                            int cpu_affinity)
 353 {
 354         struct call_rcu_data *crdp;
 355
 356         call_rcu_lock(&call_rcu_mutex);
 357         crdp = __create_call_rcu_data(flags, cpu_affinity);
 358         call_rcu_unlock(&call_rcu_mutex);
 359         return crdp;
 360 }
 361
 362 /*
 363  * Set the specified CPU to use the specified call_rcu_data structure.
 364  *
 365  * Use NULL to remove a CPU's call_rcu_data structure, but it is
 366  * the caller's responsibility to dispose of the removed structure.
 367  * Use get_cpu_call_rcu_data() to obtain a pointer to the old structure
 368  * (prior to NULLing it out, of course).
 369  */
 370
 371 int set_cpu_call_rcu_data(int cpu, struct call_rcu_data *crdp)
 372 {
 373         int warned = 0;
 374
 375         call_rcu_lock(&call_rcu_mutex);
 376         if (cpu < 0 || maxcpus <= cpu) {
 377                 if (!warned) {
 378                         fprintf(stderr, "[error] liburcu: set CPU # out of range\n");
 379                         warned = 1;
 380                 }
 381                 call_rcu_unlock(&call_rcu_mutex);
 382                 errno = EINVAL;
 383                 return -EINVAL;
 384         }
 385         alloc_cpu_call_rcu_data();
 386         call_rcu_unlock(&call_rcu_mutex);
 387         if (per_cpu_call_rcu_data == NULL) {
 388                 errno = ENOMEM;
 389                 return -ENOMEM;
 390         }
 391         per_cpu_call_rcu_data[cpu] = crdp;
 392         return 0;
 393 }
 394
 395 /*
 396  * Return a pointer to the default call_rcu_data structure, creating
 397  * one if need be.  Because we never free call_rcu_data structures,
 398  * we don't need to be in an RCU read-side critical section.
 399  */
 400
 401 struct call_rcu_data *get_default_call_rcu_data(void)
 402 {
 403         if (default_call_rcu_data != NULL)
 404                 return rcu_dereference(default_call_rcu_data);
 405         call_rcu_lock(&call_rcu_mutex);
 406         if (default_call_rcu_data != NULL) {
 407                 call_rcu_unlock(&call_rcu_mutex);
 408                 return default_call_rcu_data;
 409         }
 410         call_rcu_data_init(&default_call_rcu_data, 0, -1);
 411         call_rcu_unlock(&call_rcu_mutex);
 412         return default_call_rcu_data;
 413 }
 414
 415 /*
 416  * Return the call_rcu_data structure that applies to the currently
 417  * running thread.  Any call_rcu_data structure assigned specifically
 418  * to this thread has first priority, followed by any call_rcu_data
 419  * structure assigned to the CPU on which the thread is running,
 420  * followed by the default call_rcu_data structure.  If there is not
 421  * yet a default call_rcu_data structure, one will be created.
 422  */
 423 struct call_rcu_data *get_call_rcu_data(void)
 424 {
 425         int curcpu;
 426         static int warned = 0;
 427
 428         if (thread_call_rcu_data != NULL)
 429                 return thread_call_rcu_data;
 430         if (maxcpus <= 0)
 431                 return get_default_call_rcu_data();
 432         curcpu = sched_getcpu();
 433         if (!warned && (curcpu < 0 || maxcpus <= curcpu)) {
 434                 fprintf(stderr, "[error] liburcu: gcrd CPU # out of range\n");
 435                 warned = 1;
 436         }
 437         if (curcpu >= 0 && maxcpus > curcpu &&
 438             per_cpu_call_rcu_data != NULL &&
 439             per_cpu_call_rcu_data[curcpu] != NULL)
 440                 return per_cpu_call_rcu_data[curcpu];
 441         return get_default_call_rcu_data();
 442 }
 443
 444 /*
 445  * Return a pointer to this task's call_rcu_data if there is one.
 446  */
 447
 448 struct call_rcu_data *get_thread_call_rcu_data(void)
 449 {
 450         return thread_call_rcu_data;
 451 }
 452
 453 /*
 454  * Set this task's call_rcu_data structure as specified, regardless
 455  * of whether or not this task already had one.  (This allows switching
 456  * to and from real-time call_rcu threads, for example.)
 457  *
 458  * Use NULL to remove a thread's call_rcu_data structure, but it is
 459  * the caller's responsibility to dispose of the removed structure.
 460  * Use get_thread_call_rcu_data() to obtain a pointer to the old structure
 461  * (prior to NULLing it out, of course).
 462  */
 463
 464 void set_thread_call_rcu_data(struct call_rcu_data *crdp)
 465 {
 466         thread_call_rcu_data = crdp;
 467 }
 468
 469 /*
 470  * Create a separate call_rcu thread for each CPU.  This does not
 471  * replace a pre-existing call_rcu thread -- use the set_cpu_call_rcu_data()
 472  * function if you want that behavior.
 473  */
 474
 475 int create_all_cpu_call_rcu_data(unsigned long flags)
 476 {
 477         int i;
 478         struct call_rcu_data *crdp;
 479         int ret;
 480
 481         call_rcu_lock(&call_rcu_mutex);
 482         alloc_cpu_call_rcu_data();
 483         call_rcu_unlock(&call_rcu_mutex);
 484         if (maxcpus <= 0) {
 485                 errno = EINVAL;
 486                 return -EINVAL;
 487         }
 488         if (per_cpu_call_rcu_data == NULL) {
 489                 errno = ENOMEM;
 490                 return -ENOMEM;
 491         }
 492         for (i = 0; i < maxcpus; i++) {
 493                 call_rcu_lock(&call_rcu_mutex);
 494                 if (get_cpu_call_rcu_data(i)) {
 495                         call_rcu_unlock(&call_rcu_mutex);
 496                         continue;
 497                 }
 498                 crdp = __create_call_rcu_data(flags, i);
 499                 if (crdp == NULL) {
 500                         call_rcu_unlock(&call_rcu_mutex);
 501                         errno = ENOMEM;
 502                         return -ENOMEM;
 503                 }
 504                 call_rcu_unlock(&call_rcu_mutex);
 505                 if ((ret = set_cpu_call_rcu_data(i, crdp)) != 0) {
 506                         /* FIXME: Leaks crdp for now. */
 507                         return ret; /* Can happen on race. */
 508                 }
 509         }
 510         return 0;
 511 }
 512
 513 /*
 514  * Wake up the call_rcu thread corresponding to the specified
 515  * call_rcu_data structure.
 516  */
 517 static void wake_call_rcu_thread(struct call_rcu_data *crdp)
 518 {
 519         if (!(_CMM_LOAD_SHARED(crdp->flags) & URCU_CALL_RCU_RT))
 520                 call_rcu_wake_up(crdp);
 521 }
 522
 523 /*
 524  * Schedule a function to be invoked after a following grace period.
 525  * This is the only function that must be called -- the others are
 526  * only present to allow applications to tune their use of RCU for
 527  * maximum performance.
 528  *
 529  * Note that unless a call_rcu thread has not already been created,
 530  * the first invocation of call_rcu() will create one.  So, if you
 531  * need the first invocation of call_rcu() to be fast, make sure
 532  * to create a call_rcu thread first.  One way to accomplish this is
 533  * "get_call_rcu_data();", and another is create_all_cpu_call_rcu_data().
 534  */
 535
 536 void call_rcu(struct rcu_head *head,
 537               void (*func)(struct rcu_head *head))
 538 {
 539         struct call_rcu_data *crdp;
 540
 541         cds_wfq_node_init(&head->next);
 542         head->func = func;
 543         crdp = get_call_rcu_data();
 544         cds_wfq_enqueue(&crdp->cbs, &head->next);
 545         uatomic_inc(&crdp->qlen);
 546         wake_call_rcu_thread(crdp);
 547 }
 548
 549 /*
 550  * Free up the specified call_rcu_data structure, terminating the
 551  * associated call_rcu thread.  The caller must have previously
 552  * removed the call_rcu_data structure from per-thread or per-CPU
 553  * usage.  For example, set_cpu_call_rcu_data(cpu, NULL) for per-CPU
 554  * call_rcu_data structures or set_thread_call_rcu_data(NULL) for
 555  * per-thread call_rcu_data structures.
 556  *
 557  * We silently refuse to free up the default call_rcu_data structure
 558  * because that is where we put any leftover callbacks.  Note that
 559  * the possibility of self-spawning callbacks makes it impossible
 560  * to execute all the callbacks in finite time without putting any
 561  * newly spawned callbacks somewhere else.  The "somewhere else" of
 562  * last resort is the default call_rcu_data structure.
 563  *
 564  * We also silently refuse to free NULL pointers.  This simplifies
 565  * the calling code.
 566  */
 567 void call_rcu_data_free(struct call_rcu_data *crdp)
 568 {
 569         struct cds_wfq_node *cbs;
 570         struct cds_wfq_node **cbs_tail;
 571         struct cds_wfq_node **cbs_endprev;
 572
 573         if (crdp == NULL || crdp == default_call_rcu_data) {
 574                 return;
 575         }
 576         if ((uatomic_read(&crdp->flags) & URCU_CALL_RCU_STOPPED) == 0) {
 577                 uatomic_or(&crdp->flags, URCU_CALL_RCU_STOP);
 578                 wake_call_rcu_thread(crdp);
 579                 while ((uatomic_read(&crdp->flags) & URCU_CALL_RCU_STOPPED) == 0)
 580                         poll(NULL, 0, 1);
 581         }
 582         if (&crdp->cbs.head != _CMM_LOAD_SHARED(crdp->cbs.tail)) {
 583                 while ((cbs = _CMM_LOAD_SHARED(crdp->cbs.head)) == NULL)
 584                         poll(NULL, 0, 1);
 585                 _CMM_STORE_SHARED(crdp->cbs.head, NULL);
 586                 cbs_tail = (struct cds_wfq_node **)
 587                         uatomic_xchg(&crdp->cbs.tail, &crdp->cbs.head);
 588                 cbs_endprev = (struct cds_wfq_node **)
 589                         uatomic_xchg(&default_call_rcu_data, cbs_tail);
 590                 *cbs_endprev = cbs;
 591                 uatomic_add(&default_call_rcu_data->qlen,
 592                             uatomic_read(&crdp->qlen));
 593                 cds_list_del(&crdp->list);
 594                 free(crdp);
 595         }
 596 }
 597
 598 /*
 599  * Clean up all the per-CPU call_rcu threads.
 600  */
 601 void free_all_cpu_call_rcu_data(void)
 602 {
 603         int cpu;
 604         struct call_rcu_data *crdp;
 605
 606         if (maxcpus <= 0)
 607                 return;
 608         for (cpu = 0; cpu < maxcpus; cpu++) {
 609                 crdp = get_cpu_call_rcu_data(cpu);
 610                 if (crdp == NULL)
 611                         continue;
 612                 set_cpu_call_rcu_data(cpu, NULL);
 613                 call_rcu_data_free(crdp);
 614         }
 615 }
 616
 617 /*
 618  * Acquire the call_rcu_mutex in order to ensure that the child sees
 619  * all of the call_rcu() data structures in a consistent state.
 620  * Suitable for pthread_atfork() and friends.
 621  */
 622 void call_rcu_before_fork(void)
 623 {
 624         call_rcu_lock(&call_rcu_mutex);
 625 }
 626
 627 /*
 628  * Clean up call_rcu data structures in the parent of a successful fork()
 629  * that is not followed by exec() in the child.  Suitable for
 630  * pthread_atfork() and friends.
 631  */
 632 void call_rcu_after_fork_parent(void)
 633 {
 634         call_rcu_unlock(&call_rcu_mutex);
 635 }
 636
 637 /*
 638  * Clean up call_rcu data structures in the child of a successful fork()
 639  * that is not followed by exec().  Suitable for pthread_atfork() and
 640  * friends.
 641  */
 642 void call_rcu_after_fork_child(void)
 643 {
 644         struct call_rcu_data *crdp;
 645
 646         /* Release the mutex. */
 647         call_rcu_unlock(&call_rcu_mutex);
 648
 649         /*
 650          * Allocate a new default call_rcu_data structure in order
 651          * to get a working call_rcu thread to go with it.
 652          */
 653         default_call_rcu_data = NULL;
 654         (void)get_default_call_rcu_data();
 655
 656         /* Dispose of all of the rest of the call_rcu_data structures. */
 657         while (call_rcu_data_list.next != call_rcu_data_list.prev) {
 658                 crdp = cds_list_entry(call_rcu_data_list.prev,
 659                                       struct call_rcu_data, list);
 660                 if (crdp == default_call_rcu_data)
 661                         crdp = cds_list_entry(crdp->list.prev,
 662                                               struct call_rcu_data, list);
 663                 uatomic_set(&crdp->flags, URCU_CALL_RCU_STOPPED);
 664                 call_rcu_data_free(crdp);
 665         }
 666 }