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