linux/kernel/rcupdate.c
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   1/*
   2 * Read-Copy Update mechanism for mutual exclusion
   3 *
   4 * This program is free software; you can redistribute it and/or modify
   5 * it under the terms of the GNU General Public License as published by
   6 * the Free Software Foundation; either version 2 of the License, or
   7 * (at your option) any later version.
   8 *
   9 * This program is distributed in the hope that it will be useful,
  10 * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12 * GNU General Public License for more details.
  13 *
  14 * You should have received a copy of the GNU General Public License
  15 * along with this program; if not, write to the Free Software
  16 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  17 *
  18 * Copyright (C) IBM Corporation, 2001
  19 *
  20 * Authors: Dipankar Sarma <dipankar@in.ibm.com>
  21 *          Manfred Spraul <manfred@colorfullife.com>
  22 * 
  23 * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
  24 * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
  25 * Papers:
  26 * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
  27 * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
  28 *
  29 * For detailed explanation of Read-Copy Update mechanism see -
  30 *              http://lse.sourceforge.net/locking/rcupdate.html
  31 *
  32 */
  33#include <linux/types.h>
  34#include <linux/kernel.h>
  35#include <linux/init.h>
  36#include <linux/spinlock.h>
  37#include <linux/smp.h>
  38#include <linux/rcupdate.h>
  39#include <linux/interrupt.h>
  40#include <linux/sched.h>
  41#include <asm/atomic.h>
  42#include <linux/bitops.h>
  43#include <linux/module.h>
  44#include <linux/completion.h>
  45#include <linux/moduleparam.h>
  46#include <linux/percpu.h>
  47#include <linux/notifier.h>
  48#include <linux/rcupdate.h>
  49#include <linux/cpu.h>
  50#include <linux/mutex.h>
  51
  52/* Definition for rcupdate control block. */
  53static struct rcu_ctrlblk rcu_ctrlblk = {
  54        .cur = -300,
  55        .completed = -300,
  56        .lock = __SPIN_LOCK_UNLOCKED(&rcu_ctrlblk.lock),
  57        .cpumask = CPU_MASK_NONE,
  58};
  59static struct rcu_ctrlblk rcu_bh_ctrlblk = {
  60        .cur = -300,
  61        .completed = -300,
  62        .lock = __SPIN_LOCK_UNLOCKED(&rcu_bh_ctrlblk.lock),
  63        .cpumask = CPU_MASK_NONE,
  64};
  65
  66DEFINE_PER_CPU(struct rcu_data, rcu_data) = { 0L };
  67DEFINE_PER_CPU(struct rcu_data, rcu_bh_data) = { 0L };
  68
  69/* Fake initialization required by compiler */
  70static DEFINE_PER_CPU(struct tasklet_struct, rcu_tasklet) = {NULL};
  71static int blimit = 10;
  72static int qhimark = 10000;
  73static int qlowmark = 100;
  74
  75static atomic_t rcu_barrier_cpu_count;
  76static DEFINE_MUTEX(rcu_barrier_mutex);
  77static struct completion rcu_barrier_completion;
  78
  79#ifdef CONFIG_SMP
  80static void force_quiescent_state(struct rcu_data *rdp,
  81                        struct rcu_ctrlblk *rcp)
  82{
  83        int cpu;
  84        cpumask_t cpumask;
  85        set_need_resched();
  86        if (unlikely(!rcp->signaled)) {
  87                rcp->signaled = 1;
  88                /*
  89                 * Don't send IPI to itself. With irqs disabled,
  90                 * rdp->cpu is the current cpu.
  91                 */
  92                cpumask = rcp->cpumask;
  93                cpu_clear(rdp->cpu, cpumask);
  94                for_each_cpu_mask(cpu, cpumask)
  95                        smp_send_reschedule(cpu);
  96        }
  97}
  98#else
  99static inline void force_quiescent_state(struct rcu_data *rdp,
 100                        struct rcu_ctrlblk *rcp)
 101{
 102        set_need_resched();
 103}
 104#endif
 105
 106/**
 107 * call_rcu - Queue an RCU callback for invocation after a grace period.
 108 * @head: structure to be used for queueing the RCU updates.
 109 * @func: actual update function to be invoked after the grace period
 110 *
 111 * The update function will be invoked some time after a full grace
 112 * period elapses, in other words after all currently executing RCU
 113 * read-side critical sections have completed.  RCU read-side critical
 114 * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
 115 * and may be nested.
 116 */
 117void fastcall call_rcu(struct rcu_head *head,
 118                                void (*func)(struct rcu_head *rcu))
 119{
 120        unsigned long flags;
 121        struct rcu_data *rdp;
 122
 123        head->func = func;
 124        head->next = NULL;
 125        local_irq_save(flags);
 126        rdp = &__get_cpu_var(rcu_data);
 127        *rdp->nxttail = head;
 128        rdp->nxttail = &head->next;
 129        if (unlikely(++rdp->qlen > qhimark)) {
 130                rdp->blimit = INT_MAX;
 131                force_quiescent_state(rdp, &rcu_ctrlblk);
 132        }
 133        local_irq_restore(flags);
 134}
 135
 136/**
 137 * call_rcu_bh - Queue an RCU for invocation after a quicker grace period.
 138 * @head: structure to be used for queueing the RCU updates.
 139 * @func: actual update function to be invoked after the grace period
 140 *
 141 * The update function will be invoked some time after a full grace
 142 * period elapses, in other words after all currently executing RCU
 143 * read-side critical sections have completed. call_rcu_bh() assumes
 144 * that the read-side critical sections end on completion of a softirq
 145 * handler. This means that read-side critical sections in process
 146 * context must not be interrupted by softirqs. This interface is to be
 147 * used when most of the read-side critical sections are in softirq context.
 148 * RCU read-side critical sections are delimited by rcu_read_lock() and
 149 * rcu_read_unlock(), * if in interrupt context or rcu_read_lock_bh()
 150 * and rcu_read_unlock_bh(), if in process context. These may be nested.
 151 */
 152void fastcall call_rcu_bh(struct rcu_head *head,
 153                                void (*func)(struct rcu_head *rcu))
 154{
 155        unsigned long flags;
 156        struct rcu_data *rdp;
 157
 158        head->func = func;
 159        head->next = NULL;
 160        local_irq_save(flags);
 161        rdp = &__get_cpu_var(rcu_bh_data);
 162        *rdp->nxttail = head;
 163        rdp->nxttail = &head->next;
 164
 165        if (unlikely(++rdp->qlen > qhimark)) {
 166                rdp->blimit = INT_MAX;
 167                force_quiescent_state(rdp, &rcu_bh_ctrlblk);
 168        }
 169
 170        local_irq_restore(flags);
 171}
 172
 173/*
 174 * Return the number of RCU batches processed thus far.  Useful
 175 * for debug and statistics.
 176 */
 177long rcu_batches_completed(void)
 178{
 179        return rcu_ctrlblk.completed;
 180}
 181
 182/*
 183 * Return the number of RCU batches processed thus far.  Useful
 184 * for debug and statistics.
 185 */
 186long rcu_batches_completed_bh(void)
 187{
 188        return rcu_bh_ctrlblk.completed;
 189}
 190
 191static void rcu_barrier_callback(struct rcu_head *notused)
 192{
 193        if (atomic_dec_and_test(&rcu_barrier_cpu_count))
 194                complete(&rcu_barrier_completion);
 195}
 196
 197/*
 198 * Called with preemption disabled, and from cross-cpu IRQ context.
 199 */
 200static void rcu_barrier_func(void *notused)
 201{
 202        int cpu = smp_processor_id();
 203        struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
 204        struct rcu_head *head;
 205
 206        head = &rdp->barrier;
 207        atomic_inc(&rcu_barrier_cpu_count);
 208        call_rcu(head, rcu_barrier_callback);
 209}
 210
 211/**
 212 * rcu_barrier - Wait until all the in-flight RCUs are complete.
 213 */
 214void rcu_barrier(void)
 215{
 216        BUG_ON(in_interrupt());
 217        /* Take cpucontrol mutex to protect against CPU hotplug */
 218        mutex_lock(&rcu_barrier_mutex);
 219        init_completion(&rcu_barrier_completion);
 220        atomic_set(&rcu_barrier_cpu_count, 0);
 221        on_each_cpu(rcu_barrier_func, NULL, 0, 1);
 222        wait_for_completion(&rcu_barrier_completion);
 223        mutex_unlock(&rcu_barrier_mutex);
 224}
 225EXPORT_SYMBOL_GPL(rcu_barrier);
 226
 227/*
 228 * Invoke the completed RCU callbacks. They are expected to be in
 229 * a per-cpu list.
 230 */
 231static void rcu_do_batch(struct rcu_data *rdp)
 232{
 233        struct rcu_head *next, *list;
 234        int count = 0;
 235
 236        list = rdp->donelist;
 237        while (list) {
 238                next = rdp->donelist = list->next;
 239                list->func(list);
 240                list = next;
 241                if (++count >= rdp->blimit)
 242                        break;
 243        }
 244
 245        local_irq_disable();
 246        rdp->qlen -= count;
 247        local_irq_enable();
 248        if (rdp->blimit == INT_MAX && rdp->qlen <= qlowmark)
 249                rdp->blimit = blimit;
 250
 251        if (!rdp->donelist)
 252                rdp->donetail = &rdp->donelist;
 253        else
 254                tasklet_schedule(&per_cpu(rcu_tasklet, rdp->cpu));
 255}
 256
 257/*
 258 * Grace period handling:
 259 * The grace period handling consists out of two steps:
 260 * - A new grace period is started.
 261 *   This is done by rcu_start_batch. The start is not broadcasted to
 262 *   all cpus, they must pick this up by comparing rcp->cur with
 263 *   rdp->quiescbatch. All cpus are recorded  in the
 264 *   rcu_ctrlblk.cpumask bitmap.
 265 * - All cpus must go through a quiescent state.
 266 *   Since the start of the grace period is not broadcasted, at least two
 267 *   calls to rcu_check_quiescent_state are required:
 268 *   The first call just notices that a new grace period is running. The
 269 *   following calls check if there was a quiescent state since the beginning
 270 *   of the grace period. If so, it updates rcu_ctrlblk.cpumask. If
 271 *   the bitmap is empty, then the grace period is completed.
 272 *   rcu_check_quiescent_state calls rcu_start_batch(0) to start the next grace
 273 *   period (if necessary).
 274 */
 275/*
 276 * Register a new batch of callbacks, and start it up if there is currently no
 277 * active batch and the batch to be registered has not already occurred.
 278 * Caller must hold rcu_ctrlblk.lock.
 279 */
 280static void rcu_start_batch(struct rcu_ctrlblk *rcp)
 281{
 282        if (rcp->next_pending &&
 283                        rcp->completed == rcp->cur) {
 284                rcp->next_pending = 0;
 285                /*
 286                 * next_pending == 0 must be visible in
 287                 * __rcu_process_callbacks() before it can see new value of cur.
 288                 */
 289                smp_wmb();
 290                rcp->cur++;
 291
 292                /*
 293                 * Accessing nohz_cpu_mask before incrementing rcp->cur needs a
 294                 * Barrier  Otherwise it can cause tickless idle CPUs to be
 295                 * included in rcp->cpumask, which will extend graceperiods
 296                 * unnecessarily.
 297                 */
 298                smp_mb();
 299                cpus_andnot(rcp->cpumask, cpu_online_map, nohz_cpu_mask);
 300
 301                rcp->signaled = 0;
 302        }
 303}
 304
 305/*
 306 * cpu went through a quiescent state since the beginning of the grace period.
 307 * Clear it from the cpu mask and complete the grace period if it was the last
 308 * cpu. Start another grace period if someone has further entries pending
 309 */
 310static void cpu_quiet(int cpu, struct rcu_ctrlblk *rcp)
 311{
 312        cpu_clear(cpu, rcp->cpumask);
 313        if (cpus_empty(rcp->cpumask)) {
 314                /* batch completed ! */
 315                rcp->completed = rcp->cur;
 316                rcu_start_batch(rcp);
 317        }
 318}
 319
 320/*
 321 * Check if the cpu has gone through a quiescent state (say context
 322 * switch). If so and if it already hasn't done so in this RCU
 323 * quiescent cycle, then indicate that it has done so.
 324 */
 325static void rcu_check_quiescent_state(struct rcu_ctrlblk *rcp,
 326                                        struct rcu_data *rdp)
 327{
 328        if (rdp->quiescbatch != rcp->cur) {
 329                /* start new grace period: */
 330                rdp->qs_pending = 1;
 331                rdp->passed_quiesc = 0;
 332                rdp->quiescbatch = rcp->cur;
 333                return;
 334        }
 335
 336        /* Grace period already completed for this cpu?
 337         * qs_pending is checked instead of the actual bitmap to avoid
 338         * cacheline trashing.
 339         */
 340        if (!rdp->qs_pending)
 341                return;
 342
 343        /* 
 344         * Was there a quiescent state since the beginning of the grace
 345         * period? If no, then exit and wait for the next call.
 346         */
 347        if (!rdp->passed_quiesc)
 348                return;
 349        rdp->qs_pending = 0;
 350
 351        spin_lock(&rcp->lock);
 352        /*
 353         * rdp->quiescbatch/rcp->cur and the cpu bitmap can come out of sync
 354         * during cpu startup. Ignore the quiescent state.
 355         */
 356        if (likely(rdp->quiescbatch == rcp->cur))
 357                cpu_quiet(rdp->cpu, rcp);
 358
 359        spin_unlock(&rcp->lock);
 360}
 361
 362
 363#ifdef CONFIG_HOTPLUG_CPU
 364
 365/* warning! helper for rcu_offline_cpu. do not use elsewhere without reviewing
 366 * locking requirements, the list it's pulling from has to belong to a cpu
 367 * which is dead and hence not processing interrupts.
 368 */
 369static void rcu_move_batch(struct rcu_data *this_rdp, struct rcu_head *list,
 370                                struct rcu_head **tail)
 371{
 372        local_irq_disable();
 373        *this_rdp->nxttail = list;
 374        if (list)
 375                this_rdp->nxttail = tail;
 376        local_irq_enable();
 377}
 378
 379static void __rcu_offline_cpu(struct rcu_data *this_rdp,
 380                                struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
 381{
 382        /* if the cpu going offline owns the grace period
 383         * we can block indefinitely waiting for it, so flush
 384         * it here
 385         */
 386        spin_lock_bh(&rcp->lock);
 387        if (rcp->cur != rcp->completed)
 388                cpu_quiet(rdp->cpu, rcp);
 389        spin_unlock_bh(&rcp->lock);
 390        rcu_move_batch(this_rdp, rdp->curlist, rdp->curtail);
 391        rcu_move_batch(this_rdp, rdp->nxtlist, rdp->nxttail);
 392        rcu_move_batch(this_rdp, rdp->donelist, rdp->donetail);
 393}
 394
 395static void rcu_offline_cpu(int cpu)
 396{
 397        struct rcu_data *this_rdp = &get_cpu_var(rcu_data);
 398        struct rcu_data *this_bh_rdp = &get_cpu_var(rcu_bh_data);
 399
 400        __rcu_offline_cpu(this_rdp, &rcu_ctrlblk,
 401                                        &per_cpu(rcu_data, cpu));
 402        __rcu_offline_cpu(this_bh_rdp, &rcu_bh_ctrlblk,
 403                                        &per_cpu(rcu_bh_data, cpu));
 404        put_cpu_var(rcu_data);
 405        put_cpu_var(rcu_bh_data);
 406        tasklet_kill_immediate(&per_cpu(rcu_tasklet, cpu), cpu);
 407}
 408
 409#else
 410
 411static void rcu_offline_cpu(int cpu)
 412{
 413}
 414
 415#endif
 416
 417/*
 418 * This does the RCU processing work from tasklet context. 
 419 */
 420static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp,
 421                                        struct rcu_data *rdp)
 422{
 423        if (rdp->curlist && !rcu_batch_before(rcp->completed, rdp->batch)) {
 424                *rdp->donetail = rdp->curlist;
 425                rdp->donetail = rdp->curtail;
 426                rdp->curlist = NULL;
 427                rdp->curtail = &rdp->curlist;
 428        }
 429
 430        if (rdp->nxtlist && !rdp->curlist) {
 431                local_irq_disable();
 432                rdp->curlist = rdp->nxtlist;
 433                rdp->curtail = rdp->nxttail;
 434                rdp->nxtlist = NULL;
 435                rdp->nxttail = &rdp->nxtlist;
 436                local_irq_enable();
 437
 438                /*
 439                 * start the next batch of callbacks
 440                 */
 441
 442                /* determine batch number */
 443                rdp->batch = rcp->cur + 1;
 444                /* see the comment and corresponding wmb() in
 445                 * the rcu_start_batch()
 446                 */
 447                smp_rmb();
 448
 449                if (!rcp->next_pending) {
 450                        /* and start it/schedule start if it's a new batch */
 451                        spin_lock(&rcp->lock);
 452                        rcp->next_pending = 1;
 453                        rcu_start_batch(rcp);
 454                        spin_unlock(&rcp->lock);
 455                }
 456        }
 457
 458        rcu_check_quiescent_state(rcp, rdp);
 459        if (rdp->donelist)
 460                rcu_do_batch(rdp);
 461}
 462
 463static void rcu_process_callbacks(unsigned long unused)
 464{
 465        __rcu_process_callbacks(&rcu_ctrlblk, &__get_cpu_var(rcu_data));
 466        __rcu_process_callbacks(&rcu_bh_ctrlblk, &__get_cpu_var(rcu_bh_data));
 467}
 468
 469static int __rcu_pending(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
 470{
 471        /* This cpu has pending rcu entries and the grace period
 472         * for them has completed.
 473         */
 474        if (rdp->curlist && !rcu_batch_before(rcp->completed, rdp->batch))
 475                return 1;
 476
 477        /* This cpu has no pending entries, but there are new entries */
 478        if (!rdp->curlist && rdp->nxtlist)
 479                return 1;
 480
 481        /* This cpu has finished callbacks to invoke */
 482        if (rdp->donelist)
 483                return 1;
 484
 485        /* The rcu core waits for a quiescent state from the cpu */
 486        if (rdp->quiescbatch != rcp->cur || rdp->qs_pending)
 487                return 1;
 488
 489        /* nothing to do */
 490        return 0;
 491}
 492
 493/*
 494 * Check to see if there is any immediate RCU-related work to be done
 495 * by the current CPU, returning 1 if so.  This function is part of the
 496 * RCU implementation; it is -not- an exported member of the RCU API.
 497 */
 498int rcu_pending(int cpu)
 499{
 500        return __rcu_pending(&rcu_ctrlblk, &per_cpu(rcu_data, cpu)) ||
 501                __rcu_pending(&rcu_bh_ctrlblk, &per_cpu(rcu_bh_data, cpu));
 502}
 503
 504/*
 505 * Check to see if any future RCU-related work will need to be done
 506 * by the current CPU, even if none need be done immediately, returning
 507 * 1 if so.  This function is part of the RCU implementation; it is -not-
 508 * an exported member of the RCU API.
 509 */
 510int rcu_needs_cpu(int cpu)
 511{
 512        struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
 513        struct rcu_data *rdp_bh = &per_cpu(rcu_bh_data, cpu);
 514
 515        return (!!rdp->curlist || !!rdp_bh->curlist || rcu_pending(cpu));
 516}
 517
 518void rcu_check_callbacks(int cpu, int user)
 519{
 520        if (user || 
 521            (idle_cpu(cpu) && !in_softirq() && 
 522                                hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
 523                rcu_qsctr_inc(cpu);
 524                rcu_bh_qsctr_inc(cpu);
 525        } else if (!in_softirq())
 526                rcu_bh_qsctr_inc(cpu);
 527        tasklet_schedule(&per_cpu(rcu_tasklet, cpu));
 528}
 529
 530static void rcu_init_percpu_data(int cpu, struct rcu_ctrlblk *rcp,
 531                                                struct rcu_data *rdp)
 532{
 533        memset(rdp, 0, sizeof(*rdp));
 534        rdp->curtail = &rdp->curlist;
 535        rdp->nxttail = &rdp->nxtlist;
 536        rdp->donetail = &rdp->donelist;
 537        rdp->quiescbatch = rcp->completed;
 538        rdp->qs_pending = 0;
 539        rdp->cpu = cpu;
 540        rdp->blimit = blimit;
 541}
 542
 543static void __devinit rcu_online_cpu(int cpu)
 544{
 545        struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
 546        struct rcu_data *bh_rdp = &per_cpu(rcu_bh_data, cpu);
 547
 548        rcu_init_percpu_data(cpu, &rcu_ctrlblk, rdp);
 549        rcu_init_percpu_data(cpu, &rcu_bh_ctrlblk, bh_rdp);
 550        tasklet_init(&per_cpu(rcu_tasklet, cpu), rcu_process_callbacks, 0UL);
 551}
 552
 553static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
 554                                unsigned long action, void *hcpu)
 555{
 556        long cpu = (long)hcpu;
 557        switch (action) {
 558        case CPU_UP_PREPARE:
 559                rcu_online_cpu(cpu);
 560                break;
 561        case CPU_DEAD:
 562                rcu_offline_cpu(cpu);
 563                break;
 564        default:
 565                break;
 566        }
 567        return NOTIFY_OK;
 568}
 569
 570static struct notifier_block __cpuinitdata rcu_nb = {
 571        .notifier_call  = rcu_cpu_notify,
 572};
 573
 574/*
 575 * Initializes rcu mechanism.  Assumed to be called early.
 576 * That is before local timer(SMP) or jiffie timer (uniproc) is setup.
 577 * Note that rcu_qsctr and friends are implicitly
 578 * initialized due to the choice of ``0'' for RCU_CTR_INVALID.
 579 */
 580void __init rcu_init(void)
 581{
 582        rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE,
 583                        (void *)(long)smp_processor_id());
 584        /* Register notifier for non-boot CPUs */
 585        register_cpu_notifier(&rcu_nb);
 586}
 587
 588struct rcu_synchronize {
 589        struct rcu_head head;
 590        struct completion completion;
 591};
 592
 593/* Because of FASTCALL declaration of complete, we use this wrapper */
 594static void wakeme_after_rcu(struct rcu_head  *head)
 595{
 596        struct rcu_synchronize *rcu;
 597
 598        rcu = container_of(head, struct rcu_synchronize, head);
 599        complete(&rcu->completion);
 600}
 601
 602/**
 603 * synchronize_rcu - wait until a grace period has elapsed.
 604 *
 605 * Control will return to the caller some time after a full grace
 606 * period has elapsed, in other words after all currently executing RCU
 607 * read-side critical sections have completed.  RCU read-side critical
 608 * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
 609 * and may be nested.
 610 *
 611 * If your read-side code is not protected by rcu_read_lock(), do -not-
 612 * use synchronize_rcu().
 613 */
 614void synchronize_rcu(void)
 615{
 616        struct rcu_synchronize rcu;
 617
 618        init_completion(&rcu.completion);
 619        /* Will wake me after RCU finished */
 620        call_rcu(&rcu.head, wakeme_after_rcu);
 621
 622        /* Wait for it */
 623        wait_for_completion(&rcu.completion);
 624}
 625
 626module_param(blimit, int, 0);
 627module_param(qhimark, int, 0);
 628module_param(qlowmark, int, 0);
 629EXPORT_SYMBOL_GPL(rcu_batches_completed);
 630EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);
 631EXPORT_SYMBOL_GPL(call_rcu);
 632EXPORT_SYMBOL_GPL(call_rcu_bh);
 633EXPORT_SYMBOL_GPL(synchronize_rcu);
 634