kernel/rcupdate.c

   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. */
  53 static 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 };
  59 static 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
  66 DEFINE_PER_CPU(struct rcu_data, rcu_data) = { 0L };
  67 DEFINE_PER_CPU(struct rcu_data, rcu_bh_data) = { 0L };
  68
  69 /* Fake initialization required by compiler */
  70 static DEFINE_PER_CPU(struct tasklet_struct, rcu_tasklet) = {NULL};
  71 static int blimit = 10;
  72 static int qhimark = 10000;
  73 static int qlowmark = 100;
  74
  75 static atomic_t rcu_barrier_cpu_count;
  76 static DEFINE_MUTEX(rcu_barrier_mutex);
  77 static struct completion rcu_barrier_completion;
  78
  79 #ifdef CONFIG_SMP
  80 static 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
  99 static 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  */
 117 void 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  */
 152 void 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  */
 177 long 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  */
 186 long rcu_batches_completed_bh(void)
 187 {
 188         return rcu_bh_ctrlblk.completed;
 189 }
 190
 191 static 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  */
 200 static 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  */
 214 void 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 }
 225 EXPORT_SYMBOL_GPL(rcu_barrier);
 226
 227 /*
 228  * Invoke the completed RCU callbacks. They are expected to be in
 229  * a per-cpu list.
 230  */
 231 static 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  */
 280 static 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  */
 310 static 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  */
 325 static 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  */
 369 static 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
 379 static 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
 395 static 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
 411 static void rcu_offline_cpu(int cpu)
 412 {
 413 }
 414
 415 #endif
 416
 417 /*
 418  * This does the RCU processing work from tasklet context.
 419  */
 420 static 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
 463 static 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
 469 static 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  */
 498 int 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  */
 510 int 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
 518 void 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
 530 static 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
 543 static 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
 553 static 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
 570 static 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  */
 580 void __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
 588 struct rcu_synchronize {
 589         struct rcu_head head;
 590         struct completion completion;
 591 };
 592
 593 /* Because of FASTCALL declaration of complete, we use this wrapper */
 594 static 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  */
 614 void 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
 626 module_param(blimit, int, 0);
 627 module_param(qhimark, int, 0);
 628 module_param(qlowmark, int, 0);
 629 EXPORT_SYMBOL_GPL(rcu_batches_completed);
 630 EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);
 631 EXPORT_SYMBOL_GPL(call_rcu);
 632 EXPORT_SYMBOL_GPL(call_rcu_bh);
 633 EXPORT_SYMBOL_GPL(synchronize_rcu);