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 = list->next;
 239                 prefetch(next);
 240                 list->func(list);
 241                 list = next;
 242                 if (++count >= rdp->blimit)
 243                         break;
 244         }
 245         rdp->donelist = list;
 246
 247         local_irq_disable();
 248         rdp->qlen -= count;
 249         local_irq_enable();
 250         if (rdp->blimit == INT_MAX && rdp->qlen <= qlowmark)
 251                 rdp->blimit = blimit;
 252
 253         if (!rdp->donelist)
 254                 rdp->donetail = &rdp->donelist;
 255         else
 256                 tasklet_schedule(&per_cpu(rcu_tasklet, rdp->cpu));
 257 }
 258
 259 /*
 260  * Grace period handling:
 261  * The grace period handling consists out of two steps:
 262  * - A new grace period is started.
 263  *   This is done by rcu_start_batch. The start is not broadcasted to
 264  *   all cpus, they must pick this up by comparing rcp->cur with
 265  *   rdp->quiescbatch. All cpus are recorded  in the
 266  *   rcu_ctrlblk.cpumask bitmap.
 267  * - All cpus must go through a quiescent state.
 268  *   Since the start of the grace period is not broadcasted, at least two
 269  *   calls to rcu_check_quiescent_state are required:
 270  *   The first call just notices that a new grace period is running. The
 271  *   following calls check if there was a quiescent state since the beginning
 272  *   of the grace period. If so, it updates rcu_ctrlblk.cpumask. If
 273  *   the bitmap is empty, then the grace period is completed.
 274  *   rcu_check_quiescent_state calls rcu_start_batch(0) to start the next grace
 275  *   period (if necessary).
 276  */
 277 /*
 278  * Register a new batch of callbacks, and start it up if there is currently no
 279  * active batch and the batch to be registered has not already occurred.
 280  * Caller must hold rcu_ctrlblk.lock.
 281  */
 282 static void rcu_start_batch(struct rcu_ctrlblk *rcp)
 283 {
 284         if (rcp->next_pending &&
 285                         rcp->completed == rcp->cur) {
 286                 rcp->next_pending = 0;
 287                 /*
 288                  * next_pending == 0 must be visible in
 289                  * __rcu_process_callbacks() before it can see new value of cur.
 290                  */
 291                 smp_wmb();
 292                 rcp->cur++;
 293
 294                 /*
 295                  * Accessing nohz_cpu_mask before incrementing rcp->cur needs a
 296                  * Barrier  Otherwise it can cause tickless idle CPUs to be
 297                  * included in rcp->cpumask, which will extend graceperiods
 298                  * unnecessarily.
 299                  */
 300                 smp_mb();
 301                 cpus_andnot(rcp->cpumask, cpu_online_map, nohz_cpu_mask);
 302
 303                 rcp->signaled = 0;
 304         }
 305 }
 306
 307 /*
 308  * cpu went through a quiescent state since the beginning of the grace period.
 309  * Clear it from the cpu mask and complete the grace period if it was the last
 310  * cpu. Start another grace period if someone has further entries pending
 311  */
 312 static void cpu_quiet(int cpu, struct rcu_ctrlblk *rcp)
 313 {
 314         cpu_clear(cpu, rcp->cpumask);
 315         if (cpus_empty(rcp->cpumask)) {
 316                 /* batch completed ! */
 317                 rcp->completed = rcp->cur;
 318                 rcu_start_batch(rcp);
 319         }
 320 }
 321
 322 /*
 323  * Check if the cpu has gone through a quiescent state (say context
 324  * switch). If so and if it already hasn't done so in this RCU
 325  * quiescent cycle, then indicate that it has done so.
 326  */
 327 static void rcu_check_quiescent_state(struct rcu_ctrlblk *rcp,
 328                                         struct rcu_data *rdp)
 329 {
 330         if (rdp->quiescbatch != rcp->cur) {
 331                 /* start new grace period: */
 332                 rdp->qs_pending = 1;
 333                 rdp->passed_quiesc = 0;
 334                 rdp->quiescbatch = rcp->cur;
 335                 return;
 336         }
 337
 338         /* Grace period already completed for this cpu?
 339          * qs_pending is checked instead of the actual bitmap to avoid
 340          * cacheline trashing.
 341          */
 342         if (!rdp->qs_pending)
 343                 return;
 344
 345         /*
 346          * Was there a quiescent state since the beginning of the grace
 347          * period? If no, then exit and wait for the next call.
 348          */
 349         if (!rdp->passed_quiesc)
 350                 return;
 351         rdp->qs_pending = 0;
 352
 353         spin_lock(&rcp->lock);
 354         /*
 355          * rdp->quiescbatch/rcp->cur and the cpu bitmap can come out of sync
 356          * during cpu startup. Ignore the quiescent state.
 357          */
 358         if (likely(rdp->quiescbatch == rcp->cur))
 359                 cpu_quiet(rdp->cpu, rcp);
 360
 361         spin_unlock(&rcp->lock);
 362 }
 363
 364
 365 #ifdef CONFIG_HOTPLUG_CPU
 366
 367 /* warning! helper for rcu_offline_cpu. do not use elsewhere without reviewing
 368  * locking requirements, the list it's pulling from has to belong to a cpu
 369  * which is dead and hence not processing interrupts.
 370  */
 371 static void rcu_move_batch(struct rcu_data *this_rdp, struct rcu_head *list,
 372                                 struct rcu_head **tail)
 373 {
 374         local_irq_disable();
 375         *this_rdp->nxttail = list;
 376         if (list)
 377                 this_rdp->nxttail = tail;
 378         local_irq_enable();
 379 }
 380
 381 static void __rcu_offline_cpu(struct rcu_data *this_rdp,
 382                                 struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
 383 {
 384         /* if the cpu going offline owns the grace period
 385          * we can block indefinitely waiting for it, so flush
 386          * it here
 387          */
 388         spin_lock_bh(&rcp->lock);
 389         if (rcp->cur != rcp->completed)
 390                 cpu_quiet(rdp->cpu, rcp);
 391         spin_unlock_bh(&rcp->lock);
 392         rcu_move_batch(this_rdp, rdp->curlist, rdp->curtail);
 393         rcu_move_batch(this_rdp, rdp->nxtlist, rdp->nxttail);
 394         rcu_move_batch(this_rdp, rdp->donelist, rdp->donetail);
 395 }
 396
 397 static void rcu_offline_cpu(int cpu)
 398 {
 399         struct rcu_data *this_rdp = &get_cpu_var(rcu_data);
 400         struct rcu_data *this_bh_rdp = &get_cpu_var(rcu_bh_data);
 401
 402         __rcu_offline_cpu(this_rdp, &rcu_ctrlblk,
 403                                         &per_cpu(rcu_data, cpu));
 404         __rcu_offline_cpu(this_bh_rdp, &rcu_bh_ctrlblk,
 405                                         &per_cpu(rcu_bh_data, cpu));
 406         put_cpu_var(rcu_data);
 407         put_cpu_var(rcu_bh_data);
 408         tasklet_kill_immediate(&per_cpu(rcu_tasklet, cpu), cpu);
 409 }
 410
 411 #else
 412
 413 static void rcu_offline_cpu(int cpu)
 414 {
 415 }
 416
 417 #endif
 418
 419 /*
 420  * This does the RCU processing work from tasklet context.
 421  */
 422 static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp,
 423                                         struct rcu_data *rdp)
 424 {
 425         if (rdp->curlist && !rcu_batch_before(rcp->completed, rdp->batch)) {
 426                 *rdp->donetail = rdp->curlist;
 427                 rdp->donetail = rdp->curtail;
 428                 rdp->curlist = NULL;
 429                 rdp->curtail = &rdp->curlist;
 430         }
 431
 432         if (rdp->nxtlist && !rdp->curlist) {
 433                 local_irq_disable();
 434                 rdp->curlist = rdp->nxtlist;
 435                 rdp->curtail = rdp->nxttail;
 436                 rdp->nxtlist = NULL;
 437                 rdp->nxttail = &rdp->nxtlist;
 438                 local_irq_enable();
 439
 440                 /*
 441                  * start the next batch of callbacks
 442                  */
 443
 444                 /* determine batch number */
 445                 rdp->batch = rcp->cur + 1;
 446                 /* see the comment and corresponding wmb() in
 447                  * the rcu_start_batch()
 448                  */
 449                 smp_rmb();
 450
 451                 if (!rcp->next_pending) {
 452                         /* and start it/schedule start if it's a new batch */
 453                         spin_lock(&rcp->lock);
 454                         rcp->next_pending = 1;
 455                         rcu_start_batch(rcp);
 456                         spin_unlock(&rcp->lock);
 457                 }
 458         }
 459
 460         rcu_check_quiescent_state(rcp, rdp);
 461         if (rdp->donelist)
 462                 rcu_do_batch(rdp);
 463 }
 464
 465 static void rcu_process_callbacks(unsigned long unused)
 466 {
 467         __rcu_process_callbacks(&rcu_ctrlblk, &__get_cpu_var(rcu_data));
 468         __rcu_process_callbacks(&rcu_bh_ctrlblk, &__get_cpu_var(rcu_bh_data));
 469 }
 470
 471 static int __rcu_pending(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
 472 {
 473         /* This cpu has pending rcu entries and the grace period
 474          * for them has completed.
 475          */
 476         if (rdp->curlist && !rcu_batch_before(rcp->completed, rdp->batch))
 477                 return 1;
 478
 479         /* This cpu has no pending entries, but there are new entries */
 480         if (!rdp->curlist && rdp->nxtlist)
 481                 return 1;
 482
 483         /* This cpu has finished callbacks to invoke */
 484         if (rdp->donelist)
 485                 return 1;
 486
 487         /* The rcu core waits for a quiescent state from the cpu */
 488         if (rdp->quiescbatch != rcp->cur || rdp->qs_pending)
 489                 return 1;
 490
 491         /* nothing to do */
 492         return 0;
 493 }
 494
 495 /*
 496  * Check to see if there is any immediate RCU-related work to be done
 497  * by the current CPU, returning 1 if so.  This function is part of the
 498  * RCU implementation; it is -not- an exported member of the RCU API.
 499  */
 500 int rcu_pending(int cpu)
 501 {
 502         return __rcu_pending(&rcu_ctrlblk, &per_cpu(rcu_data, cpu)) ||
 503                 __rcu_pending(&rcu_bh_ctrlblk, &per_cpu(rcu_bh_data, cpu));
 504 }
 505
 506 /*
 507  * Check to see if any future RCU-related work will need to be done
 508  * by the current CPU, even if none need be done immediately, returning
 509  * 1 if so.  This function is part of the RCU implementation; it is -not-
 510  * an exported member of the RCU API.
 511  */
 512 int rcu_needs_cpu(int cpu)
 513 {
 514         struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
 515         struct rcu_data *rdp_bh = &per_cpu(rcu_bh_data, cpu);
 516
 517         return (!!rdp->curlist || !!rdp_bh->curlist || rcu_pending(cpu));
 518 }
 519
 520 void rcu_check_callbacks(int cpu, int user)
 521 {
 522         if (user ||
 523             (idle_cpu(cpu) && !in_softirq() &&
 524                                 hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
 525                 rcu_qsctr_inc(cpu);
 526                 rcu_bh_qsctr_inc(cpu);
 527         } else if (!in_softirq())
 528                 rcu_bh_qsctr_inc(cpu);
 529         tasklet_schedule(&per_cpu(rcu_tasklet, cpu));
 530 }
 531
 532 static void rcu_init_percpu_data(int cpu, struct rcu_ctrlblk *rcp,
 533                                                 struct rcu_data *rdp)
 534 {
 535         memset(rdp, 0, sizeof(*rdp));
 536         rdp->curtail = &rdp->curlist;
 537         rdp->nxttail = &rdp->nxtlist;
 538         rdp->donetail = &rdp->donelist;
 539         rdp->quiescbatch = rcp->completed;
 540         rdp->qs_pending = 0;
 541         rdp->cpu = cpu;
 542         rdp->blimit = blimit;
 543 }
 544
 545 static void __devinit rcu_online_cpu(int cpu)
 546 {
 547         struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
 548         struct rcu_data *bh_rdp = &per_cpu(rcu_bh_data, cpu);
 549
 550         rcu_init_percpu_data(cpu, &rcu_ctrlblk, rdp);
 551         rcu_init_percpu_data(cpu, &rcu_bh_ctrlblk, bh_rdp);
 552         tasklet_init(&per_cpu(rcu_tasklet, cpu), rcu_process_callbacks, 0UL);
 553 }
 554
 555 static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
 556                                 unsigned long action, void *hcpu)
 557 {
 558         long cpu = (long)hcpu;
 559         switch (action) {
 560         case CPU_UP_PREPARE:
 561         case CPU_UP_PREPARE_FROZEN:
 562                 rcu_online_cpu(cpu);
 563                 break;
 564         case CPU_DEAD:
 565         case CPU_DEAD_FROZEN:
 566                 rcu_offline_cpu(cpu);
 567                 break;
 568         default:
 569                 break;
 570         }
 571         return NOTIFY_OK;
 572 }
 573
 574 static struct notifier_block __cpuinitdata rcu_nb = {
 575         .notifier_call  = rcu_cpu_notify,
 576 };
 577
 578 /*
 579  * Initializes rcu mechanism.  Assumed to be called early.
 580  * That is before local timer(SMP) or jiffie timer (uniproc) is setup.
 581  * Note that rcu_qsctr and friends are implicitly
 582  * initialized due to the choice of ``0'' for RCU_CTR_INVALID.
 583  */
 584 void __init rcu_init(void)
 585 {
 586         rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE,
 587                         (void *)(long)smp_processor_id());
 588         /* Register notifier for non-boot CPUs */
 589         register_cpu_notifier(&rcu_nb);
 590 }
 591
 592 struct rcu_synchronize {
 593         struct rcu_head head;
 594         struct completion completion;
 595 };
 596
 597 /* Because of FASTCALL declaration of complete, we use this wrapper */
 598 static void wakeme_after_rcu(struct rcu_head  *head)
 599 {
 600         struct rcu_synchronize *rcu;
 601
 602         rcu = container_of(head, struct rcu_synchronize, head);
 603         complete(&rcu->completion);
 604 }
 605
 606 /**
 607  * synchronize_rcu - wait until a grace period has elapsed.
 608  *
 609  * Control will return to the caller some time after a full grace
 610  * period has elapsed, in other words after all currently executing RCU
 611  * read-side critical sections have completed.  RCU read-side critical
 612  * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
 613  * and may be nested.
 614  *
 615  * If your read-side code is not protected by rcu_read_lock(), do -not-
 616  * use synchronize_rcu().
 617  */
 618 void synchronize_rcu(void)
 619 {
 620         struct rcu_synchronize rcu;
 621
 622         init_completion(&rcu.completion);
 623         /* Will wake me after RCU finished */
 624         call_rcu(&rcu.head, wakeme_after_rcu);
 625
 626         /* Wait for it */
 627         wait_for_completion(&rcu.completion);
 628 }
 629
 630 module_param(blimit, int, 0);
 631 module_param(qhimark, int, 0);
 632 module_param(qlowmark, int, 0);
 633 EXPORT_SYMBOL_GPL(rcu_batches_completed);
 634 EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);
 635 EXPORT_SYMBOL_GPL(call_rcu);
 636 EXPORT_SYMBOL_GPL(call_rcu_bh);
 637 EXPORT_SYMBOL_GPL(synchronize_rcu);