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/cpu.h>
  49 #include <linux/mutex.h>
  50
  51 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  52 static struct lock_class_key rcu_lock_key;
  53 struct lockdep_map rcu_lock_map =
  54         STATIC_LOCKDEP_MAP_INIT("rcu_read_lock", &rcu_lock_key);
  55
  56 EXPORT_SYMBOL_GPL(rcu_lock_map);
  57 #endif
  58
  59 /* Definition for rcupdate control block. */
  60 static struct rcu_ctrlblk rcu_ctrlblk = {
  61         .cur = -300,
  62         .completed = -300,
  63         .lock = __SPIN_LOCK_UNLOCKED(&rcu_ctrlblk.lock),
  64         .cpumask = CPU_MASK_NONE,
  65 };
  66 static struct rcu_ctrlblk rcu_bh_ctrlblk = {
  67         .cur = -300,
  68         .completed = -300,
  69         .lock = __SPIN_LOCK_UNLOCKED(&rcu_bh_ctrlblk.lock),
  70         .cpumask = CPU_MASK_NONE,
  71 };
  72
  73 DEFINE_PER_CPU(struct rcu_data, rcu_data) = { 0L };
  74 DEFINE_PER_CPU(struct rcu_data, rcu_bh_data) = { 0L };
  75
  76 /* Fake initialization required by compiler */
  77 static DEFINE_PER_CPU(struct tasklet_struct, rcu_tasklet) = {NULL};
  78 static int blimit = 10;
  79 static int qhimark = 10000;
  80 static int qlowmark = 100;
  81
  82 static atomic_t rcu_barrier_cpu_count;
  83 static DEFINE_MUTEX(rcu_barrier_mutex);
  84 static struct completion rcu_barrier_completion;
  85
  86 #ifdef CONFIG_SMP
  87 static void force_quiescent_state(struct rcu_data *rdp,
  88                         struct rcu_ctrlblk *rcp)
  89 {
  90         int cpu;
  91         cpumask_t cpumask;
  92         set_need_resched();
  93         if (unlikely(!rcp->signaled)) {
  94                 rcp->signaled = 1;
  95                 /*
  96                  * Don't send IPI to itself. With irqs disabled,
  97                  * rdp->cpu is the current cpu.
  98                  */
  99                 cpumask = rcp->cpumask;
 100                 cpu_clear(rdp->cpu, cpumask);
 101                 for_each_cpu_mask(cpu, cpumask)
 102                         smp_send_reschedule(cpu);
 103         }
 104 }
 105 #else
 106 static inline void force_quiescent_state(struct rcu_data *rdp,
 107                         struct rcu_ctrlblk *rcp)
 108 {
 109         set_need_resched();
 110 }
 111 #endif
 112
 113 /**
 114  * call_rcu - Queue an RCU callback for invocation after a grace period.
 115  * @head: structure to be used for queueing the RCU updates.
 116  * @func: actual update function to be invoked after the grace period
 117  *
 118  * The update function will be invoked some time after a full grace
 119  * period elapses, in other words after all currently executing RCU
 120  * read-side critical sections have completed.  RCU read-side critical
 121  * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
 122  * and may be nested.
 123  */
 124 void fastcall call_rcu(struct rcu_head *head,
 125                                 void (*func)(struct rcu_head *rcu))
 126 {
 127         unsigned long flags;
 128         struct rcu_data *rdp;
 129
 130         head->func = func;
 131         head->next = NULL;
 132         local_irq_save(flags);
 133         rdp = &__get_cpu_var(rcu_data);
 134         *rdp->nxttail = head;
 135         rdp->nxttail = &head->next;
 136         if (unlikely(++rdp->qlen > qhimark)) {
 137                 rdp->blimit = INT_MAX;
 138                 force_quiescent_state(rdp, &rcu_ctrlblk);
 139         }
 140         local_irq_restore(flags);
 141 }
 142
 143 /**
 144  * call_rcu_bh - Queue an RCU for invocation after a quicker grace period.
 145  * @head: structure to be used for queueing the RCU updates.
 146  * @func: actual update function to be invoked after the grace period
 147  *
 148  * The update function will be invoked some time after a full grace
 149  * period elapses, in other words after all currently executing RCU
 150  * read-side critical sections have completed. call_rcu_bh() assumes
 151  * that the read-side critical sections end on completion of a softirq
 152  * handler. This means that read-side critical sections in process
 153  * context must not be interrupted by softirqs. This interface is to be
 154  * used when most of the read-side critical sections are in softirq context.
 155  * RCU read-side critical sections are delimited by rcu_read_lock() and
 156  * rcu_read_unlock(), * if in interrupt context or rcu_read_lock_bh()
 157  * and rcu_read_unlock_bh(), if in process context. These may be nested.
 158  */
 159 void fastcall call_rcu_bh(struct rcu_head *head,
 160                                 void (*func)(struct rcu_head *rcu))
 161 {
 162         unsigned long flags;
 163         struct rcu_data *rdp;
 164
 165         head->func = func;
 166         head->next = NULL;
 167         local_irq_save(flags);
 168         rdp = &__get_cpu_var(rcu_bh_data);
 169         *rdp->nxttail = head;
 170         rdp->nxttail = &head->next;
 171
 172         if (unlikely(++rdp->qlen > qhimark)) {
 173                 rdp->blimit = INT_MAX;
 174                 force_quiescent_state(rdp, &rcu_bh_ctrlblk);
 175         }
 176
 177         local_irq_restore(flags);
 178 }
 179
 180 /*
 181  * Return the number of RCU batches processed thus far.  Useful
 182  * for debug and statistics.
 183  */
 184 long rcu_batches_completed(void)
 185 {
 186         return rcu_ctrlblk.completed;
 187 }
 188
 189 /*
 190  * Return the number of RCU batches processed thus far.  Useful
 191  * for debug and statistics.
 192  */
 193 long rcu_batches_completed_bh(void)
 194 {
 195         return rcu_bh_ctrlblk.completed;
 196 }
 197
 198 static void rcu_barrier_callback(struct rcu_head *notused)
 199 {
 200         if (atomic_dec_and_test(&rcu_barrier_cpu_count))
 201                 complete(&rcu_barrier_completion);
 202 }
 203
 204 /*
 205  * Called with preemption disabled, and from cross-cpu IRQ context.
 206  */
 207 static void rcu_barrier_func(void *notused)
 208 {
 209         int cpu = smp_processor_id();
 210         struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
 211         struct rcu_head *head;
 212
 213         head = &rdp->barrier;
 214         atomic_inc(&rcu_barrier_cpu_count);
 215         call_rcu(head, rcu_barrier_callback);
 216 }
 217
 218 /**
 219  * rcu_barrier - Wait until all the in-flight RCUs are complete.
 220  */
 221 void rcu_barrier(void)
 222 {
 223         BUG_ON(in_interrupt());
 224         /* Take cpucontrol mutex to protect against CPU hotplug */
 225         mutex_lock(&rcu_barrier_mutex);
 226         init_completion(&rcu_barrier_completion);
 227         atomic_set(&rcu_barrier_cpu_count, 0);
 228         on_each_cpu(rcu_barrier_func, NULL, 0, 1);
 229         wait_for_completion(&rcu_barrier_completion);
 230         mutex_unlock(&rcu_barrier_mutex);
 231 }
 232 EXPORT_SYMBOL_GPL(rcu_barrier);
 233
 234 /*
 235  * Invoke the completed RCU callbacks. They are expected to be in
 236  * a per-cpu list.
 237  */
 238 static void rcu_do_batch(struct rcu_data *rdp)
 239 {
 240         struct rcu_head *next, *list;
 241         int count = 0;
 242
 243         list = rdp->donelist;
 244         while (list) {
 245                 next = list->next;
 246                 prefetch(next);
 247                 list->func(list);
 248                 list = next;
 249                 if (++count >= rdp->blimit)
 250                         break;
 251         }
 252         rdp->donelist = list;
 253
 254         local_irq_disable();
 255         rdp->qlen -= count;
 256         local_irq_enable();
 257         if (rdp->blimit == INT_MAX && rdp->qlen <= qlowmark)
 258                 rdp->blimit = blimit;
 259
 260         if (!rdp->donelist)
 261                 rdp->donetail = &rdp->donelist;
 262         else
 263                 tasklet_schedule(&per_cpu(rcu_tasklet, rdp->cpu));
 264 }
 265
 266 /*
 267  * Grace period handling:
 268  * The grace period handling consists out of two steps:
 269  * - A new grace period is started.
 270  *   This is done by rcu_start_batch. The start is not broadcasted to
 271  *   all cpus, they must pick this up by comparing rcp->cur with
 272  *   rdp->quiescbatch. All cpus are recorded  in the
 273  *   rcu_ctrlblk.cpumask bitmap.
 274  * - All cpus must go through a quiescent state.
 275  *   Since the start of the grace period is not broadcasted, at least two
 276  *   calls to rcu_check_quiescent_state are required:
 277  *   The first call just notices that a new grace period is running. The
 278  *   following calls check if there was a quiescent state since the beginning
 279  *   of the grace period. If so, it updates rcu_ctrlblk.cpumask. If
 280  *   the bitmap is empty, then the grace period is completed.
 281  *   rcu_check_quiescent_state calls rcu_start_batch(0) to start the next grace
 282  *   period (if necessary).
 283  */
 284 /*
 285  * Register a new batch of callbacks, and start it up if there is currently no
 286  * active batch and the batch to be registered has not already occurred.
 287  * Caller must hold rcu_ctrlblk.lock.
 288  */
 289 static void rcu_start_batch(struct rcu_ctrlblk *rcp)
 290 {
 291         if (rcp->next_pending &&
 292                         rcp->completed == rcp->cur) {
 293                 rcp->next_pending = 0;
 294                 /*
 295                  * next_pending == 0 must be visible in
 296                  * __rcu_process_callbacks() before it can see new value of cur.
 297                  */
 298                 smp_wmb();
 299                 rcp->cur++;
 300
 301                 /*
 302                  * Accessing nohz_cpu_mask before incrementing rcp->cur needs a
 303                  * Barrier  Otherwise it can cause tickless idle CPUs to be
 304                  * included in rcp->cpumask, which will extend graceperiods
 305                  * unnecessarily.
 306                  */
 307                 smp_mb();
 308                 cpus_andnot(rcp->cpumask, cpu_online_map, nohz_cpu_mask);
 309
 310                 rcp->signaled = 0;
 311         }
 312 }
 313
 314 /*
 315  * cpu went through a quiescent state since the beginning of the grace period.
 316  * Clear it from the cpu mask and complete the grace period if it was the last
 317  * cpu. Start another grace period if someone has further entries pending
 318  */
 319 static void cpu_quiet(int cpu, struct rcu_ctrlblk *rcp)
 320 {
 321         cpu_clear(cpu, rcp->cpumask);
 322         if (cpus_empty(rcp->cpumask)) {
 323                 /* batch completed ! */
 324                 rcp->completed = rcp->cur;
 325                 rcu_start_batch(rcp);
 326         }
 327 }
 328
 329 /*
 330  * Check if the cpu has gone through a quiescent state (say context
 331  * switch). If so and if it already hasn't done so in this RCU
 332  * quiescent cycle, then indicate that it has done so.
 333  */
 334 static void rcu_check_quiescent_state(struct rcu_ctrlblk *rcp,
 335                                         struct rcu_data *rdp)
 336 {
 337         if (rdp->quiescbatch != rcp->cur) {
 338                 /* start new grace period: */
 339                 rdp->qs_pending = 1;
 340                 rdp->passed_quiesc = 0;
 341                 rdp->quiescbatch = rcp->cur;
 342                 return;
 343         }
 344
 345         /* Grace period already completed for this cpu?
 346          * qs_pending is checked instead of the actual bitmap to avoid
 347          * cacheline trashing.
 348          */
 349         if (!rdp->qs_pending)
 350                 return;
 351
 352         /*
 353          * Was there a quiescent state since the beginning of the grace
 354          * period? If no, then exit and wait for the next call.
 355          */
 356         if (!rdp->passed_quiesc)
 357                 return;
 358         rdp->qs_pending = 0;
 359
 360         spin_lock(&rcp->lock);
 361         /*
 362          * rdp->quiescbatch/rcp->cur and the cpu bitmap can come out of sync
 363          * during cpu startup. Ignore the quiescent state.
 364          */
 365         if (likely(rdp->quiescbatch == rcp->cur))
 366                 cpu_quiet(rdp->cpu, rcp);
 367
 368         spin_unlock(&rcp->lock);
 369 }
 370
 371
 372 #ifdef CONFIG_HOTPLUG_CPU
 373
 374 /* warning! helper for rcu_offline_cpu. do not use elsewhere without reviewing
 375  * locking requirements, the list it's pulling from has to belong to a cpu
 376  * which is dead and hence not processing interrupts.
 377  */
 378 static void rcu_move_batch(struct rcu_data *this_rdp, struct rcu_head *list,
 379                                 struct rcu_head **tail)
 380 {
 381         local_irq_disable();
 382         *this_rdp->nxttail = list;
 383         if (list)
 384                 this_rdp->nxttail = tail;
 385         local_irq_enable();
 386 }
 387
 388 static void __rcu_offline_cpu(struct rcu_data *this_rdp,
 389                                 struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
 390 {
 391         /* if the cpu going offline owns the grace period
 392          * we can block indefinitely waiting for it, so flush
 393          * it here
 394          */
 395         spin_lock_bh(&rcp->lock);
 396         if (rcp->cur != rcp->completed)
 397                 cpu_quiet(rdp->cpu, rcp);
 398         spin_unlock_bh(&rcp->lock);
 399         rcu_move_batch(this_rdp, rdp->curlist, rdp->curtail);
 400         rcu_move_batch(this_rdp, rdp->nxtlist, rdp->nxttail);
 401         rcu_move_batch(this_rdp, rdp->donelist, rdp->donetail);
 402 }
 403
 404 static void rcu_offline_cpu(int cpu)
 405 {
 406         struct rcu_data *this_rdp = &get_cpu_var(rcu_data);
 407         struct rcu_data *this_bh_rdp = &get_cpu_var(rcu_bh_data);
 408
 409         __rcu_offline_cpu(this_rdp, &rcu_ctrlblk,
 410                                         &per_cpu(rcu_data, cpu));
 411         __rcu_offline_cpu(this_bh_rdp, &rcu_bh_ctrlblk,
 412                                         &per_cpu(rcu_bh_data, cpu));
 413         put_cpu_var(rcu_data);
 414         put_cpu_var(rcu_bh_data);
 415         tasklet_kill_immediate(&per_cpu(rcu_tasklet, cpu), cpu);
 416 }
 417
 418 #else
 419
 420 static void rcu_offline_cpu(int cpu)
 421 {
 422 }
 423
 424 #endif
 425
 426 /*
 427  * This does the RCU processing work from tasklet context.
 428  */
 429 static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp,
 430                                         struct rcu_data *rdp)
 431 {
 432         if (rdp->curlist && !rcu_batch_before(rcp->completed, rdp->batch)) {
 433                 *rdp->donetail = rdp->curlist;
 434                 rdp->donetail = rdp->curtail;
 435                 rdp->curlist = NULL;
 436                 rdp->curtail = &rdp->curlist;
 437         }
 438
 439         if (rdp->nxtlist && !rdp->curlist) {
 440                 local_irq_disable();
 441                 rdp->curlist = rdp->nxtlist;
 442                 rdp->curtail = rdp->nxttail;
 443                 rdp->nxtlist = NULL;
 444                 rdp->nxttail = &rdp->nxtlist;
 445                 local_irq_enable();
 446
 447                 /*
 448                  * start the next batch of callbacks
 449                  */
 450
 451                 /* determine batch number */
 452                 rdp->batch = rcp->cur + 1;
 453                 /* see the comment and corresponding wmb() in
 454                  * the rcu_start_batch()
 455                  */
 456                 smp_rmb();
 457
 458                 if (!rcp->next_pending) {
 459                         /* and start it/schedule start if it's a new batch */
 460                         spin_lock(&rcp->lock);
 461                         rcp->next_pending = 1;
 462                         rcu_start_batch(rcp);
 463                         spin_unlock(&rcp->lock);
 464                 }
 465         }
 466
 467         rcu_check_quiescent_state(rcp, rdp);
 468         if (rdp->donelist)
 469                 rcu_do_batch(rdp);
 470 }
 471
 472 static void rcu_process_callbacks(unsigned long unused)
 473 {
 474         __rcu_process_callbacks(&rcu_ctrlblk, &__get_cpu_var(rcu_data));
 475         __rcu_process_callbacks(&rcu_bh_ctrlblk, &__get_cpu_var(rcu_bh_data));
 476 }
 477
 478 static int __rcu_pending(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
 479 {
 480         /* This cpu has pending rcu entries and the grace period
 481          * for them has completed.
 482          */
 483         if (rdp->curlist && !rcu_batch_before(rcp->completed, rdp->batch))
 484                 return 1;
 485
 486         /* This cpu has no pending entries, but there are new entries */
 487         if (!rdp->curlist && rdp->nxtlist)
 488                 return 1;
 489
 490         /* This cpu has finished callbacks to invoke */
 491         if (rdp->donelist)
 492                 return 1;
 493
 494         /* The rcu core waits for a quiescent state from the cpu */
 495         if (rdp->quiescbatch != rcp->cur || rdp->qs_pending)
 496                 return 1;
 497
 498         /* nothing to do */
 499         return 0;
 500 }
 501
 502 /*
 503  * Check to see if there is any immediate RCU-related work to be done
 504  * by the current CPU, returning 1 if so.  This function is part of the
 505  * RCU implementation; it is -not- an exported member of the RCU API.
 506  */
 507 int rcu_pending(int cpu)
 508 {
 509         return __rcu_pending(&rcu_ctrlblk, &per_cpu(rcu_data, cpu)) ||
 510                 __rcu_pending(&rcu_bh_ctrlblk, &per_cpu(rcu_bh_data, cpu));
 511 }
 512
 513 /*
 514  * Check to see if any future RCU-related work will need to be done
 515  * by the current CPU, even if none need be done immediately, returning
 516  * 1 if so.  This function is part of the RCU implementation; it is -not-
 517  * an exported member of the RCU API.
 518  */
 519 int rcu_needs_cpu(int cpu)
 520 {
 521         struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
 522         struct rcu_data *rdp_bh = &per_cpu(rcu_bh_data, cpu);
 523
 524         return (!!rdp->curlist || !!rdp_bh->curlist || rcu_pending(cpu));
 525 }
 526
 527 void rcu_check_callbacks(int cpu, int user)
 528 {
 529         if (user ||
 530             (idle_cpu(cpu) && !in_softirq() &&
 531                                 hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
 532                 rcu_qsctr_inc(cpu);
 533                 rcu_bh_qsctr_inc(cpu);
 534         } else if (!in_softirq())
 535                 rcu_bh_qsctr_inc(cpu);
 536         tasklet_schedule(&per_cpu(rcu_tasklet, cpu));
 537 }
 538
 539 static void rcu_init_percpu_data(int cpu, struct rcu_ctrlblk *rcp,
 540                                                 struct rcu_data *rdp)
 541 {
 542         memset(rdp, 0, sizeof(*rdp));
 543         rdp->curtail = &rdp->curlist;
 544         rdp->nxttail = &rdp->nxtlist;
 545         rdp->donetail = &rdp->donelist;
 546         rdp->quiescbatch = rcp->completed;
 547         rdp->qs_pending = 0;
 548         rdp->cpu = cpu;
 549         rdp->blimit = blimit;
 550 }
 551
 552 static void __devinit rcu_online_cpu(int cpu)
 553 {
 554         struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
 555         struct rcu_data *bh_rdp = &per_cpu(rcu_bh_data, cpu);
 556
 557         rcu_init_percpu_data(cpu, &rcu_ctrlblk, rdp);
 558         rcu_init_percpu_data(cpu, &rcu_bh_ctrlblk, bh_rdp);
 559         tasklet_init(&per_cpu(rcu_tasklet, cpu), rcu_process_callbacks, 0UL);
 560 }
 561
 562 static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
 563                                 unsigned long action, void *hcpu)
 564 {
 565         long cpu = (long)hcpu;
 566         switch (action) {
 567         case CPU_UP_PREPARE:
 568         case CPU_UP_PREPARE_FROZEN:
 569                 rcu_online_cpu(cpu);
 570                 break;
 571         case CPU_DEAD:
 572         case CPU_DEAD_FROZEN:
 573                 rcu_offline_cpu(cpu);
 574                 break;
 575         default:
 576                 break;
 577         }
 578         return NOTIFY_OK;
 579 }
 580
 581 static struct notifier_block __cpuinitdata rcu_nb = {
 582         .notifier_call  = rcu_cpu_notify,
 583 };
 584
 585 /*
 586  * Initializes rcu mechanism.  Assumed to be called early.
 587  * That is before local timer(SMP) or jiffie timer (uniproc) is setup.
 588  * Note that rcu_qsctr and friends are implicitly
 589  * initialized due to the choice of ``0'' for RCU_CTR_INVALID.
 590  */
 591 void __init rcu_init(void)
 592 {
 593         rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE,
 594                         (void *)(long)smp_processor_id());
 595         /* Register notifier for non-boot CPUs */
 596         register_cpu_notifier(&rcu_nb);
 597 }
 598
 599 struct rcu_synchronize {
 600         struct rcu_head head;
 601         struct completion completion;
 602 };
 603
 604 /* Because of FASTCALL declaration of complete, we use this wrapper */
 605 static void wakeme_after_rcu(struct rcu_head  *head)
 606 {
 607         struct rcu_synchronize *rcu;
 608
 609         rcu = container_of(head, struct rcu_synchronize, head);
 610         complete(&rcu->completion);
 611 }
 612
 613 /**
 614  * synchronize_rcu - wait until a grace period has elapsed.
 615  *
 616  * Control will return to the caller some time after a full grace
 617  * period has elapsed, in other words after all currently executing RCU
 618  * read-side critical sections have completed.  RCU read-side critical
 619  * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
 620  * and may be nested.
 621  *
 622  * If your read-side code is not protected by rcu_read_lock(), do -not-
 623  * use synchronize_rcu().
 624  */
 625 void synchronize_rcu(void)
 626 {
 627         struct rcu_synchronize rcu;
 628
 629         init_completion(&rcu.completion);
 630         /* Will wake me after RCU finished */
 631         call_rcu(&rcu.head, wakeme_after_rcu);
 632
 633         /* Wait for it */
 634         wait_for_completion(&rcu.completion);
 635 }
 636
 637 module_param(blimit, int, 0);
 638 module_param(qhimark, int, 0);
 639 module_param(qlowmark, int, 0);
 640 EXPORT_SYMBOL_GPL(rcu_batches_completed);
 641 EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);
 642 EXPORT_SYMBOL_GPL(call_rcu);
 643 EXPORT_SYMBOL_GPL(call_rcu_bh);
 644 EXPORT_SYMBOL_GPL(synchronize_rcu);