kernel/rcutree_plugin.h

   1 /*
   2  * Read-Copy Update mechanism for mutual exclusion (tree-based version)
   3  * Internal non-public definitions that provide either classic
   4  * or preemptable semantics.
   5  *
   6  * This program is free software; you can redistribute it and/or modify
   7  * it under the terms of the GNU General Public License as published by
   8  * the Free Software Foundation; either version 2 of the License, or
   9  * (at your option) any later version.
  10  *
  11  * This program is distributed in the hope that it will be useful,
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14  * GNU General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU General Public License
  17  * along with this program; if not, write to the Free Software
  18  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  19  *
  20  * Copyright Red Hat, 2009
  21  * Copyright IBM Corporation, 2009
  22  *
  23  * Author: Ingo Molnar <mingo@elte.hu>
  24  *         Paul E. McKenney <paulmck@linux.vnet.ibm.com>
  25  */
  26
  27
  28 #ifdef CONFIG_TREE_PREEMPT_RCU
  29
  30 struct rcu_state rcu_preempt_state = RCU_STATE_INITIALIZER(rcu_preempt_state);
  31 DEFINE_PER_CPU(struct rcu_data, rcu_preempt_data);
  32
  33 /*
  34  * Tell them what RCU they are running.
  35  */
  36 static void __init rcu_bootup_announce(void)
  37 {
  38         printk(KERN_INFO
  39                "Experimental preemptable hierarchical RCU implementation.\n");
  40 }
  41
  42 /*
  43  * Return the number of RCU-preempt batches processed thus far
  44  * for debug and statistics.
  45  */
  46 long rcu_batches_completed_preempt(void)
  47 {
  48         return rcu_preempt_state.completed;
  49 }
  50 EXPORT_SYMBOL_GPL(rcu_batches_completed_preempt);
  51
  52 /*
  53  * Return the number of RCU batches processed thus far for debug & stats.
  54  */
  55 long rcu_batches_completed(void)
  56 {
  57         return rcu_batches_completed_preempt();
  58 }
  59 EXPORT_SYMBOL_GPL(rcu_batches_completed);
  60
  61 /*
  62  * Record a preemptable-RCU quiescent state for the specified CPU.  Note
  63  * that this just means that the task currently running on the CPU is
  64  * not in a quiescent state.  There might be any number of tasks blocked
  65  * while in an RCU read-side critical section.
  66  */
  67 static void rcu_preempt_qs(int cpu)
  68 {
  69         struct rcu_data *rdp = &per_cpu(rcu_preempt_data, cpu);
  70         rdp->passed_quiesc_completed = rdp->gpnum - 1;
  71         barrier();
  72         rdp->passed_quiesc = 1;
  73 }
  74
  75 /*
  76  * We have entered the scheduler, and the current task might soon be
  77  * context-switched away from.  If this task is in an RCU read-side
  78  * critical section, we will no longer be able to rely on the CPU to
  79  * record that fact, so we enqueue the task on the appropriate entry
  80  * of the blocked_tasks[] array.  The task will dequeue itself when
  81  * it exits the outermost enclosing RCU read-side critical section.
  82  * Therefore, the current grace period cannot be permitted to complete
  83  * until the blocked_tasks[] entry indexed by the low-order bit of
  84  * rnp->gpnum empties.
  85  *
  86  * Caller must disable preemption.
  87  */
  88 static void rcu_preempt_note_context_switch(int cpu)
  89 {
  90         struct task_struct *t = current;
  91         unsigned long flags;
  92         int phase;
  93         struct rcu_data *rdp;
  94         struct rcu_node *rnp;
  95
  96         if (t->rcu_read_lock_nesting &&
  97             (t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {
  98
  99                 /* Possibly blocking in an RCU read-side critical section. */
 100                 rdp = rcu_preempt_state.rda[cpu];
 101                 rnp = rdp->mynode;
 102                 spin_lock_irqsave(&rnp->lock, flags);
 103                 t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
 104                 t->rcu_blocked_node = rnp;
 105
 106                 /*
 107                  * If this CPU has already checked in, then this task
 108                  * will hold up the next grace period rather than the
 109                  * current grace period.  Queue the task accordingly.
 110                  * If the task is queued for the current grace period
 111                  * (i.e., this CPU has not yet passed through a quiescent
 112                  * state for the current grace period), then as long
 113                  * as that task remains queued, the current grace period
 114                  * cannot end.
 115                  *
 116                  * But first, note that the current CPU must still be
 117                  * on line!
 118                  */
 119                 WARN_ON_ONCE((rdp->grpmask & rnp->qsmaskinit) == 0);
 120                 WARN_ON_ONCE(!list_empty(&t->rcu_node_entry));
 121                 phase = (rnp->gpnum + !(rnp->qsmask & rdp->grpmask)) & 0x1;
 122                 list_add(&t->rcu_node_entry, &rnp->blocked_tasks[phase]);
 123                 spin_unlock_irqrestore(&rnp->lock, flags);
 124         }
 125
 126         /*
 127          * Either we were not in an RCU read-side critical section to
 128          * begin with, or we have now recorded that critical section
 129          * globally.  Either way, we can now note a quiescent state
 130          * for this CPU.  Again, if we were in an RCU read-side critical
 131          * section, and if that critical section was blocking the current
 132          * grace period, then the fact that the task has been enqueued
 133          * means that we continue to block the current grace period.
 134          */
 135         rcu_preempt_qs(cpu);
 136         local_irq_save(flags);
 137         t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
 138         local_irq_restore(flags);
 139 }
 140
 141 /*
 142  * Tree-preemptable RCU implementation for rcu_read_lock().
 143  * Just increment ->rcu_read_lock_nesting, shared state will be updated
 144  * if we block.
 145  */
 146 void __rcu_read_lock(void)
 147 {
 148         ACCESS_ONCE(current->rcu_read_lock_nesting)++;
 149         barrier();  /* needed if we ever invoke rcu_read_lock in rcutree.c */
 150 }
 151 EXPORT_SYMBOL_GPL(__rcu_read_lock);
 152
 153 /*
 154  * Check for preempted RCU readers blocking the current grace period
 155  * for the specified rcu_node structure.  If the caller needs a reliable
 156  * answer, it must hold the rcu_node's ->lock.
 157  */
 158 static int rcu_preempted_readers(struct rcu_node *rnp)
 159 {
 160         return !list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1]);
 161 }
 162
 163 /*
 164  * Record a quiescent state for all tasks that were previously queued
 165  * on the specified rcu_node structure and that were blocking the current
 166  * RCU grace period.  The caller must hold the specified rnp->lock with
 167  * irqs disabled, and this lock is released upon return, but irqs remain
 168  * disabled.
 169  */
 170 static void task_quiet(struct rcu_node *rnp, unsigned long flags)
 171         __releases(rnp->lock)
 172 {
 173         unsigned long mask;
 174         struct rcu_node *rnp_p;
 175
 176         if (rnp->qsmask != 0 || rcu_preempted_readers(rnp)) {
 177                 spin_unlock_irqrestore(&rnp->lock, flags);
 178                 return;  /* Still need more quiescent states! */
 179         }
 180
 181         rnp_p = rnp->parent;
 182         if (rnp_p == NULL) {
 183                 /*
 184                  * Either there is only one rcu_node in the tree,
 185                  * or tasks were kicked up to root rcu_node due to
 186                  * CPUs going offline.
 187                  */
 188                 cpu_quiet_msk_finish(&rcu_preempt_state, flags);
 189                 return;
 190         }
 191
 192         /* Report up the rest of the hierarchy. */
 193         mask = rnp->grpmask;
 194         spin_unlock(&rnp->lock);        /* irqs remain disabled. */
 195         spin_lock(&rnp_p->lock);        /* irqs already disabled. */
 196         cpu_quiet_msk(mask, &rcu_preempt_state, rnp_p, flags);
 197 }
 198
 199 /*
 200  * Handle special cases during rcu_read_unlock(), such as needing to
 201  * notify RCU core processing or task having blocked during the RCU
 202  * read-side critical section.
 203  */
 204 static void rcu_read_unlock_special(struct task_struct *t)
 205 {
 206         int empty;
 207         unsigned long flags;
 208         struct rcu_node *rnp;
 209         int special;
 210
 211         /* NMI handlers cannot block and cannot safely manipulate state. */
 212         if (in_nmi())
 213                 return;
 214
 215         local_irq_save(flags);
 216
 217         /*
 218          * If RCU core is waiting for this CPU to exit critical section,
 219          * let it know that we have done so.
 220          */
 221         special = t->rcu_read_unlock_special;
 222         if (special & RCU_READ_UNLOCK_NEED_QS) {
 223                 t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
 224                 rcu_preempt_qs(smp_processor_id());
 225         }
 226
 227         /* Hardware IRQ handlers cannot block. */
 228         if (in_irq()) {
 229                 local_irq_restore(flags);
 230                 return;
 231         }
 232
 233         /* Clean up if blocked during RCU read-side critical section. */
 234         if (special & RCU_READ_UNLOCK_BLOCKED) {
 235                 t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;
 236
 237                 /*
 238                  * Remove this task from the list it blocked on.  The
 239                  * task can migrate while we acquire the lock, but at
 240                  * most one time.  So at most two passes through loop.
 241                  */
 242                 for (;;) {
 243                         rnp = t->rcu_blocked_node;
 244                         spin_lock(&rnp->lock);  /* irqs already disabled. */
 245                         if (rnp == t->rcu_blocked_node)
 246                                 break;
 247                         spin_unlock(&rnp->lock);  /* irqs remain disabled. */
 248                 }
 249                 empty = !rcu_preempted_readers(rnp);
 250                 list_del_init(&t->rcu_node_entry);
 251                 t->rcu_blocked_node = NULL;
 252
 253                 /*
 254                  * If this was the last task on the current list, and if
 255                  * we aren't waiting on any CPUs, report the quiescent state.
 256                  * Note that task_quiet() releases rnp->lock.
 257                  */
 258                 if (empty)
 259                         spin_unlock_irqrestore(&rnp->lock, flags);
 260                 else
 261                         task_quiet(rnp, flags);
 262         } else {
 263                 local_irq_restore(flags);
 264         }
 265 }
 266
 267 /*
 268  * Tree-preemptable RCU implementation for rcu_read_unlock().
 269  * Decrement ->rcu_read_lock_nesting.  If the result is zero (outermost
 270  * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
 271  * invoke rcu_read_unlock_special() to clean up after a context switch
 272  * in an RCU read-side critical section and other special cases.
 273  */
 274 void __rcu_read_unlock(void)
 275 {
 276         struct task_struct *t = current;
 277
 278         barrier();  /* needed if we ever invoke rcu_read_unlock in rcutree.c */
 279         if (--ACCESS_ONCE(t->rcu_read_lock_nesting) == 0 &&
 280             unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
 281                 rcu_read_unlock_special(t);
 282 }
 283 EXPORT_SYMBOL_GPL(__rcu_read_unlock);
 284
 285 #ifdef CONFIG_RCU_CPU_STALL_DETECTOR
 286
 287 /*
 288  * Scan the current list of tasks blocked within RCU read-side critical
 289  * sections, printing out the tid of each.
 290  */
 291 static void rcu_print_task_stall(struct rcu_node *rnp)
 292 {
 293         unsigned long flags;
 294         struct list_head *lp;
 295         int phase;
 296         struct task_struct *t;
 297
 298         if (rcu_preempted_readers(rnp)) {
 299                 spin_lock_irqsave(&rnp->lock, flags);
 300                 phase = rnp->gpnum & 0x1;
 301                 lp = &rnp->blocked_tasks[phase];
 302                 list_for_each_entry(t, lp, rcu_node_entry)
 303                         printk(" P%d", t->pid);
 304                 spin_unlock_irqrestore(&rnp->lock, flags);
 305         }
 306 }
 307
 308 #endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
 309
 310 /*
 311  * Check that the list of blocked tasks for the newly completed grace
 312  * period is in fact empty.  It is a serious bug to complete a grace
 313  * period that still has RCU readers blocked!  This function must be
 314  * invoked -before- updating this rnp's ->gpnum, and the rnp's ->lock
 315  * must be held by the caller.
 316  */
 317 static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
 318 {
 319         WARN_ON_ONCE(rcu_preempted_readers(rnp));
 320         WARN_ON_ONCE(rnp->qsmask);
 321 }
 322
 323 #ifdef CONFIG_HOTPLUG_CPU
 324
 325 /*
 326  * Handle tasklist migration for case in which all CPUs covered by the
 327  * specified rcu_node have gone offline.  Move them up to the root
 328  * rcu_node.  The reason for not just moving them to the immediate
 329  * parent is to remove the need for rcu_read_unlock_special() to
 330  * make more than two attempts to acquire the target rcu_node's lock.
 331  * Returns true if there were tasks blocking the current RCU grace
 332  * period.
 333  *
 334  * Returns 1 if there was previously a task blocking the current grace
 335  * period on the specified rcu_node structure.
 336  *
 337  * The caller must hold rnp->lock with irqs disabled.
 338  */
 339 static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
 340                                      struct rcu_node *rnp,
 341                                      struct rcu_data *rdp)
 342 {
 343         int i;
 344         struct list_head *lp;
 345         struct list_head *lp_root;
 346         int retval;
 347         struct rcu_node *rnp_root = rcu_get_root(rsp);
 348         struct task_struct *tp;
 349
 350         if (rnp == rnp_root) {
 351                 WARN_ONCE(1, "Last CPU thought to be offlined?");
 352                 return 0;  /* Shouldn't happen: at least one CPU online. */
 353         }
 354         WARN_ON_ONCE(rnp != rdp->mynode &&
 355                      (!list_empty(&rnp->blocked_tasks[0]) ||
 356                       !list_empty(&rnp->blocked_tasks[1])));
 357
 358         /*
 359          * Move tasks up to root rcu_node.  Rely on the fact that the
 360          * root rcu_node can be at most one ahead of the rest of the
 361          * rcu_nodes in terms of gp_num value.  This fact allows us to
 362          * move the blocked_tasks[] array directly, element by element.
 363          */
 364         retval = rcu_preempted_readers(rnp);
 365         for (i = 0; i < 2; i++) {
 366                 lp = &rnp->blocked_tasks[i];
 367                 lp_root = &rnp_root->blocked_tasks[i];
 368                 while (!list_empty(lp)) {
 369                         tp = list_entry(lp->next, typeof(*tp), rcu_node_entry);
 370                         spin_lock(&rnp_root->lock); /* irqs already disabled */
 371                         list_del(&tp->rcu_node_entry);
 372                         tp->rcu_blocked_node = rnp_root;
 373                         list_add(&tp->rcu_node_entry, lp_root);
 374                         spin_unlock(&rnp_root->lock); /* irqs remain disabled */
 375                 }
 376         }
 377         return retval;
 378 }
 379
 380 /*
 381  * Do CPU-offline processing for preemptable RCU.
 382  */
 383 static void rcu_preempt_offline_cpu(int cpu)
 384 {
 385         __rcu_offline_cpu(cpu, &rcu_preempt_state);
 386 }
 387
 388 #endif /* #ifdef CONFIG_HOTPLUG_CPU */
 389
 390 /*
 391  * Check for a quiescent state from the current CPU.  When a task blocks,
 392  * the task is recorded in the corresponding CPU's rcu_node structure,
 393  * which is checked elsewhere.
 394  *
 395  * Caller must disable hard irqs.
 396  */
 397 static void rcu_preempt_check_callbacks(int cpu)
 398 {
 399         struct task_struct *t = current;
 400
 401         if (t->rcu_read_lock_nesting == 0) {
 402                 t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
 403                 rcu_preempt_qs(cpu);
 404                 return;
 405         }
 406         if (per_cpu(rcu_preempt_data, cpu).qs_pending)
 407                 t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
 408 }
 409
 410 /*
 411  * Process callbacks for preemptable RCU.
 412  */
 413 static void rcu_preempt_process_callbacks(void)
 414 {
 415         __rcu_process_callbacks(&rcu_preempt_state,
 416                                 &__get_cpu_var(rcu_preempt_data));
 417 }
 418
 419 /*
 420  * Queue a preemptable-RCU callback for invocation after a grace period.
 421  */
 422 void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
 423 {
 424         __call_rcu(head, func, &rcu_preempt_state);
 425 }
 426 EXPORT_SYMBOL_GPL(call_rcu);
 427
 428 /*
 429  * Wait for an rcu-preempt grace period.  We are supposed to expedite the
 430  * grace period, but this is the crude slow compatability hack, so just
 431  * invoke synchronize_rcu().
 432  */
 433 void synchronize_rcu_expedited(void)
 434 {
 435         synchronize_rcu();
 436 }
 437 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
 438
 439 /*
 440  * Check to see if there is any immediate preemptable-RCU-related work
 441  * to be done.
 442  */
 443 static int rcu_preempt_pending(int cpu)
 444 {
 445         return __rcu_pending(&rcu_preempt_state,
 446                              &per_cpu(rcu_preempt_data, cpu));
 447 }
 448
 449 /*
 450  * Does preemptable RCU need the CPU to stay out of dynticks mode?
 451  */
 452 static int rcu_preempt_needs_cpu(int cpu)
 453 {
 454         return !!per_cpu(rcu_preempt_data, cpu).nxtlist;
 455 }
 456
 457 /**
 458  * rcu_barrier - Wait until all in-flight call_rcu() callbacks complete.
 459  */
 460 void rcu_barrier(void)
 461 {
 462         _rcu_barrier(&rcu_preempt_state, call_rcu);
 463 }
 464 EXPORT_SYMBOL_GPL(rcu_barrier);
 465
 466 /*
 467  * Initialize preemptable RCU's per-CPU data.
 468  */
 469 static void __cpuinit rcu_preempt_init_percpu_data(int cpu)
 470 {
 471         rcu_init_percpu_data(cpu, &rcu_preempt_state, 1);
 472 }
 473
 474 /*
 475  * Move preemptable RCU's callbacks to ->orphan_cbs_list.
 476  */
 477 static void rcu_preempt_send_cbs_to_orphanage(void)
 478 {
 479         rcu_send_cbs_to_orphanage(&rcu_preempt_state);
 480 }
 481
 482 /*
 483  * Initialize preemptable RCU's state structures.
 484  */
 485 static void __init __rcu_init_preempt(void)
 486 {
 487         RCU_INIT_FLAVOR(&rcu_preempt_state, rcu_preempt_data);
 488 }
 489
 490 /*
 491  * Check for a task exiting while in a preemptable-RCU read-side
 492  * critical section, clean up if so.  No need to issue warnings,
 493  * as debug_check_no_locks_held() already does this if lockdep
 494  * is enabled.
 495  */
 496 void exit_rcu(void)
 497 {
 498         struct task_struct *t = current;
 499
 500         if (t->rcu_read_lock_nesting == 0)
 501                 return;
 502         t->rcu_read_lock_nesting = 1;
 503         rcu_read_unlock();
 504 }
 505
 506 #else /* #ifdef CONFIG_TREE_PREEMPT_RCU */
 507
 508 /*
 509  * Tell them what RCU they are running.
 510  */
 511 static void __init rcu_bootup_announce(void)
 512 {
 513         printk(KERN_INFO "Hierarchical RCU implementation.\n");
 514 }
 515
 516 /*
 517  * Return the number of RCU batches processed thus far for debug & stats.
 518  */
 519 long rcu_batches_completed(void)
 520 {
 521         return rcu_batches_completed_sched();
 522 }
 523 EXPORT_SYMBOL_GPL(rcu_batches_completed);
 524
 525 /*
 526  * Because preemptable RCU does not exist, we never have to check for
 527  * CPUs being in quiescent states.
 528  */
 529 static void rcu_preempt_note_context_switch(int cpu)
 530 {
 531 }
 532
 533 /*
 534  * Because preemptable RCU does not exist, there are never any preempted
 535  * RCU readers.
 536  */
 537 static int rcu_preempted_readers(struct rcu_node *rnp)
 538 {
 539         return 0;
 540 }
 541
 542 #ifdef CONFIG_HOTPLUG_CPU
 543
 544 /* Because preemptible RCU does not exist, no quieting of tasks. */
 545 static void task_quiet(struct rcu_node *rnp, unsigned long flags)
 546 {
 547         spin_unlock_irqrestore(&rnp->lock, flags);
 548 }
 549
 550 #endif /* #ifdef CONFIG_HOTPLUG_CPU */
 551
 552 #ifdef CONFIG_RCU_CPU_STALL_DETECTOR
 553
 554 /*
 555  * Because preemptable RCU does not exist, we never have to check for
 556  * tasks blocked within RCU read-side critical sections.
 557  */
 558 static void rcu_print_task_stall(struct rcu_node *rnp)
 559 {
 560 }
 561
 562 #endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
 563
 564 /*
 565  * Because there is no preemptable RCU, there can be no readers blocked,
 566  * so there is no need to check for blocked tasks.  So check only for
 567  * bogus qsmask values.
 568  */
 569 static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
 570 {
 571         WARN_ON_ONCE(rnp->qsmask);
 572 }
 573
 574 #ifdef CONFIG_HOTPLUG_CPU
 575
 576 /*
 577  * Because preemptable RCU does not exist, it never needs to migrate
 578  * tasks that were blocked within RCU read-side critical sections, and
 579  * such non-existent tasks cannot possibly have been blocking the current
 580  * grace period.
 581  */
 582 static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
 583                                      struct rcu_node *rnp,
 584                                      struct rcu_data *rdp)
 585 {
 586         return 0;
 587 }
 588
 589 /*
 590  * Because preemptable RCU does not exist, it never needs CPU-offline
 591  * processing.
 592  */
 593 static void rcu_preempt_offline_cpu(int cpu)
 594 {
 595 }
 596
 597 #endif /* #ifdef CONFIG_HOTPLUG_CPU */
 598
 599 /*
 600  * Because preemptable RCU does not exist, it never has any callbacks
 601  * to check.
 602  */
 603 static void rcu_preempt_check_callbacks(int cpu)
 604 {
 605 }
 606
 607 /*
 608  * Because preemptable RCU does not exist, it never has any callbacks
 609  * to process.
 610  */
 611 static void rcu_preempt_process_callbacks(void)
 612 {
 613 }
 614
 615 /*
 616  * In classic RCU, call_rcu() is just call_rcu_sched().
 617  */
 618 void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
 619 {
 620         call_rcu_sched(head, func);
 621 }
 622 EXPORT_SYMBOL_GPL(call_rcu);
 623
 624 /*
 625  * Wait for an rcu-preempt grace period, but make it happen quickly.
 626  * But because preemptable RCU does not exist, map to rcu-sched.
 627  */
 628 void synchronize_rcu_expedited(void)
 629 {
 630         synchronize_sched_expedited();
 631 }
 632 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
 633
 634 /*
 635  * Because preemptable RCU does not exist, it never has any work to do.
 636  */
 637 static int rcu_preempt_pending(int cpu)
 638 {
 639         return 0;
 640 }
 641
 642 /*
 643  * Because preemptable RCU does not exist, it never needs any CPU.
 644  */
 645 static int rcu_preempt_needs_cpu(int cpu)
 646 {
 647         return 0;
 648 }
 649
 650 /*
 651  * Because preemptable RCU does not exist, rcu_barrier() is just
 652  * another name for rcu_barrier_sched().
 653  */
 654 void rcu_barrier(void)
 655 {
 656         rcu_barrier_sched();
 657 }
 658 EXPORT_SYMBOL_GPL(rcu_barrier);
 659
 660 /*
 661  * Because preemptable RCU does not exist, there is no per-CPU
 662  * data to initialize.
 663  */
 664 static void __cpuinit rcu_preempt_init_percpu_data(int cpu)
 665 {
 666 }
 667
 668 /*
 669  * Because there is no preemptable RCU, there are no callbacks to move.
 670  */
 671 static void rcu_preempt_send_cbs_to_orphanage(void)
 672 {
 673 }
 674
 675 /*
 676  * Because preemptable RCU does not exist, it need not be initialized.
 677  */
 678 static void __init __rcu_init_preempt(void)
 679 {
 680 }
 681
 682 #endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */