kernel/rcu/tree_exp.h

   1 /*
   2  * RCU expedited grace periods
   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, you can access it online at
  16  * http://www.gnu.org/licenses/gpl-2.0.html.
  17  *
  18  * Copyright IBM Corporation, 2016
  19  *
  20  * Authors: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
  21  */
  22
  23 /* Wrapper functions for expedited grace periods.  */
  24 static void rcu_exp_gp_seq_start(struct rcu_state *rsp)
  25 {
  26         rcu_seq_start(&rsp->expedited_sequence);
  27 }
  28 static void rcu_exp_gp_seq_end(struct rcu_state *rsp)
  29 {
  30         rcu_seq_end(&rsp->expedited_sequence);
  31         smp_mb(); /* Ensure that consecutive grace periods serialize. */
  32 }
  33 static unsigned long rcu_exp_gp_seq_snap(struct rcu_state *rsp)
  34 {
  35         unsigned long s;
  36
  37         smp_mb(); /* Caller's modifications seen first by other CPUs. */
  38         s = rcu_seq_snap(&rsp->expedited_sequence);
  39         trace_rcu_exp_grace_period(rsp->name, s, TPS("snap"));
  40         return s;
  41 }
  42 static bool rcu_exp_gp_seq_done(struct rcu_state *rsp, unsigned long s)
  43 {
  44         return rcu_seq_done(&rsp->expedited_sequence, s);
  45 }
  46
  47 /*
  48  * Reset the ->expmaskinit values in the rcu_node tree to reflect any
  49  * recent CPU-online activity.  Note that these masks are not cleared
  50  * when CPUs go offline, so they reflect the union of all CPUs that have
  51  * ever been online.  This means that this function normally takes its
  52  * no-work-to-do fastpath.
  53  */
  54 static void sync_exp_reset_tree_hotplug(struct rcu_state *rsp)
  55 {
  56         bool done;
  57         unsigned long flags;
  58         unsigned long mask;
  59         unsigned long oldmask;
  60         int ncpus = READ_ONCE(rsp->ncpus);
  61         struct rcu_node *rnp;
  62         struct rcu_node *rnp_up;
  63
  64         /* If no new CPUs onlined since last time, nothing to do. */
  65         if (likely(ncpus == rsp->ncpus_snap))
  66                 return;
  67         rsp->ncpus_snap = ncpus;
  68
  69         /*
  70          * Each pass through the following loop propagates newly onlined
  71          * CPUs for the current rcu_node structure up the rcu_node tree.
  72          */
  73         rcu_for_each_leaf_node(rsp, rnp) {
  74                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
  75                 if (rnp->expmaskinit == rnp->expmaskinitnext) {
  76                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  77                         continue;  /* No new CPUs, nothing to do. */
  78                 }
  79
  80                 /* Update this node's mask, track old value for propagation. */
  81                 oldmask = rnp->expmaskinit;
  82                 rnp->expmaskinit = rnp->expmaskinitnext;
  83                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  84
  85                 /* If was already nonzero, nothing to propagate. */
  86                 if (oldmask)
  87                         continue;
  88
  89                 /* Propagate the new CPU up the tree. */
  90                 mask = rnp->grpmask;
  91                 rnp_up = rnp->parent;
  92                 done = false;
  93                 while (rnp_up) {
  94                         raw_spin_lock_irqsave_rcu_node(rnp_up, flags);
  95                         if (rnp_up->expmaskinit)
  96                                 done = true;
  97                         rnp_up->expmaskinit |= mask;
  98                         raw_spin_unlock_irqrestore_rcu_node(rnp_up, flags);
  99                         if (done)
 100                                 break;
 101                         mask = rnp_up->grpmask;
 102                         rnp_up = rnp_up->parent;
 103                 }
 104         }
 105 }
 106
 107 /*
 108  * Reset the ->expmask values in the rcu_node tree in preparation for
 109  * a new expedited grace period.
 110  */
 111 static void __maybe_unused sync_exp_reset_tree(struct rcu_state *rsp)
 112 {
 113         unsigned long flags;
 114         struct rcu_node *rnp;
 115
 116         sync_exp_reset_tree_hotplug(rsp);
 117         rcu_for_each_node_breadth_first(rsp, rnp) {
 118                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
 119                 WARN_ON_ONCE(rnp->expmask);
 120                 rnp->expmask = rnp->expmaskinit;
 121                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 122         }
 123 }
 124
 125 /*
 126  * Return non-zero if there is no RCU expedited grace period in progress
 127  * for the specified rcu_node structure, in other words, if all CPUs and
 128  * tasks covered by the specified rcu_node structure have done their bit
 129  * for the current expedited grace period.  Works only for preemptible
 130  * RCU -- other RCU implementation use other means.
 131  *
 132  * Caller must hold the rcu_state's exp_mutex.
 133  */
 134 static int sync_rcu_preempt_exp_done(struct rcu_node *rnp)
 135 {
 136         return rnp->exp_tasks == NULL &&
 137                READ_ONCE(rnp->expmask) == 0;
 138 }
 139
 140 /*
 141  * Report the exit from RCU read-side critical section for the last task
 142  * that queued itself during or before the current expedited preemptible-RCU
 143  * grace period.  This event is reported either to the rcu_node structure on
 144  * which the task was queued or to one of that rcu_node structure's ancestors,
 145  * recursively up the tree.  (Calm down, calm down, we do the recursion
 146  * iteratively!)
 147  *
 148  * Caller must hold the rcu_state's exp_mutex and the specified rcu_node
 149  * structure's ->lock.
 150  */
 151 static void __rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp,
 152                                  bool wake, unsigned long flags)
 153         __releases(rnp->lock)
 154 {
 155         unsigned long mask;
 156
 157         for (;;) {
 158                 if (!sync_rcu_preempt_exp_done(rnp)) {
 159                         if (!rnp->expmask)
 160                                 rcu_initiate_boost(rnp, flags);
 161                         else
 162                                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 163                         break;
 164                 }
 165                 if (rnp->parent == NULL) {
 166                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 167                         if (wake) {
 168                                 smp_mb(); /* EGP done before wake_up(). */
 169                                 swake_up(&rsp->expedited_wq);
 170                         }
 171                         break;
 172                 }
 173                 mask = rnp->grpmask;
 174                 raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled */
 175                 rnp = rnp->parent;
 176                 raw_spin_lock_rcu_node(rnp); /* irqs already disabled */
 177                 WARN_ON_ONCE(!(rnp->expmask & mask));
 178                 rnp->expmask &= ~mask;
 179         }
 180 }
 181
 182 /*
 183  * Report expedited quiescent state for specified node.  This is a
 184  * lock-acquisition wrapper function for __rcu_report_exp_rnp().
 185  *
 186  * Caller must hold the rcu_state's exp_mutex.
 187  */
 188 static void __maybe_unused rcu_report_exp_rnp(struct rcu_state *rsp,
 189                                               struct rcu_node *rnp, bool wake)
 190 {
 191         unsigned long flags;
 192
 193         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 194         __rcu_report_exp_rnp(rsp, rnp, wake, flags);
 195 }
 196
 197 /*
 198  * Report expedited quiescent state for multiple CPUs, all covered by the
 199  * specified leaf rcu_node structure.  Caller must hold the rcu_state's
 200  * exp_mutex.
 201  */
 202 static void rcu_report_exp_cpu_mult(struct rcu_state *rsp, struct rcu_node *rnp,
 203                                     unsigned long mask, bool wake)
 204 {
 205         unsigned long flags;
 206
 207         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 208         if (!(rnp->expmask & mask)) {
 209                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 210                 return;
 211         }
 212         rnp->expmask &= ~mask;
 213         __rcu_report_exp_rnp(rsp, rnp, wake, flags); /* Releases rnp->lock. */
 214 }
 215
 216 /*
 217  * Report expedited quiescent state for specified rcu_data (CPU).
 218  */
 219 static void rcu_report_exp_rdp(struct rcu_state *rsp, struct rcu_data *rdp,
 220                                bool wake)
 221 {
 222         rcu_report_exp_cpu_mult(rsp, rdp->mynode, rdp->grpmask, wake);
 223 }
 224
 225 /* Common code for synchronize_{rcu,sched}_expedited() work-done checking. */
 226 static bool sync_exp_work_done(struct rcu_state *rsp, atomic_long_t *stat,
 227                                unsigned long s)
 228 {
 229         if (rcu_exp_gp_seq_done(rsp, s)) {
 230                 trace_rcu_exp_grace_period(rsp->name, s, TPS("done"));
 231                 /* Ensure test happens before caller kfree(). */
 232                 smp_mb__before_atomic(); /* ^^^ */
 233                 atomic_long_inc(stat);
 234                 return true;
 235         }
 236         return false;
 237 }
 238
 239 /*
 240  * Funnel-lock acquisition for expedited grace periods.  Returns true
 241  * if some other task completed an expedited grace period that this task
 242  * can piggy-back on, and with no mutex held.  Otherwise, returns false
 243  * with the mutex held, indicating that the caller must actually do the
 244  * expedited grace period.
 245  */
 246 static bool exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
 247 {
 248         struct rcu_data *rdp = per_cpu_ptr(rsp->rda, raw_smp_processor_id());
 249         struct rcu_node *rnp = rdp->mynode;
 250         struct rcu_node *rnp_root = rcu_get_root(rsp);
 251
 252         /* Low-contention fastpath. */
 253         if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s) &&
 254             (rnp == rnp_root ||
 255              ULONG_CMP_LT(READ_ONCE(rnp_root->exp_seq_rq), s)) &&
 256             !mutex_is_locked(&rsp->exp_mutex) &&
 257             mutex_trylock(&rsp->exp_mutex))
 258                 goto fastpath;
 259
 260         /*
 261          * Each pass through the following loop works its way up
 262          * the rcu_node tree, returning if others have done the work or
 263          * otherwise falls through to acquire rsp->exp_mutex.  The mapping
 264          * from CPU to rcu_node structure can be inexact, as it is just
 265          * promoting locality and is not strictly needed for correctness.
 266          */
 267         for (; rnp != NULL; rnp = rnp->parent) {
 268                 if (sync_exp_work_done(rsp, &rdp->exp_workdone1, s))
 269                         return true;
 270
 271                 /* Work not done, either wait here or go up. */
 272                 spin_lock(&rnp->exp_lock);
 273                 if (ULONG_CMP_GE(rnp->exp_seq_rq, s)) {
 274
 275                         /* Someone else doing GP, so wait for them. */
 276                         spin_unlock(&rnp->exp_lock);
 277                         trace_rcu_exp_funnel_lock(rsp->name, rnp->level,
 278                                                   rnp->grplo, rnp->grphi,
 279                                                   TPS("wait"));
 280                         wait_event(rnp->exp_wq[(s >> 1) & 0x3],
 281                                    sync_exp_work_done(rsp,
 282                                                       &rdp->exp_workdone2, s));
 283                         return true;
 284                 }
 285                 rnp->exp_seq_rq = s; /* Followers can wait on us. */
 286                 spin_unlock(&rnp->exp_lock);
 287                 trace_rcu_exp_funnel_lock(rsp->name, rnp->level, rnp->grplo,
 288                                           rnp->grphi, TPS("nxtlvl"));
 289         }
 290         mutex_lock(&rsp->exp_mutex);
 291 fastpath:
 292         if (sync_exp_work_done(rsp, &rdp->exp_workdone3, s)) {
 293                 mutex_unlock(&rsp->exp_mutex);
 294                 return true;
 295         }
 296         rcu_exp_gp_seq_start(rsp);
 297         trace_rcu_exp_grace_period(rsp->name, s, TPS("start"));
 298         return false;
 299 }
 300
 301 /* Invoked on each online non-idle CPU for expedited quiescent state. */
 302 static void sync_sched_exp_handler(void *data)
 303 {
 304         struct rcu_data *rdp;
 305         struct rcu_node *rnp;
 306         struct rcu_state *rsp = data;
 307
 308         rdp = this_cpu_ptr(rsp->rda);
 309         rnp = rdp->mynode;
 310         if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
 311             __this_cpu_read(rcu_sched_data.cpu_no_qs.b.exp))
 312                 return;
 313         if (rcu_is_cpu_rrupt_from_idle()) {
 314                 rcu_report_exp_rdp(&rcu_sched_state,
 315                                    this_cpu_ptr(&rcu_sched_data), true);
 316                 return;
 317         }
 318         __this_cpu_write(rcu_sched_data.cpu_no_qs.b.exp, true);
 319         resched_cpu(smp_processor_id());
 320 }
 321
 322 /* Send IPI for expedited cleanup if needed at end of CPU-hotplug operation. */
 323 static void sync_sched_exp_online_cleanup(int cpu)
 324 {
 325         struct rcu_data *rdp;
 326         int ret;
 327         struct rcu_node *rnp;
 328         struct rcu_state *rsp = &rcu_sched_state;
 329
 330         rdp = per_cpu_ptr(rsp->rda, cpu);
 331         rnp = rdp->mynode;
 332         if (!(READ_ONCE(rnp->expmask) & rdp->grpmask))
 333                 return;
 334         ret = smp_call_function_single(cpu, sync_sched_exp_handler, rsp, 0);
 335         WARN_ON_ONCE(ret);
 336 }
 337
 338 /*
 339  * Select the nodes that the upcoming expedited grace period needs
 340  * to wait for.
 341  */
 342 static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
 343                                      smp_call_func_t func)
 344 {
 345         int cpu;
 346         unsigned long flags;
 347         unsigned long mask_ofl_test;
 348         unsigned long mask_ofl_ipi;
 349         int ret;
 350         struct rcu_node *rnp;
 351
 352         sync_exp_reset_tree(rsp);
 353         rcu_for_each_leaf_node(rsp, rnp) {
 354                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
 355
 356                 /* Each pass checks a CPU for identity, offline, and idle. */
 357                 mask_ofl_test = 0;
 358                 for_each_leaf_node_possible_cpu(rnp, cpu) {
 359                         struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
 360                         struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
 361
 362                         if (raw_smp_processor_id() == cpu ||
 363                             !(atomic_add_return(0, &rdtp->dynticks) & 0x1))
 364                                 mask_ofl_test |= rdp->grpmask;
 365                 }
 366                 mask_ofl_ipi = rnp->expmask & ~mask_ofl_test;
 367
 368                 /*
 369                  * Need to wait for any blocked tasks as well.  Note that
 370                  * additional blocking tasks will also block the expedited
 371                  * GP until such time as the ->expmask bits are cleared.
 372                  */
 373                 if (rcu_preempt_has_tasks(rnp))
 374                         rnp->exp_tasks = rnp->blkd_tasks.next;
 375                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 376
 377                 /* IPI the remaining CPUs for expedited quiescent state. */
 378                 for_each_leaf_node_possible_cpu(rnp, cpu) {
 379                         unsigned long mask = leaf_node_cpu_bit(rnp, cpu);
 380                         if (!(mask_ofl_ipi & mask))
 381                                 continue;
 382 retry_ipi:
 383                         ret = smp_call_function_single(cpu, func, rsp, 0);
 384                         if (!ret) {
 385                                 mask_ofl_ipi &= ~mask;
 386                                 continue;
 387                         }
 388                         /* Failed, raced with offline. */
 389                         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 390                         if (cpu_online(cpu) &&
 391                             (rnp->expmask & mask)) {
 392                                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 393                                 schedule_timeout_uninterruptible(1);
 394                                 if (cpu_online(cpu) &&
 395                                     (rnp->expmask & mask))
 396                                         goto retry_ipi;
 397                                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
 398                         }
 399                         if (!(rnp->expmask & mask))
 400                                 mask_ofl_ipi &= ~mask;
 401                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 402                 }
 403                 /* Report quiescent states for those that went offline. */
 404                 mask_ofl_test |= mask_ofl_ipi;
 405                 if (mask_ofl_test)
 406                         rcu_report_exp_cpu_mult(rsp, rnp, mask_ofl_test, false);
 407         }
 408 }
 409
 410 static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
 411 {
 412         int cpu;
 413         unsigned long jiffies_stall;
 414         unsigned long jiffies_start;
 415         unsigned long mask;
 416         int ndetected;
 417         struct rcu_node *rnp;
 418         struct rcu_node *rnp_root = rcu_get_root(rsp);
 419         int ret;
 420
 421         jiffies_stall = rcu_jiffies_till_stall_check();
 422         jiffies_start = jiffies;
 423
 424         for (;;) {
 425                 ret = swait_event_timeout(
 426                                 rsp->expedited_wq,
 427                                 sync_rcu_preempt_exp_done(rnp_root),
 428                                 jiffies_stall);
 429                 if (ret > 0 || sync_rcu_preempt_exp_done(rnp_root))
 430                         return;
 431                 if (ret < 0) {
 432                         /* Hit a signal, disable CPU stall warnings. */
 433                         swait_event(rsp->expedited_wq,
 434                                    sync_rcu_preempt_exp_done(rnp_root));
 435                         return;
 436                 }
 437                 pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {",
 438                        rsp->name);
 439                 ndetected = 0;
 440                 rcu_for_each_leaf_node(rsp, rnp) {
 441                         ndetected += rcu_print_task_exp_stall(rnp);
 442                         for_each_leaf_node_possible_cpu(rnp, cpu) {
 443                                 struct rcu_data *rdp;
 444
 445                                 mask = leaf_node_cpu_bit(rnp, cpu);
 446                                 if (!(rnp->expmask & mask))
 447                                         continue;
 448                                 ndetected++;
 449                                 rdp = per_cpu_ptr(rsp->rda, cpu);
 450                                 pr_cont(" %d-%c%c%c", cpu,
 451                                         "O."[!!cpu_online(cpu)],
 452                                         "o."[!!(rdp->grpmask & rnp->expmaskinit)],
 453                                         "N."[!!(rdp->grpmask & rnp->expmaskinitnext)]);
 454                         }
 455                 }
 456                 pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
 457                         jiffies - jiffies_start, rsp->expedited_sequence,
 458                         rnp_root->expmask, ".T"[!!rnp_root->exp_tasks]);
 459                 if (ndetected) {
 460                         pr_err("blocking rcu_node structures:");
 461                         rcu_for_each_node_breadth_first(rsp, rnp) {
 462                                 if (rnp == rnp_root)
 463                                         continue; /* printed unconditionally */
 464                                 if (sync_rcu_preempt_exp_done(rnp))
 465                                         continue;
 466                                 pr_cont(" l=%u:%d-%d:%#lx/%c",
 467                                         rnp->level, rnp->grplo, rnp->grphi,
 468                                         rnp->expmask,
 469                                         ".T"[!!rnp->exp_tasks]);
 470                         }
 471                         pr_cont("\n");
 472                 }
 473                 rcu_for_each_leaf_node(rsp, rnp) {
 474                         for_each_leaf_node_possible_cpu(rnp, cpu) {
 475                                 mask = leaf_node_cpu_bit(rnp, cpu);
 476                                 if (!(rnp->expmask & mask))
 477                                         continue;
 478                                 dump_cpu_task(cpu);
 479                         }
 480                 }
 481                 jiffies_stall = 3 * rcu_jiffies_till_stall_check() + 3;
 482         }
 483 }
 484
 485 /*
 486  * Wait for the current expedited grace period to complete, and then
 487  * wake up everyone who piggybacked on the just-completed expedited
 488  * grace period.  Also update all the ->exp_seq_rq counters as needed
 489  * in order to avoid counter-wrap problems.
 490  */
 491 static void rcu_exp_wait_wake(struct rcu_state *rsp, unsigned long s)
 492 {
 493         struct rcu_node *rnp;
 494
 495         synchronize_sched_expedited_wait(rsp);
 496         rcu_exp_gp_seq_end(rsp);
 497         trace_rcu_exp_grace_period(rsp->name, s, TPS("end"));
 498
 499         /*
 500          * Switch over to wakeup mode, allowing the next GP, but -only- the
 501          * next GP, to proceed.
 502          */
 503         mutex_lock(&rsp->exp_wake_mutex);
 504         mutex_unlock(&rsp->exp_mutex);
 505
 506         rcu_for_each_node_breadth_first(rsp, rnp) {
 507                 if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s)) {
 508                         spin_lock(&rnp->exp_lock);
 509                         /* Recheck, avoid hang in case someone just arrived. */
 510                         if (ULONG_CMP_LT(rnp->exp_seq_rq, s))
 511                                 rnp->exp_seq_rq = s;
 512                         spin_unlock(&rnp->exp_lock);
 513                 }
 514                 wake_up_all(&rnp->exp_wq[(rsp->expedited_sequence >> 1) & 0x3]);
 515         }
 516         trace_rcu_exp_grace_period(rsp->name, s, TPS("endwake"));
 517         mutex_unlock(&rsp->exp_wake_mutex);
 518 }
 519
 520 /**
 521  * synchronize_sched_expedited - Brute-force RCU-sched grace period
 522  *
 523  * Wait for an RCU-sched grace period to elapse, but use a "big hammer"
 524  * approach to force the grace period to end quickly.  This consumes
 525  * significant time on all CPUs and is unfriendly to real-time workloads,
 526  * so is thus not recommended for any sort of common-case code.  In fact,
 527  * if you are using synchronize_sched_expedited() in a loop, please
 528  * restructure your code to batch your updates, and then use a single
 529  * synchronize_sched() instead.
 530  *
 531  * This implementation can be thought of as an application of sequence
 532  * locking to expedited grace periods, but using the sequence counter to
 533  * determine when someone else has already done the work instead of for
 534  * retrying readers.
 535  */
 536 void synchronize_sched_expedited(void)
 537 {
 538         unsigned long s;
 539         struct rcu_state *rsp = &rcu_sched_state;
 540
 541         /* If only one CPU, this is automatically a grace period. */
 542         if (rcu_blocking_is_gp())
 543                 return;
 544
 545         /* If expedited grace periods are prohibited, fall back to normal. */
 546         if (rcu_gp_is_normal()) {
 547                 wait_rcu_gp(call_rcu_sched);
 548                 return;
 549         }
 550
 551         /* Take a snapshot of the sequence number.  */
 552         s = rcu_exp_gp_seq_snap(rsp);
 553         if (exp_funnel_lock(rsp, s))
 554                 return;  /* Someone else did our work for us. */
 555
 556         /* Initialize the rcu_node tree in preparation for the wait. */
 557         sync_rcu_exp_select_cpus(rsp, sync_sched_exp_handler);
 558
 559         /* Wait and clean up, including waking everyone. */
 560         rcu_exp_wait_wake(rsp, s);
 561 }
 562 EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
 563
 564 #ifdef CONFIG_PREEMPT_RCU
 565
 566 /*
 567  * Remote handler for smp_call_function_single().  If there is an
 568  * RCU read-side critical section in effect, request that the
 569  * next rcu_read_unlock() record the quiescent state up the
 570  * ->expmask fields in the rcu_node tree.  Otherwise, immediately
 571  * report the quiescent state.
 572  */
 573 static void sync_rcu_exp_handler(void *info)
 574 {
 575         struct rcu_data *rdp;
 576         struct rcu_state *rsp = info;
 577         struct task_struct *t = current;
 578
 579         /*
 580          * Within an RCU read-side critical section, request that the next
 581          * rcu_read_unlock() report.  Unless this RCU read-side critical
 582          * section has already blocked, in which case it is already set
 583          * up for the expedited grace period to wait on it.
 584          */
 585         if (t->rcu_read_lock_nesting > 0 &&
 586             !t->rcu_read_unlock_special.b.blocked) {
 587                 t->rcu_read_unlock_special.b.exp_need_qs = true;
 588                 return;
 589         }
 590
 591         /*
 592          * We are either exiting an RCU read-side critical section (negative
 593          * values of t->rcu_read_lock_nesting) or are not in one at all
 594          * (zero value of t->rcu_read_lock_nesting).  Or we are in an RCU
 595          * read-side critical section that blocked before this expedited
 596          * grace period started.  Either way, we can immediately report
 597          * the quiescent state.
 598          */
 599         rdp = this_cpu_ptr(rsp->rda);
 600         rcu_report_exp_rdp(rsp, rdp, true);
 601 }
 602
 603 /**
 604  * synchronize_rcu_expedited - Brute-force RCU grace period
 605  *
 606  * Wait for an RCU-preempt grace period, but expedite it.  The basic
 607  * idea is to IPI all non-idle non-nohz online CPUs.  The IPI handler
 608  * checks whether the CPU is in an RCU-preempt critical section, and
 609  * if so, it sets a flag that causes the outermost rcu_read_unlock()
 610  * to report the quiescent state.  On the other hand, if the CPU is
 611  * not in an RCU read-side critical section, the IPI handler reports
 612  * the quiescent state immediately.
 613  *
 614  * Although this is a greate improvement over previous expedited
 615  * implementations, it is still unfriendly to real-time workloads, so is
 616  * thus not recommended for any sort of common-case code.  In fact, if
 617  * you are using synchronize_rcu_expedited() in a loop, please restructure
 618  * your code to batch your updates, and then Use a single synchronize_rcu()
 619  * instead.
 620  */
 621 void synchronize_rcu_expedited(void)
 622 {
 623         struct rcu_state *rsp = rcu_state_p;
 624         unsigned long s;
 625
 626         /* If expedited grace periods are prohibited, fall back to normal. */
 627         if (rcu_gp_is_normal()) {
 628                 wait_rcu_gp(call_rcu);
 629                 return;
 630         }
 631
 632         s = rcu_exp_gp_seq_snap(rsp);
 633         if (exp_funnel_lock(rsp, s))
 634                 return;  /* Someone else did our work for us. */
 635
 636         /* Initialize the rcu_node tree in preparation for the wait. */
 637         sync_rcu_exp_select_cpus(rsp, sync_rcu_exp_handler);
 638
 639         /* Wait for ->blkd_tasks lists to drain, then wake everyone up. */
 640         rcu_exp_wait_wake(rsp, s);
 641 }
 642 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
 643
 644 #else /* #ifdef CONFIG_PREEMPT_RCU */
 645
 646 /*
 647  * Wait for an rcu-preempt grace period, but make it happen quickly.
 648  * But because preemptible RCU does not exist, map to rcu-sched.
 649  */
 650 void synchronize_rcu_expedited(void)
 651 {
 652         synchronize_sched_expedited();
 653 }
 654 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
 655
 656 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */