kernel/rcu/tree_exp.h

   1 /* SPDX-License-Identifier: GPL-2.0+ */
   2 /*
   3  * RCU expedited grace periods
   4  *
   5  * Copyright IBM Corporation, 2016
   6  *
   7  * Authors: Paul E. McKenney <paulmck@linux.ibm.com>
   8  */
   9
  10 #include <linux/console.h>
  11 #include <linux/lockdep.h>
  12
  13 static void rcu_exp_handler(void *unused);
  14 static int rcu_print_task_exp_stall(struct rcu_node *rnp);
  15 static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp);
  16
  17 /*
  18  * Record the start of an expedited grace period.
  19  */
  20 static void rcu_exp_gp_seq_start(void)
  21 {
  22         rcu_seq_start(&rcu_state.expedited_sequence);
  23         rcu_poll_gp_seq_start_unlocked(&rcu_state.gp_seq_polled_exp_snap);
  24 }
  25
  26 /*
  27  * Return the value that the expedited-grace-period counter will have
  28  * at the end of the current grace period.
  29  */
  30 static __maybe_unused unsigned long rcu_exp_gp_seq_endval(void)
  31 {
  32         return rcu_seq_endval(&rcu_state.expedited_sequence);
  33 }
  34
  35 /*
  36  * Record the end of an expedited grace period.
  37  */
  38 static void rcu_exp_gp_seq_end(void)
  39 {
  40         rcu_poll_gp_seq_end_unlocked(&rcu_state.gp_seq_polled_exp_snap);
  41         rcu_seq_end(&rcu_state.expedited_sequence);
  42         smp_mb(); /* Ensure that consecutive grace periods serialize. */
  43 }
  44
  45 /*
  46  * Take a snapshot of the expedited-grace-period counter, which is the
  47  * earliest value that will indicate that a full grace period has
  48  * elapsed since the current time.
  49  */
  50 static unsigned long rcu_exp_gp_seq_snap(void)
  51 {
  52         unsigned long s;
  53
  54         smp_mb(); /* Caller's modifications seen first by other CPUs. */
  55         s = rcu_seq_snap(&rcu_state.expedited_sequence);
  56         trace_rcu_exp_grace_period(rcu_state.name, s, TPS("snap"));
  57         return s;
  58 }
  59
  60 /*
  61  * Given a counter snapshot from rcu_exp_gp_seq_snap(), return true
  62  * if a full expedited grace period has elapsed since that snapshot
  63  * was taken.
  64  */
  65 static bool rcu_exp_gp_seq_done(unsigned long s)
  66 {
  67         return rcu_seq_done(&rcu_state.expedited_sequence, s);
  68 }
  69
  70 /*
  71  * Reset the ->expmaskinit values in the rcu_node tree to reflect any
  72  * recent CPU-online activity.  Note that these masks are not cleared
  73  * when CPUs go offline, so they reflect the union of all CPUs that have
  74  * ever been online.  This means that this function normally takes its
  75  * no-work-to-do fastpath.
  76  */
  77 static void sync_exp_reset_tree_hotplug(void)
  78 {
  79         bool done;
  80         unsigned long flags;
  81         unsigned long mask;
  82         unsigned long oldmask;
  83         int ncpus = smp_load_acquire(&rcu_state.ncpus); /* Order vs. locking. */
  84         struct rcu_node *rnp;
  85         struct rcu_node *rnp_up;
  86
  87         /* If no new CPUs onlined since last time, nothing to do. */
  88         if (likely(ncpus == rcu_state.ncpus_snap))
  89                 return;
  90         rcu_state.ncpus_snap = ncpus;
  91
  92         /*
  93          * Each pass through the following loop propagates newly onlined
  94          * CPUs for the current rcu_node structure up the rcu_node tree.
  95          */
  96         rcu_for_each_leaf_node(rnp) {
  97                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
  98                 if (rnp->expmaskinit == rnp->expmaskinitnext) {
  99                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 100                         continue;  /* No new CPUs, nothing to do. */
 101                 }
 102
 103                 /* Update this node's mask, track old value for propagation. */
 104                 oldmask = rnp->expmaskinit;
 105                 rnp->expmaskinit = rnp->expmaskinitnext;
 106                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 107
 108                 /* If was already nonzero, nothing to propagate. */
 109                 if (oldmask)
 110                         continue;
 111
 112                 /* Propagate the new CPU up the tree. */
 113                 mask = rnp->grpmask;
 114                 rnp_up = rnp->parent;
 115                 done = false;
 116                 while (rnp_up) {
 117                         raw_spin_lock_irqsave_rcu_node(rnp_up, flags);
 118                         if (rnp_up->expmaskinit)
 119                                 done = true;
 120                         rnp_up->expmaskinit |= mask;
 121                         raw_spin_unlock_irqrestore_rcu_node(rnp_up, flags);
 122                         if (done)
 123                                 break;
 124                         mask = rnp_up->grpmask;
 125                         rnp_up = rnp_up->parent;
 126                 }
 127         }
 128 }
 129
 130 /*
 131  * Reset the ->expmask values in the rcu_node tree in preparation for
 132  * a new expedited grace period.
 133  */
 134 static void __maybe_unused sync_exp_reset_tree(void)
 135 {
 136         unsigned long flags;
 137         struct rcu_node *rnp;
 138
 139         sync_exp_reset_tree_hotplug();
 140         rcu_for_each_node_breadth_first(rnp) {
 141                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
 142                 WARN_ON_ONCE(rnp->expmask);
 143                 WRITE_ONCE(rnp->expmask, rnp->expmaskinit);
 144                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 145         }
 146 }
 147
 148 /*
 149  * Return non-zero if there is no RCU expedited grace period in progress
 150  * for the specified rcu_node structure, in other words, if all CPUs and
 151  * tasks covered by the specified rcu_node structure have done their bit
 152  * for the current expedited grace period.
 153  */
 154 static bool sync_rcu_exp_done(struct rcu_node *rnp)
 155 {
 156         raw_lockdep_assert_held_rcu_node(rnp);
 157         return READ_ONCE(rnp->exp_tasks) == NULL &&
 158                READ_ONCE(rnp->expmask) == 0;
 159 }
 160
 161 /*
 162  * Like sync_rcu_exp_done(), but where the caller does not hold the
 163  * rcu_node's ->lock.
 164  */
 165 static bool sync_rcu_exp_done_unlocked(struct rcu_node *rnp)
 166 {
 167         unsigned long flags;
 168         bool ret;
 169
 170         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 171         ret = sync_rcu_exp_done(rnp);
 172         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 173
 174         return ret;
 175 }
 176
 177 /*
 178  * Report the exit from RCU read-side critical section for the last task
 179  * that queued itself during or before the current expedited preemptible-RCU
 180  * grace period.  This event is reported either to the rcu_node structure on
 181  * which the task was queued or to one of that rcu_node structure's ancestors,
 182  * recursively up the tree.  (Calm down, calm down, we do the recursion
 183  * iteratively!)
 184  */
 185 static void __rcu_report_exp_rnp(struct rcu_node *rnp,
 186                                  bool wake, unsigned long flags)
 187         __releases(rnp->lock)
 188 {
 189         unsigned long mask;
 190
 191         raw_lockdep_assert_held_rcu_node(rnp);
 192         for (;;) {
 193                 if (!sync_rcu_exp_done(rnp)) {
 194                         if (!rnp->expmask)
 195                                 rcu_initiate_boost(rnp, flags);
 196                         else
 197                                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 198                         break;
 199                 }
 200                 if (rnp->parent == NULL) {
 201                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 202                         if (wake)
 203                                 swake_up_one_online(&rcu_state.expedited_wq);
 204
 205                         break;
 206                 }
 207                 mask = rnp->grpmask;
 208                 raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled */
 209                 rnp = rnp->parent;
 210                 raw_spin_lock_rcu_node(rnp); /* irqs already disabled */
 211                 WARN_ON_ONCE(!(rnp->expmask & mask));
 212                 WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask);
 213         }
 214 }
 215
 216 /*
 217  * Report expedited quiescent state for specified node.  This is a
 218  * lock-acquisition wrapper function for __rcu_report_exp_rnp().
 219  */
 220 static void __maybe_unused rcu_report_exp_rnp(struct rcu_node *rnp, bool wake)
 221 {
 222         unsigned long flags;
 223
 224         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 225         __rcu_report_exp_rnp(rnp, wake, flags);
 226 }
 227
 228 /*
 229  * Report expedited quiescent state for multiple CPUs, all covered by the
 230  * specified leaf rcu_node structure.
 231  */
 232 static void rcu_report_exp_cpu_mult(struct rcu_node *rnp,
 233                                     unsigned long mask, bool wake)
 234 {
 235         int cpu;
 236         unsigned long flags;
 237         struct rcu_data *rdp;
 238
 239         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 240         if (!(rnp->expmask & mask)) {
 241                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 242                 return;
 243         }
 244         WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask);
 245         for_each_leaf_node_cpu_mask(rnp, cpu, mask) {
 246                 rdp = per_cpu_ptr(&rcu_data, cpu);
 247                 if (!IS_ENABLED(CONFIG_NO_HZ_FULL) || !rdp->rcu_forced_tick_exp)
 248                         continue;
 249                 rdp->rcu_forced_tick_exp = false;
 250                 tick_dep_clear_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
 251         }
 252         __rcu_report_exp_rnp(rnp, wake, flags); /* Releases rnp->lock. */
 253 }
 254
 255 /*
 256  * Report expedited quiescent state for specified rcu_data (CPU).
 257  */
 258 static void rcu_report_exp_rdp(struct rcu_data *rdp)
 259 {
 260         WRITE_ONCE(rdp->cpu_no_qs.b.exp, false);
 261         rcu_report_exp_cpu_mult(rdp->mynode, rdp->grpmask, true);
 262 }
 263
 264 /* Common code for work-done checking. */
 265 static bool sync_exp_work_done(unsigned long s)
 266 {
 267         if (rcu_exp_gp_seq_done(s)) {
 268                 trace_rcu_exp_grace_period(rcu_state.name, s, TPS("done"));
 269                 /*
 270                  * Order GP completion with preceding accesses. Order also GP
 271                  * completion with post GP update side accesses. Pairs with
 272                  * rcu_seq_end().
 273                  */
 274                 smp_mb();
 275                 return true;
 276         }
 277         return false;
 278 }
 279
 280 /*
 281  * Funnel-lock acquisition for expedited grace periods.  Returns true
 282  * if some other task completed an expedited grace period that this task
 283  * can piggy-back on, and with no mutex held.  Otherwise, returns false
 284  * with the mutex held, indicating that the caller must actually do the
 285  * expedited grace period.
 286  */
 287 static bool exp_funnel_lock(unsigned long s)
 288 {
 289         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, raw_smp_processor_id());
 290         struct rcu_node *rnp = rdp->mynode;
 291         struct rcu_node *rnp_root = rcu_get_root();
 292
 293         /* Low-contention fastpath. */
 294         if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s) &&
 295             (rnp == rnp_root ||
 296              ULONG_CMP_LT(READ_ONCE(rnp_root->exp_seq_rq), s)) &&
 297             mutex_trylock(&rcu_state.exp_mutex))
 298                 goto fastpath;
 299
 300         /*
 301          * Each pass through the following loop works its way up
 302          * the rcu_node tree, returning if others have done the work or
 303          * otherwise falls through to acquire ->exp_mutex.  The mapping
 304          * from CPU to rcu_node structure can be inexact, as it is just
 305          * promoting locality and is not strictly needed for correctness.
 306          */
 307         for (; rnp != NULL; rnp = rnp->parent) {
 308                 if (sync_exp_work_done(s))
 309                         return true;
 310
 311                 /* Work not done, either wait here or go up. */
 312                 spin_lock(&rnp->exp_lock);
 313                 if (ULONG_CMP_GE(rnp->exp_seq_rq, s)) {
 314
 315                         /* Someone else doing GP, so wait for them. */
 316                         spin_unlock(&rnp->exp_lock);
 317                         trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
 318                                                   rnp->grplo, rnp->grphi,
 319                                                   TPS("wait"));
 320                         wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
 321                                    sync_exp_work_done(s));
 322                         return true;
 323                 }
 324                 WRITE_ONCE(rnp->exp_seq_rq, s); /* Followers can wait on us. */
 325                 spin_unlock(&rnp->exp_lock);
 326                 trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
 327                                           rnp->grplo, rnp->grphi, TPS("nxtlvl"));
 328         }
 329         mutex_lock(&rcu_state.exp_mutex);
 330 fastpath:
 331         if (sync_exp_work_done(s)) {
 332                 mutex_unlock(&rcu_state.exp_mutex);
 333                 return true;
 334         }
 335         rcu_exp_gp_seq_start();
 336         trace_rcu_exp_grace_period(rcu_state.name, s, TPS("start"));
 337         return false;
 338 }
 339
 340 /*
 341  * Select the CPUs within the specified rcu_node that the upcoming
 342  * expedited grace period needs to wait for.
 343  */
 344 static void __sync_rcu_exp_select_node_cpus(struct rcu_exp_work *rewp)
 345 {
 346         int cpu;
 347         unsigned long flags;
 348         unsigned long mask_ofl_test;
 349         unsigned long mask_ofl_ipi;
 350         int ret;
 351         struct rcu_node *rnp = container_of(rewp, struct rcu_node, rew);
 352
 353         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 354
 355         /* Each pass checks a CPU for identity, offline, and idle. */
 356         mask_ofl_test = 0;
 357         for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) {
 358                 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 359                 unsigned long mask = rdp->grpmask;
 360                 int snap;
 361
 362                 if (raw_smp_processor_id() == cpu ||
 363                     !(rnp->qsmaskinitnext & mask)) {
 364                         mask_ofl_test |= mask;
 365                 } else {
 366                         /*
 367                          * Full ordering between remote CPU's post idle accesses
 368                          * and updater's accesses prior to current GP (and also
 369                          * the started GP sequence number) is enforced by
 370                          * rcu_seq_start() implicit barrier, relayed by kworkers
 371                          * locking and even further by smp_mb__after_unlock_lock()
 372                          * barriers chained all the way throughout the rnp locking
 373                          * tree since sync_exp_reset_tree() and up to the current
 374                          * leaf rnp locking.
 375                          *
 376                          * Ordering between remote CPU's pre idle accesses and
 377                          * post grace period updater's accesses is enforced by the
 378                          * below acquire semantic.
 379                          */
 380                         snap = ct_rcu_watching_cpu_acquire(cpu);
 381                         if (rcu_watching_snap_in_eqs(snap))
 382                                 mask_ofl_test |= mask;
 383                         else
 384                                 rdp->exp_watching_snap = snap;
 385                 }
 386         }
 387         mask_ofl_ipi = rnp->expmask & ~mask_ofl_test;
 388
 389         /*
 390          * Need to wait for any blocked tasks as well.  Note that
 391          * additional blocking tasks will also block the expedited GP
 392          * until such time as the ->expmask bits are cleared.
 393          */
 394         if (rcu_preempt_has_tasks(rnp))
 395                 WRITE_ONCE(rnp->exp_tasks, rnp->blkd_tasks.next);
 396         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 397
 398         /* IPI the remaining CPUs for expedited quiescent state. */
 399         for_each_leaf_node_cpu_mask(rnp, cpu, mask_ofl_ipi) {
 400                 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 401                 unsigned long mask = rdp->grpmask;
 402
 403 retry_ipi:
 404                 if (rcu_watching_snap_stopped_since(rdp, rdp->exp_watching_snap)) {
 405                         mask_ofl_test |= mask;
 406                         continue;
 407                 }
 408                 if (get_cpu() == cpu) {
 409                         mask_ofl_test |= mask;
 410                         put_cpu();
 411                         continue;
 412                 }
 413                 ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
 414                 put_cpu();
 415                 /* The CPU will report the QS in response to the IPI. */
 416                 if (!ret)
 417                         continue;
 418
 419                 /* Failed, raced with CPU hotplug operation. */
 420                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
 421                 if ((rnp->qsmaskinitnext & mask) &&
 422                     (rnp->expmask & mask)) {
 423                         /* Online, so delay for a bit and try again. */
 424                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 425                         trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("selectofl"));
 426                         schedule_timeout_idle(1);
 427                         goto retry_ipi;
 428                 }
 429                 /* CPU really is offline, so we must report its QS. */
 430                 if (rnp->expmask & mask)
 431                         mask_ofl_test |= mask;
 432                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 433         }
 434         /* Report quiescent states for those that went offline. */
 435         if (mask_ofl_test)
 436                 rcu_report_exp_cpu_mult(rnp, mask_ofl_test, false);
 437 }
 438
 439 static void rcu_exp_sel_wait_wake(unsigned long s);
 440
 441 static void sync_rcu_exp_select_node_cpus(struct kthread_work *wp)
 442 {
 443         struct rcu_exp_work *rewp =
 444                 container_of(wp, struct rcu_exp_work, rew_work);
 445
 446         __sync_rcu_exp_select_node_cpus(rewp);
 447 }
 448
 449 static inline bool rcu_exp_worker_started(void)
 450 {
 451         return !!READ_ONCE(rcu_exp_gp_kworker);
 452 }
 453
 454 static inline bool rcu_exp_par_worker_started(struct rcu_node *rnp)
 455 {
 456         return !!READ_ONCE(rnp->exp_kworker);
 457 }
 458
 459 static inline void sync_rcu_exp_select_cpus_queue_work(struct rcu_node *rnp)
 460 {
 461         kthread_init_work(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
 462         /*
 463          * Use rcu_exp_par_gp_kworker, because flushing a work item from
 464          * another work item on the same kthread worker can result in
 465          * deadlock.
 466          */
 467         kthread_queue_work(READ_ONCE(rnp->exp_kworker), &rnp->rew.rew_work);
 468 }
 469
 470 static inline void sync_rcu_exp_select_cpus_flush_work(struct rcu_node *rnp)
 471 {
 472         kthread_flush_work(&rnp->rew.rew_work);
 473 }
 474
 475 /*
 476  * Work-queue handler to drive an expedited grace period forward.
 477  */
 478 static void wait_rcu_exp_gp(struct kthread_work *wp)
 479 {
 480         struct rcu_exp_work *rewp;
 481
 482         rewp = container_of(wp, struct rcu_exp_work, rew_work);
 483         rcu_exp_sel_wait_wake(rewp->rew_s);
 484 }
 485
 486 static inline void synchronize_rcu_expedited_queue_work(struct rcu_exp_work *rew)
 487 {
 488         kthread_init_work(&rew->rew_work, wait_rcu_exp_gp);
 489         kthread_queue_work(rcu_exp_gp_kworker, &rew->rew_work);
 490 }
 491
 492 /*
 493  * Select the nodes that the upcoming expedited grace period needs
 494  * to wait for.
 495  */
 496 static void sync_rcu_exp_select_cpus(void)
 497 {
 498         struct rcu_node *rnp;
 499
 500         trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("reset"));
 501         sync_exp_reset_tree();
 502         trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("select"));
 503
 504         /* Schedule work for each leaf rcu_node structure. */
 505         rcu_for_each_leaf_node(rnp) {
 506                 rnp->exp_need_flush = false;
 507                 if (!READ_ONCE(rnp->expmask))
 508                         continue; /* Avoid early boot non-existent wq. */
 509                 if (!rcu_exp_par_worker_started(rnp) ||
 510                     rcu_scheduler_active != RCU_SCHEDULER_RUNNING ||
 511                     rcu_is_last_leaf_node(rnp)) {
 512                         /* No worker started yet or last leaf, do direct call. */
 513                         sync_rcu_exp_select_node_cpus(&rnp->rew.rew_work);
 514                         continue;
 515                 }
 516                 sync_rcu_exp_select_cpus_queue_work(rnp);
 517                 rnp->exp_need_flush = true;
 518         }
 519
 520         /* Wait for jobs (if any) to complete. */
 521         rcu_for_each_leaf_node(rnp)
 522                 if (rnp->exp_need_flush)
 523                         sync_rcu_exp_select_cpus_flush_work(rnp);
 524 }
 525
 526 /*
 527  * Wait for the expedited grace period to elapse, within time limit.
 528  * If the time limit is exceeded without the grace period elapsing,
 529  * return false, otherwise return true.
 530  */
 531 static bool synchronize_rcu_expedited_wait_once(long tlimit)
 532 {
 533         int t;
 534         struct rcu_node *rnp_root = rcu_get_root();
 535
 536         t = swait_event_timeout_exclusive(rcu_state.expedited_wq,
 537                                           sync_rcu_exp_done_unlocked(rnp_root),
 538                                           tlimit);
 539         // Workqueues should not be signaled.
 540         if (t > 0 || sync_rcu_exp_done_unlocked(rnp_root))
 541                 return true;
 542         WARN_ON(t < 0);  /* workqueues should not be signaled. */
 543         return false;
 544 }
 545
 546 /*
 547  * Print out an expedited RCU CPU stall warning message.
 548  */
 549 static void synchronize_rcu_expedited_stall(unsigned long jiffies_start, unsigned long j)
 550 {
 551         int cpu;
 552         unsigned long mask;
 553         int ndetected;
 554         struct rcu_node *rnp;
 555         struct rcu_node *rnp_root = rcu_get_root();
 556
 557         if (READ_ONCE(csd_lock_suppress_rcu_stall) && csd_lock_is_stuck()) {
 558                 pr_err("INFO: %s detected expedited stalls, but suppressed full report due to a stuck CSD-lock.\n", rcu_state.name);
 559                 return;
 560         }
 561         pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {", rcu_state.name);
 562         ndetected = 0;
 563         rcu_for_each_leaf_node(rnp) {
 564                 ndetected += rcu_print_task_exp_stall(rnp);
 565                 for_each_leaf_node_possible_cpu(rnp, cpu) {
 566                         struct rcu_data *rdp;
 567
 568                         mask = leaf_node_cpu_bit(rnp, cpu);
 569                         if (!(READ_ONCE(rnp->expmask) & mask))
 570                                 continue;
 571                         ndetected++;
 572                         rdp = per_cpu_ptr(&rcu_data, cpu);
 573                         pr_cont(" %d-%c%c%c%c", cpu,
 574                                 "O."[!!cpu_online(cpu)],
 575                                 "o."[!!(rdp->grpmask & rnp->expmaskinit)],
 576                                 "N."[!!(rdp->grpmask & rnp->expmaskinitnext)],
 577                                 "D."[!!data_race(rdp->cpu_no_qs.b.exp)]);
 578                 }
 579         }
 580         pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
 581                 j - jiffies_start, rcu_state.expedited_sequence, data_race(rnp_root->expmask),
 582                 ".T"[!!data_race(rnp_root->exp_tasks)]);
 583         if (ndetected) {
 584                 pr_err("blocking rcu_node structures (internal RCU debug):");
 585                 rcu_for_each_node_breadth_first(rnp) {
 586                         if (rnp == rnp_root)
 587                                 continue; /* printed unconditionally */
 588                         if (sync_rcu_exp_done_unlocked(rnp))
 589                                 continue;
 590                         pr_cont(" l=%u:%d-%d:%#lx/%c",
 591                                 rnp->level, rnp->grplo, rnp->grphi, data_race(rnp->expmask),
 592                                 ".T"[!!data_race(rnp->exp_tasks)]);
 593                 }
 594                 pr_cont("\n");
 595         }
 596         rcu_for_each_leaf_node(rnp) {
 597                 for_each_leaf_node_possible_cpu(rnp, cpu) {
 598                         mask = leaf_node_cpu_bit(rnp, cpu);
 599                         if (!(READ_ONCE(rnp->expmask) & mask))
 600                                 continue;
 601                         dump_cpu_task(cpu);
 602                 }
 603                 rcu_exp_print_detail_task_stall_rnp(rnp);
 604         }
 605 }
 606
 607 /*
 608  * Wait for the expedited grace period to elapse, issuing any needed
 609  * RCU CPU stall warnings along the way.
 610  */
 611 static void synchronize_rcu_expedited_wait(void)
 612 {
 613         int cpu;
 614         unsigned long j;
 615         unsigned long jiffies_stall;
 616         unsigned long jiffies_start;
 617         unsigned long mask;
 618         struct rcu_data *rdp;
 619         struct rcu_node *rnp;
 620         unsigned long flags;
 621
 622         trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("startwait"));
 623         jiffies_stall = rcu_exp_jiffies_till_stall_check();
 624         jiffies_start = jiffies;
 625         if (tick_nohz_full_enabled() && rcu_inkernel_boot_has_ended()) {
 626                 if (synchronize_rcu_expedited_wait_once(1))
 627                         return;
 628                 rcu_for_each_leaf_node(rnp) {
 629                         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 630                         mask = READ_ONCE(rnp->expmask);
 631                         for_each_leaf_node_cpu_mask(rnp, cpu, mask) {
 632                                 rdp = per_cpu_ptr(&rcu_data, cpu);
 633                                 if (rdp->rcu_forced_tick_exp)
 634                                         continue;
 635                                 rdp->rcu_forced_tick_exp = true;
 636                                 if (cpu_online(cpu))
 637                                         tick_dep_set_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
 638                         }
 639                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 640                 }
 641                 j = READ_ONCE(jiffies_till_first_fqs);
 642                 if (synchronize_rcu_expedited_wait_once(j + HZ))
 643                         return;
 644         }
 645
 646         for (;;) {
 647                 unsigned long j;
 648
 649                 if (synchronize_rcu_expedited_wait_once(jiffies_stall))
 650                         return;
 651                 if (rcu_stall_is_suppressed())
 652                         continue;
 653
 654                 nbcon_cpu_emergency_enter();
 655
 656                 j = jiffies;
 657                 rcu_stall_notifier_call_chain(RCU_STALL_NOTIFY_EXP, (void *)(j - jiffies_start));
 658                 trace_rcu_stall_warning(rcu_state.name, TPS("ExpeditedStall"));
 659                 synchronize_rcu_expedited_stall(jiffies_start, j);
 660                 jiffies_stall = 3 * rcu_exp_jiffies_till_stall_check() + 3;
 661
 662                 nbcon_cpu_emergency_exit();
 663
 664                 panic_on_rcu_stall();
 665         }
 666 }
 667
 668 /*
 669  * Wait for the current expedited grace period to complete, and then
 670  * wake up everyone who piggybacked on the just-completed expedited
 671  * grace period.  Also update all the ->exp_seq_rq counters as needed
 672  * in order to avoid counter-wrap problems.
 673  */
 674 static void rcu_exp_wait_wake(unsigned long s)
 675 {
 676         struct rcu_node *rnp;
 677
 678         synchronize_rcu_expedited_wait();
 679
 680         // Switch over to wakeup mode, allowing the next GP to proceed.
 681         // End the previous grace period only after acquiring the mutex
 682         // to ensure that only one GP runs concurrently with wakeups.
 683         mutex_lock(&rcu_state.exp_wake_mutex);
 684         rcu_exp_gp_seq_end();
 685         trace_rcu_exp_grace_period(rcu_state.name, s, TPS("end"));
 686
 687         rcu_for_each_node_breadth_first(rnp) {
 688                 if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s)) {
 689                         spin_lock(&rnp->exp_lock);
 690                         /* Recheck, avoid hang in case someone just arrived. */
 691                         if (ULONG_CMP_LT(rnp->exp_seq_rq, s))
 692                                 WRITE_ONCE(rnp->exp_seq_rq, s);
 693                         spin_unlock(&rnp->exp_lock);
 694                 }
 695                 smp_mb(); /* All above changes before wakeup. */
 696                 wake_up_all(&rnp->exp_wq[rcu_seq_ctr(s) & 0x3]);
 697         }
 698         trace_rcu_exp_grace_period(rcu_state.name, s, TPS("endwake"));
 699         mutex_unlock(&rcu_state.exp_wake_mutex);
 700 }
 701
 702 /*
 703  * Common code to drive an expedited grace period forward, used by
 704  * workqueues and mid-boot-time tasks.
 705  */
 706 static void rcu_exp_sel_wait_wake(unsigned long s)
 707 {
 708         /* Initialize the rcu_node tree in preparation for the wait. */
 709         sync_rcu_exp_select_cpus();
 710
 711         /* Wait and clean up, including waking everyone. */
 712         rcu_exp_wait_wake(s);
 713 }
 714
 715 #ifdef CONFIG_PREEMPT_RCU
 716
 717 /*
 718  * Remote handler for smp_call_function_single().  If there is an
 719  * RCU read-side critical section in effect, request that the
 720  * next rcu_read_unlock() record the quiescent state up the
 721  * ->expmask fields in the rcu_node tree.  Otherwise, immediately
 722  * report the quiescent state.
 723  */
 724 static void rcu_exp_handler(void *unused)
 725 {
 726         int depth = rcu_preempt_depth();
 727         unsigned long flags;
 728         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 729         struct rcu_node *rnp = rdp->mynode;
 730         struct task_struct *t = current;
 731
 732         /*
 733          * First, the common case of not being in an RCU read-side
 734          * critical section.  If also enabled or idle, immediately
 735          * report the quiescent state, otherwise defer.
 736          */
 737         if (!depth) {
 738                 if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
 739                     rcu_is_cpu_rrupt_from_idle()) {
 740                         rcu_report_exp_rdp(rdp);
 741                 } else {
 742                         WRITE_ONCE(rdp->cpu_no_qs.b.exp, true);
 743                         set_tsk_need_resched(t);
 744                         set_preempt_need_resched();
 745                 }
 746                 return;
 747         }
 748
 749         /*
 750          * Second, the less-common case of being in an RCU read-side
 751          * critical section.  In this case we can count on a future
 752          * rcu_read_unlock().  However, this rcu_read_unlock() might
 753          * execute on some other CPU, but in that case there will be
 754          * a future context switch.  Either way, if the expedited
 755          * grace period is still waiting on this CPU, set ->deferred_qs
 756          * so that the eventual quiescent state will be reported.
 757          * Note that there is a large group of race conditions that
 758          * can have caused this quiescent state to already have been
 759          * reported, so we really do need to check ->expmask.
 760          */
 761         if (depth > 0) {
 762                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
 763                 if (rnp->expmask & rdp->grpmask) {
 764                         WRITE_ONCE(rdp->cpu_no_qs.b.exp, true);
 765                         t->rcu_read_unlock_special.b.exp_hint = true;
 766                 }
 767                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 768                 return;
 769         }
 770
 771         // Finally, negative nesting depth should not happen.
 772         WARN_ON_ONCE(1);
 773 }
 774
 775 /* PREEMPTION=y, so no PREEMPTION=n expedited grace period to clean up after. */
 776 static void sync_sched_exp_online_cleanup(int cpu)
 777 {
 778 }
 779
 780 /*
 781  * Scan the current list of tasks blocked within RCU read-side critical
 782  * sections, printing out the tid of each that is blocking the current
 783  * expedited grace period.
 784  */
 785 static int rcu_print_task_exp_stall(struct rcu_node *rnp)
 786 {
 787         unsigned long flags;
 788         int ndetected = 0;
 789         struct task_struct *t;
 790
 791         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 792         if (!rnp->exp_tasks) {
 793                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 794                 return 0;
 795         }
 796         t = list_entry(rnp->exp_tasks->prev,
 797                        struct task_struct, rcu_node_entry);
 798         list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
 799                 pr_cont(" P%d", t->pid);
 800                 ndetected++;
 801         }
 802         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 803         return ndetected;
 804 }
 805
 806 /*
 807  * Scan the current list of tasks blocked within RCU read-side critical
 808  * sections, dumping the stack of each that is blocking the current
 809  * expedited grace period.
 810  */
 811 static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp)
 812 {
 813         unsigned long flags;
 814         struct task_struct *t;
 815
 816         if (!rcu_exp_stall_task_details)
 817                 return;
 818         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 819         if (!READ_ONCE(rnp->exp_tasks)) {
 820                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 821                 return;
 822         }
 823         t = list_entry(rnp->exp_tasks->prev,
 824                        struct task_struct, rcu_node_entry);
 825         list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
 826                 /*
 827                  * We could be printing a lot while holding a spinlock.
 828                  * Avoid triggering hard lockup.
 829                  */
 830                 touch_nmi_watchdog();
 831                 sched_show_task(t);
 832         }
 833         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 834 }
 835
 836 #else /* #ifdef CONFIG_PREEMPT_RCU */
 837
 838 /* Request an expedited quiescent state. */
 839 static void rcu_exp_need_qs(void)
 840 {
 841         __this_cpu_write(rcu_data.cpu_no_qs.b.exp, true);
 842         /* Store .exp before .rcu_urgent_qs. */
 843         smp_store_release(this_cpu_ptr(&rcu_data.rcu_urgent_qs), true);
 844         set_tsk_need_resched(current);
 845         set_preempt_need_resched();
 846 }
 847
 848 /* Invoked on each online non-idle CPU for expedited quiescent state. */
 849 static void rcu_exp_handler(void *unused)
 850 {
 851         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 852         struct rcu_node *rnp = rdp->mynode;
 853         bool preempt_bh_enabled = !(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));
 854
 855         if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
 856             __this_cpu_read(rcu_data.cpu_no_qs.b.exp))
 857                 return;
 858         if (rcu_is_cpu_rrupt_from_idle() ||
 859             (IS_ENABLED(CONFIG_PREEMPT_COUNT) && preempt_bh_enabled)) {
 860                 rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));
 861                 return;
 862         }
 863         rcu_exp_need_qs();
 864 }
 865
 866 /* Send IPI for expedited cleanup if needed at end of CPU-hotplug operation. */
 867 static void sync_sched_exp_online_cleanup(int cpu)
 868 {
 869         unsigned long flags;
 870         int my_cpu;
 871         struct rcu_data *rdp;
 872         int ret;
 873         struct rcu_node *rnp;
 874
 875         rdp = per_cpu_ptr(&rcu_data, cpu);
 876         rnp = rdp->mynode;
 877         my_cpu = get_cpu();
 878         /* Quiescent state either not needed or already requested, leave. */
 879         if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
 880             READ_ONCE(rdp->cpu_no_qs.b.exp)) {
 881                 put_cpu();
 882                 return;
 883         }
 884         /* Quiescent state needed on current CPU, so set it up locally. */
 885         if (my_cpu == cpu) {
 886                 local_irq_save(flags);
 887                 rcu_exp_need_qs();
 888                 local_irq_restore(flags);
 889                 put_cpu();
 890                 return;
 891         }
 892         /* Quiescent state needed on some other CPU, send IPI. */
 893         ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
 894         put_cpu();
 895         WARN_ON_ONCE(ret);
 896 }
 897
 898 /*
 899  * Because preemptible RCU does not exist, we never have to check for
 900  * tasks blocked within RCU read-side critical sections that are
 901  * blocking the current expedited grace period.
 902  */
 903 static int rcu_print_task_exp_stall(struct rcu_node *rnp)
 904 {
 905         return 0;
 906 }
 907
 908 /*
 909  * Because preemptible RCU does not exist, we never have to print out
 910  * tasks blocked within RCU read-side critical sections that are blocking
 911  * the current expedited grace period.
 912  */
 913 static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp)
 914 {
 915 }
 916
 917 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 918
 919 /**
 920  * synchronize_rcu_expedited - Brute-force RCU grace period
 921  *
 922  * Wait for an RCU grace period, but expedite it.  The basic idea is to
 923  * IPI all non-idle non-nohz online CPUs.  The IPI handler checks whether
 924  * the CPU is in an RCU critical section, and if so, it sets a flag that
 925  * causes the outermost rcu_read_unlock() to report the quiescent state
 926  * for RCU-preempt or asks the scheduler for help for RCU-sched.  On the
 927  * other hand, if the CPU is not in an RCU read-side critical section,
 928  * the IPI handler reports the quiescent state immediately.
 929  *
 930  * Although this is a great improvement over previous expedited
 931  * implementations, it is still unfriendly to real-time workloads, so is
 932  * thus not recommended for any sort of common-case code.  In fact, if
 933  * you are using synchronize_rcu_expedited() in a loop, please restructure
 934  * your code to batch your updates, and then use a single synchronize_rcu()
 935  * instead.
 936  *
 937  * This has the same semantics as (but is more brutal than) synchronize_rcu().
 938  */
 939 void synchronize_rcu_expedited(void)
 940 {
 941         unsigned long flags;
 942         struct rcu_exp_work rew;
 943         struct rcu_node *rnp;
 944         unsigned long s;
 945
 946         RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
 947                          lock_is_held(&rcu_lock_map) ||
 948                          lock_is_held(&rcu_sched_lock_map),
 949                          "Illegal synchronize_rcu_expedited() in RCU read-side critical section");
 950
 951         /* Is the state is such that the call is a grace period? */
 952         if (rcu_blocking_is_gp()) {
 953                 // Note well that this code runs with !PREEMPT && !SMP.
 954                 // In addition, all code that advances grace periods runs
 955                 // at process level.  Therefore, this expedited GP overlaps
 956                 // with other expedited GPs only by being fully nested within
 957                 // them, which allows reuse of ->gp_seq_polled_exp_snap.
 958                 rcu_poll_gp_seq_start_unlocked(&rcu_state.gp_seq_polled_exp_snap);
 959                 rcu_poll_gp_seq_end_unlocked(&rcu_state.gp_seq_polled_exp_snap);
 960
 961                 local_irq_save(flags);
 962                 WARN_ON_ONCE(num_online_cpus() > 1);
 963                 rcu_state.expedited_sequence += (1 << RCU_SEQ_CTR_SHIFT);
 964                 local_irq_restore(flags);
 965                 return;  // Context allows vacuous grace periods.
 966         }
 967
 968         /* If expedited grace periods are prohibited, fall back to normal. */
 969         if (rcu_gp_is_normal()) {
 970                 synchronize_rcu_normal();
 971                 return;
 972         }
 973
 974         /* Take a snapshot of the sequence number.  */
 975         s = rcu_exp_gp_seq_snap();
 976         if (exp_funnel_lock(s))
 977                 return;  /* Someone else did our work for us. */
 978
 979         /* Ensure that load happens before action based on it. */
 980         if (unlikely((rcu_scheduler_active == RCU_SCHEDULER_INIT) || !rcu_exp_worker_started())) {
 981                 /* Direct call during scheduler init and early_initcalls(). */
 982                 rcu_exp_sel_wait_wake(s);
 983         } else {
 984                 /* Marshall arguments & schedule the expedited grace period. */
 985                 rew.rew_s = s;
 986                 synchronize_rcu_expedited_queue_work(&rew);
 987         }
 988
 989         /* Wait for expedited grace period to complete. */
 990         rnp = rcu_get_root();
 991         wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
 992                    sync_exp_work_done(s));
 993
 994         /* Let the next expedited grace period start. */
 995         mutex_unlock(&rcu_state.exp_mutex);
 996 }
 997 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
 998
 999 /*
1000  * Ensure that start_poll_synchronize_rcu_expedited() has the expedited
1001  * RCU grace periods that it needs.
1002  */
1003 static void sync_rcu_do_polled_gp(struct work_struct *wp)
1004 {
1005         unsigned long flags;
1006         int i = 0;
1007         struct rcu_node *rnp = container_of(wp, struct rcu_node, exp_poll_wq);
1008         unsigned long s;
1009
1010         raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags);
1011         s = rnp->exp_seq_poll_rq;
1012         rnp->exp_seq_poll_rq = RCU_GET_STATE_COMPLETED;
1013         raw_spin_unlock_irqrestore(&rnp->exp_poll_lock, flags);
1014         if (s == RCU_GET_STATE_COMPLETED)
1015                 return;
1016         while (!poll_state_synchronize_rcu(s)) {
1017                 synchronize_rcu_expedited();
1018                 if (i == 10 || i == 20)
1019                         pr_info("%s: i = %d s = %lx gp_seq_polled = %lx\n", __func__, i, s, READ_ONCE(rcu_state.gp_seq_polled));
1020                 i++;
1021         }
1022         raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags);
1023         s = rnp->exp_seq_poll_rq;
1024         if (poll_state_synchronize_rcu(s))
1025                 rnp->exp_seq_poll_rq = RCU_GET_STATE_COMPLETED;
1026         raw_spin_unlock_irqrestore(&rnp->exp_poll_lock, flags);
1027 }
1028
1029 /**
1030  * start_poll_synchronize_rcu_expedited - Snapshot current RCU state and start expedited grace period
1031  *
1032  * Returns a cookie to pass to a call to cond_synchronize_rcu(),
1033  * cond_synchronize_rcu_expedited(), or poll_state_synchronize_rcu(),
1034  * allowing them to determine whether or not any sort of grace period has
1035  * elapsed in the meantime.  If the needed expedited grace period is not
1036  * already slated to start, initiates that grace period.
1037  */
1038 unsigned long start_poll_synchronize_rcu_expedited(void)
1039 {
1040         unsigned long flags;
1041         struct rcu_data *rdp;
1042         struct rcu_node *rnp;
1043         unsigned long s;
1044
1045         s = get_state_synchronize_rcu();
1046         rdp = per_cpu_ptr(&rcu_data, raw_smp_processor_id());
1047         rnp = rdp->mynode;
1048         if (rcu_init_invoked())
1049                 raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags);
1050         if (!poll_state_synchronize_rcu(s)) {
1051                 if (rcu_init_invoked()) {
1052                         rnp->exp_seq_poll_rq = s;
1053                         queue_work(rcu_gp_wq, &rnp->exp_poll_wq);
1054                 }
1055         }
1056         if (rcu_init_invoked())
1057                 raw_spin_unlock_irqrestore(&rnp->exp_poll_lock, flags);
1058
1059         return s;
1060 }
1061 EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_expedited);
1062
1063 /**
1064  * start_poll_synchronize_rcu_expedited_full - Take a full snapshot and start expedited grace period
1065  * @rgosp: Place to put snapshot of grace-period state
1066  *
1067  * Places the normal and expedited grace-period states in rgosp.  This
1068  * state value can be passed to a later call to cond_synchronize_rcu_full()
1069  * or poll_state_synchronize_rcu_full() to determine whether or not a
1070  * grace period (whether normal or expedited) has elapsed in the meantime.
1071  * If the needed expedited grace period is not already slated to start,
1072  * initiates that grace period.
1073  */
1074 void start_poll_synchronize_rcu_expedited_full(struct rcu_gp_oldstate *rgosp)
1075 {
1076         get_state_synchronize_rcu_full(rgosp);
1077         (void)start_poll_synchronize_rcu_expedited();
1078 }
1079 EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_expedited_full);
1080
1081 /**
1082  * cond_synchronize_rcu_expedited - Conditionally wait for an expedited RCU grace period
1083  *
1084  * @oldstate: value from get_state_synchronize_rcu(), start_poll_synchronize_rcu(), or start_poll_synchronize_rcu_expedited()
1085  *
1086  * If any type of full RCU grace period has elapsed since the earlier
1087  * call to get_state_synchronize_rcu(), start_poll_synchronize_rcu(),
1088  * or start_poll_synchronize_rcu_expedited(), just return.  Otherwise,
1089  * invoke synchronize_rcu_expedited() to wait for a full grace period.
1090  *
1091  * Yes, this function does not take counter wrap into account.
1092  * But counter wrap is harmless.  If the counter wraps, we have waited for
1093  * more than 2 billion grace periods (and way more on a 64-bit system!),
1094  * so waiting for a couple of additional grace periods should be just fine.
1095  *
1096  * This function provides the same memory-ordering guarantees that
1097  * would be provided by a synchronize_rcu() that was invoked at the call
1098  * to the function that provided @oldstate and that returned at the end
1099  * of this function.
1100  */
1101 void cond_synchronize_rcu_expedited(unsigned long oldstate)
1102 {
1103         if (!poll_state_synchronize_rcu(oldstate))
1104                 synchronize_rcu_expedited();
1105 }
1106 EXPORT_SYMBOL_GPL(cond_synchronize_rcu_expedited);
1107
1108 /**
1109  * cond_synchronize_rcu_expedited_full - Conditionally wait for an expedited RCU grace period
1110  * @rgosp: value from get_state_synchronize_rcu_full(), start_poll_synchronize_rcu_full(), or start_poll_synchronize_rcu_expedited_full()
1111  *
1112  * If a full RCU grace period has elapsed since the call to
1113  * get_state_synchronize_rcu_full(), start_poll_synchronize_rcu_full(),
1114  * or start_poll_synchronize_rcu_expedited_full() from which @rgosp was
1115  * obtained, just return.  Otherwise, invoke synchronize_rcu_expedited()
1116  * to wait for a full grace period.
1117  *
1118  * Yes, this function does not take counter wrap into account.
1119  * But counter wrap is harmless.  If the counter wraps, we have waited for
1120  * more than 2 billion grace periods (and way more on a 64-bit system!),
1121  * so waiting for a couple of additional grace periods should be just fine.
1122  *
1123  * This function provides the same memory-ordering guarantees that
1124  * would be provided by a synchronize_rcu() that was invoked at the call
1125  * to the function that provided @rgosp and that returned at the end of
1126  * this function.
1127  */
1128 void cond_synchronize_rcu_expedited_full(struct rcu_gp_oldstate *rgosp)
1129 {
1130         if (!poll_state_synchronize_rcu_full(rgosp))
1131                 synchronize_rcu_expedited();
1132 }
1133 EXPORT_SYMBOL_GPL(cond_synchronize_rcu_expedited_full);