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 <[email protected]>
   8  */
   9
  10 #include <linux/lockdep.h>
  11
  12 static void rcu_exp_handler(void *unused);
  13 static int rcu_print_task_exp_stall(struct rcu_node *rnp);
  14 static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp);
  15
  16 /*
  17  * Record the start of an expedited grace period.
  18  */
  19 static void rcu_exp_gp_seq_start(void)
  20 {
  21         rcu_seq_start(&rcu_state.expedited_sequence);
  22         rcu_poll_gp_seq_start_unlocked(&rcu_state.gp_seq_polled_exp_snap);
  23 }
  24
  25 /*
  26  * Return the value that the expedited-grace-period counter will have
  27  * at the end of the current grace period.
  28  */
  29 static __maybe_unused unsigned long rcu_exp_gp_seq_endval(void)
  30 {
  31         return rcu_seq_endval(&rcu_state.expedited_sequence);
  32 }
  33
  34 /*
  35  * Record the end of an expedited grace period.
  36  */
  37 static void rcu_exp_gp_seq_end(void)
  38 {
  39         rcu_poll_gp_seq_end_unlocked(&rcu_state.gp_seq_polled_exp_snap);
  40         rcu_seq_end(&rcu_state.expedited_sequence);
  41         smp_mb(); /* Ensure that consecutive grace periods serialize. */
  42 }
  43
  44 /*
  45  * Take a snapshot of the expedited-grace-period counter, which is the
  46  * earliest value that will indicate that a full grace period has
  47  * elapsed since the current time.
  48  */
  49 static unsigned long rcu_exp_gp_seq_snap(void)
  50 {
  51         unsigned long s;
  52
  53         smp_mb(); /* Caller's modifications seen first by other CPUs. */
  54         s = rcu_seq_snap(&rcu_state.expedited_sequence);
  55         trace_rcu_exp_grace_period(rcu_state.name, s, TPS("snap"));
  56         return s;
  57 }
  58
  59 /*
  60  * Given a counter snapshot from rcu_exp_gp_seq_snap(), return true
  61  * if a full expedited grace period has elapsed since that snapshot
  62  * was taken.
  63  */
  64 static bool rcu_exp_gp_seq_done(unsigned long s)
  65 {
  66         return rcu_seq_done(&rcu_state.expedited_sequence, s);
  67 }
  68
  69 /*
  70  * Reset the ->expmaskinit values in the rcu_node tree to reflect any
  71  * recent CPU-online activity.  Note that these masks are not cleared
  72  * when CPUs go offline, so they reflect the union of all CPUs that have
  73  * ever been online.  This means that this function normally takes its
  74  * no-work-to-do fastpath.
  75  */
  76 static void sync_exp_reset_tree_hotplug(void)
  77 {
  78         bool done;
  79         unsigned long flags;
  80         unsigned long mask;
  81         unsigned long oldmask;
  82         int ncpus = smp_load_acquire(&rcu_state.ncpus); /* Order vs. locking. */
  83         struct rcu_node *rnp;
  84         struct rcu_node *rnp_up;
  85
  86         /* If no new CPUs onlined since last time, nothing to do. */
  87         if (likely(ncpus == rcu_state.ncpus_snap))
  88                 return;
  89         rcu_state.ncpus_snap = ncpus;
  90
  91         /*
  92          * Each pass through the following loop propagates newly onlined
  93          * CPUs for the current rcu_node structure up the rcu_node tree.
  94          */
  95         rcu_for_each_leaf_node(rnp) {
  96                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
  97                 if (rnp->expmaskinit == rnp->expmaskinitnext) {
  98                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  99                         continue;  /* No new CPUs, nothing to do. */
 100                 }
 101
 102                 /* Update this node's mask, track old value for propagation. */
 103                 oldmask = rnp->expmaskinit;
 104                 rnp->expmaskinit = rnp->expmaskinitnext;
 105                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 106
 107                 /* If was already nonzero, nothing to propagate. */
 108                 if (oldmask)
 109                         continue;
 110
 111                 /* Propagate the new CPU up the tree. */
 112                 mask = rnp->grpmask;
 113                 rnp_up = rnp->parent;
 114                 done = false;
 115                 while (rnp_up) {
 116                         raw_spin_lock_irqsave_rcu_node(rnp_up, flags);
 117                         if (rnp_up->expmaskinit)
 118                                 done = true;
 119                         rnp_up->expmaskinit |= mask;
 120                         raw_spin_unlock_irqrestore_rcu_node(rnp_up, flags);
 121                         if (done)
 122                                 break;
 123                         mask = rnp_up->grpmask;
 124                         rnp_up = rnp_up->parent;
 125                 }
 126         }
 127 }
 128
 129 /*
 130  * Reset the ->expmask values in the rcu_node tree in preparation for
 131  * a new expedited grace period.
 132  */
 133 static void __maybe_unused sync_exp_reset_tree(void)
 134 {
 135         unsigned long flags;
 136         struct rcu_node *rnp;
 137
 138         sync_exp_reset_tree_hotplug();
 139         rcu_for_each_node_breadth_first(rnp) {
 140                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
 141                 WARN_ON_ONCE(rnp->expmask);
 142                 WRITE_ONCE(rnp->expmask, rnp->expmaskinit);
 143                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 144         }
 145 }
 146
 147 /*
 148  * Return non-zero if there is no RCU expedited grace period in progress
 149  * for the specified rcu_node structure, in other words, if all CPUs and
 150  * tasks covered by the specified rcu_node structure have done their bit
 151  * for the current expedited grace period.
 152  */
 153 static bool sync_rcu_exp_done(struct rcu_node *rnp)
 154 {
 155         raw_lockdep_assert_held_rcu_node(rnp);
 156         return READ_ONCE(rnp->exp_tasks) == NULL &&
 157                READ_ONCE(rnp->expmask) == 0;
 158 }
 159
 160 /*
 161  * Like sync_rcu_exp_done(), but where the caller does not hold the
 162  * rcu_node's ->lock.
 163  */
 164 static bool sync_rcu_exp_done_unlocked(struct rcu_node *rnp)
 165 {
 166         unsigned long flags;
 167         bool ret;
 168
 169         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 170         ret = sync_rcu_exp_done(rnp);
 171         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 172
 173         return ret;
 174 }
 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                                 smp_mb(); /* EGP done before wake_up(). */
 204                                 swake_up_one(&rcu_state.expedited_wq);
 205                         }
 206                         break;
 207                 }
 208                 mask = rnp->grpmask;
 209                 raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled */
 210                 rnp = rnp->parent;
 211                 raw_spin_lock_rcu_node(rnp); /* irqs already disabled */
 212                 WARN_ON_ONCE(!(rnp->expmask & mask));
 213                 WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask);
 214         }
 215 }
 216
 217 /*
 218  * Report expedited quiescent state for specified node.  This is a
 219  * lock-acquisition wrapper function for __rcu_report_exp_rnp().
 220  */
 221 static void __maybe_unused rcu_report_exp_rnp(struct rcu_node *rnp, bool wake)
 222 {
 223         unsigned long flags;
 224
 225         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 226         __rcu_report_exp_rnp(rnp, wake, flags);
 227 }
 228
 229 /*
 230  * Report expedited quiescent state for multiple CPUs, all covered by the
 231  * specified leaf rcu_node structure.
 232  */
 233 static void rcu_report_exp_cpu_mult(struct rcu_node *rnp,
 234                                     unsigned long mask, bool wake)
 235 {
 236         int cpu;
 237         unsigned long flags;
 238         struct rcu_data *rdp;
 239
 240         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 241         if (!(rnp->expmask & mask)) {
 242                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 243                 return;
 244         }
 245         WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask);
 246         for_each_leaf_node_cpu_mask(rnp, cpu, mask) {
 247                 rdp = per_cpu_ptr(&rcu_data, cpu);
 248                 if (!IS_ENABLED(CONFIG_NO_HZ_FULL) || !rdp->rcu_forced_tick_exp)
 249                         continue;
 250                 rdp->rcu_forced_tick_exp = false;
 251                 tick_dep_clear_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
 252         }
 253         __rcu_report_exp_rnp(rnp, wake, flags); /* Releases rnp->lock. */
 254 }
 255
 256 /*
 257  * Report expedited quiescent state for specified rcu_data (CPU).
 258  */
 259 static void rcu_report_exp_rdp(struct rcu_data *rdp)
 260 {
 261         WRITE_ONCE(rdp->cpu_no_qs.b.exp, false);
 262         rcu_report_exp_cpu_mult(rdp->mynode, rdp->grpmask, true);
 263 }
 264
 265 /* Common code for work-done checking. */
 266 static bool sync_exp_work_done(unsigned long s)
 267 {
 268         if (rcu_exp_gp_seq_done(s)) {
 269                 trace_rcu_exp_grace_period(rcu_state.name, s, TPS("done"));
 270                 smp_mb(); /* Ensure test happens before caller kfree(). */
 271                 return true;
 272         }
 273         return false;
 274 }
 275
 276 /*
 277  * Funnel-lock acquisition for expedited grace periods.  Returns true
 278  * if some other task completed an expedited grace period that this task
 279  * can piggy-back on, and with no mutex held.  Otherwise, returns false
 280  * with the mutex held, indicating that the caller must actually do the
 281  * expedited grace period.
 282  */
 283 static bool exp_funnel_lock(unsigned long s)
 284 {
 285         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, raw_smp_processor_id());
 286         struct rcu_node *rnp = rdp->mynode;
 287         struct rcu_node *rnp_root = rcu_get_root();
 288
 289         /* Low-contention fastpath. */
 290         if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s) &&
 291             (rnp == rnp_root ||
 292              ULONG_CMP_LT(READ_ONCE(rnp_root->exp_seq_rq), s)) &&
 293             mutex_trylock(&rcu_state.exp_mutex))
 294                 goto fastpath;
 295
 296         /*
 297          * Each pass through the following loop works its way up
 298          * the rcu_node tree, returning if others have done the work or
 299          * otherwise falls through to acquire ->exp_mutex.  The mapping
 300          * from CPU to rcu_node structure can be inexact, as it is just
 301          * promoting locality and is not strictly needed for correctness.
 302          */
 303         for (; rnp != NULL; rnp = rnp->parent) {
 304                 if (sync_exp_work_done(s))
 305                         return true;
 306
 307                 /* Work not done, either wait here or go up. */
 308                 spin_lock(&rnp->exp_lock);
 309                 if (ULONG_CMP_GE(rnp->exp_seq_rq, s)) {
 310
 311                         /* Someone else doing GP, so wait for them. */
 312                         spin_unlock(&rnp->exp_lock);
 313                         trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
 314                                                   rnp->grplo, rnp->grphi,
 315                                                   TPS("wait"));
 316                         wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
 317                                    sync_exp_work_done(s));
 318                         return true;
 319                 }
 320                 WRITE_ONCE(rnp->exp_seq_rq, s); /* Followers can wait on us. */
 321                 spin_unlock(&rnp->exp_lock);
 322                 trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
 323                                           rnp->grplo, rnp->grphi, TPS("nxtlvl"));
 324         }
 325         mutex_lock(&rcu_state.exp_mutex);
 326 fastpath:
 327         if (sync_exp_work_done(s)) {
 328                 mutex_unlock(&rcu_state.exp_mutex);
 329                 return true;
 330         }
 331         rcu_exp_gp_seq_start();
 332         trace_rcu_exp_grace_period(rcu_state.name, s, TPS("start"));
 333         return false;
 334 }
 335
 336 /*
 337  * Select the CPUs within the specified rcu_node that the upcoming
 338  * expedited grace period needs to wait for.
 339  */
 340 static void __sync_rcu_exp_select_node_cpus(struct rcu_exp_work *rewp)
 341 {
 342         int cpu;
 343         unsigned long flags;
 344         unsigned long mask_ofl_test;
 345         unsigned long mask_ofl_ipi;
 346         int ret;
 347         struct rcu_node *rnp = container_of(rewp, struct rcu_node, rew);
 348
 349         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 350
 351         /* Each pass checks a CPU for identity, offline, and idle. */
 352         mask_ofl_test = 0;
 353         for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) {
 354                 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 355                 unsigned long mask = rdp->grpmask;
 356                 int snap;
 357
 358                 if (raw_smp_processor_id() == cpu ||
 359                     !(rnp->qsmaskinitnext & mask)) {
 360                         mask_ofl_test |= mask;
 361                 } else {
 362                         snap = rcu_dynticks_snap(cpu);
 363                         if (rcu_dynticks_in_eqs(snap))
 364                                 mask_ofl_test |= mask;
 365                         else
 366                                 rdp->exp_dynticks_snap = snap;
 367                 }
 368         }
 369         mask_ofl_ipi = rnp->expmask & ~mask_ofl_test;
 370
 371         /*
 372          * Need to wait for any blocked tasks as well.  Note that
 373          * additional blocking tasks will also block the expedited GP
 374          * until such time as the ->expmask bits are cleared.
 375          */
 376         if (rcu_preempt_has_tasks(rnp))
 377                 WRITE_ONCE(rnp->exp_tasks, rnp->blkd_tasks.next);
 378         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 379
 380         /* IPI the remaining CPUs for expedited quiescent state. */
 381         for_each_leaf_node_cpu_mask(rnp, cpu, mask_ofl_ipi) {
 382                 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 383                 unsigned long mask = rdp->grpmask;
 384
 385 retry_ipi:
 386                 if (rcu_dynticks_in_eqs_since(rdp, rdp->exp_dynticks_snap)) {
 387                         mask_ofl_test |= mask;
 388                         continue;
 389                 }
 390                 if (get_cpu() == cpu) {
 391                         mask_ofl_test |= mask;
 392                         put_cpu();
 393                         continue;
 394                 }
 395                 ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
 396                 put_cpu();
 397                 /* The CPU will report the QS in response to the IPI. */
 398                 if (!ret)
 399                         continue;
 400
 401                 /* Failed, raced with CPU hotplug operation. */
 402                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
 403                 if ((rnp->qsmaskinitnext & mask) &&
 404                     (rnp->expmask & mask)) {
 405                         /* Online, so delay for a bit and try again. */
 406                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 407                         trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("selectofl"));
 408                         schedule_timeout_idle(1);
 409                         goto retry_ipi;
 410                 }
 411                 /* CPU really is offline, so we must report its QS. */
 412                 if (rnp->expmask & mask)
 413                         mask_ofl_test |= mask;
 414                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 415         }
 416         /* Report quiescent states for those that went offline. */
 417         if (mask_ofl_test)
 418                 rcu_report_exp_cpu_mult(rnp, mask_ofl_test, false);
 419 }
 420
 421 static void rcu_exp_sel_wait_wake(unsigned long s);
 422
 423 #ifdef CONFIG_RCU_EXP_KTHREAD
 424 static void sync_rcu_exp_select_node_cpus(struct kthread_work *wp)
 425 {
 426         struct rcu_exp_work *rewp =
 427                 container_of(wp, struct rcu_exp_work, rew_work);
 428
 429         __sync_rcu_exp_select_node_cpus(rewp);
 430 }
 431
 432 static inline bool rcu_gp_par_worker_started(void)
 433 {
 434         return !!READ_ONCE(rcu_exp_par_gp_kworker);
 435 }
 436
 437 static inline void sync_rcu_exp_select_cpus_queue_work(struct rcu_node *rnp)
 438 {
 439         kthread_init_work(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
 440         /*
 441          * Use rcu_exp_par_gp_kworker, because flushing a work item from
 442          * another work item on the same kthread worker can result in
 443          * deadlock.
 444          */
 445         kthread_queue_work(rcu_exp_par_gp_kworker, &rnp->rew.rew_work);
 446 }
 447
 448 static inline void sync_rcu_exp_select_cpus_flush_work(struct rcu_node *rnp)
 449 {
 450         kthread_flush_work(&rnp->rew.rew_work);
 451 }
 452
 453 /*
 454  * Work-queue handler to drive an expedited grace period forward.
 455  */
 456 static void wait_rcu_exp_gp(struct kthread_work *wp)
 457 {
 458         struct rcu_exp_work *rewp;
 459
 460         rewp = container_of(wp, struct rcu_exp_work, rew_work);
 461         rcu_exp_sel_wait_wake(rewp->rew_s);
 462 }
 463
 464 static inline void synchronize_rcu_expedited_queue_work(struct rcu_exp_work *rew)
 465 {
 466         kthread_init_work(&rew->rew_work, wait_rcu_exp_gp);
 467         kthread_queue_work(rcu_exp_gp_kworker, &rew->rew_work);
 468 }
 469
 470 static inline void synchronize_rcu_expedited_destroy_work(struct rcu_exp_work *rew)
 471 {
 472 }
 473 #else /* !CONFIG_RCU_EXP_KTHREAD */
 474 static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
 475 {
 476         struct rcu_exp_work *rewp =
 477                 container_of(wp, struct rcu_exp_work, rew_work);
 478
 479         __sync_rcu_exp_select_node_cpus(rewp);
 480 }
 481
 482 static inline bool rcu_gp_par_worker_started(void)
 483 {
 484         return !!READ_ONCE(rcu_par_gp_wq);
 485 }
 486
 487 static inline void sync_rcu_exp_select_cpus_queue_work(struct rcu_node *rnp)
 488 {
 489         int cpu = find_next_bit(&rnp->ffmask, BITS_PER_LONG, -1);
 490
 491         INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
 492         /* If all offline, queue the work on an unbound CPU. */
 493         if (unlikely(cpu > rnp->grphi - rnp->grplo))
 494                 cpu = WORK_CPU_UNBOUND;
 495         else
 496                 cpu += rnp->grplo;
 497         queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
 498 }
 499
 500 static inline void sync_rcu_exp_select_cpus_flush_work(struct rcu_node *rnp)
 501 {
 502         flush_work(&rnp->rew.rew_work);
 503 }
 504
 505 /*
 506  * Work-queue handler to drive an expedited grace period forward.
 507  */
 508 static void wait_rcu_exp_gp(struct work_struct *wp)
 509 {
 510         struct rcu_exp_work *rewp;
 511
 512         rewp = container_of(wp, struct rcu_exp_work, rew_work);
 513         rcu_exp_sel_wait_wake(rewp->rew_s);
 514 }
 515
 516 static inline void synchronize_rcu_expedited_queue_work(struct rcu_exp_work *rew)
 517 {
 518         INIT_WORK_ONSTACK(&rew->rew_work, wait_rcu_exp_gp);
 519         queue_work(rcu_gp_wq, &rew->rew_work);
 520 }
 521
 522 static inline void synchronize_rcu_expedited_destroy_work(struct rcu_exp_work *rew)
 523 {
 524         destroy_work_on_stack(&rew->rew_work);
 525 }
 526 #endif /* CONFIG_RCU_EXP_KTHREAD */
 527
 528 /*
 529  * Select the nodes that the upcoming expedited grace period needs
 530  * to wait for.
 531  */
 532 static void sync_rcu_exp_select_cpus(void)
 533 {
 534         struct rcu_node *rnp;
 535
 536         trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("reset"));
 537         sync_exp_reset_tree();
 538         trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("select"));
 539
 540         /* Schedule work for each leaf rcu_node structure. */
 541         rcu_for_each_leaf_node(rnp) {
 542                 rnp->exp_need_flush = false;
 543                 if (!READ_ONCE(rnp->expmask))
 544                         continue; /* Avoid early boot non-existent wq. */
 545                 if (!rcu_gp_par_worker_started() ||
 546                     rcu_scheduler_active != RCU_SCHEDULER_RUNNING ||
 547                     rcu_is_last_leaf_node(rnp)) {
 548                         /* No worker started yet or last leaf, do direct call. */
 549                         sync_rcu_exp_select_node_cpus(&rnp->rew.rew_work);
 550                         continue;
 551                 }
 552                 sync_rcu_exp_select_cpus_queue_work(rnp);
 553                 rnp->exp_need_flush = true;
 554         }
 555
 556         /* Wait for jobs (if any) to complete. */
 557         rcu_for_each_leaf_node(rnp)
 558                 if (rnp->exp_need_flush)
 559                         sync_rcu_exp_select_cpus_flush_work(rnp);
 560 }
 561
 562 /*
 563  * Wait for the expedited grace period to elapse, within time limit.
 564  * If the time limit is exceeded without the grace period elapsing,
 565  * return false, otherwise return true.
 566  */
 567 static bool synchronize_rcu_expedited_wait_once(long tlimit)
 568 {
 569         int t;
 570         struct rcu_node *rnp_root = rcu_get_root();
 571
 572         t = swait_event_timeout_exclusive(rcu_state.expedited_wq,
 573                                           sync_rcu_exp_done_unlocked(rnp_root),
 574                                           tlimit);
 575         // Workqueues should not be signaled.
 576         if (t > 0 || sync_rcu_exp_done_unlocked(rnp_root))
 577                 return true;
 578         WARN_ON(t < 0);  /* workqueues should not be signaled. */
 579         return false;
 580 }
 581
 582 /*
 583  * Wait for the expedited grace period to elapse, issuing any needed
 584  * RCU CPU stall warnings along the way.
 585  */
 586 static void synchronize_rcu_expedited_wait(void)
 587 {
 588         int cpu;
 589         unsigned long j;
 590         unsigned long jiffies_stall;
 591         unsigned long jiffies_start;
 592         unsigned long mask;
 593         int ndetected;
 594         struct rcu_data *rdp;
 595         struct rcu_node *rnp;
 596         struct rcu_node *rnp_root = rcu_get_root();
 597
 598         trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("startwait"));
 599         jiffies_stall = rcu_exp_jiffies_till_stall_check();
 600         jiffies_start = jiffies;
 601         if (tick_nohz_full_enabled() && rcu_inkernel_boot_has_ended()) {
 602                 if (synchronize_rcu_expedited_wait_once(1))
 603                         return;
 604                 rcu_for_each_leaf_node(rnp) {
 605                         mask = READ_ONCE(rnp->expmask);
 606                         for_each_leaf_node_cpu_mask(rnp, cpu, mask) {
 607                                 rdp = per_cpu_ptr(&rcu_data, cpu);
 608                                 if (rdp->rcu_forced_tick_exp)
 609                                         continue;
 610                                 rdp->rcu_forced_tick_exp = true;
 611                                 preempt_disable();
 612                                 if (cpu_online(cpu))
 613                                         tick_dep_set_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
 614                                 preempt_enable();
 615                         }
 616                 }
 617                 j = READ_ONCE(jiffies_till_first_fqs);
 618                 if (synchronize_rcu_expedited_wait_once(j + HZ))
 619                         return;
 620         }
 621
 622         for (;;) {
 623                 if (synchronize_rcu_expedited_wait_once(jiffies_stall))
 624                         return;
 625                 if (rcu_stall_is_suppressed())
 626                         continue;
 627                 trace_rcu_stall_warning(rcu_state.name, TPS("ExpeditedStall"));
 628                 pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {",
 629                        rcu_state.name);
 630                 ndetected = 0;
 631                 rcu_for_each_leaf_node(rnp) {
 632                         ndetected += rcu_print_task_exp_stall(rnp);
 633                         for_each_leaf_node_possible_cpu(rnp, cpu) {
 634                                 struct rcu_data *rdp;
 635
 636                                 mask = leaf_node_cpu_bit(rnp, cpu);
 637                                 if (!(READ_ONCE(rnp->expmask) & mask))
 638                                         continue;
 639                                 ndetected++;
 640                                 rdp = per_cpu_ptr(&rcu_data, cpu);
 641                                 pr_cont(" %d-%c%c%c%c", cpu,
 642                                         "O."[!!cpu_online(cpu)],
 643                                         "o."[!!(rdp->grpmask & rnp->expmaskinit)],
 644                                         "N."[!!(rdp->grpmask & rnp->expmaskinitnext)],
 645                                         "D."[!!(rdp->cpu_no_qs.b.exp)]);
 646                         }
 647                 }
 648                 pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
 649                         jiffies - jiffies_start, rcu_state.expedited_sequence,
 650                         data_race(rnp_root->expmask),
 651                         ".T"[!!data_race(rnp_root->exp_tasks)]);
 652                 if (ndetected) {
 653                         pr_err("blocking rcu_node structures (internal RCU debug):");
 654                         rcu_for_each_node_breadth_first(rnp) {
 655                                 if (rnp == rnp_root)
 656                                         continue; /* printed unconditionally */
 657                                 if (sync_rcu_exp_done_unlocked(rnp))
 658                                         continue;
 659                                 pr_cont(" l=%u:%d-%d:%#lx/%c",
 660                                         rnp->level, rnp->grplo, rnp->grphi,
 661                                         data_race(rnp->expmask),
 662                                         ".T"[!!data_race(rnp->exp_tasks)]);
 663                         }
 664                         pr_cont("\n");
 665                 }
 666                 rcu_for_each_leaf_node(rnp) {
 667                         for_each_leaf_node_possible_cpu(rnp, cpu) {
 668                                 mask = leaf_node_cpu_bit(rnp, cpu);
 669                                 if (!(READ_ONCE(rnp->expmask) & mask))
 670                                         continue;
 671                                 preempt_disable(); // For smp_processor_id() in dump_cpu_task().
 672                                 dump_cpu_task(cpu);
 673                                 preempt_enable();
 674                         }
 675                         rcu_exp_print_detail_task_stall_rnp(rnp);
 676                 }
 677                 jiffies_stall = 3 * rcu_exp_jiffies_till_stall_check() + 3;
 678                 panic_on_rcu_stall();
 679         }
 680 }
 681
 682 /*
 683  * Wait for the current expedited grace period to complete, and then
 684  * wake up everyone who piggybacked on the just-completed expedited
 685  * grace period.  Also update all the ->exp_seq_rq counters as needed
 686  * in order to avoid counter-wrap problems.
 687  */
 688 static void rcu_exp_wait_wake(unsigned long s)
 689 {
 690         struct rcu_node *rnp;
 691
 692         synchronize_rcu_expedited_wait();
 693
 694         // Switch over to wakeup mode, allowing the next GP to proceed.
 695         // End the previous grace period only after acquiring the mutex
 696         // to ensure that only one GP runs concurrently with wakeups.
 697         mutex_lock(&rcu_state.exp_wake_mutex);
 698         rcu_exp_gp_seq_end();
 699         trace_rcu_exp_grace_period(rcu_state.name, s, TPS("end"));
 700
 701         rcu_for_each_node_breadth_first(rnp) {
 702                 if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s)) {
 703                         spin_lock(&rnp->exp_lock);
 704                         /* Recheck, avoid hang in case someone just arrived. */
 705                         if (ULONG_CMP_LT(rnp->exp_seq_rq, s))
 706                                 WRITE_ONCE(rnp->exp_seq_rq, s);
 707                         spin_unlock(&rnp->exp_lock);
 708                 }
 709                 smp_mb(); /* All above changes before wakeup. */
 710                 wake_up_all(&rnp->exp_wq[rcu_seq_ctr(s) & 0x3]);
 711         }
 712         trace_rcu_exp_grace_period(rcu_state.name, s, TPS("endwake"));
 713         mutex_unlock(&rcu_state.exp_wake_mutex);
 714 }
 715
 716 /*
 717  * Common code to drive an expedited grace period forward, used by
 718  * workqueues and mid-boot-time tasks.
 719  */
 720 static void rcu_exp_sel_wait_wake(unsigned long s)
 721 {
 722         /* Initialize the rcu_node tree in preparation for the wait. */
 723         sync_rcu_exp_select_cpus();
 724
 725         /* Wait and clean up, including waking everyone. */
 726         rcu_exp_wait_wake(s);
 727 }
 728
 729 #ifdef CONFIG_PREEMPT_RCU
 730
 731 /*
 732  * Remote handler for smp_call_function_single().  If there is an
 733  * RCU read-side critical section in effect, request that the
 734  * next rcu_read_unlock() record the quiescent state up the
 735  * ->expmask fields in the rcu_node tree.  Otherwise, immediately
 736  * report the quiescent state.
 737  */
 738 static void rcu_exp_handler(void *unused)
 739 {
 740         int depth = rcu_preempt_depth();
 741         unsigned long flags;
 742         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 743         struct rcu_node *rnp = rdp->mynode;
 744         struct task_struct *t = current;
 745
 746         /*
 747          * First, the common case of not being in an RCU read-side
 748          * critical section.  If also enabled or idle, immediately
 749          * report the quiescent state, otherwise defer.
 750          */
 751         if (!depth) {
 752                 if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
 753                     rcu_is_cpu_rrupt_from_idle()) {
 754                         rcu_report_exp_rdp(rdp);
 755                 } else {
 756                         WRITE_ONCE(rdp->cpu_no_qs.b.exp, true);
 757                         set_tsk_need_resched(t);
 758                         set_preempt_need_resched();
 759                 }
 760                 return;
 761         }
 762
 763         /*
 764          * Second, the less-common case of being in an RCU read-side
 765          * critical section.  In this case we can count on a future
 766          * rcu_read_unlock().  However, this rcu_read_unlock() might
 767          * execute on some other CPU, but in that case there will be
 768          * a future context switch.  Either way, if the expedited
 769          * grace period is still waiting on this CPU, set ->deferred_qs
 770          * so that the eventual quiescent state will be reported.
 771          * Note that there is a large group of race conditions that
 772          * can have caused this quiescent state to already have been
 773          * reported, so we really do need to check ->expmask.
 774          */
 775         if (depth > 0) {
 776                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
 777                 if (rnp->expmask & rdp->grpmask) {
 778                         WRITE_ONCE(rdp->cpu_no_qs.b.exp, true);
 779                         t->rcu_read_unlock_special.b.exp_hint = true;
 780                 }
 781                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 782                 return;
 783         }
 784
 785         // Finally, negative nesting depth should not happen.
 786         WARN_ON_ONCE(1);
 787 }
 788
 789 /* PREEMPTION=y, so no PREEMPTION=n expedited grace period to clean up after. */
 790 static void sync_sched_exp_online_cleanup(int cpu)
 791 {
 792 }
 793
 794 /*
 795  * Scan the current list of tasks blocked within RCU read-side critical
 796  * sections, printing out the tid of each that is blocking the current
 797  * expedited grace period.
 798  */
 799 static int rcu_print_task_exp_stall(struct rcu_node *rnp)
 800 {
 801         unsigned long flags;
 802         int ndetected = 0;
 803         struct task_struct *t;
 804
 805         if (!READ_ONCE(rnp->exp_tasks))
 806                 return 0;
 807         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 808         t = list_entry(rnp->exp_tasks->prev,
 809                        struct task_struct, rcu_node_entry);
 810         list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
 811                 pr_cont(" P%d", t->pid);
 812                 ndetected++;
 813         }
 814         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 815         return ndetected;
 816 }
 817
 818 /*
 819  * Scan the current list of tasks blocked within RCU read-side critical
 820  * sections, dumping the stack of each that is blocking the current
 821  * expedited grace period.
 822  */
 823 static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp)
 824 {
 825         unsigned long flags;
 826         struct task_struct *t;
 827
 828         if (!rcu_exp_stall_task_details)
 829                 return;
 830         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 831         if (!READ_ONCE(rnp->exp_tasks)) {
 832                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 833                 return;
 834         }
 835         t = list_entry(rnp->exp_tasks->prev,
 836                        struct task_struct, rcu_node_entry);
 837         list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
 838                 /*
 839                  * We could be printing a lot while holding a spinlock.
 840                  * Avoid triggering hard lockup.
 841                  */
 842                 touch_nmi_watchdog();
 843                 sched_show_task(t);
 844         }
 845         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 846 }
 847
 848 #else /* #ifdef CONFIG_PREEMPT_RCU */
 849
 850 /* Request an expedited quiescent state. */
 851 static void rcu_exp_need_qs(void)
 852 {
 853         __this_cpu_write(rcu_data.cpu_no_qs.b.exp, true);
 854         /* Store .exp before .rcu_urgent_qs. */
 855         smp_store_release(this_cpu_ptr(&rcu_data.rcu_urgent_qs), true);
 856         set_tsk_need_resched(current);
 857         set_preempt_need_resched();
 858 }
 859
 860 /* Invoked on each online non-idle CPU for expedited quiescent state. */
 861 static void rcu_exp_handler(void *unused)
 862 {
 863         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 864         struct rcu_node *rnp = rdp->mynode;
 865         bool preempt_bh_enabled = !(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));
 866
 867         if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
 868             __this_cpu_read(rcu_data.cpu_no_qs.b.exp))
 869                 return;
 870         if (rcu_is_cpu_rrupt_from_idle() ||
 871             (IS_ENABLED(CONFIG_PREEMPT_COUNT) && preempt_bh_enabled)) {
 872                 rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));
 873                 return;
 874         }
 875         rcu_exp_need_qs();
 876 }
 877
 878 /* Send IPI for expedited cleanup if needed at end of CPU-hotplug operation. */
 879 static void sync_sched_exp_online_cleanup(int cpu)
 880 {
 881         unsigned long flags;
 882         int my_cpu;
 883         struct rcu_data *rdp;
 884         int ret;
 885         struct rcu_node *rnp;
 886
 887         rdp = per_cpu_ptr(&rcu_data, cpu);
 888         rnp = rdp->mynode;
 889         my_cpu = get_cpu();
 890         /* Quiescent state either not needed or already requested, leave. */
 891         if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
 892             READ_ONCE(rdp->cpu_no_qs.b.exp)) {
 893                 put_cpu();
 894                 return;
 895         }
 896         /* Quiescent state needed on current CPU, so set it up locally. */
 897         if (my_cpu == cpu) {
 898                 local_irq_save(flags);
 899                 rcu_exp_need_qs();
 900                 local_irq_restore(flags);
 901                 put_cpu();
 902                 return;
 903         }
 904         /* Quiescent state needed on some other CPU, send IPI. */
 905         ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
 906         put_cpu();
 907         WARN_ON_ONCE(ret);
 908 }
 909
 910 /*
 911  * Because preemptible RCU does not exist, we never have to check for
 912  * tasks blocked within RCU read-side critical sections that are
 913  * blocking the current expedited grace period.
 914  */
 915 static int rcu_print_task_exp_stall(struct rcu_node *rnp)
 916 {
 917         return 0;
 918 }
 919
 920 /*
 921  * Because preemptible RCU does not exist, we never have to print out
 922  * tasks blocked within RCU read-side critical sections that are blocking
 923  * the current expedited grace period.
 924  */
 925 static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp)
 926 {
 927 }
 928
 929 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 930
 931 /**
 932  * synchronize_rcu_expedited - Brute-force RCU grace period
 933  *
 934  * Wait for an RCU grace period, but expedite it.  The basic idea is to
 935  * IPI all non-idle non-nohz online CPUs.  The IPI handler checks whether
 936  * the CPU is in an RCU critical section, and if so, it sets a flag that
 937  * causes the outermost rcu_read_unlock() to report the quiescent state
 938  * for RCU-preempt or asks the scheduler for help for RCU-sched.  On the
 939  * other hand, if the CPU is not in an RCU read-side critical section,
 940  * the IPI handler reports the quiescent state immediately.
 941  *
 942  * Although this is a great improvement over previous expedited
 943  * implementations, it is still unfriendly to real-time workloads, so is
 944  * thus not recommended for any sort of common-case code.  In fact, if
 945  * you are using synchronize_rcu_expedited() in a loop, please restructure
 946  * your code to batch your updates, and then use a single synchronize_rcu()
 947  * instead.
 948  *
 949  * This has the same semantics as (but is more brutal than) synchronize_rcu().
 950  */
 951 void synchronize_rcu_expedited(void)
 952 {
 953         bool boottime = (rcu_scheduler_active == RCU_SCHEDULER_INIT);
 954         unsigned long flags;
 955         struct rcu_exp_work rew;
 956         struct rcu_node *rnp;
 957         unsigned long s;
 958
 959         RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
 960                          lock_is_held(&rcu_lock_map) ||
 961                          lock_is_held(&rcu_sched_lock_map),
 962                          "Illegal synchronize_rcu_expedited() in RCU read-side critical section");
 963
 964         /* Is the state is such that the call is a grace period? */
 965         if (rcu_blocking_is_gp()) {
 966                 // Note well that this code runs with !PREEMPT && !SMP.
 967                 // In addition, all code that advances grace periods runs
 968                 // at process level.  Therefore, this expedited GP overlaps
 969                 // with other expedited GPs only by being fully nested within
 970                 // them, which allows reuse of ->gp_seq_polled_exp_snap.
 971                 rcu_poll_gp_seq_start_unlocked(&rcu_state.gp_seq_polled_exp_snap);
 972                 rcu_poll_gp_seq_end_unlocked(&rcu_state.gp_seq_polled_exp_snap);
 973
 974                 local_irq_save(flags);
 975                 WARN_ON_ONCE(num_online_cpus() > 1);
 976                 rcu_state.expedited_sequence += (1 << RCU_SEQ_CTR_SHIFT);
 977                 local_irq_restore(flags);
 978                 return;  // Context allows vacuous grace periods.
 979         }
 980
 981         /* If expedited grace periods are prohibited, fall back to normal. */
 982         if (rcu_gp_is_normal()) {
 983                 wait_rcu_gp(call_rcu_hurry);
 984                 return;
 985         }
 986
 987         /* Take a snapshot of the sequence number.  */
 988         s = rcu_exp_gp_seq_snap();
 989         if (exp_funnel_lock(s))
 990                 return;  /* Someone else did our work for us. */
 991
 992         /* Ensure that load happens before action based on it. */
 993         if (unlikely(boottime)) {
 994                 /* Direct call during scheduler init and early_initcalls(). */
 995                 rcu_exp_sel_wait_wake(s);
 996         } else {
 997                 /* Marshall arguments & schedule the expedited grace period. */
 998                 rew.rew_s = s;
 999                 synchronize_rcu_expedited_queue_work(&rew);
1000         }
1001
1002         /* Wait for expedited grace period to complete. */
1003         rnp = rcu_get_root();
1004         wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
1005                    sync_exp_work_done(s));
1006         smp_mb(); /* Work actions happen before return. */
1007
1008         /* Let the next expedited grace period start. */
1009         mutex_unlock(&rcu_state.exp_mutex);
1010
1011         if (likely(!boottime))
1012                 synchronize_rcu_expedited_destroy_work(&rew);
1013 }
1014 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
1015
1016 /*
1017  * Ensure that start_poll_synchronize_rcu_expedited() has the expedited
1018  * RCU grace periods that it needs.
1019  */
1020 static void sync_rcu_do_polled_gp(struct work_struct *wp)
1021 {
1022         unsigned long flags;
1023         int i = 0;
1024         struct rcu_node *rnp = container_of(wp, struct rcu_node, exp_poll_wq);
1025         unsigned long s;
1026
1027         raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags);
1028         s = rnp->exp_seq_poll_rq;
1029         rnp->exp_seq_poll_rq = RCU_GET_STATE_COMPLETED;
1030         raw_spin_unlock_irqrestore(&rnp->exp_poll_lock, flags);
1031         if (s == RCU_GET_STATE_COMPLETED)
1032                 return;
1033         while (!poll_state_synchronize_rcu(s)) {
1034                 synchronize_rcu_expedited();
1035                 if (i == 10 || i == 20)
1036                         pr_info("%s: i = %d s = %lx gp_seq_polled = %lx\n", __func__, i, s, READ_ONCE(rcu_state.gp_seq_polled));
1037                 i++;
1038         }
1039         raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags);
1040         s = rnp->exp_seq_poll_rq;
1041         if (poll_state_synchronize_rcu(s))
1042                 rnp->exp_seq_poll_rq = RCU_GET_STATE_COMPLETED;
1043         raw_spin_unlock_irqrestore(&rnp->exp_poll_lock, flags);
1044 }
1045
1046 /**
1047  * start_poll_synchronize_rcu_expedited - Snapshot current RCU state and start expedited grace period
1048  *
1049  * Returns a cookie to pass to a call to cond_synchronize_rcu(),
1050  * cond_synchronize_rcu_expedited(), or poll_state_synchronize_rcu(),
1051  * allowing them to determine whether or not any sort of grace period has
1052  * elapsed in the meantime.  If the needed expedited grace period is not
1053  * already slated to start, initiates that grace period.
1054  */
1055 unsigned long start_poll_synchronize_rcu_expedited(void)
1056 {
1057         unsigned long flags;
1058         struct rcu_data *rdp;
1059         struct rcu_node *rnp;
1060         unsigned long s;
1061
1062         s = get_state_synchronize_rcu();
1063         rdp = per_cpu_ptr(&rcu_data, raw_smp_processor_id());
1064         rnp = rdp->mynode;
1065         if (rcu_init_invoked())
1066                 raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags);
1067         if (!poll_state_synchronize_rcu(s)) {
1068                 rnp->exp_seq_poll_rq = s;
1069                 if (rcu_init_invoked())
1070                         queue_work(rcu_gp_wq, &rnp->exp_poll_wq);
1071         }
1072         if (rcu_init_invoked())
1073                 raw_spin_unlock_irqrestore(&rnp->exp_poll_lock, flags);
1074
1075         return s;
1076 }
1077 EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_expedited);
1078
1079 /**
1080  * start_poll_synchronize_rcu_expedited_full - Take a full snapshot and start expedited grace period
1081  * @rgosp: Place to put snapshot of grace-period state
1082  *
1083  * Places the normal and expedited grace-period states in rgosp.  This
1084  * state value can be passed to a later call to cond_synchronize_rcu_full()
1085  * or poll_state_synchronize_rcu_full() to determine whether or not a
1086  * grace period (whether normal or expedited) has elapsed in the meantime.
1087  * If the needed expedited grace period is not already slated to start,
1088  * initiates that grace period.
1089  */
1090 void start_poll_synchronize_rcu_expedited_full(struct rcu_gp_oldstate *rgosp)
1091 {
1092         get_state_synchronize_rcu_full(rgosp);
1093         (void)start_poll_synchronize_rcu_expedited();
1094 }
1095 EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_expedited_full);
1096
1097 /**
1098  * cond_synchronize_rcu_expedited - Conditionally wait for an expedited RCU grace period
1099  *
1100  * @oldstate: value from get_state_synchronize_rcu(), start_poll_synchronize_rcu(), or start_poll_synchronize_rcu_expedited()
1101  *
1102  * If any type of full RCU grace period has elapsed since the earlier
1103  * call to get_state_synchronize_rcu(), start_poll_synchronize_rcu(),
1104  * or start_poll_synchronize_rcu_expedited(), just return.  Otherwise,
1105  * invoke synchronize_rcu_expedited() to wait for a full grace period.
1106  *
1107  * Yes, this function does not take counter wrap into account.
1108  * But counter wrap is harmless.  If the counter wraps, we have waited for
1109  * more than 2 billion grace periods (and way more on a 64-bit system!),
1110  * so waiting for a couple of additional grace periods should be just fine.
1111  *
1112  * This function provides the same memory-ordering guarantees that
1113  * would be provided by a synchronize_rcu() that was invoked at the call
1114  * to the function that provided @oldstate and that returned at the end
1115  * of this function.
1116  */
1117 void cond_synchronize_rcu_expedited(unsigned long oldstate)
1118 {
1119         if (!poll_state_synchronize_rcu(oldstate))
1120                 synchronize_rcu_expedited();
1121 }
1122 EXPORT_SYMBOL_GPL(cond_synchronize_rcu_expedited);
1123
1124 /**
1125  * cond_synchronize_rcu_expedited_full - Conditionally wait for an expedited RCU grace period
1126  * @rgosp: value from get_state_synchronize_rcu_full(), start_poll_synchronize_rcu_full(), or start_poll_synchronize_rcu_expedited_full()
1127  *
1128  * If a full RCU grace period has elapsed since the call to
1129  * get_state_synchronize_rcu_full(), start_poll_synchronize_rcu_full(),
1130  * or start_poll_synchronize_rcu_expedited_full() from which @rgosp was
1131  * obtained, just return.  Otherwise, invoke synchronize_rcu_expedited()
1132  * to wait for a full grace period.
1133  *
1134  * Yes, this function does not take counter wrap into account.
1135  * But counter wrap is harmless.  If the counter wraps, we have waited for
1136  * more than 2 billion grace periods (and way more on a 64-bit system!),
1137  * so waiting for a couple of additional grace periods should be just fine.
1138  *
1139  * This function provides the same memory-ordering guarantees that
1140  * would be provided by a synchronize_rcu() that was invoked at the call
1141  * to the function that provided @rgosp and that returned at the end of
1142  * this function.
1143  */
1144 void cond_synchronize_rcu_expedited_full(struct rcu_gp_oldstate *rgosp)
1145 {
1146         if (!poll_state_synchronize_rcu_full(rgosp))
1147                 synchronize_rcu_expedited();
1148 }
1149 EXPORT_SYMBOL_GPL(cond_synchronize_rcu_expedited_full);