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