kernel/rcu/tree_stall.h

   1 // SPDX-License-Identifier: GPL-2.0+
   2 /*
   3  * RCU CPU stall warnings for normal RCU grace periods
   4  *
   5  * Copyright IBM Corporation, 2019
   6  *
   7  * Author: Paul E. McKenney <paulmck@linux.ibm.com>
   8  */
   9
  10 #include <linux/kvm_para.h>
  11 #include <linux/rcu_notifier.h>
  12
  13 //////////////////////////////////////////////////////////////////////////////
  14 //
  15 // Controlling CPU stall warnings, including delay calculation.
  16
  17 /* panic() on RCU Stall sysctl. */
  18 int sysctl_panic_on_rcu_stall __read_mostly;
  19 int sysctl_max_rcu_stall_to_panic __read_mostly;
  20
  21 #ifdef CONFIG_PROVE_RCU
  22 #define RCU_STALL_DELAY_DELTA           (5 * HZ)
  23 #else
  24 #define RCU_STALL_DELAY_DELTA           0
  25 #endif
  26 #define RCU_STALL_MIGHT_DIV             8
  27 #define RCU_STALL_MIGHT_MIN             (2 * HZ)
  28
  29 int rcu_exp_jiffies_till_stall_check(void)
  30 {
  31         int cpu_stall_timeout = READ_ONCE(rcu_exp_cpu_stall_timeout);
  32         int exp_stall_delay_delta = 0;
  33         int till_stall_check;
  34
  35         // Zero says to use rcu_cpu_stall_timeout, but in milliseconds.
  36         if (!cpu_stall_timeout)
  37                 cpu_stall_timeout = jiffies_to_msecs(rcu_jiffies_till_stall_check());
  38
  39         // Limit check must be consistent with the Kconfig limits for
  40         // CONFIG_RCU_EXP_CPU_STALL_TIMEOUT, so check the allowed range.
  41         // The minimum clamped value is "2UL", because at least one full
  42         // tick has to be guaranteed.
  43         till_stall_check = clamp(msecs_to_jiffies(cpu_stall_timeout), 2UL, 300UL * HZ);
  44
  45         if (cpu_stall_timeout && jiffies_to_msecs(till_stall_check) != cpu_stall_timeout)
  46                 WRITE_ONCE(rcu_exp_cpu_stall_timeout, jiffies_to_msecs(till_stall_check));
  47
  48 #ifdef CONFIG_PROVE_RCU
  49         /* Add extra ~25% out of till_stall_check. */
  50         exp_stall_delay_delta = ((till_stall_check * 25) / 100) + 1;
  51 #endif
  52
  53         return till_stall_check + exp_stall_delay_delta;
  54 }
  55 EXPORT_SYMBOL_GPL(rcu_exp_jiffies_till_stall_check);
  56
  57 /* Limit-check stall timeouts specified at boottime and runtime. */
  58 int rcu_jiffies_till_stall_check(void)
  59 {
  60         int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout);
  61
  62         /*
  63          * Limit check must be consistent with the Kconfig limits
  64          * for CONFIG_RCU_CPU_STALL_TIMEOUT.
  65          */
  66         if (till_stall_check < 3) {
  67                 WRITE_ONCE(rcu_cpu_stall_timeout, 3);
  68                 till_stall_check = 3;
  69         } else if (till_stall_check > 300) {
  70                 WRITE_ONCE(rcu_cpu_stall_timeout, 300);
  71                 till_stall_check = 300;
  72         }
  73         return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
  74 }
  75 EXPORT_SYMBOL_GPL(rcu_jiffies_till_stall_check);
  76
  77 /**
  78  * rcu_gp_might_be_stalled - Is it likely that the grace period is stalled?
  79  *
  80  * Returns @true if the current grace period is sufficiently old that
  81  * it is reasonable to assume that it might be stalled.  This can be
  82  * useful when deciding whether to allocate memory to enable RCU-mediated
  83  * freeing on the one hand or just invoking synchronize_rcu() on the other.
  84  * The latter is preferable when the grace period is stalled.
  85  *
  86  * Note that sampling of the .gp_start and .gp_seq fields must be done
  87  * carefully to avoid false positives at the beginnings and ends of
  88  * grace periods.
  89  */
  90 bool rcu_gp_might_be_stalled(void)
  91 {
  92         unsigned long d = rcu_jiffies_till_stall_check() / RCU_STALL_MIGHT_DIV;
  93         unsigned long j = jiffies;
  94
  95         if (d < RCU_STALL_MIGHT_MIN)
  96                 d = RCU_STALL_MIGHT_MIN;
  97         smp_mb(); // jiffies before .gp_seq to avoid false positives.
  98         if (!rcu_gp_in_progress())
  99                 return false;
 100         // Long delays at this point avoids false positive, but a delay
 101         // of ULONG_MAX/4 jiffies voids your no-false-positive warranty.
 102         smp_mb(); // .gp_seq before second .gp_start
 103         // And ditto here.
 104         return !time_before(j, READ_ONCE(rcu_state.gp_start) + d);
 105 }
 106
 107 /* Don't do RCU CPU stall warnings during long sysrq printouts. */
 108 void rcu_sysrq_start(void)
 109 {
 110         if (!rcu_cpu_stall_suppress)
 111                 rcu_cpu_stall_suppress = 2;
 112 }
 113
 114 void rcu_sysrq_end(void)
 115 {
 116         if (rcu_cpu_stall_suppress == 2)
 117                 rcu_cpu_stall_suppress = 0;
 118 }
 119
 120 /* Don't print RCU CPU stall warnings during a kernel panic. */
 121 static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
 122 {
 123         rcu_cpu_stall_suppress = 1;
 124         return NOTIFY_DONE;
 125 }
 126
 127 static struct notifier_block rcu_panic_block = {
 128         .notifier_call = rcu_panic,
 129 };
 130
 131 static int __init check_cpu_stall_init(void)
 132 {
 133         atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block);
 134         return 0;
 135 }
 136 early_initcall(check_cpu_stall_init);
 137
 138 /* If so specified via sysctl, panic, yielding cleaner stall-warning output. */
 139 static void panic_on_rcu_stall(void)
 140 {
 141         static int cpu_stall;
 142
 143         if (++cpu_stall < sysctl_max_rcu_stall_to_panic)
 144                 return;
 145
 146         if (sysctl_panic_on_rcu_stall)
 147                 panic("RCU Stall\n");
 148 }
 149
 150 /**
 151  * rcu_cpu_stall_reset - restart stall-warning timeout for current grace period
 152  *
 153  * To perform the reset request from the caller, disable stall detection until
 154  * 3 fqs loops have passed. This is required to ensure a fresh jiffies is
 155  * loaded.  It should be safe to do from the fqs loop as enough timer
 156  * interrupts and context switches should have passed.
 157  *
 158  * The caller must disable hard irqs.
 159  */
 160 void rcu_cpu_stall_reset(void)
 161 {
 162         WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, 3);
 163         WRITE_ONCE(rcu_state.jiffies_stall, ULONG_MAX);
 164 }
 165
 166 //////////////////////////////////////////////////////////////////////////////
 167 //
 168 // Interaction with RCU grace periods
 169
 170 /* Start of new grace period, so record stall time (and forcing times). */
 171 static void record_gp_stall_check_time(void)
 172 {
 173         unsigned long j = jiffies;
 174         unsigned long j1;
 175
 176         WRITE_ONCE(rcu_state.gp_start, j);
 177         j1 = rcu_jiffies_till_stall_check();
 178         smp_mb(); // ->gp_start before ->jiffies_stall and caller's ->gp_seq.
 179         WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, 0);
 180         WRITE_ONCE(rcu_state.jiffies_stall, j + j1);
 181         rcu_state.jiffies_resched = j + j1 / 2;
 182         rcu_state.n_force_qs_gpstart = READ_ONCE(rcu_state.n_force_qs);
 183 }
 184
 185 /* Zero ->ticks_this_gp and snapshot the number of RCU softirq handlers. */
 186 static void zero_cpu_stall_ticks(struct rcu_data *rdp)
 187 {
 188         rdp->ticks_this_gp = 0;
 189         rdp->softirq_snap = kstat_softirqs_cpu(RCU_SOFTIRQ, smp_processor_id());
 190         WRITE_ONCE(rdp->last_fqs_resched, jiffies);
 191 }
 192
 193 /*
 194  * If too much time has passed in the current grace period, and if
 195  * so configured, go kick the relevant kthreads.
 196  */
 197 static void rcu_stall_kick_kthreads(void)
 198 {
 199         unsigned long j;
 200
 201         if (!READ_ONCE(rcu_kick_kthreads))
 202                 return;
 203         j = READ_ONCE(rcu_state.jiffies_kick_kthreads);
 204         if (time_after(jiffies, j) && rcu_state.gp_kthread &&
 205             (rcu_gp_in_progress() || READ_ONCE(rcu_state.gp_flags))) {
 206                 WARN_ONCE(1, "Kicking %s grace-period kthread\n",
 207                           rcu_state.name);
 208                 rcu_ftrace_dump(DUMP_ALL);
 209                 wake_up_process(rcu_state.gp_kthread);
 210                 WRITE_ONCE(rcu_state.jiffies_kick_kthreads, j + HZ);
 211         }
 212 }
 213
 214 /*
 215  * Handler for the irq_work request posted about halfway into the RCU CPU
 216  * stall timeout, and used to detect excessive irq disabling.  Set state
 217  * appropriately, but just complain if there is unexpected state on entry.
 218  */
 219 static void rcu_iw_handler(struct irq_work *iwp)
 220 {
 221         struct rcu_data *rdp;
 222         struct rcu_node *rnp;
 223
 224         rdp = container_of(iwp, struct rcu_data, rcu_iw);
 225         rnp = rdp->mynode;
 226         raw_spin_lock_rcu_node(rnp);
 227         if (!WARN_ON_ONCE(!rdp->rcu_iw_pending)) {
 228                 rdp->rcu_iw_gp_seq = rnp->gp_seq;
 229                 rdp->rcu_iw_pending = false;
 230         }
 231         raw_spin_unlock_rcu_node(rnp);
 232 }
 233
 234 //////////////////////////////////////////////////////////////////////////////
 235 //
 236 // Printing RCU CPU stall warnings
 237
 238 #ifdef CONFIG_PREEMPT_RCU
 239
 240 /*
 241  * Dump detailed information for all tasks blocking the current RCU
 242  * grace period on the specified rcu_node structure.
 243  */
 244 static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
 245 {
 246         unsigned long flags;
 247         struct task_struct *t;
 248
 249         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 250         if (!rcu_preempt_blocked_readers_cgp(rnp)) {
 251                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 252                 return;
 253         }
 254         t = list_entry(rnp->gp_tasks->prev,
 255                        struct task_struct, rcu_node_entry);
 256         list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
 257                 /*
 258                  * We could be printing a lot while holding a spinlock.
 259                  * Avoid triggering hard lockup.
 260                  */
 261                 touch_nmi_watchdog();
 262                 sched_show_task(t);
 263         }
 264         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 265 }
 266
 267 // Communicate task state back to the RCU CPU stall warning request.
 268 struct rcu_stall_chk_rdr {
 269         int nesting;
 270         union rcu_special rs;
 271         bool on_blkd_list;
 272 };
 273
 274 /*
 275  * Report out the state of a not-running task that is stalling the
 276  * current RCU grace period.
 277  */
 278 static int check_slow_task(struct task_struct *t, void *arg)
 279 {
 280         struct rcu_stall_chk_rdr *rscrp = arg;
 281
 282         if (task_curr(t))
 283                 return -EBUSY; // It is running, so decline to inspect it.
 284         rscrp->nesting = t->rcu_read_lock_nesting;
 285         rscrp->rs = t->rcu_read_unlock_special;
 286         rscrp->on_blkd_list = !list_empty(&t->rcu_node_entry);
 287         return 0;
 288 }
 289
 290 /*
 291  * Scan the current list of tasks blocked within RCU read-side critical
 292  * sections, printing out the tid of each of the first few of them.
 293  */
 294 static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
 295         __releases(rnp->lock)
 296 {
 297         int i = 0;
 298         int ndetected = 0;
 299         struct rcu_stall_chk_rdr rscr;
 300         struct task_struct *t;
 301         struct task_struct *ts[8];
 302
 303         lockdep_assert_irqs_disabled();
 304         if (!rcu_preempt_blocked_readers_cgp(rnp)) {
 305                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 306                 return 0;
 307         }
 308         pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
 309                rnp->level, rnp->grplo, rnp->grphi);
 310         t = list_entry(rnp->gp_tasks->prev,
 311                        struct task_struct, rcu_node_entry);
 312         list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
 313                 get_task_struct(t);
 314                 ts[i++] = t;
 315                 if (i >= ARRAY_SIZE(ts))
 316                         break;
 317         }
 318         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 319         while (i) {
 320                 t = ts[--i];
 321                 if (task_call_func(t, check_slow_task, &rscr))
 322                         pr_cont(" P%d", t->pid);
 323                 else
 324                         pr_cont(" P%d/%d:%c%c%c%c",
 325                                 t->pid, rscr.nesting,
 326                                 ".b"[rscr.rs.b.blocked],
 327                                 ".q"[rscr.rs.b.need_qs],
 328                                 ".e"[rscr.rs.b.exp_hint],
 329                                 ".l"[rscr.on_blkd_list]);
 330                 lockdep_assert_irqs_disabled();
 331                 put_task_struct(t);
 332                 ndetected++;
 333         }
 334         pr_cont("\n");
 335         return ndetected;
 336 }
 337
 338 #else /* #ifdef CONFIG_PREEMPT_RCU */
 339
 340 /*
 341  * Because preemptible RCU does not exist, we never have to check for
 342  * tasks blocked within RCU read-side critical sections.
 343  */
 344 static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
 345 {
 346 }
 347
 348 /*
 349  * Because preemptible RCU does not exist, we never have to check for
 350  * tasks blocked within RCU read-side critical sections.
 351  */
 352 static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
 353         __releases(rnp->lock)
 354 {
 355         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 356         return 0;
 357 }
 358 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 359
 360 /*
 361  * Dump stacks of all tasks running on stalled CPUs.  First try using
 362  * NMIs, but fall back to manual remote stack tracing on architectures
 363  * that don't support NMI-based stack dumps.  The NMI-triggered stack
 364  * traces are more accurate because they are printed by the target CPU.
 365  */
 366 static void rcu_dump_cpu_stacks(void)
 367 {
 368         int cpu;
 369         unsigned long flags;
 370         struct rcu_node *rnp;
 371
 372         rcu_for_each_leaf_node(rnp) {
 373                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
 374                 for_each_leaf_node_possible_cpu(rnp, cpu)
 375                         if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
 376                                 if (cpu_is_offline(cpu))
 377                                         pr_err("Offline CPU %d blocking current GP.\n", cpu);
 378                                 else
 379                                         dump_cpu_task(cpu);
 380                         }
 381                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 382         }
 383 }
 384
 385 static const char * const gp_state_names[] = {
 386         [RCU_GP_IDLE] = "RCU_GP_IDLE",
 387         [RCU_GP_WAIT_GPS] = "RCU_GP_WAIT_GPS",
 388         [RCU_GP_DONE_GPS] = "RCU_GP_DONE_GPS",
 389         [RCU_GP_ONOFF] = "RCU_GP_ONOFF",
 390         [RCU_GP_INIT] = "RCU_GP_INIT",
 391         [RCU_GP_WAIT_FQS] = "RCU_GP_WAIT_FQS",
 392         [RCU_GP_DOING_FQS] = "RCU_GP_DOING_FQS",
 393         [RCU_GP_CLEANUP] = "RCU_GP_CLEANUP",
 394         [RCU_GP_CLEANED] = "RCU_GP_CLEANED",
 395 };
 396
 397 /*
 398  * Convert a ->gp_state value to a character string.
 399  */
 400 static const char *gp_state_getname(short gs)
 401 {
 402         if (gs < 0 || gs >= ARRAY_SIZE(gp_state_names))
 403                 return "???";
 404         return gp_state_names[gs];
 405 }
 406
 407 /* Is the RCU grace-period kthread being starved of CPU time? */
 408 static bool rcu_is_gp_kthread_starving(unsigned long *jp)
 409 {
 410         unsigned long j = jiffies - READ_ONCE(rcu_state.gp_activity);
 411
 412         if (jp)
 413                 *jp = j;
 414         return j > 2 * HZ;
 415 }
 416
 417 static bool rcu_is_rcuc_kthread_starving(struct rcu_data *rdp, unsigned long *jp)
 418 {
 419         int cpu;
 420         struct task_struct *rcuc;
 421         unsigned long j;
 422
 423         rcuc = rdp->rcu_cpu_kthread_task;
 424         if (!rcuc)
 425                 return false;
 426
 427         cpu = task_cpu(rcuc);
 428         if (cpu_is_offline(cpu) || idle_cpu(cpu))
 429                 return false;
 430
 431         j = jiffies - READ_ONCE(rdp->rcuc_activity);
 432
 433         if (jp)
 434                 *jp = j;
 435         return j > 2 * HZ;
 436 }
 437
 438 static void print_cpu_stat_info(int cpu)
 439 {
 440         struct rcu_snap_record rsr, *rsrp;
 441         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 442         struct kernel_cpustat *kcsp = &kcpustat_cpu(cpu);
 443
 444         if (!rcu_cpu_stall_cputime)
 445                 return;
 446
 447         rsrp = &rdp->snap_record;
 448         if (rsrp->gp_seq != rdp->gp_seq)
 449                 return;
 450
 451         rsr.cputime_irq     = kcpustat_field(kcsp, CPUTIME_IRQ, cpu);
 452         rsr.cputime_softirq = kcpustat_field(kcsp, CPUTIME_SOFTIRQ, cpu);
 453         rsr.cputime_system  = kcpustat_field(kcsp, CPUTIME_SYSTEM, cpu);
 454
 455         pr_err("\t         hardirqs   softirqs   csw/system\n");
 456         pr_err("\t number: %8ld %10d %12lld\n",
 457                 kstat_cpu_irqs_sum(cpu) - rsrp->nr_hardirqs,
 458                 kstat_cpu_softirqs_sum(cpu) - rsrp->nr_softirqs,
 459                 nr_context_switches_cpu(cpu) - rsrp->nr_csw);
 460         pr_err("\tcputime: %8lld %10lld %12lld   ==> %d(ms)\n",
 461                 div_u64(rsr.cputime_irq - rsrp->cputime_irq, NSEC_PER_MSEC),
 462                 div_u64(rsr.cputime_softirq - rsrp->cputime_softirq, NSEC_PER_MSEC),
 463                 div_u64(rsr.cputime_system - rsrp->cputime_system, NSEC_PER_MSEC),
 464                 jiffies_to_msecs(jiffies - rsrp->jiffies));
 465 }
 466
 467 /*
 468  * Print out diagnostic information for the specified stalled CPU.
 469  *
 470  * If the specified CPU is aware of the current RCU grace period, then
 471  * print the number of scheduling clock interrupts the CPU has taken
 472  * during the time that it has been aware.  Otherwise, print the number
 473  * of RCU grace periods that this CPU is ignorant of, for example, "1"
 474  * if the CPU was aware of the previous grace period.
 475  *
 476  * Also print out idle info.
 477  */
 478 static void print_cpu_stall_info(int cpu)
 479 {
 480         unsigned long delta;
 481         bool falsepositive;
 482         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 483         char *ticks_title;
 484         unsigned long ticks_value;
 485         bool rcuc_starved;
 486         unsigned long j;
 487         char buf[32];
 488
 489         /*
 490          * We could be printing a lot while holding a spinlock.  Avoid
 491          * triggering hard lockup.
 492          */
 493         touch_nmi_watchdog();
 494
 495         ticks_value = rcu_seq_ctr(rcu_state.gp_seq - rdp->gp_seq);
 496         if (ticks_value) {
 497                 ticks_title = "GPs behind";
 498         } else {
 499                 ticks_title = "ticks this GP";
 500                 ticks_value = rdp->ticks_this_gp;
 501         }
 502         delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
 503         falsepositive = rcu_is_gp_kthread_starving(NULL) &&
 504                         rcu_dynticks_in_eqs(rcu_dynticks_snap(cpu));
 505         rcuc_starved = rcu_is_rcuc_kthread_starving(rdp, &j);
 506         if (rcuc_starved)
 507                 sprintf(buf, " rcuc=%ld jiffies(starved)", j);
 508         pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%04x/%ld/%#lx softirq=%u/%u fqs=%ld%s%s\n",
 509                cpu,
 510                "O."[!!cpu_online(cpu)],
 511                "o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)],
 512                "N."[!!(rdp->grpmask & rdp->mynode->qsmaskinitnext)],
 513                !IS_ENABLED(CONFIG_IRQ_WORK) ? '?' :
 514                         rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' :
 515                                 "!."[!delta],
 516                ticks_value, ticks_title,
 517                rcu_dynticks_snap(cpu) & 0xffff,
 518                ct_dynticks_nesting_cpu(cpu), ct_dynticks_nmi_nesting_cpu(cpu),
 519                rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
 520                data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
 521                rcuc_starved ? buf : "",
 522                falsepositive ? " (false positive?)" : "");
 523
 524         print_cpu_stat_info(cpu);
 525 }
 526
 527 /* Complain about starvation of grace-period kthread.  */
 528 static void rcu_check_gp_kthread_starvation(void)
 529 {
 530         int cpu;
 531         struct task_struct *gpk = rcu_state.gp_kthread;
 532         unsigned long j;
 533
 534         if (rcu_is_gp_kthread_starving(&j)) {
 535                 cpu = gpk ? task_cpu(gpk) : -1;
 536                 pr_err("%s kthread starved for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x ->cpu=%d\n",
 537                        rcu_state.name, j,
 538                        (long)rcu_seq_current(&rcu_state.gp_seq),
 539                        data_race(READ_ONCE(rcu_state.gp_flags)),
 540                        gp_state_getname(rcu_state.gp_state),
 541                        data_race(READ_ONCE(rcu_state.gp_state)),
 542                        gpk ? data_race(READ_ONCE(gpk->__state)) : ~0, cpu);
 543                 if (gpk) {
 544                         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 545
 546                         pr_err("\tUnless %s kthread gets sufficient CPU time, OOM is now expected behavior.\n", rcu_state.name);
 547                         pr_err("RCU grace-period kthread stack dump:\n");
 548                         sched_show_task(gpk);
 549                         if (cpu_is_offline(cpu)) {
 550                                 pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu);
 551                         } else if (!(data_race(READ_ONCE(rdp->mynode->qsmask)) & rdp->grpmask)) {
 552                                 pr_err("Stack dump where RCU GP kthread last ran:\n");
 553                                 dump_cpu_task(cpu);
 554                         }
 555                         wake_up_process(gpk);
 556                 }
 557         }
 558 }
 559
 560 /* Complain about missing wakeups from expired fqs wait timer */
 561 static void rcu_check_gp_kthread_expired_fqs_timer(void)
 562 {
 563         struct task_struct *gpk = rcu_state.gp_kthread;
 564         short gp_state;
 565         unsigned long jiffies_fqs;
 566         int cpu;
 567
 568         /*
 569          * Order reads of .gp_state and .jiffies_force_qs.
 570          * Matching smp_wmb() is present in rcu_gp_fqs_loop().
 571          */
 572         gp_state = smp_load_acquire(&rcu_state.gp_state);
 573         jiffies_fqs = READ_ONCE(rcu_state.jiffies_force_qs);
 574
 575         if (gp_state == RCU_GP_WAIT_FQS &&
 576             time_after(jiffies, jiffies_fqs + RCU_STALL_MIGHT_MIN) &&
 577             gpk && !READ_ONCE(gpk->on_rq)) {
 578                 cpu = task_cpu(gpk);
 579                 pr_err("%s kthread timer wakeup didn't happen for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x\n",
 580                        rcu_state.name, (jiffies - jiffies_fqs),
 581                        (long)rcu_seq_current(&rcu_state.gp_seq),
 582                        data_race(rcu_state.gp_flags),
 583                        gp_state_getname(RCU_GP_WAIT_FQS), RCU_GP_WAIT_FQS,
 584                        data_race(READ_ONCE(gpk->__state)));
 585                 pr_err("\tPossible timer handling issue on cpu=%d timer-softirq=%u\n",
 586                        cpu, kstat_softirqs_cpu(TIMER_SOFTIRQ, cpu));
 587         }
 588 }
 589
 590 static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
 591 {
 592         int cpu;
 593         unsigned long flags;
 594         unsigned long gpa;
 595         unsigned long j;
 596         int ndetected = 0;
 597         struct rcu_node *rnp;
 598         long totqlen = 0;
 599
 600         lockdep_assert_irqs_disabled();
 601
 602         /* Kick and suppress, if so configured. */
 603         rcu_stall_kick_kthreads();
 604         if (rcu_stall_is_suppressed())
 605                 return;
 606
 607         /*
 608          * OK, time to rat on our buddy...
 609          * See Documentation/RCU/stallwarn.rst for info on how to debug
 610          * RCU CPU stall warnings.
 611          */
 612         trace_rcu_stall_warning(rcu_state.name, TPS("StallDetected"));
 613         pr_err("INFO: %s detected stalls on CPUs/tasks:\n", rcu_state.name);
 614         rcu_for_each_leaf_node(rnp) {
 615                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
 616                 if (rnp->qsmask != 0) {
 617                         for_each_leaf_node_possible_cpu(rnp, cpu)
 618                                 if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
 619                                         print_cpu_stall_info(cpu);
 620                                         ndetected++;
 621                                 }
 622                 }
 623                 ndetected += rcu_print_task_stall(rnp, flags); // Releases rnp->lock.
 624                 lockdep_assert_irqs_disabled();
 625         }
 626
 627         for_each_possible_cpu(cpu)
 628                 totqlen += rcu_get_n_cbs_cpu(cpu);
 629         pr_err("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu ncpus=%d)\n",
 630                smp_processor_id(), (long)(jiffies - gps),
 631                (long)rcu_seq_current(&rcu_state.gp_seq), totqlen, rcu_state.n_online_cpus);
 632         if (ndetected) {
 633                 rcu_dump_cpu_stacks();
 634
 635                 /* Complain about tasks blocking the grace period. */
 636                 rcu_for_each_leaf_node(rnp)
 637                         rcu_print_detail_task_stall_rnp(rnp);
 638         } else {
 639                 if (rcu_seq_current(&rcu_state.gp_seq) != gp_seq) {
 640                         pr_err("INFO: Stall ended before state dump start\n");
 641                 } else {
 642                         j = jiffies;
 643                         gpa = data_race(READ_ONCE(rcu_state.gp_activity));
 644                         pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld, root ->qsmask %#lx\n",
 645                                rcu_state.name, j - gpa, j, gpa,
 646                                data_race(READ_ONCE(jiffies_till_next_fqs)),
 647                                data_race(READ_ONCE(rcu_get_root()->qsmask)));
 648                 }
 649         }
 650         /* Rewrite if needed in case of slow consoles. */
 651         if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
 652                 WRITE_ONCE(rcu_state.jiffies_stall,
 653                            jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
 654
 655         rcu_check_gp_kthread_expired_fqs_timer();
 656         rcu_check_gp_kthread_starvation();
 657
 658         panic_on_rcu_stall();
 659
 660         rcu_force_quiescent_state();  /* Kick them all. */
 661 }
 662
 663 static void print_cpu_stall(unsigned long gps)
 664 {
 665         int cpu;
 666         unsigned long flags;
 667         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 668         struct rcu_node *rnp = rcu_get_root();
 669         long totqlen = 0;
 670
 671         lockdep_assert_irqs_disabled();
 672
 673         /* Kick and suppress, if so configured. */
 674         rcu_stall_kick_kthreads();
 675         if (rcu_stall_is_suppressed())
 676                 return;
 677
 678         /*
 679          * OK, time to rat on ourselves...
 680          * See Documentation/RCU/stallwarn.rst for info on how to debug
 681          * RCU CPU stall warnings.
 682          */
 683         trace_rcu_stall_warning(rcu_state.name, TPS("SelfDetected"));
 684         pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name);
 685         raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags);
 686         print_cpu_stall_info(smp_processor_id());
 687         raw_spin_unlock_irqrestore_rcu_node(rdp->mynode, flags);
 688         for_each_possible_cpu(cpu)
 689                 totqlen += rcu_get_n_cbs_cpu(cpu);
 690         pr_err("\t(t=%lu jiffies g=%ld q=%lu ncpus=%d)\n",
 691                 jiffies - gps,
 692                 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen, rcu_state.n_online_cpus);
 693
 694         rcu_check_gp_kthread_expired_fqs_timer();
 695         rcu_check_gp_kthread_starvation();
 696
 697         rcu_dump_cpu_stacks();
 698
 699         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 700         /* Rewrite if needed in case of slow consoles. */
 701         if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
 702                 WRITE_ONCE(rcu_state.jiffies_stall,
 703                            jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
 704         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 705
 706         panic_on_rcu_stall();
 707
 708         /*
 709          * Attempt to revive the RCU machinery by forcing a context switch.
 710          *
 711          * A context switch would normally allow the RCU state machine to make
 712          * progress and it could be we're stuck in kernel space without context
 713          * switches for an entirely unreasonable amount of time.
 714          */
 715         set_tsk_need_resched(current);
 716         set_preempt_need_resched();
 717 }
 718
 719 static void check_cpu_stall(struct rcu_data *rdp)
 720 {
 721         bool self_detected;
 722         unsigned long gs1;
 723         unsigned long gs2;
 724         unsigned long gps;
 725         unsigned long j;
 726         unsigned long jn;
 727         unsigned long js;
 728         struct rcu_node *rnp;
 729
 730         lockdep_assert_irqs_disabled();
 731         if ((rcu_stall_is_suppressed() && !READ_ONCE(rcu_kick_kthreads)) ||
 732             !rcu_gp_in_progress())
 733                 return;
 734         rcu_stall_kick_kthreads();
 735
 736         /*
 737          * Check if it was requested (via rcu_cpu_stall_reset()) that the FQS
 738          * loop has to set jiffies to ensure a non-stale jiffies value. This
 739          * is required to have good jiffies value after coming out of long
 740          * breaks of jiffies updates. Not doing so can cause false positives.
 741          */
 742         if (READ_ONCE(rcu_state.nr_fqs_jiffies_stall) > 0)
 743                 return;
 744
 745         j = jiffies;
 746
 747         /*
 748          * Lots of memory barriers to reject false positives.
 749          *
 750          * The idea is to pick up rcu_state.gp_seq, then
 751          * rcu_state.jiffies_stall, then rcu_state.gp_start, and finally
 752          * another copy of rcu_state.gp_seq.  These values are updated in
 753          * the opposite order with memory barriers (or equivalent) during
 754          * grace-period initialization and cleanup.  Now, a false positive
 755          * can occur if we get an new value of rcu_state.gp_start and a old
 756          * value of rcu_state.jiffies_stall.  But given the memory barriers,
 757          * the only way that this can happen is if one grace period ends
 758          * and another starts between these two fetches.  This is detected
 759          * by comparing the second fetch of rcu_state.gp_seq with the
 760          * previous fetch from rcu_state.gp_seq.
 761          *
 762          * Given this check, comparisons of jiffies, rcu_state.jiffies_stall,
 763          * and rcu_state.gp_start suffice to forestall false positives.
 764          */
 765         gs1 = READ_ONCE(rcu_state.gp_seq);
 766         smp_rmb(); /* Pick up ->gp_seq first... */
 767         js = READ_ONCE(rcu_state.jiffies_stall);
 768         smp_rmb(); /* ...then ->jiffies_stall before the rest... */
 769         gps = READ_ONCE(rcu_state.gp_start);
 770         smp_rmb(); /* ...and finally ->gp_start before ->gp_seq again. */
 771         gs2 = READ_ONCE(rcu_state.gp_seq);
 772         if (gs1 != gs2 ||
 773             ULONG_CMP_LT(j, js) ||
 774             ULONG_CMP_GE(gps, js))
 775                 return; /* No stall or GP completed since entering function. */
 776         rnp = rdp->mynode;
 777         jn = jiffies + ULONG_MAX / 2;
 778         self_detected = READ_ONCE(rnp->qsmask) & rdp->grpmask;
 779         if (rcu_gp_in_progress() &&
 780             (self_detected || ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY)) &&
 781             cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
 782                 /*
 783                  * If a virtual machine is stopped by the host it can look to
 784                  * the watchdog like an RCU stall. Check to see if the host
 785                  * stopped the vm.
 786                  */
 787                 if (kvm_check_and_clear_guest_paused())
 788                         return;
 789
 790                 rcu_stall_notifier_call_chain(RCU_STALL_NOTIFY_NORM, (void *)j - gps);
 791                 if (self_detected) {
 792                         /* We haven't checked in, so go dump stack. */
 793                         print_cpu_stall(gps);
 794                 } else {
 795                         /* They had a few time units to dump stack, so complain. */
 796                         print_other_cpu_stall(gs2, gps);
 797                 }
 798
 799                 if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
 800                         rcu_ftrace_dump(DUMP_ALL);
 801
 802                 if (READ_ONCE(rcu_state.jiffies_stall) == jn) {
 803                         jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
 804                         WRITE_ONCE(rcu_state.jiffies_stall, jn);
 805                 }
 806         }
 807 }
 808
 809 //////////////////////////////////////////////////////////////////////////////
 810 //
 811 // RCU forward-progress mechanisms, including for callback invocation.
 812
 813
 814 /*
 815  * Check to see if a failure to end RCU priority inversion was due to
 816  * a CPU not passing through a quiescent state.  When this happens, there
 817  * is nothing that RCU priority boosting can do to help, so we shouldn't
 818  * count this as an RCU priority boosting failure.  A return of true says
 819  * RCU priority boosting is to blame, and false says otherwise.  If false
 820  * is returned, the first of the CPUs to blame is stored through cpup.
 821  * If there was no CPU blocking the current grace period, but also nothing
 822  * in need of being boosted, *cpup is set to -1.  This can happen in case
 823  * of vCPU preemption while the last CPU is reporting its quiscent state,
 824  * for example.
 825  *
 826  * If cpup is NULL, then a lockless quick check is carried out, suitable
 827  * for high-rate usage.  On the other hand, if cpup is non-NULL, each
 828  * rcu_node structure's ->lock is acquired, ruling out high-rate usage.
 829  */
 830 bool rcu_check_boost_fail(unsigned long gp_state, int *cpup)
 831 {
 832         bool atb = false;
 833         int cpu;
 834         unsigned long flags;
 835         struct rcu_node *rnp;
 836
 837         rcu_for_each_leaf_node(rnp) {
 838                 if (!cpup) {
 839                         if (data_race(READ_ONCE(rnp->qsmask))) {
 840                                 return false;
 841                         } else {
 842                                 if (READ_ONCE(rnp->gp_tasks))
 843                                         atb = true;
 844                                 continue;
 845                         }
 846                 }
 847                 *cpup = -1;
 848                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
 849                 if (rnp->gp_tasks)
 850                         atb = true;
 851                 if (!rnp->qsmask) {
 852                         // No CPUs without quiescent states for this rnp.
 853                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 854                         continue;
 855                 }
 856                 // Find the first holdout CPU.
 857                 for_each_leaf_node_possible_cpu(rnp, cpu) {
 858                         if (rnp->qsmask & (1UL << (cpu - rnp->grplo))) {
 859                                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 860                                 *cpup = cpu;
 861                                 return false;
 862                         }
 863                 }
 864                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 865         }
 866         // Can't blame CPUs, so must blame RCU priority boosting.
 867         return atb;
 868 }
 869 EXPORT_SYMBOL_GPL(rcu_check_boost_fail);
 870
 871 /*
 872  * Show the state of the grace-period kthreads.
 873  */
 874 void show_rcu_gp_kthreads(void)
 875 {
 876         unsigned long cbs = 0;
 877         int cpu;
 878         unsigned long j;
 879         unsigned long ja;
 880         unsigned long jr;
 881         unsigned long js;
 882         unsigned long jw;
 883         struct rcu_data *rdp;
 884         struct rcu_node *rnp;
 885         struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);
 886
 887         j = jiffies;
 888         ja = j - data_race(READ_ONCE(rcu_state.gp_activity));
 889         jr = j - data_race(READ_ONCE(rcu_state.gp_req_activity));
 890         js = j - data_race(READ_ONCE(rcu_state.gp_start));
 891         jw = j - data_race(READ_ONCE(rcu_state.gp_wake_time));
 892         pr_info("%s: wait state: %s(%d) ->state: %#x ->rt_priority %u delta ->gp_start %lu ->gp_activity %lu ->gp_req_activity %lu ->gp_wake_time %lu ->gp_wake_seq %ld ->gp_seq %ld ->gp_seq_needed %ld ->gp_max %lu ->gp_flags %#x\n",
 893                 rcu_state.name, gp_state_getname(rcu_state.gp_state),
 894                 data_race(READ_ONCE(rcu_state.gp_state)),
 895                 t ? data_race(READ_ONCE(t->__state)) : 0x1ffff, t ? t->rt_priority : 0xffU,
 896                 js, ja, jr, jw, (long)data_race(READ_ONCE(rcu_state.gp_wake_seq)),
 897                 (long)data_race(READ_ONCE(rcu_state.gp_seq)),
 898                 (long)data_race(READ_ONCE(rcu_get_root()->gp_seq_needed)),
 899                 data_race(READ_ONCE(rcu_state.gp_max)),
 900                 data_race(READ_ONCE(rcu_state.gp_flags)));
 901         rcu_for_each_node_breadth_first(rnp) {
 902                 if (ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq), READ_ONCE(rnp->gp_seq_needed)) &&
 903                     !data_race(READ_ONCE(rnp->qsmask)) && !data_race(READ_ONCE(rnp->boost_tasks)) &&
 904                     !data_race(READ_ONCE(rnp->exp_tasks)) && !data_race(READ_ONCE(rnp->gp_tasks)))
 905                         continue;
 906                 pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld ->qsmask %#lx %c%c%c%c ->n_boosts %ld\n",
 907                         rnp->grplo, rnp->grphi,
 908                         (long)data_race(READ_ONCE(rnp->gp_seq)),
 909                         (long)data_race(READ_ONCE(rnp->gp_seq_needed)),
 910                         data_race(READ_ONCE(rnp->qsmask)),
 911                         ".b"[!!data_race(READ_ONCE(rnp->boost_kthread_task))],
 912                         ".B"[!!data_race(READ_ONCE(rnp->boost_tasks))],
 913                         ".E"[!!data_race(READ_ONCE(rnp->exp_tasks))],
 914                         ".G"[!!data_race(READ_ONCE(rnp->gp_tasks))],
 915                         data_race(READ_ONCE(rnp->n_boosts)));
 916                 if (!rcu_is_leaf_node(rnp))
 917                         continue;
 918                 for_each_leaf_node_possible_cpu(rnp, cpu) {
 919                         rdp = per_cpu_ptr(&rcu_data, cpu);
 920                         if (READ_ONCE(rdp->gpwrap) ||
 921                             ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq),
 922                                          READ_ONCE(rdp->gp_seq_needed)))
 923                                 continue;
 924                         pr_info("\tcpu %d ->gp_seq_needed %ld\n",
 925                                 cpu, (long)data_race(READ_ONCE(rdp->gp_seq_needed)));
 926                 }
 927         }
 928         for_each_possible_cpu(cpu) {
 929                 rdp = per_cpu_ptr(&rcu_data, cpu);
 930                 cbs += data_race(READ_ONCE(rdp->n_cbs_invoked));
 931                 if (rcu_segcblist_is_offloaded(&rdp->cblist))
 932                         show_rcu_nocb_state(rdp);
 933         }
 934         pr_info("RCU callbacks invoked since boot: %lu\n", cbs);
 935         show_rcu_tasks_gp_kthreads();
 936 }
 937 EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);
 938
 939 /*
 940  * This function checks for grace-period requests that fail to motivate
 941  * RCU to come out of its idle mode.
 942  */
 943 static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
 944                                      const unsigned long gpssdelay)
 945 {
 946         unsigned long flags;
 947         unsigned long j;
 948         struct rcu_node *rnp_root = rcu_get_root();
 949         static atomic_t warned = ATOMIC_INIT(0);
 950
 951         if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() ||
 952             ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
 953                          READ_ONCE(rnp_root->gp_seq_needed)) ||
 954             !smp_load_acquire(&rcu_state.gp_kthread)) // Get stable kthread.
 955                 return;
 956         j = jiffies; /* Expensive access, and in common case don't get here. */
 957         if (time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
 958             time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
 959             atomic_read(&warned))
 960                 return;
 961
 962         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 963         j = jiffies;
 964         if (rcu_gp_in_progress() ||
 965             ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
 966                          READ_ONCE(rnp_root->gp_seq_needed)) ||
 967             time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
 968             time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
 969             atomic_read(&warned)) {
 970                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 971                 return;
 972         }
 973         /* Hold onto the leaf lock to make others see warned==1. */
 974
 975         if (rnp_root != rnp)
 976                 raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
 977         j = jiffies;
 978         if (rcu_gp_in_progress() ||
 979             ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
 980                          READ_ONCE(rnp_root->gp_seq_needed)) ||
 981             time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
 982             time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
 983             atomic_xchg(&warned, 1)) {
 984                 if (rnp_root != rnp)
 985                         /* irqs remain disabled. */
 986                         raw_spin_unlock_rcu_node(rnp_root);
 987                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 988                 return;
 989         }
 990         WARN_ON(1);
 991         if (rnp_root != rnp)
 992                 raw_spin_unlock_rcu_node(rnp_root);
 993         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 994         show_rcu_gp_kthreads();
 995 }
 996
 997 /*
 998  * Do a forward-progress check for rcutorture.  This is normally invoked
 999  * due to an OOM event.  The argument "j" gives the time period during
1000  * which rcutorture would like progress to have been made.
1001  */
1002 void rcu_fwd_progress_check(unsigned long j)
1003 {
1004         unsigned long cbs;
1005         int cpu;
1006         unsigned long max_cbs = 0;
1007         int max_cpu = -1;
1008         struct rcu_data *rdp;
1009
1010         if (rcu_gp_in_progress()) {
1011                 pr_info("%s: GP age %lu jiffies\n",
1012                         __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_start)));
1013                 show_rcu_gp_kthreads();
1014         } else {
1015                 pr_info("%s: Last GP end %lu jiffies ago\n",
1016                         __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_end)));
1017                 preempt_disable();
1018                 rdp = this_cpu_ptr(&rcu_data);
1019                 rcu_check_gp_start_stall(rdp->mynode, rdp, j);
1020                 preempt_enable();
1021         }
1022         for_each_possible_cpu(cpu) {
1023                 cbs = rcu_get_n_cbs_cpu(cpu);
1024                 if (!cbs)
1025                         continue;
1026                 if (max_cpu < 0)
1027                         pr_info("%s: callbacks", __func__);
1028                 pr_cont(" %d: %lu", cpu, cbs);
1029                 if (cbs <= max_cbs)
1030                         continue;
1031                 max_cbs = cbs;
1032                 max_cpu = cpu;
1033         }
1034         if (max_cpu >= 0)
1035                 pr_cont("\n");
1036 }
1037 EXPORT_SYMBOL_GPL(rcu_fwd_progress_check);
1038
1039 /* Commandeer a sysrq key to dump RCU's tree. */
1040 static bool sysrq_rcu;
1041 module_param(sysrq_rcu, bool, 0444);
1042
1043 /* Dump grace-period-request information due to commandeered sysrq. */
1044 static void sysrq_show_rcu(u8 key)
1045 {
1046         show_rcu_gp_kthreads();
1047 }
1048
1049 static const struct sysrq_key_op sysrq_rcudump_op = {
1050         .handler = sysrq_show_rcu,
1051         .help_msg = "show-rcu(y)",
1052         .action_msg = "Show RCU tree",
1053         .enable_mask = SYSRQ_ENABLE_DUMP,
1054 };
1055
1056 static int __init rcu_sysrq_init(void)
1057 {
1058         if (sysrq_rcu)
1059                 return register_sysrq_key('y', &sysrq_rcudump_op);
1060         return 0;
1061 }
1062 early_initcall(rcu_sysrq_init);
1063
1064 #ifdef CONFIG_RCU_CPU_STALL_NOTIFIER
1065
1066 //////////////////////////////////////////////////////////////////////////////
1067 //
1068 // RCU CPU stall-warning notifiers
1069
1070 static ATOMIC_NOTIFIER_HEAD(rcu_cpu_stall_notifier_list);
1071
1072 /**
1073  * rcu_stall_chain_notifier_register - Add an RCU CPU stall notifier
1074  * @n: Entry to add.
1075  *
1076  * Adds an RCU CPU stall notifier to an atomic notifier chain.
1077  * The @action passed to a notifier will be @RCU_STALL_NOTIFY_NORM or
1078  * friends.  The @data will be the duration of the stalled grace period,
1079  * in jiffies, coerced to a void* pointer.
1080  *
1081  * Returns 0 on success, %-EEXIST on error.
1082  */
1083 int rcu_stall_chain_notifier_register(struct notifier_block *n)
1084 {
1085         int rcsn = rcu_cpu_stall_notifiers;
1086
1087         WARN(1, "Adding %pS() to RCU stall notifier list (%s).\n", n->notifier_call,
1088              rcsn ? "possibly suppressing RCU CPU stall warnings" : "failed, so all is well");
1089         if (rcsn)
1090                 return atomic_notifier_chain_register(&rcu_cpu_stall_notifier_list, n);
1091         return -EEXIST;
1092 }
1093 EXPORT_SYMBOL_GPL(rcu_stall_chain_notifier_register);
1094
1095 /**
1096  * rcu_stall_chain_notifier_unregister - Remove an RCU CPU stall notifier
1097  * @n: Entry to add.
1098  *
1099  * Removes an RCU CPU stall notifier from an atomic notifier chain.
1100  *
1101  * Returns zero on success, %-ENOENT on failure.
1102  */
1103 int rcu_stall_chain_notifier_unregister(struct notifier_block *n)
1104 {
1105         return atomic_notifier_chain_unregister(&rcu_cpu_stall_notifier_list, n);
1106 }
1107 EXPORT_SYMBOL_GPL(rcu_stall_chain_notifier_unregister);
1108
1109 /*
1110  * rcu_stall_notifier_call_chain - Call functions in an RCU CPU stall notifier chain
1111  * @val: Value passed unmodified to notifier function
1112  * @v: Pointer passed unmodified to notifier function
1113  *
1114  * Calls each function in the RCU CPU stall notifier chain in turn, which
1115  * is an atomic call chain.  See atomic_notifier_call_chain() for more
1116  * information.
1117  *
1118  * This is for use within RCU, hence the omission of the extra asterisk
1119  * to indicate a non-kerneldoc format header comment.
1120  */
1121 int rcu_stall_notifier_call_chain(unsigned long val, void *v)
1122 {
1123         return atomic_notifier_call_chain(&rcu_cpu_stall_notifier_list, val, v);
1124 }
1125
1126 #endif // #ifdef CONFIG_RCU_CPU_STALL_NOTIFIER