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