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/console.h>
  11 #include <linux/kvm_para.h>
  12 #include <linux/rcu_notifier.h>
  13 #include <linux/smp.h>
  14
  15 //////////////////////////////////////////////////////////////////////////////
  16 //
  17 // Controlling CPU stall warnings, including delay calculation.
  18
  19 /* panic() on RCU Stall sysctl. */
  20 int sysctl_panic_on_rcu_stall __read_mostly;
  21 int sysctl_max_rcu_stall_to_panic __read_mostly;
  22
  23 #ifdef CONFIG_PROVE_RCU
  24 #define RCU_STALL_DELAY_DELTA           (5 * HZ)
  25 #else
  26 #define RCU_STALL_DELAY_DELTA           0
  27 #endif
  28 #define RCU_STALL_MIGHT_DIV             8
  29 #define RCU_STALL_MIGHT_MIN             (2 * HZ)
  30
  31 int rcu_exp_jiffies_till_stall_check(void)
  32 {
  33         int cpu_stall_timeout = READ_ONCE(rcu_exp_cpu_stall_timeout);
  34         int exp_stall_delay_delta = 0;
  35         int till_stall_check;
  36
  37         // Zero says to use rcu_cpu_stall_timeout, but in milliseconds.
  38         if (!cpu_stall_timeout)
  39                 cpu_stall_timeout = jiffies_to_msecs(rcu_jiffies_till_stall_check());
  40
  41         // Limit check must be consistent with the Kconfig limits for
  42         // CONFIG_RCU_EXP_CPU_STALL_TIMEOUT, so check the allowed range.
  43         // The minimum clamped value is "2UL", because at least one full
  44         // tick has to be guaranteed.
  45         till_stall_check = clamp(msecs_to_jiffies(cpu_stall_timeout), 2UL, 300UL * HZ);
  46
  47         if (cpu_stall_timeout && jiffies_to_msecs(till_stall_check) != cpu_stall_timeout)
  48                 WRITE_ONCE(rcu_exp_cpu_stall_timeout, jiffies_to_msecs(till_stall_check));
  49
  50 #ifdef CONFIG_PROVE_RCU
  51         /* Add extra ~25% out of till_stall_check. */
  52         exp_stall_delay_delta = ((till_stall_check * 25) / 100) + 1;
  53 #endif
  54
  55         return till_stall_check + exp_stall_delay_delta;
  56 }
  57 EXPORT_SYMBOL_GPL(rcu_exp_jiffies_till_stall_check);
  58
  59 /* Limit-check stall timeouts specified at boottime and runtime. */
  60 int rcu_jiffies_till_stall_check(void)
  61 {
  62         int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout);
  63
  64         /*
  65          * Limit check must be consistent with the Kconfig limits
  66          * for CONFIG_RCU_CPU_STALL_TIMEOUT.
  67          */
  68         if (till_stall_check < 3) {
  69                 WRITE_ONCE(rcu_cpu_stall_timeout, 3);
  70                 till_stall_check = 3;
  71         } else if (till_stall_check > 300) {
  72                 WRITE_ONCE(rcu_cpu_stall_timeout, 300);
  73                 till_stall_check = 300;
  74         }
  75         return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
  76 }
  77 EXPORT_SYMBOL_GPL(rcu_jiffies_till_stall_check);
  78
  79 /**
  80  * rcu_gp_might_be_stalled - Is it likely that the grace period is stalled?
  81  *
  82  * Returns @true if the current grace period is sufficiently old that
  83  * it is reasonable to assume that it might be stalled.  This can be
  84  * useful when deciding whether to allocate memory to enable RCU-mediated
  85  * freeing on the one hand or just invoking synchronize_rcu() on the other.
  86  * The latter is preferable when the grace period is stalled.
  87  *
  88  * Note that sampling of the .gp_start and .gp_seq fields must be done
  89  * carefully to avoid false positives at the beginnings and ends of
  90  * grace periods.
  91  */
  92 bool rcu_gp_might_be_stalled(void)
  93 {
  94         unsigned long d = rcu_jiffies_till_stall_check() / RCU_STALL_MIGHT_DIV;
  95         unsigned long j = jiffies;
  96
  97         if (d < RCU_STALL_MIGHT_MIN)
  98                 d = RCU_STALL_MIGHT_MIN;
  99         smp_mb(); // jiffies before .gp_seq to avoid false positives.
 100         if (!rcu_gp_in_progress())
 101                 return false;
 102         // Long delays at this point avoids false positive, but a delay
 103         // of ULONG_MAX/4 jiffies voids your no-false-positive warranty.
 104         smp_mb(); // .gp_seq before second .gp_start
 105         // And ditto here.
 106         return !time_before(j, READ_ONCE(rcu_state.gp_start) + d);
 107 }
 108
 109 /* Don't do RCU CPU stall warnings during long sysrq printouts. */
 110 void rcu_sysrq_start(void)
 111 {
 112         if (!rcu_cpu_stall_suppress)
 113                 rcu_cpu_stall_suppress = 2;
 114 }
 115
 116 void rcu_sysrq_end(void)
 117 {
 118         if (rcu_cpu_stall_suppress == 2)
 119                 rcu_cpu_stall_suppress = 0;
 120 }
 121
 122 /* Don't print RCU CPU stall warnings during a kernel panic. */
 123 static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
 124 {
 125         rcu_cpu_stall_suppress = 1;
 126         return NOTIFY_DONE;
 127 }
 128
 129 static struct notifier_block rcu_panic_block = {
 130         .notifier_call = rcu_panic,
 131 };
 132
 133 static int __init check_cpu_stall_init(void)
 134 {
 135         atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block);
 136         return 0;
 137 }
 138 early_initcall(check_cpu_stall_init);
 139
 140 /* If so specified via sysctl, panic, yielding cleaner stall-warning output. */
 141 static void panic_on_rcu_stall(void)
 142 {
 143         static int cpu_stall;
 144
 145         if (++cpu_stall < sysctl_max_rcu_stall_to_panic)
 146                 return;
 147
 148         if (sysctl_panic_on_rcu_stall)
 149                 panic("RCU Stall\n");
 150 }
 151
 152 /**
 153  * rcu_cpu_stall_reset - restart stall-warning timeout for current grace period
 154  *
 155  * To perform the reset request from the caller, disable stall detection until
 156  * 3 fqs loops have passed. This is required to ensure a fresh jiffies is
 157  * loaded.  It should be safe to do from the fqs loop as enough timer
 158  * interrupts and context switches should have passed.
 159  *
 160  * The caller must disable hard irqs.
 161  */
 162 void rcu_cpu_stall_reset(void)
 163 {
 164         WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, 3);
 165         WRITE_ONCE(rcu_state.jiffies_stall, ULONG_MAX);
 166 }
 167
 168 //////////////////////////////////////////////////////////////////////////////
 169 //
 170 // Interaction with RCU grace periods
 171
 172 /* Start of new grace period, so record stall time (and forcing times). */
 173 static void record_gp_stall_check_time(void)
 174 {
 175         unsigned long j = jiffies;
 176         unsigned long j1;
 177
 178         WRITE_ONCE(rcu_state.gp_start, j);
 179         j1 = rcu_jiffies_till_stall_check();
 180         smp_mb(); // ->gp_start before ->jiffies_stall and caller's ->gp_seq.
 181         WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, 0);
 182         WRITE_ONCE(rcu_state.jiffies_stall, j + j1);
 183         rcu_state.jiffies_resched = j + j1 / 2;
 184         rcu_state.n_force_qs_gpstart = READ_ONCE(rcu_state.n_force_qs);
 185 }
 186
 187 /* Zero ->ticks_this_gp and snapshot the number of RCU softirq handlers. */
 188 static void zero_cpu_stall_ticks(struct rcu_data *rdp)
 189 {
 190         rdp->ticks_this_gp = 0;
 191         rdp->softirq_snap = kstat_softirqs_cpu(RCU_SOFTIRQ, smp_processor_id());
 192         WRITE_ONCE(rdp->last_fqs_resched, jiffies);
 193 }
 194
 195 /*
 196  * If too much time has passed in the current grace period, and if
 197  * so configured, go kick the relevant kthreads.
 198  */
 199 static void rcu_stall_kick_kthreads(void)
 200 {
 201         unsigned long j;
 202
 203         if (!READ_ONCE(rcu_kick_kthreads))
 204                 return;
 205         j = READ_ONCE(rcu_state.jiffies_kick_kthreads);
 206         if (time_after(jiffies, j) && rcu_state.gp_kthread &&
 207             (rcu_gp_in_progress() || READ_ONCE(rcu_state.gp_flags))) {
 208                 WARN_ONCE(1, "Kicking %s grace-period kthread\n",
 209                           rcu_state.name);
 210                 rcu_ftrace_dump(DUMP_ALL);
 211                 wake_up_process(rcu_state.gp_kthread);
 212                 WRITE_ONCE(rcu_state.jiffies_kick_kthreads, j + HZ);
 213         }
 214 }
 215
 216 /*
 217  * Handler for the irq_work request posted about halfway into the RCU CPU
 218  * stall timeout, and used to detect excessive irq disabling.  Set state
 219  * appropriately, but just complain if there is unexpected state on entry.
 220  */
 221 static void rcu_iw_handler(struct irq_work *iwp)
 222 {
 223         struct rcu_data *rdp;
 224         struct rcu_node *rnp;
 225
 226         rdp = container_of(iwp, struct rcu_data, rcu_iw);
 227         rnp = rdp->mynode;
 228         raw_spin_lock_rcu_node(rnp);
 229         if (!WARN_ON_ONCE(!rdp->rcu_iw_pending)) {
 230                 rdp->rcu_iw_gp_seq = rnp->gp_seq;
 231                 rdp->rcu_iw_pending = false;
 232         }
 233         raw_spin_unlock_rcu_node(rnp);
 234 }
 235
 236 //////////////////////////////////////////////////////////////////////////////
 237 //
 238 // Printing RCU CPU stall warnings
 239
 240 #ifdef CONFIG_PREEMPT_RCU
 241
 242 /*
 243  * Dump detailed information for all tasks blocking the current RCU
 244  * grace period on the specified rcu_node structure.
 245  */
 246 static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
 247 {
 248         unsigned long flags;
 249         struct task_struct *t;
 250
 251         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 252         if (!rcu_preempt_blocked_readers_cgp(rnp)) {
 253                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 254                 return;
 255         }
 256         t = list_entry(rnp->gp_tasks->prev,
 257                        struct task_struct, rcu_node_entry);
 258         list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
 259                 /*
 260                  * We could be printing a lot while holding a spinlock.
 261                  * Avoid triggering hard lockup.
 262                  */
 263                 touch_nmi_watchdog();
 264                 sched_show_task(t);
 265         }
 266         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 267 }
 268
 269 // Communicate task state back to the RCU CPU stall warning request.
 270 struct rcu_stall_chk_rdr {
 271         int nesting;
 272         union rcu_special rs;
 273         bool on_blkd_list;
 274 };
 275
 276 /*
 277  * Report out the state of a not-running task that is stalling the
 278  * current RCU grace period.
 279  */
 280 static int check_slow_task(struct task_struct *t, void *arg)
 281 {
 282         struct rcu_stall_chk_rdr *rscrp = arg;
 283
 284         if (task_curr(t))
 285                 return -EBUSY; // It is running, so decline to inspect it.
 286         rscrp->nesting = t->rcu_read_lock_nesting;
 287         rscrp->rs = t->rcu_read_unlock_special;
 288         rscrp->on_blkd_list = !list_empty(&t->rcu_node_entry);
 289         return 0;
 290 }
 291
 292 /*
 293  * Scan the current list of tasks blocked within RCU read-side critical
 294  * sections, printing out the tid of each of the first few of them.
 295  */
 296 static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
 297         __releases(rnp->lock)
 298 {
 299         int i = 0;
 300         int ndetected = 0;
 301         struct rcu_stall_chk_rdr rscr;
 302         struct task_struct *t;
 303         struct task_struct *ts[8];
 304
 305         lockdep_assert_irqs_disabled();
 306         if (!rcu_preempt_blocked_readers_cgp(rnp)) {
 307                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 308                 return 0;
 309         }
 310         pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
 311                rnp->level, rnp->grplo, rnp->grphi);
 312         t = list_entry(rnp->gp_tasks->prev,
 313                        struct task_struct, rcu_node_entry);
 314         list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
 315                 get_task_struct(t);
 316                 ts[i++] = t;
 317                 if (i >= ARRAY_SIZE(ts))
 318                         break;
 319         }
 320         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 321         while (i) {
 322                 t = ts[--i];
 323                 if (task_call_func(t, check_slow_task, &rscr))
 324                         pr_cont(" P%d", t->pid);
 325                 else
 326                         pr_cont(" P%d/%d:%c%c%c%c",
 327                                 t->pid, rscr.nesting,
 328                                 ".b"[rscr.rs.b.blocked],
 329                                 ".q"[rscr.rs.b.need_qs],
 330                                 ".e"[rscr.rs.b.exp_hint],
 331                                 ".l"[rscr.on_blkd_list]);
 332                 lockdep_assert_irqs_disabled();
 333                 put_task_struct(t);
 334                 ndetected++;
 335         }
 336         pr_cont("\n");
 337         return ndetected;
 338 }
 339
 340 #else /* #ifdef CONFIG_PREEMPT_RCU */
 341
 342 /*
 343  * Because preemptible RCU does not exist, we never have to check for
 344  * tasks blocked within RCU read-side critical sections.
 345  */
 346 static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
 347 {
 348 }
 349
 350 /*
 351  * Because preemptible RCU does not exist, we never have to check for
 352  * tasks blocked within RCU read-side critical sections.
 353  */
 354 static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
 355         __releases(rnp->lock)
 356 {
 357         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 358         return 0;
 359 }
 360 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 361
 362 /*
 363  * Dump stacks of all tasks running on stalled CPUs.  First try using
 364  * NMIs, but fall back to manual remote stack tracing on architectures
 365  * that don't support NMI-based stack dumps.  The NMI-triggered stack
 366  * traces are more accurate because they are printed by the target CPU.
 367  */
 368 static void rcu_dump_cpu_stacks(void)
 369 {
 370         int cpu;
 371         unsigned long flags;
 372         struct rcu_node *rnp;
 373
 374         rcu_for_each_leaf_node(rnp) {
 375                 printk_deferred_enter();
 376                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
 377                 for_each_leaf_node_possible_cpu(rnp, cpu)
 378                         if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
 379                                 if (cpu_is_offline(cpu))
 380                                         pr_err("Offline CPU %d blocking current GP.\n", cpu);
 381                                 else
 382                                         dump_cpu_task(cpu);
 383                         }
 384                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 385                 printk_deferred_exit();
 386         }
 387 }
 388
 389 static const char * const gp_state_names[] = {
 390         [RCU_GP_IDLE] = "RCU_GP_IDLE",
 391         [RCU_GP_WAIT_GPS] = "RCU_GP_WAIT_GPS",
 392         [RCU_GP_DONE_GPS] = "RCU_GP_DONE_GPS",
 393         [RCU_GP_ONOFF] = "RCU_GP_ONOFF",
 394         [RCU_GP_INIT] = "RCU_GP_INIT",
 395         [RCU_GP_WAIT_FQS] = "RCU_GP_WAIT_FQS",
 396         [RCU_GP_DOING_FQS] = "RCU_GP_DOING_FQS",
 397         [RCU_GP_CLEANUP] = "RCU_GP_CLEANUP",
 398         [RCU_GP_CLEANED] = "RCU_GP_CLEANED",
 399 };
 400
 401 /*
 402  * Convert a ->gp_state value to a character string.
 403  */
 404 static const char *gp_state_getname(short gs)
 405 {
 406         if (gs < 0 || gs >= ARRAY_SIZE(gp_state_names))
 407                 return "???";
 408         return gp_state_names[gs];
 409 }
 410
 411 /* Is the RCU grace-period kthread being starved of CPU time? */
 412 static bool rcu_is_gp_kthread_starving(unsigned long *jp)
 413 {
 414         unsigned long j = jiffies - READ_ONCE(rcu_state.gp_activity);
 415
 416         if (jp)
 417                 *jp = j;
 418         return j > 2 * HZ;
 419 }
 420
 421 static bool rcu_is_rcuc_kthread_starving(struct rcu_data *rdp, unsigned long *jp)
 422 {
 423         int cpu;
 424         struct task_struct *rcuc;
 425         unsigned long j;
 426
 427         rcuc = rdp->rcu_cpu_kthread_task;
 428         if (!rcuc)
 429                 return false;
 430
 431         cpu = task_cpu(rcuc);
 432         if (cpu_is_offline(cpu) || idle_cpu(cpu))
 433                 return false;
 434
 435         j = jiffies - READ_ONCE(rdp->rcuc_activity);
 436
 437         if (jp)
 438                 *jp = j;
 439         return j > 2 * HZ;
 440 }
 441
 442 static void print_cpu_stat_info(int cpu)
 443 {
 444         struct rcu_snap_record rsr, *rsrp;
 445         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 446         struct kernel_cpustat *kcsp = &kcpustat_cpu(cpu);
 447
 448         if (!rcu_cpu_stall_cputime)
 449                 return;
 450
 451         rsrp = &rdp->snap_record;
 452         if (rsrp->gp_seq != rdp->gp_seq)
 453                 return;
 454
 455         rsr.cputime_irq     = kcpustat_field(kcsp, CPUTIME_IRQ, cpu);
 456         rsr.cputime_softirq = kcpustat_field(kcsp, CPUTIME_SOFTIRQ, cpu);
 457         rsr.cputime_system  = kcpustat_field(kcsp, CPUTIME_SYSTEM, cpu);
 458
 459         pr_err("\t         hardirqs   softirqs   csw/system\n");
 460         pr_err("\t number: %8ld %10d %12lld\n",
 461                 kstat_cpu_irqs_sum(cpu) - rsrp->nr_hardirqs,
 462                 kstat_cpu_softirqs_sum(cpu) - rsrp->nr_softirqs,
 463                 nr_context_switches_cpu(cpu) - rsrp->nr_csw);
 464         pr_err("\tcputime: %8lld %10lld %12lld   ==> %d(ms)\n",
 465                 div_u64(rsr.cputime_irq - rsrp->cputime_irq, NSEC_PER_MSEC),
 466                 div_u64(rsr.cputime_softirq - rsrp->cputime_softirq, NSEC_PER_MSEC),
 467                 div_u64(rsr.cputime_system - rsrp->cputime_system, NSEC_PER_MSEC),
 468                 jiffies_to_msecs(jiffies - rsrp->jiffies));
 469 }
 470
 471 /*
 472  * Print out diagnostic information for the specified stalled CPU.
 473  *
 474  * If the specified CPU is aware of the current RCU grace period, then
 475  * print the number of scheduling clock interrupts the CPU has taken
 476  * during the time that it has been aware.  Otherwise, print the number
 477  * of RCU grace periods that this CPU is ignorant of, for example, "1"
 478  * if the CPU was aware of the previous grace period.
 479  *
 480  * Also print out idle info.
 481  */
 482 static void print_cpu_stall_info(int cpu)
 483 {
 484         unsigned long delta;
 485         bool falsepositive;
 486         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 487         char *ticks_title;
 488         unsigned long ticks_value;
 489         bool rcuc_starved;
 490         unsigned long j;
 491         char buf[32];
 492
 493         /*
 494          * We could be printing a lot while holding a spinlock.  Avoid
 495          * triggering hard lockup.
 496          */
 497         touch_nmi_watchdog();
 498
 499         ticks_value = rcu_seq_ctr(rcu_state.gp_seq - rdp->gp_seq);
 500         if (ticks_value) {
 501                 ticks_title = "GPs behind";
 502         } else {
 503                 ticks_title = "ticks this GP";
 504                 ticks_value = rdp->ticks_this_gp;
 505         }
 506         delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
 507         falsepositive = rcu_is_gp_kthread_starving(NULL) &&
 508                         rcu_watching_snap_in_eqs(ct_rcu_watching_cpu(cpu));
 509         rcuc_starved = rcu_is_rcuc_kthread_starving(rdp, &j);
 510         if (rcuc_starved)
 511                 // Print signed value, as negative values indicate a probable bug.
 512                 snprintf(buf, sizeof(buf), " rcuc=%ld jiffies(starved)", j);
 513         pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%04x/%ld/%#lx softirq=%u/%u fqs=%ld%s%s\n",
 514                cpu,
 515                "O."[!!cpu_online(cpu)],
 516                "o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)],
 517                "N."[!!(rdp->grpmask & rdp->mynode->qsmaskinitnext)],
 518                !IS_ENABLED(CONFIG_IRQ_WORK) ? '?' :
 519                         rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' :
 520                                 "!."[!delta],
 521                ticks_value, ticks_title,
 522                ct_rcu_watching_cpu(cpu) & 0xffff,
 523                ct_nesting_cpu(cpu), ct_nmi_nesting_cpu(cpu),
 524                rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
 525                data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
 526                rcuc_starved ? buf : "",
 527                falsepositive ? " (false positive?)" : "");
 528
 529         print_cpu_stat_info(cpu);
 530 }
 531
 532 /* Complain about starvation of grace-period kthread.  */
 533 static void rcu_check_gp_kthread_starvation(void)
 534 {
 535         int cpu;
 536         struct task_struct *gpk = rcu_state.gp_kthread;
 537         unsigned long j;
 538
 539         if (rcu_is_gp_kthread_starving(&j)) {
 540                 cpu = gpk ? task_cpu(gpk) : -1;
 541                 pr_err("%s kthread starved for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x ->cpu=%d\n",
 542                        rcu_state.name, j,
 543                        (long)rcu_seq_current(&rcu_state.gp_seq),
 544                        data_race(READ_ONCE(rcu_state.gp_flags)),
 545                        gp_state_getname(rcu_state.gp_state),
 546                        data_race(READ_ONCE(rcu_state.gp_state)),
 547                        gpk ? data_race(READ_ONCE(gpk->__state)) : ~0, cpu);
 548                 if (gpk) {
 549                         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 550
 551                         pr_err("\tUnless %s kthread gets sufficient CPU time, OOM is now expected behavior.\n", rcu_state.name);
 552                         pr_err("RCU grace-period kthread stack dump:\n");
 553                         sched_show_task(gpk);
 554                         if (cpu_is_offline(cpu)) {
 555                                 pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu);
 556                         } else if (!(data_race(READ_ONCE(rdp->mynode->qsmask)) & rdp->grpmask)) {
 557                                 pr_err("Stack dump where RCU GP kthread last ran:\n");
 558                                 dump_cpu_task(cpu);
 559                         }
 560                         wake_up_process(gpk);
 561                 }
 562         }
 563 }
 564
 565 /* Complain about missing wakeups from expired fqs wait timer */
 566 static void rcu_check_gp_kthread_expired_fqs_timer(void)
 567 {
 568         struct task_struct *gpk = rcu_state.gp_kthread;
 569         short gp_state;
 570         unsigned long jiffies_fqs;
 571         int cpu;
 572
 573         /*
 574          * Order reads of .gp_state and .jiffies_force_qs.
 575          * Matching smp_wmb() is present in rcu_gp_fqs_loop().
 576          */
 577         gp_state = smp_load_acquire(&rcu_state.gp_state);
 578         jiffies_fqs = READ_ONCE(rcu_state.jiffies_force_qs);
 579
 580         if (gp_state == RCU_GP_WAIT_FQS &&
 581             time_after(jiffies, jiffies_fqs + RCU_STALL_MIGHT_MIN) &&
 582             gpk && !READ_ONCE(gpk->on_rq)) {
 583                 cpu = task_cpu(gpk);
 584                 pr_err("%s kthread timer wakeup didn't happen for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x\n",
 585                        rcu_state.name, (jiffies - jiffies_fqs),
 586                        (long)rcu_seq_current(&rcu_state.gp_seq),
 587                        data_race(READ_ONCE(rcu_state.gp_flags)), // Diagnostic read
 588                        gp_state_getname(RCU_GP_WAIT_FQS), RCU_GP_WAIT_FQS,
 589                        data_race(READ_ONCE(gpk->__state)));
 590                 pr_err("\tPossible timer handling issue on cpu=%d timer-softirq=%u\n",
 591                        cpu, kstat_softirqs_cpu(TIMER_SOFTIRQ, cpu));
 592         }
 593 }
 594
 595 static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
 596 {
 597         int cpu;
 598         unsigned long flags;
 599         unsigned long gpa;
 600         unsigned long j;
 601         int ndetected = 0;
 602         struct rcu_node *rnp;
 603         long totqlen = 0;
 604
 605         lockdep_assert_irqs_disabled();
 606
 607         /* Kick and suppress, if so configured. */
 608         rcu_stall_kick_kthreads();
 609         if (rcu_stall_is_suppressed())
 610                 return;
 611
 612         nbcon_cpu_emergency_enter();
 613
 614         /*
 615          * OK, time to rat on our buddy...
 616          * See Documentation/RCU/stallwarn.rst for info on how to debug
 617          * RCU CPU stall warnings.
 618          */
 619         trace_rcu_stall_warning(rcu_state.name, TPS("StallDetected"));
 620         pr_err("INFO: %s detected stalls on CPUs/tasks:\n", rcu_state.name);
 621         rcu_for_each_leaf_node(rnp) {
 622                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
 623                 if (rnp->qsmask != 0) {
 624                         for_each_leaf_node_possible_cpu(rnp, cpu)
 625                                 if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
 626                                         print_cpu_stall_info(cpu);
 627                                         ndetected++;
 628                                 }
 629                 }
 630                 ndetected += rcu_print_task_stall(rnp, flags); // Releases rnp->lock.
 631                 lockdep_assert_irqs_disabled();
 632         }
 633
 634         for_each_possible_cpu(cpu)
 635                 totqlen += rcu_get_n_cbs_cpu(cpu);
 636         pr_err("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu ncpus=%d)\n",
 637                smp_processor_id(), (long)(jiffies - gps),
 638                (long)rcu_seq_current(&rcu_state.gp_seq), totqlen,
 639                data_race(rcu_state.n_online_cpus)); // Diagnostic read
 640         if (ndetected) {
 641                 rcu_dump_cpu_stacks();
 642
 643                 /* Complain about tasks blocking the grace period. */
 644                 rcu_for_each_leaf_node(rnp)
 645                         rcu_print_detail_task_stall_rnp(rnp);
 646         } else {
 647                 if (rcu_seq_current(&rcu_state.gp_seq) != gp_seq) {
 648                         pr_err("INFO: Stall ended before state dump start\n");
 649                 } else {
 650                         j = jiffies;
 651                         gpa = data_race(READ_ONCE(rcu_state.gp_activity));
 652                         pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld, root ->qsmask %#lx\n",
 653                                rcu_state.name, j - gpa, j, gpa,
 654                                data_race(READ_ONCE(jiffies_till_next_fqs)),
 655                                data_race(READ_ONCE(rcu_get_root()->qsmask)));
 656                 }
 657         }
 658         /* Rewrite if needed in case of slow consoles. */
 659         if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
 660                 WRITE_ONCE(rcu_state.jiffies_stall,
 661                            jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
 662
 663         rcu_check_gp_kthread_expired_fqs_timer();
 664         rcu_check_gp_kthread_starvation();
 665
 666         nbcon_cpu_emergency_exit();
 667
 668         panic_on_rcu_stall();
 669
 670         rcu_force_quiescent_state();  /* Kick them all. */
 671 }
 672
 673 static void print_cpu_stall(unsigned long gps)
 674 {
 675         int cpu;
 676         unsigned long flags;
 677         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 678         struct rcu_node *rnp = rcu_get_root();
 679         long totqlen = 0;
 680
 681         lockdep_assert_irqs_disabled();
 682
 683         /* Kick and suppress, if so configured. */
 684         rcu_stall_kick_kthreads();
 685         if (rcu_stall_is_suppressed())
 686                 return;
 687
 688         nbcon_cpu_emergency_enter();
 689
 690         /*
 691          * OK, time to rat on ourselves...
 692          * See Documentation/RCU/stallwarn.rst for info on how to debug
 693          * RCU CPU stall warnings.
 694          */
 695         trace_rcu_stall_warning(rcu_state.name, TPS("SelfDetected"));
 696         pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name);
 697         raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags);
 698         print_cpu_stall_info(smp_processor_id());
 699         raw_spin_unlock_irqrestore_rcu_node(rdp->mynode, flags);
 700         for_each_possible_cpu(cpu)
 701                 totqlen += rcu_get_n_cbs_cpu(cpu);
 702         pr_err("\t(t=%lu jiffies g=%ld q=%lu ncpus=%d)\n",
 703                 jiffies - gps,
 704                 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen,
 705                 data_race(rcu_state.n_online_cpus)); // Diagnostic read
 706
 707         rcu_check_gp_kthread_expired_fqs_timer();
 708         rcu_check_gp_kthread_starvation();
 709
 710         rcu_dump_cpu_stacks();
 711
 712         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 713         /* Rewrite if needed in case of slow consoles. */
 714         if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
 715                 WRITE_ONCE(rcu_state.jiffies_stall,
 716                            jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
 717         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 718
 719         nbcon_cpu_emergency_exit();
 720
 721         panic_on_rcu_stall();
 722
 723         /*
 724          * Attempt to revive the RCU machinery by forcing a context switch.
 725          *
 726          * A context switch would normally allow the RCU state machine to make
 727          * progress and it could be we're stuck in kernel space without context
 728          * switches for an entirely unreasonable amount of time.
 729          */
 730         set_tsk_need_resched(current);
 731         set_preempt_need_resched();
 732 }
 733
 734 static bool csd_lock_suppress_rcu_stall;
 735 module_param(csd_lock_suppress_rcu_stall, bool, 0644);
 736
 737 static void check_cpu_stall(struct rcu_data *rdp)
 738 {
 739         bool self_detected;
 740         unsigned long gs1;
 741         unsigned long gs2;
 742         unsigned long gps;
 743         unsigned long j;
 744         unsigned long jn;
 745         unsigned long js;
 746         struct rcu_node *rnp;
 747
 748         lockdep_assert_irqs_disabled();
 749         if ((rcu_stall_is_suppressed() && !READ_ONCE(rcu_kick_kthreads)) ||
 750             !rcu_gp_in_progress())
 751                 return;
 752         rcu_stall_kick_kthreads();
 753
 754         /*
 755          * Check if it was requested (via rcu_cpu_stall_reset()) that the FQS
 756          * loop has to set jiffies to ensure a non-stale jiffies value. This
 757          * is required to have good jiffies value after coming out of long
 758          * breaks of jiffies updates. Not doing so can cause false positives.
 759          */
 760         if (READ_ONCE(rcu_state.nr_fqs_jiffies_stall) > 0)
 761                 return;
 762
 763         j = jiffies;
 764
 765         /*
 766          * Lots of memory barriers to reject false positives.
 767          *
 768          * The idea is to pick up rcu_state.gp_seq, then
 769          * rcu_state.jiffies_stall, then rcu_state.gp_start, and finally
 770          * another copy of rcu_state.gp_seq.  These values are updated in
 771          * the opposite order with memory barriers (or equivalent) during
 772          * grace-period initialization and cleanup.  Now, a false positive
 773          * can occur if we get an new value of rcu_state.gp_start and a old
 774          * value of rcu_state.jiffies_stall.  But given the memory barriers,
 775          * the only way that this can happen is if one grace period ends
 776          * and another starts between these two fetches.  This is detected
 777          * by comparing the second fetch of rcu_state.gp_seq with the
 778          * previous fetch from rcu_state.gp_seq.
 779          *
 780          * Given this check, comparisons of jiffies, rcu_state.jiffies_stall,
 781          * and rcu_state.gp_start suffice to forestall false positives.
 782          */
 783         gs1 = READ_ONCE(rcu_state.gp_seq);
 784         smp_rmb(); /* Pick up ->gp_seq first... */
 785         js = READ_ONCE(rcu_state.jiffies_stall);
 786         smp_rmb(); /* ...then ->jiffies_stall before the rest... */
 787         gps = READ_ONCE(rcu_state.gp_start);
 788         smp_rmb(); /* ...and finally ->gp_start before ->gp_seq again. */
 789         gs2 = READ_ONCE(rcu_state.gp_seq);
 790         if (gs1 != gs2 ||
 791             ULONG_CMP_LT(j, js) ||
 792             ULONG_CMP_GE(gps, js))
 793                 return; /* No stall or GP completed since entering function. */
 794         rnp = rdp->mynode;
 795         jn = jiffies + ULONG_MAX / 2;
 796         self_detected = READ_ONCE(rnp->qsmask) & rdp->grpmask;
 797         if (rcu_gp_in_progress() &&
 798             (self_detected || ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY)) &&
 799             cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
 800                 /*
 801                  * If a virtual machine is stopped by the host it can look to
 802                  * the watchdog like an RCU stall. Check to see if the host
 803                  * stopped the vm.
 804                  */
 805                 if (kvm_check_and_clear_guest_paused())
 806                         return;
 807
 808                 rcu_stall_notifier_call_chain(RCU_STALL_NOTIFY_NORM, (void *)j - gps);
 809                 if (READ_ONCE(csd_lock_suppress_rcu_stall) && csd_lock_is_stuck()) {
 810                         pr_err("INFO: %s detected stall, but suppressed full report due to a stuck CSD-lock.\n", rcu_state.name);
 811                 } else if (self_detected) {
 812                         /* We haven't checked in, so go dump stack. */
 813                         print_cpu_stall(gps);
 814                 } else {
 815                         /* They had a few time units to dump stack, so complain. */
 816                         print_other_cpu_stall(gs2, gps);
 817                 }
 818
 819                 if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
 820                         rcu_ftrace_dump(DUMP_ALL);
 821
 822                 if (READ_ONCE(rcu_state.jiffies_stall) == jn) {
 823                         jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
 824                         WRITE_ONCE(rcu_state.jiffies_stall, jn);
 825                 }
 826         }
 827 }
 828
 829 //////////////////////////////////////////////////////////////////////////////
 830 //
 831 // RCU forward-progress mechanisms, including for callback invocation.
 832
 833
 834 /*
 835  * Check to see if a failure to end RCU priority inversion was due to
 836  * a CPU not passing through a quiescent state.  When this happens, there
 837  * is nothing that RCU priority boosting can do to help, so we shouldn't
 838  * count this as an RCU priority boosting failure.  A return of true says
 839  * RCU priority boosting is to blame, and false says otherwise.  If false
 840  * is returned, the first of the CPUs to blame is stored through cpup.
 841  * If there was no CPU blocking the current grace period, but also nothing
 842  * in need of being boosted, *cpup is set to -1.  This can happen in case
 843  * of vCPU preemption while the last CPU is reporting its quiscent state,
 844  * for example.
 845  *
 846  * If cpup is NULL, then a lockless quick check is carried out, suitable
 847  * for high-rate usage.  On the other hand, if cpup is non-NULL, each
 848  * rcu_node structure's ->lock is acquired, ruling out high-rate usage.
 849  */
 850 bool rcu_check_boost_fail(unsigned long gp_state, int *cpup)
 851 {
 852         bool atb = false;
 853         int cpu;
 854         unsigned long flags;
 855         struct rcu_node *rnp;
 856
 857         rcu_for_each_leaf_node(rnp) {
 858                 if (!cpup) {
 859                         if (data_race(READ_ONCE(rnp->qsmask))) {
 860                                 return false;
 861                         } else {
 862                                 if (READ_ONCE(rnp->gp_tasks))
 863                                         atb = true;
 864                                 continue;
 865                         }
 866                 }
 867                 *cpup = -1;
 868                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
 869                 if (rnp->gp_tasks)
 870                         atb = true;
 871                 if (!rnp->qsmask) {
 872                         // No CPUs without quiescent states for this rnp.
 873                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 874                         continue;
 875                 }
 876                 // Find the first holdout CPU.
 877                 for_each_leaf_node_possible_cpu(rnp, cpu) {
 878                         if (rnp->qsmask & (1UL << (cpu - rnp->grplo))) {
 879                                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 880                                 *cpup = cpu;
 881                                 return false;
 882                         }
 883                 }
 884                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 885         }
 886         // Can't blame CPUs, so must blame RCU priority boosting.
 887         return atb;
 888 }
 889 EXPORT_SYMBOL_GPL(rcu_check_boost_fail);
 890
 891 /*
 892  * Show the state of the grace-period kthreads.
 893  */
 894 void show_rcu_gp_kthreads(void)
 895 {
 896         unsigned long cbs = 0;
 897         int cpu;
 898         unsigned long j;
 899         unsigned long ja;
 900         unsigned long jr;
 901         unsigned long js;
 902         unsigned long jw;
 903         struct rcu_data *rdp;
 904         struct rcu_node *rnp;
 905         struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);
 906
 907         j = jiffies;
 908         ja = j - data_race(READ_ONCE(rcu_state.gp_activity));
 909         jr = j - data_race(READ_ONCE(rcu_state.gp_req_activity));
 910         js = j - data_race(READ_ONCE(rcu_state.gp_start));
 911         jw = j - data_race(READ_ONCE(rcu_state.gp_wake_time));
 912         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",
 913                 rcu_state.name, gp_state_getname(rcu_state.gp_state),
 914                 data_race(READ_ONCE(rcu_state.gp_state)),
 915                 t ? data_race(READ_ONCE(t->__state)) : 0x1ffff, t ? t->rt_priority : 0xffU,
 916                 js, ja, jr, jw, (long)data_race(READ_ONCE(rcu_state.gp_wake_seq)),
 917                 (long)data_race(READ_ONCE(rcu_state.gp_seq)),
 918                 (long)data_race(READ_ONCE(rcu_get_root()->gp_seq_needed)),
 919                 data_race(READ_ONCE(rcu_state.gp_max)),
 920                 data_race(READ_ONCE(rcu_state.gp_flags)));
 921         rcu_for_each_node_breadth_first(rnp) {
 922                 if (ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq), READ_ONCE(rnp->gp_seq_needed)) &&
 923                     !data_race(READ_ONCE(rnp->qsmask)) && !data_race(READ_ONCE(rnp->boost_tasks)) &&
 924                     !data_race(READ_ONCE(rnp->exp_tasks)) && !data_race(READ_ONCE(rnp->gp_tasks)))
 925                         continue;
 926                 pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld ->qsmask %#lx %c%c%c%c ->n_boosts %ld\n",
 927                         rnp->grplo, rnp->grphi,
 928                         (long)data_race(READ_ONCE(rnp->gp_seq)),
 929                         (long)data_race(READ_ONCE(rnp->gp_seq_needed)),
 930                         data_race(READ_ONCE(rnp->qsmask)),
 931                         ".b"[!!data_race(READ_ONCE(rnp->boost_kthread_task))],
 932                         ".B"[!!data_race(READ_ONCE(rnp->boost_tasks))],
 933                         ".E"[!!data_race(READ_ONCE(rnp->exp_tasks))],
 934                         ".G"[!!data_race(READ_ONCE(rnp->gp_tasks))],
 935                         data_race(READ_ONCE(rnp->n_boosts)));
 936                 if (!rcu_is_leaf_node(rnp))
 937                         continue;
 938                 for_each_leaf_node_possible_cpu(rnp, cpu) {
 939                         rdp = per_cpu_ptr(&rcu_data, cpu);
 940                         if (READ_ONCE(rdp->gpwrap) ||
 941                             ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq),
 942                                          READ_ONCE(rdp->gp_seq_needed)))
 943                                 continue;
 944                         pr_info("\tcpu %d ->gp_seq_needed %ld\n",
 945                                 cpu, (long)data_race(READ_ONCE(rdp->gp_seq_needed)));
 946                 }
 947         }
 948         for_each_possible_cpu(cpu) {
 949                 rdp = per_cpu_ptr(&rcu_data, cpu);
 950                 cbs += data_race(READ_ONCE(rdp->n_cbs_invoked));
 951                 if (rcu_segcblist_is_offloaded(&rdp->cblist))
 952                         show_rcu_nocb_state(rdp);
 953         }
 954         pr_info("RCU callbacks invoked since boot: %lu\n", cbs);
 955         show_rcu_tasks_gp_kthreads();
 956 }
 957 EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);
 958
 959 /*
 960  * This function checks for grace-period requests that fail to motivate
 961  * RCU to come out of its idle mode.
 962  */
 963 static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
 964                                      const unsigned long gpssdelay)
 965 {
 966         unsigned long flags;
 967         unsigned long j;
 968         struct rcu_node *rnp_root = rcu_get_root();
 969         static atomic_t warned = ATOMIC_INIT(0);
 970
 971         if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() ||
 972             ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
 973                          READ_ONCE(rnp_root->gp_seq_needed)) ||
 974             !smp_load_acquire(&rcu_state.gp_kthread)) // Get stable kthread.
 975                 return;
 976         j = jiffies; /* Expensive access, and in common case don't get here. */
 977         if (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                 return;
 981
 982         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 983         j = jiffies;
 984         if (rcu_gp_in_progress() ||
 985             ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
 986                          READ_ONCE(rnp_root->gp_seq_needed)) ||
 987             time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
 988             time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
 989             atomic_read(&warned)) {
 990                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 991                 return;
 992         }
 993         /* Hold onto the leaf lock to make others see warned==1. */
 994
 995         if (rnp_root != rnp)
 996                 raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
 997         j = jiffies;
 998         if (rcu_gp_in_progress() ||
 999             ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
1000                          READ_ONCE(rnp_root->gp_seq_needed)) ||
1001             time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
1002             time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
1003             atomic_xchg(&warned, 1)) {
1004                 if (rnp_root != rnp)
1005                         /* irqs remain disabled. */
1006                         raw_spin_unlock_rcu_node(rnp_root);
1007                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1008                 return;
1009         }
1010         WARN_ON(1);
1011         if (rnp_root != rnp)
1012                 raw_spin_unlock_rcu_node(rnp_root);
1013         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1014         show_rcu_gp_kthreads();
1015 }
1016
1017 /*
1018  * Do a forward-progress check for rcutorture.  This is normally invoked
1019  * due to an OOM event.  The argument "j" gives the time period during
1020  * which rcutorture would like progress to have been made.
1021  */
1022 void rcu_fwd_progress_check(unsigned long j)
1023 {
1024         unsigned long cbs;
1025         int cpu;
1026         unsigned long max_cbs = 0;
1027         int max_cpu = -1;
1028         struct rcu_data *rdp;
1029
1030         if (rcu_gp_in_progress()) {
1031                 pr_info("%s: GP age %lu jiffies\n",
1032                         __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_start)));
1033                 show_rcu_gp_kthreads();
1034         } else {
1035                 pr_info("%s: Last GP end %lu jiffies ago\n",
1036                         __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_end)));
1037                 preempt_disable();
1038                 rdp = this_cpu_ptr(&rcu_data);
1039                 rcu_check_gp_start_stall(rdp->mynode, rdp, j);
1040                 preempt_enable();
1041         }
1042         for_each_possible_cpu(cpu) {
1043                 cbs = rcu_get_n_cbs_cpu(cpu);
1044                 if (!cbs)
1045                         continue;
1046                 if (max_cpu < 0)
1047                         pr_info("%s: callbacks", __func__);
1048                 pr_cont(" %d: %lu", cpu, cbs);
1049                 if (cbs <= max_cbs)
1050                         continue;
1051                 max_cbs = cbs;
1052                 max_cpu = cpu;
1053         }
1054         if (max_cpu >= 0)
1055                 pr_cont("\n");
1056 }
1057 EXPORT_SYMBOL_GPL(rcu_fwd_progress_check);
1058
1059 /* Commandeer a sysrq key to dump RCU's tree. */
1060 static bool sysrq_rcu;
1061 module_param(sysrq_rcu, bool, 0444);
1062
1063 /* Dump grace-period-request information due to commandeered sysrq. */
1064 static void sysrq_show_rcu(u8 key)
1065 {
1066         show_rcu_gp_kthreads();
1067 }
1068
1069 static const struct sysrq_key_op sysrq_rcudump_op = {
1070         .handler = sysrq_show_rcu,
1071         .help_msg = "show-rcu(y)",
1072         .action_msg = "Show RCU tree",
1073         .enable_mask = SYSRQ_ENABLE_DUMP,
1074 };
1075
1076 static int __init rcu_sysrq_init(void)
1077 {
1078         if (sysrq_rcu)
1079                 return register_sysrq_key('y', &sysrq_rcudump_op);
1080         return 0;
1081 }
1082 early_initcall(rcu_sysrq_init);
1083
1084 #ifdef CONFIG_RCU_CPU_STALL_NOTIFIER
1085
1086 //////////////////////////////////////////////////////////////////////////////
1087 //
1088 // RCU CPU stall-warning notifiers
1089
1090 static ATOMIC_NOTIFIER_HEAD(rcu_cpu_stall_notifier_list);
1091
1092 /**
1093  * rcu_stall_chain_notifier_register - Add an RCU CPU stall notifier
1094  * @n: Entry to add.
1095  *
1096  * Adds an RCU CPU stall notifier to an atomic notifier chain.
1097  * The @action passed to a notifier will be @RCU_STALL_NOTIFY_NORM or
1098  * friends.  The @data will be the duration of the stalled grace period,
1099  * in jiffies, coerced to a void* pointer.
1100  *
1101  * Returns 0 on success, %-EEXIST on error.
1102  */
1103 int rcu_stall_chain_notifier_register(struct notifier_block *n)
1104 {
1105         int rcsn = rcu_cpu_stall_notifiers;
1106
1107         WARN(1, "Adding %pS() to RCU stall notifier list (%s).\n", n->notifier_call,
1108              rcsn ? "possibly suppressing RCU CPU stall warnings" : "failed, so all is well");
1109         if (rcsn)
1110                 return atomic_notifier_chain_register(&rcu_cpu_stall_notifier_list, n);
1111         return -EEXIST;
1112 }
1113 EXPORT_SYMBOL_GPL(rcu_stall_chain_notifier_register);
1114
1115 /**
1116  * rcu_stall_chain_notifier_unregister - Remove an RCU CPU stall notifier
1117  * @n: Entry to add.
1118  *
1119  * Removes an RCU CPU stall notifier from an atomic notifier chain.
1120  *
1121  * Returns zero on success, %-ENOENT on failure.
1122  */
1123 int rcu_stall_chain_notifier_unregister(struct notifier_block *n)
1124 {
1125         return atomic_notifier_chain_unregister(&rcu_cpu_stall_notifier_list, n);
1126 }
1127 EXPORT_SYMBOL_GPL(rcu_stall_chain_notifier_unregister);
1128
1129 /*
1130  * rcu_stall_notifier_call_chain - Call functions in an RCU CPU stall notifier chain
1131  * @val: Value passed unmodified to notifier function
1132  * @v: Pointer passed unmodified to notifier function
1133  *
1134  * Calls each function in the RCU CPU stall notifier chain in turn, which
1135  * is an atomic call chain.  See atomic_notifier_call_chain() for more
1136  * information.
1137  *
1138  * This is for use within RCU, hence the omission of the extra asterisk
1139  * to indicate a non-kerneldoc format header comment.
1140  */
1141 int rcu_stall_notifier_call_chain(unsigned long val, void *v)
1142 {
1143         return atomic_notifier_call_chain(&rcu_cpu_stall_notifier_list, val, v);
1144 }
1145
1146 #endif // #ifdef CONFIG_RCU_CPU_STALL_NOTIFIER