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