kernel/rcu/tree_stall.h

   1 // SPDX-License-Identifier: GPL-2.0+
   2 /*
   3  * RCU CPU stall warnings for normal RCU grace periods
   4  *
   5  * Copyright IBM Corporation, 2019
   6  *
   7  * Author: Paul E. McKenney <paulmck@linux.ibm.com>
   8  */
   9
  10 #include <linux/kvm_para.h>
  11
  12 //////////////////////////////////////////////////////////////////////////////
  13 //
  14 // Controlling CPU stall warnings, including delay calculation.
  15
  16 /* panic() on RCU Stall sysctl. */
  17 int sysctl_panic_on_rcu_stall __read_mostly;
  18 int sysctl_max_rcu_stall_to_panic __read_mostly;
  19
  20 #ifdef CONFIG_PROVE_RCU
  21 #define RCU_STALL_DELAY_DELTA           (5 * HZ)
  22 #else
  23 #define RCU_STALL_DELAY_DELTA           0
  24 #endif
  25 #define RCU_STALL_MIGHT_DIV             8
  26 #define RCU_STALL_MIGHT_MIN             (2 * HZ)
  27
  28 int rcu_exp_jiffies_till_stall_check(void)
  29 {
  30         int cpu_stall_timeout = READ_ONCE(rcu_exp_cpu_stall_timeout);
  31         int exp_stall_delay_delta = 0;
  32         int till_stall_check;
  33
  34         // Zero says to use rcu_cpu_stall_timeout, but in milliseconds.
  35         if (!cpu_stall_timeout)
  36                 cpu_stall_timeout = jiffies_to_msecs(rcu_jiffies_till_stall_check());
  37
  38         // Limit check must be consistent with the Kconfig limits for
  39         // CONFIG_RCU_EXP_CPU_STALL_TIMEOUT, so check the allowed range.
  40         // The minimum clamped value is "2UL", because at least one full
  41         // tick has to be guaranteed.
  42         till_stall_check = clamp(msecs_to_jiffies(cpu_stall_timeout), 2UL, 21UL * HZ);
  43
  44         if (cpu_stall_timeout && jiffies_to_msecs(till_stall_check) != cpu_stall_timeout)
  45                 WRITE_ONCE(rcu_exp_cpu_stall_timeout, jiffies_to_msecs(till_stall_check));
  46
  47 #ifdef CONFIG_PROVE_RCU
  48         /* Add extra ~25% out of till_stall_check. */
  49         exp_stall_delay_delta = ((till_stall_check * 25) / 100) + 1;
  50 #endif
  51
  52         return till_stall_check + exp_stall_delay_delta;
  53 }
  54 EXPORT_SYMBOL_GPL(rcu_exp_jiffies_till_stall_check);
  55
  56 /* Limit-check stall timeouts specified at boottime and runtime. */
  57 int rcu_jiffies_till_stall_check(void)
  58 {
  59         int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout);
  60
  61         /*
  62          * Limit check must be consistent with the Kconfig limits
  63          * for CONFIG_RCU_CPU_STALL_TIMEOUT.
  64          */
  65         if (till_stall_check < 3) {
  66                 WRITE_ONCE(rcu_cpu_stall_timeout, 3);
  67                 till_stall_check = 3;
  68         } else if (till_stall_check > 300) {
  69                 WRITE_ONCE(rcu_cpu_stall_timeout, 300);
  70                 till_stall_check = 300;
  71         }
  72         return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
  73 }
  74 EXPORT_SYMBOL_GPL(rcu_jiffies_till_stall_check);
  75
  76 /**
  77  * rcu_gp_might_be_stalled - Is it likely that the grace period is stalled?
  78  *
  79  * Returns @true if the current grace period is sufficiently old that
  80  * it is reasonable to assume that it might be stalled.  This can be
  81  * useful when deciding whether to allocate memory to enable RCU-mediated
  82  * freeing on the one hand or just invoking synchronize_rcu() on the other.
  83  * The latter is preferable when the grace period is stalled.
  84  *
  85  * Note that sampling of the .gp_start and .gp_seq fields must be done
  86  * carefully to avoid false positives at the beginnings and ends of
  87  * grace periods.
  88  */
  89 bool rcu_gp_might_be_stalled(void)
  90 {
  91         unsigned long d = rcu_jiffies_till_stall_check() / RCU_STALL_MIGHT_DIV;
  92         unsigned long j = jiffies;
  93
  94         if (d < RCU_STALL_MIGHT_MIN)
  95                 d = RCU_STALL_MIGHT_MIN;
  96         smp_mb(); // jiffies before .gp_seq to avoid false positives.
  97         if (!rcu_gp_in_progress())
  98                 return false;
  99         // Long delays at this point avoids false positive, but a delay
 100         // of ULONG_MAX/4 jiffies voids your no-false-positive warranty.
 101         smp_mb(); // .gp_seq before second .gp_start
 102         // And ditto here.
 103         return !time_before(j, READ_ONCE(rcu_state.gp_start) + d);
 104 }
 105
 106 /* Don't do RCU CPU stall warnings during long sysrq printouts. */
 107 void rcu_sysrq_start(void)
 108 {
 109         if (!rcu_cpu_stall_suppress)
 110                 rcu_cpu_stall_suppress = 2;
 111 }
 112
 113 void rcu_sysrq_end(void)
 114 {
 115         if (rcu_cpu_stall_suppress == 2)
 116                 rcu_cpu_stall_suppress = 0;
 117 }
 118
 119 /* Don't print RCU CPU stall warnings during a kernel panic. */
 120 static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
 121 {
 122         rcu_cpu_stall_suppress = 1;
 123         return NOTIFY_DONE;
 124 }
 125
 126 static struct notifier_block rcu_panic_block = {
 127         .notifier_call = rcu_panic,
 128 };
 129
 130 static int __init check_cpu_stall_init(void)
 131 {
 132         atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block);
 133         return 0;
 134 }
 135 early_initcall(check_cpu_stall_init);
 136
 137 /* If so specified via sysctl, panic, yielding cleaner stall-warning output. */
 138 static void panic_on_rcu_stall(void)
 139 {
 140         static int cpu_stall;
 141
 142         if (++cpu_stall < sysctl_max_rcu_stall_to_panic)
 143                 return;
 144
 145         if (sysctl_panic_on_rcu_stall)
 146                 panic("RCU Stall\n");
 147 }
 148
 149 /**
 150  * rcu_cpu_stall_reset - restart stall-warning timeout for current grace period
 151  *
 152  * The caller must disable hard irqs.
 153  */
 154 void rcu_cpu_stall_reset(void)
 155 {
 156         WRITE_ONCE(rcu_state.jiffies_stall,
 157                    jiffies + rcu_jiffies_till_stall_check());
 158 }
 159
 160 //////////////////////////////////////////////////////////////////////////////
 161 //
 162 // Interaction with RCU grace periods
 163
 164 /* Start of new grace period, so record stall time (and forcing times). */
 165 static void record_gp_stall_check_time(void)
 166 {
 167         unsigned long j = jiffies;
 168         unsigned long j1;
 169
 170         WRITE_ONCE(rcu_state.gp_start, j);
 171         j1 = rcu_jiffies_till_stall_check();
 172         smp_mb(); // ->gp_start before ->jiffies_stall and caller's ->gp_seq.
 173         WRITE_ONCE(rcu_state.jiffies_stall, j + j1);
 174         rcu_state.jiffies_resched = j + j1 / 2;
 175         rcu_state.n_force_qs_gpstart = READ_ONCE(rcu_state.n_force_qs);
 176 }
 177
 178 /* Zero ->ticks_this_gp and snapshot the number of RCU softirq handlers. */
 179 static void zero_cpu_stall_ticks(struct rcu_data *rdp)
 180 {
 181         rdp->ticks_this_gp = 0;
 182         rdp->softirq_snap = kstat_softirqs_cpu(RCU_SOFTIRQ, smp_processor_id());
 183         WRITE_ONCE(rdp->last_fqs_resched, jiffies);
 184 }
 185
 186 /*
 187  * If too much time has passed in the current grace period, and if
 188  * so configured, go kick the relevant kthreads.
 189  */
 190 static void rcu_stall_kick_kthreads(void)
 191 {
 192         unsigned long j;
 193
 194         if (!READ_ONCE(rcu_kick_kthreads))
 195                 return;
 196         j = READ_ONCE(rcu_state.jiffies_kick_kthreads);
 197         if (time_after(jiffies, j) && rcu_state.gp_kthread &&
 198             (rcu_gp_in_progress() || READ_ONCE(rcu_state.gp_flags))) {
 199                 WARN_ONCE(1, "Kicking %s grace-period kthread\n",
 200                           rcu_state.name);
 201                 rcu_ftrace_dump(DUMP_ALL);
 202                 wake_up_process(rcu_state.gp_kthread);
 203                 WRITE_ONCE(rcu_state.jiffies_kick_kthreads, j + HZ);
 204         }
 205 }
 206
 207 /*
 208  * Handler for the irq_work request posted about halfway into the RCU CPU
 209  * stall timeout, and used to detect excessive irq disabling.  Set state
 210  * appropriately, but just complain if there is unexpected state on entry.
 211  */
 212 static void rcu_iw_handler(struct irq_work *iwp)
 213 {
 214         struct rcu_data *rdp;
 215         struct rcu_node *rnp;
 216
 217         rdp = container_of(iwp, struct rcu_data, rcu_iw);
 218         rnp = rdp->mynode;
 219         raw_spin_lock_rcu_node(rnp);
 220         if (!WARN_ON_ONCE(!rdp->rcu_iw_pending)) {
 221                 rdp->rcu_iw_gp_seq = rnp->gp_seq;
 222                 rdp->rcu_iw_pending = false;
 223         }
 224         raw_spin_unlock_rcu_node(rnp);
 225 }
 226
 227 //////////////////////////////////////////////////////////////////////////////
 228 //
 229 // Printing RCU CPU stall warnings
 230
 231 #ifdef CONFIG_PREEMPT_RCU
 232
 233 /*
 234  * Dump detailed information for all tasks blocking the current RCU
 235  * grace period on the specified rcu_node structure.
 236  */
 237 static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
 238 {
 239         unsigned long flags;
 240         struct task_struct *t;
 241
 242         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 243         if (!rcu_preempt_blocked_readers_cgp(rnp)) {
 244                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 245                 return;
 246         }
 247         t = list_entry(rnp->gp_tasks->prev,
 248                        struct task_struct, rcu_node_entry);
 249         list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
 250                 /*
 251                  * We could be printing a lot while holding a spinlock.
 252                  * Avoid triggering hard lockup.
 253                  */
 254                 touch_nmi_watchdog();
 255                 sched_show_task(t);
 256         }
 257         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 258 }
 259
 260 // Communicate task state back to the RCU CPU stall warning request.
 261 struct rcu_stall_chk_rdr {
 262         int nesting;
 263         union rcu_special rs;
 264         bool on_blkd_list;
 265 };
 266
 267 /*
 268  * Report out the state of a not-running task that is stalling the
 269  * current RCU grace period.
 270  */
 271 static int check_slow_task(struct task_struct *t, void *arg)
 272 {
 273         struct rcu_stall_chk_rdr *rscrp = arg;
 274
 275         if (task_curr(t))
 276                 return -EBUSY; // It is running, so decline to inspect it.
 277         rscrp->nesting = t->rcu_read_lock_nesting;
 278         rscrp->rs = t->rcu_read_unlock_special;
 279         rscrp->on_blkd_list = !list_empty(&t->rcu_node_entry);
 280         return 0;
 281 }
 282
 283 /*
 284  * Scan the current list of tasks blocked within RCU read-side critical
 285  * sections, printing out the tid of each of the first few of them.
 286  */
 287 static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
 288         __releases(rnp->lock)
 289 {
 290         int i = 0;
 291         int ndetected = 0;
 292         struct rcu_stall_chk_rdr rscr;
 293         struct task_struct *t;
 294         struct task_struct *ts[8];
 295
 296         lockdep_assert_irqs_disabled();
 297         if (!rcu_preempt_blocked_readers_cgp(rnp)) {
 298                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 299                 return 0;
 300         }
 301         pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
 302                rnp->level, rnp->grplo, rnp->grphi);
 303         t = list_entry(rnp->gp_tasks->prev,
 304                        struct task_struct, rcu_node_entry);
 305         list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
 306                 get_task_struct(t);
 307                 ts[i++] = t;
 308                 if (i >= ARRAY_SIZE(ts))
 309                         break;
 310         }
 311         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 312         while (i) {
 313                 t = ts[--i];
 314                 if (task_call_func(t, check_slow_task, &rscr))
 315                         pr_cont(" P%d", t->pid);
 316                 else
 317                         pr_cont(" P%d/%d:%c%c%c%c",
 318                                 t->pid, rscr.nesting,
 319                                 ".b"[rscr.rs.b.blocked],
 320                                 ".q"[rscr.rs.b.need_qs],
 321                                 ".e"[rscr.rs.b.exp_hint],
 322                                 ".l"[rscr.on_blkd_list]);
 323                 lockdep_assert_irqs_disabled();
 324                 put_task_struct(t);
 325                 ndetected++;
 326         }
 327         pr_cont("\n");
 328         return ndetected;
 329 }
 330
 331 #else /* #ifdef CONFIG_PREEMPT_RCU */
 332
 333 /*
 334  * Because preemptible RCU does not exist, we never have to check for
 335  * tasks blocked within RCU read-side critical sections.
 336  */
 337 static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
 338 {
 339 }
 340
 341 /*
 342  * Because preemptible RCU does not exist, we never have to check for
 343  * tasks blocked within RCU read-side critical sections.
 344  */
 345 static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
 346         __releases(rnp->lock)
 347 {
 348         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 349         return 0;
 350 }
 351 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 352
 353 /*
 354  * Dump stacks of all tasks running on stalled CPUs.  First try using
 355  * NMIs, but fall back to manual remote stack tracing on architectures
 356  * that don't support NMI-based stack dumps.  The NMI-triggered stack
 357  * traces are more accurate because they are printed by the target CPU.
 358  */
 359 static void rcu_dump_cpu_stacks(void)
 360 {
 361         int cpu;
 362         unsigned long flags;
 363         struct rcu_node *rnp;
 364
 365         rcu_for_each_leaf_node(rnp) {
 366                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
 367                 for_each_leaf_node_possible_cpu(rnp, cpu)
 368                         if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
 369                                 if (cpu_is_offline(cpu))
 370                                         pr_err("Offline CPU %d blocking current GP.\n", cpu);
 371                                 else if (!trigger_single_cpu_backtrace(cpu))
 372                                         dump_cpu_task(cpu);
 373                         }
 374                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 375         }
 376 }
 377
 378 static const char * const gp_state_names[] = {
 379         [RCU_GP_IDLE] = "RCU_GP_IDLE",
 380         [RCU_GP_WAIT_GPS] = "RCU_GP_WAIT_GPS",
 381         [RCU_GP_DONE_GPS] = "RCU_GP_DONE_GPS",
 382         [RCU_GP_ONOFF] = "RCU_GP_ONOFF",
 383         [RCU_GP_INIT] = "RCU_GP_INIT",
 384         [RCU_GP_WAIT_FQS] = "RCU_GP_WAIT_FQS",
 385         [RCU_GP_DOING_FQS] = "RCU_GP_DOING_FQS",
 386         [RCU_GP_CLEANUP] = "RCU_GP_CLEANUP",
 387         [RCU_GP_CLEANED] = "RCU_GP_CLEANED",
 388 };
 389
 390 /*
 391  * Convert a ->gp_state value to a character string.
 392  */
 393 static const char *gp_state_getname(short gs)
 394 {
 395         if (gs < 0 || gs >= ARRAY_SIZE(gp_state_names))
 396                 return "???";
 397         return gp_state_names[gs];
 398 }
 399
 400 /* Is the RCU grace-period kthread being starved of CPU time? */
 401 static bool rcu_is_gp_kthread_starving(unsigned long *jp)
 402 {
 403         unsigned long j = jiffies - READ_ONCE(rcu_state.gp_activity);
 404
 405         if (jp)
 406                 *jp = j;
 407         return j > 2 * HZ;
 408 }
 409
 410 static bool rcu_is_rcuc_kthread_starving(struct rcu_data *rdp, unsigned long *jp)
 411 {
 412         unsigned long j = jiffies - READ_ONCE(rdp->rcuc_activity);
 413
 414         if (jp)
 415                 *jp = j;
 416         return j > 2 * HZ;
 417 }
 418
 419 /*
 420  * Print out diagnostic information for the specified stalled CPU.
 421  *
 422  * If the specified CPU is aware of the current RCU grace period, then
 423  * print the number of scheduling clock interrupts the CPU has taken
 424  * during the time that it has been aware.  Otherwise, print the number
 425  * of RCU grace periods that this CPU is ignorant of, for example, "1"
 426  * if the CPU was aware of the previous grace period.
 427  *
 428  * Also print out idle info.
 429  */
 430 static void print_cpu_stall_info(int cpu)
 431 {
 432         unsigned long delta;
 433         bool falsepositive;
 434         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 435         char *ticks_title;
 436         unsigned long ticks_value;
 437
 438         /*
 439          * We could be printing a lot while holding a spinlock.  Avoid
 440          * triggering hard lockup.
 441          */
 442         touch_nmi_watchdog();
 443
 444         ticks_value = rcu_seq_ctr(rcu_state.gp_seq - rdp->gp_seq);
 445         if (ticks_value) {
 446                 ticks_title = "GPs behind";
 447         } else {
 448                 ticks_title = "ticks this GP";
 449                 ticks_value = rdp->ticks_this_gp;
 450         }
 451         delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
 452         falsepositive = rcu_is_gp_kthread_starving(NULL) &&
 453                         rcu_dynticks_in_eqs(rcu_dynticks_snap(rdp));
 454         pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%03x/%ld/%#lx softirq=%u/%u fqs=%ld %s\n",
 455                cpu,
 456                "O."[!!cpu_online(cpu)],
 457                "o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)],
 458                "N."[!!(rdp->grpmask & rdp->mynode->qsmaskinitnext)],
 459                !IS_ENABLED(CONFIG_IRQ_WORK) ? '?' :
 460                         rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' :
 461                                 "!."[!delta],
 462                ticks_value, ticks_title,
 463                rcu_dynticks_snap(rdp) & 0xfff,
 464                rdp->dynticks_nesting, rdp->dynticks_nmi_nesting,
 465                rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
 466                data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
 467                falsepositive ? " (false positive?)" : "");
 468 }
 469
 470 static void rcuc_kthread_dump(struct rcu_data *rdp)
 471 {
 472         int cpu;
 473         unsigned long j;
 474         struct task_struct *rcuc;
 475
 476         rcuc = rdp->rcu_cpu_kthread_task;
 477         if (!rcuc)
 478                 return;
 479
 480         cpu = task_cpu(rcuc);
 481         if (cpu_is_offline(cpu) || idle_cpu(cpu))
 482                 return;
 483
 484         if (!rcu_is_rcuc_kthread_starving(rdp, &j))
 485                 return;
 486
 487         pr_err("%s kthread starved for %ld jiffies\n", rcuc->comm, j);
 488         sched_show_task(rcuc);
 489         if (!trigger_single_cpu_backtrace(cpu))
 490                 dump_cpu_task(cpu);
 491 }
 492
 493 /* Complain about starvation of grace-period kthread.  */
 494 static void rcu_check_gp_kthread_starvation(void)
 495 {
 496         int cpu;
 497         struct task_struct *gpk = rcu_state.gp_kthread;
 498         unsigned long j;
 499
 500         if (rcu_is_gp_kthread_starving(&j)) {
 501                 cpu = gpk ? task_cpu(gpk) : -1;
 502                 pr_err("%s kthread starved for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x ->cpu=%d\n",
 503                        rcu_state.name, j,
 504                        (long)rcu_seq_current(&rcu_state.gp_seq),
 505                        data_race(READ_ONCE(rcu_state.gp_flags)),
 506                        gp_state_getname(rcu_state.gp_state),
 507                        data_race(READ_ONCE(rcu_state.gp_state)),
 508                        gpk ? data_race(READ_ONCE(gpk->__state)) : ~0, cpu);
 509                 if (gpk) {
 510                         pr_err("\tUnless %s kthread gets sufficient CPU time, OOM is now expected behavior.\n", rcu_state.name);
 511                         pr_err("RCU grace-period kthread stack dump:\n");
 512                         sched_show_task(gpk);
 513                         if (cpu >= 0) {
 514                                 if (cpu_is_offline(cpu)) {
 515                                         pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu);
 516                                 } else  {
 517                                         pr_err("Stack dump where RCU GP kthread last ran:\n");
 518                                         if (!trigger_single_cpu_backtrace(cpu))
 519                                                 dump_cpu_task(cpu);
 520                                 }
 521                         }
 522                         wake_up_process(gpk);
 523                 }
 524         }
 525 }
 526
 527 /* Complain about missing wakeups from expired fqs wait timer */
 528 static void rcu_check_gp_kthread_expired_fqs_timer(void)
 529 {
 530         struct task_struct *gpk = rcu_state.gp_kthread;
 531         short gp_state;
 532         unsigned long jiffies_fqs;
 533         int cpu;
 534
 535         /*
 536          * Order reads of .gp_state and .jiffies_force_qs.
 537          * Matching smp_wmb() is present in rcu_gp_fqs_loop().
 538          */
 539         gp_state = smp_load_acquire(&rcu_state.gp_state);
 540         jiffies_fqs = READ_ONCE(rcu_state.jiffies_force_qs);
 541
 542         if (gp_state == RCU_GP_WAIT_FQS &&
 543             time_after(jiffies, jiffies_fqs + RCU_STALL_MIGHT_MIN) &&
 544             gpk && !READ_ONCE(gpk->on_rq)) {
 545                 cpu = task_cpu(gpk);
 546                 pr_err("%s kthread timer wakeup didn't happen for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x\n",
 547                        rcu_state.name, (jiffies - jiffies_fqs),
 548                        (long)rcu_seq_current(&rcu_state.gp_seq),
 549                        data_race(rcu_state.gp_flags),
 550                        gp_state_getname(RCU_GP_WAIT_FQS), RCU_GP_WAIT_FQS,
 551                        data_race(READ_ONCE(gpk->__state)));
 552                 pr_err("\tPossible timer handling issue on cpu=%d timer-softirq=%u\n",
 553                        cpu, kstat_softirqs_cpu(TIMER_SOFTIRQ, cpu));
 554         }
 555 }
 556
 557 static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
 558 {
 559         int cpu;
 560         unsigned long flags;
 561         unsigned long gpa;
 562         unsigned long j;
 563         int ndetected = 0;
 564         struct rcu_node *rnp;
 565         long totqlen = 0;
 566
 567         lockdep_assert_irqs_disabled();
 568
 569         /* Kick and suppress, if so configured. */
 570         rcu_stall_kick_kthreads();
 571         if (rcu_stall_is_suppressed())
 572                 return;
 573
 574         /*
 575          * OK, time to rat on our buddy...
 576          * See Documentation/RCU/stallwarn.rst for info on how to debug
 577          * RCU CPU stall warnings.
 578          */
 579         trace_rcu_stall_warning(rcu_state.name, TPS("StallDetected"));
 580         pr_err("INFO: %s detected stalls on CPUs/tasks:\n", rcu_state.name);
 581         rcu_for_each_leaf_node(rnp) {
 582                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
 583                 if (rnp->qsmask != 0) {
 584                         for_each_leaf_node_possible_cpu(rnp, cpu)
 585                                 if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
 586                                         print_cpu_stall_info(cpu);
 587                                         ndetected++;
 588                                 }
 589                 }
 590                 ndetected += rcu_print_task_stall(rnp, flags); // Releases rnp->lock.
 591                 lockdep_assert_irqs_disabled();
 592         }
 593
 594         for_each_possible_cpu(cpu)
 595                 totqlen += rcu_get_n_cbs_cpu(cpu);
 596         pr_cont("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu ncpus=%d)\n",
 597                smp_processor_id(), (long)(jiffies - gps),
 598                (long)rcu_seq_current(&rcu_state.gp_seq), totqlen, rcu_state.n_online_cpus);
 599         if (ndetected) {
 600                 rcu_dump_cpu_stacks();
 601
 602                 /* Complain about tasks blocking the grace period. */
 603                 rcu_for_each_leaf_node(rnp)
 604                         rcu_print_detail_task_stall_rnp(rnp);
 605         } else {
 606                 if (rcu_seq_current(&rcu_state.gp_seq) != gp_seq) {
 607                         pr_err("INFO: Stall ended before state dump start\n");
 608                 } else {
 609                         j = jiffies;
 610                         gpa = data_race(READ_ONCE(rcu_state.gp_activity));
 611                         pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld, root ->qsmask %#lx\n",
 612                                rcu_state.name, j - gpa, j, gpa,
 613                                data_race(READ_ONCE(jiffies_till_next_fqs)),
 614                                data_race(READ_ONCE(rcu_get_root()->qsmask)));
 615                 }
 616         }
 617         /* Rewrite if needed in case of slow consoles. */
 618         if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
 619                 WRITE_ONCE(rcu_state.jiffies_stall,
 620                            jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
 621
 622         rcu_check_gp_kthread_expired_fqs_timer();
 623         rcu_check_gp_kthread_starvation();
 624
 625         panic_on_rcu_stall();
 626
 627         rcu_force_quiescent_state();  /* Kick them all. */
 628 }
 629
 630 static void print_cpu_stall(unsigned long gps)
 631 {
 632         int cpu;
 633         unsigned long flags;
 634         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 635         struct rcu_node *rnp = rcu_get_root();
 636         long totqlen = 0;
 637
 638         lockdep_assert_irqs_disabled();
 639
 640         /* Kick and suppress, if so configured. */
 641         rcu_stall_kick_kthreads();
 642         if (rcu_stall_is_suppressed())
 643                 return;
 644
 645         /*
 646          * OK, time to rat on ourselves...
 647          * See Documentation/RCU/stallwarn.rst for info on how to debug
 648          * RCU CPU stall warnings.
 649          */
 650         trace_rcu_stall_warning(rcu_state.name, TPS("SelfDetected"));
 651         pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name);
 652         raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags);
 653         print_cpu_stall_info(smp_processor_id());
 654         raw_spin_unlock_irqrestore_rcu_node(rdp->mynode, flags);
 655         for_each_possible_cpu(cpu)
 656                 totqlen += rcu_get_n_cbs_cpu(cpu);
 657         pr_cont("\t(t=%lu jiffies g=%ld q=%lu ncpus=%d)\n",
 658                 jiffies - gps,
 659                 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen, rcu_state.n_online_cpus);
 660
 661         rcu_check_gp_kthread_expired_fqs_timer();
 662         rcu_check_gp_kthread_starvation();
 663
 664         if (!use_softirq)
 665                 rcuc_kthread_dump(rdp);
 666
 667         rcu_dump_cpu_stacks();
 668
 669         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 670         /* Rewrite if needed in case of slow consoles. */
 671         if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
 672                 WRITE_ONCE(rcu_state.jiffies_stall,
 673                            jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
 674         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 675
 676         panic_on_rcu_stall();
 677
 678         /*
 679          * Attempt to revive the RCU machinery by forcing a context switch.
 680          *
 681          * A context switch would normally allow the RCU state machine to make
 682          * progress and it could be we're stuck in kernel space without context
 683          * switches for an entirely unreasonable amount of time.
 684          */
 685         set_tsk_need_resched(current);
 686         set_preempt_need_resched();
 687 }
 688
 689 static void check_cpu_stall(struct rcu_data *rdp)
 690 {
 691         bool didstall = false;
 692         unsigned long gs1;
 693         unsigned long gs2;
 694         unsigned long gps;
 695         unsigned long j;
 696         unsigned long jn;
 697         unsigned long js;
 698         struct rcu_node *rnp;
 699
 700         lockdep_assert_irqs_disabled();
 701         if ((rcu_stall_is_suppressed() && !READ_ONCE(rcu_kick_kthreads)) ||
 702             !rcu_gp_in_progress())
 703                 return;
 704         rcu_stall_kick_kthreads();
 705         j = jiffies;
 706
 707         /*
 708          * Lots of memory barriers to reject false positives.
 709          *
 710          * The idea is to pick up rcu_state.gp_seq, then
 711          * rcu_state.jiffies_stall, then rcu_state.gp_start, and finally
 712          * another copy of rcu_state.gp_seq.  These values are updated in
 713          * the opposite order with memory barriers (or equivalent) during
 714          * grace-period initialization and cleanup.  Now, a false positive
 715          * can occur if we get an new value of rcu_state.gp_start and a old
 716          * value of rcu_state.jiffies_stall.  But given the memory barriers,
 717          * the only way that this can happen is if one grace period ends
 718          * and another starts between these two fetches.  This is detected
 719          * by comparing the second fetch of rcu_state.gp_seq with the
 720          * previous fetch from rcu_state.gp_seq.
 721          *
 722          * Given this check, comparisons of jiffies, rcu_state.jiffies_stall,
 723          * and rcu_state.gp_start suffice to forestall false positives.
 724          */
 725         gs1 = READ_ONCE(rcu_state.gp_seq);
 726         smp_rmb(); /* Pick up ->gp_seq first... */
 727         js = READ_ONCE(rcu_state.jiffies_stall);
 728         smp_rmb(); /* ...then ->jiffies_stall before the rest... */
 729         gps = READ_ONCE(rcu_state.gp_start);
 730         smp_rmb(); /* ...and finally ->gp_start before ->gp_seq again. */
 731         gs2 = READ_ONCE(rcu_state.gp_seq);
 732         if (gs1 != gs2 ||
 733             ULONG_CMP_LT(j, js) ||
 734             ULONG_CMP_GE(gps, js))
 735                 return; /* No stall or GP completed since entering function. */
 736         rnp = rdp->mynode;
 737         jn = jiffies + ULONG_MAX / 2;
 738         if (rcu_gp_in_progress() &&
 739             (READ_ONCE(rnp->qsmask) & rdp->grpmask) &&
 740             cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
 741
 742                 /*
 743                  * If a virtual machine is stopped by the host it can look to
 744                  * the watchdog like an RCU stall. Check to see if the host
 745                  * stopped the vm.
 746                  */
 747                 if (kvm_check_and_clear_guest_paused())
 748                         return;
 749
 750                 /* We haven't checked in, so go dump stack. */
 751                 print_cpu_stall(gps);
 752                 if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
 753                         rcu_ftrace_dump(DUMP_ALL);
 754                 didstall = true;
 755
 756         } else if (rcu_gp_in_progress() &&
 757                    ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
 758                    cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
 759
 760                 /*
 761                  * If a virtual machine is stopped by the host it can look to
 762                  * the watchdog like an RCU stall. Check to see if the host
 763                  * stopped the vm.
 764                  */
 765                 if (kvm_check_and_clear_guest_paused())
 766                         return;
 767
 768                 /* They had a few time units to dump stack, so complain. */
 769                 print_other_cpu_stall(gs2, gps);
 770                 if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
 771                         rcu_ftrace_dump(DUMP_ALL);
 772                 didstall = true;
 773         }
 774         if (didstall && READ_ONCE(rcu_state.jiffies_stall) == jn) {
 775                 jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
 776                 WRITE_ONCE(rcu_state.jiffies_stall, jn);
 777         }
 778 }
 779
 780 //////////////////////////////////////////////////////////////////////////////
 781 //
 782 // RCU forward-progress mechanisms, including of callback invocation.
 783
 784
 785 /*
 786  * Check to see if a failure to end RCU priority inversion was due to
 787  * a CPU not passing through a quiescent state.  When this happens, there
 788  * is nothing that RCU priority boosting can do to help, so we shouldn't
 789  * count this as an RCU priority boosting failure.  A return of true says
 790  * RCU priority boosting is to blame, and false says otherwise.  If false
 791  * is returned, the first of the CPUs to blame is stored through cpup.
 792  * If there was no CPU blocking the current grace period, but also nothing
 793  * in need of being boosted, *cpup is set to -1.  This can happen in case
 794  * of vCPU preemption while the last CPU is reporting its quiscent state,
 795  * for example.
 796  *
 797  * If cpup is NULL, then a lockless quick check is carried out, suitable
 798  * for high-rate usage.  On the other hand, if cpup is non-NULL, each
 799  * rcu_node structure's ->lock is acquired, ruling out high-rate usage.
 800  */
 801 bool rcu_check_boost_fail(unsigned long gp_state, int *cpup)
 802 {
 803         bool atb = false;
 804         int cpu;
 805         unsigned long flags;
 806         struct rcu_node *rnp;
 807
 808         rcu_for_each_leaf_node(rnp) {
 809                 if (!cpup) {
 810                         if (data_race(READ_ONCE(rnp->qsmask))) {
 811                                 return false;
 812                         } else {
 813                                 if (READ_ONCE(rnp->gp_tasks))
 814                                         atb = true;
 815                                 continue;
 816                         }
 817                 }
 818                 *cpup = -1;
 819                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
 820                 if (rnp->gp_tasks)
 821                         atb = true;
 822                 if (!rnp->qsmask) {
 823                         // No CPUs without quiescent states for this rnp.
 824                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 825                         continue;
 826                 }
 827                 // Find the first holdout CPU.
 828                 for_each_leaf_node_possible_cpu(rnp, cpu) {
 829                         if (rnp->qsmask & (1UL << (cpu - rnp->grplo))) {
 830                                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 831                                 *cpup = cpu;
 832                                 return false;
 833                         }
 834                 }
 835                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 836         }
 837         // Can't blame CPUs, so must blame RCU priority boosting.
 838         return atb;
 839 }
 840 EXPORT_SYMBOL_GPL(rcu_check_boost_fail);
 841
 842 /*
 843  * Show the state of the grace-period kthreads.
 844  */
 845 void show_rcu_gp_kthreads(void)
 846 {
 847         unsigned long cbs = 0;
 848         int cpu;
 849         unsigned long j;
 850         unsigned long ja;
 851         unsigned long jr;
 852         unsigned long js;
 853         unsigned long jw;
 854         struct rcu_data *rdp;
 855         struct rcu_node *rnp;
 856         struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);
 857
 858         j = jiffies;
 859         ja = j - data_race(READ_ONCE(rcu_state.gp_activity));
 860         jr = j - data_race(READ_ONCE(rcu_state.gp_req_activity));
 861         js = j - data_race(READ_ONCE(rcu_state.gp_start));
 862         jw = j - data_race(READ_ONCE(rcu_state.gp_wake_time));
 863         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",
 864                 rcu_state.name, gp_state_getname(rcu_state.gp_state),
 865                 data_race(READ_ONCE(rcu_state.gp_state)),
 866                 t ? data_race(READ_ONCE(t->__state)) : 0x1ffff, t ? t->rt_priority : 0xffU,
 867                 js, ja, jr, jw, (long)data_race(READ_ONCE(rcu_state.gp_wake_seq)),
 868                 (long)data_race(READ_ONCE(rcu_state.gp_seq)),
 869                 (long)data_race(READ_ONCE(rcu_get_root()->gp_seq_needed)),
 870                 data_race(READ_ONCE(rcu_state.gp_max)),
 871                 data_race(READ_ONCE(rcu_state.gp_flags)));
 872         rcu_for_each_node_breadth_first(rnp) {
 873                 if (ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq), READ_ONCE(rnp->gp_seq_needed)) &&
 874                     !data_race(READ_ONCE(rnp->qsmask)) && !data_race(READ_ONCE(rnp->boost_tasks)) &&
 875                     !data_race(READ_ONCE(rnp->exp_tasks)) && !data_race(READ_ONCE(rnp->gp_tasks)))
 876                         continue;
 877                 pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld ->qsmask %#lx %c%c%c%c ->n_boosts %ld\n",
 878                         rnp->grplo, rnp->grphi,
 879                         (long)data_race(READ_ONCE(rnp->gp_seq)),
 880                         (long)data_race(READ_ONCE(rnp->gp_seq_needed)),
 881                         data_race(READ_ONCE(rnp->qsmask)),
 882                         ".b"[!!data_race(READ_ONCE(rnp->boost_kthread_task))],
 883                         ".B"[!!data_race(READ_ONCE(rnp->boost_tasks))],
 884                         ".E"[!!data_race(READ_ONCE(rnp->exp_tasks))],
 885                         ".G"[!!data_race(READ_ONCE(rnp->gp_tasks))],
 886                         data_race(READ_ONCE(rnp->n_boosts)));
 887                 if (!rcu_is_leaf_node(rnp))
 888                         continue;
 889                 for_each_leaf_node_possible_cpu(rnp, cpu) {
 890                         rdp = per_cpu_ptr(&rcu_data, cpu);
 891                         if (READ_ONCE(rdp->gpwrap) ||
 892                             ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq),
 893                                          READ_ONCE(rdp->gp_seq_needed)))
 894                                 continue;
 895                         pr_info("\tcpu %d ->gp_seq_needed %ld\n",
 896                                 cpu, (long)data_race(READ_ONCE(rdp->gp_seq_needed)));
 897                 }
 898         }
 899         for_each_possible_cpu(cpu) {
 900                 rdp = per_cpu_ptr(&rcu_data, cpu);
 901                 cbs += data_race(READ_ONCE(rdp->n_cbs_invoked));
 902                 if (rcu_segcblist_is_offloaded(&rdp->cblist))
 903                         show_rcu_nocb_state(rdp);
 904         }
 905         pr_info("RCU callbacks invoked since boot: %lu\n", cbs);
 906         show_rcu_tasks_gp_kthreads();
 907 }
 908 EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);
 909
 910 /*
 911  * This function checks for grace-period requests that fail to motivate
 912  * RCU to come out of its idle mode.
 913  */
 914 static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
 915                                      const unsigned long gpssdelay)
 916 {
 917         unsigned long flags;
 918         unsigned long j;
 919         struct rcu_node *rnp_root = rcu_get_root();
 920         static atomic_t warned = ATOMIC_INIT(0);
 921
 922         if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() ||
 923             ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
 924                          READ_ONCE(rnp_root->gp_seq_needed)) ||
 925             !smp_load_acquire(&rcu_state.gp_kthread)) // Get stable kthread.
 926                 return;
 927         j = jiffies; /* Expensive access, and in common case don't get here. */
 928         if (time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
 929             time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
 930             atomic_read(&warned))
 931                 return;
 932
 933         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 934         j = jiffies;
 935         if (rcu_gp_in_progress() ||
 936             ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
 937                          READ_ONCE(rnp_root->gp_seq_needed)) ||
 938             time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
 939             time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
 940             atomic_read(&warned)) {
 941                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 942                 return;
 943         }
 944         /* Hold onto the leaf lock to make others see warned==1. */
 945
 946         if (rnp_root != rnp)
 947                 raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
 948         j = jiffies;
 949         if (rcu_gp_in_progress() ||
 950             ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
 951                          READ_ONCE(rnp_root->gp_seq_needed)) ||
 952             time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
 953             time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
 954             atomic_xchg(&warned, 1)) {
 955                 if (rnp_root != rnp)
 956                         /* irqs remain disabled. */
 957                         raw_spin_unlock_rcu_node(rnp_root);
 958                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 959                 return;
 960         }
 961         WARN_ON(1);
 962         if (rnp_root != rnp)
 963                 raw_spin_unlock_rcu_node(rnp_root);
 964         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 965         show_rcu_gp_kthreads();
 966 }
 967
 968 /*
 969  * Do a forward-progress check for rcutorture.  This is normally invoked
 970  * due to an OOM event.  The argument "j" gives the time period during
 971  * which rcutorture would like progress to have been made.
 972  */
 973 void rcu_fwd_progress_check(unsigned long j)
 974 {
 975         unsigned long cbs;
 976         int cpu;
 977         unsigned long max_cbs = 0;
 978         int max_cpu = -1;
 979         struct rcu_data *rdp;
 980
 981         if (rcu_gp_in_progress()) {
 982                 pr_info("%s: GP age %lu jiffies\n",
 983                         __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_start)));
 984                 show_rcu_gp_kthreads();
 985         } else {
 986                 pr_info("%s: Last GP end %lu jiffies ago\n",
 987                         __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_end)));
 988                 preempt_disable();
 989                 rdp = this_cpu_ptr(&rcu_data);
 990                 rcu_check_gp_start_stall(rdp->mynode, rdp, j);
 991                 preempt_enable();
 992         }
 993         for_each_possible_cpu(cpu) {
 994                 cbs = rcu_get_n_cbs_cpu(cpu);
 995                 if (!cbs)
 996                         continue;
 997                 if (max_cpu < 0)
 998                         pr_info("%s: callbacks", __func__);
 999                 pr_cont(" %d: %lu", cpu, cbs);
1000                 if (cbs <= max_cbs)
1001                         continue;
1002                 max_cbs = cbs;
1003                 max_cpu = cpu;
1004         }
1005         if (max_cpu >= 0)
1006                 pr_cont("\n");
1007 }
1008 EXPORT_SYMBOL_GPL(rcu_fwd_progress_check);
1009
1010 /* Commandeer a sysrq key to dump RCU's tree. */
1011 static bool sysrq_rcu;
1012 module_param(sysrq_rcu, bool, 0444);
1013
1014 /* Dump grace-period-request information due to commandeered sysrq. */
1015 static void sysrq_show_rcu(int key)
1016 {
1017         show_rcu_gp_kthreads();
1018 }
1019
1020 static const struct sysrq_key_op sysrq_rcudump_op = {
1021         .handler = sysrq_show_rcu,
1022         .help_msg = "show-rcu(y)",
1023         .action_msg = "Show RCU tree",
1024         .enable_mask = SYSRQ_ENABLE_DUMP,
1025 };
1026
1027 static int __init rcu_sysrq_init(void)
1028 {
1029         if (sysrq_rcu)
1030                 return register_sysrq_key('y', &sysrq_rcudump_op);
1031         return 0;
1032 }
1033 early_initcall(rcu_sysrq_init);