1/* SPDX-License-Identifier: GPL-2.0+ */
   2/*
   3 * RCU expedited grace periods
   4 *
   5 * Copyright IBM Corporation, 2016
   6 *
   7 * Authors: Paul E. McKenney <paulmck@linux.ibm.com>
   8 */
   9
  10#include <linux/lockdep.h>
  11
  12static void rcu_exp_handler(void *unused);
  13static int rcu_print_task_exp_stall(struct rcu_node *rnp);
  14static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp);
  15
  16/*
  17 * Record the start of an expedited grace period.
  18 */
  19static void rcu_exp_gp_seq_start(void)
  20{
  21	rcu_seq_start(&rcu_state.expedited_sequence);
  22	rcu_poll_gp_seq_start_unlocked(&rcu_state.gp_seq_polled_exp_snap);
  23}
  24
  25/*
  26 * Return the value that the expedited-grace-period counter will have
  27 * at the end of the current grace period.
  28 */
  29static __maybe_unused unsigned long rcu_exp_gp_seq_endval(void)
  30{
  31	return rcu_seq_endval(&rcu_state.expedited_sequence);
  32}
  33
  34/*
  35 * Record the end of an expedited grace period.
  36 */
  37static void rcu_exp_gp_seq_end(void)
  38{
  39	rcu_poll_gp_seq_end_unlocked(&rcu_state.gp_seq_polled_exp_snap);
  40	rcu_seq_end(&rcu_state.expedited_sequence);
  41	smp_mb(); /* Ensure that consecutive grace periods serialize. */
  42}
  43
  44/*
  45 * Take a snapshot of the expedited-grace-period counter, which is the
  46 * earliest value that will indicate that a full grace period has
  47 * elapsed since the current time.
  48 */
  49static unsigned long rcu_exp_gp_seq_snap(void)
  50{
  51	unsigned long s;
  52
  53	smp_mb(); /* Caller's modifications seen first by other CPUs. */
  54	s = rcu_seq_snap(&rcu_state.expedited_sequence);
  55	trace_rcu_exp_grace_period(rcu_state.name, s, TPS("snap"));
  56	return s;
  57}
  58
  59/*
  60 * Given a counter snapshot from rcu_exp_gp_seq_snap(), return true
  61 * if a full expedited grace period has elapsed since that snapshot
  62 * was taken.
  63 */
  64static bool rcu_exp_gp_seq_done(unsigned long s)
  65{
  66	return rcu_seq_done(&rcu_state.expedited_sequence, s);
  67}
  68
  69/*
  70 * Reset the ->expmaskinit values in the rcu_node tree to reflect any
  71 * recent CPU-online activity.  Note that these masks are not cleared
  72 * when CPUs go offline, so they reflect the union of all CPUs that have
  73 * ever been online.  This means that this function normally takes its
  74 * no-work-to-do fastpath.
  75 */
  76static void sync_exp_reset_tree_hotplug(void)
  77{
  78	bool done;
  79	unsigned long flags;
  80	unsigned long mask;
  81	unsigned long oldmask;
  82	int ncpus = smp_load_acquire(&rcu_state.ncpus); /* Order vs. locking. */
  83	struct rcu_node *rnp;
  84	struct rcu_node *rnp_up;
  85
  86	/* If no new CPUs onlined since last time, nothing to do. */
  87	if (likely(ncpus == rcu_state.ncpus_snap))
  88		return;
  89	rcu_state.ncpus_snap = ncpus;
  90
  91	/*
  92	 * Each pass through the following loop propagates newly onlined
  93	 * CPUs for the current rcu_node structure up the rcu_node tree.
  94	 */
  95	rcu_for_each_leaf_node(rnp) {
  96		raw_spin_lock_irqsave_rcu_node(rnp, flags);
  97		if (rnp->expmaskinit == rnp->expmaskinitnext) {
  98			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  99			continue;  /* No new CPUs, nothing to do. */
 100		}
 101
 102		/* Update this node's mask, track old value for propagation. */
 103		oldmask = rnp->expmaskinit;
 104		rnp->expmaskinit = rnp->expmaskinitnext;
 105		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 106
 107		/* If was already nonzero, nothing to propagate. */
 108		if (oldmask)
 109			continue;
 110
 111		/* Propagate the new CPU up the tree. */
 112		mask = rnp->grpmask;
 113		rnp_up = rnp->parent;
 114		done = false;
 115		while (rnp_up) {
 116			raw_spin_lock_irqsave_rcu_node(rnp_up, flags);
 117			if (rnp_up->expmaskinit)
 118				done = true;
 119			rnp_up->expmaskinit |= mask;
 120			raw_spin_unlock_irqrestore_rcu_node(rnp_up, flags);
 121			if (done)
 122				break;
 123			mask = rnp_up->grpmask;
 124			rnp_up = rnp_up->parent;
 125		}
 126	}
 127}
 128
 129/*
 130 * Reset the ->expmask values in the rcu_node tree in preparation for
 131 * a new expedited grace period.
 132 */
 133static void __maybe_unused sync_exp_reset_tree(void)
 134{
 135	unsigned long flags;
 136	struct rcu_node *rnp;
 137
 138	sync_exp_reset_tree_hotplug();
 139	rcu_for_each_node_breadth_first(rnp) {
 140		raw_spin_lock_irqsave_rcu_node(rnp, flags);
 141		WARN_ON_ONCE(rnp->expmask);
 142		WRITE_ONCE(rnp->expmask, rnp->expmaskinit);
 143		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 144	}
 145}
 146
 147/*
 148 * Return non-zero if there is no RCU expedited grace period in progress
 149 * for the specified rcu_node structure, in other words, if all CPUs and
 150 * tasks covered by the specified rcu_node structure have done their bit
 151 * for the current expedited grace period.
 152 */
 153static bool sync_rcu_exp_done(struct rcu_node *rnp)
 154{
 155	raw_lockdep_assert_held_rcu_node(rnp);
 156	return READ_ONCE(rnp->exp_tasks) == NULL &&
 157	       READ_ONCE(rnp->expmask) == 0;
 158}
 159
 160/*
 161 * Like sync_rcu_exp_done(), but where the caller does not hold the
 162 * rcu_node's ->lock.
 163 */
 164static bool sync_rcu_exp_done_unlocked(struct rcu_node *rnp)
 165{
 166	unsigned long flags;
 167	bool ret;
 168
 169	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 170	ret = sync_rcu_exp_done(rnp);
 171	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 172
 173	return ret;
 174}
 175
 
 176/*
 177 * Report the exit from RCU read-side critical section for the last task
 178 * that queued itself during or before the current expedited preemptible-RCU
 179 * grace period.  This event is reported either to the rcu_node structure on
 180 * which the task was queued or to one of that rcu_node structure's ancestors,
 181 * recursively up the tree.  (Calm down, calm down, we do the recursion
 182 * iteratively!)
 183 */
 184static void __rcu_report_exp_rnp(struct rcu_node *rnp,
 185				 bool wake, unsigned long flags)
 186	__releases(rnp->lock)
 187{
 188	unsigned long mask;
 189
 190	raw_lockdep_assert_held_rcu_node(rnp);
 191	for (;;) {
 192		if (!sync_rcu_exp_done(rnp)) {
 193			if (!rnp->expmask)
 194				rcu_initiate_boost(rnp, flags);
 195			else
 196				raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 197			break;
 198		}
 199		if (rnp->parent == NULL) {
 200			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 201			if (wake)
 202				swake_up_one_online(&rcu_state.expedited_wq);
 203
 
 204			break;
 205		}
 206		mask = rnp->grpmask;
 207		raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled */
 208		rnp = rnp->parent;
 209		raw_spin_lock_rcu_node(rnp); /* irqs already disabled */
 210		WARN_ON_ONCE(!(rnp->expmask & mask));
 211		WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask);
 212	}
 213}
 214
 215/*
 216 * Report expedited quiescent state for specified node.  This is a
 217 * lock-acquisition wrapper function for __rcu_report_exp_rnp().
 218 */
 219static void __maybe_unused rcu_report_exp_rnp(struct rcu_node *rnp, bool wake)
 220{
 221	unsigned long flags;
 222
 223	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 224	__rcu_report_exp_rnp(rnp, wake, flags);
 225}
 226
 227/*
 228 * Report expedited quiescent state for multiple CPUs, all covered by the
 229 * specified leaf rcu_node structure.
 230 */
 231static void rcu_report_exp_cpu_mult(struct rcu_node *rnp,
 232				    unsigned long mask, bool wake)
 233{
 234	int cpu;
 235	unsigned long flags;
 236	struct rcu_data *rdp;
 237
 238	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 239	if (!(rnp->expmask & mask)) {
 240		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 241		return;
 242	}
 243	WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask);
 244	for_each_leaf_node_cpu_mask(rnp, cpu, mask) {
 245		rdp = per_cpu_ptr(&rcu_data, cpu);
 246		if (!IS_ENABLED(CONFIG_NO_HZ_FULL) || !rdp->rcu_forced_tick_exp)
 247			continue;
 248		rdp->rcu_forced_tick_exp = false;
 249		tick_dep_clear_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
 250	}
 251	__rcu_report_exp_rnp(rnp, wake, flags); /* Releases rnp->lock. */
 252}
 253
 254/*
 255 * Report expedited quiescent state for specified rcu_data (CPU).
 256 */
 257static void rcu_report_exp_rdp(struct rcu_data *rdp)
 258{
 259	WRITE_ONCE(rdp->cpu_no_qs.b.exp, false);
 260	rcu_report_exp_cpu_mult(rdp->mynode, rdp->grpmask, true);
 261}
 262
 263/* Common code for work-done checking. */
 264static bool sync_exp_work_done(unsigned long s)
 265{
 266	if (rcu_exp_gp_seq_done(s)) {
 267		trace_rcu_exp_grace_period(rcu_state.name, s, TPS("done"));
 268		smp_mb(); /* Ensure test happens before caller kfree(). */
 269		return true;
 270	}
 271	return false;
 272}
 273
 274/*
 275 * Funnel-lock acquisition for expedited grace periods.  Returns true
 276 * if some other task completed an expedited grace period that this task
 277 * can piggy-back on, and with no mutex held.  Otherwise, returns false
 278 * with the mutex held, indicating that the caller must actually do the
 279 * expedited grace period.
 280 */
 281static bool exp_funnel_lock(unsigned long s)
 282{
 283	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, raw_smp_processor_id());
 284	struct rcu_node *rnp = rdp->mynode;
 285	struct rcu_node *rnp_root = rcu_get_root();
 286
 287	/* Low-contention fastpath. */
 288	if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s) &&
 289	    (rnp == rnp_root ||
 290	     ULONG_CMP_LT(READ_ONCE(rnp_root->exp_seq_rq), s)) &&
 291	    mutex_trylock(&rcu_state.exp_mutex))
 292		goto fastpath;
 293
 294	/*
 295	 * Each pass through the following loop works its way up
 296	 * the rcu_node tree, returning if others have done the work or
 297	 * otherwise falls through to acquire ->exp_mutex.  The mapping
 298	 * from CPU to rcu_node structure can be inexact, as it is just
 299	 * promoting locality and is not strictly needed for correctness.
 300	 */
 301	for (; rnp != NULL; rnp = rnp->parent) {
 302		if (sync_exp_work_done(s))
 303			return true;
 304
 305		/* Work not done, either wait here or go up. */
 306		spin_lock(&rnp->exp_lock);
 307		if (ULONG_CMP_GE(rnp->exp_seq_rq, s)) {
 308
 309			/* Someone else doing GP, so wait for them. */
 310			spin_unlock(&rnp->exp_lock);
 311			trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
 312						  rnp->grplo, rnp->grphi,
 313						  TPS("wait"));
 314			wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
 315				   sync_exp_work_done(s));
 316			return true;
 317		}
 318		WRITE_ONCE(rnp->exp_seq_rq, s); /* Followers can wait on us. */
 319		spin_unlock(&rnp->exp_lock);
 320		trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
 321					  rnp->grplo, rnp->grphi, TPS("nxtlvl"));
 322	}
 323	mutex_lock(&rcu_state.exp_mutex);
 324fastpath:
 325	if (sync_exp_work_done(s)) {
 326		mutex_unlock(&rcu_state.exp_mutex);
 327		return true;
 328	}
 329	rcu_exp_gp_seq_start();
 330	trace_rcu_exp_grace_period(rcu_state.name, s, TPS("start"));
 331	return false;
 332}
 333
 334/*
 335 * Select the CPUs within the specified rcu_node that the upcoming
 336 * expedited grace period needs to wait for.
 337 */
 338static void __sync_rcu_exp_select_node_cpus(struct rcu_exp_work *rewp)
 339{
 340	int cpu;
 341	unsigned long flags;
 342	unsigned long mask_ofl_test;
 343	unsigned long mask_ofl_ipi;
 344	int ret;
 
 
 345	struct rcu_node *rnp = container_of(rewp, struct rcu_node, rew);
 346
 347	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 348
 349	/* Each pass checks a CPU for identity, offline, and idle. */
 350	mask_ofl_test = 0;
 351	for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) {
 352		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 353		unsigned long mask = rdp->grpmask;
 354		int snap;
 355
 356		if (raw_smp_processor_id() == cpu ||
 357		    !(rnp->qsmaskinitnext & mask)) {
 358			mask_ofl_test |= mask;
 359		} else {
 360			snap = rcu_dynticks_snap(cpu);
 361			if (rcu_dynticks_in_eqs(snap))
 362				mask_ofl_test |= mask;
 363			else
 364				rdp->exp_dynticks_snap = snap;
 365		}
 366	}
 367	mask_ofl_ipi = rnp->expmask & ~mask_ofl_test;
 368
 369	/*
 370	 * Need to wait for any blocked tasks as well.	Note that
 371	 * additional blocking tasks will also block the expedited GP
 372	 * until such time as the ->expmask bits are cleared.
 373	 */
 374	if (rcu_preempt_has_tasks(rnp))
 375		WRITE_ONCE(rnp->exp_tasks, rnp->blkd_tasks.next);
 376	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 377
 378	/* IPI the remaining CPUs for expedited quiescent state. */
 379	for_each_leaf_node_cpu_mask(rnp, cpu, mask_ofl_ipi) {
 380		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 381		unsigned long mask = rdp->grpmask;
 382
 383retry_ipi:
 384		if (rcu_dynticks_in_eqs_since(rdp, rdp->exp_dynticks_snap)) {
 385			mask_ofl_test |= mask;
 386			continue;
 387		}
 388		if (get_cpu() == cpu) {
 389			mask_ofl_test |= mask;
 390			put_cpu();
 391			continue;
 392		}
 393		ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
 394		put_cpu();
 395		/* The CPU will report the QS in response to the IPI. */
 396		if (!ret)
 397			continue;
 398
 399		/* Failed, raced with CPU hotplug operation. */
 400		raw_spin_lock_irqsave_rcu_node(rnp, flags);
 401		if ((rnp->qsmaskinitnext & mask) &&
 402		    (rnp->expmask & mask)) {
 403			/* Online, so delay for a bit and try again. */
 404			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 405			trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("selectofl"));
 406			schedule_timeout_idle(1);
 407			goto retry_ipi;
 408		}
 409		/* CPU really is offline, so we must report its QS. */
 410		if (rnp->expmask & mask)
 411			mask_ofl_test |= mask;
 412		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 413	}
 414	/* Report quiescent states for those that went offline. */
 415	if (mask_ofl_test)
 416		rcu_report_exp_cpu_mult(rnp, mask_ofl_test, false);
 417}
 418
 419static void rcu_exp_sel_wait_wake(unsigned long s);
 420
 421static void sync_rcu_exp_select_node_cpus(struct kthread_work *wp)
 422{
 423	struct rcu_exp_work *rewp =
 424		container_of(wp, struct rcu_exp_work, rew_work);
 425
 426	__sync_rcu_exp_select_node_cpus(rewp);
 427}
 428
 429static inline bool rcu_exp_worker_started(void)
 430{
 431	return !!READ_ONCE(rcu_exp_gp_kworker);
 432}
 433
 434static inline bool rcu_exp_par_worker_started(struct rcu_node *rnp)
 435{
 436	return !!READ_ONCE(rnp->exp_kworker);
 437}
 438
 439static inline void sync_rcu_exp_select_cpus_queue_work(struct rcu_node *rnp)
 440{
 441	kthread_init_work(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
 442	/*
 443	 * Use rcu_exp_par_gp_kworker, because flushing a work item from
 444	 * another work item on the same kthread worker can result in
 445	 * deadlock.
 446	 */
 447	kthread_queue_work(READ_ONCE(rnp->exp_kworker), &rnp->rew.rew_work);
 448}
 449
 450static inline void sync_rcu_exp_select_cpus_flush_work(struct rcu_node *rnp)
 451{
 452	kthread_flush_work(&rnp->rew.rew_work);
 453}
 454
 455/*
 456 * Work-queue handler to drive an expedited grace period forward.
 457 */
 458static void wait_rcu_exp_gp(struct kthread_work *wp)
 459{
 460	struct rcu_exp_work *rewp;
 461
 462	rewp = container_of(wp, struct rcu_exp_work, rew_work);
 463	rcu_exp_sel_wait_wake(rewp->rew_s);
 464}
 465
 466static inline void synchronize_rcu_expedited_queue_work(struct rcu_exp_work *rew)
 467{
 468	kthread_init_work(&rew->rew_work, wait_rcu_exp_gp);
 469	kthread_queue_work(rcu_exp_gp_kworker, &rew->rew_work);
 470}
 471
 472/*
 473 * Select the nodes that the upcoming expedited grace period needs
 474 * to wait for.
 475 */
 476static void sync_rcu_exp_select_cpus(void)
 477{
 
 478	struct rcu_node *rnp;
 479
 480	trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("reset"));
 481	sync_exp_reset_tree();
 482	trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("select"));
 483
 484	/* Schedule work for each leaf rcu_node structure. */
 485	rcu_for_each_leaf_node(rnp) {
 486		rnp->exp_need_flush = false;
 487		if (!READ_ONCE(rnp->expmask))
 488			continue; /* Avoid early boot non-existent wq. */
 489		if (!rcu_exp_par_worker_started(rnp) ||
 490		    rcu_scheduler_active != RCU_SCHEDULER_RUNNING ||
 491		    rcu_is_last_leaf_node(rnp)) {
 492			/* No worker started yet or last leaf, do direct call. */
 493			sync_rcu_exp_select_node_cpus(&rnp->rew.rew_work);
 494			continue;
 495		}
 496		sync_rcu_exp_select_cpus_queue_work(rnp);
 
 
 
 
 
 
 
 497		rnp->exp_need_flush = true;
 498	}
 499
 500	/* Wait for jobs (if any) to complete. */
 501	rcu_for_each_leaf_node(rnp)
 502		if (rnp->exp_need_flush)
 503			sync_rcu_exp_select_cpus_flush_work(rnp);
 504}
 505
 506/*
 507 * Wait for the expedited grace period to elapse, within time limit.
 508 * If the time limit is exceeded without the grace period elapsing,
 509 * return false, otherwise return true.
 510 */
 511static bool synchronize_rcu_expedited_wait_once(long tlimit)
 512{
 513	int t;
 514	struct rcu_node *rnp_root = rcu_get_root();
 515
 516	t = swait_event_timeout_exclusive(rcu_state.expedited_wq,
 517					  sync_rcu_exp_done_unlocked(rnp_root),
 518					  tlimit);
 519	// Workqueues should not be signaled.
 520	if (t > 0 || sync_rcu_exp_done_unlocked(rnp_root))
 521		return true;
 522	WARN_ON(t < 0);  /* workqueues should not be signaled. */
 523	return false;
 524}
 525
 526/*
 527 * Wait for the expedited grace period to elapse, issuing any needed
 528 * RCU CPU stall warnings along the way.
 529 */
 530static void synchronize_rcu_expedited_wait(void)
 531{
 532	int cpu;
 533	unsigned long j;
 534	unsigned long jiffies_stall;
 535	unsigned long jiffies_start;
 536	unsigned long mask;
 537	int ndetected;
 538	struct rcu_data *rdp;
 539	struct rcu_node *rnp;
 540	struct rcu_node *rnp_root = rcu_get_root();
 541	unsigned long flags;
 542
 543	trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("startwait"));
 544	jiffies_stall = rcu_exp_jiffies_till_stall_check();
 545	jiffies_start = jiffies;
 546	if (tick_nohz_full_enabled() && rcu_inkernel_boot_has_ended()) {
 547		if (synchronize_rcu_expedited_wait_once(1))
 548			return;
 549		rcu_for_each_leaf_node(rnp) {
 550			raw_spin_lock_irqsave_rcu_node(rnp, flags);
 551			mask = READ_ONCE(rnp->expmask);
 552			for_each_leaf_node_cpu_mask(rnp, cpu, mask) {
 553				rdp = per_cpu_ptr(&rcu_data, cpu);
 554				if (rdp->rcu_forced_tick_exp)
 555					continue;
 556				rdp->rcu_forced_tick_exp = true;
 557				if (cpu_online(cpu))
 558					tick_dep_set_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
 559			}
 560			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 561		}
 562		j = READ_ONCE(jiffies_till_first_fqs);
 563		if (synchronize_rcu_expedited_wait_once(j + HZ))
 564			return;
 
 565	}
 566
 567	for (;;) {
 568		unsigned long j;
 569
 570		if (synchronize_rcu_expedited_wait_once(jiffies_stall))
 571			return;
 572		if (rcu_stall_is_suppressed())
 573			continue;
 574		j = jiffies;
 575		rcu_stall_notifier_call_chain(RCU_STALL_NOTIFY_EXP, (void *)(j - jiffies_start));
 576		trace_rcu_stall_warning(rcu_state.name, TPS("ExpeditedStall"));
 577		pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {",
 578		       rcu_state.name);
 579		ndetected = 0;
 580		rcu_for_each_leaf_node(rnp) {
 581			ndetected += rcu_print_task_exp_stall(rnp);
 582			for_each_leaf_node_possible_cpu(rnp, cpu) {
 583				struct rcu_data *rdp;
 584
 585				mask = leaf_node_cpu_bit(rnp, cpu);
 586				if (!(READ_ONCE(rnp->expmask) & mask))
 587					continue;
 588				ndetected++;
 589				rdp = per_cpu_ptr(&rcu_data, cpu);
 590				pr_cont(" %d-%c%c%c%c", cpu,
 591					"O."[!!cpu_online(cpu)],
 592					"o."[!!(rdp->grpmask & rnp->expmaskinit)],
 593					"N."[!!(rdp->grpmask & rnp->expmaskinitnext)],
 594					"D."[!!data_race(rdp->cpu_no_qs.b.exp)]);
 595			}
 596		}
 597		pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
 598			j - jiffies_start, rcu_state.expedited_sequence,
 599			data_race(rnp_root->expmask),
 600			".T"[!!data_race(rnp_root->exp_tasks)]);
 601		if (ndetected) {
 602			pr_err("blocking rcu_node structures (internal RCU debug):");
 603			rcu_for_each_node_breadth_first(rnp) {
 604				if (rnp == rnp_root)
 605					continue; /* printed unconditionally */
 606				if (sync_rcu_exp_done_unlocked(rnp))
 607					continue;
 608				pr_cont(" l=%u:%d-%d:%#lx/%c",
 609					rnp->level, rnp->grplo, rnp->grphi,
 610					data_race(rnp->expmask),
 611					".T"[!!data_race(rnp->exp_tasks)]);
 612			}
 613			pr_cont("\n");
 614		}
 615		rcu_for_each_leaf_node(rnp) {
 616			for_each_leaf_node_possible_cpu(rnp, cpu) {
 617				mask = leaf_node_cpu_bit(rnp, cpu);
 618				if (!(READ_ONCE(rnp->expmask) & mask))
 619					continue;
 620				preempt_disable(); // For smp_processor_id() in dump_cpu_task().
 621				dump_cpu_task(cpu);
 622				preempt_enable();
 623			}
 624			rcu_exp_print_detail_task_stall_rnp(rnp);
 625		}
 626		jiffies_stall = 3 * rcu_exp_jiffies_till_stall_check() + 3;
 627		panic_on_rcu_stall();
 628	}
 629}
 630
 631/*
 632 * Wait for the current expedited grace period to complete, and then
 633 * wake up everyone who piggybacked on the just-completed expedited
 634 * grace period.  Also update all the ->exp_seq_rq counters as needed
 635 * in order to avoid counter-wrap problems.
 636 */
 637static void rcu_exp_wait_wake(unsigned long s)
 638{
 639	struct rcu_node *rnp;
 640
 641	synchronize_rcu_expedited_wait();
 642
 643	// Switch over to wakeup mode, allowing the next GP to proceed.
 644	// End the previous grace period only after acquiring the mutex
 645	// to ensure that only one GP runs concurrently with wakeups.
 646	mutex_lock(&rcu_state.exp_wake_mutex);
 647	rcu_exp_gp_seq_end();
 648	trace_rcu_exp_grace_period(rcu_state.name, s, TPS("end"));
 649
 650	rcu_for_each_node_breadth_first(rnp) {
 651		if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s)) {
 652			spin_lock(&rnp->exp_lock);
 653			/* Recheck, avoid hang in case someone just arrived. */
 654			if (ULONG_CMP_LT(rnp->exp_seq_rq, s))
 655				WRITE_ONCE(rnp->exp_seq_rq, s);
 656			spin_unlock(&rnp->exp_lock);
 657		}
 658		smp_mb(); /* All above changes before wakeup. */
 659		wake_up_all(&rnp->exp_wq[rcu_seq_ctr(s) & 0x3]);
 660	}
 661	trace_rcu_exp_grace_period(rcu_state.name, s, TPS("endwake"));
 662	mutex_unlock(&rcu_state.exp_wake_mutex);
 663}
 664
 665/*
 666 * Common code to drive an expedited grace period forward, used by
 667 * workqueues and mid-boot-time tasks.
 668 */
 669static void rcu_exp_sel_wait_wake(unsigned long s)
 670{
 671	/* Initialize the rcu_node tree in preparation for the wait. */
 672	sync_rcu_exp_select_cpus();
 673
 674	/* Wait and clean up, including waking everyone. */
 675	rcu_exp_wait_wake(s);
 676}
 677
 
 
 
 
 
 
 
 
 
 
 
 678#ifdef CONFIG_PREEMPT_RCU
 679
 680/*
 681 * Remote handler for smp_call_function_single().  If there is an
 682 * RCU read-side critical section in effect, request that the
 683 * next rcu_read_unlock() record the quiescent state up the
 684 * ->expmask fields in the rcu_node tree.  Otherwise, immediately
 685 * report the quiescent state.
 686 */
 687static void rcu_exp_handler(void *unused)
 688{
 689	int depth = rcu_preempt_depth();
 690	unsigned long flags;
 691	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 692	struct rcu_node *rnp = rdp->mynode;
 693	struct task_struct *t = current;
 694
 695	/*
 696	 * First, the common case of not being in an RCU read-side
 697	 * critical section.  If also enabled or idle, immediately
 698	 * report the quiescent state, otherwise defer.
 699	 */
 700	if (!depth) {
 701		if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
 702		    rcu_is_cpu_rrupt_from_idle()) {
 703			rcu_report_exp_rdp(rdp);
 704		} else {
 705			WRITE_ONCE(rdp->cpu_no_qs.b.exp, true);
 706			set_tsk_need_resched(t);
 707			set_preempt_need_resched();
 708		}
 709		return;
 710	}
 711
 712	/*
 713	 * Second, the less-common case of being in an RCU read-side
 714	 * critical section.  In this case we can count on a future
 715	 * rcu_read_unlock().  However, this rcu_read_unlock() might
 716	 * execute on some other CPU, but in that case there will be
 717	 * a future context switch.  Either way, if the expedited
 718	 * grace period is still waiting on this CPU, set ->deferred_qs
 719	 * so that the eventual quiescent state will be reported.
 720	 * Note that there is a large group of race conditions that
 721	 * can have caused this quiescent state to already have been
 722	 * reported, so we really do need to check ->expmask.
 723	 */
 724	if (depth > 0) {
 725		raw_spin_lock_irqsave_rcu_node(rnp, flags);
 726		if (rnp->expmask & rdp->grpmask) {
 727			WRITE_ONCE(rdp->cpu_no_qs.b.exp, true);
 728			t->rcu_read_unlock_special.b.exp_hint = true;
 729		}
 730		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 731		return;
 732	}
 733
 734	// Finally, negative nesting depth should not happen.
 735	WARN_ON_ONCE(1);
 736}
 737
 738/* PREEMPTION=y, so no PREEMPTION=n expedited grace period to clean up after. */
 739static void sync_sched_exp_online_cleanup(int cpu)
 740{
 741}
 742
 743/*
 744 * Scan the current list of tasks blocked within RCU read-side critical
 745 * sections, printing out the tid of each that is blocking the current
 746 * expedited grace period.
 747 */
 748static int rcu_print_task_exp_stall(struct rcu_node *rnp)
 749{
 750	unsigned long flags;
 751	int ndetected = 0;
 752	struct task_struct *t;
 753
 754	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 755	if (!rnp->exp_tasks) {
 756		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 757		return 0;
 758	}
 759	t = list_entry(rnp->exp_tasks->prev,
 760		       struct task_struct, rcu_node_entry);
 761	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
 762		pr_cont(" P%d", t->pid);
 763		ndetected++;
 764	}
 765	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 766	return ndetected;
 767}
 768
 769/*
 770 * Scan the current list of tasks blocked within RCU read-side critical
 771 * sections, dumping the stack of each that is blocking the current
 772 * expedited grace period.
 773 */
 774static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp)
 775{
 776	unsigned long flags;
 777	struct task_struct *t;
 778
 779	if (!rcu_exp_stall_task_details)
 780		return;
 781	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 782	if (!READ_ONCE(rnp->exp_tasks)) {
 783		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 784		return;
 785	}
 786	t = list_entry(rnp->exp_tasks->prev,
 787		       struct task_struct, rcu_node_entry);
 788	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
 789		/*
 790		 * We could be printing a lot while holding a spinlock.
 791		 * Avoid triggering hard lockup.
 792		 */
 793		touch_nmi_watchdog();
 794		sched_show_task(t);
 795	}
 796	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 797}
 798
 799#else /* #ifdef CONFIG_PREEMPT_RCU */
 800
 801/* Request an expedited quiescent state. */
 802static void rcu_exp_need_qs(void)
 803{
 804	__this_cpu_write(rcu_data.cpu_no_qs.b.exp, true);
 805	/* Store .exp before .rcu_urgent_qs. */
 806	smp_store_release(this_cpu_ptr(&rcu_data.rcu_urgent_qs), true);
 807	set_tsk_need_resched(current);
 808	set_preempt_need_resched();
 809}
 810
 811/* Invoked on each online non-idle CPU for expedited quiescent state. */
 812static void rcu_exp_handler(void *unused)
 813{
 814	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 815	struct rcu_node *rnp = rdp->mynode;
 816	bool preempt_bh_enabled = !(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));
 817
 818	if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
 819	    __this_cpu_read(rcu_data.cpu_no_qs.b.exp))
 820		return;
 821	if (rcu_is_cpu_rrupt_from_idle() ||
 822	    (IS_ENABLED(CONFIG_PREEMPT_COUNT) && preempt_bh_enabled)) {
 823		rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));
 824		return;
 825	}
 826	rcu_exp_need_qs();
 827}
 828
 829/* Send IPI for expedited cleanup if needed at end of CPU-hotplug operation. */
 830static void sync_sched_exp_online_cleanup(int cpu)
 831{
 832	unsigned long flags;
 833	int my_cpu;
 834	struct rcu_data *rdp;
 835	int ret;
 836	struct rcu_node *rnp;
 837
 838	rdp = per_cpu_ptr(&rcu_data, cpu);
 839	rnp = rdp->mynode;
 840	my_cpu = get_cpu();
 841	/* Quiescent state either not needed or already requested, leave. */
 842	if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
 843	    READ_ONCE(rdp->cpu_no_qs.b.exp)) {
 844		put_cpu();
 845		return;
 846	}
 847	/* Quiescent state needed on current CPU, so set it up locally. */
 848	if (my_cpu == cpu) {
 849		local_irq_save(flags);
 850		rcu_exp_need_qs();
 851		local_irq_restore(flags);
 852		put_cpu();
 853		return;
 854	}
 855	/* Quiescent state needed on some other CPU, send IPI. */
 856	ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
 857	put_cpu();
 858	WARN_ON_ONCE(ret);
 859}
 860
 861/*
 862 * Because preemptible RCU does not exist, we never have to check for
 863 * tasks blocked within RCU read-side critical sections that are
 864 * blocking the current expedited grace period.
 865 */
 866static int rcu_print_task_exp_stall(struct rcu_node *rnp)
 867{
 868	return 0;
 869}
 870
 871/*
 872 * Because preemptible RCU does not exist, we never have to print out
 873 * tasks blocked within RCU read-side critical sections that are blocking
 874 * the current expedited grace period.
 875 */
 876static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp)
 877{
 878}
 879
 880#endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 881
 882/**
 883 * synchronize_rcu_expedited - Brute-force RCU grace period
 884 *
 885 * Wait for an RCU grace period, but expedite it.  The basic idea is to
 886 * IPI all non-idle non-nohz online CPUs.  The IPI handler checks whether
 887 * the CPU is in an RCU critical section, and if so, it sets a flag that
 888 * causes the outermost rcu_read_unlock() to report the quiescent state
 889 * for RCU-preempt or asks the scheduler for help for RCU-sched.  On the
 890 * other hand, if the CPU is not in an RCU read-side critical section,
 891 * the IPI handler reports the quiescent state immediately.
 892 *
 893 * Although this is a great improvement over previous expedited
 894 * implementations, it is still unfriendly to real-time workloads, so is
 895 * thus not recommended for any sort of common-case code.  In fact, if
 896 * you are using synchronize_rcu_expedited() in a loop, please restructure
 897 * your code to batch your updates, and then use a single synchronize_rcu()
 898 * instead.
 899 *
 900 * This has the same semantics as (but is more brutal than) synchronize_rcu().
 901 */
 902void synchronize_rcu_expedited(void)
 903{
 904	unsigned long flags;
 905	struct rcu_exp_work rew;
 906	struct rcu_node *rnp;
 907	unsigned long s;
 908
 909	RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
 910			 lock_is_held(&rcu_lock_map) ||
 911			 lock_is_held(&rcu_sched_lock_map),
 912			 "Illegal synchronize_rcu_expedited() in RCU read-side critical section");
 913
 914	/* Is the state is such that the call is a grace period? */
 915	if (rcu_blocking_is_gp()) {
 916		// Note well that this code runs with !PREEMPT && !SMP.
 917		// In addition, all code that advances grace periods runs
 918		// at process level.  Therefore, this expedited GP overlaps
 919		// with other expedited GPs only by being fully nested within
 920		// them, which allows reuse of ->gp_seq_polled_exp_snap.
 921		rcu_poll_gp_seq_start_unlocked(&rcu_state.gp_seq_polled_exp_snap);
 922		rcu_poll_gp_seq_end_unlocked(&rcu_state.gp_seq_polled_exp_snap);
 923
 924		local_irq_save(flags);
 925		WARN_ON_ONCE(num_online_cpus() > 1);
 926		rcu_state.expedited_sequence += (1 << RCU_SEQ_CTR_SHIFT);
 927		local_irq_restore(flags);
 928		return;  // Context allows vacuous grace periods.
 929	}
 930
 931	/* If expedited grace periods are prohibited, fall back to normal. */
 932	if (rcu_gp_is_normal()) {
 933		wait_rcu_gp(call_rcu_hurry);
 934		return;
 935	}
 936
 937	/* Take a snapshot of the sequence number.  */
 938	s = rcu_exp_gp_seq_snap();
 939	if (exp_funnel_lock(s))
 940		return;  /* Someone else did our work for us. */
 941
 942	/* Ensure that load happens before action based on it. */
 943	if (unlikely((rcu_scheduler_active == RCU_SCHEDULER_INIT) || !rcu_exp_worker_started())) {
 944		/* Direct call during scheduler init and early_initcalls(). */
 945		rcu_exp_sel_wait_wake(s);
 946	} else {
 947		/* Marshall arguments & schedule the expedited grace period. */
 948		rew.rew_s = s;
 949		synchronize_rcu_expedited_queue_work(&rew);
 
 950	}
 951
 952	/* Wait for expedited grace period to complete. */
 953	rnp = rcu_get_root();
 954	wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
 955		   sync_exp_work_done(s));
 956	smp_mb(); /* Work actions happen before return. */
 957
 958	/* Let the next expedited grace period start. */
 959	mutex_unlock(&rcu_state.exp_mutex);
 960}
 961EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
 962
 963/*
 964 * Ensure that start_poll_synchronize_rcu_expedited() has the expedited
 965 * RCU grace periods that it needs.
 966 */
 967static void sync_rcu_do_polled_gp(struct work_struct *wp)
 968{
 969	unsigned long flags;
 970	int i = 0;
 971	struct rcu_node *rnp = container_of(wp, struct rcu_node, exp_poll_wq);
 972	unsigned long s;
 973
 974	raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags);
 975	s = rnp->exp_seq_poll_rq;
 976	rnp->exp_seq_poll_rq = RCU_GET_STATE_COMPLETED;
 977	raw_spin_unlock_irqrestore(&rnp->exp_poll_lock, flags);
 978	if (s == RCU_GET_STATE_COMPLETED)
 979		return;
 980	while (!poll_state_synchronize_rcu(s)) {
 981		synchronize_rcu_expedited();
 982		if (i == 10 || i == 20)
 983			pr_info("%s: i = %d s = %lx gp_seq_polled = %lx\n", __func__, i, s, READ_ONCE(rcu_state.gp_seq_polled));
 984		i++;
 985	}
 986	raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags);
 987	s = rnp->exp_seq_poll_rq;
 988	if (poll_state_synchronize_rcu(s))
 989		rnp->exp_seq_poll_rq = RCU_GET_STATE_COMPLETED;
 990	raw_spin_unlock_irqrestore(&rnp->exp_poll_lock, flags);
 991}
 992
 993/**
 994 * start_poll_synchronize_rcu_expedited - Snapshot current RCU state and start expedited grace period
 995 *
 996 * Returns a cookie to pass to a call to cond_synchronize_rcu(),
 997 * cond_synchronize_rcu_expedited(), or poll_state_synchronize_rcu(),
 998 * allowing them to determine whether or not any sort of grace period has
 999 * elapsed in the meantime.  If the needed expedited grace period is not
1000 * already slated to start, initiates that grace period.
1001 */
1002unsigned long start_poll_synchronize_rcu_expedited(void)
1003{
1004	unsigned long flags;
1005	struct rcu_data *rdp;
1006	struct rcu_node *rnp;
1007	unsigned long s;
1008
1009	s = get_state_synchronize_rcu();
1010	rdp = per_cpu_ptr(&rcu_data, raw_smp_processor_id());
1011	rnp = rdp->mynode;
1012	if (rcu_init_invoked())
1013		raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags);
1014	if (!poll_state_synchronize_rcu(s)) {
1015		if (rcu_init_invoked()) {
1016			rnp->exp_seq_poll_rq = s;
1017			queue_work(rcu_gp_wq, &rnp->exp_poll_wq);
1018		}
1019	}
1020	if (rcu_init_invoked())
1021		raw_spin_unlock_irqrestore(&rnp->exp_poll_lock, flags);
1022
1023	return s;
 
1024}
1025EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_expedited);
1026
1027/**
1028 * start_poll_synchronize_rcu_expedited_full - Take a full snapshot and start expedited grace period
1029 * @rgosp: Place to put snapshot of grace-period state
1030 *
1031 * Places the normal and expedited grace-period states in rgosp.  This
1032 * state value can be passed to a later call to cond_synchronize_rcu_full()
1033 * or poll_state_synchronize_rcu_full() to determine whether or not a
1034 * grace period (whether normal or expedited) has elapsed in the meantime.
1035 * If the needed expedited grace period is not already slated to start,
1036 * initiates that grace period.
1037 */
1038void start_poll_synchronize_rcu_expedited_full(struct rcu_gp_oldstate *rgosp)
1039{
1040	get_state_synchronize_rcu_full(rgosp);
1041	(void)start_poll_synchronize_rcu_expedited();
1042}
1043EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_expedited_full);
1044
1045/**
1046 * cond_synchronize_rcu_expedited - Conditionally wait for an expedited RCU grace period
1047 *
1048 * @oldstate: value from get_state_synchronize_rcu(), start_poll_synchronize_rcu(), or start_poll_synchronize_rcu_expedited()
1049 *
1050 * If any type of full RCU grace period has elapsed since the earlier
1051 * call to get_state_synchronize_rcu(), start_poll_synchronize_rcu(),
1052 * or start_poll_synchronize_rcu_expedited(), just return.  Otherwise,
1053 * invoke synchronize_rcu_expedited() to wait for a full grace period.
1054 *
1055 * Yes, this function does not take counter wrap into account.
1056 * But counter wrap is harmless.  If the counter wraps, we have waited for
1057 * more than 2 billion grace periods (and way more on a 64-bit system!),
1058 * so waiting for a couple of additional grace periods should be just fine.
1059 *
1060 * This function provides the same memory-ordering guarantees that
1061 * would be provided by a synchronize_rcu() that was invoked at the call
1062 * to the function that provided @oldstate and that returned at the end
1063 * of this function.
1064 */
1065void cond_synchronize_rcu_expedited(unsigned long oldstate)
1066{
1067	if (!poll_state_synchronize_rcu(oldstate))
1068		synchronize_rcu_expedited();
1069}
1070EXPORT_SYMBOL_GPL(cond_synchronize_rcu_expedited);
1071
1072/**
1073 * cond_synchronize_rcu_expedited_full - Conditionally wait for an expedited RCU grace period
1074 * @rgosp: value from get_state_synchronize_rcu_full(), start_poll_synchronize_rcu_full(), or start_poll_synchronize_rcu_expedited_full()
1075 *
1076 * If a full RCU grace period has elapsed since the call to
1077 * get_state_synchronize_rcu_full(), start_poll_synchronize_rcu_full(),
1078 * or start_poll_synchronize_rcu_expedited_full() from which @rgosp was
1079 * obtained, just return.  Otherwise, invoke synchronize_rcu_expedited()
1080 * to wait for a full grace period.
1081 *
1082 * Yes, this function does not take counter wrap into account.
1083 * But counter wrap is harmless.  If the counter wraps, we have waited for
1084 * more than 2 billion grace periods (and way more on a 64-bit system!),
1085 * so waiting for a couple of additional grace periods should be just fine.
1086 *
1087 * This function provides the same memory-ordering guarantees that
1088 * would be provided by a synchronize_rcu() that was invoked at the call
1089 * to the function that provided @rgosp and that returned at the end of
1090 * this function.
1091 */
1092void cond_synchronize_rcu_expedited_full(struct rcu_gp_oldstate *rgosp)
1093{
1094	if (!poll_state_synchronize_rcu_full(rgosp))
1095		synchronize_rcu_expedited();
1096}
1097EXPORT_SYMBOL_GPL(cond_synchronize_rcu_expedited_full);