tree_plugin.h

   1/* SPDX-License-Identifier: GPL-2.0+ */
   2/*
   3 * Read-Copy Update mechanism for mutual exclusion (tree-based version)
   4 * Internal non-public definitions that provide either classic
   5 * or preemptible semantics.
   6 *
   7 * Copyright Red Hat, 2009
   8 * Copyright IBM Corporation, 2009
   9 *
  10 * Author: Ingo Molnar <mingo@elte.hu>
  11 *	   Paul E. McKenney <paulmck@linux.ibm.com>
  12 */
  13
  14#include "../locking/rtmutex_common.h"
  15
  16static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
  17{
  18	/*
  19	 * In order to read the offloaded state of an rdp in a safe
  20	 * and stable way and prevent from its value to be changed
  21	 * under us, we must either hold the barrier mutex, the cpu
  22	 * hotplug lock (read or write) or the nocb lock. Local
  23	 * non-preemptible reads are also safe. NOCB kthreads and
  24	 * timers have their own means of synchronization against the
  25	 * offloaded state updaters.
  26	 */
  27	RCU_NOCB_LOCKDEP_WARN(
  28		!(lockdep_is_held(&rcu_state.barrier_mutex) ||
  29		  (IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_held()) ||
  30		  lockdep_is_held(&rdp->nocb_lock) ||
  31		  lockdep_is_held(&rcu_state.nocb_mutex) ||
  32		  ((!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) || softirq_count()) &&
  33		   rdp == this_cpu_ptr(&rcu_data)) ||
  34		  rcu_current_is_nocb_kthread(rdp)),
  35		"Unsafe read of RCU_NOCB offloaded state"
  36	);
  37
  38	return rcu_segcblist_is_offloaded(&rdp->cblist);
  39}
  40
  41/*
  42 * Check the RCU kernel configuration parameters and print informative
  43 * messages about anything out of the ordinary.
  44 */
  45static void __init rcu_bootup_announce_oddness(void)
  46{
  47	if (IS_ENABLED(CONFIG_RCU_TRACE))
  48		pr_info("\tRCU event tracing is enabled.\n");
  49	if ((IS_ENABLED(CONFIG_64BIT) && RCU_FANOUT != 64) ||
  50	    (!IS_ENABLED(CONFIG_64BIT) && RCU_FANOUT != 32))
  51		pr_info("\tCONFIG_RCU_FANOUT set to non-default value of %d.\n",
  52			RCU_FANOUT);
  53	if (rcu_fanout_exact)
  54		pr_info("\tHierarchical RCU autobalancing is disabled.\n");
  55	if (IS_ENABLED(CONFIG_PROVE_RCU))
  56		pr_info("\tRCU lockdep checking is enabled.\n");
  57	if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
  58		pr_info("\tRCU strict (and thus non-scalable) grace periods are enabled.\n");
  59	if (RCU_NUM_LVLS >= 4)
  60		pr_info("\tFour(or more)-level hierarchy is enabled.\n");
  61	if (RCU_FANOUT_LEAF != 16)
  62		pr_info("\tBuild-time adjustment of leaf fanout to %d.\n",
  63			RCU_FANOUT_LEAF);
  64	if (rcu_fanout_leaf != RCU_FANOUT_LEAF)
  65		pr_info("\tBoot-time adjustment of leaf fanout to %d.\n",
  66			rcu_fanout_leaf);
  67	if (nr_cpu_ids != NR_CPUS)
  68		pr_info("\tRCU restricting CPUs from NR_CPUS=%d to nr_cpu_ids=%u.\n", NR_CPUS, nr_cpu_ids);
  69#ifdef CONFIG_RCU_BOOST
  70	pr_info("\tRCU priority boosting: priority %d delay %d ms.\n",
  71		kthread_prio, CONFIG_RCU_BOOST_DELAY);
  72#endif
  73	if (blimit != DEFAULT_RCU_BLIMIT)
  74		pr_info("\tBoot-time adjustment of callback invocation limit to %ld.\n", blimit);
  75	if (qhimark != DEFAULT_RCU_QHIMARK)
  76		pr_info("\tBoot-time adjustment of callback high-water mark to %ld.\n", qhimark);
  77	if (qlowmark != DEFAULT_RCU_QLOMARK)
  78		pr_info("\tBoot-time adjustment of callback low-water mark to %ld.\n", qlowmark);
  79	if (qovld != DEFAULT_RCU_QOVLD)
  80		pr_info("\tBoot-time adjustment of callback overload level to %ld.\n", qovld);
  81	if (jiffies_till_first_fqs != ULONG_MAX)
  82		pr_info("\tBoot-time adjustment of first FQS scan delay to %ld jiffies.\n", jiffies_till_first_fqs);
  83	if (jiffies_till_next_fqs != ULONG_MAX)
  84		pr_info("\tBoot-time adjustment of subsequent FQS scan delay to %ld jiffies.\n", jiffies_till_next_fqs);
  85	if (jiffies_till_sched_qs != ULONG_MAX)
  86		pr_info("\tBoot-time adjustment of scheduler-enlistment delay to %ld jiffies.\n", jiffies_till_sched_qs);
  87	if (rcu_kick_kthreads)
  88		pr_info("\tKick kthreads if too-long grace period.\n");
  89	if (IS_ENABLED(CONFIG_DEBUG_OBJECTS_RCU_HEAD))
  90		pr_info("\tRCU callback double-/use-after-free debug is enabled.\n");
  91	if (gp_preinit_delay)
  92		pr_info("\tRCU debug GP pre-init slowdown %d jiffies.\n", gp_preinit_delay);
  93	if (gp_init_delay)
  94		pr_info("\tRCU debug GP init slowdown %d jiffies.\n", gp_init_delay);
  95	if (gp_cleanup_delay)
  96		pr_info("\tRCU debug GP cleanup slowdown %d jiffies.\n", gp_cleanup_delay);
  97	if (nohz_full_patience_delay < 0) {
  98		pr_info("\tRCU NOCB CPU patience negative (%d), resetting to zero.\n", nohz_full_patience_delay);
  99		nohz_full_patience_delay = 0;
 100	} else if (nohz_full_patience_delay > 5 * MSEC_PER_SEC) {
 101		pr_info("\tRCU NOCB CPU patience too large (%d), resetting to %ld.\n", nohz_full_patience_delay, 5 * MSEC_PER_SEC);
 102		nohz_full_patience_delay = 5 * MSEC_PER_SEC;
 103	} else if (nohz_full_patience_delay) {
 104		pr_info("\tRCU NOCB CPU patience set to %d milliseconds.\n", nohz_full_patience_delay);
 105	}
 106	nohz_full_patience_delay_jiffies = msecs_to_jiffies(nohz_full_patience_delay);
 107	if (!use_softirq)
 108		pr_info("\tRCU_SOFTIRQ processing moved to rcuc kthreads.\n");
 109	if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG))
 110		pr_info("\tRCU debug extended QS entry/exit.\n");
 111	rcupdate_announce_bootup_oddness();
 112}
 113
 114#ifdef CONFIG_PREEMPT_RCU
 115
 116static void rcu_report_exp_rnp(struct rcu_node *rnp, bool wake);
 117static void rcu_read_unlock_special(struct task_struct *t);
 118
 119/*
 120 * Tell them what RCU they are running.
 121 */
 122static void __init rcu_bootup_announce(void)
 123{
 124	pr_info("Preemptible hierarchical RCU implementation.\n");
 125	rcu_bootup_announce_oddness();
 126}
 127
 128/* Flags for rcu_preempt_ctxt_queue() decision table. */
 129#define RCU_GP_TASKS	0x8
 130#define RCU_EXP_TASKS	0x4
 131#define RCU_GP_BLKD	0x2
 132#define RCU_EXP_BLKD	0x1
 133
 134/*
 135 * Queues a task preempted within an RCU-preempt read-side critical
 136 * section into the appropriate location within the ->blkd_tasks list,
 137 * depending on the states of any ongoing normal and expedited grace
 138 * periods.  The ->gp_tasks pointer indicates which element the normal
 139 * grace period is waiting on (NULL if none), and the ->exp_tasks pointer
 140 * indicates which element the expedited grace period is waiting on (again,
 141 * NULL if none).  If a grace period is waiting on a given element in the
 142 * ->blkd_tasks list, it also waits on all subsequent elements.  Thus,
 143 * adding a task to the tail of the list blocks any grace period that is
 144 * already waiting on one of the elements.  In contrast, adding a task
 145 * to the head of the list won't block any grace period that is already
 146 * waiting on one of the elements.
 147 *
 148 * This queuing is imprecise, and can sometimes make an ongoing grace
 149 * period wait for a task that is not strictly speaking blocking it.
 150 * Given the choice, we needlessly block a normal grace period rather than
 151 * blocking an expedited grace period.
 152 *
 153 * Note that an endless sequence of expedited grace periods still cannot
 154 * indefinitely postpone a normal grace period.  Eventually, all of the
 155 * fixed number of preempted tasks blocking the normal grace period that are
 156 * not also blocking the expedited grace period will resume and complete
 157 * their RCU read-side critical sections.  At that point, the ->gp_tasks
 158 * pointer will equal the ->exp_tasks pointer, at which point the end of
 159 * the corresponding expedited grace period will also be the end of the
 160 * normal grace period.
 161 */
 162static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp)
 163	__releases(rnp->lock) /* But leaves rrupts disabled. */
 164{
 165	int blkd_state = (rnp->gp_tasks ? RCU_GP_TASKS : 0) +
 166			 (rnp->exp_tasks ? RCU_EXP_TASKS : 0) +
 167			 (rnp->qsmask & rdp->grpmask ? RCU_GP_BLKD : 0) +
 168			 (rnp->expmask & rdp->grpmask ? RCU_EXP_BLKD : 0);
 169	struct task_struct *t = current;
 170
 171	raw_lockdep_assert_held_rcu_node(rnp);
 172	WARN_ON_ONCE(rdp->mynode != rnp);
 173	WARN_ON_ONCE(!rcu_is_leaf_node(rnp));
 174	/* RCU better not be waiting on newly onlined CPUs! */
 175	WARN_ON_ONCE(rnp->qsmaskinitnext & ~rnp->qsmaskinit & rnp->qsmask &
 176		     rdp->grpmask);
 177
 178	/*
 179	 * Decide where to queue the newly blocked task.  In theory,
 180	 * this could be an if-statement.  In practice, when I tried
 181	 * that, it was quite messy.
 182	 */
 183	switch (blkd_state) {
 184	case 0:
 185	case                RCU_EXP_TASKS:
 186	case                RCU_EXP_TASKS | RCU_GP_BLKD:
 187	case RCU_GP_TASKS:
 188	case RCU_GP_TASKS | RCU_EXP_TASKS:
 189
 190		/*
 191		 * Blocking neither GP, or first task blocking the normal
 192		 * GP but not blocking the already-waiting expedited GP.
 193		 * Queue at the head of the list to avoid unnecessarily
 194		 * blocking the already-waiting GPs.
 195		 */
 196		list_add(&t->rcu_node_entry, &rnp->blkd_tasks);
 197		break;
 198
 199	case                                              RCU_EXP_BLKD:
 200	case                                RCU_GP_BLKD:
 201	case                                RCU_GP_BLKD | RCU_EXP_BLKD:
 202	case RCU_GP_TASKS |                               RCU_EXP_BLKD:
 203	case RCU_GP_TASKS |                 RCU_GP_BLKD | RCU_EXP_BLKD:
 204	case RCU_GP_TASKS | RCU_EXP_TASKS | RCU_GP_BLKD | RCU_EXP_BLKD:
 205
 206		/*
 207		 * First task arriving that blocks either GP, or first task
 208		 * arriving that blocks the expedited GP (with the normal
 209		 * GP already waiting), or a task arriving that blocks
 210		 * both GPs with both GPs already waiting.  Queue at the
 211		 * tail of the list to avoid any GP waiting on any of the
 212		 * already queued tasks that are not blocking it.
 213		 */
 214		list_add_tail(&t->rcu_node_entry, &rnp->blkd_tasks);
 215		break;
 216
 217	case                RCU_EXP_TASKS |               RCU_EXP_BLKD:
 218	case                RCU_EXP_TASKS | RCU_GP_BLKD | RCU_EXP_BLKD:
 219	case RCU_GP_TASKS | RCU_EXP_TASKS |               RCU_EXP_BLKD:
 220
 221		/*
 222		 * Second or subsequent task blocking the expedited GP.
 223		 * The task either does not block the normal GP, or is the
 224		 * first task blocking the normal GP.  Queue just after
 225		 * the first task blocking the expedited GP.
 226		 */
 227		list_add(&t->rcu_node_entry, rnp->exp_tasks);
 228		break;
 229
 230	case RCU_GP_TASKS |                 RCU_GP_BLKD:
 231	case RCU_GP_TASKS | RCU_EXP_TASKS | RCU_GP_BLKD:
 232
 233		/*
 234		 * Second or subsequent task blocking the normal GP.
 235		 * The task does not block the expedited GP. Queue just
 236		 * after the first task blocking the normal GP.
 237		 */
 238		list_add(&t->rcu_node_entry, rnp->gp_tasks);
 239		break;
 240
 241	default:
 242
 243		/* Yet another exercise in excessive paranoia. */
 244		WARN_ON_ONCE(1);
 245		break;
 246	}
 247
 248	/*
 249	 * We have now queued the task.  If it was the first one to
 250	 * block either grace period, update the ->gp_tasks and/or
 251	 * ->exp_tasks pointers, respectively, to reference the newly
 252	 * blocked tasks.
 253	 */
 254	if (!rnp->gp_tasks && (blkd_state & RCU_GP_BLKD)) {
 255		WRITE_ONCE(rnp->gp_tasks, &t->rcu_node_entry);
 256		WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq);
 257	}
 258	if (!rnp->exp_tasks && (blkd_state & RCU_EXP_BLKD))
 259		WRITE_ONCE(rnp->exp_tasks, &t->rcu_node_entry);
 260	WARN_ON_ONCE(!(blkd_state & RCU_GP_BLKD) !=
 261		     !(rnp->qsmask & rdp->grpmask));
 262	WARN_ON_ONCE(!(blkd_state & RCU_EXP_BLKD) !=
 263		     !(rnp->expmask & rdp->grpmask));
 264	raw_spin_unlock_rcu_node(rnp); /* interrupts remain disabled. */
 265
 266	/*
 267	 * Report the quiescent state for the expedited GP.  This expedited
 268	 * GP should not be able to end until we report, so there should be
 269	 * no need to check for a subsequent expedited GP.  (Though we are
 270	 * still in a quiescent state in any case.)
 271	 *
 272	 * Interrupts are disabled, so ->cpu_no_qs.b.exp cannot change.
 273	 */
 274	if (blkd_state & RCU_EXP_BLKD && rdp->cpu_no_qs.b.exp)
 275		rcu_report_exp_rdp(rdp);
 276	else
 277		WARN_ON_ONCE(rdp->cpu_no_qs.b.exp);
 278	ASSERT_EXCLUSIVE_WRITER_SCOPED(rdp->cpu_no_qs.b.exp);
 279}
 280
 281/*
 282 * Record a preemptible-RCU quiescent state for the specified CPU.
 283 * Note that this does not necessarily mean that the task currently running
 284 * on the CPU is in a quiescent state:  Instead, it means that the current
 285 * grace period need not wait on any RCU read-side critical section that
 286 * starts later on this CPU.  It also means that if the current task is
 287 * in an RCU read-side critical section, it has already added itself to
 288 * some leaf rcu_node structure's ->blkd_tasks list.  In addition to the
 289 * current task, there might be any number of other tasks blocked while
 290 * in an RCU read-side critical section.
 291 *
 292 * Unlike non-preemptible-RCU, quiescent state reports for expedited
 293 * grace periods are handled separately via deferred quiescent states
 294 * and context switch events.
 295 *
 296 * Callers to this function must disable preemption.
 297 */
 298static void rcu_qs(void)
 299{
 300	RCU_LOCKDEP_WARN(preemptible(), "rcu_qs() invoked with preemption enabled!!!\n");
 301	if (__this_cpu_read(rcu_data.cpu_no_qs.b.norm)) {
 302		trace_rcu_grace_period(TPS("rcu_preempt"),
 303				       __this_cpu_read(rcu_data.gp_seq),
 304				       TPS("cpuqs"));
 305		__this_cpu_write(rcu_data.cpu_no_qs.b.norm, false);
 306		barrier(); /* Coordinate with rcu_flavor_sched_clock_irq(). */
 307		WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, false);
 308	}
 309}
 310
 311/*
 312 * We have entered the scheduler, and the current task might soon be
 313 * context-switched away from.  If this task is in an RCU read-side
 314 * critical section, we will no longer be able to rely on the CPU to
 315 * record that fact, so we enqueue the task on the blkd_tasks list.
 316 * The task will dequeue itself when it exits the outermost enclosing
 317 * RCU read-side critical section.  Therefore, the current grace period
 318 * cannot be permitted to complete until the blkd_tasks list entries
 319 * predating the current grace period drain, in other words, until
 320 * rnp->gp_tasks becomes NULL.
 321 *
 322 * Caller must disable interrupts.
 323 */
 324void rcu_note_context_switch(bool preempt)
 325{
 326	struct task_struct *t = current;
 327	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 328	struct rcu_node *rnp;
 329
 330	trace_rcu_utilization(TPS("Start context switch"));
 331	lockdep_assert_irqs_disabled();
 332	WARN_ONCE(!preempt && rcu_preempt_depth() > 0, "Voluntary context switch within RCU read-side critical section!");
 333	if (rcu_preempt_depth() > 0 &&
 334	    !t->rcu_read_unlock_special.b.blocked) {
 335
 336		/* Possibly blocking in an RCU read-side critical section. */
 337		rnp = rdp->mynode;
 338		raw_spin_lock_rcu_node(rnp);
 339		t->rcu_read_unlock_special.b.blocked = true;
 340		t->rcu_blocked_node = rnp;
 341
 342		/*
 343		 * Verify the CPU's sanity, trace the preemption, and
 344		 * then queue the task as required based on the states
 345		 * of any ongoing and expedited grace periods.
 346		 */
 347		WARN_ON_ONCE(!rcu_rdp_cpu_online(rdp));
 348		WARN_ON_ONCE(!list_empty(&t->rcu_node_entry));
 349		trace_rcu_preempt_task(rcu_state.name,
 350				       t->pid,
 351				       (rnp->qsmask & rdp->grpmask)
 352				       ? rnp->gp_seq
 353				       : rcu_seq_snap(&rnp->gp_seq));
 354		rcu_preempt_ctxt_queue(rnp, rdp);
 355	} else {
 356		rcu_preempt_deferred_qs(t);
 357	}
 358
 359	/*
 360	 * Either we were not in an RCU read-side critical section to
 361	 * begin with, or we have now recorded that critical section
 362	 * globally.  Either way, we can now note a quiescent state
 363	 * for this CPU.  Again, if we were in an RCU read-side critical
 364	 * section, and if that critical section was blocking the current
 365	 * grace period, then the fact that the task has been enqueued
 366	 * means that we continue to block the current grace period.
 367	 */
 368	rcu_qs();
 369	if (rdp->cpu_no_qs.b.exp)
 370		rcu_report_exp_rdp(rdp);
 371	rcu_tasks_qs(current, preempt);
 372	trace_rcu_utilization(TPS("End context switch"));
 373}
 374EXPORT_SYMBOL_GPL(rcu_note_context_switch);
 375
 376/*
 377 * Check for preempted RCU readers blocking the current grace period
 378 * for the specified rcu_node structure.  If the caller needs a reliable
 379 * answer, it must hold the rcu_node's ->lock.
 380 */
 381static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
 382{
 383	return READ_ONCE(rnp->gp_tasks) != NULL;
 384}
 385
 386/* limit value for ->rcu_read_lock_nesting. */
 387#define RCU_NEST_PMAX (INT_MAX / 2)
 388
 389static void rcu_preempt_read_enter(void)
 390{
 391	WRITE_ONCE(current->rcu_read_lock_nesting, READ_ONCE(current->rcu_read_lock_nesting) + 1);
 392}
 393
 394static int rcu_preempt_read_exit(void)
 395{
 396	int ret = READ_ONCE(current->rcu_read_lock_nesting) - 1;
 397
 398	WRITE_ONCE(current->rcu_read_lock_nesting, ret);
 399	return ret;
 400}
 401
 402static void rcu_preempt_depth_set(int val)
 403{
 404	WRITE_ONCE(current->rcu_read_lock_nesting, val);
 405}
 406
 407/*
 408 * Preemptible RCU implementation for rcu_read_lock().
 409 * Just increment ->rcu_read_lock_nesting, shared state will be updated
 410 * if we block.
 411 */
 412void __rcu_read_lock(void)
 413{
 414	rcu_preempt_read_enter();
 415	if (IS_ENABLED(CONFIG_PROVE_LOCKING))
 416		WARN_ON_ONCE(rcu_preempt_depth() > RCU_NEST_PMAX);
 417	if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread)
 418		WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, true);
 419	barrier();  /* critical section after entry code. */
 420}
 421EXPORT_SYMBOL_GPL(__rcu_read_lock);
 422
 423/*
 424 * Preemptible RCU implementation for rcu_read_unlock().
 425 * Decrement ->rcu_read_lock_nesting.  If the result is zero (outermost
 426 * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
 427 * invoke rcu_read_unlock_special() to clean up after a context switch
 428 * in an RCU read-side critical section and other special cases.
 429 */
 430void __rcu_read_unlock(void)
 431{
 432	struct task_struct *t = current;
 433
 434	barrier();  // critical section before exit code.
 435	if (rcu_preempt_read_exit() == 0) {
 436		barrier();  // critical-section exit before .s check.
 437		if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
 438			rcu_read_unlock_special(t);
 439	}
 440	if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {
 441		int rrln = rcu_preempt_depth();
 442
 443		WARN_ON_ONCE(rrln < 0 || rrln > RCU_NEST_PMAX);
 444	}
 445}
 446EXPORT_SYMBOL_GPL(__rcu_read_unlock);
 447
 448/*
 449 * Advance a ->blkd_tasks-list pointer to the next entry, instead
 450 * returning NULL if at the end of the list.
 451 */
 452static struct list_head *rcu_next_node_entry(struct task_struct *t,
 453					     struct rcu_node *rnp)
 454{
 455	struct list_head *np;
 456
 457	np = t->rcu_node_entry.next;
 458	if (np == &rnp->blkd_tasks)
 459		np = NULL;
 460	return np;
 461}
 462
 463/*
 464 * Return true if the specified rcu_node structure has tasks that were
 465 * preempted within an RCU read-side critical section.
 466 */
 467static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
 468{
 469	return !list_empty(&rnp->blkd_tasks);
 470}
 471
 472/*
 473 * Report deferred quiescent states.  The deferral time can
 474 * be quite short, for example, in the case of the call from
 475 * rcu_read_unlock_special().
 476 */
 477static notrace void
 478rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
 479{
 480	bool empty_exp;
 481	bool empty_norm;
 482	bool empty_exp_now;
 483	struct list_head *np;
 484	bool drop_boost_mutex = false;
 485	struct rcu_data *rdp;
 486	struct rcu_node *rnp;
 487	union rcu_special special;
 488
 489	rdp = this_cpu_ptr(&rcu_data);
 490	if (rdp->defer_qs_pending == DEFER_QS_PENDING)
 491		rdp->defer_qs_pending = DEFER_QS_IDLE;
 492
 493	/*
 494	 * If RCU core is waiting for this CPU to exit its critical section,
 495	 * report the fact that it has exited.  Because irqs are disabled,
 496	 * t->rcu_read_unlock_special cannot change.
 497	 */
 498	special = t->rcu_read_unlock_special;
 499	if (!special.s && !rdp->cpu_no_qs.b.exp) {
 500		local_irq_restore(flags);
 501		return;
 502	}
 503	t->rcu_read_unlock_special.s = 0;
 504	if (special.b.need_qs) {
 505		if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)) {
 506			rdp->cpu_no_qs.b.norm = false;
 507			rcu_report_qs_rdp(rdp);
 508			udelay(rcu_unlock_delay);
 509		} else {
 510			rcu_qs();
 511		}
 512	}
 513
 514	/*
 515	 * Respond to a request by an expedited grace period for a
 516	 * quiescent state from this CPU.  Note that requests from
 517	 * tasks are handled when removing the task from the
 518	 * blocked-tasks list below.
 519	 */
 520	if (rdp->cpu_no_qs.b.exp)
 521		rcu_report_exp_rdp(rdp);
 522
 523	/* Clean up if blocked during RCU read-side critical section. */
 524	if (special.b.blocked) {
 525
 526		/*
 527		 * Remove this task from the list it blocked on.  The task
 528		 * now remains queued on the rcu_node corresponding to the
 529		 * CPU it first blocked on, so there is no longer any need
 530		 * to loop.  Retain a WARN_ON_ONCE() out of sheer paranoia.
 531		 */
 532		rnp = t->rcu_blocked_node;
 533		raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
 534		WARN_ON_ONCE(rnp != t->rcu_blocked_node);
 535		WARN_ON_ONCE(!rcu_is_leaf_node(rnp));
 536		empty_norm = !rcu_preempt_blocked_readers_cgp(rnp);
 537		WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq &&
 538			     (!empty_norm || rnp->qsmask));
 539		empty_exp = sync_rcu_exp_done(rnp);
 540		np = rcu_next_node_entry(t, rnp);
 541		list_del_init(&t->rcu_node_entry);
 542		t->rcu_blocked_node = NULL;
 543		trace_rcu_unlock_preempted_task(TPS("rcu_preempt"),
 544						rnp->gp_seq, t->pid);
 545		if (&t->rcu_node_entry == rnp->gp_tasks)
 546			WRITE_ONCE(rnp->gp_tasks, np);
 547		if (&t->rcu_node_entry == rnp->exp_tasks)
 548			WRITE_ONCE(rnp->exp_tasks, np);
 549		if (IS_ENABLED(CONFIG_RCU_BOOST)) {
 550			/* Snapshot ->boost_mtx ownership w/rnp->lock held. */
 551			drop_boost_mutex = rt_mutex_owner(&rnp->boost_mtx.rtmutex) == t;
 552			if (&t->rcu_node_entry == rnp->boost_tasks)
 553				WRITE_ONCE(rnp->boost_tasks, np);
 554		}
 555
 556		/*
 557		 * If this was the last task on the current list, and if
 558		 * we aren't waiting on any CPUs, report the quiescent state.
 559		 * Note that rcu_report_unblock_qs_rnp() releases rnp->lock,
 560		 * so we must take a snapshot of the expedited state.
 561		 */
 562		empty_exp_now = sync_rcu_exp_done(rnp);
 563		if (!empty_norm && !rcu_preempt_blocked_readers_cgp(rnp)) {
 564			trace_rcu_quiescent_state_report(TPS("preempt_rcu"),
 565							 rnp->gp_seq,
 566							 0, rnp->qsmask,
 567							 rnp->level,
 568							 rnp->grplo,
 569							 rnp->grphi,
 570							 !!rnp->gp_tasks);
 571			rcu_report_unblock_qs_rnp(rnp, flags);
 572		} else {
 573			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 574		}
 575
 576		/*
 577		 * If this was the last task on the expedited lists,
 578		 * then we need to report up the rcu_node hierarchy.
 579		 */
 580		if (!empty_exp && empty_exp_now)
 581			rcu_report_exp_rnp(rnp, true);
 582
 583		/* Unboost if we were boosted. */
 584		if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex)
 585			rt_mutex_futex_unlock(&rnp->boost_mtx.rtmutex);
 586	} else {
 587		local_irq_restore(flags);
 588	}
 589}
 590
 591/*
 592 * Is a deferred quiescent-state pending, and are we also not in
 593 * an RCU read-side critical section?  It is the caller's responsibility
 594 * to ensure it is otherwise safe to report any deferred quiescent
 595 * states.  The reason for this is that it is safe to report a
 596 * quiescent state during context switch even though preemption
 597 * is disabled.  This function cannot be expected to understand these
 598 * nuances, so the caller must handle them.
 599 */
 600static notrace bool rcu_preempt_need_deferred_qs(struct task_struct *t)
 601{
 602	return (__this_cpu_read(rcu_data.cpu_no_qs.b.exp) ||
 603		READ_ONCE(t->rcu_read_unlock_special.s)) &&
 604	       rcu_preempt_depth() == 0;
 605}
 606
 607/*
 608 * Report a deferred quiescent state if needed and safe to do so.
 609 * As with rcu_preempt_need_deferred_qs(), "safe" involves only
 610 * not being in an RCU read-side critical section.  The caller must
 611 * evaluate safety in terms of interrupt, softirq, and preemption
 612 * disabling.
 613 */
 614notrace void rcu_preempt_deferred_qs(struct task_struct *t)
 615{
 616	unsigned long flags;
 617
 618	if (!rcu_preempt_need_deferred_qs(t))
 619		return;
 620	local_irq_save(flags);
 621	rcu_preempt_deferred_qs_irqrestore(t, flags);
 622}
 623
 624/*
 625 * Minimal handler to give the scheduler a chance to re-evaluate.
 626 */
 627static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
 628{
 629	struct rcu_data *rdp;
 630
 631	lockdep_assert_irqs_disabled();
 632	rdp = container_of(iwp, struct rcu_data, defer_qs_iw);
 633
 634	/*
 635	 * If the IRQ work handler happens to run in the middle of RCU read-side
 636	 * critical section, it could be ineffective in getting the scheduler's
 637	 * attention to report a deferred quiescent state (the whole point of the
 638	 * IRQ work). For this reason, requeue the IRQ work.
 639	 *
 640	 * Basically, we want to avoid following situation:
 641	 * 1. rcu_read_unlock() queues IRQ work (state -> DEFER_QS_PENDING)
 642	 * 2. CPU enters new rcu_read_lock()
 643	 * 3. IRQ work runs but cannot report QS due to rcu_preempt_depth() > 0
 644	 * 4. rcu_read_unlock() does not re-queue work (state still PENDING)
 645	 * 5. Deferred QS reporting does not happen.
 646	 */
 647	if (rcu_preempt_depth() > 0)
 648		WRITE_ONCE(rdp->defer_qs_pending, DEFER_QS_IDLE);
 649}
 650
 651/*
 652 * Check if expedited grace period processing during unlock is needed.
 653 *
 654 * This function determines whether expedited handling is required based on:
 655 * 1. Task blocking an expedited grace period (based on a heuristic, could be
 656 *    false-positive, see below.)
 657 * 2. CPU participating in an expedited grace period
 658 * 3. Strict grace period mode requiring expedited handling
 659 * 4. RCU priority deboosting needs when interrupts were disabled
 660 *
 661 * @t: The task being checked
 662 * @rdp: The per-CPU RCU data
 663 * @rnp: The RCU node for this CPU
 664 * @irqs_were_disabled: Whether interrupts were disabled before rcu_read_unlock()
 665 *
 666 * Returns true if expedited processing of the rcu_read_unlock() is needed.
 667 */
 668static bool rcu_unlock_needs_exp_handling(struct task_struct *t,
 669				      struct rcu_data *rdp,
 670				      struct rcu_node *rnp,
 671				      bool irqs_were_disabled)
 672{
 673	/*
 674	 * Check if this task is blocking an expedited grace period. If the
 675	 * task was preempted within an RCU read-side critical section and is
 676	 * on the expedited grace period blockers list (exp_tasks), we need
 677	 * expedited handling to unblock the expedited GP. This is not an exact
 678	 * check because 't' might not be on the exp_tasks list at all - its
 679	 * just a fast heuristic that can be false-positive sometimes.
 680	 */
 681	if (t->rcu_blocked_node && READ_ONCE(t->rcu_blocked_node->exp_tasks))
 682		return true;
 683
 684	/*
 685	 * Check if this CPU is participating in an expedited grace period.
 686	 * The expmask bitmap tracks which CPUs need to check in for the
 687	 * current expedited GP. If our CPU's bit is set, we need expedited
 688	 * handling to help complete the expedited GP.
 689	 */
 690	if (rdp->grpmask & READ_ONCE(rnp->expmask))
 691		return true;
 692
 693	/*
 694	 * In CONFIG_RCU_STRICT_GRACE_PERIOD=y kernels, all grace periods
 695	 * are treated as short for testing purposes even if that means
 696	 * disturbing the system more. Check if either:
 697	 * - This CPU has not yet reported a quiescent state, or
 698	 * - This task was preempted within an RCU critical section
 699	 * In either case, require expedited handling for strict GP mode.
 700	 */
 701	if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) &&
 702	    ((rdp->grpmask & READ_ONCE(rnp->qsmask)) || t->rcu_blocked_node))
 703		return true;
 704
 705	/*
 706	 * RCU priority boosting case: If a task is subject to RCU priority
 707	 * boosting and exits an RCU read-side critical section with interrupts
 708	 * disabled, we need expedited handling to ensure timely deboosting.
 709	 * Without this, a low-priority task could incorrectly run at high
 710	 * real-time priority for an extended period degrading real-time
 711	 * responsiveness. This applies to all CONFIG_RCU_BOOST=y kernels,
 712	 * not just to PREEMPT_RT.
 713	 */
 714	if (IS_ENABLED(CONFIG_RCU_BOOST) && irqs_were_disabled && t->rcu_blocked_node)
 715		return true;
 716
 717	return false;
 718}
 719
 720/*
 721 * Handle special cases during rcu_read_unlock(), such as needing to
 722 * notify RCU core processing or task having blocked during the RCU
 723 * read-side critical section.
 724 */
 725static void rcu_read_unlock_special(struct task_struct *t)
 726{
 727	unsigned long flags;
 728	bool irqs_were_disabled;
 729	bool preempt_bh_were_disabled =
 730			!!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));
 731
 732	/* NMI handlers cannot block and cannot safely manipulate state. */
 733	if (in_nmi())
 734		return;
 735
 736	local_irq_save(flags);
 737	irqs_were_disabled = irqs_disabled_flags(flags);
 738	if (preempt_bh_were_disabled || irqs_were_disabled) {
 739		bool needs_exp; // Expedited handling needed.
 740		struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 741		struct rcu_node *rnp = rdp->mynode;
 742
 743		needs_exp = rcu_unlock_needs_exp_handling(t, rdp, rnp, irqs_were_disabled);
 744
 745		// Need to defer quiescent state until everything is enabled.
 746		if (use_softirq && (in_hardirq() || (needs_exp && !irqs_were_disabled))) {
 747			// Using softirq, safe to awaken, and either the
 748			// wakeup is free or there is either an expedited
 749			// GP in flight or a potential need to deboost.
 750			if (rdp->defer_qs_pending != DEFER_QS_PENDING) {
 751				rdp->defer_qs_pending = DEFER_QS_PENDING;
 752				raise_softirq_irqoff(RCU_SOFTIRQ);
 753			}
 754		} else {
 755			// Enabling BH or preempt does reschedule, so...
 756			// Also if no expediting and no possible deboosting,
 757			// slow is OK.  Plus nohz_full CPUs eventually get
 758			// tick enabled.
 759			set_need_resched_current();
 760			if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled &&
 761			    needs_exp && rdp->defer_qs_pending != DEFER_QS_PENDING &&
 762			    cpu_online(rdp->cpu)) {
 763				// Get scheduler to re-evaluate and call hooks.
 764				// If !IRQ_WORK, FQS scan will eventually IPI.
 765				rdp->defer_qs_pending = DEFER_QS_PENDING;
 766				irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
 767			}
 768		}
 769		local_irq_restore(flags);
 770		return;
 771	}
 772	rcu_preempt_deferred_qs_irqrestore(t, flags);
 773}
 774
 775/*
 776 * Check that the list of blocked tasks for the newly completed grace
 777 * period is in fact empty.  It is a serious bug to complete a grace
 778 * period that still has RCU readers blocked!  This function must be
 779 * invoked -before- updating this rnp's ->gp_seq.
 780 *
 781 * Also, if there are blocked tasks on the list, they automatically
 782 * block the newly created grace period, so set up ->gp_tasks accordingly.
 783 */
 784static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
 785{
 786	struct task_struct *t;
 787
 788	RCU_LOCKDEP_WARN(preemptible(), "rcu_preempt_check_blocked_tasks() invoked with preemption enabled!!!\n");
 789	raw_lockdep_assert_held_rcu_node(rnp);
 790	if (WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp)))
 791		dump_blkd_tasks(rnp, 10);
 792	if (rcu_preempt_has_tasks(rnp) &&
 793	    (rnp->qsmaskinit || rnp->wait_blkd_tasks)) {
 794		WRITE_ONCE(rnp->gp_tasks, rnp->blkd_tasks.next);
 795		t = container_of(rnp->gp_tasks, struct task_struct,
 796				 rcu_node_entry);
 797		trace_rcu_unlock_preempted_task(TPS("rcu_preempt-GPS"),
 798						rnp->gp_seq, t->pid);
 799	}
 800	WARN_ON_ONCE(rnp->qsmask);
 801}
 802
 803/*
 804 * Check for a quiescent state from the current CPU, including voluntary
 805 * context switches for Tasks RCU.  When a task blocks, the task is
 806 * recorded in the corresponding CPU's rcu_node structure, which is checked
 807 * elsewhere, hence this function need only check for quiescent states
 808 * related to the current CPU, not to those related to tasks.
 809 */
 810static void rcu_flavor_sched_clock_irq(int user)
 811{
 812	struct task_struct *t = current;
 813
 814	lockdep_assert_irqs_disabled();
 815	if (rcu_preempt_depth() > 0 ||
 816	    (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK))) {
 817		/* No QS, force context switch if deferred. */
 818		if (rcu_preempt_need_deferred_qs(t))
 819			set_need_resched_current();
 820	} else if (rcu_preempt_need_deferred_qs(t)) {
 821		rcu_preempt_deferred_qs(t); /* Report deferred QS. */
 822		return;
 823	} else if (!WARN_ON_ONCE(rcu_preempt_depth())) {
 824		rcu_qs(); /* Report immediate QS. */
 825		return;
 826	}
 827
 828	/* If GP is oldish, ask for help from rcu_read_unlock_special(). */
 829	if (rcu_preempt_depth() > 0 &&
 830	    __this_cpu_read(rcu_data.core_needs_qs) &&
 831	    __this_cpu_read(rcu_data.cpu_no_qs.b.norm) &&
 832	    !t->rcu_read_unlock_special.b.need_qs &&
 833	    time_after(jiffies, rcu_state.gp_start + HZ))
 834		t->rcu_read_unlock_special.b.need_qs = true;
 835}
 836
 837/*
 838 * Check for a task exiting while in a preemptible-RCU read-side
 839 * critical section, clean up if so.  No need to issue warnings, as
 840 * debug_check_no_locks_held() already does this if lockdep is enabled.
 841 * Besides, if this function does anything other than just immediately
 842 * return, there was a bug of some sort.  Spewing warnings from this
 843 * function is like as not to simply obscure important prior warnings.
 844 */
 845void exit_rcu(void)
 846{
 847	struct task_struct *t = current;
 848
 849	if (unlikely(!list_empty(&current->rcu_node_entry))) {
 850		rcu_preempt_depth_set(1);
 851		barrier();
 852		WRITE_ONCE(t->rcu_read_unlock_special.b.blocked, true);
 853	} else if (unlikely(rcu_preempt_depth())) {
 854		rcu_preempt_depth_set(1);
 855	} else {
 856		return;
 857	}
 858	__rcu_read_unlock();
 859	rcu_preempt_deferred_qs(current);
 860}
 861
 862/*
 863 * Dump the blocked-tasks state, but limit the list dump to the
 864 * specified number of elements.
 865 */
 866static void
 867dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
 868{
 869	int cpu;
 870	int i;
 871	struct list_head *lhp;
 872	struct rcu_data *rdp;
 873	struct rcu_node *rnp1;
 874
 875	raw_lockdep_assert_held_rcu_node(rnp);
 876	pr_info("%s: grp: %d-%d level: %d ->gp_seq %ld ->completedqs %ld\n",
 877		__func__, rnp->grplo, rnp->grphi, rnp->level,
 878		(long)READ_ONCE(rnp->gp_seq), (long)rnp->completedqs);
 879	for (rnp1 = rnp; rnp1; rnp1 = rnp1->parent)
 880		pr_info("%s: %d:%d ->qsmask %#lx ->qsmaskinit %#lx ->qsmaskinitnext %#lx\n",
 881			__func__, rnp1->grplo, rnp1->grphi, rnp1->qsmask, rnp1->qsmaskinit, rnp1->qsmaskinitnext);
 882	pr_info("%s: ->gp_tasks %p ->boost_tasks %p ->exp_tasks %p\n",
 883		__func__, READ_ONCE(rnp->gp_tasks), data_race(rnp->boost_tasks),
 884		READ_ONCE(rnp->exp_tasks));
 885	pr_info("%s: ->blkd_tasks", __func__);
 886	i = 0;
 887	list_for_each(lhp, &rnp->blkd_tasks) {
 888		pr_cont(" %p", lhp);
 889		if (++i >= ncheck)
 890			break;
 891	}
 892	pr_cont("\n");
 893	for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++) {
 894		rdp = per_cpu_ptr(&rcu_data, cpu);
 895		pr_info("\t%d: %c online: %ld(%d) offline: %ld(%d)\n",
 896			cpu, ".o"[rcu_rdp_cpu_online(rdp)],
 897			(long)rdp->rcu_onl_gp_seq, rdp->rcu_onl_gp_state,
 898			(long)rdp->rcu_ofl_gp_seq, rdp->rcu_ofl_gp_state);
 899	}
 900}
 901
 902static void rcu_preempt_deferred_qs_init(struct rcu_data *rdp)
 903{
 904	rdp->defer_qs_iw = IRQ_WORK_INIT_HARD(rcu_preempt_deferred_qs_handler);
 905}
 906#else /* #ifdef CONFIG_PREEMPT_RCU */
 907
 908/*
 909 * If strict grace periods are enabled, and if the calling
 910 * __rcu_read_unlock() marks the beginning of a quiescent state, immediately
 911 * report that quiescent state and, if requested, spin for a bit.
 912 */
 913void rcu_read_unlock_strict(void)
 914{
 915	struct rcu_data *rdp;
 916
 917	if (irqs_disabled() || in_atomic_preempt_off() || !rcu_state.gp_kthread)
 918		return;
 919
 920	/*
 921	 * rcu_report_qs_rdp() can only be invoked with a stable rdp and
 922	 * from the local CPU.
 923	 *
 924	 * The in_atomic_preempt_off() check ensures that we come here holding
 925	 * the last preempt_count (which will get dropped once we return to
 926	 * __rcu_read_unlock().
 927	 */
 928	rdp = this_cpu_ptr(&rcu_data);
 929	rdp->cpu_no_qs.b.norm = false;
 930	rcu_report_qs_rdp(rdp);
 931	udelay(rcu_unlock_delay);
 932}
 933EXPORT_SYMBOL_GPL(rcu_read_unlock_strict);
 934
 935/*
 936 * Tell them what RCU they are running.
 937 */
 938static void __init rcu_bootup_announce(void)
 939{
 940	pr_info("Hierarchical RCU implementation.\n");
 941	rcu_bootup_announce_oddness();
 942}
 943
 944/*
 945 * Note a quiescent state for PREEMPTION=n.  Because we do not need to know
 946 * how many quiescent states passed, just if there was at least one since
 947 * the start of the grace period, this just sets a flag.  The caller must
 948 * have disabled preemption.
 949 */
 950static void rcu_qs(void)
 951{
 952	RCU_LOCKDEP_WARN(preemptible(), "rcu_qs() invoked with preemption enabled!!!");
 953	if (!__this_cpu_read(rcu_data.cpu_no_qs.s))
 954		return;
 955	trace_rcu_grace_period(TPS("rcu_sched"),
 956			       __this_cpu_read(rcu_data.gp_seq), TPS("cpuqs"));
 957	__this_cpu_write(rcu_data.cpu_no_qs.b.norm, false);
 958	if (__this_cpu_read(rcu_data.cpu_no_qs.b.exp))
 959		rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));
 960}
 961
 962/*
 963 * Register an urgently needed quiescent state.  If there is an
 964 * emergency, invoke rcu_momentary_eqs() to do a heavy-weight
 965 * dyntick-idle quiescent state visible to other CPUs, which will in
 966 * some cases serve for expedited as well as normal grace periods.
 967 * Either way, register a lightweight quiescent state.
 968 */
 969void rcu_all_qs(void)
 970{
 971	unsigned long flags;
 972
 973	if (!raw_cpu_read(rcu_data.rcu_urgent_qs))
 974		return;
 975	preempt_disable();  // For CONFIG_PREEMPT_COUNT=y kernels
 976	/* Load rcu_urgent_qs before other flags. */
 977	if (!smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs))) {
 978		preempt_enable();
 979		return;
 980	}
 981	this_cpu_write(rcu_data.rcu_urgent_qs, false);
 982	if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs))) {
 983		local_irq_save(flags);
 984		rcu_momentary_eqs();
 985		local_irq_restore(flags);
 986	}
 987	rcu_qs();
 988	preempt_enable();
 989}
 990EXPORT_SYMBOL_GPL(rcu_all_qs);
 991
 992/*
 993 * Note a PREEMPTION=n context switch. The caller must have disabled interrupts.
 994 */
 995void rcu_note_context_switch(bool preempt)
 996{
 997	trace_rcu_utilization(TPS("Start context switch"));
 998	rcu_qs();
 999	/* Load rcu_urgent_qs before other flags. */
1000	if (!smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs)))
1001		goto out;
1002	this_cpu_write(rcu_data.rcu_urgent_qs, false);
1003	if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs)))
1004		rcu_momentary_eqs();
1005out:
1006	rcu_tasks_qs(current, preempt);
1007	trace_rcu_utilization(TPS("End context switch"));
1008}
1009EXPORT_SYMBOL_GPL(rcu_note_context_switch);
1010
1011/*
1012 * Because preemptible RCU does not exist, there are never any preempted
1013 * RCU readers.
1014 */
1015static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
1016{
1017	return 0;
1018}
1019
1020/*
1021 * Because there is no preemptible RCU, there can be no readers blocked.
1022 */
1023static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
1024{
1025	return false;
1026}
1027
1028/*
1029 * Because there is no preemptible RCU, there can be no deferred quiescent
1030 * states.
1031 */
1032static notrace bool rcu_preempt_need_deferred_qs(struct task_struct *t)
1033{
1034	return false;
1035}
1036
1037// Except that we do need to respond to a request by an expedited
1038// grace period for a quiescent state from this CPU.  Note that in
1039// non-preemptible kernels, there can be no context switches within RCU
1040// read-side critical sections, which in turn means that the leaf rcu_node
1041// structure's blocked-tasks list is always empty.  is therefore no need to
1042// actually check it.  Instead, a quiescent state from this CPU suffices,
1043// and this function is only called from such a quiescent state.
1044notrace void rcu_preempt_deferred_qs(struct task_struct *t)
1045{
1046	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
1047
1048	if (READ_ONCE(rdp->cpu_no_qs.b.exp))
1049		rcu_report_exp_rdp(rdp);
1050}
1051
1052/*
1053 * Because there is no preemptible RCU, there can be no readers blocked,
1054 * so there is no need to check for blocked tasks.  So check only for
1055 * bogus qsmask values.
1056 */
1057static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
1058{
1059	WARN_ON_ONCE(rnp->qsmask);
1060}
1061
1062/*
1063 * Check to see if this CPU is in a non-context-switch quiescent state,
1064 * namely user mode and idle loop.
1065 */
1066static void rcu_flavor_sched_clock_irq(int user)
1067{
1068	if (user || rcu_is_cpu_rrupt_from_idle() ||
1069	     (IS_ENABLED(CONFIG_PREEMPT_COUNT) &&
1070	      (preempt_count() == HARDIRQ_OFFSET))) {
1071
1072		/*
1073		 * Get here if this CPU took its interrupt from user
1074		 * mode, from the idle loop without this being a nested
1075		 * interrupt, or while not holding the task preempt count
1076		 * (with PREEMPT_COUNT=y). In this case, the CPU is in a
1077		 * quiescent state, so note it.
1078		 *
1079		 * No memory barrier is required here because rcu_qs()
1080		 * references only CPU-local variables that other CPUs
1081		 * neither access nor modify, at least not while the
1082		 * corresponding CPU is online.
1083		 */
1084		rcu_qs();
1085	}
1086}
1087
1088/*
1089 * Because preemptible RCU does not exist, tasks cannot possibly exit
1090 * while in preemptible RCU read-side critical sections.
1091 */
1092void exit_rcu(void)
1093{
1094}
1095
1096/*
1097 * Dump the guaranteed-empty blocked-tasks state.  Trust but verify.
1098 */
1099static void
1100dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
1101{
1102	WARN_ON_ONCE(!list_empty(&rnp->blkd_tasks));
1103}
1104
1105static void rcu_preempt_deferred_qs_init(struct rcu_data *rdp) { }
1106
1107#endif /* #else #ifdef CONFIG_PREEMPT_RCU */
1108
1109/*
1110 * If boosting, set rcuc kthreads to realtime priority.
1111 */
1112static void rcu_cpu_kthread_setup(unsigned int cpu)
1113{
1114	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
1115#ifdef CONFIG_RCU_BOOST
1116	struct sched_param sp;
1117
1118	sp.sched_priority = kthread_prio;
1119	sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
1120#endif /* #ifdef CONFIG_RCU_BOOST */
1121
1122	WRITE_ONCE(rdp->rcuc_activity, jiffies);
1123}
1124
1125static bool rcu_is_callbacks_nocb_kthread(struct rcu_data *rdp)
1126{
1127#ifdef CONFIG_RCU_NOCB_CPU
1128	return rdp->nocb_cb_kthread == current;
1129#else
1130	return false;
1131#endif
1132}
1133
1134/*
1135 * Is the current CPU running the RCU-callbacks kthread?
1136 * Caller must have preemption disabled.
1137 */
1138static bool rcu_is_callbacks_kthread(struct rcu_data *rdp)
1139{
1140	return rdp->rcu_cpu_kthread_task == current ||
1141			rcu_is_callbacks_nocb_kthread(rdp);
1142}
1143
1144#ifdef CONFIG_RCU_BOOST
1145
1146/*
1147 * Carry out RCU priority boosting on the task indicated by ->exp_tasks
1148 * or ->boost_tasks, advancing the pointer to the next task in the
1149 * ->blkd_tasks list.
1150 *
1151 * Note that irqs must be enabled: boosting the task can block.
1152 * Returns 1 if there are more tasks needing to be boosted.
1153 */
1154static int rcu_boost(struct rcu_node *rnp)
1155{
1156	unsigned long flags;
1157	struct task_struct *t;
1158	struct list_head *tb;
1159
1160	if (READ_ONCE(rnp->exp_tasks) == NULL &&
1161	    READ_ONCE(rnp->boost_tasks) == NULL)
1162		return 0;  /* Nothing left to boost. */
1163
1164	raw_spin_lock_irqsave_rcu_node(rnp, flags);
1165
1166	/*
1167	 * Recheck under the lock: all tasks in need of boosting
1168	 * might exit their RCU read-side critical sections on their own.
1169	 */
1170	if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL) {
1171		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1172		return 0;
1173	}
1174
1175	/*
1176	 * Preferentially boost tasks blocking expedited grace periods.
1177	 * This cannot starve the normal grace periods because a second
1178	 * expedited grace period must boost all blocked tasks, including
1179	 * those blocking the pre-existing normal grace period.
1180	 */
1181	if (rnp->exp_tasks != NULL)
1182		tb = rnp->exp_tasks;
1183	else
1184		tb = rnp->boost_tasks;
1185
1186	/*
1187	 * We boost task t by manufacturing an rt_mutex that appears to
1188	 * be held by task t.  We leave a pointer to that rt_mutex where
1189	 * task t can find it, and task t will release the mutex when it
1190	 * exits its outermost RCU read-side critical section.  Then
1191	 * simply acquiring this artificial rt_mutex will boost task
1192	 * t's priority.  (Thanks to tglx for suggesting this approach!)
1193	 *
1194	 * Note that task t must acquire rnp->lock to remove itself from
1195	 * the ->blkd_tasks list, which it will do from exit() if from
1196	 * nowhere else.  We therefore are guaranteed that task t will
1197	 * stay around at least until we drop rnp->lock.  Note that
1198	 * rnp->lock also resolves races between our priority boosting
1199	 * and task t's exiting its outermost RCU read-side critical
1200	 * section.
1201	 */
1202	t = container_of(tb, struct task_struct, rcu_node_entry);
1203	rt_mutex_init_proxy_locked(&rnp->boost_mtx.rtmutex, t);
1204	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1205	/* Lock only for side effect: boosts task t's priority. */
1206	rt_mutex_lock(&rnp->boost_mtx);
1207	rt_mutex_unlock(&rnp->boost_mtx);  /* Then keep lockdep happy. */
1208	rnp->n_boosts++;
1209
1210	return READ_ONCE(rnp->exp_tasks) != NULL ||
1211	       READ_ONCE(rnp->boost_tasks) != NULL;
1212}
1213
1214/*
1215 * Priority-boosting kthread, one per leaf rcu_node.
1216 */
1217static int rcu_boost_kthread(void *arg)
1218{
1219	struct rcu_node *rnp = (struct rcu_node *)arg;
1220	int spincnt = 0;
1221	int more2boost;
1222
1223	trace_rcu_utilization(TPS("Start boost kthread@init"));
1224	for (;;) {
1225		WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_WAITING);
1226		trace_rcu_utilization(TPS("End boost kthread@rcu_wait"));
1227		rcu_wait(READ_ONCE(rnp->boost_tasks) ||
1228			 READ_ONCE(rnp->exp_tasks));
1229		trace_rcu_utilization(TPS("Start boost kthread@rcu_wait"));
1230		WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_RUNNING);
1231		more2boost = rcu_boost(rnp);
1232		if (more2boost)
1233			spincnt++;
1234		else
1235			spincnt = 0;
1236		if (spincnt > 10) {
1237			WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_YIELDING);
1238			trace_rcu_utilization(TPS("End boost kthread@rcu_yield"));
1239			schedule_timeout_idle(2);
1240			trace_rcu_utilization(TPS("Start boost kthread@rcu_yield"));
1241			spincnt = 0;
1242		}
1243	}
1244	/* NOTREACHED */
1245	trace_rcu_utilization(TPS("End boost kthread@notreached"));
1246	return 0;
1247}
1248
1249/*
1250 * Check to see if it is time to start boosting RCU readers that are
1251 * blocking the current grace period, and, if so, tell the per-rcu_node
1252 * kthread to start boosting them.  If there is an expedited grace
1253 * period in progress, it is always time to boost.
1254 *
1255 * The caller must hold rnp->lock, which this function releases.
1256 * The ->boost_kthread_task is immortal, so we don't need to worry
1257 * about it going away.
1258 */
1259static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
1260	__releases(rnp->lock)
1261{
1262	raw_lockdep_assert_held_rcu_node(rnp);
1263	if (!rnp->boost_kthread_task ||
1264	    (!rcu_preempt_blocked_readers_cgp(rnp) && !rnp->exp_tasks)) {
1265		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1266		return;
1267	}
1268	if (rnp->exp_tasks != NULL ||
1269	    (rnp->gp_tasks != NULL &&
1270	     rnp->boost_tasks == NULL &&
1271	     rnp->qsmask == 0 &&
1272	     (!time_after(rnp->boost_time, jiffies) || rcu_state.cbovld ||
1273	      IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)))) {
1274		if (rnp->exp_tasks == NULL)
1275			WRITE_ONCE(rnp->boost_tasks, rnp->gp_tasks);
1276		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1277		rcu_wake_cond(rnp->boost_kthread_task,
1278			      READ_ONCE(rnp->boost_kthread_status));
1279	} else {
1280		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1281	}
1282}
1283
1284#define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY * HZ, 1000)
1285
1286/*
1287 * Do priority-boost accounting for the start of a new grace period.
1288 */
1289static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
1290{
1291	rnp->boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
1292}
1293
1294/*
1295 * Create an RCU-boost kthread for the specified node if one does not
1296 * already exist.  We only create this kthread for preemptible RCU.
1297 */
1298static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
1299{
1300	unsigned long flags;
1301	int rnp_index = rnp - rcu_get_root();
1302	struct sched_param sp;
1303	struct task_struct *t;
1304
1305	if (rnp->boost_kthread_task)
1306		return;
1307
1308	t = kthread_create(rcu_boost_kthread, (void *)rnp,
1309			   "rcub/%d", rnp_index);
1310	if (WARN_ON_ONCE(IS_ERR(t)))
1311		return;
1312
1313	raw_spin_lock_irqsave_rcu_node(rnp, flags);
1314	rnp->boost_kthread_task = t;
1315	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1316
1317	sp.sched_priority = kthread_prio;
1318	sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
1319	rcu_thread_affine_rnp(t, rnp);
1320	wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
1321}
1322
1323#else /* #ifdef CONFIG_RCU_BOOST */
1324
1325static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
1326	__releases(rnp->lock)
1327{
1328	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1329}
1330
1331static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
1332{
1333}
1334
1335static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
1336{
1337}
1338
1339#endif /* #else #ifdef CONFIG_RCU_BOOST */
1340
1341/*
1342 * Is this CPU a NO_HZ_FULL CPU that should ignore RCU so that the
1343 * grace-period kthread will do force_quiescent_state() processing?
1344 * The idea is to avoid waking up RCU core processing on such a
1345 * CPU unless the grace period has extended for too long.
1346 *
1347 * This code relies on the fact that all NO_HZ_FULL CPUs are also
1348 * RCU_NOCB_CPU CPUs.
1349 */
1350static bool rcu_nohz_full_cpu(void)
1351{
1352#ifdef CONFIG_NO_HZ_FULL
1353	if (tick_nohz_full_cpu(smp_processor_id()) &&
1354	    (!rcu_gp_in_progress() ||
1355	     time_before(jiffies, READ_ONCE(rcu_state.gp_start) + HZ)))
1356		return true;
1357#endif /* #ifdef CONFIG_NO_HZ_FULL */
1358	return false;
1359}
1360
1361/*
1362 * Bind the RCU grace-period kthreads to the housekeeping CPU.
1363 */
1364static void rcu_bind_gp_kthread(void)
1365{
1366	if (!tick_nohz_full_enabled())
1367		return;
1368	housekeeping_affine(current, HK_TYPE_RCU);
1369}