/* SPDX-License-Identifier: GPL-2.0+ */

  /*
   * Read-Copy Update mechanism for mutual exclusion (tree-based version)
   * Internal non-public definitions that provide either classic

   * or preemptible semantics.

   *

   * Copyright Red Hat, 2009
   * Copyright IBM Corporation, 2009
   *
   * Author: Ingo Molnar <mingo@elte.hu>

   *	   Paul E. McKenney <paulmck@linux.ibm.com>

   */

  #include "../locking/rtmutex_common.h"

  #ifdef CONFIG_RCU_NOCB_CPU
  static cpumask_var_t rcu_nocb_mask; /* CPUs to have callbacks offloaded. */

  static bool __read_mostly rcu_nocb_poll;    /* Offload kthread are to poll. */

  #endif /* #ifdef CONFIG_RCU_NOCB_CPU */

  /*
   * Check the RCU kernel configuration parameters and print informative

   * messages about anything out of the ordinary.

   */
  static void __init rcu_bootup_announce_oddness(void)
  {

  	if (IS_ENABLED(CONFIG_RCU_TRACE))

  		pr_info("\tRCU event tracing is enabled.
  ");

  	if ((IS_ENABLED(CONFIG_64BIT) && RCU_FANOUT != 64) ||
  	    (!IS_ENABLED(CONFIG_64BIT) && RCU_FANOUT != 32))

  		pr_info("\tCONFIG_RCU_FANOUT set to non-default value of %d.
  ",
  			RCU_FANOUT);

  	if (rcu_fanout_exact)

  		pr_info("\tHierarchical RCU autobalancing is disabled.
  ");
  	if (IS_ENABLED(CONFIG_RCU_FAST_NO_HZ))
  		pr_info("\tRCU dyntick-idle grace-period acceleration is enabled.
  ");

  	if (IS_ENABLED(CONFIG_PROVE_RCU))

  		pr_info("\tRCU lockdep checking is enabled.
  ");

  	if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
  		pr_info("\tRCU strict (and thus non-scalable) grace periods enabled.
  ");

  	if (RCU_NUM_LVLS >= 4)
  		pr_info("\tFour(or more)-level hierarchy is enabled.
  ");

  	if (RCU_FANOUT_LEAF != 16)

  		pr_info("\tBuild-time adjustment of leaf fanout to %d.
  ",

  			RCU_FANOUT_LEAF);
  	if (rcu_fanout_leaf != RCU_FANOUT_LEAF)

  		pr_info("\tBoot-time adjustment of leaf fanout to %d.
  ",
  			rcu_fanout_leaf);

  	if (nr_cpu_ids != NR_CPUS)

  		pr_info("\tRCU restricting CPUs from NR_CPUS=%d to nr_cpu_ids=%u.
  ", NR_CPUS, nr_cpu_ids);

  #ifdef CONFIG_RCU_BOOST

  	pr_info("\tRCU priority boosting: priority %d delay %d ms.
  ",
  		kthread_prio, CONFIG_RCU_BOOST_DELAY);

  #endif
  	if (blimit != DEFAULT_RCU_BLIMIT)
  		pr_info("\tBoot-time adjustment of callback invocation limit to %ld.
  ", blimit);
  	if (qhimark != DEFAULT_RCU_QHIMARK)
  		pr_info("\tBoot-time adjustment of callback high-water mark to %ld.
  ", qhimark);
  	if (qlowmark != DEFAULT_RCU_QLOMARK)
  		pr_info("\tBoot-time adjustment of callback low-water mark to %ld.
  ", qlowmark);

  	if (qovld != DEFAULT_RCU_QOVLD)

  		pr_info("\tBoot-time adjustment of callback overload level to %ld.
  ", qovld);

  	if (jiffies_till_first_fqs != ULONG_MAX)
  		pr_info("\tBoot-time adjustment of first FQS scan delay to %ld jiffies.
  ", jiffies_till_first_fqs);
  	if (jiffies_till_next_fqs != ULONG_MAX)
  		pr_info("\tBoot-time adjustment of subsequent FQS scan delay to %ld jiffies.
  ", jiffies_till_next_fqs);

  	if (jiffies_till_sched_qs != ULONG_MAX)
  		pr_info("\tBoot-time adjustment of scheduler-enlistment delay to %ld jiffies.
  ", jiffies_till_sched_qs);

  	if (rcu_kick_kthreads)
  		pr_info("\tKick kthreads if too-long grace period.
  ");
  	if (IS_ENABLED(CONFIG_DEBUG_OBJECTS_RCU_HEAD))
  		pr_info("\tRCU callback double-/use-after-free debug enabled.
  ");

  	if (gp_preinit_delay)

  		pr_info("\tRCU debug GP pre-init slowdown %d jiffies.
  ", gp_preinit_delay);

  	if (gp_init_delay)

  		pr_info("\tRCU debug GP init slowdown %d jiffies.
  ", gp_init_delay);

  	if (gp_cleanup_delay)

  		pr_info("\tRCU debug GP init slowdown %d jiffies.
  ", gp_cleanup_delay);

  	if (!use_softirq)
  		pr_info("\tRCU_SOFTIRQ processing moved to rcuc kthreads.
  ");

  	if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG))
  		pr_info("\tRCU debug extended QS entry/exit.
  ");

  	rcupdate_announce_bootup_oddness();

  }

  #ifdef CONFIG_PREEMPT_RCU

  static void rcu_report_exp_rnp(struct rcu_node *rnp, bool wake);

  static void rcu_read_unlock_special(struct task_struct *t);

  /*
   * Tell them what RCU they are running.
   */

  static void __init rcu_bootup_announce(void)

  {

  	pr_info("Preemptible hierarchical RCU implementation.
  ");

  	rcu_bootup_announce_oddness();

  }

  /* Flags for rcu_preempt_ctxt_queue() decision table. */
  #define RCU_GP_TASKS	0x8
  #define RCU_EXP_TASKS	0x4
  #define RCU_GP_BLKD	0x2
  #define RCU_EXP_BLKD	0x1
  
  /*
   * Queues a task preempted within an RCU-preempt read-side critical
   * section into the appropriate location within the ->blkd_tasks list,
   * depending on the states of any ongoing normal and expedited grace
   * periods.  The ->gp_tasks pointer indicates which element the normal
   * grace period is waiting on (NULL if none), and the ->exp_tasks pointer
   * indicates which element the expedited grace period is waiting on (again,
   * NULL if none).  If a grace period is waiting on a given element in the
   * ->blkd_tasks list, it also waits on all subsequent elements.  Thus,
   * adding a task to the tail of the list blocks any grace period that is
   * already waiting on one of the elements.  In contrast, adding a task
   * to the head of the list won't block any grace period that is already
   * waiting on one of the elements.
   *
   * This queuing is imprecise, and can sometimes make an ongoing grace
   * period wait for a task that is not strictly speaking blocking it.
   * Given the choice, we needlessly block a normal grace period rather than
   * blocking an expedited grace period.
   *
   * Note that an endless sequence of expedited grace periods still cannot
   * indefinitely postpone a normal grace period.  Eventually, all of the
   * fixed number of preempted tasks blocking the normal grace period that are
   * not also blocking the expedited grace period will resume and complete
   * their RCU read-side critical sections.  At that point, the ->gp_tasks
   * pointer will equal the ->exp_tasks pointer, at which point the end of
   * the corresponding expedited grace period will also be the end of the
   * normal grace period.
   */

  static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp)
  	__releases(rnp->lock) /* But leaves rrupts disabled. */

  {
  	int blkd_state = (rnp->gp_tasks ? RCU_GP_TASKS : 0) +
  			 (rnp->exp_tasks ? RCU_EXP_TASKS : 0) +
  			 (rnp->qsmask & rdp->grpmask ? RCU_GP_BLKD : 0) +
  			 (rnp->expmask & rdp->grpmask ? RCU_EXP_BLKD : 0);
  	struct task_struct *t = current;

  	raw_lockdep_assert_held_rcu_node(rnp);

  	WARN_ON_ONCE(rdp->mynode != rnp);

  	WARN_ON_ONCE(!rcu_is_leaf_node(rnp));

  	/* RCU better not be waiting on newly onlined CPUs! */
  	WARN_ON_ONCE(rnp->qsmaskinitnext & ~rnp->qsmaskinit & rnp->qsmask &
  		     rdp->grpmask);

  	/*
  	 * Decide where to queue the newly blocked task.  In theory,
  	 * this could be an if-statement.  In practice, when I tried
  	 * that, it was quite messy.
  	 */
  	switch (blkd_state) {
  	case 0:
  	case                RCU_EXP_TASKS:
  	case                RCU_EXP_TASKS + RCU_GP_BLKD:
  	case RCU_GP_TASKS:
  	case RCU_GP_TASKS + RCU_EXP_TASKS:
  
  		/*
  		 * Blocking neither GP, or first task blocking the normal
  		 * GP but not blocking the already-waiting expedited GP.
  		 * Queue at the head of the list to avoid unnecessarily
  		 * blocking the already-waiting GPs.
  		 */
  		list_add(&t->rcu_node_entry, &rnp->blkd_tasks);
  		break;
  
  	case                                              RCU_EXP_BLKD:
  	case                                RCU_GP_BLKD:
  	case                                RCU_GP_BLKD + RCU_EXP_BLKD:
  	case RCU_GP_TASKS +                               RCU_EXP_BLKD:
  	case RCU_GP_TASKS +                 RCU_GP_BLKD + RCU_EXP_BLKD:
  	case RCU_GP_TASKS + RCU_EXP_TASKS + RCU_GP_BLKD + RCU_EXP_BLKD:
  
  		/*
  		 * First task arriving that blocks either GP, or first task
  		 * arriving that blocks the expedited GP (with the normal
  		 * GP already waiting), or a task arriving that blocks
  		 * both GPs with both GPs already waiting.  Queue at the
  		 * tail of the list to avoid any GP waiting on any of the
  		 * already queued tasks that are not blocking it.
  		 */
  		list_add_tail(&t->rcu_node_entry, &rnp->blkd_tasks);
  		break;
  
  	case                RCU_EXP_TASKS +               RCU_EXP_BLKD:
  	case                RCU_EXP_TASKS + RCU_GP_BLKD + RCU_EXP_BLKD:
  	case RCU_GP_TASKS + RCU_EXP_TASKS +               RCU_EXP_BLKD:
  
  		/*
  		 * Second or subsequent task blocking the expedited GP.
  		 * The task either does not block the normal GP, or is the
  		 * first task blocking the normal GP.  Queue just after
  		 * the first task blocking the expedited GP.
  		 */
  		list_add(&t->rcu_node_entry, rnp->exp_tasks);
  		break;
  
  	case RCU_GP_TASKS +                 RCU_GP_BLKD:
  	case RCU_GP_TASKS + RCU_EXP_TASKS + RCU_GP_BLKD:
  
  		/*
  		 * Second or subsequent task blocking the normal GP.
  		 * The task does not block the expedited GP. Queue just
  		 * after the first task blocking the normal GP.
  		 */
  		list_add(&t->rcu_node_entry, rnp->gp_tasks);
  		break;
  
  	default:
  
  		/* Yet another exercise in excessive paranoia. */
  		WARN_ON_ONCE(1);
  		break;
  	}
  
  	/*
  	 * We have now queued the task.  If it was the first one to
  	 * block either grace period, update the ->gp_tasks and/or
  	 * ->exp_tasks pointers, respectively, to reference the newly
  	 * blocked tasks.
  	 */

  	if (!rnp->gp_tasks && (blkd_state & RCU_GP_BLKD)) {

  		WRITE_ONCE(rnp->gp_tasks, &t->rcu_node_entry);

  		WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq);

  	}

  	if (!rnp->exp_tasks && (blkd_state & RCU_EXP_BLKD))

  		WRITE_ONCE(rnp->exp_tasks, &t->rcu_node_entry);

  	WARN_ON_ONCE(!(blkd_state & RCU_GP_BLKD) !=
  		     !(rnp->qsmask & rdp->grpmask));
  	WARN_ON_ONCE(!(blkd_state & RCU_EXP_BLKD) !=
  		     !(rnp->expmask & rdp->grpmask));

  	raw_spin_unlock_rcu_node(rnp); /* interrupts remain disabled. */

  
  	/*
  	 * Report the quiescent state for the expedited GP.  This expedited
  	 * GP should not be able to end until we report, so there should be
  	 * no need to check for a subsequent expedited GP.  (Though we are
  	 * still in a quiescent state in any case.)
  	 */

  	if (blkd_state & RCU_EXP_BLKD && rdp->exp_deferred_qs)

  		rcu_report_exp_rdp(rdp);

  	else

  		WARN_ON_ONCE(rdp->exp_deferred_qs);

  }

  /*

   * Record a preemptible-RCU quiescent state for the specified CPU.
   * Note that this does not necessarily mean that the task currently running
   * on the CPU is in a quiescent state:  Instead, it means that the current
   * grace period need not wait on any RCU read-side critical section that
   * starts later on this CPU.  It also means that if the current task is
   * in an RCU read-side critical section, it has already added itself to
   * some leaf rcu_node structure's ->blkd_tasks list.  In addition to the
   * current task, there might be any number of other tasks blocked while
   * in an RCU read-side critical section.

   *

   * Callers to this function must disable preemption.

   */

  static void rcu_qs(void)

  {

  	RCU_LOCKDEP_WARN(preemptible(), "rcu_qs() invoked with preemption enabled!!!
  ");

  	if (__this_cpu_read(rcu_data.cpu_no_qs.s)) {

  		trace_rcu_grace_period(TPS("rcu_preempt"),

  				       __this_cpu_read(rcu_data.gp_seq),

  				       TPS("cpuqs"));

  		__this_cpu_write(rcu_data.cpu_no_qs.b.norm, false);

  		barrier(); /* Coordinate with rcu_flavor_sched_clock_irq(). */

  		WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, false);

  	}

  }
  
  /*

   * We have entered the scheduler, and the current task might soon be
   * context-switched away from.  If this task is in an RCU read-side
   * critical section, we will no longer be able to rely on the CPU to

   * record that fact, so we enqueue the task on the blkd_tasks list.
   * The task will dequeue itself when it exits the outermost enclosing
   * RCU read-side critical section.  Therefore, the current grace period
   * cannot be permitted to complete until the blkd_tasks list entries
   * predating the current grace period drain, in other words, until
   * rnp->gp_tasks becomes NULL.

   *

   * Caller must disable interrupts.

   */

  void rcu_note_context_switch(bool preempt)

  {
  	struct task_struct *t = current;

  	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);

  	struct rcu_node *rnp;

  	trace_rcu_utilization(TPS("Start context switch"));

  	lockdep_assert_irqs_disabled();

  	WARN_ON_ONCE(!preempt && rcu_preempt_depth() > 0);
  	if (rcu_preempt_depth() > 0 &&

  	    !t->rcu_read_unlock_special.b.blocked) {

  
  		/* Possibly blocking in an RCU read-side critical section. */

  		rnp = rdp->mynode;

  		raw_spin_lock_rcu_node(rnp);

  		t->rcu_read_unlock_special.b.blocked = true;

  		t->rcu_blocked_node = rnp;

  
  		/*

  		 * Verify the CPU's sanity, trace the preemption, and
  		 * then queue the task as required based on the states
  		 * of any ongoing and expedited grace periods.

  		 */

  		WARN_ON_ONCE((rdp->grpmask & rcu_rnp_online_cpus(rnp)) == 0);

  		WARN_ON_ONCE(!list_empty(&t->rcu_node_entry));

  		trace_rcu_preempt_task(rcu_state.name,

  				       t->pid,
  				       (rnp->qsmask & rdp->grpmask)

  				       ? rnp->gp_seq
  				       : rcu_seq_snap(&rnp->gp_seq));

  		rcu_preempt_ctxt_queue(rnp, rdp);

  	} else {
  		rcu_preempt_deferred_qs(t);

  	}
  
  	/*
  	 * Either we were not in an RCU read-side critical section to
  	 * begin with, or we have now recorded that critical section
  	 * globally.  Either way, we can now note a quiescent state
  	 * for this CPU.  Again, if we were in an RCU read-side critical
  	 * section, and if that critical section was blocking the current
  	 * grace period, then the fact that the task has been enqueued
  	 * means that we continue to block the current grace period.
  	 */

  	rcu_qs();

  	if (rdp->exp_deferred_qs)

  		rcu_report_exp_rdp(rdp);

  	rcu_tasks_qs(current, preempt);

  	trace_rcu_utilization(TPS("End context switch"));

  }

  EXPORT_SYMBOL_GPL(rcu_note_context_switch);

  
  /*

   * Check for preempted RCU readers blocking the current grace period
   * for the specified rcu_node structure.  If the caller needs a reliable
   * answer, it must hold the rcu_node's ->lock.
   */

  static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)

  {

  	return READ_ONCE(rnp->gp_tasks) != NULL;

  }

  /* limit value for ->rcu_read_lock_nesting. */

  #define RCU_NEST_PMAX (INT_MAX / 2)

  static void rcu_preempt_read_enter(void)
  {
  	current->rcu_read_lock_nesting++;
  }

  static int rcu_preempt_read_exit(void)

  {

  	return --current->rcu_read_lock_nesting;

  }
  
  static void rcu_preempt_depth_set(int val)
  {
  	current->rcu_read_lock_nesting = val;
  }

  /*

   * Preemptible RCU implementation for rcu_read_lock().
   * Just increment ->rcu_read_lock_nesting, shared state will be updated
   * if we block.
   */
  void __rcu_read_lock(void)
  {

  	rcu_preempt_read_enter();

  	if (IS_ENABLED(CONFIG_PROVE_LOCKING))

  		WARN_ON_ONCE(rcu_preempt_depth() > RCU_NEST_PMAX);

  	if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread)
  		WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, true);

  	barrier();  /* critical section after entry code. */
  }
  EXPORT_SYMBOL_GPL(__rcu_read_lock);
  
  /*
   * Preemptible RCU implementation for rcu_read_unlock().
   * Decrement ->rcu_read_lock_nesting.  If the result is zero (outermost
   * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
   * invoke rcu_read_unlock_special() to clean up after a context switch
   * in an RCU read-side critical section and other special cases.
   */
  void __rcu_read_unlock(void)
  {
  	struct task_struct *t = current;

  	if (rcu_preempt_read_exit() == 0) {

  		barrier();  /* critical section before exit code. */

  		if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
  			rcu_read_unlock_special(t);

  	}

  	if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {

  		int rrln = rcu_preempt_depth();

  		WARN_ON_ONCE(rrln < 0 || rrln > RCU_NEST_PMAX);

  	}

  }
  EXPORT_SYMBOL_GPL(__rcu_read_unlock);
  
  /*

   * Advance a ->blkd_tasks-list pointer to the next entry, instead
   * returning NULL if at the end of the list.
   */
  static struct list_head *rcu_next_node_entry(struct task_struct *t,
  					     struct rcu_node *rnp)
  {
  	struct list_head *np;
  
  	np = t->rcu_node_entry.next;
  	if (np == &rnp->blkd_tasks)
  		np = NULL;
  	return np;
  }
  
  /*

   * Return true if the specified rcu_node structure has tasks that were
   * preempted within an RCU read-side critical section.
   */
  static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
  {
  	return !list_empty(&rnp->blkd_tasks);
  }
  
  /*

   * Report deferred quiescent states.  The deferral time can
   * be quite short, for example, in the case of the call from
   * rcu_read_unlock_special().

   */

  static void
  rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)

  {

  	bool empty_exp;
  	bool empty_norm;
  	bool empty_exp_now;

  	struct list_head *np;

  	bool drop_boost_mutex = false;

  	struct rcu_data *rdp;

  	struct rcu_node *rnp;

  	union rcu_special special;

  	/*

  	 * If RCU core is waiting for this CPU to exit its critical section,
  	 * report the fact that it has exited.  Because irqs are disabled,

  	 * t->rcu_read_unlock_special cannot change.

  	 */
  	special = t->rcu_read_unlock_special;

  	rdp = this_cpu_ptr(&rcu_data);

  	if (!special.s && !rdp->exp_deferred_qs) {

  		local_irq_restore(flags);
  		return;
  	}

  	t->rcu_read_unlock_special.s = 0;

  	if (special.b.need_qs) {

  		if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)) {

  			rcu_report_qs_rdp(rdp);

  			udelay(rcu_unlock_delay);
  		} else {

  			rcu_qs();

  		}

  	}

  	/*

  	 * Respond to a request by an expedited grace period for a
  	 * quiescent state from this CPU.  Note that requests from
  	 * tasks are handled when removing the task from the
  	 * blocked-tasks list below.

  	 */

  	if (rdp->exp_deferred_qs)

  		rcu_report_exp_rdp(rdp);

  	/* Clean up if blocked during RCU read-side critical section. */

  	if (special.b.blocked) {

  		/*

  		 * Remove this task from the list it blocked on.  The task

  		 * now remains queued on the rcu_node corresponding to the
  		 * CPU it first blocked on, so there is no longer any need
  		 * to loop.  Retain a WARN_ON_ONCE() out of sheer paranoia.

  		 */

  		rnp = t->rcu_blocked_node;
  		raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
  		WARN_ON_ONCE(rnp != t->rcu_blocked_node);

  		WARN_ON_ONCE(!rcu_is_leaf_node(rnp));

  		empty_norm = !rcu_preempt_blocked_readers_cgp(rnp);

  		WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq &&

  			     (!empty_norm || rnp->qsmask));

  		empty_exp = sync_rcu_exp_done(rnp);

  		smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */

  		np = rcu_next_node_entry(t, rnp);

  		list_del_init(&t->rcu_node_entry);

  		t->rcu_blocked_node = NULL;

  		trace_rcu_unlock_preempted_task(TPS("rcu_preempt"),

  						rnp->gp_seq, t->pid);

  		if (&t->rcu_node_entry == rnp->gp_tasks)

  			WRITE_ONCE(rnp->gp_tasks, np);

  		if (&t->rcu_node_entry == rnp->exp_tasks)

  			WRITE_ONCE(rnp->exp_tasks, np);

  		if (IS_ENABLED(CONFIG_RCU_BOOST)) {

  			/* Snapshot ->boost_mtx ownership w/rnp->lock held. */
  			drop_boost_mutex = rt_mutex_owner(&rnp->boost_mtx) == t;

  			if (&t->rcu_node_entry == rnp->boost_tasks)

  				WRITE_ONCE(rnp->boost_tasks, np);

  		}

  
  		/*
  		 * If this was the last task on the current list, and if
  		 * we aren't waiting on any CPUs, report the quiescent state.

  		 * Note that rcu_report_unblock_qs_rnp() releases rnp->lock,
  		 * so we must take a snapshot of the expedited state.

  		 */

  		empty_exp_now = sync_rcu_exp_done(rnp);

  		if (!empty_norm && !rcu_preempt_blocked_readers_cgp(rnp)) {

  			trace_rcu_quiescent_state_report(TPS("preempt_rcu"),

  							 rnp->gp_seq,

  							 0, rnp->qsmask,
  							 rnp->level,
  							 rnp->grplo,
  							 rnp->grphi,
  							 !!rnp->gp_tasks);

  			rcu_report_unblock_qs_rnp(rnp, flags);

  		} else {

  			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

  		}

  		/* Unboost if we were boosted. */

  		if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex)

  			rt_mutex_futex_unlock(&rnp->boost_mtx);

  		/*
  		 * If this was the last task on the expedited lists,
  		 * then we need to report up the rcu_node hierarchy.
  		 */

  		if (!empty_exp && empty_exp_now)

  			rcu_report_exp_rnp(rnp, true);

  	} else {
  		local_irq_restore(flags);

  	}

  }

  /*

   * Is a deferred quiescent-state pending, and are we also not in
   * an RCU read-side critical section?  It is the caller's responsibility
   * to ensure it is otherwise safe to report any deferred quiescent
   * states.  The reason for this is that it is safe to report a
   * quiescent state during context switch even though preemption
   * is disabled.  This function cannot be expected to understand these
   * nuances, so the caller must handle them.
   */
  static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
  {

  	return (__this_cpu_read(rcu_data.exp_deferred_qs) ||

  		READ_ONCE(t->rcu_read_unlock_special.s)) &&

  	       rcu_preempt_depth() == 0;

  }
  
  /*
   * Report a deferred quiescent state if needed and safe to do so.
   * As with rcu_preempt_need_deferred_qs(), "safe" involves only
   * not being in an RCU read-side critical section.  The caller must
   * evaluate safety in terms of interrupt, softirq, and preemption
   * disabling.
   */
  static void rcu_preempt_deferred_qs(struct task_struct *t)
  {
  	unsigned long flags;

  
  	if (!rcu_preempt_need_deferred_qs(t))
  		return;

  	local_irq_save(flags);
  	rcu_preempt_deferred_qs_irqrestore(t, flags);

  }
  
  /*

   * Minimal handler to give the scheduler a chance to re-evaluate.
   */
  static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
  {
  	struct rcu_data *rdp;
  
  	rdp = container_of(iwp, struct rcu_data, defer_qs_iw);
  	rdp->defer_qs_iw_pending = false;
  }
  
  /*

   * Handle special cases during rcu_read_unlock(), such as needing to
   * notify RCU core processing or task having blocked during the RCU
   * read-side critical section.
   */
  static void rcu_read_unlock_special(struct task_struct *t)
  {
  	unsigned long flags;
  	bool preempt_bh_were_disabled =
  			!!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));
  	bool irqs_were_disabled;
  
  	/* NMI handlers cannot block and cannot safely manipulate state. */
  	if (in_nmi())
  		return;
  
  	local_irq_save(flags);
  	irqs_were_disabled = irqs_disabled_flags(flags);

  	if (preempt_bh_were_disabled || irqs_were_disabled) {

  		bool exp;
  		struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
  		struct rcu_node *rnp = rdp->mynode;

  		exp = (t->rcu_blocked_node &&
  		       READ_ONCE(t->rcu_blocked_node->exp_tasks)) ||
  		      (rdp->grpmask & READ_ONCE(rnp->expmask));

  		// Need to defer quiescent state until everything is enabled.

  		if (use_softirq && (in_irq() || (exp && !irqs_were_disabled))) {
  			// Using softirq, safe to awaken, and either the
  			// wakeup is free or there is an expedited GP.

  			raise_softirq_irqoff(RCU_SOFTIRQ);
  		} else {

  			// Enabling BH or preempt does reschedule, so...

  			// Also if no expediting, slow is OK.
  			// Plus nohz_full CPUs eventually get tick enabled.

  			set_tsk_need_resched(current);
  			set_preempt_need_resched();

  			if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled &&

  			    !rdp->defer_qs_iw_pending && exp) {
  				// Get scheduler to re-evaluate and call hooks.
  				// If !IRQ_WORK, FQS scan will eventually IPI.
  				init_irq_work(&rdp->defer_qs_iw,
  					      rcu_preempt_deferred_qs_handler);
  				rdp->defer_qs_iw_pending = true;
  				irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
  			}

  		}

  		local_irq_restore(flags);
  		return;
  	}
  	rcu_preempt_deferred_qs_irqrestore(t, flags);
  }
  
  /*

   * Check that the list of blocked tasks for the newly completed grace
   * period is in fact empty.  It is a serious bug to complete a grace
   * period that still has RCU readers blocked!  This function must be

   * invoked -before- updating this rnp's ->gp_seq.

   *
   * Also, if there are blocked tasks on the list, they automatically
   * block the newly created grace period, so set up ->gp_tasks accordingly.

   */

  static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)

  {

  	struct task_struct *t;

  	RCU_LOCKDEP_WARN(preemptible(), "rcu_preempt_check_blocked_tasks() invoked with preemption enabled!!!
  ");

  	raw_lockdep_assert_held_rcu_node(rnp);

  	if (WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp)))

  		dump_blkd_tasks(rnp, 10);

  	if (rcu_preempt_has_tasks(rnp) &&
  	    (rnp->qsmaskinit || rnp->wait_blkd_tasks)) {

  		WRITE_ONCE(rnp->gp_tasks, rnp->blkd_tasks.next);

  		t = container_of(rnp->gp_tasks, struct task_struct,
  				 rcu_node_entry);
  		trace_rcu_unlock_preempted_task(TPS("rcu_preempt-GPS"),

  						rnp->gp_seq, t->pid);

  	}

  	WARN_ON_ONCE(rnp->qsmask);

  }

  /*

   * Check for a quiescent state from the current CPU, including voluntary
   * context switches for Tasks RCU.  When a task blocks, the task is
   * recorded in the corresponding CPU's rcu_node structure, which is checked
   * elsewhere, hence this function need only check for quiescent states
   * related to the current CPU, not to those related to tasks.

   */

  static void rcu_flavor_sched_clock_irq(int user)

  {
  	struct task_struct *t = current;

  	if (user || rcu_is_cpu_rrupt_from_idle()) {
  		rcu_note_voluntary_context_switch(current);
  	}

  	if (rcu_preempt_depth() > 0 ||

  	    (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK))) {
  		/* No QS, force context switch if deferred. */

  		if (rcu_preempt_need_deferred_qs(t)) {
  			set_tsk_need_resched(t);
  			set_preempt_need_resched();
  		}

  	} else if (rcu_preempt_need_deferred_qs(t)) {
  		rcu_preempt_deferred_qs(t); /* Report deferred QS. */
  		return;

  	} else if (!WARN_ON_ONCE(rcu_preempt_depth())) {

  		rcu_qs(); /* Report immediate QS. */

  		return;
  	}

  
  	/* If GP is oldish, ask for help from rcu_read_unlock_special(). */

  	if (rcu_preempt_depth() > 0 &&

  	    __this_cpu_read(rcu_data.core_needs_qs) &&
  	    __this_cpu_read(rcu_data.cpu_no_qs.b.norm) &&

  	    !t->rcu_read_unlock_special.b.need_qs &&

  	    time_after(jiffies, rcu_state.gp_start + HZ))

  		t->rcu_read_unlock_special.b.need_qs = true;

  }

  /*
   * Check for a task exiting while in a preemptible-RCU read-side

   * critical section, clean up if so.  No need to issue warnings, as
   * debug_check_no_locks_held() already does this if lockdep is enabled.
   * Besides, if this function does anything other than just immediately
   * return, there was a bug of some sort.  Spewing warnings from this
   * function is like as not to simply obscure important prior warnings.

   */
  void exit_rcu(void)
  {
  	struct task_struct *t = current;

  	if (unlikely(!list_empty(&current->rcu_node_entry))) {

  		rcu_preempt_depth_set(1);

  		barrier();

  		WRITE_ONCE(t->rcu_read_unlock_special.b.blocked, true);

  	} else if (unlikely(rcu_preempt_depth())) {
  		rcu_preempt_depth_set(1);

  	} else {

  		return;

  	}

  	__rcu_read_unlock();

  	rcu_preempt_deferred_qs(current);

  }

  /*
   * Dump the blocked-tasks state, but limit the list dump to the
   * specified number of elements.
   */

  static void

  dump_blkd_tasks(struct rcu_node *rnp, int ncheck)

  {

  	int cpu;

  	int i;
  	struct list_head *lhp;

  	bool onl;
  	struct rcu_data *rdp;

  	struct rcu_node *rnp1;

  	raw_lockdep_assert_held_rcu_node(rnp);

  	pr_info("%s: grp: %d-%d level: %d ->gp_seq %ld ->completedqs %ld
  ",

  		__func__, rnp->grplo, rnp->grphi, rnp->level,

  		(long)READ_ONCE(rnp->gp_seq), (long)rnp->completedqs);

  	for (rnp1 = rnp; rnp1; rnp1 = rnp1->parent)
  		pr_info("%s: %d:%d ->qsmask %#lx ->qsmaskinit %#lx ->qsmaskinitnext %#lx
  ",
  			__func__, rnp1->grplo, rnp1->grphi, rnp1->qsmask, rnp1->qsmaskinit, rnp1->qsmaskinitnext);

  	pr_info("%s: ->gp_tasks %p ->boost_tasks %p ->exp_tasks %p
  ",

  		__func__, READ_ONCE(rnp->gp_tasks), data_race(rnp->boost_tasks),

  		READ_ONCE(rnp->exp_tasks));

  	pr_info("%s: ->blkd_tasks", __func__);

  	i = 0;
  	list_for_each(lhp, &rnp->blkd_tasks) {
  		pr_cont(" %p", lhp);

  		if (++i >= ncheck)

  			break;
  	}
  	pr_cont("
  ");

  	for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++) {

  		rdp = per_cpu_ptr(&rcu_data, cpu);

  		onl = !!(rdp->grpmask & rcu_rnp_online_cpus(rnp));
  		pr_info("\t%d: %c online: %ld(%d) offline: %ld(%d)
  ",
  			cpu, ".o"[onl],
  			(long)rdp->rcu_onl_gp_seq, rdp->rcu_onl_gp_flags,
  			(long)rdp->rcu_ofl_gp_seq, rdp->rcu_ofl_gp_flags);
  	}

  }

  #else /* #ifdef CONFIG_PREEMPT_RCU */

  
  /*

   * If strict grace periods are enabled, and if the calling
   * __rcu_read_unlock() marks the beginning of a quiescent state, immediately
   * report that quiescent state and, if requested, spin for a bit.
   */
  void rcu_read_unlock_strict(void)
  {
  	struct rcu_data *rdp;
  
  	if (!IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) ||
  	   irqs_disabled() || preempt_count() || !rcu_state.gp_kthread)
  		return;
  	rdp = this_cpu_ptr(&rcu_data);

  	rcu_report_qs_rdp(rdp);

  	udelay(rcu_unlock_delay);
  }
  EXPORT_SYMBOL_GPL(rcu_read_unlock_strict);
  
  /*

   * Tell them what RCU they are running.
   */

  static void __init rcu_bootup_announce(void)

  {

  	pr_info("Hierarchical RCU implementation.
  ");

  	rcu_bootup_announce_oddness();

  }

  /*

   * Note a quiescent state for PREEMPTION=n.  Because we do not need to know

   * how many quiescent states passed, just if there was at least one since
   * the start of the grace period, this just sets a flag.  The caller must
   * have disabled preemption.
   */
  static void rcu_qs(void)

  {

  	RCU_LOCKDEP_WARN(preemptible(), "rcu_qs() invoked with preemption enabled!!!");
  	if (!__this_cpu_read(rcu_data.cpu_no_qs.s))
  		return;
  	trace_rcu_grace_period(TPS("rcu_sched"),
  			       __this_cpu_read(rcu_data.gp_seq), TPS("cpuqs"));
  	__this_cpu_write(rcu_data.cpu_no_qs.b.norm, false);
  	if (!__this_cpu_read(rcu_data.cpu_no_qs.b.exp))
  		return;
  	__this_cpu_write(rcu_data.cpu_no_qs.b.exp, false);

  	rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));

  }

  /*

   * Register an urgently needed quiescent state.  If there is an
   * emergency, invoke rcu_momentary_dyntick_idle() to do a heavy-weight
   * dyntick-idle quiescent state visible to other CPUs, which will in
   * some cases serve for expedited as well as normal grace periods.
   * Either way, register a lightweight quiescent state.

   */
  void rcu_all_qs(void)
  {
  	unsigned long flags;

  	if (!raw_cpu_read(rcu_data.rcu_urgent_qs))

  		return;
  	preempt_disable();
  	/* Load rcu_urgent_qs before other flags. */

  	if (!smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs))) {

  		preempt_enable();
  		return;
  	}

  	this_cpu_write(rcu_data.rcu_urgent_qs, false);

  	if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs))) {

  		local_irq_save(flags);
  		rcu_momentary_dyntick_idle();
  		local_irq_restore(flags);
  	}

  	rcu_qs();

  	preempt_enable();
  }
  EXPORT_SYMBOL_GPL(rcu_all_qs);
  
  /*

   * Note a PREEMPTION=n context switch. The caller must have disabled interrupts.

   */

  void rcu_note_context_switch(bool preempt)

  {

  	trace_rcu_utilization(TPS("Start context switch"));
  	rcu_qs();
  	/* Load rcu_urgent_qs before other flags. */

  	if (!smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs)))

  		goto out;

  	this_cpu_write(rcu_data.rcu_urgent_qs, false);
  	if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs)))

  		rcu_momentary_dyntick_idle();

  	rcu_tasks_qs(current, preempt);

  out:
  	trace_rcu_utilization(TPS("End context switch"));

  }

  EXPORT_SYMBOL_GPL(rcu_note_context_switch);

  
  /*

   * Because preemptible RCU does not exist, there are never any preempted

   * RCU readers.
   */

  static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)

  {
  	return 0;
  }

  /*
   * Because there is no preemptible RCU, there can be no readers blocked.
   */
  static bool rcu_preempt_has_tasks(struct rcu_node *rnp)

  {

  	return false;

  }

  /*

   * Because there is no preemptible RCU, there can be no deferred quiescent
   * states.
   */
  static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
  {
  	return false;
  }
  static void rcu_preempt_deferred_qs(struct task_struct *t) { }
  
  /*

   * Because there is no preemptible RCU, there can be no readers blocked,

   * so there is no need to check for blocked tasks.  So check only for
   * bogus qsmask values.

   */

  static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)

  {

  	WARN_ON_ONCE(rnp->qsmask);

  }

  /*

   * Check to see if this CPU is in a non-context-switch quiescent state,
   * namely user mode and idle loop.

   */

  static void rcu_flavor_sched_clock_irq(int user)

  {

  	if (user || rcu_is_cpu_rrupt_from_idle()) {

  		/*
  		 * Get here if this CPU took its interrupt from user
  		 * mode or from the idle loop, and if this is not a
  		 * nested interrupt.  In this case, the CPU is in
  		 * a quiescent state, so note it.
  		 *
  		 * No memory barrier is required here because rcu_qs()
  		 * references only CPU-local variables that other CPUs
  		 * neither access nor modify, at least not while the
  		 * corresponding CPU is online.
  		 */
  
  		rcu_qs();
  	}

  }

  /*
   * Because preemptible RCU does not exist, tasks cannot possibly exit
   * while in preemptible RCU read-side critical sections.
   */
  void exit_rcu(void)
  {
  }

  /*
   * Dump the guaranteed-empty blocked-tasks state.  Trust but verify.
   */

  static void

  dump_blkd_tasks(struct rcu_node *rnp, int ncheck)

  {
  	WARN_ON_ONCE(!list_empty(&rnp->blkd_tasks));
  }

  #endif /* #else #ifdef CONFIG_PREEMPT_RCU */

  /*
   * If boosting, set rcuc kthreads to realtime priority.
   */
  static void rcu_cpu_kthread_setup(unsigned int cpu)
  {

  #ifdef CONFIG_RCU_BOOST

  	struct sched_param sp;

  	sp.sched_priority = kthread_prio;
  	sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
  #endif /* #ifdef CONFIG_RCU_BOOST */

  }

  #ifdef CONFIG_RCU_BOOST

  /*
   * Carry out RCU priority boosting on the task indicated by ->exp_tasks
   * or ->boost_tasks, advancing the pointer to the next task in the
   * ->blkd_tasks list.
   *
   * Note that irqs must be enabled: boosting the task can block.
   * Returns 1 if there are more tasks needing to be boosted.
   */
  static int rcu_boost(struct rcu_node *rnp)
  {
  	unsigned long flags;

  	struct task_struct *t;
  	struct list_head *tb;

  	if (READ_ONCE(rnp->exp_tasks) == NULL &&
  	    READ_ONCE(rnp->boost_tasks) == NULL)

  		return 0;  /* Nothing left to boost. */

  	raw_spin_lock_irqsave_rcu_node(rnp, flags);

  
  	/*
  	 * Recheck under the lock: all tasks in need of boosting
  	 * might exit their RCU read-side critical sections on their own.
  	 */
  	if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL) {

  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

  		return 0;
  	}
  
  	/*
  	 * Preferentially boost tasks blocking expedited grace periods.
  	 * This cannot starve the normal grace periods because a second
  	 * expedited grace period must boost all blocked tasks, including
  	 * those blocking the pre-existing normal grace period.
  	 */

  	if (rnp->exp_tasks != NULL)

  		tb = rnp->exp_tasks;

  	else

  		tb = rnp->boost_tasks;
  
  	/*
  	 * We boost task t by manufacturing an rt_mutex that appears to
  	 * be held by task t.  We leave a pointer to that rt_mutex where
  	 * task t can find it, and task t will release the mutex when it
  	 * exits its outermost RCU read-side critical section.  Then
  	 * simply acquiring this artificial rt_mutex will boost task
  	 * t's priority.  (Thanks to tglx for suggesting this approach!)
  	 *
  	 * Note that task t must acquire rnp->lock to remove itself from
  	 * the ->blkd_tasks list, which it will do from exit() if from
  	 * nowhere else.  We therefore are guaranteed that task t will
  	 * stay around at least until we drop rnp->lock.  Note that
  	 * rnp->lock also resolves races between our priority boosting
  	 * and task t's exiting its outermost RCU read-side critical
  	 * section.
  	 */
  	t = container_of(tb, struct task_struct, rcu_node_entry);

  	rt_mutex_init_proxy_locked(&rnp->boost_mtx, t);

  	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

  	/* Lock only for side effect: boosts task t's priority. */
  	rt_mutex_lock(&rnp->boost_mtx);
  	rt_mutex_unlock(&rnp->boost_mtx);  /* Then keep lockdep happy. */

  	return READ_ONCE(rnp->exp_tasks) != NULL ||
  	       READ_ONCE(rnp->boost_tasks) != NULL;

  }
  
  /*

   * Priority-boosting kthread, one per leaf rcu_node.

   */
  static int rcu_boost_kthread(void *arg)
  {
  	struct rcu_node *rnp = (struct rcu_node *)arg;
  	int spincnt = 0;
  	int more2boost;

  	trace_rcu_utilization(TPS("Start boost kthread@init"));

  	for (;;) {

  		WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_WAITING);

  		trace_rcu_utilization(TPS("End boost kthread@rcu_wait"));

  		rcu_wait(READ_ONCE(rnp->boost_tasks) ||
  			 READ_ONCE(rnp->exp_tasks));

  		trace_rcu_utilization(TPS("Start boost kthread@rcu_wait"));

  		WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_RUNNING);

  		more2boost = rcu_boost(rnp);
  		if (more2boost)
  			spincnt++;
  		else
  			spincnt = 0;
  		if (spincnt > 10) {

  			WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_YIELDING);

  			trace_rcu_utilization(TPS("End boost kthread@rcu_yield"));

  			schedule_timeout_idle(2);

  			trace_rcu_utilization(TPS("Start boost kthread@rcu_yield"));

  			spincnt = 0;
  		}
  	}

  	/* NOTREACHED */

  	trace_rcu_utilization(TPS("End boost kthread@notreached"));

  	return 0;
  }
  
  /*
   * Check to see if it is time to start boosting RCU readers that are
   * blocking the current grace period, and, if so, tell the per-rcu_node
   * kthread to start boosting them.  If there is an expedited grace
   * period in progress, it is always time to boost.
   *

   * The caller must hold rnp->lock, which this function releases.
   * The ->boost_kthread_task is immortal, so we don't need to worry
   * about it going away.

   */

  static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)

  	__releases(rnp->lock)

  {

  	raw_lockdep_assert_held_rcu_node(rnp);

  	if (!rcu_preempt_blocked_readers_cgp(rnp) && rnp->exp_tasks == NULL) {

  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

  		return;

  	}

  	if (rnp->exp_tasks != NULL ||
  	    (rnp->gp_tasks != NULL &&
  	     rnp->boost_tasks == NULL &&
  	     rnp->qsmask == 0 &&

  	     (!time_after(rnp->boost_time, jiffies) || rcu_state.cbovld))) {

  		if (rnp->exp_tasks == NULL)

  			WRITE_ONCE(rnp->boost_tasks, rnp->gp_tasks);

  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

  		rcu_wake_cond(rnp->boost_kthread_task,

  			      READ_ONCE(rnp->boost_kthread_status));

  	} else {

  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

  	}

  }

  /*

   * Is the current CPU running the RCU-callbacks kthread?
   * Caller must have preemption disabled.
   */
  static bool rcu_is_callbacks_kthread(void)
  {

  	return __this_cpu_read(rcu_data.rcu_cpu_kthread_task) == current;

  }

  #define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY * HZ, 1000)
  
  /*
   * Do priority-boost accounting for the start of a new grace period.
   */
  static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
  {
  	rnp->boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
  }
  
  /*

   * Create an RCU-boost kthread for the specified node if one does not
   * already exist.  We only create this kthread for preemptible RCU.
   * Returns zero if all is well, a negated errno otherwise.
   */

  static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)

  {

  	int rnp_index = rnp - rcu_get_root();

  	unsigned long flags;
  	struct sched_param sp;
  	struct task_struct *t;

  	if (!IS_ENABLED(CONFIG_PREEMPT_RCU))

  		return;

  	if (!rcu_scheduler_fully_active || rcu_rnp_online_cpus(rnp) == 0)

  		return;

  	rcu_state.boost = 1;

  	if (rnp->boost_kthread_task != NULL)

  		return;

  	t = kthread_create(rcu_boost_kthread, (void *)rnp,

  			   "rcub/%d", rnp_index);

  	if (WARN_ON_ONCE(IS_ERR(t)))
  		return;

  	raw_spin_lock_irqsave_rcu_node(rnp, flags);

  	rnp->boost_kthread_task = t;

  	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

  	sp.sched_priority = kthread_prio;

  	sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);

  	wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */

  }

  /*
   * Set the per-rcu_node kthread's affinity to cover all CPUs that are
   * served by the rcu_node in question.  The CPU hotplug lock is still
   * held, so the value of rnp->qsmaskinit will be stable.
   *
   * We don't include outgoingcpu in the affinity set, use -1 if there is
   * no outgoing CPU.  If there are no CPUs left in the affinity set,
   * this function allows the kthread to execute on any CPU.
   */

  static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)

  {

  	struct task_struct *t = rnp->boost_kthread_task;

  	unsigned long mask = rcu_rnp_online_cpus(rnp);

  	cpumask_var_t cm;
  	int cpu;

  	if (!t)

  		return;

  	if (!zalloc_cpumask_var(&cm, GFP_KERNEL))

  		return;

  	for_each_leaf_node_possible_cpu(rnp, cpu)
  		if ((mask & leaf_node_cpu_bit(rnp, cpu)) &&
  		    cpu != outgoingcpu)

  			cpumask_set_cpu(cpu, cm);

  	if (cpumask_weight(cm) == 0)

  		cpumask_setall(cm);

  	set_cpus_allowed_ptr(t, cm);

  	free_cpumask_var(cm);
  }

  /*

   * Spawn boost kthreads -- called as soon as the scheduler is running.

   */

  static void __init rcu_spawn_boost_kthreads(void)

  {

  	struct rcu_node *rnp;

  	rcu_for_each_leaf_node(rnp)

  		rcu_spawn_one_boost_kthread(rnp);

  }

  static void rcu_prepare_kthreads(int cpu)

  {

  	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);

  	struct rcu_node *rnp = rdp->mynode;
  
  	/* Fire up the incoming CPU's kthread and leaf rcu_node kthread. */

  	if (rcu_scheduler_fully_active)

  		rcu_spawn_one_boost_kthread(rnp);

  }

  #else /* #ifdef CONFIG_RCU_BOOST */

  static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)

  	__releases(rnp->lock)

  {

  	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

  }

  static bool rcu_is_callbacks_kthread(void)
  {
  	return false;
  }

  static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
  {
  }

  static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)

  {
  }

  static void __init rcu_spawn_boost_kthreads(void)

  {

  }

  static void rcu_prepare_kthreads(int cpu)

  {
  }

  #endif /* #else #ifdef CONFIG_RCU_BOOST */

  #if !defined(CONFIG_RCU_FAST_NO_HZ)
  
  /*

   * Check to see if any future non-offloaded RCU-related work will need
   * to be done by the current CPU, even if none need be done immediately,
   * returning 1 if so.  This function is part of the RCU implementation;
   * it is -not- an exported member of the RCU API.

   *

   * Because we not have RCU_FAST_NO_HZ, just check whether or not this
   * CPU has RCU callbacks queued.

   */

  int rcu_needs_cpu(u64 basemono, u64 *nextevt)

  {

  	*nextevt = KTIME_MAX;

  	return !rcu_segcblist_empty(&this_cpu_ptr(&rcu_data)->cblist) &&
  	       !rcu_segcblist_is_offloaded(&this_cpu_ptr(&rcu_data)->cblist);

  }
  
  /*
   * Because we do not have RCU_FAST_NO_HZ, don't bother cleaning up
   * after it.
   */

  static void rcu_cleanup_after_idle(void)

  {
  }
  
  /*

   * Do the idle-entry grace-period work, which, because CONFIG_RCU_FAST_NO_HZ=n,

   * is nothing.
   */

  static void rcu_prepare_for_idle(void)

  {
  }

  #else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */

  /*
   * This code is invoked when a CPU goes idle, at which point we want
   * to have the CPU do everything required for RCU so that it can enter

   * the energy-efficient dyntick-idle mode.

   *

   * The following preprocessor symbol controls this:

   *

   * RCU_IDLE_GP_DELAY gives the number of jiffies that a CPU is permitted
   *	to sleep in dyntick-idle mode with RCU callbacks pending.  This
   *	is sized to be roughly one RCU grace period.  Those energy-efficiency
   *	benchmarkers who might otherwise be tempted to set this to a large
   *	number, be warned: Setting RCU_IDLE_GP_DELAY too high can hang your
   *	system.  And if you are -that- concerned about energy efficiency,
   *	just power the system down and be done with it!
   *

   * The value below works well in practice.  If future workloads require

   * adjustment, they can be converted into kernel config parameters, though
   * making the state machine smarter might be a better option.
   */

  #define RCU_IDLE_GP_DELAY 4		/* Roughly one grace period. */

  static int rcu_idle_gp_delay = RCU_IDLE_GP_DELAY;
  module_param(rcu_idle_gp_delay, int, 0644);

  /*

   * Try to advance callbacks on the current CPU, but only if it has been
   * awhile since the last time we did so.  Afterwards, if there are any
   * callbacks ready for immediate invocation, return true.

   */

  static bool __maybe_unused rcu_try_advance_all_cbs(void)

  {

  	bool cbs_ready = false;

  	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);

  	struct rcu_node *rnp;

  	/* Exit early if we advanced recently. */

  	if (jiffies == rdp->last_advance_all)

  		return false;

  	rdp->last_advance_all = jiffies;

  	rnp = rdp->mynode;

  	/*
  	 * Don't bother checking unless a grace period has
  	 * completed since we last checked and there are
  	 * callbacks not yet ready to invoke.
  	 */
  	if ((rcu_seq_completed_gp(rdp->gp_seq,
  				  rcu_seq_current(&rnp->gp_seq)) ||
  	     unlikely(READ_ONCE(rdp->gpwrap))) &&
  	    rcu_segcblist_pend_cbs(&rdp->cblist))
  		note_gp_changes(rdp);
  
  	if (rcu_segcblist_ready_cbs(&rdp->cblist))
  		cbs_ready = true;

  	return cbs_ready;

  }
  
  /*

   * Allow the CPU to enter dyntick-idle mode unless it has callbacks ready
   * to invoke.  If the CPU has callbacks, try to advance them.  Tell the

   * caller about what to set the timeout.

   *

   * The caller must have disabled interrupts.

   */

  int rcu_needs_cpu(u64 basemono, u64 *nextevt)

  {

  	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);

  	unsigned long dj;

  	lockdep_assert_irqs_disabled();

  	/* If no non-offloaded callbacks, RCU doesn't need the CPU. */
  	if (rcu_segcblist_empty(&rdp->cblist) ||
  	    rcu_segcblist_is_offloaded(&this_cpu_ptr(&rcu_data)->cblist)) {

  		*nextevt = KTIME_MAX;

  		return 0;
  	}

  
  	/* Attempt to advance callbacks. */
  	if (rcu_try_advance_all_cbs()) {
  		/* Some ready to invoke, so initiate later invocation. */
  		invoke_rcu_core();

  		return 1;
  	}

  	rdp->last_accelerate = jiffies;

  	/* Request timer and round. */
  	dj = round_up(rcu_idle_gp_delay + jiffies, rcu_idle_gp_delay) - jiffies;

  	*nextevt = basemono + dj * TICK_NSEC;

  	return 0;
  }
  
  /*

   * Prepare a CPU for idle from an RCU perspective.  The first major task is to
   * sense whether nohz mode has been enabled or disabled via sysfs.  The second
   * major task is to accelerate (that is, assign grace-period numbers to) any
   * recently arrived callbacks.

   *
   * The caller must have disabled interrupts.

   */

  static void rcu_prepare_for_idle(void)

  {

  	bool needwake;

  	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);

  	struct rcu_node *rnp;

  	int tne;

  	lockdep_assert_irqs_disabled();

  	if (rcu_segcblist_is_offloaded(&rdp->cblist))

  		return;

  	/* Handle nohz enablement switches conservatively. */

  	tne = READ_ONCE(tick_nohz_active);

  	if (tne != rdp->tick_nohz_enabled_snap) {

  		if (!rcu_segcblist_empty(&rdp->cblist))

  			invoke_rcu_core(); /* force nohz to see update. */