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

   * or preemptible semantics.

   *
   * This program is free software; you can redistribute it and/or modify
   * it under the terms of the GNU General Public License as published by
   * the Free Software Foundation; either version 2 of the License, or
   * (at your option) any later version.
   *
   * This program is distributed in the hope that it will be useful,
   * but WITHOUT ANY WARRANTY; without even the implied warranty of
   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   * GNU General Public License for more details.
   *
   * You should have received a copy of the GNU General Public License
   * along with this program; if not, write to the Free Software
   * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
   *
   * Copyright Red Hat, 2009
   * Copyright IBM Corporation, 2009
   *
   * Author: Ingo Molnar <mingo@elte.hu>
   *	   Paul E. McKenney <paulmck@linux.vnet.ibm.com>
   */

  #include <linux/delay.h>

  #include <linux/stop_machine.h>

  #define RCU_KTHREAD_PRIO 1
  
  #ifdef CONFIG_RCU_BOOST
  #define RCU_BOOST_PRIO CONFIG_RCU_BOOST_PRIO
  #else
  #define RCU_BOOST_PRIO RCU_KTHREAD_PRIO
  #endif

  /*
   * Check the RCU kernel configuration parameters and print informative
   * messages about anything out of the ordinary.  If you like #ifdef, you
   * will love this function.
   */
  static void __init rcu_bootup_announce_oddness(void)
  {
  #ifdef CONFIG_RCU_TRACE
  	printk(KERN_INFO "\tRCU debugfs-based tracing is enabled.
  ");
  #endif
  #if (defined(CONFIG_64BIT) && CONFIG_RCU_FANOUT != 64) || (!defined(CONFIG_64BIT) && CONFIG_RCU_FANOUT != 32)
  	printk(KERN_INFO "\tCONFIG_RCU_FANOUT set to non-default value of %d
  ",
  	       CONFIG_RCU_FANOUT);
  #endif
  #ifdef CONFIG_RCU_FANOUT_EXACT
  	printk(KERN_INFO "\tHierarchical RCU autobalancing is disabled.
  ");
  #endif
  #ifdef CONFIG_RCU_FAST_NO_HZ
  	printk(KERN_INFO
  	       "\tRCU dyntick-idle grace-period acceleration is enabled.
  ");
  #endif
  #ifdef CONFIG_PROVE_RCU
  	printk(KERN_INFO "\tRCU lockdep checking is enabled.
  ");
  #endif
  #ifdef CONFIG_RCU_TORTURE_TEST_RUNNABLE
  	printk(KERN_INFO "\tRCU torture testing starts during boot.
  ");
  #endif

  #if defined(CONFIG_TREE_PREEMPT_RCU) && !defined(CONFIG_RCU_CPU_STALL_VERBOSE)

  	printk(KERN_INFO "\tVerbose stalled-CPUs detection is disabled.
  ");
  #endif
  #if NUM_RCU_LVL_4 != 0
  	printk(KERN_INFO "\tExperimental four-level hierarchy is enabled.
  ");
  #endif
  }

  #ifdef CONFIG_TREE_PREEMPT_RCU

  struct rcu_state rcu_preempt_state = RCU_STATE_INITIALIZER(rcu_preempt);

  DEFINE_PER_CPU(struct rcu_data, rcu_preempt_data);

  static struct rcu_state *rcu_state = &rcu_preempt_state;

  static void rcu_read_unlock_special(struct task_struct *t);

  static int rcu_preempted_readers_exp(struct rcu_node *rnp);

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

  static void __init rcu_bootup_announce(void)

  {

  	printk(KERN_INFO "Preemptible hierarchical RCU implementation.
  ");

  	rcu_bootup_announce_oddness();

  }
  
  /*
   * Return the number of RCU-preempt batches processed thus far
   * for debug and statistics.
   */
  long rcu_batches_completed_preempt(void)
  {
  	return rcu_preempt_state.completed;
  }
  EXPORT_SYMBOL_GPL(rcu_batches_completed_preempt);
  
  /*
   * Return the number of RCU batches processed thus far for debug & stats.
   */
  long rcu_batches_completed(void)
  {
  	return rcu_batches_completed_preempt();
  }
  EXPORT_SYMBOL_GPL(rcu_batches_completed);
  
  /*

   * Force a quiescent state for preemptible RCU.
   */
  void rcu_force_quiescent_state(void)
  {
  	force_quiescent_state(&rcu_preempt_state, 0);
  }
  EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);
  
  /*

   * Record a preemptible-RCU quiescent state for the specified CPU.  Note

   * that this just means that the task currently running on the CPU is
   * not in a quiescent state.  There might be any number of tasks blocked
   * while in an RCU read-side critical section.

   *
   * Unlike the other rcu_*_qs() functions, callers to this function
   * must disable irqs in order to protect the assignment to
   * ->rcu_read_unlock_special.

   */

  static void rcu_preempt_qs(int cpu)

  {
  	struct rcu_data *rdp = &per_cpu(rcu_preempt_data, cpu);

  	rdp->passed_quiesce_gpnum = rdp->gpnum;

  	barrier();

  	if (rdp->passed_quiesce == 0)

  		trace_rcu_grace_period("rcu_preempt", rdp->gpnum, "cpuqs");

  	rdp->passed_quiesce = 1;

  	current->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;

  }
  
  /*

   * 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 preemption.

   */

  static void rcu_preempt_note_context_switch(int cpu)

  {
  	struct task_struct *t = current;

  	unsigned long flags;

  	struct rcu_data *rdp;
  	struct rcu_node *rnp;

  	if (t->rcu_read_lock_nesting > 0 &&

  	    (t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {
  
  		/* Possibly blocking in an RCU read-side critical section. */

  		rdp = per_cpu_ptr(rcu_preempt_state.rda, cpu);

  		rnp = rdp->mynode;

  		raw_spin_lock_irqsave(&rnp->lock, flags);

  		t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;

  		t->rcu_blocked_node = rnp;

  
  		/*
  		 * If this CPU has already checked in, then this task
  		 * will hold up the next grace period rather than the
  		 * current grace period.  Queue the task accordingly.
  		 * If the task is queued for the current grace period
  		 * (i.e., this CPU has not yet passed through a quiescent
  		 * state for the current grace period), then as long
  		 * as that task remains queued, the current grace period

  		 * cannot end.  Note that there is some uncertainty as
  		 * to exactly when the current grace period started.
  		 * We take a conservative approach, which can result
  		 * in unnecessarily waiting on tasks that started very
  		 * slightly after the current grace period began.  C'est
  		 * la vie!!!

  		 *
  		 * But first, note that the current CPU must still be
  		 * on line!

  		 */

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

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

  		if ((rnp->qsmask & rdp->grpmask) && rnp->gp_tasks != NULL) {
  			list_add(&t->rcu_node_entry, rnp->gp_tasks->prev);
  			rnp->gp_tasks = &t->rcu_node_entry;

  #ifdef CONFIG_RCU_BOOST
  			if (rnp->boost_tasks != NULL)
  				rnp->boost_tasks = rnp->gp_tasks;
  #endif /* #ifdef CONFIG_RCU_BOOST */

  		} else {
  			list_add(&t->rcu_node_entry, &rnp->blkd_tasks);
  			if (rnp->qsmask & rdp->grpmask)
  				rnp->gp_tasks = &t->rcu_node_entry;
  		}

  		trace_rcu_preempt_task(rdp->rsp->name,
  				       t->pid,
  				       (rnp->qsmask & rdp->grpmask)
  				       ? rnp->gpnum
  				       : rnp->gpnum + 1);

  		raw_spin_unlock_irqrestore(&rnp->lock, flags);

  	} else if (t->rcu_read_lock_nesting < 0 &&
  		   t->rcu_read_unlock_special) {
  
  		/*
  		 * Complete exit from RCU read-side critical section on
  		 * behalf of preempted instance of __rcu_read_unlock().
  		 */
  		rcu_read_unlock_special(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.
  	 */

  	local_irq_save(flags);

  	rcu_preempt_qs(cpu);

  	local_irq_restore(flags);

  }
  
  /*

   * Tree-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)
  {

  	current->rcu_read_lock_nesting++;

  	barrier();  /* needed if we ever invoke rcu_read_lock in rcutree.c */
  }
  EXPORT_SYMBOL_GPL(__rcu_read_lock);

  /*
   * 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 rnp->gp_tasks != NULL;

  }

  /*
   * Record a quiescent state for all tasks that were previously queued
   * on the specified rcu_node structure and that were blocking the current
   * RCU grace period.  The caller must hold the specified rnp->lock with
   * irqs disabled, and this lock is released upon return, but irqs remain
   * disabled.
   */

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

  	__releases(rnp->lock)
  {
  	unsigned long mask;
  	struct rcu_node *rnp_p;

  	if (rnp->qsmask != 0 || rcu_preempt_blocked_readers_cgp(rnp)) {

  		raw_spin_unlock_irqrestore(&rnp->lock, flags);

  		return;  /* Still need more quiescent states! */
  	}
  
  	rnp_p = rnp->parent;
  	if (rnp_p == NULL) {
  		/*
  		 * Either there is only one rcu_node in the tree,
  		 * or tasks were kicked up to root rcu_node due to
  		 * CPUs going offline.
  		 */

  		rcu_report_qs_rsp(&rcu_preempt_state, flags);

  		return;
  	}
  
  	/* Report up the rest of the hierarchy. */
  	mask = rnp->grpmask;

  	raw_spin_unlock(&rnp->lock);	/* irqs remain disabled. */
  	raw_spin_lock(&rnp_p->lock);	/* irqs already disabled. */

  	rcu_report_qs_rnp(mask, &rcu_preempt_state, rnp_p, flags);

  }
  
  /*

   * 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;
  }
  
  /*

   * 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 noinline void rcu_read_unlock_special(struct task_struct *t)

  {
  	int empty;

  	int empty_exp;

  	int empty_exp_now;

  	unsigned long flags;

  	struct list_head *np;

  #ifdef CONFIG_RCU_BOOST
  	struct rt_mutex *rbmp = NULL;
  #endif /* #ifdef CONFIG_RCU_BOOST */

  	struct rcu_node *rnp;
  	int special;
  
  	/* NMI handlers cannot block and cannot safely manipulate state. */
  	if (in_nmi())
  		return;
  
  	local_irq_save(flags);
  
  	/*
  	 * If RCU core is waiting for this CPU to exit critical section,
  	 * let it know that we have done so.
  	 */
  	special = t->rcu_read_unlock_special;
  	if (special & RCU_READ_UNLOCK_NEED_QS) {

  		rcu_preempt_qs(smp_processor_id());

  	}
  
  	/* Hardware IRQ handlers cannot block. */

  	if (in_irq() || in_serving_softirq()) {

  		local_irq_restore(flags);
  		return;
  	}
  
  	/* Clean up if blocked during RCU read-side critical section. */
  	if (special & RCU_READ_UNLOCK_BLOCKED) {
  		t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;

  		/*
  		 * Remove this task from the list it blocked on.  The
  		 * task can migrate while we acquire the lock, but at
  		 * most one time.  So at most two passes through loop.
  		 */
  		for (;;) {

  			rnp = t->rcu_blocked_node;

  			raw_spin_lock(&rnp->lock);  /* irqs already disabled. */

  			if (rnp == t->rcu_blocked_node)

  				break;

  			raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */

  		}

  		empty = !rcu_preempt_blocked_readers_cgp(rnp);

  		empty_exp = !rcu_preempted_readers_exp(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("rcu_preempt",
  						rnp->gpnum, t->pid);

  		if (&t->rcu_node_entry == rnp->gp_tasks)
  			rnp->gp_tasks = np;
  		if (&t->rcu_node_entry == rnp->exp_tasks)
  			rnp->exp_tasks = np;

  #ifdef CONFIG_RCU_BOOST
  		if (&t->rcu_node_entry == rnp->boost_tasks)
  			rnp->boost_tasks = np;

  		/* Snapshot/clear ->rcu_boost_mutex with rcu_node lock held. */
  		if (t->rcu_boost_mutex) {
  			rbmp = t->rcu_boost_mutex;
  			t->rcu_boost_mutex = NULL;

  		}

  #endif /* #ifdef CONFIG_RCU_BOOST */

  
  		/*
  		 * 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 = !rcu_preempted_readers_exp(rnp);

  		if (!empty && !rcu_preempt_blocked_readers_cgp(rnp)) {
  			trace_rcu_quiescent_state_report("preempt_rcu",
  							 rnp->gpnum,
  							 0, rnp->qsmask,
  							 rnp->level,
  							 rnp->grplo,
  							 rnp->grphi,
  							 !!rnp->gp_tasks);

  			rcu_report_unblock_qs_rnp(rnp, flags);

  		} else
  			raw_spin_unlock_irqrestore(&rnp->lock, flags);

  #ifdef CONFIG_RCU_BOOST
  		/* Unboost if we were boosted. */

  		if (rbmp)
  			rt_mutex_unlock(rbmp);

  #endif /* #ifdef CONFIG_RCU_BOOST */

  		/*
  		 * 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(&rcu_preempt_state, rnp, true);

  	} else {
  		local_irq_restore(flags);

  	}

  }
  
  /*

   * Tree-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 (t->rcu_read_lock_nesting != 1)
  		--t->rcu_read_lock_nesting;
  	else {

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

  		t->rcu_read_lock_nesting = INT_MIN;
  		barrier();  /* assign before ->rcu_read_unlock_special load */

  		if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
  			rcu_read_unlock_special(t);

  		barrier();  /* ->rcu_read_unlock_special load before assign */
  		t->rcu_read_lock_nesting = 0;

  	}

  #ifdef CONFIG_PROVE_LOCKING

  	{
  		int rrln = ACCESS_ONCE(t->rcu_read_lock_nesting);
  
  		WARN_ON_ONCE(rrln < 0 && rrln > INT_MIN / 2);
  	}

  #endif /* #ifdef CONFIG_PROVE_LOCKING */

  }
  EXPORT_SYMBOL_GPL(__rcu_read_unlock);

  #ifdef CONFIG_RCU_CPU_STALL_VERBOSE
  
  /*
   * Dump detailed information for all tasks blocking the current RCU
   * grace period on the specified rcu_node structure.
   */
  static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
  {
  	unsigned long flags;

  	struct task_struct *t;

  	if (!rcu_preempt_blocked_readers_cgp(rnp))

  		return;
  	raw_spin_lock_irqsave(&rnp->lock, flags);
  	t = list_entry(rnp->gp_tasks,
  		       struct task_struct, rcu_node_entry);
  	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry)
  		sched_show_task(t);
  	raw_spin_unlock_irqrestore(&rnp->lock, flags);

  }
  
  /*
   * Dump detailed information for all tasks blocking the current RCU
   * grace period.
   */
  static void rcu_print_detail_task_stall(struct rcu_state *rsp)
  {
  	struct rcu_node *rnp = rcu_get_root(rsp);
  
  	rcu_print_detail_task_stall_rnp(rnp);
  	rcu_for_each_leaf_node(rsp, rnp)
  		rcu_print_detail_task_stall_rnp(rnp);
  }
  
  #else /* #ifdef CONFIG_RCU_CPU_STALL_VERBOSE */
  
  static void rcu_print_detail_task_stall(struct rcu_state *rsp)
  {
  }
  
  #endif /* #else #ifdef CONFIG_RCU_CPU_STALL_VERBOSE */

  /*
   * Scan the current list of tasks blocked within RCU read-side critical
   * sections, printing out the tid of each.
   */

  static int rcu_print_task_stall(struct rcu_node *rnp)

  {

  	struct task_struct *t;

  	int ndetected = 0;

  	if (!rcu_preempt_blocked_readers_cgp(rnp))

  		return 0;

  	t = list_entry(rnp->gp_tasks,
  		       struct task_struct, rcu_node_entry);

  	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {

  		printk(" P%d", t->pid);

  		ndetected++;
  	}
  	return ndetected;

  }

  /*
   * Suppress preemptible RCU's CPU stall warnings by pushing the
   * time of the next stall-warning message comfortably far into the
   * future.
   */
  static void rcu_preempt_stall_reset(void)
  {
  	rcu_preempt_state.jiffies_stall = jiffies + ULONG_MAX / 2;
  }

  /*

   * 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 ->gpnum, and the rnp's ->lock
   * must be held by the caller.

   *
   * 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)
  {

  	WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp));

  	if (!list_empty(&rnp->blkd_tasks))
  		rnp->gp_tasks = rnp->blkd_tasks.next;

  	WARN_ON_ONCE(rnp->qsmask);

  }

  #ifdef CONFIG_HOTPLUG_CPU
  
  /*

   * Handle tasklist migration for case in which all CPUs covered by the
   * specified rcu_node have gone offline.  Move them up to the root
   * rcu_node.  The reason for not just moving them to the immediate
   * parent is to remove the need for rcu_read_unlock_special() to
   * make more than two attempts to acquire the target rcu_node's lock.

   * Returns true if there were tasks blocking the current RCU grace
   * period.

   *

   * Returns 1 if there was previously a task blocking the current grace
   * period on the specified rcu_node structure.
   *

   * The caller must hold rnp->lock with irqs disabled.
   */

  static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
  				     struct rcu_node *rnp,
  				     struct rcu_data *rdp)

  {

  	struct list_head *lp;
  	struct list_head *lp_root;

  	int retval = 0;

  	struct rcu_node *rnp_root = rcu_get_root(rsp);

  	struct task_struct *t;

  	if (rnp == rnp_root) {
  		WARN_ONCE(1, "Last CPU thought to be offlined?");

  		return 0;  /* Shouldn't happen: at least one CPU online. */

  	}

  
  	/* If we are on an internal node, complain bitterly. */
  	WARN_ON_ONCE(rnp != rdp->mynode);

  
  	/*

  	 * Move tasks up to root rcu_node.  Don't try to get fancy for
  	 * this corner-case operation -- just put this node's tasks
  	 * at the head of the root node's list, and update the root node's
  	 * ->gp_tasks and ->exp_tasks pointers to those of this node's,
  	 * if non-NULL.  This might result in waiting for more tasks than
  	 * absolutely necessary, but this is a good performance/complexity
  	 * tradeoff.

  	 */

  	if (rcu_preempt_blocked_readers_cgp(rnp))

  		retval |= RCU_OFL_TASKS_NORM_GP;
  	if (rcu_preempted_readers_exp(rnp))
  		retval |= RCU_OFL_TASKS_EXP_GP;

  	lp = &rnp->blkd_tasks;
  	lp_root = &rnp_root->blkd_tasks;
  	while (!list_empty(lp)) {
  		t = list_entry(lp->next, typeof(*t), rcu_node_entry);
  		raw_spin_lock(&rnp_root->lock); /* irqs already disabled */
  		list_del(&t->rcu_node_entry);
  		t->rcu_blocked_node = rnp_root;
  		list_add(&t->rcu_node_entry, lp_root);
  		if (&t->rcu_node_entry == rnp->gp_tasks)
  			rnp_root->gp_tasks = rnp->gp_tasks;
  		if (&t->rcu_node_entry == rnp->exp_tasks)
  			rnp_root->exp_tasks = rnp->exp_tasks;

  #ifdef CONFIG_RCU_BOOST
  		if (&t->rcu_node_entry == rnp->boost_tasks)
  			rnp_root->boost_tasks = rnp->boost_tasks;
  #endif /* #ifdef CONFIG_RCU_BOOST */

  		raw_spin_unlock(&rnp_root->lock); /* irqs still disabled */

  	}

  
  #ifdef CONFIG_RCU_BOOST
  	/* In case root is being boosted and leaf is not. */
  	raw_spin_lock(&rnp_root->lock); /* irqs already disabled */
  	if (rnp_root->boost_tasks != NULL &&
  	    rnp_root->boost_tasks != rnp_root->gp_tasks)
  		rnp_root->boost_tasks = rnp_root->gp_tasks;
  	raw_spin_unlock(&rnp_root->lock); /* irqs still disabled */
  #endif /* #ifdef CONFIG_RCU_BOOST */

  	rnp->gp_tasks = NULL;
  	rnp->exp_tasks = NULL;

  	return retval;

  }
  
  /*

   * Do CPU-offline processing for preemptible RCU.

   */
  static void rcu_preempt_offline_cpu(int cpu)
  {
  	__rcu_offline_cpu(cpu, &rcu_preempt_state);
  }
  
  #endif /* #ifdef CONFIG_HOTPLUG_CPU */

  /*
   * Check for a quiescent state from the current CPU.  When a task blocks,
   * the task is recorded in the corresponding CPU's rcu_node structure,
   * which is checked elsewhere.
   *
   * Caller must disable hard irqs.
   */
  static void rcu_preempt_check_callbacks(int cpu)
  {
  	struct task_struct *t = current;
  
  	if (t->rcu_read_lock_nesting == 0) {

  		rcu_preempt_qs(cpu);

  		return;
  	}

  	if (t->rcu_read_lock_nesting > 0 &&
  	    per_cpu(rcu_preempt_data, cpu).qs_pending)

  		t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;

  }
  
  /*

   * Process callbacks for preemptible RCU.

   */
  static void rcu_preempt_process_callbacks(void)
  {
  	__rcu_process_callbacks(&rcu_preempt_state,
  				&__get_cpu_var(rcu_preempt_data));
  }

  #ifdef CONFIG_RCU_BOOST

  static void rcu_preempt_do_callbacks(void)
  {
  	rcu_do_batch(&rcu_preempt_state, &__get_cpu_var(rcu_preempt_data));
  }

  #endif /* #ifdef CONFIG_RCU_BOOST */

  /*

   * Queue a preemptible-RCU callback for invocation after a grace period.

   */
  void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
  {
  	__call_rcu(head, func, &rcu_preempt_state);
  }
  EXPORT_SYMBOL_GPL(call_rcu);

  /**
   * synchronize_rcu - wait until a grace period has elapsed.
   *
   * Control will return to the caller some time after a full grace
   * period has elapsed, in other words after all currently executing RCU

   * read-side critical sections have completed.  Note, however, that
   * upon return from synchronize_rcu(), the caller might well be executing
   * concurrently with new RCU read-side critical sections that began while
   * synchronize_rcu() was waiting.  RCU read-side critical sections are
   * delimited by rcu_read_lock() and rcu_read_unlock(), and may be nested.

   */
  void synchronize_rcu(void)
  {

  	if (!rcu_scheduler_active)
  		return;

  	wait_rcu_gp(call_rcu);

  }
  EXPORT_SYMBOL_GPL(synchronize_rcu);

  static DECLARE_WAIT_QUEUE_HEAD(sync_rcu_preempt_exp_wq);
  static long sync_rcu_preempt_exp_count;
  static DEFINE_MUTEX(sync_rcu_preempt_exp_mutex);
  
  /*
   * Return non-zero if there are any tasks in RCU read-side critical
   * sections blocking the current preemptible-RCU expedited grace period.
   * If there is no preemptible-RCU expedited grace period currently in
   * progress, returns zero unconditionally.
   */
  static int rcu_preempted_readers_exp(struct rcu_node *rnp)
  {

  	return rnp->exp_tasks != NULL;

  }
  
  /*
   * return non-zero if there is no RCU expedited grace period in progress
   * for the specified rcu_node structure, in other words, if all CPUs and
   * tasks covered by the specified rcu_node structure have done their bit
   * for the current expedited grace period.  Works only for preemptible
   * RCU -- other RCU implementation use other means.
   *
   * Caller must hold sync_rcu_preempt_exp_mutex.
   */
  static int sync_rcu_preempt_exp_done(struct rcu_node *rnp)
  {
  	return !rcu_preempted_readers_exp(rnp) &&
  	       ACCESS_ONCE(rnp->expmask) == 0;
  }
  
  /*
   * Report the exit from RCU read-side critical section for the last task
   * that queued itself during or before the current expedited preemptible-RCU
   * grace period.  This event is reported either to the rcu_node structure on
   * which the task was queued or to one of that rcu_node structure's ancestors,
   * recursively up the tree.  (Calm down, calm down, we do the recursion
   * iteratively!)
   *

   * Most callers will set the "wake" flag, but the task initiating the
   * expedited grace period need not wake itself.
   *

   * Caller must hold sync_rcu_preempt_exp_mutex.
   */

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

  {
  	unsigned long flags;
  	unsigned long mask;

  	raw_spin_lock_irqsave(&rnp->lock, flags);

  	for (;;) {

  		if (!sync_rcu_preempt_exp_done(rnp)) {
  			raw_spin_unlock_irqrestore(&rnp->lock, flags);

  			break;

  		}

  		if (rnp->parent == NULL) {

  			raw_spin_unlock_irqrestore(&rnp->lock, flags);

  			if (wake)
  				wake_up(&sync_rcu_preempt_exp_wq);

  			break;
  		}
  		mask = rnp->grpmask;

  		raw_spin_unlock(&rnp->lock); /* irqs remain disabled */

  		rnp = rnp->parent;

  		raw_spin_lock(&rnp->lock); /* irqs already disabled */

  		rnp->expmask &= ~mask;
  	}

  }
  
  /*
   * Snapshot the tasks blocking the newly started preemptible-RCU expedited
   * grace period for the specified rcu_node structure.  If there are no such
   * tasks, report it up the rcu_node hierarchy.
   *
   * Caller must hold sync_rcu_preempt_exp_mutex and rsp->onofflock.
   */
  static void
  sync_rcu_preempt_exp_init(struct rcu_state *rsp, struct rcu_node *rnp)
  {

  	unsigned long flags;

  	int must_wait = 0;

  	raw_spin_lock_irqsave(&rnp->lock, flags);
  	if (list_empty(&rnp->blkd_tasks))
  		raw_spin_unlock_irqrestore(&rnp->lock, flags);
  	else {

  		rnp->exp_tasks = rnp->blkd_tasks.next;

  		rcu_initiate_boost(rnp, flags);  /* releases rnp->lock */

  		must_wait = 1;
  	}

  	if (!must_wait)

  		rcu_report_exp_rnp(rsp, rnp, false); /* Don't wake self. */

  }

  /*

   * Wait for an rcu-preempt grace period, but expedite it.  The basic idea
   * is to invoke synchronize_sched_expedited() to push all the tasks to

   * the ->blkd_tasks lists and wait for this list to drain.

   */
  void synchronize_rcu_expedited(void)
  {

  	unsigned long flags;
  	struct rcu_node *rnp;
  	struct rcu_state *rsp = &rcu_preempt_state;
  	long snap;
  	int trycount = 0;
  
  	smp_mb(); /* Caller's modifications seen first by other CPUs. */
  	snap = ACCESS_ONCE(sync_rcu_preempt_exp_count) + 1;
  	smp_mb(); /* Above access cannot bleed into critical section. */
  
  	/*
  	 * Acquire lock, falling back to synchronize_rcu() if too many
  	 * lock-acquisition failures.  Of course, if someone does the
  	 * expedited grace period for us, just leave.
  	 */
  	while (!mutex_trylock(&sync_rcu_preempt_exp_mutex)) {
  		if (trycount++ < 10)
  			udelay(trycount * num_online_cpus());
  		else {
  			synchronize_rcu();
  			return;
  		}
  		if ((ACCESS_ONCE(sync_rcu_preempt_exp_count) - snap) > 0)
  			goto mb_ret; /* Others did our work for us. */
  	}
  	if ((ACCESS_ONCE(sync_rcu_preempt_exp_count) - snap) > 0)
  		goto unlock_mb_ret; /* Others did our work for us. */

  	/* force all RCU readers onto ->blkd_tasks lists. */

  	synchronize_sched_expedited();

  	raw_spin_lock_irqsave(&rsp->onofflock, flags);

  
  	/* Initialize ->expmask for all non-leaf rcu_node structures. */
  	rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) {

  		raw_spin_lock(&rnp->lock); /* irqs already disabled. */

  		rnp->expmask = rnp->qsmaskinit;

  		raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */

  	}

  	/* Snapshot current state of ->blkd_tasks lists. */

  	rcu_for_each_leaf_node(rsp, rnp)
  		sync_rcu_preempt_exp_init(rsp, rnp);
  	if (NUM_RCU_NODES > 1)
  		sync_rcu_preempt_exp_init(rsp, rcu_get_root(rsp));

  	raw_spin_unlock_irqrestore(&rsp->onofflock, flags);

  	/* Wait for snapshotted ->blkd_tasks lists to drain. */

  	rnp = rcu_get_root(rsp);
  	wait_event(sync_rcu_preempt_exp_wq,
  		   sync_rcu_preempt_exp_done(rnp));
  
  	/* Clean up and exit. */
  	smp_mb(); /* ensure expedited GP seen before counter increment. */
  	ACCESS_ONCE(sync_rcu_preempt_exp_count)++;
  unlock_mb_ret:
  	mutex_unlock(&sync_rcu_preempt_exp_mutex);
  mb_ret:
  	smp_mb(); /* ensure subsequent action seen after grace period. */

  }
  EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
  
  /*

   * Check to see if there is any immediate preemptible-RCU-related work

   * to be done.
   */
  static int rcu_preempt_pending(int cpu)
  {
  	return __rcu_pending(&rcu_preempt_state,
  			     &per_cpu(rcu_preempt_data, cpu));
  }
  
  /*

   * Does preemptible RCU need the CPU to stay out of dynticks mode?

   */
  static int rcu_preempt_needs_cpu(int cpu)
  {
  	return !!per_cpu(rcu_preempt_data, cpu).nxtlist;
  }

  /**
   * rcu_barrier - Wait until all in-flight call_rcu() callbacks complete.
   */
  void rcu_barrier(void)
  {
  	_rcu_barrier(&rcu_preempt_state, call_rcu);
  }
  EXPORT_SYMBOL_GPL(rcu_barrier);

  /*

   * Initialize preemptible RCU's per-CPU data.

   */
  static void __cpuinit rcu_preempt_init_percpu_data(int cpu)
  {
  	rcu_init_percpu_data(cpu, &rcu_preempt_state, 1);
  }
  
  /*

   * Move preemptible RCU's callbacks from dying CPU to other online CPU.

   */

  static void rcu_preempt_send_cbs_to_online(void)

  {

  	rcu_send_cbs_to_online(&rcu_preempt_state);

  }
  
  /*

   * Initialize preemptible RCU's state structures.

   */
  static void __init __rcu_init_preempt(void)
  {

  	rcu_init_one(&rcu_preempt_state, &rcu_preempt_data);

  }
  
  /*

   * 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.
   */
  void exit_rcu(void)
  {
  	struct task_struct *t = current;
  
  	if (t->rcu_read_lock_nesting == 0)
  		return;
  	t->rcu_read_lock_nesting = 1;

  	__rcu_read_unlock();

  }
  
  #else /* #ifdef CONFIG_TREE_PREEMPT_RCU */

  static struct rcu_state *rcu_state = &rcu_sched_state;

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

  static void __init rcu_bootup_announce(void)

  {
  	printk(KERN_INFO "Hierarchical RCU implementation.
  ");

  	rcu_bootup_announce_oddness();

  }
  
  /*
   * Return the number of RCU batches processed thus far for debug & stats.
   */
  long rcu_batches_completed(void)
  {
  	return rcu_batches_completed_sched();
  }
  EXPORT_SYMBOL_GPL(rcu_batches_completed);
  
  /*

   * Force a quiescent state for RCU, which, because there is no preemptible
   * RCU, becomes the same as rcu-sched.
   */
  void rcu_force_quiescent_state(void)
  {
  	rcu_sched_force_quiescent_state();
  }
  EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);
  
  /*

   * Because preemptible RCU does not exist, we never have to check for

   * CPUs being in quiescent states.
   */

  static void rcu_preempt_note_context_switch(int cpu)

  {
  }

  /*

   * 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;
  }

  #ifdef CONFIG_HOTPLUG_CPU
  
  /* Because preemptible RCU does not exist, no quieting of tasks. */

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

  {

  	raw_spin_unlock_irqrestore(&rnp->lock, flags);

  }
  
  #endif /* #ifdef CONFIG_HOTPLUG_CPU */

  /*

   * Because preemptible RCU does not exist, we never have to check for

   * tasks blocked within RCU read-side critical sections.
   */

  static void rcu_print_detail_task_stall(struct rcu_state *rsp)
  {
  }
  
  /*

   * Because preemptible RCU does not exist, we never have to check for

   * tasks blocked within RCU read-side critical sections.
   */

  static int rcu_print_task_stall(struct rcu_node *rnp)

  {

  	return 0;

  }

  /*
   * Because preemptible RCU does not exist, there is no need to suppress
   * its CPU stall warnings.
   */
  static void rcu_preempt_stall_reset(void)
  {
  }

  /*

   * 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);

  }

  #ifdef CONFIG_HOTPLUG_CPU
  
  /*

   * Because preemptible RCU does not exist, it never needs to migrate

   * tasks that were blocked within RCU read-side critical sections, and
   * such non-existent tasks cannot possibly have been blocking the current
   * grace period.

   */

  static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
  				     struct rcu_node *rnp,
  				     struct rcu_data *rdp)

  {

  	return 0;

  }
  
  /*

   * Because preemptible RCU does not exist, it never needs CPU-offline

   * processing.
   */
  static void rcu_preempt_offline_cpu(int cpu)
  {
  }
  
  #endif /* #ifdef CONFIG_HOTPLUG_CPU */

  /*

   * Because preemptible RCU does not exist, it never has any callbacks

   * to check.
   */

  static void rcu_preempt_check_callbacks(int cpu)

  {
  }
  
  /*

   * Because preemptible RCU does not exist, it never has any callbacks

   * to process.
   */

  static void rcu_preempt_process_callbacks(void)

  {
  }
  
  /*

   * Wait for an rcu-preempt grace period, but make it happen quickly.

   * But because preemptible RCU does not exist, map to rcu-sched.

   */
  void synchronize_rcu_expedited(void)
  {
  	synchronize_sched_expedited();
  }
  EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);

  #ifdef CONFIG_HOTPLUG_CPU
  
  /*

   * Because preemptible RCU does not exist, there is never any need to

   * report on tasks preempted in RCU read-side critical sections during
   * expedited RCU grace periods.
   */

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

  {

  }
  
  #endif /* #ifdef CONFIG_HOTPLUG_CPU */

  /*

   * Because preemptible RCU does not exist, it never has any work to do.

   */
  static int rcu_preempt_pending(int cpu)
  {
  	return 0;
  }
  
  /*

   * Because preemptible RCU does not exist, it never needs any CPU.

   */
  static int rcu_preempt_needs_cpu(int cpu)
  {
  	return 0;
  }
  
  /*

   * Because preemptible RCU does not exist, rcu_barrier() is just

   * another name for rcu_barrier_sched().
   */
  void rcu_barrier(void)
  {
  	rcu_barrier_sched();
  }
  EXPORT_SYMBOL_GPL(rcu_barrier);
  
  /*

   * Because preemptible RCU does not exist, there is no per-CPU

   * data to initialize.
   */
  static void __cpuinit rcu_preempt_init_percpu_data(int cpu)
  {
  }

  /*

   * Because there is no preemptible RCU, there are no callbacks to move.

   */

  static void rcu_preempt_send_cbs_to_online(void)

  {
  }
  
  /*

   * Because preemptible RCU does not exist, it need not be initialized.

   */
  static void __init __rcu_init_preempt(void)
  {
  }

  #endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */

  #ifdef CONFIG_RCU_BOOST
  
  #include "rtmutex_common.h"

  #ifdef CONFIG_RCU_TRACE
  
  static void rcu_initiate_boost_trace(struct rcu_node *rnp)
  {
  	if (list_empty(&rnp->blkd_tasks))
  		rnp->n_balk_blkd_tasks++;
  	else if (rnp->exp_tasks == NULL && rnp->gp_tasks == NULL)
  		rnp->n_balk_exp_gp_tasks++;
  	else if (rnp->gp_tasks != NULL && rnp->boost_tasks != NULL)
  		rnp->n_balk_boost_tasks++;
  	else if (rnp->gp_tasks != NULL && rnp->qsmask != 0)
  		rnp->n_balk_notblocked++;
  	else if (rnp->gp_tasks != NULL &&

  		 ULONG_CMP_LT(jiffies, rnp->boost_time))

  		rnp->n_balk_notyet++;
  	else
  		rnp->n_balk_nos++;
  }
  
  #else /* #ifdef CONFIG_RCU_TRACE */
  
  static void rcu_initiate_boost_trace(struct rcu_node *rnp)
  {
  }
  
  #endif /* #else #ifdef CONFIG_RCU_TRACE */

  /*
   * 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 rt_mutex mtx;
  	struct task_struct *t;
  	struct list_head *tb;
  
  	if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL)
  		return 0;  /* Nothing left to boost. */
  
  	raw_spin_lock_irqsave(&rnp->lock, 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(&rnp->lock, 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;

  		rnp->n_exp_boosts++;
  	} else {

  		tb = rnp->boost_tasks;

  		rnp->n_normal_boosts++;
  	}
  	rnp->n_tasks_boosted++;

  
  	/*
  	 * 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(&mtx, t);
  	t->rcu_boost_mutex = &mtx;

  	raw_spin_unlock_irqrestore(&rnp->lock, flags);
  	rt_mutex_lock(&mtx);  /* Side effect: boosts task t's priority. */
  	rt_mutex_unlock(&mtx);  /* Keep lockdep happy. */

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

  }
  
  /*
   * Timer handler to initiate waking up of boost kthreads that
   * have yielded the CPU due to excessive numbers of tasks to
   * boost.  We wake up the per-rcu_node kthread, which in turn
   * will wake up the booster kthread.
   */
  static void rcu_boost_kthread_timer(unsigned long arg)
  {

  	invoke_rcu_node_kthread((struct rcu_node *)arg);

  }
  
  /*
   * Priority-boosting kthread.  One per leaf rcu_node and one for the
   * root 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("Start boost kthread@init");

  	for (;;) {

  		rnp->boost_kthread_status = RCU_KTHREAD_WAITING;

  		trace_rcu_utilization("End boost kthread@rcu_wait");

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

  		trace_rcu_utilization("Start boost kthread@rcu_wait");

  		rnp->boost_kthread_status = RCU_KTHREAD_RUNNING;

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

  			trace_rcu_utilization("End boost kthread@rcu_yield");

  			rcu_yield(rcu_boost_kthread_timer, (unsigned long)rnp);

  			trace_rcu_utilization("Start boost kthread@rcu_yield");

  			spincnt = 0;
  		}
  	}

  	/* NOTREACHED */

  	trace_rcu_utilization("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,
   * but irqs remain disabled.  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)

  {
  	struct task_struct *t;

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

  		raw_spin_unlock_irqrestore(&rnp->lock, flags);

  		return;

  	}

  	if (rnp->exp_tasks != NULL ||
  	    (rnp->gp_tasks != NULL &&
  	     rnp->boost_tasks == NULL &&
  	     rnp->qsmask == 0 &&
  	     ULONG_CMP_GE(jiffies, rnp->boost_time))) {
  		if (rnp->exp_tasks == NULL)
  			rnp->boost_tasks = rnp->gp_tasks;

  		raw_spin_unlock_irqrestore(&rnp->lock, flags);

  		t = rnp->boost_kthread_task;
  		if (t != NULL)
  			wake_up_process(t);

  	} else {

  		rcu_initiate_boost_trace(rnp);

  		raw_spin_unlock_irqrestore(&rnp->lock, flags);
  	}

  }

  /*

   * Wake up the per-CPU kthread to invoke RCU callbacks.
   */
  static void invoke_rcu_callbacks_kthread(void)
  {
  	unsigned long flags;
  
  	local_irq_save(flags);
  	__this_cpu_write(rcu_cpu_has_work, 1);

  	if (__this_cpu_read(rcu_cpu_kthread_task) != NULL &&
  	    current != __this_cpu_read(rcu_cpu_kthread_task))
  		wake_up_process(__this_cpu_read(rcu_cpu_kthread_task));

  	local_irq_restore(flags);
  }
  
  /*

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

   * Set the affinity of the boost kthread.  The CPU-hotplug locks are
   * held, so no one should be messing with the existence of the boost
   * kthread.
   */

  static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp,
  					  cpumask_var_t cm)
  {

  	struct task_struct *t;

  	t = rnp->boost_kthread_task;
  	if (t != NULL)
  		set_cpus_allowed_ptr(rnp->boost_kthread_task, cm);

  }
  
  #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 int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
  						 struct rcu_node *rnp,
  						 int rnp_index)
  {
  	unsigned long flags;
  	struct sched_param sp;
  	struct task_struct *t;
  
  	if (&rcu_preempt_state != rsp)
  		return 0;

  	rsp->boost = 1;

  	if (rnp->boost_kthread_task != NULL)
  		return 0;
  	t = kthread_create(rcu_boost_kthread, (void *)rnp,

  			   "rcub/%d", rnp_index);

  	if (IS_ERR(t))
  		return PTR_ERR(t);
  	raw_spin_lock_irqsave(&rnp->lock, flags);
  	rnp->boost_kthread_task = t;
  	raw_spin_unlock_irqrestore(&rnp->lock, flags);

  	sp.sched_priority = RCU_BOOST_PRIO;

  	sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);

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

  	return 0;
  }

  #ifdef CONFIG_HOTPLUG_CPU
  
  /*
   * Stop the RCU's per-CPU kthread when its CPU goes offline,.
   */
  static void rcu_stop_cpu_kthread(int cpu)
  {
  	struct task_struct *t;
  
  	/* Stop the CPU's kthread. */
  	t = per_cpu(rcu_cpu_kthread_task, cpu);
  	if (t != NULL) {
  		per_cpu(rcu_cpu_kthread_task, cpu) = NULL;
  		kthread_stop(t);
  	}
  }
  
  #endif /* #ifdef CONFIG_HOTPLUG_CPU */
  
  static void rcu_kthread_do_work(void)
  {
  	rcu_do_batch(&rcu_sched_state, &__get_cpu_var(rcu_sched_data));
  	rcu_do_batch(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));
  	rcu_preempt_do_callbacks();
  }
  
  /*
   * Wake up the specified per-rcu_node-structure kthread.
   * Because the per-rcu_node kthreads are immortal, we don't need
   * to do anything to keep them alive.
   */
  static void invoke_rcu_node_kthread(struct rcu_node *rnp)
  {
  	struct task_struct *t;
  
  	t = rnp->node_kthread_task;
  	if (t != NULL)
  		wake_up_process(t);
  }
  
  /*
   * Set the specified CPU's kthread to run RT or not, as specified by
   * the to_rt argument.  The CPU-hotplug locks are held, so the task
   * is not going away.
   */
  static void rcu_cpu_kthread_setrt(int cpu, int to_rt)
  {
  	int policy;
  	struct sched_param sp;
  	struct task_struct *t;
  
  	t = per_cpu(rcu_cpu_kthread_task, cpu);
  	if (t == NULL)
  		return;
  	if (to_rt) {
  		policy = SCHED_FIFO;
  		sp.sched_priority = RCU_KTHREAD_PRIO;
  	} else {
  		policy = SCHED_NORMAL;
  		sp.sched_priority = 0;
  	}
  	sched_setscheduler_nocheck(t, policy, &sp);
  }
  
  /*
   * Timer handler to initiate the waking up of per-CPU kthreads that
   * have yielded the CPU due to excess numbers of RCU callbacks.
   * We wake up the per-rcu_node kthread, which in turn will wake up
   * the booster kthread.
   */
  static void rcu_cpu_kthread_timer(unsigned long arg)
  {
  	struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, arg);
  	struct rcu_node *rnp = rdp->mynode;
  
  	atomic_or(rdp->grpmask, &rnp->wakemask);
  	invoke_rcu_node_kthread(rnp);
  }
  
  /*
   * Drop to non-real-time priority and yield, but only after posting a
   * timer that will cause us to regain our real-time priority if we
   * remain preempted.  Either way, we restore our real-time priority
   * before returning.
   */
  static void rcu_yield(void (*f)(unsigned long), unsigned long arg)
  {
  	struct sched_param sp;
  	struct timer_list yield_timer;

  	int prio = current->rt_priority;

  
  	setup_timer_on_stack(&yield_timer, f, arg);
  	mod_timer(&yield_timer, jiffies + 2);
  	sp.sched_priority = 0;
  	sched_setscheduler_nocheck(current, SCHED_NORMAL, &sp);
  	set_user_nice(current, 19);
  	schedule();

  	set_user_nice(current, 0);
  	sp.sched_priority = prio;

  	sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
  	del_timer(&yield_timer);
  }
  
  /*
   * Handle cases where the rcu_cpu_kthread() ends up on the wrong CPU.
   * This can happen while the corresponding CPU is either coming online
   * or going offline.  We cannot wait until the CPU is fully online
   * before starting the kthread, because the various notifier functions
   * can wait for RCU grace periods.  So we park rcu_cpu_kthread() until
   * the corresponding CPU is online.
   *
   * Return 1 if the kthread needs to stop, 0 otherwise.
   *
   * Caller must disable bh.  This function can momentarily enable it.
   */
  static int rcu_cpu_kthread_should_stop(int cpu)
  {
  	while (cpu_is_offline(cpu) ||
  	       !cpumask_equal(&current->cpus_allowed, cpumask_of(cpu)) ||
  	       smp_processor_id() != cpu) {
  		if (kthread_should_stop())
  			return 1;
  		per_cpu(rcu_cpu_kthread_status, cpu) = RCU_KTHREAD_OFFCPU;
  		per_cpu(rcu_cpu_kthread_cpu, cpu) = raw_smp_processor_id();
  		local_bh_enable();
  		schedule_timeout_uninterruptible(1);
  		if (!cpumask_equal(&current->cpus_allowed, cpumask_of(cpu)))
  			set_cpus_allowed_ptr(current, cpumask_of(cpu));
  		local_bh_disable();
  	}
  	per_cpu(rcu_cpu_kthread_cpu, cpu) = cpu;
  	return 0;
  }
  
  /*
   * Per-CPU kernel thread that invokes RCU callbacks.  This replaces the

   * RCU softirq used in flavors and configurations of RCU that do not
   * support RCU priority boosting.

   */
  static int rcu_cpu_kthread(void *arg)
  {
  	int cpu = (int)(long)arg;
  	unsigned long flags;
  	int spincnt = 0;
  	unsigned int *statusp = &per_cpu(rcu_cpu_kthread_status, cpu);
  	char work;
  	char *workp = &per_cpu(rcu_cpu_has_work, cpu);

  	trace_rcu_utilization("Start CPU kthread@init");

  	for (;;) {
  		*statusp = RCU_KTHREAD_WAITING;

  		trace_rcu_utilization("End CPU kthread@rcu_wait");

  		rcu_wait(*workp != 0 || kthread_should_stop());

  		trace_rcu_utilization("Start CPU kthread@rcu_wait");

  		local_bh_disable();
  		if (rcu_cpu_kthread_should_stop(cpu)) {
  			local_bh_enable();
  			break;
  		}
  		*statusp = RCU_KTHREAD_RUNNING;
  		per_cpu(rcu_cpu_kthread_loops, cpu)++;
  		local_irq_save(flags);
  		work = *workp;
  		*workp = 0;
  		local_irq_restore(flags);
  		if (work)
  			rcu_kthread_do_work();
  		local_bh_enable();
  		if (*workp != 0)
  			spincnt++;
  		else
  			spincnt = 0;
  		if (spincnt > 10) {
  			*statusp = RCU_KTHREAD_YIELDING;

  			trace_rcu_utilization("End CPU kthread@rcu_yield");

  			rcu_yield(rcu_cpu_kthread_timer, (unsigned long)cpu);

  			trace_rcu_utilization("Start CPU kthread@rcu_yield");

  			spincnt = 0;
  		}
  	}
  	*statusp = RCU_KTHREAD_STOPPED;

  	trace_rcu_utilization("End CPU kthread@term");

  	return 0;
  }
  
  /*
   * Spawn a per-CPU kthread, setting up affinity and priority.
   * Because the CPU hotplug lock is held, no other CPU will be attempting
   * to manipulate rcu_cpu_kthread_task.  There might be another CPU
   * attempting to access it during boot, but the locking in kthread_bind()
   * will enforce sufficient ordering.
   *
   * Please note that we cannot simply refuse to wake up the per-CPU
   * kthread because kthreads are created in TASK_UNINTERRUPTIBLE state,
   * which can result in softlockup complaints if the task ends up being
   * idle for more than a couple of minutes.
   *
   * However, please note also that we cannot bind the per-CPU kthread to its
   * CPU until that CPU is fully online.  We also cannot wait until the
   * CPU is fully online before we create its per-CPU kthread, as this would
   * deadlock the system when CPU notifiers tried waiting for grace
   * periods.  So we bind the per-CPU kthread to its CPU only if the CPU
   * is online.  If its CPU is not yet fully online, then the code in
   * rcu_cpu_kthread() will wait until it is fully online, and then do
   * the binding.
   */
  static int __cpuinit rcu_spawn_one_cpu_kthread(int cpu)
  {
  	struct sched_param sp;
  	struct task_struct *t;

  	if (!rcu_scheduler_fully_active ||

  	    per_cpu(rcu_cpu_kthread_task, cpu) != NULL)
  		return 0;

  	t = kthread_create_on_node(rcu_cpu_kthread,
  				   (void *)(long)cpu,
  				   cpu_to_node(cpu),

  				   "rcuc/%d", cpu);

  	if (IS_ERR(t))
  		return PTR_ERR(t);
  	if (cpu_online(cpu))
  		kthread_bind(t, cpu);
  	per_cpu(rcu_cpu_kthread_cpu, cpu) = cpu;
  	WARN_ON_ONCE(per_cpu(rcu_cpu_kthread_task, cpu) != NULL);
  	sp.sched_priority = RCU_KTHREAD_PRIO;
  	sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
  	per_cpu(rcu_cpu_kthread_task, cpu) = t;
  	wake_up_process(t); /* Get to TASK_INTERRUPTIBLE quickly. */
  	return 0;
  }
  
  /*
   * Per-rcu_node kthread, which is in charge of waking up the per-CPU
   * kthreads when needed.  We ignore requests to wake up kthreads
   * for offline CPUs, which is OK because force_quiescent_state()
   * takes care of this case.
   */
  static int rcu_node_kthread(void *arg)
  {
  	int cpu;
  	unsigned long flags;
  	unsigned long mask;
  	struct rcu_node *rnp = (struct rcu_node *)arg;
  	struct sched_param sp;
  	struct task_struct *t;
  
  	for (;;) {
  		rnp->node_kthread_status = RCU_KTHREAD_WAITING;
  		rcu_wait(atomic_read(&rnp->wakemask) != 0);
  		rnp->node_kthread_status = RCU_KTHREAD_RUNNING;
  		raw_spin_lock_irqsave(&rnp->lock, flags);
  		mask = atomic_xchg(&rnp->wakemask, 0);
  		rcu_initiate_boost(rnp, flags); /* releases rnp->lock. */
  		for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask >>= 1) {
  			if ((mask & 0x1) == 0)
  				continue;
  			preempt_disable();
  			t = per_cpu(rcu_cpu_kthread_task, cpu);
  			if (!cpu_online(cpu) || t == NULL) {
  				preempt_enable();
  				continue;
  			}
  			per_cpu(rcu_cpu_has_work, cpu) = 1;
  			sp.sched_priority = RCU_KTHREAD_PRIO;
  			sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
  			preempt_enable();
  		}
  	}
  	/* NOTREACHED */
  	rnp->node_kthread_status = RCU_KTHREAD_STOPPED;
  	return 0;
  }
  
  /*
   * 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_node_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
  {
  	cpumask_var_t cm;
  	int cpu;
  	unsigned long mask = rnp->qsmaskinit;
  
  	if (rnp->node_kthread_task == NULL)
  		return;
  	if (!alloc_cpumask_var(&cm, GFP_KERNEL))
  		return;
  	cpumask_clear(cm);
  	for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask >>= 1)
  		if ((mask & 0x1) && cpu != outgoingcpu)
  			cpumask_set_cpu(cpu, cm);
  	if (cpumask_weight(cm) == 0) {
  		cpumask_setall(cm);
  		for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++)
  			cpumask_clear_cpu(cpu, cm);
  		WARN_ON_ONCE(cpumask_weight(cm) == 0);
  	}
  	set_cpus_allowed_ptr(rnp->node_kthread_task, cm);
  	rcu_boost_kthread_setaffinity(rnp, cm);
  	free_cpumask_var(cm);
  }
  
  /*
   * Spawn a per-rcu_node kthread, setting priority and affinity.
   * Called during boot before online/offline can happen, or, if
   * during runtime, with the main CPU-hotplug locks held.  So only
   * one of these can be executing at a time.
   */
  static int __cpuinit rcu_spawn_one_node_kthread(struct rcu_state *rsp,
  						struct rcu_node *rnp)
  {
  	unsigned long flags;
  	int rnp_index = rnp - &rsp->node[0];
  	struct sched_param sp;
  	struct task_struct *t;

  	if (!rcu_scheduler_fully_active ||

  	    rnp->qsmaskinit == 0)
  		return 0;
  	if (rnp->node_kthread_task == NULL) {
  		t = kthread_create(rcu_node_kthread, (void *)rnp,

  				   "rcun/%d", rnp_index);

  		if (IS_ERR(t))
  			return PTR_ERR(t);
  		raw_spin_lock_irqsave(&rnp->lock, flags);
  		rnp->node_kthread_task = t;
  		raw_spin_unlock_irqrestore(&rnp->lock, flags);
  		sp.sched_priority = 99;
  		sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
  		wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
  	}
  	return rcu_spawn_one_boost_kthread(rsp, rnp, rnp_index);
  }
  
  /*
   * Spawn all kthreads -- called as soon as the scheduler is running.
   */
  static int __init rcu_spawn_kthreads(void)
  {
  	int cpu;
  	struct rcu_node *rnp;

  	rcu_scheduler_fully_active = 1;

  	for_each_possible_cpu(cpu) {
  		per_cpu(rcu_cpu_has_work, cpu) = 0;
  		if (cpu_online(cpu))
  			(void)rcu_spawn_one_cpu_kthread(cpu);
  	}
  	rnp = rcu_get_root(rcu_state);
  	(void)rcu_spawn_one_node_kthread(rcu_state, rnp);
  	if (NUM_RCU_NODES > 1) {
  		rcu_for_each_leaf_node(rcu_state, rnp)
  			(void)rcu_spawn_one_node_kthread(rcu_state, rnp);
  	}
  	return 0;
  }
  early_initcall(rcu_spawn_kthreads);
  
  static void __cpuinit rcu_prepare_kthreads(int cpu)
  {
  	struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);
  	struct rcu_node *rnp = rdp->mynode;
  
  	/* Fire up the incoming CPU's kthread and leaf rcu_node kthread. */

  	if (rcu_scheduler_fully_active) {

  		(void)rcu_spawn_one_cpu_kthread(cpu);
  		if (rnp->node_kthread_task == NULL)
  			(void)rcu_spawn_one_node_kthread(rcu_state, rnp);
  	}
  }

  #else /* #ifdef CONFIG_RCU_BOOST */

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

  {

  	raw_spin_unlock_irqrestore(&rnp->lock, flags);

  }

  static void invoke_rcu_callbacks_kthread(void)

  {

  	WARN_ON_ONCE(1);

  }

  static bool rcu_is_callbacks_kthread(void)
  {
  	return false;
  }

  static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
  {
  }

  #ifdef CONFIG_HOTPLUG_CPU
  
  static void rcu_stop_cpu_kthread(int cpu)
  {
  }
  
  #endif /* #ifdef CONFIG_HOTPLUG_CPU */
  
  static void rcu_node_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
  {
  }
  
  static void rcu_cpu_kthread_setrt(int cpu, int to_rt)
  {
  }

  static int __init rcu_scheduler_really_started(void)
  {
  	rcu_scheduler_fully_active = 1;
  	return 0;
  }
  early_initcall(rcu_scheduler_really_started);

  static void __cpuinit rcu_prepare_kthreads(int cpu)
  {
  }

  #endif /* #else #ifdef CONFIG_RCU_BOOST */

  #ifndef CONFIG_SMP
  
  void synchronize_sched_expedited(void)
  {
  	cond_resched();
  }
  EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
  
  #else /* #ifndef CONFIG_SMP */

  static atomic_t sync_sched_expedited_started = ATOMIC_INIT(0);
  static atomic_t sync_sched_expedited_done = ATOMIC_INIT(0);

  
  static int synchronize_sched_expedited_cpu_stop(void *data)
  {
  	/*
  	 * There must be a full memory barrier on each affected CPU
  	 * between the time that try_stop_cpus() is called and the
  	 * time that it returns.
  	 *
  	 * In the current initial implementation of cpu_stop, the
  	 * above condition is already met when the control reaches
  	 * this point and the following smp_mb() is not strictly
  	 * necessary.  Do smp_mb() anyway for documentation and
  	 * robustness against future implementation changes.
  	 */
  	smp_mb(); /* See above comment block. */
  	return 0;
  }
  
  /*
   * Wait for an rcu-sched grace period to elapse, but use "big hammer"
   * approach to force grace period to end quickly.  This consumes
   * significant time on all CPUs, and is thus not recommended for
   * any sort of common-case code.
   *
   * Note that it is illegal to call this function while holding any
   * lock that is acquired by a CPU-hotplug notifier.  Failing to
   * observe this restriction will result in deadlock.

   *

   * This implementation can be thought of as an application of ticket
   * locking to RCU, with sync_sched_expedited_started and
   * sync_sched_expedited_done taking on the roles of the halves
   * of the ticket-lock word.  Each task atomically increments
   * sync_sched_expedited_started upon entry, snapshotting the old value,
   * then attempts to stop all the CPUs.  If this succeeds, then each
   * CPU will have executed a context switch, resulting in an RCU-sched
   * grace period.  We are then done, so we use atomic_cmpxchg() to
   * update sync_sched_expedited_done to match our snapshot -- but
   * only if someone else has not already advanced past our snapshot.
   *
   * On the other hand, if try_stop_cpus() fails, we check the value
   * of sync_sched_expedited_done.  If it has advanced past our
   * initial snapshot, then someone else must have forced a grace period
   * some time after we took our snapshot.  In this case, our work is
   * done for us, and we can simply return.  Otherwise, we try again,
   * but keep our initial snapshot for purposes of checking for someone
   * doing our work for us.
   *
   * If we fail too many times in a row, we fall back to synchronize_sched().

   */
  void synchronize_sched_expedited(void)
  {

  	int firstsnap, s, snap, trycount = 0;

  	/* Note that atomic_inc_return() implies full memory barrier. */
  	firstsnap = snap = atomic_inc_return(&sync_sched_expedited_started);

  	get_online_cpus();

  
  	/*
  	 * Each pass through the following loop attempts to force a
  	 * context switch on each CPU.
  	 */

  	while (try_stop_cpus(cpu_online_mask,
  			     synchronize_sched_expedited_cpu_stop,
  			     NULL) == -EAGAIN) {
  		put_online_cpus();

  
  		/* No joy, try again later.  Or just synchronize_sched(). */

  		if (trycount++ < 10)
  			udelay(trycount * num_online_cpus());
  		else {
  			synchronize_sched();
  			return;
  		}

  
  		/* Check to see if someone else did our work for us. */
  		s = atomic_read(&sync_sched_expedited_done);
  		if (UINT_CMP_GE((unsigned)s, (unsigned)firstsnap)) {

  			smp_mb(); /* ensure test happens before caller kfree */
  			return;
  		}

  
  		/*
  		 * Refetching sync_sched_expedited_started allows later
  		 * callers to piggyback on our grace period.  We subtract
  		 * 1 to get the same token that the last incrementer got.
  		 * We retry after they started, so our grace period works
  		 * for them, and they started after our first try, so their
  		 * grace period works for us.
  		 */

  		get_online_cpus();

  		snap = atomic_read(&sync_sched_expedited_started);

  		smp_mb(); /* ensure read is before try_stop_cpus(). */

  	}

  
  	/*
  	 * Everyone up to our most recent fetch is covered by our grace
  	 * period.  Update the counter, but only if our work is still
  	 * relevant -- which it won't be if someone who started later
  	 * than we did beat us to the punch.
  	 */
  	do {
  		s = atomic_read(&sync_sched_expedited_done);
  		if (UINT_CMP_GE((unsigned)s, (unsigned)snap)) {
  			smp_mb(); /* ensure test happens before caller kfree */
  			break;
  		}
  	} while (atomic_cmpxchg(&sync_sched_expedited_done, s, snap) != s);

  	put_online_cpus();
  }
  EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
  
  #endif /* #else #ifndef CONFIG_SMP */

  #if !defined(CONFIG_RCU_FAST_NO_HZ)
  
  /*
   * Check to see if any future 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 this CPU needs
   * any flavor of RCU.

   */
  int rcu_needs_cpu(int cpu)
  {

  	return rcu_cpu_has_callbacks(cpu);
  }
  
  /*

   * Because we do not have RCU_FAST_NO_HZ, don't bother initializing for it.
   */
  static void rcu_prepare_for_idle_init(int cpu)
  {
  }
  
  /*
   * Because we do not have RCU_FAST_NO_HZ, don't bother cleaning up
   * after it.
   */
  static void rcu_cleanup_after_idle(int cpu)
  {
  }
  
  /*

   * Do the idle-entry grace-period work, which, because CONFIG_RCU_FAST_NO_HZ=y,
   * is nothing.
   */
  static void rcu_prepare_for_idle(int cpu)
  {
  }

  #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.  This is handled by a
   * state machine implemented by rcu_prepare_for_idle() below.
   *
   * The following three proprocessor symbols control this state machine:
   *
   * RCU_IDLE_FLUSHES gives the maximum number of times that we will attempt
   *	to satisfy RCU.  Beyond this point, it is better to incur a periodic
   *	scheduling-clock interrupt than to loop through the state machine
   *	at full power.
   * RCU_IDLE_OPT_FLUSHES gives the number of RCU_IDLE_FLUSHES that are
   *	optional if RCU does not need anything immediately from this
   *	CPU, even if this CPU still has RCU callbacks queued.  The first
   *	times through the state machine are mandatory: we need to give
   *	the state machine a chance to communicate a quiescent state
   *	to the RCU core.
   * 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 values below work 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_FLUSHES 5		/* Number of dyntick-idle tries. */
  #define RCU_IDLE_OPT_FLUSHES 3		/* Optional dyntick-idle tries. */

  #define RCU_IDLE_GP_DELAY 6		/* Roughly one grace period. */

  static DEFINE_PER_CPU(int, rcu_dyntick_drain);

  static DEFINE_PER_CPU(unsigned long, rcu_dyntick_holdoff);

  static DEFINE_PER_CPU(struct hrtimer, rcu_idle_gp_timer);
  static ktime_t rcu_idle_gp_wait;

  
  /*

   * Allow the CPU to enter dyntick-idle mode if either: (1) There are no
   * callbacks on this CPU, (2) this CPU has not yet attempted to enter
   * dyntick-idle mode, or (3) this CPU is in the process of attempting to
   * enter dyntick-idle mode.  Otherwise, if we have recently tried and failed
   * to enter dyntick-idle mode, we refuse to try to enter it.  After all,
   * it is better to incur scheduling-clock interrupts than to spin
   * continuously for the same time duration!
   */
  int rcu_needs_cpu(int cpu)
  {
  	/* If no callbacks, RCU doesn't need the CPU. */
  	if (!rcu_cpu_has_callbacks(cpu))
  		return 0;
  	/* Otherwise, RCU needs the CPU only if it recently tried and failed. */
  	return per_cpu(rcu_dyntick_holdoff, cpu) == jiffies;
  }
  
  /*

   * Timer handler used to force CPU to start pushing its remaining RCU
   * callbacks in the case where it entered dyntick-idle mode with callbacks
   * pending.  The hander doesn't really need to do anything because the
   * real work is done upon re-entry to idle, or by the next scheduling-clock
   * interrupt should idle not be re-entered.
   */
  static enum hrtimer_restart rcu_idle_gp_timer_func(struct hrtimer *hrtp)
  {
  	trace_rcu_prep_idle("Timer");
  	return HRTIMER_NORESTART;
  }
  
  /*
   * Initialize the timer used to pull CPUs out of dyntick-idle mode.
   */
  static void rcu_prepare_for_idle_init(int cpu)
  {
  	static int firsttime = 1;
  	struct hrtimer *hrtp = &per_cpu(rcu_idle_gp_timer, cpu);
  
  	hrtimer_init(hrtp, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
  	hrtp->function = rcu_idle_gp_timer_func;
  	if (firsttime) {
  		unsigned int upj = jiffies_to_usecs(RCU_IDLE_GP_DELAY);
  
  		rcu_idle_gp_wait = ns_to_ktime(upj * (u64)1000);
  		firsttime = 0;
  	}
  }
  
  /*
   * Clean up for exit from idle.  Because we are exiting from idle, there
   * is no longer any point to rcu_idle_gp_timer, so cancel it.  This will
   * do nothing if this timer is not active, so just cancel it unconditionally.
   */
  static void rcu_cleanup_after_idle(int cpu)
  {
  	hrtimer_cancel(&per_cpu(rcu_idle_gp_timer, cpu));
  }
  
  /*

   * Check to see if any RCU-related work can be done by the current CPU,
   * and if so, schedule a softirq to get it done.  This function is part
   * of the RCU implementation; it is -not- an exported member of the RCU API.

   *

   * The idea is for the current CPU to clear out all work required by the
   * RCU core for the current grace period, so that this CPU can be permitted
   * to enter dyntick-idle mode.  In some cases, it will need to be awakened
   * at the end of the grace period by whatever CPU ends the grace period.
   * This allows CPUs to go dyntick-idle more quickly, and to reduce the
   * number of wakeups by a modest integer factor.

   *
   * Because it is not legal to invoke rcu_process_callbacks() with irqs
   * disabled, we do one pass of force_quiescent_state(), then do a

   * invoke_rcu_core() to cause rcu_process_callbacks() to be invoked

   * later.  The per-cpu rcu_dyntick_drain variable controls the sequencing.

   *
   * The caller must have disabled interrupts.

   */

  static void rcu_prepare_for_idle(int cpu)

  {

  	unsigned long flags;
  
  	local_irq_save(flags);

  	/*

  	 * If there are no callbacks on this CPU, enter dyntick-idle mode.
  	 * Also reset state to avoid prejudicing later attempts.

  	 */

  	if (!rcu_cpu_has_callbacks(cpu)) {
  		per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;

  		per_cpu(rcu_dyntick_drain, cpu) = 0;

  		local_irq_restore(flags);

  		trace_rcu_prep_idle("No callbacks");

  		return;

  	}

  
  	/*
  	 * If in holdoff mode, just return.  We will presumably have
  	 * refrained from disabling the scheduling-clock tick.
  	 */

  	if (per_cpu(rcu_dyntick_holdoff, cpu) == jiffies) {

  		local_irq_restore(flags);

  		trace_rcu_prep_idle("In holdoff");

  		return;

  	}

  
  	/* Check and update the rcu_dyntick_drain sequencing. */
  	if (per_cpu(rcu_dyntick_drain, cpu) <= 0) {
  		/* First time through, initialize the counter. */

  		per_cpu(rcu_dyntick_drain, cpu) = RCU_IDLE_FLUSHES;
  	} else if (per_cpu(rcu_dyntick_drain, cpu) <= RCU_IDLE_OPT_FLUSHES &&
  		   !rcu_pending(cpu)) {

  		/* Can we go dyntick-idle despite still having callbacks? */

  		trace_rcu_prep_idle("Dyntick with callbacks");
  		per_cpu(rcu_dyntick_drain, cpu) = 0;
  		per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;
  		hrtimer_start(&per_cpu(rcu_idle_gp_timer, cpu),
  			      rcu_idle_gp_wait, HRTIMER_MODE_REL);
  		return; /* Nothing more to do immediately. */
  	} else if (--per_cpu(rcu_dyntick_drain, cpu) <= 0) {

  		/* We have hit the limit, so time to give up. */

  		per_cpu(rcu_dyntick_holdoff, cpu) = jiffies;

  		local_irq_restore(flags);

  		trace_rcu_prep_idle("Begin holdoff");

  		invoke_rcu_core();  /* Force the CPU out of dyntick-idle. */
  		return;

  	}

  	/*
  	 * Do one step of pushing the remaining RCU callbacks through
  	 * the RCU core state machine.
  	 */
  #ifdef CONFIG_TREE_PREEMPT_RCU
  	if (per_cpu(rcu_preempt_data, cpu).nxtlist) {

  		local_irq_restore(flags);

  		rcu_preempt_qs(cpu);
  		force_quiescent_state(&rcu_preempt_state, 0);

  		local_irq_save(flags);

  	}
  #endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */

  	if (per_cpu(rcu_sched_data, cpu).nxtlist) {

  		local_irq_restore(flags);

  		rcu_sched_qs(cpu);
  		force_quiescent_state(&rcu_sched_state, 0);

  		local_irq_save(flags);

  	}
  	if (per_cpu(rcu_bh_data, cpu).nxtlist) {

  		local_irq_restore(flags);

  		rcu_bh_qs(cpu);
  		force_quiescent_state(&rcu_bh_state, 0);

  		local_irq_save(flags);

  	}

  	/*
  	 * If RCU callbacks are still pending, RCU still needs this CPU.
  	 * So try forcing the callbacks through the grace period.
  	 */

  	if (rcu_cpu_has_callbacks(cpu)) {

  		local_irq_restore(flags);

  		trace_rcu_prep_idle("More callbacks");

  		invoke_rcu_core();

  	} else {
  		local_irq_restore(flags);

  		trace_rcu_prep_idle("Callbacks drained");

  	}

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