/*
   * Read-Copy Update mechanism for mutual exclusion
   *
   * 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 IBM Corporation, 2008
   *
   * Authors: Dipankar Sarma <dipankar@in.ibm.com>
   *	    Manfred Spraul <manfred@colorfullife.com>
   *	    Paul E. McKenney <paulmck@linux.vnet.ibm.com> Hierarchical version
   *
   * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
   * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
   *
   * For detailed explanation of Read-Copy Update mechanism see -

   *	Documentation/RCU

   */
  #include <linux/types.h>
  #include <linux/kernel.h>
  #include <linux/init.h>
  #include <linux/spinlock.h>
  #include <linux/smp.h>
  #include <linux/rcupdate.h>
  #include <linux/interrupt.h>
  #include <linux/sched.h>

  #include <linux/nmi.h>

  #include <asm/atomic.h>
  #include <linux/bitops.h>
  #include <linux/module.h>
  #include <linux/completion.h>
  #include <linux/moduleparam.h>
  #include <linux/percpu.h>
  #include <linux/notifier.h>
  #include <linux/cpu.h>
  #include <linux/mutex.h>
  #include <linux/time.h>

  #include <linux/kernel_stat.h>

  #include "rcutree.h"

  /* Data structures. */

  static struct lock_class_key rcu_node_class[NUM_RCU_LVLS];

  #define RCU_STATE_INITIALIZER(name) { \
  	.level = { &name.node[0] }, \
  	.levelcnt = { \
  		NUM_RCU_LVL_0,  /* root of hierarchy. */ \
  		NUM_RCU_LVL_1, \
  		NUM_RCU_LVL_2, \

  		NUM_RCU_LVL_3, \
  		NUM_RCU_LVL_4, /* == MAX_RCU_LVLS */ \

  	}, \

  	.signaled = RCU_GP_IDLE, \

  	.gpnum = -300, \
  	.completed = -300, \
  	.onofflock = __SPIN_LOCK_UNLOCKED(&name.onofflock), \

  	.orphan_cbs_list = NULL, \
  	.orphan_cbs_tail = &name.orphan_cbs_list, \
  	.orphan_qlen = 0, \

  	.fqslock = __SPIN_LOCK_UNLOCKED(&name.fqslock), \
  	.n_force_qs = 0, \
  	.n_force_qs_ngp = 0, \
  }

  struct rcu_state rcu_sched_state = RCU_STATE_INITIALIZER(rcu_sched_state);
  DEFINE_PER_CPU(struct rcu_data, rcu_sched_data);

  struct rcu_state rcu_bh_state = RCU_STATE_INITIALIZER(rcu_bh_state);
  DEFINE_PER_CPU(struct rcu_data, rcu_bh_data);

  static int rcu_scheduler_active __read_mostly;

  /*

   * Return true if an RCU grace period is in progress.  The ACCESS_ONCE()s
   * permit this function to be invoked without holding the root rcu_node
   * structure's ->lock, but of course results can be subject to change.
   */
  static int rcu_gp_in_progress(struct rcu_state *rsp)
  {
  	return ACCESS_ONCE(rsp->completed) != ACCESS_ONCE(rsp->gpnum);
  }
  
  /*

   * Note a quiescent state.  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.

   */

  void rcu_sched_qs(int cpu)

  {

  	struct rcu_data *rdp;

  	rdp = &per_cpu(rcu_sched_data, cpu);

  	rdp->passed_quiesc_completed = rdp->gpnum - 1;

  	barrier();
  	rdp->passed_quiesc = 1;
  	rcu_preempt_note_context_switch(cpu);

  }

  void rcu_bh_qs(int cpu)

  {

  	struct rcu_data *rdp;

  	rdp = &per_cpu(rcu_bh_data, cpu);

  	rdp->passed_quiesc_completed = rdp->gpnum - 1;

  	barrier();
  	rdp->passed_quiesc = 1;

  }

  
  #ifdef CONFIG_NO_HZ

  DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
  	.dynticks_nesting = 1,
  	.dynticks = 1,
  };

  #endif /* #ifdef CONFIG_NO_HZ */
  
  static int blimit = 10;		/* Maximum callbacks per softirq. */
  static int qhimark = 10000;	/* If this many pending, ignore blimit. */
  static int qlowmark = 100;	/* Once only this many pending, use blimit. */

  module_param(blimit, int, 0);
  module_param(qhimark, int, 0);
  module_param(qlowmark, int, 0);

  static void force_quiescent_state(struct rcu_state *rsp, int relaxed);

  static int rcu_pending(int cpu);

  
  /*

   * Return the number of RCU-sched batches processed thus far for debug & stats.

   */

  long rcu_batches_completed_sched(void)

  {

  	return rcu_sched_state.completed;

  }

  EXPORT_SYMBOL_GPL(rcu_batches_completed_sched);

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

   * Force a quiescent state for RCU BH.
   */
  void rcu_bh_force_quiescent_state(void)
  {
  	force_quiescent_state(&rcu_bh_state, 0);
  }
  EXPORT_SYMBOL_GPL(rcu_bh_force_quiescent_state);
  
  /*
   * Force a quiescent state for RCU-sched.
   */
  void rcu_sched_force_quiescent_state(void)
  {
  	force_quiescent_state(&rcu_sched_state, 0);
  }
  EXPORT_SYMBOL_GPL(rcu_sched_force_quiescent_state);
  
  /*

   * Does the CPU have callbacks ready to be invoked?
   */
  static int
  cpu_has_callbacks_ready_to_invoke(struct rcu_data *rdp)
  {
  	return &rdp->nxtlist != rdp->nxttail[RCU_DONE_TAIL];
  }
  
  /*
   * Does the current CPU require a yet-as-unscheduled grace period?
   */
  static int
  cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
  {

  	return *rdp->nxttail[RCU_DONE_TAIL] && !rcu_gp_in_progress(rsp);

  }
  
  /*
   * Return the root node of the specified rcu_state structure.
   */
  static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
  {
  	return &rsp->node[0];
  }
  
  #ifdef CONFIG_SMP
  
  /*
   * If the specified CPU is offline, tell the caller that it is in
   * a quiescent state.  Otherwise, whack it with a reschedule IPI.
   * Grace periods can end up waiting on an offline CPU when that
   * CPU is in the process of coming online -- it will be added to the
   * rcu_node bitmasks before it actually makes it online.  The same thing
   * can happen while a CPU is in the process of coming online.  Because this
   * race is quite rare, we check for it after detecting that the grace
   * period has been delayed rather than checking each and every CPU
   * each and every time we start a new grace period.
   */
  static int rcu_implicit_offline_qs(struct rcu_data *rdp)
  {
  	/*
  	 * If the CPU is offline, it is in a quiescent state.  We can
  	 * trust its state not to change because interrupts are disabled.
  	 */
  	if (cpu_is_offline(rdp->cpu)) {
  		rdp->offline_fqs++;
  		return 1;
  	}

  	/* If preemptable RCU, no point in sending reschedule IPI. */
  	if (rdp->preemptable)
  		return 0;

  	/* The CPU is online, so send it a reschedule IPI. */
  	if (rdp->cpu != smp_processor_id())
  		smp_send_reschedule(rdp->cpu);
  	else
  		set_need_resched();
  	rdp->resched_ipi++;
  	return 0;
  }
  
  #endif /* #ifdef CONFIG_SMP */
  
  #ifdef CONFIG_NO_HZ

  
  /**
   * rcu_enter_nohz - inform RCU that current CPU is entering nohz
   *
   * Enter nohz mode, in other words, -leave- the mode in which RCU
   * read-side critical sections can occur.  (Though RCU read-side
   * critical sections can occur in irq handlers in nohz mode, a possibility
   * handled by rcu_irq_enter() and rcu_irq_exit()).
   */
  void rcu_enter_nohz(void)
  {
  	unsigned long flags;
  	struct rcu_dynticks *rdtp;
  
  	smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
  	local_irq_save(flags);
  	rdtp = &__get_cpu_var(rcu_dynticks);
  	rdtp->dynticks++;
  	rdtp->dynticks_nesting--;

  	WARN_ON_ONCE(rdtp->dynticks & 0x1);

  	local_irq_restore(flags);
  }
  
  /*
   * rcu_exit_nohz - inform RCU that current CPU is leaving nohz
   *
   * Exit nohz mode, in other words, -enter- the mode in which RCU
   * read-side critical sections normally occur.
   */
  void rcu_exit_nohz(void)
  {
  	unsigned long flags;
  	struct rcu_dynticks *rdtp;
  
  	local_irq_save(flags);
  	rdtp = &__get_cpu_var(rcu_dynticks);
  	rdtp->dynticks++;
  	rdtp->dynticks_nesting++;

  	WARN_ON_ONCE(!(rdtp->dynticks & 0x1));

  	local_irq_restore(flags);
  	smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
  }
  
  /**
   * rcu_nmi_enter - inform RCU of entry to NMI context
   *
   * If the CPU was idle with dynamic ticks active, and there is no
   * irq handler running, this updates rdtp->dynticks_nmi to let the
   * RCU grace-period handling know that the CPU is active.
   */
  void rcu_nmi_enter(void)
  {
  	struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
  
  	if (rdtp->dynticks & 0x1)
  		return;
  	rdtp->dynticks_nmi++;

  	WARN_ON_ONCE(!(rdtp->dynticks_nmi & 0x1));

  	smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
  }
  
  /**
   * rcu_nmi_exit - inform RCU of exit from NMI context
   *
   * If the CPU was idle with dynamic ticks active, and there is no
   * irq handler running, this updates rdtp->dynticks_nmi to let the
   * RCU grace-period handling know that the CPU is no longer active.
   */
  void rcu_nmi_exit(void)
  {
  	struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
  
  	if (rdtp->dynticks & 0x1)
  		return;
  	smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
  	rdtp->dynticks_nmi++;

  	WARN_ON_ONCE(rdtp->dynticks_nmi & 0x1);

  }
  
  /**
   * rcu_irq_enter - inform RCU of entry to hard irq context
   *
   * If the CPU was idle with dynamic ticks active, this updates the
   * rdtp->dynticks to let the RCU handling know that the CPU is active.
   */
  void rcu_irq_enter(void)
  {
  	struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
  
  	if (rdtp->dynticks_nesting++)
  		return;
  	rdtp->dynticks++;

  	WARN_ON_ONCE(!(rdtp->dynticks & 0x1));

  	smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
  }
  
  /**
   * rcu_irq_exit - inform RCU of exit from hard irq context
   *
   * If the CPU was idle with dynamic ticks active, update the rdp->dynticks
   * to put let the RCU handling be aware that the CPU is going back to idle
   * with no ticks.
   */
  void rcu_irq_exit(void)
  {
  	struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
  
  	if (--rdtp->dynticks_nesting)
  		return;
  	smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
  	rdtp->dynticks++;

  	WARN_ON_ONCE(rdtp->dynticks & 0x1);

  
  	/* If the interrupt queued a callback, get out of dyntick mode. */

  	if (__get_cpu_var(rcu_sched_data).nxtlist ||

  	    __get_cpu_var(rcu_bh_data).nxtlist)
  		set_need_resched();
  }

  #ifdef CONFIG_SMP
  
  /*

   * Snapshot the specified CPU's dynticks counter so that we can later
   * credit them with an implicit quiescent state.  Return 1 if this CPU

   * is in dynticks idle mode, which is an extended quiescent state.

   */
  static int dyntick_save_progress_counter(struct rcu_data *rdp)
  {
  	int ret;
  	int snap;
  	int snap_nmi;
  
  	snap = rdp->dynticks->dynticks;
  	snap_nmi = rdp->dynticks->dynticks_nmi;
  	smp_mb();	/* Order sampling of snap with end of grace period. */
  	rdp->dynticks_snap = snap;
  	rdp->dynticks_nmi_snap = snap_nmi;
  	ret = ((snap & 0x1) == 0) && ((snap_nmi & 0x1) == 0);
  	if (ret)
  		rdp->dynticks_fqs++;
  	return ret;
  }
  
  /*
   * Return true if the specified CPU has passed through a quiescent
   * state by virtue of being in or having passed through an dynticks
   * idle state since the last call to dyntick_save_progress_counter()
   * for this same CPU.
   */
  static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
  {
  	long curr;
  	long curr_nmi;
  	long snap;
  	long snap_nmi;
  
  	curr = rdp->dynticks->dynticks;
  	snap = rdp->dynticks_snap;
  	curr_nmi = rdp->dynticks->dynticks_nmi;
  	snap_nmi = rdp->dynticks_nmi_snap;
  	smp_mb(); /* force ordering with cpu entering/leaving dynticks. */
  
  	/*
  	 * If the CPU passed through or entered a dynticks idle phase with
  	 * no active irq/NMI handlers, then we can safely pretend that the CPU
  	 * already acknowledged the request to pass through a quiescent
  	 * state.  Either way, that CPU cannot possibly be in an RCU
  	 * read-side critical section that started before the beginning
  	 * of the current RCU grace period.
  	 */
  	if ((curr != snap || (curr & 0x1) == 0) &&
  	    (curr_nmi != snap_nmi || (curr_nmi & 0x1) == 0)) {
  		rdp->dynticks_fqs++;
  		return 1;
  	}
  
  	/* Go check for the CPU being offline. */
  	return rcu_implicit_offline_qs(rdp);
  }
  
  #endif /* #ifdef CONFIG_SMP */
  
  #else /* #ifdef CONFIG_NO_HZ */

  #ifdef CONFIG_SMP

  static int dyntick_save_progress_counter(struct rcu_data *rdp)
  {
  	return 0;
  }
  
  static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
  {
  	return rcu_implicit_offline_qs(rdp);
  }
  
  #endif /* #ifdef CONFIG_SMP */
  
  #endif /* #else #ifdef CONFIG_NO_HZ */
  
  #ifdef CONFIG_RCU_CPU_STALL_DETECTOR
  
  static void record_gp_stall_check_time(struct rcu_state *rsp)
  {
  	rsp->gp_start = jiffies;
  	rsp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_CHECK;
  }
  
  static void print_other_cpu_stall(struct rcu_state *rsp)
  {
  	int cpu;
  	long delta;
  	unsigned long flags;
  	struct rcu_node *rnp = rcu_get_root(rsp);

  
  	/* Only let one CPU complain about others per time interval. */
  
  	spin_lock_irqsave(&rnp->lock, flags);
  	delta = jiffies - rsp->jiffies_stall;

  	if (delta < RCU_STALL_RAT_DELAY || !rcu_gp_in_progress(rsp)) {

  		spin_unlock_irqrestore(&rnp->lock, flags);
  		return;
  	}
  	rsp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_RECHECK;

  
  	/*
  	 * Now rat on any tasks that got kicked up to the root rcu_node
  	 * due to CPU offlining.
  	 */
  	rcu_print_task_stall(rnp);

  	spin_unlock_irqrestore(&rnp->lock, flags);
  
  	/* OK, time to rat on our buddy... */
  
  	printk(KERN_ERR "INFO: RCU detected CPU stalls:");

  	rcu_for_each_leaf_node(rsp, rnp) {

  		rcu_print_task_stall(rnp);

  		if (rnp->qsmask == 0)

  			continue;

  		for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
  			if (rnp->qsmask & (1UL << cpu))
  				printk(" %d", rnp->grplo + cpu);

  	}
  	printk(" (detected by %d, t=%ld jiffies)
  ",
  	       smp_processor_id(), (long)(jiffies - rsp->gp_start));

  	trigger_all_cpu_backtrace();

  	force_quiescent_state(rsp, 0);  /* Kick them all. */
  }
  
  static void print_cpu_stall(struct rcu_state *rsp)
  {
  	unsigned long flags;
  	struct rcu_node *rnp = rcu_get_root(rsp);
  
  	printk(KERN_ERR "INFO: RCU detected CPU %d stall (t=%lu jiffies)
  ",
  			smp_processor_id(), jiffies - rsp->gp_start);

  	trigger_all_cpu_backtrace();

  	spin_lock_irqsave(&rnp->lock, flags);
  	if ((long)(jiffies - rsp->jiffies_stall) >= 0)
  		rsp->jiffies_stall =
  			jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
  	spin_unlock_irqrestore(&rnp->lock, flags);

  	set_need_resched();  /* kick ourselves to get things going. */
  }
  
  static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
  {
  	long delta;
  	struct rcu_node *rnp;
  
  	delta = jiffies - rsp->jiffies_stall;
  	rnp = rdp->mynode;
  	if ((rnp->qsmask & rdp->grpmask) && delta >= 0) {
  
  		/* We haven't checked in, so go dump stack. */
  		print_cpu_stall(rsp);

  	} else if (rcu_gp_in_progress(rsp) && delta >= RCU_STALL_RAT_DELAY) {

  
  		/* They had two time units to dump stack, so complain. */
  		print_other_cpu_stall(rsp);
  	}
  }
  
  #else /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
  
  static void record_gp_stall_check_time(struct rcu_state *rsp)
  {
  }
  
  static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
  {
  }
  
  #endif /* #else #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
  
  /*
   * Update CPU-local rcu_data state to record the newly noticed grace period.
   * This is used both when we started the grace period and when we notice

   * that someone else started the grace period.  The caller must hold the
   * ->lock of the leaf rcu_node structure corresponding to the current CPU,
   *  and must have irqs disabled.

   */

  static void __note_new_gpnum(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_data *rdp)
  {
  	if (rdp->gpnum != rnp->gpnum) {
  		rdp->qs_pending = 1;
  		rdp->passed_quiesc = 0;
  		rdp->gpnum = rnp->gpnum;
  	}
  }

  static void note_new_gpnum(struct rcu_state *rsp, struct rcu_data *rdp)
  {

  	unsigned long flags;
  	struct rcu_node *rnp;
  
  	local_irq_save(flags);
  	rnp = rdp->mynode;
  	if (rdp->gpnum == ACCESS_ONCE(rnp->gpnum) || /* outside lock. */
  	    !spin_trylock(&rnp->lock)) { /* irqs already off, retry later. */
  		local_irq_restore(flags);
  		return;
  	}
  	__note_new_gpnum(rsp, rnp, rdp);
  	spin_unlock_irqrestore(&rnp->lock, flags);

  }
  
  /*
   * Did someone else start a new RCU grace period start since we last
   * checked?  Update local state appropriately if so.  Must be called
   * on the CPU corresponding to rdp.
   */
  static int
  check_for_new_grace_period(struct rcu_state *rsp, struct rcu_data *rdp)
  {
  	unsigned long flags;
  	int ret = 0;
  
  	local_irq_save(flags);
  	if (rdp->gpnum != rsp->gpnum) {
  		note_new_gpnum(rsp, rdp);
  		ret = 1;
  	}
  	local_irq_restore(flags);
  	return ret;
  }
  
  /*

   * Advance this CPU's callbacks, but only if the current grace period
   * has ended.  This may be called only from the CPU to whom the rdp
   * belongs.  In addition, the corresponding leaf rcu_node structure's
   * ->lock must be held by the caller, with irqs disabled.
   */
  static void
  __rcu_process_gp_end(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_data *rdp)
  {
  	/* Did another grace period end? */
  	if (rdp->completed != rnp->completed) {
  
  		/* Advance callbacks.  No harm if list empty. */
  		rdp->nxttail[RCU_DONE_TAIL] = rdp->nxttail[RCU_WAIT_TAIL];
  		rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_READY_TAIL];
  		rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
  
  		/* Remember that we saw this grace-period completion. */
  		rdp->completed = rnp->completed;
  	}
  }
  
  /*
   * Advance this CPU's callbacks, but only if the current grace period
   * has ended.  This may be called only from the CPU to whom the rdp
   * belongs.
   */
  static void
  rcu_process_gp_end(struct rcu_state *rsp, struct rcu_data *rdp)
  {
  	unsigned long flags;
  	struct rcu_node *rnp;
  
  	local_irq_save(flags);
  	rnp = rdp->mynode;
  	if (rdp->completed == ACCESS_ONCE(rnp->completed) || /* outside lock. */
  	    !spin_trylock(&rnp->lock)) { /* irqs already off, retry later. */
  		local_irq_restore(flags);
  		return;
  	}
  	__rcu_process_gp_end(rsp, rnp, rdp);
  	spin_unlock_irqrestore(&rnp->lock, flags);
  }
  
  /*
   * Do per-CPU grace-period initialization for running CPU.  The caller
   * must hold the lock of the leaf rcu_node structure corresponding to
   * this CPU.
   */
  static void
  rcu_start_gp_per_cpu(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_data *rdp)
  {
  	/* Prior grace period ended, so advance callbacks for current CPU. */
  	__rcu_process_gp_end(rsp, rnp, rdp);
  
  	/*
  	 * Because this CPU just now started the new grace period, we know
  	 * that all of its callbacks will be covered by this upcoming grace
  	 * period, even the ones that were registered arbitrarily recently.
  	 * Therefore, advance all outstanding callbacks to RCU_WAIT_TAIL.
  	 *
  	 * Other CPUs cannot be sure exactly when the grace period started.
  	 * Therefore, their recently registered callbacks must pass through
  	 * an additional RCU_NEXT_READY stage, so that they will be handled
  	 * by the next RCU grace period.
  	 */
  	rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
  	rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];

  
  	/* Set state so that this CPU will detect the next quiescent state. */
  	__note_new_gpnum(rsp, rnp, rdp);

  }
  
  /*

   * Start a new RCU grace period if warranted, re-initializing the hierarchy
   * in preparation for detecting the next grace period.  The caller must hold
   * the root node's ->lock, which is released before return.  Hard irqs must
   * be disabled.
   */
  static void
  rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
  	__releases(rcu_get_root(rsp)->lock)
  {
  	struct rcu_data *rdp = rsp->rda[smp_processor_id()];
  	struct rcu_node *rnp = rcu_get_root(rsp);

  	if (!cpu_needs_another_gp(rsp, rdp) || rsp->fqs_active) {

  		if (cpu_needs_another_gp(rsp, rdp))
  			rsp->fqs_need_gp = 1;

  		if (rnp->completed == rsp->completed) {
  			spin_unlock_irqrestore(&rnp->lock, flags);
  			return;
  		}
  		spin_unlock(&rnp->lock);	 /* irqs remain disabled. */
  
  		/*
  		 * Propagate new ->completed value to rcu_node structures
  		 * so that other CPUs don't have to wait until the start
  		 * of the next grace period to process their callbacks.
  		 */
  		rcu_for_each_node_breadth_first(rsp, rnp) {
  			spin_lock(&rnp->lock);	 /* irqs already disabled. */
  			rnp->completed = rsp->completed;
  			spin_unlock(&rnp->lock); /* irqs remain disabled. */
  		}
  		local_irq_restore(flags);

  		return;
  	}
  
  	/* Advance to a new grace period and initialize state. */
  	rsp->gpnum++;

  	WARN_ON_ONCE(rsp->signaled == RCU_GP_INIT);

  	rsp->signaled = RCU_GP_INIT; /* Hold off force_quiescent_state. */
  	rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;

  	record_gp_stall_check_time(rsp);

  	/* Special-case the common single-level case. */
  	if (NUM_RCU_NODES == 1) {

  		rcu_preempt_check_blocked_tasks(rnp);

  		rnp->qsmask = rnp->qsmaskinit;

  		rnp->gpnum = rsp->gpnum;

  		rnp->completed = rsp->completed;

  		rsp->signaled = RCU_SIGNAL_INIT; /* force_quiescent_state OK. */

  		rcu_start_gp_per_cpu(rsp, rnp, rdp);

  		spin_unlock_irqrestore(&rnp->lock, flags);
  		return;
  	}
  
  	spin_unlock(&rnp->lock);  /* leave irqs disabled. */
  
  
  	/* Exclude any concurrent CPU-hotplug operations. */
  	spin_lock(&rsp->onofflock);  /* irqs already disabled. */
  
  	/*

  	 * Set the quiescent-state-needed bits in all the rcu_node
  	 * structures for all currently online CPUs in breadth-first
  	 * order, starting from the root rcu_node structure.  This
  	 * operation relies on the layout of the hierarchy within the
  	 * rsp->node[] array.  Note that other CPUs will access only
  	 * the leaves of the hierarchy, which still indicate that no
  	 * grace period is in progress, at least until the corresponding
  	 * leaf node has been initialized.  In addition, we have excluded
  	 * CPU-hotplug operations.

  	 *
  	 * Note that the grace period cannot complete until we finish
  	 * the initialization process, as there will be at least one
  	 * qsmask bit set in the root node until that time, namely the

  	 * one corresponding to this CPU, due to the fact that we have
  	 * irqs disabled.

  	 */

  	rcu_for_each_node_breadth_first(rsp, rnp) {

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

  		rcu_preempt_check_blocked_tasks(rnp);

  		rnp->qsmask = rnp->qsmaskinit;

  		rnp->gpnum = rsp->gpnum;

  		rnp->completed = rsp->completed;
  		if (rnp == rdp->mynode)
  			rcu_start_gp_per_cpu(rsp, rnp, rdp);

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

  	}

  	rnp = rcu_get_root(rsp);
  	spin_lock(&rnp->lock);			/* irqs already disabled. */

  	rsp->signaled = RCU_SIGNAL_INIT; /* force_quiescent_state now OK. */

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

  	spin_unlock_irqrestore(&rsp->onofflock, flags);
  }
  
  /*

   * Report a full set of quiescent states to the specified rcu_state
   * data structure.  This involves cleaning up after the prior grace
   * period and letting rcu_start_gp() start up the next grace period
   * if one is needed.  Note that the caller must hold rnp->lock, as
   * required by rcu_start_gp(), which will release it.

   */

  static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)

  	__releases(rcu_get_root(rsp)->lock)

  {

  	WARN_ON_ONCE(!rcu_gp_in_progress(rsp));

  	rsp->completed = rsp->gpnum;

  	rsp->signaled = RCU_GP_IDLE;

  	rcu_start_gp(rsp, flags);  /* releases root node's rnp->lock. */
  }
  
  /*

   * Similar to rcu_report_qs_rdp(), for which it is a helper function.
   * Allows quiescent states for a group of CPUs to be reported at one go
   * to the specified rcu_node structure, though all the CPUs in the group
   * must be represented by the same rcu_node structure (which need not be
   * a leaf rcu_node structure, though it often will be).  That structure's
   * lock must be held upon entry, and it is released before return.

   */
  static void

  rcu_report_qs_rnp(unsigned long mask, struct rcu_state *rsp,
  		  struct rcu_node *rnp, unsigned long flags)

  	__releases(rnp->lock)
  {

  	struct rcu_node *rnp_c;

  	/* Walk up the rcu_node hierarchy. */
  	for (;;) {
  		if (!(rnp->qsmask & mask)) {
  
  			/* Our bit has already been cleared, so done. */
  			spin_unlock_irqrestore(&rnp->lock, flags);
  			return;
  		}
  		rnp->qsmask &= ~mask;

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

  
  			/* Other bits still set at this level, so done. */
  			spin_unlock_irqrestore(&rnp->lock, flags);
  			return;
  		}
  		mask = rnp->grpmask;
  		if (rnp->parent == NULL) {
  
  			/* No more levels.  Exit loop holding root lock. */
  
  			break;
  		}
  		spin_unlock_irqrestore(&rnp->lock, flags);

  		rnp_c = rnp;

  		rnp = rnp->parent;
  		spin_lock_irqsave(&rnp->lock, flags);

  		WARN_ON_ONCE(rnp_c->qsmask);

  	}
  
  	/*
  	 * Get here if we are the last CPU to pass through a quiescent

  	 * state for this grace period.  Invoke rcu_report_qs_rsp()

  	 * to clean up and start the next grace period if one is needed.

  	 */

  	rcu_report_qs_rsp(rsp, flags); /* releases rnp->lock. */

  }
  
  /*

   * Record a quiescent state for the specified CPU to that CPU's rcu_data
   * structure.  This must be either called from the specified CPU, or
   * called when the specified CPU is known to be offline (and when it is
   * also known that no other CPU is concurrently trying to help the offline
   * CPU).  The lastcomp argument is used to make sure we are still in the
   * grace period of interest.  We don't want to end the current grace period
   * based on quiescent states detected in an earlier grace period!

   */
  static void

  rcu_report_qs_rdp(int cpu, struct rcu_state *rsp, struct rcu_data *rdp, long lastcomp)

  {
  	unsigned long flags;
  	unsigned long mask;
  	struct rcu_node *rnp;
  
  	rnp = rdp->mynode;
  	spin_lock_irqsave(&rnp->lock, flags);

  	if (lastcomp != rnp->completed) {

  
  		/*
  		 * Someone beat us to it for this grace period, so leave.
  		 * The race with GP start is resolved by the fact that we
  		 * hold the leaf rcu_node lock, so that the per-CPU bits
  		 * cannot yet be initialized -- so we would simply find our

  		 * CPU's bit already cleared in rcu_report_qs_rnp() if this
  		 * race occurred.

  		 */
  		rdp->passed_quiesc = 0;	/* try again later! */
  		spin_unlock_irqrestore(&rnp->lock, flags);
  		return;
  	}
  	mask = rdp->grpmask;
  	if ((rnp->qsmask & mask) == 0) {
  		spin_unlock_irqrestore(&rnp->lock, flags);
  	} else {
  		rdp->qs_pending = 0;
  
  		/*
  		 * This GP can't end until cpu checks in, so all of our
  		 * callbacks can be processed during the next GP.
  		 */

  		rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];

  		rcu_report_qs_rnp(mask, rsp, rnp, flags); /* rlses rnp->lock */

  	}
  }
  
  /*
   * Check to see if there is a new grace period of which this CPU
   * is not yet aware, and if so, set up local rcu_data state for it.
   * Otherwise, see if this CPU has just passed through its first
   * quiescent state for this grace period, and record that fact if so.
   */
  static void
  rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
  {
  	/* If there is now a new grace period, record and return. */
  	if (check_for_new_grace_period(rsp, rdp))
  		return;
  
  	/*
  	 * Does this CPU still need to do its part for current grace period?
  	 * If no, return and let the other CPUs do their part as well.
  	 */
  	if (!rdp->qs_pending)
  		return;
  
  	/*
  	 * Was there a quiescent state since the beginning of the grace
  	 * period? If no, then exit and wait for the next call.
  	 */
  	if (!rdp->passed_quiesc)
  		return;

  	/*
  	 * Tell RCU we are done (but rcu_report_qs_rdp() will be the
  	 * judge of that).
  	 */
  	rcu_report_qs_rdp(rdp->cpu, rsp, rdp, rdp->passed_quiesc_completed);

  }
  
  #ifdef CONFIG_HOTPLUG_CPU
  
  /*

   * Move a dying CPU's RCU callbacks to the ->orphan_cbs_list for the
   * specified flavor of RCU.  The callbacks will be adopted by the next
   * _rcu_barrier() invocation or by the CPU_DEAD notifier, whichever
   * comes first.  Because this is invoked from the CPU_DYING notifier,
   * irqs are already disabled.
   */
  static void rcu_send_cbs_to_orphanage(struct rcu_state *rsp)
  {
  	int i;
  	struct rcu_data *rdp = rsp->rda[smp_processor_id()];
  
  	if (rdp->nxtlist == NULL)
  		return;  /* irqs disabled, so comparison is stable. */
  	spin_lock(&rsp->onofflock);  /* irqs already disabled. */
  	*rsp->orphan_cbs_tail = rdp->nxtlist;
  	rsp->orphan_cbs_tail = rdp->nxttail[RCU_NEXT_TAIL];
  	rdp->nxtlist = NULL;
  	for (i = 0; i < RCU_NEXT_SIZE; i++)
  		rdp->nxttail[i] = &rdp->nxtlist;
  	rsp->orphan_qlen += rdp->qlen;
  	rdp->qlen = 0;
  	spin_unlock(&rsp->onofflock);  /* irqs remain disabled. */
  }
  
  /*
   * Adopt previously orphaned RCU callbacks.
   */
  static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
  {
  	unsigned long flags;
  	struct rcu_data *rdp;
  
  	spin_lock_irqsave(&rsp->onofflock, flags);
  	rdp = rsp->rda[smp_processor_id()];
  	if (rsp->orphan_cbs_list == NULL) {
  		spin_unlock_irqrestore(&rsp->onofflock, flags);
  		return;
  	}
  	*rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_cbs_list;
  	rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_cbs_tail;
  	rdp->qlen += rsp->orphan_qlen;
  	rsp->orphan_cbs_list = NULL;
  	rsp->orphan_cbs_tail = &rsp->orphan_cbs_list;
  	rsp->orphan_qlen = 0;
  	spin_unlock_irqrestore(&rsp->onofflock, flags);
  }
  
  /*

   * Remove the outgoing CPU from the bitmasks in the rcu_node hierarchy
   * and move all callbacks from the outgoing CPU to the current one.
   */
  static void __rcu_offline_cpu(int cpu, struct rcu_state *rsp)
  {

  	unsigned long flags;

  	unsigned long mask;

  	int need_report = 0;

  	struct rcu_data *rdp = rsp->rda[cpu];

  	struct rcu_node *rnp;
  
  	/* Exclude any attempts to start a new grace period. */
  	spin_lock_irqsave(&rsp->onofflock, flags);
  
  	/* Remove the outgoing CPU from the masks in the rcu_node hierarchy. */

  	rnp = rdp->mynode;	/* this is the outgoing CPU's rnp. */

  	mask = rdp->grpmask;	/* rnp->grplo is constant. */
  	do {
  		spin_lock(&rnp->lock);		/* irqs already disabled. */
  		rnp->qsmaskinit &= ~mask;
  		if (rnp->qsmaskinit != 0) {

  			if (rnp != rdp->mynode)
  				spin_unlock(&rnp->lock); /* irqs remain disabled. */

  			break;
  		}

  		if (rnp == rdp->mynode)

  			need_report = rcu_preempt_offline_tasks(rsp, rnp, rdp);

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

  		mask = rnp->grpmask;

  		rnp = rnp->parent;
  	} while (rnp != NULL);

  	/*
  	 * We still hold the leaf rcu_node structure lock here, and
  	 * irqs are still disabled.  The reason for this subterfuge is

  	 * because invoking rcu_report_unblock_qs_rnp() with ->onofflock
  	 * held leads to deadlock.

  	 */
  	spin_unlock(&rsp->onofflock); /* irqs remain disabled. */
  	rnp = rdp->mynode;

  	if (need_report & RCU_OFL_TASKS_NORM_GP)

  		rcu_report_unblock_qs_rnp(rnp, flags);

  	else
  		spin_unlock_irqrestore(&rnp->lock, flags);

  	if (need_report & RCU_OFL_TASKS_EXP_GP)
  		rcu_report_exp_rnp(rsp, rnp);

  	rcu_adopt_orphan_cbs(rsp);

  }
  
  /*
   * Remove the specified CPU from the RCU hierarchy and move any pending
   * callbacks that it might have to the current CPU.  This code assumes
   * that at least one CPU in the system will remain running at all times.
   * Any attempt to offline -all- CPUs is likely to strand RCU callbacks.
   */
  static void rcu_offline_cpu(int cpu)
  {

  	__rcu_offline_cpu(cpu, &rcu_sched_state);

  	__rcu_offline_cpu(cpu, &rcu_bh_state);

  	rcu_preempt_offline_cpu(cpu);

  }
  
  #else /* #ifdef CONFIG_HOTPLUG_CPU */

  static void rcu_send_cbs_to_orphanage(struct rcu_state *rsp)
  {
  }
  
  static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
  {
  }

  static void rcu_offline_cpu(int cpu)
  {
  }
  
  #endif /* #else #ifdef CONFIG_HOTPLUG_CPU */
  
  /*
   * Invoke any RCU callbacks that have made it to the end of their grace
   * period.  Thottle as specified by rdp->blimit.
   */

  static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)

  {
  	unsigned long flags;
  	struct rcu_head *next, *list, **tail;
  	int count;
  
  	/* If no callbacks are ready, just return.*/
  	if (!cpu_has_callbacks_ready_to_invoke(rdp))
  		return;
  
  	/*
  	 * Extract the list of ready callbacks, disabling to prevent
  	 * races with call_rcu() from interrupt handlers.
  	 */
  	local_irq_save(flags);
  	list = rdp->nxtlist;
  	rdp->nxtlist = *rdp->nxttail[RCU_DONE_TAIL];
  	*rdp->nxttail[RCU_DONE_TAIL] = NULL;
  	tail = rdp->nxttail[RCU_DONE_TAIL];
  	for (count = RCU_NEXT_SIZE - 1; count >= 0; count--)
  		if (rdp->nxttail[count] == rdp->nxttail[RCU_DONE_TAIL])
  			rdp->nxttail[count] = &rdp->nxtlist;
  	local_irq_restore(flags);
  
  	/* Invoke callbacks. */
  	count = 0;
  	while (list) {
  		next = list->next;
  		prefetch(next);
  		list->func(list);
  		list = next;
  		if (++count >= rdp->blimit)
  			break;
  	}
  
  	local_irq_save(flags);
  
  	/* Update count, and requeue any remaining callbacks. */
  	rdp->qlen -= count;
  	if (list != NULL) {
  		*tail = rdp->nxtlist;
  		rdp->nxtlist = list;
  		for (count = 0; count < RCU_NEXT_SIZE; count++)
  			if (&rdp->nxtlist == rdp->nxttail[count])
  				rdp->nxttail[count] = tail;
  			else
  				break;
  	}
  
  	/* Reinstate batch limit if we have worked down the excess. */
  	if (rdp->blimit == LONG_MAX && rdp->qlen <= qlowmark)
  		rdp->blimit = blimit;

  	/* Reset ->qlen_last_fqs_check trigger if enough CBs have drained. */
  	if (rdp->qlen == 0 && rdp->qlen_last_fqs_check != 0) {
  		rdp->qlen_last_fqs_check = 0;
  		rdp->n_force_qs_snap = rsp->n_force_qs;
  	} else if (rdp->qlen < rdp->qlen_last_fqs_check - qhimark)
  		rdp->qlen_last_fqs_check = rdp->qlen;

  	local_irq_restore(flags);
  
  	/* Re-raise the RCU softirq if there are callbacks remaining. */
  	if (cpu_has_callbacks_ready_to_invoke(rdp))
  		raise_softirq(RCU_SOFTIRQ);
  }
  
  /*
   * Check to see if this CPU is in a non-context-switch quiescent state
   * (user mode or idle loop for rcu, non-softirq execution for rcu_bh).
   * Also schedule the RCU softirq handler.
   *
   * This function must be called with hardirqs disabled.  It is normally
   * invoked from the scheduling-clock interrupt.  If rcu_pending returns
   * false, there is no point in invoking rcu_check_callbacks().
   */
  void rcu_check_callbacks(int cpu, int user)
  {

  	if (!rcu_pending(cpu))
  		return; /* if nothing for RCU to do. */

  	if (user ||

  	    (idle_cpu(cpu) && rcu_scheduler_active &&
  	     !in_softirq() && hardirq_count() <= (1 << HARDIRQ_SHIFT))) {

  
  		/*
  		 * 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 both

  		 * rcu_sched_qs() and rcu_bh_qs() reference only CPU-local
  		 * variables that other CPUs neither access nor modify,
  		 * at least not while the corresponding CPU is online.

  		 */

  		rcu_sched_qs(cpu);
  		rcu_bh_qs(cpu);

  
  	} else if (!in_softirq()) {
  
  		/*
  		 * Get here if this CPU did not take its interrupt from
  		 * softirq, in other words, if it is not interrupting
  		 * a rcu_bh read-side critical section.  This is an _bh

  		 * critical section, so note it.

  		 */

  		rcu_bh_qs(cpu);

  	}

  	rcu_preempt_check_callbacks(cpu);

  	raise_softirq(RCU_SOFTIRQ);
  }
  
  #ifdef CONFIG_SMP
  
  /*
   * Scan the leaf rcu_node structures, processing dyntick state for any that
   * have not yet encountered a quiescent state, using the function specified.

   * The caller must have suppressed start of new grace periods.

   */

  static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))

  {
  	unsigned long bit;
  	int cpu;
  	unsigned long flags;
  	unsigned long mask;

  	struct rcu_node *rnp;

  	rcu_for_each_leaf_node(rsp, rnp) {

  		mask = 0;

  		spin_lock_irqsave(&rnp->lock, flags);

  		if (!rcu_gp_in_progress(rsp)) {

  			spin_unlock_irqrestore(&rnp->lock, flags);

  			return;

  		}

  		if (rnp->qsmask == 0) {
  			spin_unlock_irqrestore(&rnp->lock, flags);

  			continue;
  		}

  		cpu = rnp->grplo;

  		bit = 1;

  		for (; cpu <= rnp->grphi; cpu++, bit <<= 1) {
  			if ((rnp->qsmask & bit) != 0 && f(rsp->rda[cpu]))

  				mask |= bit;
  		}

  		if (mask != 0) {

  			/* rcu_report_qs_rnp() releases rnp->lock. */
  			rcu_report_qs_rnp(mask, rsp, rnp, flags);

  			continue;
  		}

  		spin_unlock_irqrestore(&rnp->lock, flags);

  	}

  }
  
  /*
   * Force quiescent states on reluctant CPUs, and also detect which
   * CPUs are in dyntick-idle mode.
   */
  static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
  {
  	unsigned long flags;

  	struct rcu_node *rnp = rcu_get_root(rsp);

  	if (!rcu_gp_in_progress(rsp))

  		return;  /* No grace period in progress, nothing to force. */
  	if (!spin_trylock_irqsave(&rsp->fqslock, flags)) {
  		rsp->n_force_qs_lh++; /* Inexact, can lose counts.  Tough! */
  		return;	/* Someone else is already on the job. */
  	}
  	if (relaxed &&

  	    (long)(rsp->jiffies_force_qs - jiffies) >= 0)

  		goto unlock_fqs_ret; /* no emergency and done recently. */

  	rsp->n_force_qs++;

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

  	rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;

  	if(!rcu_gp_in_progress(rsp)) {

  		rsp->n_force_qs_ngp++;

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

  		goto unlock_fqs_ret;  /* no GP in progress, time updated. */

  	}

  	rsp->fqs_active = 1;

  	switch (rsp->signaled) {

  	case RCU_GP_IDLE:

  	case RCU_GP_INIT:

  		break; /* grace period idle or initializing, ignore. */

  
  	case RCU_SAVE_DYNTICK:

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

  		if (RCU_SIGNAL_INIT != RCU_SAVE_DYNTICK)
  			break; /* So gcc recognizes the dead code. */
  
  		/* Record dyntick-idle state. */

  		force_qs_rnp(rsp, dyntick_save_progress_counter);

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

  		if (rcu_gp_in_progress(rsp))

  			rsp->signaled = RCU_FORCE_QS;

  		break;

  
  	case RCU_FORCE_QS:
  
  		/* Check dyntick-idle state, send IPI to laggarts. */

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

  		force_qs_rnp(rsp, rcu_implicit_dynticks_qs);

  
  		/* Leave state in case more forcing is required. */

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

  		break;

  	}

  	rsp->fqs_active = 0;

  	if (rsp->fqs_need_gp) {
  		spin_unlock(&rsp->fqslock); /* irqs remain disabled */
  		rsp->fqs_need_gp = 0;
  		rcu_start_gp(rsp, flags); /* releases rnp->lock */
  		return;
  	}

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

  unlock_fqs_ret:

  	spin_unlock_irqrestore(&rsp->fqslock, flags);
  }
  
  #else /* #ifdef CONFIG_SMP */
  
  static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
  {
  	set_need_resched();
  }
  
  #endif /* #else #ifdef CONFIG_SMP */
  
  /*
   * This does the RCU processing work from softirq context for the
   * specified rcu_state and rcu_data structures.  This may be called
   * only from the CPU to whom the rdp belongs.
   */
  static void
  __rcu_process_callbacks(struct rcu_state *rsp, struct rcu_data *rdp)
  {
  	unsigned long flags;

  	WARN_ON_ONCE(rdp->beenonline == 0);

  	/*
  	 * If an RCU GP has gone long enough, go check for dyntick
  	 * idle CPUs and, if needed, send resched IPIs.
  	 */

  	if ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0)

  		force_quiescent_state(rsp, 1);
  
  	/*
  	 * Advance callbacks in response to end of earlier grace
  	 * period that some other CPU ended.
  	 */
  	rcu_process_gp_end(rsp, rdp);
  
  	/* Update RCU state based on any recent quiescent states. */
  	rcu_check_quiescent_state(rsp, rdp);
  
  	/* Does this CPU require a not-yet-started grace period? */
  	if (cpu_needs_another_gp(rsp, rdp)) {
  		spin_lock_irqsave(&rcu_get_root(rsp)->lock, flags);
  		rcu_start_gp(rsp, flags);  /* releases above lock */
  	}
  
  	/* If there are callbacks ready, invoke them. */

  	rcu_do_batch(rsp, rdp);

  }
  
  /*
   * Do softirq processing for the current CPU.
   */
  static void rcu_process_callbacks(struct softirq_action *unused)
  {
  	/*
  	 * Memory references from any prior RCU read-side critical sections
  	 * executed by the interrupted code must be seen before any RCU
  	 * grace-period manipulations below.
  	 */
  	smp_mb(); /* See above block comment. */

  	__rcu_process_callbacks(&rcu_sched_state,
  				&__get_cpu_var(rcu_sched_data));

  	__rcu_process_callbacks(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));

  	rcu_preempt_process_callbacks();

  
  	/*
  	 * Memory references from any later RCU read-side critical sections
  	 * executed by the interrupted code must be seen after any RCU
  	 * grace-period manipulations above.
  	 */
  	smp_mb(); /* See above block comment. */
  }
  
  static void
  __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
  	   struct rcu_state *rsp)
  {
  	unsigned long flags;
  	struct rcu_data *rdp;
  
  	head->func = func;
  	head->next = NULL;
  
  	smp_mb(); /* Ensure RCU update seen before callback registry. */
  
  	/*
  	 * Opportunistically note grace-period endings and beginnings.
  	 * Note that we might see a beginning right after we see an
  	 * end, but never vice versa, since this CPU has to pass through
  	 * a quiescent state betweentimes.
  	 */
  	local_irq_save(flags);
  	rdp = rsp->rda[smp_processor_id()];
  	rcu_process_gp_end(rsp, rdp);
  	check_for_new_grace_period(rsp, rdp);
  
  	/* Add the callback to our list. */
  	*rdp->nxttail[RCU_NEXT_TAIL] = head;
  	rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
  
  	/* Start a new grace period if one not already started. */

  	if (!rcu_gp_in_progress(rsp)) {

  		unsigned long nestflag;
  		struct rcu_node *rnp_root = rcu_get_root(rsp);
  
  		spin_lock_irqsave(&rnp_root->lock, nestflag);
  		rcu_start_gp(rsp, nestflag);  /* releases rnp_root->lock. */
  	}

  	/*
  	 * Force the grace period if too many callbacks or too long waiting.
  	 * Enforce hysteresis, and don't invoke force_quiescent_state()
  	 * if some other CPU has recently done so.  Also, don't bother
  	 * invoking force_quiescent_state() if the newly enqueued callback
  	 * is the only one waiting for a grace period to complete.
  	 */
  	if (unlikely(++rdp->qlen > rdp->qlen_last_fqs_check + qhimark)) {

  		rdp->blimit = LONG_MAX;

  		if (rsp->n_force_qs == rdp->n_force_qs_snap &&
  		    *rdp->nxttail[RCU_DONE_TAIL] != head)
  			force_quiescent_state(rsp, 0);
  		rdp->n_force_qs_snap = rsp->n_force_qs;
  		rdp->qlen_last_fqs_check = rdp->qlen;

  	} else if ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0)

  		force_quiescent_state(rsp, 1);
  	local_irq_restore(flags);
  }
  
  /*

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

   */

  void call_rcu_sched(struct rcu_head *head, void (*func)(struct rcu_head *rcu))

  {

  	__call_rcu(head, func, &rcu_sched_state);

  }

  EXPORT_SYMBOL_GPL(call_rcu_sched);

  
  /*
   * Queue an RCU for invocation after a quicker grace period.
   */
  void call_rcu_bh(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
  {
  	__call_rcu(head, func, &rcu_bh_state);
  }
  EXPORT_SYMBOL_GPL(call_rcu_bh);

  /**
   * synchronize_sched - wait until an rcu-sched grace period has elapsed.
   *
   * Control will return to the caller some time after a full rcu-sched
   * grace period has elapsed, in other words after all currently executing
   * rcu-sched read-side critical sections have completed.   These read-side
   * critical sections are delimited by rcu_read_lock_sched() and
   * rcu_read_unlock_sched(), and may be nested.  Note that preempt_disable(),
   * local_irq_disable(), and so on may be used in place of
   * rcu_read_lock_sched().
   *
   * This means that all preempt_disable code sequences, including NMI and
   * hardware-interrupt handlers, in progress on entry will have completed
   * before this primitive returns.  However, this does not guarantee that
   * softirq handlers will have completed, since in some kernels, these
   * handlers can run in process context, and can block.
   *
   * This primitive provides the guarantees made by the (now removed)
   * synchronize_kernel() API.  In contrast, synchronize_rcu() only
   * guarantees that rcu_read_lock() sections will have completed.
   * In "classic RCU", these two guarantees happen to be one and
   * the same, but can differ in realtime RCU implementations.
   */
  void synchronize_sched(void)
  {
  	struct rcu_synchronize rcu;
  
  	if (rcu_blocking_is_gp())
  		return;
  
  	init_completion(&rcu.completion);
  	/* Will wake me after RCU finished. */
  	call_rcu_sched(&rcu.head, wakeme_after_rcu);
  	/* Wait for it. */
  	wait_for_completion(&rcu.completion);
  }
  EXPORT_SYMBOL_GPL(synchronize_sched);
  
  /**
   * synchronize_rcu_bh - wait until an rcu_bh grace period has elapsed.
   *
   * Control will return to the caller some time after a full rcu_bh grace
   * period has elapsed, in other words after all currently executing rcu_bh
   * read-side critical sections have completed.  RCU read-side critical
   * sections are delimited by rcu_read_lock_bh() and rcu_read_unlock_bh(),
   * and may be nested.
   */
  void synchronize_rcu_bh(void)
  {
  	struct rcu_synchronize rcu;
  
  	if (rcu_blocking_is_gp())
  		return;
  
  	init_completion(&rcu.completion);
  	/* Will wake me after RCU finished. */
  	call_rcu_bh(&rcu.head, wakeme_after_rcu);
  	/* Wait for it. */
  	wait_for_completion(&rcu.completion);
  }
  EXPORT_SYMBOL_GPL(synchronize_rcu_bh);

  /*
   * Check to see if there is any immediate RCU-related work to be done
   * by the current CPU, for the specified type of RCU, returning 1 if so.
   * The checks are in order of increasing expense: checks that can be
   * carried out against CPU-local state are performed first.  However,
   * we must check for CPU stalls first, else we might not get a chance.
   */
  static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
  {

  	struct rcu_node *rnp = rdp->mynode;

  	rdp->n_rcu_pending++;
  
  	/* Check for CPU stalls, if enabled. */
  	check_cpu_stall(rsp, rdp);
  
  	/* Is the RCU core waiting for a quiescent state from this CPU? */

  	if (rdp->qs_pending) {
  		rdp->n_rp_qs_pending++;

  		return 1;

  	}

  
  	/* Does this CPU have callbacks ready to invoke? */

  	if (cpu_has_callbacks_ready_to_invoke(rdp)) {
  		rdp->n_rp_cb_ready++;

  		return 1;

  	}

  
  	/* Has RCU gone idle with this CPU needing another grace period? */

  	if (cpu_needs_another_gp(rsp, rdp)) {
  		rdp->n_rp_cpu_needs_gp++;

  		return 1;

  	}

  
  	/* Has another RCU grace period completed?  */

  	if (ACCESS_ONCE(rnp->completed) != rdp->completed) { /* outside lock */

  		rdp->n_rp_gp_completed++;

  		return 1;

  	}

  
  	/* Has a new RCU grace period started? */

  	if (ACCESS_ONCE(rnp->gpnum) != rdp->gpnum) { /* outside lock */

  		rdp->n_rp_gp_started++;

  		return 1;

  	}

  
  	/* Has an RCU GP gone long enough to send resched IPIs &c? */

  	if (rcu_gp_in_progress(rsp) &&

  	    ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0)) {
  		rdp->n_rp_need_fqs++;

  		return 1;

  	}

  
  	/* nothing to do */

  	rdp->n_rp_need_nothing++;

  	return 0;
  }
  
  /*
   * Check to see if there is any immediate RCU-related work to be done
   * by the current CPU, returning 1 if so.  This function is part of the
   * RCU implementation; it is -not- an exported member of the RCU API.
   */

  static int rcu_pending(int cpu)

  {

  	return __rcu_pending(&rcu_sched_state, &per_cpu(rcu_sched_data, cpu)) ||

  	       __rcu_pending(&rcu_bh_state, &per_cpu(rcu_bh_data, cpu)) ||
  	       rcu_preempt_pending(cpu);

  }
  
  /*
   * 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.
   */
  int rcu_needs_cpu(int cpu)
  {
  	/* RCU callbacks either ready or pending? */

  	return per_cpu(rcu_sched_data, cpu).nxtlist ||

  	       per_cpu(rcu_bh_data, cpu).nxtlist ||
  	       rcu_preempt_needs_cpu(cpu);

  }

  /*
   * This function is invoked towards the end of the scheduler's initialization
   * process.  Before this is called, the idle task might contain
   * RCU read-side critical sections (during which time, this idle
   * task is booting the system).  After this function is called, the
   * idle tasks are prohibited from containing RCU read-side critical
   * sections.
   */
  void rcu_scheduler_starting(void)
  {
  	WARN_ON(num_online_cpus() != 1);
  	WARN_ON(nr_context_switches() > 0);
  	rcu_scheduler_active = 1;
  }

  static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL};
  static atomic_t rcu_barrier_cpu_count;
  static DEFINE_MUTEX(rcu_barrier_mutex);
  static struct completion rcu_barrier_completion;

  
  static void rcu_barrier_callback(struct rcu_head *notused)
  {
  	if (atomic_dec_and_test(&rcu_barrier_cpu_count))
  		complete(&rcu_barrier_completion);
  }
  
  /*
   * Called with preemption disabled, and from cross-cpu IRQ context.
   */
  static void rcu_barrier_func(void *type)
  {
  	int cpu = smp_processor_id();
  	struct rcu_head *head = &per_cpu(rcu_barrier_head, cpu);
  	void (*call_rcu_func)(struct rcu_head *head,
  			      void (*func)(struct rcu_head *head));
  
  	atomic_inc(&rcu_barrier_cpu_count);
  	call_rcu_func = type;
  	call_rcu_func(head, rcu_barrier_callback);
  }

  /*
   * Orchestrate the specified type of RCU barrier, waiting for all
   * RCU callbacks of the specified type to complete.
   */

  static void _rcu_barrier(struct rcu_state *rsp,
  			 void (*call_rcu_func)(struct rcu_head *head,

  					       void (*func)(struct rcu_head *head)))
  {
  	BUG_ON(in_interrupt());

  	/* Take mutex to serialize concurrent rcu_barrier() requests. */

  	mutex_lock(&rcu_barrier_mutex);
  	init_completion(&rcu_barrier_completion);
  	/*
  	 * Initialize rcu_barrier_cpu_count to 1, then invoke
  	 * rcu_barrier_func() on each CPU, so that each CPU also has
  	 * incremented rcu_barrier_cpu_count.  Only then is it safe to
  	 * decrement rcu_barrier_cpu_count -- otherwise the first CPU
  	 * might complete its grace period before all of the other CPUs
  	 * did their increment, causing this function to return too
  	 * early.
  	 */
  	atomic_set(&rcu_barrier_cpu_count, 1);

  	preempt_disable(); /* stop CPU_DYING from filling orphan_cbs_list */
  	rcu_adopt_orphan_cbs(rsp);

  	on_each_cpu(rcu_barrier_func, (void *)call_rcu_func, 1);

  	preempt_enable(); /* CPU_DYING can again fill orphan_cbs_list */

  	if (atomic_dec_and_test(&rcu_barrier_cpu_count))
  		complete(&rcu_barrier_completion);
  	wait_for_completion(&rcu_barrier_completion);
  	mutex_unlock(&rcu_barrier_mutex);

  }

  
  /**
   * rcu_barrier_bh - Wait until all in-flight call_rcu_bh() callbacks complete.
   */
  void rcu_barrier_bh(void)
  {

  	_rcu_barrier(&rcu_bh_state, call_rcu_bh);

  }
  EXPORT_SYMBOL_GPL(rcu_barrier_bh);
  
  /**
   * rcu_barrier_sched - Wait for in-flight call_rcu_sched() callbacks.
   */
  void rcu_barrier_sched(void)
  {

  	_rcu_barrier(&rcu_sched_state, call_rcu_sched);

  }
  EXPORT_SYMBOL_GPL(rcu_barrier_sched);

  /*

   * Do boot-time initialization of a CPU's per-CPU RCU data.

   */

  static void __init
  rcu_boot_init_percpu_data(int cpu, struct rcu_state *rsp)

  {
  	unsigned long flags;
  	int i;

  	struct rcu_data *rdp = rsp->rda[cpu];
  	struct rcu_node *rnp = rcu_get_root(rsp);
  
  	/* Set up local state, ensuring consistent view of global state. */
  	spin_lock_irqsave(&rnp->lock, flags);
  	rdp->grpmask = 1UL << (cpu - rdp->mynode->grplo);
  	rdp->nxtlist = NULL;
  	for (i = 0; i < RCU_NEXT_SIZE; i++)
  		rdp->nxttail[i] = &rdp->nxtlist;
  	rdp->qlen = 0;
  #ifdef CONFIG_NO_HZ
  	rdp->dynticks = &per_cpu(rcu_dynticks, cpu);
  #endif /* #ifdef CONFIG_NO_HZ */
  	rdp->cpu = cpu;
  	spin_unlock_irqrestore(&rnp->lock, flags);
  }
  
  /*
   * Initialize a CPU's per-CPU RCU data.  Note that only one online or
   * offline event can be happening at a given time.  Note also that we
   * can accept some slop in the rsp->completed access due to the fact
   * that this CPU cannot possibly have any RCU callbacks in flight yet.

   */

  static void __cpuinit

  rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptable)

  {
  	unsigned long flags;

  	unsigned long mask;
  	struct rcu_data *rdp = rsp->rda[cpu];
  	struct rcu_node *rnp = rcu_get_root(rsp);
  
  	/* Set up local state, ensuring consistent view of global state. */
  	spin_lock_irqsave(&rnp->lock, flags);

  	rdp->passed_quiesc = 0;  /* We could be racing with new GP, */
  	rdp->qs_pending = 1;	 /*  so set up to respond to current GP. */
  	rdp->beenonline = 1;	 /* We have now been online. */

  	rdp->preemptable = preemptable;

  	rdp->qlen_last_fqs_check = 0;
  	rdp->n_force_qs_snap = rsp->n_force_qs;

  	rdp->blimit = blimit;

  	spin_unlock(&rnp->lock);		/* irqs remain disabled. */
  
  	/*
  	 * A new grace period might start here.  If so, we won't be part
  	 * of it, but that is OK, as we are currently in a quiescent state.
  	 */
  
  	/* Exclude any attempts to start a new GP on large systems. */
  	spin_lock(&rsp->onofflock);		/* irqs already disabled. */
  
  	/* Add CPU to rcu_node bitmasks. */
  	rnp = rdp->mynode;
  	mask = rdp->grpmask;
  	do {
  		/* Exclude any attempts to start a new GP on small systems. */
  		spin_lock(&rnp->lock);	/* irqs already disabled. */
  		rnp->qsmaskinit |= mask;
  		mask = rnp->grpmask;

  		if (rnp == rdp->mynode) {
  			rdp->gpnum = rnp->completed; /* if GP in progress... */
  			rdp->completed = rnp->completed;
  			rdp->passed_quiesc_completed = rnp->completed - 1;
  		}

  		spin_unlock(&rnp->lock); /* irqs already disabled. */
  		rnp = rnp->parent;
  	} while (rnp != NULL && !(rnp->qsmaskinit & mask));

  	spin_unlock_irqrestore(&rsp->onofflock, flags);

  }
  
  static void __cpuinit rcu_online_cpu(int cpu)
  {

  	rcu_init_percpu_data(cpu, &rcu_sched_state, 0);
  	rcu_init_percpu_data(cpu, &rcu_bh_state, 0);
  	rcu_preempt_init_percpu_data(cpu);

  }
  
  /*

   * Handle CPU online/offline notification events.

   */

  static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
  				    unsigned long action, void *hcpu)

  {
  	long cpu = (long)hcpu;
  
  	switch (action) {
  	case CPU_UP_PREPARE:
  	case CPU_UP_PREPARE_FROZEN:
  		rcu_online_cpu(cpu);
  		break;

  	case CPU_DYING:
  	case CPU_DYING_FROZEN:
  		/*

  		 * preempt_disable() in _rcu_barrier() prevents stop_machine(),

  		 * so when "on_each_cpu(rcu_barrier_func, (void *)type, 1);"

  		 * returns, all online cpus have queued rcu_barrier_func().
  		 * The dying CPU clears its cpu_online_mask bit and
  		 * moves all of its RCU callbacks to ->orphan_cbs_list
  		 * in the context of stop_machine(), so subsequent calls
  		 * to _rcu_barrier() will adopt these callbacks and only
  		 * then queue rcu_barrier_func() on all remaining CPUs.

  		 */

  		rcu_send_cbs_to_orphanage(&rcu_bh_state);
  		rcu_send_cbs_to_orphanage(&rcu_sched_state);
  		rcu_preempt_send_cbs_to_orphanage();

  		break;

  	case CPU_DEAD:
  	case CPU_DEAD_FROZEN:
  	case CPU_UP_CANCELED:
  	case CPU_UP_CANCELED_FROZEN:
  		rcu_offline_cpu(cpu);
  		break;
  	default:
  		break;
  	}
  	return NOTIFY_OK;
  }
  
  /*
   * Compute the per-level fanout, either using the exact fanout specified
   * or balancing the tree, depending on CONFIG_RCU_FANOUT_EXACT.
   */
  #ifdef CONFIG_RCU_FANOUT_EXACT
  static void __init rcu_init_levelspread(struct rcu_state *rsp)
  {
  	int i;
  
  	for (i = NUM_RCU_LVLS - 1; i >= 0; i--)
  		rsp->levelspread[i] = CONFIG_RCU_FANOUT;
  }
  #else /* #ifdef CONFIG_RCU_FANOUT_EXACT */
  static void __init rcu_init_levelspread(struct rcu_state *rsp)
  {
  	int ccur;
  	int cprv;
  	int i;
  
  	cprv = NR_CPUS;
  	for (i = NUM_RCU_LVLS - 1; i >= 0; i--) {
  		ccur = rsp->levelcnt[i];
  		rsp->levelspread[i] = (cprv + ccur - 1) / ccur;
  		cprv = ccur;
  	}
  }
  #endif /* #else #ifdef CONFIG_RCU_FANOUT_EXACT */
  
  /*
   * Helper function for rcu_init() that initializes one rcu_state structure.
   */
  static void __init rcu_init_one(struct rcu_state *rsp)
  {
  	int cpustride = 1;
  	int i;
  	int j;
  	struct rcu_node *rnp;
  
  	/* Initialize the level-tracking arrays. */
  
  	for (i = 1; i < NUM_RCU_LVLS; i++)
  		rsp->level[i] = rsp->level[i - 1] + rsp->levelcnt[i - 1];
  	rcu_init_levelspread(rsp);
  
  	/* Initialize the elements themselves, starting from the leaves. */
  
  	for (i = NUM_RCU_LVLS - 1; i >= 0; i--) {
  		cpustride *= rsp->levelspread[i];
  		rnp = rsp->level[i];
  		for (j = 0; j < rsp->levelcnt[i]; j++, rnp++) {

  			spin_lock_init(&rnp->lock);

  			lockdep_set_class(&rnp->lock, &rcu_node_class[i]);

  			rnp->gpnum = 0;

  			rnp->qsmask = 0;
  			rnp->qsmaskinit = 0;
  			rnp->grplo = j * cpustride;
  			rnp->grphi = (j + 1) * cpustride - 1;
  			if (rnp->grphi >= NR_CPUS)
  				rnp->grphi = NR_CPUS - 1;
  			if (i == 0) {
  				rnp->grpnum = 0;
  				rnp->grpmask = 0;
  				rnp->parent = NULL;
  			} else {
  				rnp->grpnum = j % rsp->levelspread[i - 1];
  				rnp->grpmask = 1UL << rnp->grpnum;
  				rnp->parent = rsp->level[i - 1] +
  					      j / rsp->levelspread[i - 1];
  			}
  			rnp->level = i;

  			INIT_LIST_HEAD(&rnp->blocked_tasks[0]);
  			INIT_LIST_HEAD(&rnp->blocked_tasks[1]);

  			INIT_LIST_HEAD(&rnp->blocked_tasks[2]);
  			INIT_LIST_HEAD(&rnp->blocked_tasks[3]);

  		}
  	}
  }
  
  /*

   * Helper macro for __rcu_init() and __rcu_init_preempt().  To be used
   * nowhere else!  Assigns leaf node pointers into each CPU's rcu_data
   * structure.

   */

  #define RCU_INIT_FLAVOR(rsp, rcu_data) \

  do { \

  	int i; \
  	int j; \
  	struct rcu_node *rnp; \
  	\

  	rcu_init_one(rsp); \

  	rnp = (rsp)->level[NUM_RCU_LVLS - 1]; \
  	j = 0; \
  	for_each_possible_cpu(i) { \
  		if (i > rnp[j].grphi) \
  			j++; \
  		per_cpu(rcu_data, i).mynode = &rnp[j]; \
  		(rsp)->rda[i] = &per_cpu(rcu_data, i); \

  		rcu_boot_init_percpu_data(i, rsp); \

  	} \
  } while (0)

  void __init rcu_init(void)

  {

  	int i;

  	rcu_bootup_announce();

  #ifdef CONFIG_RCU_CPU_STALL_DETECTOR
  	printk(KERN_INFO "RCU-based detection of stalled CPUs is enabled.
  ");
  #endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */

  #if NUM_RCU_LVL_4 != 0
  	printk(KERN_INFO "Experimental four-level hierarchy is enabled.
  ");
  #endif /* #if NUM_RCU_LVL_4 != 0 */

  	RCU_INIT_FLAVOR(&rcu_sched_state, rcu_sched_data);
  	RCU_INIT_FLAVOR(&rcu_bh_state, rcu_bh_data);

  	__rcu_init_preempt();

  	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);

  
  	/*
  	 * We don't need protection against CPU-hotplug here because
  	 * this is called early in boot, before either interrupts
  	 * or the scheduler are operational.
  	 */
  	cpu_notifier(rcu_cpu_notify, 0);
  	for_each_online_cpu(i)
  		rcu_cpu_notify(NULL, CPU_UP_PREPARE, (void *)(long)i);

  }

  #include "rcutree_plugin.h"