/*
   * 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, 2001
   *
   * Authors: Dipankar Sarma <dipankar@in.ibm.com>
   *	    Manfred Spraul <manfred@colorfullife.com>
   *
   * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
   * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
   * Papers:
   * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
   * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
   *
   * 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 <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>

  
  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  static struct lock_class_key rcu_lock_key;
  struct lockdep_map rcu_lock_map =
  	STATIC_LOCKDEP_MAP_INIT("rcu_read_lock", &rcu_lock_key);
  EXPORT_SYMBOL_GPL(rcu_lock_map);
  #endif
  
  
  /* Definition for rcupdate control block. */
  static struct rcu_ctrlblk rcu_ctrlblk = {
  	.cur = -300,
  	.completed = -300,

  	.pending = -300,

  	.lock = __SPIN_LOCK_UNLOCKED(&rcu_ctrlblk.lock),

  	.cpumask = CPU_BITS_NONE,

  };
  static struct rcu_ctrlblk rcu_bh_ctrlblk = {
  	.cur = -300,
  	.completed = -300,

  	.pending = -300,

  	.lock = __SPIN_LOCK_UNLOCKED(&rcu_bh_ctrlblk.lock),

  	.cpumask = CPU_BITS_NONE,

  };
  
  DEFINE_PER_CPU(struct rcu_data, rcu_data) = { 0L };
  DEFINE_PER_CPU(struct rcu_data, rcu_bh_data) = { 0L };
  
  static int blimit = 10;
  static int qhimark = 10000;
  static int qlowmark = 100;
  
  #ifdef CONFIG_SMP
  static void force_quiescent_state(struct rcu_data *rdp,
  			struct rcu_ctrlblk *rcp)
  {
  	int cpu;

  	unsigned long flags;

  	set_need_resched();

  	spin_lock_irqsave(&rcp->lock, flags);

  	if (unlikely(!rcp->signaled)) {
  		rcp->signaled = 1;
  		/*
  		 * Don't send IPI to itself. With irqs disabled,
  		 * rdp->cpu is the current cpu.

  		 *

  		 * cpu_online_mask is updated by the _cpu_down()

  		 * using __stop_machine(). Since we're in irqs disabled
  		 * section, __stop_machine() is not exectuting, hence

  		 * the cpu_online_mask is stable.

  		 *
  		 * However,  a cpu might have been offlined _just_ before
  		 * we disabled irqs while entering here.
  		 * And rcu subsystem might not yet have handled the CPU_DEAD
  		 * notification, leading to the offlined cpu's bit
  		 * being set in the rcp->cpumask.
  		 *

  		 * Hence cpumask = (rcp->cpumask & cpu_online_mask) to prevent

  		 * sending smp_reschedule() to an offlined CPU.

  		 */

  		for_each_cpu_and(cpu,
  				  to_cpumask(rcp->cpumask), cpu_online_mask) {
  			if (cpu != rdp->cpu)
  				smp_send_reschedule(cpu);
  		}

  	}

  	spin_unlock_irqrestore(&rcp->lock, flags);

  }
  #else
  static inline void force_quiescent_state(struct rcu_data *rdp,
  			struct rcu_ctrlblk *rcp)
  {
  	set_need_resched();
  }
  #endif

  static void __call_rcu(struct rcu_head *head, struct rcu_ctrlblk *rcp,
  		struct rcu_data *rdp)
  {
  	long batch;

  
  	head->next = NULL;
  	smp_mb(); /* Read of rcu->cur must happen after any change by caller. */

  
  	/*
  	 * Determine the batch number of this callback.
  	 *
  	 * Using ACCESS_ONCE to avoid the following error when gcc eliminates
  	 * local variable "batch" and emits codes like this:
  	 *	1) rdp->batch = rcp->cur + 1 # gets old value
  	 *	......
  	 *	2)rcu_batch_after(rcp->cur + 1, rdp->batch) # gets new value
  	 * then [*nxttail[0], *nxttail[1]) may contain callbacks
  	 * that batch# = rdp->batch, see the comment of struct rcu_data.
  	 */
  	batch = ACCESS_ONCE(rcp->cur) + 1;
  
  	if (rdp->nxtlist && rcu_batch_after(batch, rdp->batch)) {
  		/* process callbacks */
  		rdp->nxttail[0] = rdp->nxttail[1];
  		rdp->nxttail[1] = rdp->nxttail[2];
  		if (rcu_batch_after(batch - 1, rdp->batch))
  			rdp->nxttail[0] = rdp->nxttail[2];
  	}
  
  	rdp->batch = batch;
  	*rdp->nxttail[2] = head;
  	rdp->nxttail[2] = &head->next;
  
  	if (unlikely(++rdp->qlen > qhimark)) {
  		rdp->blimit = INT_MAX;
  		force_quiescent_state(rdp, &rcu_ctrlblk);
  	}
  }

  #ifdef CONFIG_RCU_CPU_STALL_DETECTOR
  
  static void record_gp_stall_check_time(struct rcu_ctrlblk *rcp)
  {
  	rcp->gp_start = jiffies;
  	rcp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_CHECK;
  }
  
  static void print_other_cpu_stall(struct rcu_ctrlblk *rcp)
  {
  	int cpu;
  	long delta;
  	unsigned long flags;
  
  	/* Only let one CPU complain about others per time interval. */
  
  	spin_lock_irqsave(&rcp->lock, flags);
  	delta = jiffies - rcp->jiffies_stall;
  	if (delta < 2 || rcp->cur != rcp->completed) {
  		spin_unlock_irqrestore(&rcp->lock, flags);
  		return;
  	}
  	rcp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
  	spin_unlock_irqrestore(&rcp->lock, flags);
  
  	/* OK, time to rat on our buddy... */

  	printk(KERN_ERR "INFO: RCU detected CPU stalls:");

  	for_each_possible_cpu(cpu) {

  		if (cpumask_test_cpu(cpu, to_cpumask(rcp->cpumask)))

  			printk(" %d", cpu);
  	}
  	printk(" (detected by %d, t=%ld jiffies)
  ",
  	       smp_processor_id(), (long)(jiffies - rcp->gp_start));
  }
  
  static void print_cpu_stall(struct rcu_ctrlblk *rcp)
  {
  	unsigned long flags;

  	printk(KERN_ERR "INFO: RCU detected CPU %d stall (t=%lu/%lu jiffies)
  ",

  			smp_processor_id(), jiffies,
  			jiffies - rcp->gp_start);
  	dump_stack();
  	spin_lock_irqsave(&rcp->lock, flags);
  	if ((long)(jiffies - rcp->jiffies_stall) >= 0)
  		rcp->jiffies_stall =
  			jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
  	spin_unlock_irqrestore(&rcp->lock, flags);
  	set_need_resched();  /* kick ourselves to get things going. */
  }
  
  static void check_cpu_stall(struct rcu_ctrlblk *rcp)
  {
  	long delta;
  
  	delta = jiffies - rcp->jiffies_stall;

  	if (cpumask_test_cpu(smp_processor_id(), to_cpumask(rcp->cpumask)) &&
  		delta >= 0) {

  
  		/* We haven't checked in, so go dump stack. */
  		print_cpu_stall(rcp);
  
  	} else if (rcp->cur != rcp->completed && delta >= 2) {
  
  		/* They had two seconds to dump stack, so complain. */
  		print_other_cpu_stall(rcp);
  	}
  }
  
  #else /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
  
  static void record_gp_stall_check_time(struct rcu_ctrlblk *rcp)
  {
  }

  static inline void check_cpu_stall(struct rcu_ctrlblk *rcp)

  {
  }
  
  #endif /* #else #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */

  /**
   * call_rcu - Queue an RCU callback for invocation after a grace period.
   * @head: structure to be used for queueing the RCU updates.
   * @func: actual update function to be invoked after the grace period
   *
   * The update function will be invoked some time after a full grace
   * period elapses, in other words after all currently executing RCU
   * read-side critical sections have completed.  RCU read-side critical
   * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
   * and may be nested.
   */
  void call_rcu(struct rcu_head *head,
  				void (*func)(struct rcu_head *rcu))
  {
  	unsigned long flags;

  
  	head->func = func;

  	local_irq_save(flags);

  	__call_rcu(head, &rcu_ctrlblk, &__get_cpu_var(rcu_data));

  	local_irq_restore(flags);
  }
  EXPORT_SYMBOL_GPL(call_rcu);
  
  /**
   * call_rcu_bh - Queue an RCU for invocation after a quicker grace period.
   * @head: structure to be used for queueing the RCU updates.
   * @func: actual update function to be invoked after the grace period
   *
   * The update function will be invoked some time after a full grace
   * period elapses, in other words after all currently executing RCU
   * read-side critical sections have completed. call_rcu_bh() assumes
   * that the read-side critical sections end on completion of a softirq
   * handler. This means that read-side critical sections in process
   * context must not be interrupted by softirqs. This interface is to be
   * used when most of the read-side critical sections are in softirq context.
   * RCU read-side critical sections are delimited by rcu_read_lock() and
   * rcu_read_unlock(), * if in interrupt context or rcu_read_lock_bh()
   * and rcu_read_unlock_bh(), if in process context. These may be nested.
   */
  void call_rcu_bh(struct rcu_head *head,
  				void (*func)(struct rcu_head *rcu))
  {
  	unsigned long flags;

  
  	head->func = func;

  	local_irq_save(flags);

  	__call_rcu(head, &rcu_bh_ctrlblk, &__get_cpu_var(rcu_bh_data));

  	local_irq_restore(flags);
  }
  EXPORT_SYMBOL_GPL(call_rcu_bh);
  
  /*
   * Return the number of RCU batches processed thus far.  Useful
   * for debug and statistics.
   */
  long rcu_batches_completed(void)
  {
  	return rcu_ctrlblk.completed;
  }
  EXPORT_SYMBOL_GPL(rcu_batches_completed);
  
  /*
   * Return the number of RCU batches processed thus far.  Useful
   * for debug and statistics.
   */
  long rcu_batches_completed_bh(void)
  {
  	return rcu_bh_ctrlblk.completed;
  }
  EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);
  
  /* Raises the softirq for processing rcu_callbacks. */
  static inline void raise_rcu_softirq(void)
  {
  	raise_softirq(RCU_SOFTIRQ);

  }
  
  /*
   * Invoke the completed RCU callbacks. They are expected to be in
   * a per-cpu list.
   */
  static void rcu_do_batch(struct rcu_data *rdp)
  {

  	unsigned long flags;

  	struct rcu_head *next, *list;
  	int count = 0;
  
  	list = rdp->donelist;
  	while (list) {
  		next = list->next;
  		prefetch(next);
  		list->func(list);
  		list = next;
  		if (++count >= rdp->blimit)
  			break;
  	}
  	rdp->donelist = list;

  	local_irq_save(flags);

  	rdp->qlen -= count;

  	local_irq_restore(flags);

  	if (rdp->blimit == INT_MAX && rdp->qlen <= qlowmark)
  		rdp->blimit = blimit;
  
  	if (!rdp->donelist)
  		rdp->donetail = &rdp->donelist;
  	else
  		raise_rcu_softirq();
  }
  
  /*
   * Grace period handling:
   * The grace period handling consists out of two steps:
   * - A new grace period is started.
   *   This is done by rcu_start_batch. The start is not broadcasted to
   *   all cpus, they must pick this up by comparing rcp->cur with
   *   rdp->quiescbatch. All cpus are recorded  in the
   *   rcu_ctrlblk.cpumask bitmap.
   * - All cpus must go through a quiescent state.
   *   Since the start of the grace period is not broadcasted, at least two
   *   calls to rcu_check_quiescent_state are required:
   *   The first call just notices that a new grace period is running. The
   *   following calls check if there was a quiescent state since the beginning
   *   of the grace period. If so, it updates rcu_ctrlblk.cpumask. If
   *   the bitmap is empty, then the grace period is completed.
   *   rcu_check_quiescent_state calls rcu_start_batch(0) to start the next grace
   *   period (if necessary).
   */

  /*
   * Register a new batch of callbacks, and start it up if there is currently no
   * active batch and the batch to be registered has not already occurred.
   * Caller must hold rcu_ctrlblk.lock.
   */
  static void rcu_start_batch(struct rcu_ctrlblk *rcp)
  {

  	if (rcp->cur != rcp->pending &&

  			rcp->completed == rcp->cur) {

  		rcp->cur++;

  		record_gp_stall_check_time(rcp);

  
  		/*
  		 * Accessing nohz_cpu_mask before incrementing rcp->cur needs a
  		 * Barrier  Otherwise it can cause tickless idle CPUs to be
  		 * included in rcp->cpumask, which will extend graceperiods
  		 * unnecessarily.
  		 */
  		smp_mb();

  		cpumask_andnot(to_cpumask(rcp->cpumask),
  			       cpu_online_mask, nohz_cpu_mask);

  
  		rcp->signaled = 0;
  	}
  }
  
  /*
   * cpu went through a quiescent state since the beginning of the grace period.
   * Clear it from the cpu mask and complete the grace period if it was the last
   * cpu. Start another grace period if someone has further entries pending
   */
  static void cpu_quiet(int cpu, struct rcu_ctrlblk *rcp)
  {

  	cpumask_clear_cpu(cpu, to_cpumask(rcp->cpumask));
  	if (cpumask_empty(to_cpumask(rcp->cpumask))) {

  		/* batch completed ! */
  		rcp->completed = rcp->cur;
  		rcu_start_batch(rcp);
  	}
  }
  
  /*
   * Check if the cpu has gone through a quiescent state (say context
   * switch). If so and if it already hasn't done so in this RCU
   * quiescent cycle, then indicate that it has done so.
   */
  static void rcu_check_quiescent_state(struct rcu_ctrlblk *rcp,
  					struct rcu_data *rdp)
  {

  	unsigned long flags;

  	if (rdp->quiescbatch != rcp->cur) {
  		/* start new grace period: */
  		rdp->qs_pending = 1;
  		rdp->passed_quiesc = 0;
  		rdp->quiescbatch = rcp->cur;
  		return;
  	}
  
  	/* Grace period already completed for this cpu?
  	 * qs_pending is checked instead of the actual bitmap to avoid
  	 * cacheline trashing.
  	 */
  	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;
  	rdp->qs_pending = 0;

  	spin_lock_irqsave(&rcp->lock, flags);

  	/*
  	 * rdp->quiescbatch/rcp->cur and the cpu bitmap can come out of sync
  	 * during cpu startup. Ignore the quiescent state.
  	 */
  	if (likely(rdp->quiescbatch == rcp->cur))
  		cpu_quiet(rdp->cpu, rcp);

  	spin_unlock_irqrestore(&rcp->lock, flags);

  }
  
  
  #ifdef CONFIG_HOTPLUG_CPU
  
  /* warning! helper for rcu_offline_cpu. do not use elsewhere without reviewing
   * locking requirements, the list it's pulling from has to belong to a cpu
   * which is dead and hence not processing interrupts.
   */
  static void rcu_move_batch(struct rcu_data *this_rdp, struct rcu_head *list,

  				struct rcu_head **tail, long batch)

  {

  	unsigned long flags;

  	if (list) {

  		local_irq_save(flags);

  		this_rdp->batch = batch;
  		*this_rdp->nxttail[2] = list;
  		this_rdp->nxttail[2] = tail;

  		local_irq_restore(flags);

  	}

  }
  
  static void __rcu_offline_cpu(struct rcu_data *this_rdp,
  				struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
  {

  	unsigned long flags;

  	/*
  	 * if the cpu going offline owns the grace period

  	 * we can block indefinitely waiting for it, so flush
  	 * it here
  	 */

  	spin_lock_irqsave(&rcp->lock, flags);

  	if (rcp->cur != rcp->completed)
  		cpu_quiet(rdp->cpu, rcp);

  	rcu_move_batch(this_rdp, rdp->donelist, rdp->donetail, rcp->cur + 1);
  	rcu_move_batch(this_rdp, rdp->nxtlist, rdp->nxttail[2], rcp->cur + 1);

  	spin_unlock(&rcp->lock);

  	this_rdp->qlen += rdp->qlen;

  	local_irq_restore(flags);

  }
  
  static void rcu_offline_cpu(int cpu)
  {
  	struct rcu_data *this_rdp = &get_cpu_var(rcu_data);
  	struct rcu_data *this_bh_rdp = &get_cpu_var(rcu_bh_data);
  
  	__rcu_offline_cpu(this_rdp, &rcu_ctrlblk,
  					&per_cpu(rcu_data, cpu));
  	__rcu_offline_cpu(this_bh_rdp, &rcu_bh_ctrlblk,
  					&per_cpu(rcu_bh_data, cpu));
  	put_cpu_var(rcu_data);
  	put_cpu_var(rcu_bh_data);
  }
  
  #else
  
  static void rcu_offline_cpu(int cpu)
  {
  }
  
  #endif
  
  /*
   * This does the RCU processing work from softirq context.
   */
  static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp,
  					struct rcu_data *rdp)
  {

  	unsigned long flags;

  	long completed_snap;

  	if (rdp->nxtlist) {

  		local_irq_save(flags);

  		completed_snap = ACCESS_ONCE(rcp->completed);

  
  		/*

  		 * move the other grace-period-completed entries to
  		 * [rdp->nxtlist, *rdp->nxttail[0]) temporarily
  		 */

  		if (!rcu_batch_before(completed_snap, rdp->batch))

  			rdp->nxttail[0] = rdp->nxttail[1] = rdp->nxttail[2];

  		else if (!rcu_batch_before(completed_snap, rdp->batch - 1))

  			rdp->nxttail[0] = rdp->nxttail[1];
  
  		/*
  		 * the grace period for entries in
  		 * [rdp->nxtlist, *rdp->nxttail[0]) has completed and
  		 * move these entries to donelist

  		 */

  		if (rdp->nxttail[0] != &rdp->nxtlist) {
  			*rdp->donetail = rdp->nxtlist;
  			rdp->donetail = rdp->nxttail[0];
  			rdp->nxtlist = *rdp->nxttail[0];
  			*rdp->donetail = NULL;
  
  			if (rdp->nxttail[1] == rdp->nxttail[0])
  				rdp->nxttail[1] = &rdp->nxtlist;
  			if (rdp->nxttail[2] == rdp->nxttail[0])
  				rdp->nxttail[2] = &rdp->nxtlist;
  			rdp->nxttail[0] = &rdp->nxtlist;
  		}

  		local_irq_restore(flags);

  		if (rcu_batch_after(rdp->batch, rcp->pending)) {

  			unsigned long flags2;

  			/* and start it/schedule start if it's a new batch */

  			spin_lock_irqsave(&rcp->lock, flags2);

  			if (rcu_batch_after(rdp->batch, rcp->pending)) {
  				rcp->pending = rdp->batch;
  				rcu_start_batch(rcp);
  			}

  			spin_unlock_irqrestore(&rcp->lock, flags2);

  		}
  	}
  
  	rcu_check_quiescent_state(rcp, rdp);
  	if (rdp->donelist)
  		rcu_do_batch(rdp);
  }
  
  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 see before any RCU
  	 * grace-period manupulations below.
  	 */
  
  	smp_mb(); /* See above block comment. */

  	__rcu_process_callbacks(&rcu_ctrlblk, &__get_cpu_var(rcu_data));
  	__rcu_process_callbacks(&rcu_bh_ctrlblk, &__get_cpu_var(rcu_bh_data));

  
  	/*
  	 * Memory references from any later RCU read-side critical sections
  	 * executed by the interrupted code must be see after any RCU
  	 * grace-period manupulations above.
  	 */
  
  	smp_mb(); /* See above block comment. */

  }
  
  static int __rcu_pending(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
  {

  	/* Check for CPU stalls, if enabled. */

  	check_cpu_stall(rcp);

  	if (rdp->nxtlist) {

  		long completed_snap = ACCESS_ONCE(rcp->completed);

  		/*
  		 * This cpu has pending rcu entries and the grace period
  		 * for them has completed.
  		 */

  		if (!rcu_batch_before(completed_snap, rdp->batch))

  			return 1;

  		if (!rcu_batch_before(completed_snap, rdp->batch - 1) &&

  				rdp->nxttail[0] != rdp->nxttail[1])
  			return 1;
  		if (rdp->nxttail[0] != &rdp->nxtlist)
  			return 1;

  		/*
  		 * This cpu has pending rcu entries and the new batch
  		 * for then hasn't been started nor scheduled start
  		 */
  		if (rcu_batch_after(rdp->batch, rcp->pending))
  			return 1;
  	}

  
  	/* This cpu has finished callbacks to invoke */
  	if (rdp->donelist)
  		return 1;
  
  	/* The rcu core waits for a quiescent state from the cpu */
  	if (rdp->quiescbatch != rcp->cur || rdp->qs_pending)
  		return 1;
  
  	/* nothing to do */
  	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.
   */
  int rcu_pending(int cpu)
  {
  	return __rcu_pending(&rcu_ctrlblk, &per_cpu(rcu_data, cpu)) ||
  		__rcu_pending(&rcu_bh_ctrlblk, &per_cpu(rcu_bh_data, 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)
  {
  	struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
  	struct rcu_data *rdp_bh = &per_cpu(rcu_bh_data, cpu);

  	return !!rdp->nxtlist || !!rdp_bh->nxtlist || rcu_pending(cpu);

  }

  /*
   * Top-level function driving RCU grace-period detection, normally
   * invoked from the scheduler-clock interrupt.  This function simply
   * increments counters that are read only from softirq by this same
   * CPU, so there are no memory barriers required.
   */

  void rcu_check_callbacks(int cpu, int user)
  {
  	if (user ||
  	    (idle_cpu(cpu) && !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 count it.
  		 *
  		 * Also do a memory barrier.  This is needed to handle
  		 * the case where writes from a preempt-disable section
  		 * of code get reordered into schedule() by this CPU's
  		 * write buffer.  The memory barrier makes sure that
  		 * the rcu_qsctr_inc() and rcu_bh_qsctr_inc() are see
  		 * by other CPUs to happen after any such write.
  		 */
  
  		smp_mb();  /* See above block comment. */

  		rcu_qsctr_inc(cpu);
  		rcu_bh_qsctr_inc(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 count it.  The memory barrier
  		 * is needed for the same reason as is the above one.
  		 */
  
  		smp_mb();  /* See above block comment. */

  		rcu_bh_qsctr_inc(cpu);

  	}

  	raise_rcu_softirq();
  }

  static void __cpuinit rcu_init_percpu_data(int cpu, struct rcu_ctrlblk *rcp,

  						struct rcu_data *rdp)
  {

  	unsigned long flags;

  
  	spin_lock_irqsave(&rcp->lock, flags);

  	memset(rdp, 0, sizeof(*rdp));

  	rdp->nxttail[0] = rdp->nxttail[1] = rdp->nxttail[2] = &rdp->nxtlist;

  	rdp->donetail = &rdp->donelist;
  	rdp->quiescbatch = rcp->completed;
  	rdp->qs_pending = 0;
  	rdp->cpu = cpu;
  	rdp->blimit = blimit;

  	spin_unlock_irqrestore(&rcp->lock, flags);

  }
  
  static void __cpuinit rcu_online_cpu(int cpu)
  {
  	struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
  	struct rcu_data *bh_rdp = &per_cpu(rcu_bh_data, cpu);
  
  	rcu_init_percpu_data(cpu, &rcu_ctrlblk, rdp);
  	rcu_init_percpu_data(cpu, &rcu_bh_ctrlblk, bh_rdp);

  	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);

  }
  
  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_DEAD:
  	case CPU_DEAD_FROZEN:
  		rcu_offline_cpu(cpu);
  		break;
  	default:
  		break;
  	}
  	return NOTIFY_OK;
  }
  
  static struct notifier_block __cpuinitdata rcu_nb = {
  	.notifier_call	= rcu_cpu_notify,
  };
  
  /*
   * Initializes rcu mechanism.  Assumed to be called early.
   * That is before local timer(SMP) or jiffie timer (uniproc) is setup.
   * Note that rcu_qsctr and friends are implicitly
   * initialized due to the choice of ``0'' for RCU_CTR_INVALID.
   */
  void __init __rcu_init(void)
  {

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

  	rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE,
  			(void *)(long)smp_processor_id());
  	/* Register notifier for non-boot CPUs */
  	register_cpu_notifier(&rcu_nb);
  }
  
  module_param(blimit, int, 0);
  module_param(qhimark, int, 0);
  module_param(qlowmark, int, 0);