/*
   * 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 <linux/atomic.h>

  #include <linux/bitops.h>

  #include <linux/export.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 <linux/wait.h>
  #include <linux/kthread.h>

  #include <linux/prefetch.h>

  #include "rcutree.h"

  #include <trace/events/rcu.h>
  
  #include "rcu.h"

  /* Data structures. */

  static struct lock_class_key rcu_node_class[NUM_RCU_LVLS];

  #define RCU_STATE_INITIALIZER(structname) { \

  	.level = { &structname##_state.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 */ \

  	}, \

  	.fqs_state = RCU_GP_IDLE, \

  	.gpnum = -300, \
  	.completed = -300, \

  	.onofflock = __RAW_SPIN_LOCK_UNLOCKED(&structname##_state.onofflock), \
  	.fqslock = __RAW_SPIN_LOCK_UNLOCKED(&structname##_state.fqslock), \

  	.n_force_qs = 0, \
  	.n_force_qs_ngp = 0, \

  	.name = #structname, \

  }

  struct rcu_state rcu_sched_state = RCU_STATE_INITIALIZER(rcu_sched);

  DEFINE_PER_CPU(struct rcu_data, rcu_sched_data);

  struct rcu_state rcu_bh_state = RCU_STATE_INITIALIZER(rcu_bh);

  DEFINE_PER_CPU(struct rcu_data, rcu_bh_data);

  static struct rcu_state *rcu_state;

  /*
   * The rcu_scheduler_active variable transitions from zero to one just
   * before the first task is spawned.  So when this variable is zero, RCU
   * can assume that there is but one task, allowing RCU to (for example)
   * optimized synchronize_sched() to a simple barrier().  When this variable
   * is one, RCU must actually do all the hard work required to detect real
   * grace periods.  This variable is also used to suppress boot-time false
   * positives from lockdep-RCU error checking.
   */

  int rcu_scheduler_active __read_mostly;
  EXPORT_SYMBOL_GPL(rcu_scheduler_active);

  /*
   * The rcu_scheduler_fully_active variable transitions from zero to one
   * during the early_initcall() processing, which is after the scheduler
   * is capable of creating new tasks.  So RCU processing (for example,
   * creating tasks for RCU priority boosting) must be delayed until after
   * rcu_scheduler_fully_active transitions from zero to one.  We also
   * currently delay invocation of any RCU callbacks until after this point.
   *
   * It might later prove better for people registering RCU callbacks during
   * early boot to take responsibility for these callbacks, but one step at
   * a time.
   */
  static int rcu_scheduler_fully_active __read_mostly;

  #ifdef CONFIG_RCU_BOOST

  /*

   * Control variables for per-CPU and per-rcu_node kthreads.  These
   * handle all flavors of RCU.
   */
  static DEFINE_PER_CPU(struct task_struct *, rcu_cpu_kthread_task);

  DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_status);

  DEFINE_PER_CPU(int, rcu_cpu_kthread_cpu);

  DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_loops);

  DEFINE_PER_CPU(char, rcu_cpu_has_work);

  #endif /* #ifdef CONFIG_RCU_BOOST */

  static void rcu_node_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu);

  static void invoke_rcu_core(void);
  static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp);

  /*

   * Track the rcutorture test sequence number and the update version
   * number within a given test.  The rcutorture_testseq is incremented
   * on every rcutorture module load and unload, so has an odd value
   * when a test is running.  The rcutorture_vernum is set to zero
   * when rcutorture starts and is incremented on each rcutorture update.
   * These variables enable correlating rcutorture output with the
   * RCU tracing information.
   */
  unsigned long rcutorture_testseq;
  unsigned long rcutorture_vernum;
  
  /*

   * 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.

   * The caller must have disabled preemption.

   */

  void rcu_sched_qs(int cpu)

  {

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

  	rdp->passed_quiesce_gpnum = rdp->gpnum;

  	barrier();

  	if (rdp->passed_quiesce == 0)

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

  	rdp->passed_quiesce = 1;

  }

  void rcu_bh_qs(int cpu)

  {

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

  	rdp->passed_quiesce_gpnum = rdp->gpnum;

  	barrier();

  	if (rdp->passed_quiesce == 0)

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

  	rdp->passed_quiesce = 1;

  }

  /*
   * Note a context switch.  This is a quiescent state for RCU-sched,
   * and requires special handling for preemptible RCU.

   * The caller must have disabled preemption.

   */
  void rcu_note_context_switch(int cpu)
  {

  	trace_rcu_utilization("Start context switch");

  	rcu_sched_qs(cpu);
  	rcu_preempt_note_context_switch(cpu);

  	trace_rcu_utilization("End context switch");

  }

  EXPORT_SYMBOL_GPL(rcu_note_context_switch);

  DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {

  	.dynticks_nesting = DYNTICK_TASK_NESTING,

  	.dynticks = ATOMIC_INIT(1),

  };

  static int blimit = 10;		/* Maximum callbacks per rcu_do_batch. */

  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);

  int rcu_cpu_stall_suppress __read_mostly;

  module_param(rcu_cpu_stall_suppress, int, 0644);

  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);
  
  /*

   * Record the number of times rcutorture tests have been initiated and
   * terminated.  This information allows the debugfs tracing stats to be
   * correlated to the rcutorture messages, even when the rcutorture module
   * is being repeatedly loaded and unloaded.  In other words, we cannot
   * store this state in rcutorture itself.
   */
  void rcutorture_record_test_transition(void)
  {
  	rcutorture_testseq++;
  	rcutorture_vernum = 0;
  }
  EXPORT_SYMBOL_GPL(rcutorture_record_test_transition);
  
  /*
   * Record the number of writer passes through the current rcutorture test.
   * This is also used to correlate debugfs tracing stats with the rcutorture
   * messages.
   */
  void rcutorture_record_progress(unsigned long vernum)
  {
  	rcutorture_vernum++;
  }
  EXPORT_SYMBOL_GPL(rcutorture_record_progress);
  
  /*

   * 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)) {

  		trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, "ofl");

  		rdp->offline_fqs++;
  		return 1;
  	}

  	/*
  	 * The CPU is online, so send it a reschedule IPI.  This forces
  	 * it through the scheduler, and (inefficiently) also handles cases
  	 * where idle loops fail to inform RCU about the CPU being idle.
  	 */

  	if (rdp->cpu != smp_processor_id())
  		smp_send_reschedule(rdp->cpu);
  	else
  		set_need_resched();
  	rdp->resched_ipi++;
  	return 0;
  }
  
  #endif /* #ifdef CONFIG_SMP */

  /*
   * rcu_idle_enter_common - inform RCU that current CPU is moving towards idle
   *
   * If the new value of the ->dynticks_nesting counter now is zero,
   * we really have entered idle, and must do the appropriate accounting.
   * The caller must have disabled interrupts.
   */

  static void rcu_idle_enter_common(struct rcu_dynticks *rdtp, long long oldval)

  {

  	trace_rcu_dyntick("Start", oldval, 0);

  	if (!is_idle_task(current)) {

  		struct task_struct *idle = idle_task(smp_processor_id());

  		trace_rcu_dyntick("Error on entry: not idle task", oldval, 0);

  		ftrace_dump(DUMP_ALL);

  		WARN_ONCE(1, "Current pid: %d comm: %s / Idle pid: %d comm: %s",
  			  current->pid, current->comm,
  			  idle->pid, idle->comm); /* must be idle task! */

  	}

  	rcu_prepare_for_idle(smp_processor_id());

  	/* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */
  	smp_mb__before_atomic_inc();  /* See above. */
  	atomic_inc(&rdtp->dynticks);
  	smp_mb__after_atomic_inc();  /* Force ordering with next sojourn. */
  	WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1);
  }

  
  /**

   * rcu_idle_enter - inform RCU that current CPU is entering idle

   *

   * Enter idle 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 idle, a possibility
   * handled by irq_enter() and irq_exit().)
   *
   * We crowbar the ->dynticks_nesting field to zero to allow for
   * the possibility of usermode upcalls having messed up our count
   * of interrupt nesting level during the prior busy period.

   */

  void rcu_idle_enter(void)

  {
  	unsigned long flags;

  	long long oldval;

  	struct rcu_dynticks *rdtp;

  	local_irq_save(flags);
  	rdtp = &__get_cpu_var(rcu_dynticks);

  	oldval = rdtp->dynticks_nesting;

  	rdtp->dynticks_nesting = 0;

  	rcu_idle_enter_common(rdtp, oldval);

  	local_irq_restore(flags);
  }

  /**
   * rcu_irq_exit - inform RCU that current CPU is exiting irq towards idle
   *
   * Exit from an interrupt handler, which might possibly result in entering
   * idle mode, in other words, leaving the mode in which read-side critical
   * sections can occur.

   *

   * This code assumes that the idle loop never does anything that might
   * result in unbalanced calls to irq_enter() and irq_exit().  If your
   * architecture violates this assumption, RCU will give you what you
   * deserve, good and hard.  But very infrequently and irreproducibly.
   *
   * Use things like work queues to work around this limitation.
   *
   * You have been warned.

   */

  void rcu_irq_exit(void)

  {
  	unsigned long flags;

  	long long oldval;

  	struct rcu_dynticks *rdtp;
  
  	local_irq_save(flags);
  	rdtp = &__get_cpu_var(rcu_dynticks);

  	oldval = rdtp->dynticks_nesting;

  	rdtp->dynticks_nesting--;
  	WARN_ON_ONCE(rdtp->dynticks_nesting < 0);

  	if (rdtp->dynticks_nesting)
  		trace_rcu_dyntick("--=", oldval, rdtp->dynticks_nesting);
  	else
  		rcu_idle_enter_common(rdtp, oldval);

  	local_irq_restore(flags);
  }
  
  /*
   * rcu_idle_exit_common - inform RCU that current CPU is moving away from idle
   *
   * If the new value of the ->dynticks_nesting counter was previously zero,
   * we really have exited idle, and must do the appropriate accounting.
   * The caller must have disabled interrupts.
   */
  static void rcu_idle_exit_common(struct rcu_dynticks *rdtp, long long oldval)
  {

  	smp_mb__before_atomic_inc();  /* Force ordering w/previous sojourn. */
  	atomic_inc(&rdtp->dynticks);
  	/* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
  	smp_mb__after_atomic_inc();  /* See above. */
  	WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));

  	rcu_cleanup_after_idle(smp_processor_id());

  	trace_rcu_dyntick("End", oldval, rdtp->dynticks_nesting);

  	if (!is_idle_task(current)) {

  		struct task_struct *idle = idle_task(smp_processor_id());

  		trace_rcu_dyntick("Error on exit: not idle task",
  				  oldval, rdtp->dynticks_nesting);

  		ftrace_dump(DUMP_ALL);

  		WARN_ONCE(1, "Current pid: %d comm: %s / Idle pid: %d comm: %s",
  			  current->pid, current->comm,
  			  idle->pid, idle->comm); /* must be idle task! */

  	}
  }
  
  /**
   * rcu_idle_exit - inform RCU that current CPU is leaving idle
   *
   * Exit idle mode, in other words, -enter- the mode in which RCU
   * read-side critical sections can occur.
   *

   * We crowbar the ->dynticks_nesting field to DYNTICK_TASK_NESTING to
   * allow for the possibility of usermode upcalls messing up our count

   * of interrupt nesting level during the busy period that is just
   * now starting.
   */
  void rcu_idle_exit(void)
  {
  	unsigned long flags;
  	struct rcu_dynticks *rdtp;
  	long long oldval;
  
  	local_irq_save(flags);
  	rdtp = &__get_cpu_var(rcu_dynticks);
  	oldval = rdtp->dynticks_nesting;
  	WARN_ON_ONCE(oldval != 0);

  	rdtp->dynticks_nesting = DYNTICK_TASK_NESTING;

  	rcu_idle_exit_common(rdtp, oldval);
  	local_irq_restore(flags);
  }
  
  /**
   * rcu_irq_enter - inform RCU that current CPU is entering irq away from idle
   *
   * Enter an interrupt handler, which might possibly result in exiting
   * idle mode, in other words, entering the mode in which read-side critical
   * sections can occur.
   *
   * Note that the Linux kernel is fully capable of entering an interrupt
   * handler that it never exits, for example when doing upcalls to
   * user mode!  This code assumes that the idle loop never does upcalls to
   * user mode.  If your architecture does do upcalls from the idle loop (or
   * does anything else that results in unbalanced calls to the irq_enter()
   * and irq_exit() functions), RCU will give you what you deserve, good
   * and hard.  But very infrequently and irreproducibly.
   *
   * Use things like work queues to work around this limitation.
   *
   * You have been warned.
   */
  void rcu_irq_enter(void)
  {
  	unsigned long flags;
  	struct rcu_dynticks *rdtp;
  	long long oldval;
  
  	local_irq_save(flags);
  	rdtp = &__get_cpu_var(rcu_dynticks);
  	oldval = rdtp->dynticks_nesting;
  	rdtp->dynticks_nesting++;
  	WARN_ON_ONCE(rdtp->dynticks_nesting == 0);

  	if (oldval)
  		trace_rcu_dyntick("++=", oldval, rdtp->dynticks_nesting);
  	else
  		rcu_idle_exit_common(rdtp, oldval);

  	local_irq_restore(flags);

  }
  
  /**
   * 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_nmi_nesting == 0 &&
  	    (atomic_read(&rdtp->dynticks) & 0x1))

  		return;

  	rdtp->dynticks_nmi_nesting++;
  	smp_mb__before_atomic_inc();  /* Force delay from prior write. */
  	atomic_inc(&rdtp->dynticks);
  	/* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
  	smp_mb__after_atomic_inc();  /* See above. */
  	WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));

  }
  
  /**
   * 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_nmi_nesting == 0 ||
  	    --rdtp->dynticks_nmi_nesting != 0)

  		return;

  	/* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */
  	smp_mb__before_atomic_inc();  /* See above. */
  	atomic_inc(&rdtp->dynticks);
  	smp_mb__after_atomic_inc();  /* Force delay to next write. */
  	WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1);

  }

  #ifdef CONFIG_PROVE_RCU

  /**

   * rcu_is_cpu_idle - see if RCU thinks that the current CPU is idle

   *

   * If the current CPU is in its idle loop and is neither in an interrupt

   * or NMI handler, return true.

   */

  int rcu_is_cpu_idle(void)

  {

  	int ret;
  
  	preempt_disable();
  	ret = (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0;
  	preempt_enable();
  	return ret;

  }

  EXPORT_SYMBOL(rcu_is_cpu_idle);

  #endif /* #ifdef CONFIG_PROVE_RCU */

  /**

   * rcu_is_cpu_rrupt_from_idle - see if idle or immediately interrupted from idle

   *

   * If the current CPU is idle or running at a first-level (not nested)
   * interrupt from idle, return true.  The caller must have at least
   * disabled preemption.

   */

  int rcu_is_cpu_rrupt_from_idle(void)

  {

  	return __get_cpu_var(rcu_dynticks).dynticks_nesting <= 1;

  }

  #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)
  {

  	rdp->dynticks_snap = atomic_add_return(0, &rdp->dynticks->dynticks);

  	return (rdp->dynticks_snap & 0x1) == 0;

  }
  
  /*
   * 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)
  {

  	unsigned int curr;
  	unsigned int snap;

  	curr = (unsigned int)atomic_add_return(0, &rdp->dynticks->dynticks);
  	snap = (unsigned int)rdp->dynticks_snap;

  
  	/*
  	 * 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 & 0x1) == 0 || UINT_CMP_GE(curr, snap + 2)) {

  		trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, "dti");

  		rdp->dynticks_fqs++;
  		return 1;
  	}
  
  	/* Go check for the CPU being offline. */
  	return rcu_implicit_offline_qs(rdp);
  }
  
  #endif /* #ifdef CONFIG_SMP */

  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;

  	int ndetected;

  	struct rcu_node *rnp = rcu_get_root(rsp);

  
  	/* Only let one CPU complain about others per time interval. */

  	raw_spin_lock_irqsave(&rnp->lock, flags);

  	delta = jiffies - rsp->jiffies_stall;

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

  		raw_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.
  	 */

  	ndetected = rcu_print_task_stall(rnp);

  	raw_spin_unlock_irqrestore(&rnp->lock, flags);

  	/*
  	 * OK, time to rat on our buddy...
  	 * See Documentation/RCU/stallwarn.txt for info on how to debug
  	 * RCU CPU stall warnings.
  	 */

  	printk(KERN_ERR "INFO: %s detected stalls on CPUs/tasks: {",
  	       rsp->name);

  	rcu_for_each_leaf_node(rsp, rnp) {

  		raw_spin_lock_irqsave(&rnp->lock, flags);

  		ndetected += rcu_print_task_stall(rnp);

  		raw_spin_unlock_irqrestore(&rnp->lock, flags);

  		if (rnp->qsmask == 0)

  			continue;

  		for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)

  			if (rnp->qsmask & (1UL << cpu)) {

  				printk(" %d", rnp->grplo + cpu);

  				ndetected++;
  			}

  	}

  	printk("} (detected by %d, t=%ld jiffies)
  ",

  	       smp_processor_id(), (long)(jiffies - rsp->gp_start));

  	if (ndetected == 0)
  		printk(KERN_ERR "INFO: Stall ended before state dump start
  ");
  	else if (!trigger_all_cpu_backtrace())

  		dump_stack();

  	/* If so configured, complain about tasks blocking the grace period. */
  
  	rcu_print_detail_task_stall(rsp);

  	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);

  	/*
  	 * OK, time to rat on ourselves...
  	 * See Documentation/RCU/stallwarn.txt for info on how to debug
  	 * RCU CPU stall warnings.
  	 */

  	printk(KERN_ERR "INFO: %s detected stall on CPU %d (t=%lu jiffies)
  ",
  	       rsp->name, smp_processor_id(), jiffies - rsp->gp_start);

  	if (!trigger_all_cpu_backtrace())
  		dump_stack();

  	raw_spin_lock_irqsave(&rnp->lock, flags);

  	if (ULONG_CMP_GE(jiffies, rsp->jiffies_stall))

  		rsp->jiffies_stall =
  			jiffies + RCU_SECONDS_TILL_STALL_RECHECK;

  	raw_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)
  {

  	unsigned long j;
  	unsigned long js;

  	struct rcu_node *rnp;

  	if (rcu_cpu_stall_suppress)

  		return;

  	j = ACCESS_ONCE(jiffies);
  	js = ACCESS_ONCE(rsp->jiffies_stall);

  	rnp = rdp->mynode;

  	if ((ACCESS_ONCE(rnp->qsmask) & rdp->grpmask) && ULONG_CMP_GE(j, js)) {

  
  		/* We haven't checked in, so go dump stack. */
  		print_cpu_stall(rsp);

  	} else if (rcu_gp_in_progress(rsp) &&
  		   ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY)) {

  		/* They had a few time units to dump stack, so complain. */

  		print_other_cpu_stall(rsp);
  	}
  }

  static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
  {

  	rcu_cpu_stall_suppress = 1;

  	return NOTIFY_DONE;
  }

  /**
   * rcu_cpu_stall_reset - prevent further stall warnings in current grace period
   *
   * Set the stall-warning timeout way off into the future, thus preventing
   * any RCU CPU stall-warning messages from appearing in the current set of
   * RCU grace periods.
   *
   * The caller must disable hard irqs.
   */
  void rcu_cpu_stall_reset(void)
  {
  	rcu_sched_state.jiffies_stall = jiffies + ULONG_MAX / 2;
  	rcu_bh_state.jiffies_stall = jiffies + ULONG_MAX / 2;
  	rcu_preempt_stall_reset();
  }

  static struct notifier_block rcu_panic_block = {
  	.notifier_call = rcu_panic,
  };
  
  static void __init check_cpu_stall_init(void)
  {
  	atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block);
  }

  /*
   * 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) {

  		/*
  		 * If the current grace period is waiting for this CPU,
  		 * set up to detect a quiescent state, otherwise don't
  		 * go looking for one.
  		 */

  		rdp->gpnum = rnp->gpnum;

  		trace_rcu_grace_period(rsp->name, rdp->gpnum, "cpustart");

  		if (rnp->qsmask & rdp->grpmask) {
  			rdp->qs_pending = 1;

  			rdp->passed_quiesce = 0;

  		} else
  			rdp->qs_pending = 0;

  	}
  }

  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. */

  	    !raw_spin_trylock(&rnp->lock)) { /* irqs already off, so later. */

  		local_irq_restore(flags);
  		return;
  	}
  	__note_new_gpnum(rsp, rnp, rdp);

  	raw_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;

  		trace_rcu_grace_period(rsp->name, rdp->gpnum, "cpuend");

  
  		/*

  		 * If we were in an extended quiescent state, we may have

  		 * missed some grace periods that others CPUs handled on

  		 * our behalf. Catch up with this state to avoid noting

  		 * spurious new grace periods.  If another grace period
  		 * has started, then rnp->gpnum will have advanced, so
  		 * we will detect this later on.

  		 */

  		if (ULONG_CMP_LT(rdp->gpnum, rdp->completed))

  			rdp->gpnum = rdp->completed;
  
  		/*

  		 * If RCU does not need a quiescent state from this CPU,
  		 * then make sure that this CPU doesn't go looking for one.

  		 */

  		if ((rnp->qsmask & rdp->grpmask) == 0)

  			rdp->qs_pending = 0;

  	}
  }
  
  /*
   * 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. */

  	    !raw_spin_trylock(&rnp->lock)) { /* irqs already off, so later. */

  		local_irq_restore(flags);
  		return;
  	}
  	__rcu_process_gp_end(rsp, rnp, rdp);

  	raw_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 = this_cpu_ptr(rsp->rda);

  	struct rcu_node *rnp = rcu_get_root(rsp);

  	if (!rcu_scheduler_fully_active ||

  	    !cpu_needs_another_gp(rsp, rdp)) {
  		/*
  		 * Either the scheduler hasn't yet spawned the first
  		 * non-idle task or this CPU does not need another
  		 * grace period.  Either way, don't start a new grace
  		 * period.
  		 */
  		raw_spin_unlock_irqrestore(&rnp->lock, flags);
  		return;
  	}

  	if (rsp->fqs_active) {

  		/*

  		 * This CPU needs a grace period, but force_quiescent_state()
  		 * is running.  Tell it to start one on this CPU's behalf.

  		 */

  		rsp->fqs_need_gp = 1;
  		raw_spin_unlock_irqrestore(&rnp->lock, flags);

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

  	trace_rcu_grace_period(rsp->name, rsp->gpnum, "start");

  	WARN_ON_ONCE(rsp->fqs_state == RCU_GP_INIT);
  	rsp->fqs_state = 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->fqs_state = RCU_SIGNAL_INIT; /* force_quiescent_state OK */

  		rcu_start_gp_per_cpu(rsp, rnp, rdp);

  		rcu_preempt_boost_start_gp(rnp);

  		trace_rcu_grace_period_init(rsp->name, rnp->gpnum,
  					    rnp->level, rnp->grplo,
  					    rnp->grphi, rnp->qsmask);

  		raw_spin_unlock_irqrestore(&rnp->lock, flags);

  		return;
  	}

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

  
  
  	/* Exclude any concurrent CPU-hotplug operations. */

  	raw_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) {

  		raw_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);

  		rcu_preempt_boost_start_gp(rnp);

  		trace_rcu_grace_period_init(rsp->name, rnp->gpnum,
  					    rnp->level, rnp->grplo,
  					    rnp->grphi, rnp->qsmask);

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

  	}

  	rnp = rcu_get_root(rsp);

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

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

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

  {

  	unsigned long gp_duration;

  	struct rcu_node *rnp = rcu_get_root(rsp);
  	struct rcu_data *rdp = this_cpu_ptr(rsp->rda);

  	WARN_ON_ONCE(!rcu_gp_in_progress(rsp));

  
  	/*
  	 * Ensure that all grace-period and pre-grace-period activity
  	 * is seen before the assignment to rsp->completed.
  	 */
  	smp_mb(); /* See above block comment. */

  	gp_duration = jiffies - rsp->gp_start;
  	if (gp_duration > rsp->gp_max)
  		rsp->gp_max = gp_duration;

  
  	/*
  	 * We know the grace period is complete, but to everyone else
  	 * it appears to still be ongoing.  But it is also the case
  	 * that to everyone else it looks like there is nothing that
  	 * they can do to advance the grace period.  It is therefore
  	 * safe for us to drop the lock in order to mark the grace
  	 * period as completed in all of the rcu_node structures.
  	 *
  	 * But if this CPU needs another grace period, it will take
  	 * care of this while initializing the next grace period.
  	 * We use RCU_WAIT_TAIL instead of the usual RCU_DONE_TAIL
  	 * because the callbacks have not yet been advanced: Those
  	 * callbacks are waiting on the grace period that just now
  	 * completed.
  	 */
  	if (*rdp->nxttail[RCU_WAIT_TAIL] == NULL) {
  		raw_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) {
  			raw_spin_lock(&rnp->lock); /* irqs already disabled. */
  			rnp->completed = rsp->gpnum;
  			raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
  		}
  		rnp = rcu_get_root(rsp);
  		raw_spin_lock(&rnp->lock); /* irqs already disabled. */
  	}
  
  	rsp->completed = rsp->gpnum;  /* Declare the grace period complete. */

  	trace_rcu_grace_period(rsp->name, rsp->completed, "end");

  	rsp->fqs_state = 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. */

  			raw_spin_unlock_irqrestore(&rnp->lock, flags);

  			return;
  		}
  		rnp->qsmask &= ~mask;

  		trace_rcu_quiescent_state_report(rsp->name, rnp->gpnum,
  						 mask, rnp->qsmask, rnp->level,
  						 rnp->grplo, rnp->grphi,
  						 !!rnp->gp_tasks);

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

  
  			/* Other bits still set at this level, so done. */

  			raw_spin_unlock_irqrestore(&rnp->lock, flags);

  			return;
  		}
  		mask = rnp->grpmask;
  		if (rnp->parent == NULL) {
  
  			/* No more levels.  Exit loop holding root lock. */
  
  			break;
  		}

  		raw_spin_unlock_irqrestore(&rnp->lock, flags);

  		rnp_c = rnp;

  		rnp = rnp->parent;

  		raw_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 lastgp)

  {
  	unsigned long flags;
  	unsigned long mask;
  	struct rcu_node *rnp;
  
  	rnp = rdp->mynode;

  	raw_spin_lock_irqsave(&rnp->lock, flags);

  	if (lastgp != rnp->gpnum || rnp->completed == rnp->gpnum) {

  
  		/*

  		 * The grace period in which this quiescent state was
  		 * recorded has ended, so don't report it upwards.
  		 * We will instead need a new quiescent state that lies
  		 * within the current grace period.

  		 */

  		rdp->passed_quiesce = 0;	/* need qs for new gp. */

  		raw_spin_unlock_irqrestore(&rnp->lock, flags);

  		return;
  	}
  	mask = rdp->grpmask;
  	if ((rnp->qsmask & mask) == 0) {

  		raw_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_quiesce)

  		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_quiesce_gpnum);

  }
  
  #ifdef CONFIG_HOTPLUG_CPU
  
  /*

   * Move a dying CPU's RCU callbacks to online CPU's callback list.
   * Synchronization is not required because this function executes
   * in stop_machine() context.

   */

  static void rcu_send_cbs_to_online(struct rcu_state *rsp)

  {
  	int i;

  	/* current DYING CPU is cleared in the cpu_online_mask */
  	int receive_cpu = cpumask_any(cpu_online_mask);

  	struct rcu_data *rdp = this_cpu_ptr(rsp->rda);

  	struct rcu_data *receive_rdp = per_cpu_ptr(rsp->rda, receive_cpu);

  
  	if (rdp->nxtlist == NULL)
  		return;  /* irqs disabled, so comparison is stable. */

  
  	*receive_rdp->nxttail[RCU_NEXT_TAIL] = rdp->nxtlist;
  	receive_rdp->nxttail[RCU_NEXT_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
  	receive_rdp->qlen += rdp->qlen;
  	receive_rdp->n_cbs_adopted += rdp->qlen;
  	rdp->n_cbs_orphaned += rdp->qlen;

  	rdp->nxtlist = NULL;
  	for (i = 0; i < RCU_NEXT_SIZE; i++)
  		rdp->nxttail[i] = &rdp->nxtlist;

  	rdp->qlen = 0;

  }
  
  /*

   * Remove the outgoing CPU from the bitmasks in the rcu_node hierarchy
   * and move all callbacks from the outgoing CPU to the current one.

   * There can only be one CPU hotplug operation at a time, so no other
   * CPU can be attempting to update rcu_cpu_kthread_task.

   */
  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 = per_cpu_ptr(rsp->rda, cpu);

  	struct rcu_node *rnp;

  	rcu_stop_cpu_kthread(cpu);

  
  	/* Exclude any attempts to start a new grace period. */

  	raw_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 {

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

  		rnp->qsmaskinit &= ~mask;
  		if (rnp->qsmaskinit != 0) {

  			if (rnp != rdp->mynode)

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

  			else
  				trace_rcu_grace_period(rsp->name,
  						       rnp->gpnum + 1 -
  						       !!(rnp->qsmask & mask),
  						       "cpuofl");

  			break;
  		}

  		if (rnp == rdp->mynode) {
  			trace_rcu_grace_period(rsp->name,
  					       rnp->gpnum + 1 -
  					       !!(rnp->qsmask & mask),
  					       "cpuofl");

  			need_report = rcu_preempt_offline_tasks(rsp, rnp, rdp);

  		} else

  			raw_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.

  	 */

  	raw_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

  		raw_spin_unlock_irqrestore(&rnp->lock, flags);

  	if (need_report & RCU_OFL_TASKS_EXP_GP)

  		rcu_report_exp_rnp(rsp, rnp, true);

  	rcu_node_kthread_setaffinity(rnp, -1);

  }
  
  /*
   * 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_online(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 bl, count;

  
  	/* If no callbacks are ready, just return.*/

  	if (!cpu_has_callbacks_ready_to_invoke(rdp)) {

  		trace_rcu_batch_start(rsp->name, 0, 0);

  		trace_rcu_batch_end(rsp->name, 0, !!ACCESS_ONCE(rdp->nxtlist),
  				    need_resched(), is_idle_task(current),
  				    rcu_is_callbacks_kthread());

  		return;

  	}

  
  	/*
  	 * Extract the list of ready callbacks, disabling to prevent
  	 * races with call_rcu() from interrupt handlers.
  	 */
  	local_irq_save(flags);

  	bl = rdp->blimit;

  	trace_rcu_batch_start(rsp->name, rdp->qlen, bl);

  	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);

  		debug_rcu_head_unqueue(list);

  		__rcu_reclaim(rsp->name, list);

  		list = next;

  		/* Stop only if limit reached and CPU has something to do. */
  		if (++count >= bl &&
  		    (need_resched() ||
  		     (!is_idle_task(current) && !rcu_is_callbacks_kthread())))

  			break;
  	}
  
  	local_irq_save(flags);

  	trace_rcu_batch_end(rsp->name, count, !!list, need_resched(),
  			    is_idle_task(current),
  			    rcu_is_callbacks_kthread());

  
  	/* Update count, and requeue any remaining callbacks. */
  	rdp->qlen -= count;

  	rdp->n_cbs_invoked += 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-invoke RCU core processing if there are callbacks remaining. */

  	if (cpu_has_callbacks_ready_to_invoke(rdp))

  		invoke_rcu_core();

  }
  
  /*
   * 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 RCU core processing.

   *

   * This function must be called from hardirq context.  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)
  {

  	trace_rcu_utilization("Start scheduler-tick");

  	if (user || rcu_is_cpu_rrupt_from_idle()) {

  
  		/*
  		 * Get here if this CPU took its interrupt from user
  		 * mode or from the idle loop, and if this is not a
  		 * nested interrupt.  In this case, the CPU is in

  		 * a quiescent state, so note it.

  		 *
  		 * No memory barrier is required here because 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);

  	if (rcu_pending(cpu))

  		invoke_rcu_core();

  	trace_rcu_utilization("End scheduler-tick");

  }
  
  #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.

   * Also initiate boosting for any threads blocked on the root rcu_node.
   *

   * 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;

  		raw_spin_lock_irqsave(&rnp->lock, flags);

  		if (!rcu_gp_in_progress(rsp)) {

  			raw_spin_unlock_irqrestore(&rnp->lock, flags);

  			return;

  		}

  		if (rnp->qsmask == 0) {

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

  			continue;
  		}

  		cpu = rnp->grplo;

  		bit = 1;

  		for (; cpu <= rnp->grphi; cpu++, bit <<= 1) {

  			if ((rnp->qsmask & bit) != 0 &&
  			    f(per_cpu_ptr(rsp->rda, cpu)))

  				mask |= bit;
  		}

  		if (mask != 0) {

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

  			continue;
  		}

  		raw_spin_unlock_irqrestore(&rnp->lock, flags);

  	}

  	rnp = rcu_get_root(rsp);

  	if (rnp->qsmask == 0) {
  		raw_spin_lock_irqsave(&rnp->lock, flags);
  		rcu_initiate_boost(rnp, flags); /* releases rnp->lock. */
  	}

  }
  
  /*
   * 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);

  	trace_rcu_utilization("Start fqs");
  	if (!rcu_gp_in_progress(rsp)) {
  		trace_rcu_utilization("End fqs");

  		return;  /* No grace period in progress, nothing to force. */

  	}

  	if (!raw_spin_trylock_irqsave(&rsp->fqslock, flags)) {

  		rsp->n_force_qs_lh++; /* Inexact, can lose counts.  Tough! */

  		trace_rcu_utilization("End fqs");

  		return;	/* Someone else is already on the job. */
  	}

  	if (relaxed && ULONG_CMP_GE(rsp->jiffies_force_qs, jiffies))

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

  	rsp->n_force_qs++;

  	raw_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++;

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

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

  	}

  	rsp->fqs_active = 1;

  	switch (rsp->fqs_state) {

  	case RCU_GP_IDLE:

  	case RCU_GP_INIT:

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

  
  	case RCU_SAVE_DYNTICK:

  		if (RCU_SIGNAL_INIT != RCU_SAVE_DYNTICK)
  			break; /* So gcc recognizes the dead code. */

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

  		/* Record dyntick-idle state. */

  		force_qs_rnp(rsp, dyntick_save_progress_counter);

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

  		if (rcu_gp_in_progress(rsp))

  			rsp->fqs_state = RCU_FORCE_QS;

  		break;

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

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

  		force_qs_rnp(rsp, rcu_implicit_dynticks_qs);

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

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

  		break;

  	}

  	rsp->fqs_active = 0;

  	if (rsp->fqs_need_gp) {

  		raw_spin_unlock(&rsp->fqslock); /* irqs remain disabled */

  		rsp->fqs_need_gp = 0;
  		rcu_start_gp(rsp, flags); /* releases rnp->lock */

  		trace_rcu_utilization("End fqs");

  		return;
  	}

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

  unlock_fqs_ret:

  	raw_spin_unlock_irqrestore(&rsp->fqslock, flags);

  	trace_rcu_utilization("End fqs");

  }
  
  #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 core processing work 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 (ULONG_CMP_LT(ACCESS_ONCE(rsp->jiffies_force_qs), jiffies))

  		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)) {

  		raw_spin_lock_irqsave(&rcu_get_root(rsp)->lock, flags);

  		rcu_start_gp(rsp, flags);  /* releases above lock */
  	}
  
  	/* If there are callbacks ready, invoke them. */

  	if (cpu_has_callbacks_ready_to_invoke(rdp))

  		invoke_rcu_callbacks(rsp, rdp);

  }

  /*

   * Do RCU core processing for the current CPU.

   */

  static void rcu_process_callbacks(struct softirq_action *unused)

  {

  	trace_rcu_utilization("Start RCU core");

  	__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();

  	trace_rcu_utilization("End RCU core");

  }

  /*

   * Schedule RCU callback invocation.  If the specified type of RCU
   * does not support RCU priority boosting, just do a direct call,
   * otherwise wake up the per-CPU kernel kthread.  Note that because we
   * are running on the current CPU with interrupts disabled, the
   * rcu_cpu_kthread_task cannot disappear out from under us.

   */

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

  {

  	if (unlikely(!ACCESS_ONCE(rcu_scheduler_fully_active)))
  		return;

  	if (likely(!rsp->boost)) {
  		rcu_do_batch(rsp, rdp);

  		return;
  	}

  	invoke_rcu_callbacks_kthread();

  }

  static void invoke_rcu_core(void)

  {
  	raise_softirq(RCU_SOFTIRQ);
  }

  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;

  	debug_rcu_head_queue(head);

  	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 = this_cpu_ptr(rsp->rda);