text: /*
   * 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 <linux/delay.h>
  #include <linux/stop_machine.h>
  #include <linux/random.h>
  
  #include "rcutree.h"
  #include <trace/events/rcu.h>
  
  #include "rcu.h"
  
  /* Data structures. */
  
  static struct lock_class_key rcu_node_class[RCU_NUM_LVLS];
  static struct lock_class_key rcu_fqs_class[RCU_NUM_LVLS];
  
  #define RCU_STATE_INITIALIZER(sname, sabbr, cr) { \
  	.level = { &sname##_state.node[0] }, \
  	.call = cr, \
  	.fqs_state = RCU_GP_IDLE, \
  	.gpnum = 0UL - 300UL, \
  	.completed = 0UL - 300UL, \
  	.orphan_lock = __RAW_SPIN_LOCK_UNLOCKED(&sname##_state.orphan_lock), \
  	.orphan_nxttail = &sname##_state.orphan_nxtlist, \
  	.orphan_donetail = &sname##_state.orphan_donelist, \
  	.barrier_mutex = __MUTEX_INITIALIZER(sname##_state.barrier_mutex), \
  	.onoff_mutex = __MUTEX_INITIALIZER(sname##_state.onoff_mutex), \
  	.name = #sname, \
  	.abbr = sabbr, \
  }
  
  struct rcu_state rcu_sched_state =
  	RCU_STATE_INITIALIZER(rcu_sched, 's', call_rcu_sched);
  DEFINE_PER_CPU(struct rcu_data, rcu_sched_data);
  
  struct rcu_state rcu_bh_state = RCU_STATE_INITIALIZER(rcu_bh, 'b', call_rcu_bh);
  DEFINE_PER_CPU(struct rcu_data, rcu_bh_data);
  
  static struct rcu_state *rcu_state;
  LIST_HEAD(rcu_struct_flavors);
  
  /* Increase (but not decrease) the CONFIG_RCU_FANOUT_LEAF at boot time. */
  static int rcu_fanout_leaf = CONFIG_RCU_FANOUT_LEAF;
  module_param(rcu_fanout_leaf, int, 0444);
  int rcu_num_lvls __read_mostly = RCU_NUM_LVLS;
  static int num_rcu_lvl[] = {  /* Number of rcu_nodes at specified level. */
  	NUM_RCU_LVL_0,
  	NUM_RCU_LVL_1,
  	NUM_RCU_LVL_2,
  	NUM_RCU_LVL_3,
  	NUM_RCU_LVL_4,
  };
  int rcu_num_nodes __read_mostly = NUM_RCU_NODES; /* Total # rcu_nodes in use. */
  
  /*
   * 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)
   * optimize 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(unsigned int, rcu_cpu_kthread_loops);
  DEFINE_PER_CPU(char, rcu_cpu_has_work);
  
  #endif /* #ifdef CONFIG_RCU_BOOST */
  
  static void rcu_boost_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);
  
  	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);
  
  	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_EXIT_IDLE,
  	.dynticks = ATOMIC_INIT(1),
  };
  
  static long blimit = 10;	/* Maximum callbacks per rcu_do_batch. */
  static long qhimark = 10000;	/* If this many pending, ignore blimit. */
  static long qlowmark = 100;	/* Once only this many pending, use blimit. */
  
  module_param(blimit, long, 0444);
  module_param(qhimark, long, 0444);
  module_param(qlowmark, long, 0444);
  
  static ulong jiffies_till_first_fqs = RCU_JIFFIES_TILL_FORCE_QS;
  static ulong jiffies_till_next_fqs = RCU_JIFFIES_TILL_FORCE_QS;
  
  module_param(jiffies_till_first_fqs, ulong, 0644);
  module_param(jiffies_till_next_fqs, ulong, 0644);
  
  static void rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
  				  struct rcu_data *rdp);
  static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *));
  static void force_quiescent_state(struct rcu_state *rsp);
  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);
  }
  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);
  }
  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] &&
  	       rdp->nxttail[RCU_DONE_TAIL] != NULL;
  }
  
  /*
   * Does the current CPU require a not-yet-started grace period?
   * The caller must have disabled interrupts to prevent races with
   * normal callback registry.
   */
  static int
  cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
  {
  	int i;
  
  	if (rcu_gp_in_progress(rsp))
  		return 0;  /* No, a grace period is already in progress. */
  	if (rcu_nocb_needs_gp(rsp))
  		return 1;  /* Yes, a no-CBs CPU needs one. */
  	if (!rdp->nxttail[RCU_NEXT_TAIL])
  		return 0;  /* No, this is a no-CBs (or offline) CPU. */
  	if (*rdp->nxttail[RCU_NEXT_READY_TAIL])
  		return 1;  /* Yes, this CPU has newly registered callbacks. */
  	for (i = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL; i++)
  		if (rdp->nxttail[i - 1] != rdp->nxttail[i] &&
  		    ULONG_CMP_LT(ACCESS_ONCE(rsp->completed),
  				 rdp->nxtcompleted[i]))
  			return 1;  /* Yes, CBs for future grace period. */
  	return 0; /* No grace period needed. */
  }
  
  /*
   * 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];
  }
  
  /*
   * rcu_eqs_enter_common - current CPU is moving towards extended quiescent state
   *
   * 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_eqs_enter_common(struct rcu_dynticks *rdtp, long long oldval,
  				bool user)
  {
  	trace_rcu_dyntick("Start", oldval, rdtp->dynticks_nesting);
  	if (!user && !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_ORIG);
  		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);
  
  	/*
  	 * It is illegal to enter an extended quiescent state while
  	 * in an RCU read-side critical section.
  	 */
  	rcu_lockdep_assert(!lock_is_held(&rcu_lock_map),
  			   "Illegal idle entry in RCU read-side critical section.");
  	rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map),
  			   "Illegal idle entry in RCU-bh read-side critical section.");
  	rcu_lockdep_assert(!lock_is_held(&rcu_sched_lock_map),
  			   "Illegal idle entry in RCU-sched read-side critical section.");
  }
  
  /*
   * Enter an RCU extended quiescent state, which can be either the
   * idle loop or adaptive-tickless usermode execution.
   */
  static void rcu_eqs_enter(bool user)
  {
  	long long oldval;
  	struct rcu_dynticks *rdtp;
  
  	rdtp = &__get_cpu_var(rcu_dynticks);
  	oldval = rdtp->dynticks_nesting;
  	WARN_ON_ONCE((oldval & DYNTICK_TASK_NEST_MASK) == 0);
  	if ((oldval & DYNTICK_TASK_NEST_MASK) == DYNTICK_TASK_NEST_VALUE)
  		rdtp->dynticks_nesting = 0;
  	else
  		rdtp->dynticks_nesting -= DYNTICK_TASK_NEST_VALUE;
  	rcu_eqs_enter_common(rdtp, oldval, user);
  }
  
  /**
   * 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;
  
  	local_irq_save(flags);
  	rcu_eqs_enter(false);
  	local_irq_restore(flags);
  }
  EXPORT_SYMBOL_GPL(rcu_idle_enter);
  
  #ifdef CONFIG_RCU_USER_QS
  /**
   * rcu_user_enter - inform RCU that we are resuming userspace.
   *
   * Enter RCU idle mode right before resuming userspace.  No use of RCU
   * is permitted between this call and rcu_user_exit(). This way the
   * CPU doesn't need to maintain the tick for RCU maintenance purposes
   * when the CPU runs in userspace.
   */
  void rcu_user_enter(void)
  {
  	rcu_eqs_enter(1);
  }
  
  /**
   * rcu_user_enter_after_irq - inform RCU that we are going to resume userspace
   * after the current irq returns.
   *
   * This is similar to rcu_user_enter() but in the context of a non-nesting
   * irq. After this call, RCU enters into idle mode when the interrupt
   * returns.
   */
  void rcu_user_enter_after_irq(void)
  {
  	unsigned long flags;
  	struct rcu_dynticks *rdtp;
  
  	local_irq_save(flags);
  	rdtp = &__get_cpu_var(rcu_dynticks);
  	/* Ensure this irq is interrupting a non-idle RCU state.  */
  	WARN_ON_ONCE(!(rdtp->dynticks_nesting & DYNTICK_TASK_MASK));
  	rdtp->dynticks_nesting = 1;
  	local_irq_restore(flags);
  }
  #endif /* CONFIG_RCU_USER_QS */
  
  /**
   * 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_eqs_enter_common(rdtp, oldval, true);
  	local_irq_restore(flags);
  }
  
  /*
   * rcu_eqs_exit_common - current CPU moving away from extended quiescent state
   *
   * 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_eqs_exit_common(struct rcu_dynticks *rdtp, long long oldval,
  			       int user)
  {
  	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 (!user && !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_ORIG);
  		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! */
  	}
  }
  
  /*
   * Exit an RCU extended quiescent state, which can be either the
   * idle loop or adaptive-tickless usermode execution.
   */
  static void rcu_eqs_exit(bool user)
  {
  	struct rcu_dynticks *rdtp;
  	long long oldval;
  
  	rdtp = &__get_cpu_var(rcu_dynticks);
  	oldval = rdtp->dynticks_nesting;
  	WARN_ON_ONCE(oldval < 0);
  	if (oldval & DYNTICK_TASK_NEST_MASK)
  		rdtp->dynticks_nesting += DYNTICK_TASK_NEST_VALUE;
  	else
  		rdtp->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
  	rcu_eqs_exit_common(rdtp, oldval, user);
  }
  
  /**
   * 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_NEST 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;
  
  	local_irq_save(flags);
  	rcu_eqs_exit(false);
  	local_irq_restore(flags);
  }
  EXPORT_SYMBOL_GPL(rcu_idle_exit);
  
  #ifdef CONFIG_RCU_USER_QS
  /**
   * rcu_user_exit - inform RCU that we are exiting userspace.
   *
   * Exit RCU idle mode while entering the kernel because it can
   * run a RCU read side critical section anytime.
   */
  void rcu_user_exit(void)
  {
  	rcu_eqs_exit(1);
  }
  
  /**
   * rcu_user_exit_after_irq - inform RCU that we won't resume to userspace
   * idle mode after the current non-nesting irq returns.
   *
   * This is similar to rcu_user_exit() but in the context of an irq.
   * This is called when the irq has interrupted a userspace RCU idle mode
   * context. When the current non-nesting interrupt returns after this call,
   * the CPU won't restore the RCU idle mode.
   */
  void rcu_user_exit_after_irq(void)
  {
  	unsigned long flags;
  	struct rcu_dynticks *rdtp;
  
  	local_irq_save(flags);
  	rdtp = &__get_cpu_var(rcu_dynticks);
  	/* Ensure we are interrupting an RCU idle mode. */
  	WARN_ON_ONCE(rdtp->dynticks_nesting & DYNTICK_TASK_NEST_MASK);
  	rdtp->dynticks_nesting += DYNTICK_TASK_EXIT_IDLE;
  	local_irq_restore(flags);
  }
  #endif /* CONFIG_RCU_USER_QS */
  
  /**
   * 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_eqs_exit_common(rdtp, oldval, true);
  	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);
  }
  
  /**
   * 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);
  
  #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU)
  
  /*
   * Is the current CPU online?  Disable preemption to avoid false positives
   * that could otherwise happen due to the current CPU number being sampled,
   * this task being preempted, its old CPU being taken offline, resuming
   * on some other CPU, then determining that its old CPU is now offline.
   * It is OK to use RCU on an offline processor during initial boot, hence
   * the check for rcu_scheduler_fully_active.  Note also that it is OK
   * for a CPU coming online to use RCU for one jiffy prior to marking itself
   * online in the cpu_online_mask.  Similarly, it is OK for a CPU going
   * offline to continue to use RCU for one jiffy after marking itself
   * offline in the cpu_online_mask.  This leniency is necessary given the
   * non-atomic nature of the online and offline processing, for example,
   * the fact that a CPU enters the scheduler after completing the CPU_DYING
   * notifiers.
   *
   * This is also why RCU internally marks CPUs online during the
   * CPU_UP_PREPARE phase and offline during the CPU_DEAD phase.
   *
   * Disable checking if in an NMI handler because we cannot safely report
   * errors from NMI handlers anyway.
   */
  bool rcu_lockdep_current_cpu_online(void)
  {
  	struct rcu_data *rdp;
  	struct rcu_node *rnp;
  	bool ret;
  
  	if (in_nmi())
  		return 1;
  	preempt_disable();
  	rdp = &__get_cpu_var(rcu_sched_data);
  	rnp = rdp->mynode;
  	ret = (rdp->grpmask & rnp->qsmaskinit) ||
  	      !rcu_scheduler_fully_active;
  	preempt_enable();
  	return ret;
  }
  EXPORT_SYMBOL_GPL(rcu_lockdep_current_cpu_online);
  
  #endif /* #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU) */
  
  /**
   * 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.
   */
  static int rcu_is_cpu_rrupt_from_idle(void)
  {
  	return __get_cpu_var(rcu_dynticks).dynticks_nesting <= 1;
  }
  
  /*
   * 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, or by virtue of having been offline.
   */
  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;
  	}
  
  	/*
  	 * Check for the CPU being offline, but only if the grace period
  	 * is old enough.  We don't need to worry about the CPU changing
  	 * state: If we see it offline even once, it has been through a
  	 * quiescent state.
  	 *
  	 * The reason for insisting that the grace period be at least
  	 * one jiffy old is that CPUs that are not quite online and that
  	 * have just gone offline can still execute RCU read-side critical
  	 * sections.
  	 */
  	if (ULONG_CMP_GE(rdp->rsp->gp_start + 2, jiffies))
  		return 0;  /* Grace period is not old enough. */
  	barrier();
  	if (cpu_is_offline(rdp->cpu)) {
  		trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, "ofl");
  		rdp->offline_fqs++;
  		return 1;
  	}
  
  	/*
  	 * There is a possibility that a CPU in adaptive-ticks state
  	 * might run in the kernel with the scheduling-clock tick disabled
  	 * for an extended time period.  Invoke rcu_kick_nohz_cpu() to
  	 * force the CPU to restart the scheduling-clock tick in this
  	 * CPU is in this state.
  	 */
  	rcu_kick_nohz_cpu(rdp->cpu);
  
  	return 0;
  }
  
  static void record_gp_stall_check_time(struct rcu_state *rsp)
  {
  	rsp->gp_start = jiffies;
  	rsp->jiffies_stall = jiffies + rcu_jiffies_till_stall_check();
  }
  
  /*
   * Dump stacks of all tasks running on stalled CPUs.  This is a fallback
   * for architectures that do not implement trigger_all_cpu_backtrace().
   * The NMI-triggered stack traces are more accurate because they are
   * printed by the target CPU.
   */
  static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
  {
  	int cpu;
  	unsigned long flags;
  	struct rcu_node *rnp;
  
  	rcu_for_each_leaf_node(rsp, rnp) {
  		raw_spin_lock_irqsave(&rnp->lock, flags);
  		if (rnp->qsmask != 0) {
  			for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
  				if (rnp->qsmask & (1UL << cpu))
  					dump_cpu_task(rnp->grplo + cpu);
  		}
  		raw_spin_unlock_irqrestore(&rnp->lock, flags);
  	}
  }
  
  static void print_other_cpu_stall(struct rcu_state *rsp)
  {
  	int cpu;
  	long delta;
  	unsigned long flags;
  	int ndetected = 0;
  	struct rcu_node *rnp = rcu_get_root(rsp);
  	long totqlen = 0;
  
  	/* 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 + 3 * rcu_jiffies_till_stall_check() + 3;
  	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);
  	print_cpu_stall_info_begin();
  	rcu_for_each_leaf_node(rsp, rnp) {
  		raw_spin_lock_irqsave(&rnp->lock, flags);
  		ndetected += rcu_print_task_stall(rnp);
  		if (rnp->qsmask != 0) {
  			for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
  				if (rnp->qsmask & (1UL << cpu)) {
  					print_cpu_stall_info(rsp,
  							     rnp->grplo + cpu);
  					ndetected++;
  				}
  		}
  		raw_spin_unlock_irqrestore(&rnp->lock, flags);
  	}
  
  	/*
  	 * Now rat on any tasks that got kicked up to the root rcu_node
  	 * due to CPU offlining.
  	 */
  	rnp = rcu_get_root(rsp);
  	raw_spin_lock_irqsave(&rnp->lock, flags);
  	ndetected += rcu_print_task_stall(rnp);
  	raw_spin_unlock_irqrestore(&rnp->lock, flags);
  
  	print_cpu_stall_info_end();
  	for_each_possible_cpu(cpu)
  		totqlen += per_cpu_ptr(rsp->rda, cpu)->qlen;
  	pr_cont("(detected by %d, t=%ld jiffies, g=%lu, c=%lu, q=%lu)\n",
  	       smp_processor_id(), (long)(jiffies - rsp->gp_start),
  	       rsp->gpnum, rsp->completed, totqlen);
  	if (ndetected == 0)
  		printk(KERN_ERR "INFO: Stall ended before state dump start\n");
  	else if (!trigger_all_cpu_backtrace())
  		rcu_dump_cpu_stacks(rsp);
  
  	/* Complain about tasks blocking the grace period. */
  
  	rcu_print_detail_task_stall(rsp);
  
  	force_quiescent_state(rsp);  /* Kick them all. */
  }
  
  static void print_cpu_stall(struct rcu_state *rsp)
  {
  	int cpu;
  	unsigned long flags;
  	struct rcu_node *rnp = rcu_get_root(rsp);
  	long totqlen = 0;
  
  	/*
  	 * 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 self-detected stall on CPU", rsp->name);
  	print_cpu_stall_info_begin();
  	print_cpu_stall_info(rsp, smp_processor_id());
  	print_cpu_stall_info_end();
  	for_each_possible_cpu(cpu)
  		totqlen += per_cpu_ptr(rsp->rda, cpu)->qlen;
  	pr_cont(" (t=%lu jiffies g=%lu c=%lu q=%lu)\n",
  		jiffies - rsp->gp_start, rsp->gpnum, rsp->completed, totqlen);
  	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 +
  				     3 * rcu_jiffies_till_stall_check() + 3;
  	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 (rcu_gp_in_progress(rsp) &&
  	    (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);
  	}
  }
  
  /**
   * 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)
  {
  	struct rcu_state *rsp;
  
  	for_each_rcu_flavor(rsp)
  		rsp->jiffies_stall = jiffies + ULONG_MAX / 2;
  }
  
  /*
   * 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");
  		rdp->passed_quiesce = 0;
  		rdp->qs_pending = !!(rnp->qsmask & rdp->grpmask);
  		zero_cpu_stall_ticks(rdp);
  	}
  }
  
  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;
  }
  
  /*
   * Initialize the specified rcu_data structure's callback list to empty.
   */
  static void init_callback_list(struct rcu_data *rdp)
  {
  	int i;
  
  	if (init_nocb_callback_list(rdp))
  		return;
  	rdp->nxtlist = NULL;
  	for (i = 0; i < RCU_NEXT_SIZE; i++)
  		rdp->nxttail[i] = &rdp->nxtlist;
  }
  
  /*
   * Determine the value that ->completed will have at the end of the
   * next subsequent grace period.  This is used to tag callbacks so that
   * a CPU can invoke callbacks in a timely fashion even if that CPU has
   * been dyntick-idle for an extended period with callbacks under the
   * influence of RCU_FAST_NO_HZ.
   *
   * The caller must hold rnp->lock with interrupts disabled.
   */
  static unsigned long rcu_cbs_completed(struct rcu_state *rsp,
  				       struct rcu_node *rnp)
  {
  	/*
  	 * If RCU is idle, we just wait for the next grace period.
  	 * But we can only be sure that RCU is idle if we are looking
  	 * at the root rcu_node structure -- otherwise, a new grace
  	 * period might have started, but just not yet gotten around
  	 * to initializing the current non-root rcu_node structure.
  	 */
  	if (rcu_get_root(rsp) == rnp && rnp->gpnum == rnp->completed)
  		return rnp->completed + 1;
  
  	/*
  	 * Otherwise, wait for a possible partial grace period and
  	 * then the subsequent full grace period.
  	 */
  	return rnp->completed + 2;
  }
  
  /*
   * Trace-event helper function for rcu_start_future_gp() and
   * rcu_nocb_wait_gp().
   */
  static void trace_rcu_future_gp(struct rcu_node *rnp, struct rcu_data *rdp,
  				unsigned long c, char *s)
  {
  	trace_rcu_future_grace_period(rdp->rsp->name, rnp->gpnum,
  				      rnp->completed, c, rnp->level,
  				      rnp->grplo, rnp->grphi, s);
  }
  
  /*
   * Start some future grace period, as needed to handle newly arrived
   * callbacks.  The required future grace periods are recorded in each
   * rcu_node structure's ->need_future_gp field.
   *
   * The caller must hold the specified rcu_node structure's ->lock.
   */
  static unsigned long __maybe_unused
  rcu_start_future_gp(struct rcu_node *rnp, struct rcu_data *rdp)
  {
  	unsigned long c;
  	int i;
  	struct rcu_node *rnp_root = rcu_get_root(rdp->rsp);
  
  	/*
  	 * Pick up grace-period number for new callbacks.  If this
  	 * grace period is already marked as needed, return to the caller.
  	 */
  	c = rcu_cbs_completed(rdp->rsp, rnp);
  	trace_rcu_future_gp(rnp, rdp, c, "Startleaf");
  	if (rnp->need_future_gp[c & 0x1]) {
  		trace_rcu_future_gp(rnp, rdp, c, "Prestartleaf");
  		return c;
  	}
  
  	/*
  	 * If either this rcu_node structure or the root rcu_node structure
  	 * believe that a grace period is in progress, then we must wait
  	 * for the one following, which is in "c".  Because our request
  	 * will be noticed at the end of the current grace period, we don't
  	 * need to explicitly start one.
  	 */
  	if (rnp->gpnum != rnp->completed ||
  	    ACCESS_ONCE(rnp->gpnum) != ACCESS_ONCE(rnp->completed)) {
  		rnp->need_future_gp[c & 0x1]++;
  		trace_rcu_future_gp(rnp, rdp, c, "Startedleaf");
  		return c;
  	}
  
  	/*
  	 * There might be no grace period in progress.  If we don't already
  	 * hold it, acquire the root rcu_node structure's lock in order to
  	 * start one (if needed).
  	 */
  	if (rnp != rnp_root)
  		raw_spin_lock(&rnp_root->lock);
  
  	/*
  	 * Get a new grace-period number.  If there really is no grace
  	 * period in progress, it will be smaller than the one we obtained
  	 * earlier.  Adjust callbacks as needed.  Note that even no-CBs
  	 * CPUs have a ->nxtcompleted[] array, so no no-CBs checks needed.
  	 */
  	c = rcu_cbs_completed(rdp->rsp, rnp_root);
  	for (i = RCU_DONE_TAIL; i < RCU_NEXT_TAIL; i++)
  		if (ULONG_CMP_LT(c, rdp->nxtcompleted[i]))
  			rdp->nxtcompleted[i] = c;
  
  	/*
  	 * If the needed for the required grace period is already
  	 * recorded, trace and leave.
  	 */
  	if (rnp_root->need_future_gp[c & 0x1]) {
  		trace_rcu_future_gp(rnp, rdp, c, "Prestartedroot");
  		goto unlock_out;
  	}
  
  	/* Record the need for the future grace period. */
  	rnp_root->need_future_gp[c & 0x1]++;
  
  	/* If a grace period is not already in progress, start one. */
  	if (rnp_root->gpnum != rnp_root->completed) {
  		trace_rcu_future_gp(rnp, rdp, c, "Startedleafroot");
  	} else {
  		trace_rcu_future_gp(rnp, rdp, c, "Startedroot");
  		rcu_start_gp_advanced(rdp->rsp, rnp_root, rdp);
  	}
  unlock_out:
  	if (rnp != rnp_root)
  		raw_spin_unlock(&rnp_root->lock);
  	return c;
  }
  
  /*
   * Clean up any old requests for the just-ended grace period.  Also return
   * whether any additional grace periods have been requested.  Also invoke
   * rcu_nocb_gp_cleanup() in order to wake up any no-callbacks kthreads
   * waiting for this grace period to complete.
   */
  static int rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
  {
  	int c = rnp->completed;
  	int needmore;
  	struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
  
  	rcu_nocb_gp_cleanup(rsp, rnp);
  	rnp->need_future_gp[c & 0x1] = 0;
  	needmore = rnp->need_future_gp[(c + 1) & 0x1];
  	trace_rcu_future_gp(rnp, rdp, c, needmore ? "CleanupMore" : "Cleanup");
  	return needmore;
  }
  
  /*
   * If there is room, assign a ->completed number to any callbacks on
   * this CPU that have not already been assigned.  Also accelerate any
   * callbacks that were previously assigned a ->completed number that has
   * since proven to be too conservative, which can happen if callbacks get
   * assigned a ->completed number while RCU is idle, but with reference to
   * a non-root rcu_node structure.  This function is idempotent, so it does
   * not hurt to call it repeatedly.
   *
   * The caller must hold rnp->lock with interrupts disabled.
   */
  static void rcu_accelerate_cbs(struct rcu_state *rsp, struct rcu_node *rnp,
  			       struct rcu_data *rdp)
  {
  	unsigned long c;
  	int i;
  
  	/* If the CPU has no callbacks, nothing to do. */
  	if (!rdp->nxttail[RCU_NEXT_TAIL] || !*rdp->nxttail[RCU_DONE_TAIL])
  		return;
  
  	/*
  	 * Starting from the sublist containing the callbacks most
  	 * recently assigned a ->completed number and working down, find the
  	 * first sublist that is not assignable to an upcoming grace period.
  	 * Such a sublist has something in it (first two tests) and has
  	 * a ->completed number assigned that will complete sooner than
  	 * the ->completed number for newly arrived callbacks (last test).
  	 *
  	 * The key point is that any later sublist can be assigned the
  	 * same ->completed number as the newly arrived callbacks, which
  	 * means that the callbacks in any of these later sublist can be
  	 * grouped into a single sublist, whether or not they have already
  	 * been assigned a ->completed number.
  	 */
  	c = rcu_cbs_completed(rsp, rnp);
  	for (i = RCU_NEXT_TAIL - 1; i > RCU_DONE_TAIL; i--)
  		if (rdp->nxttail[i] != rdp->nxttail[i - 1] &&
  		    !ULONG_CMP_GE(rdp->nxtcompleted[i], c))
  			break;
  
  	/*
  	 * If there are no sublist for unassigned callbacks, leave.
  	 * At the same time, advance "i" one sublist, so that "i" will
  	 * index into the sublist where all the remaining callbacks should
  	 * be grouped into.
  	 */
  	if (++i >= RCU_NEXT_TAIL)
  		return;
  
  	/*
  	 * Assign all subsequent callbacks' ->completed number to the next
  	 * full grace period and group them all in the sublist initially
  	 * indexed by "i".
  	 */
  	for (; i <= RCU_NEXT_TAIL; i++) {
  		rdp->nxttail[i] = rdp->nxttail[RCU_NEXT_TAIL];
  		rdp->nxtcompleted[i] = c;
  	}
  	/* Record any needed additional grace periods. */
  	rcu_start_future_gp(rnp, rdp);
  
  	/* Trace depending on how much we were able to accelerate. */
  	if (!*rdp->nxttail[RCU_WAIT_TAIL])
  		trace_rcu_grace_period(rsp->name, rdp->gpnum, "AccWaitCB");
  	else
  		trace_rcu_grace_period(rsp->name, rdp->gpnum, "AccReadyCB");
  }
  
  /*
   * Move any callbacks whose grace period has completed to the
   * RCU_DONE_TAIL sublist, then compact the remaining sublists and
   * assign ->completed numbers to any callbacks in the RCU_NEXT_TAIL
   * sublist.  This function is idempotent, so it does not hurt to
   * invoke it repeatedly.  As long as it is not invoked -too- often...
   *
   * The caller must hold rnp->lock with interrupts disabled.
   */
  static void rcu_advance_cbs(struct rcu_state *rsp, struct rcu_node *rnp,
  			    struct rcu_data *rdp)
  {
  	int i, j;
  
  	/* If the CPU has no callbacks, nothing to do. */
  	if (!rdp->nxttail[RCU_NEXT_TAIL] || !*rdp->nxttail[RCU_DONE_TAIL])
  		return;
  
  	/*
  	 * Find all callbacks whose ->completed numbers indicate that they
  	 * are ready to invoke, and put them into the RCU_DONE_TAIL sublist.
  	 */
  	for (i = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL; i++) {
  		if (ULONG_CMP_LT(rnp->completed, rdp->nxtcompleted[i]))
  			break;
  		rdp->nxttail[RCU_DONE_TAIL] = rdp->nxttail[i];
  	}
  	/* Clean up any sublist tail pointers that were misordered above. */
  	for (j = RCU_WAIT_TAIL; j < i; j++)
  		rdp->nxttail[j] = rdp->nxttail[RCU_DONE_TAIL];
  
  	/* Copy down callbacks to fill in empty sublists. */
  	for (j = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL; i++, j++) {
  		if (rdp->nxttail[j] == rdp->nxttail[RCU_NEXT_TAIL])
  			break;
  		rdp->nxttail[j] = rdp->nxttail[i];
  		rdp->nxtcompleted[j] = rdp->nxtcompleted[i];
  	}
  
  	/* Classify any remaining callbacks. */
  	rcu_accelerate_cbs(rsp, rnp, rdp);
  }
  
  /*
   * 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) {
  
  		/* No, so just accelerate recent callbacks. */
  		rcu_accelerate_cbs(rsp, rnp, rdp);
  
  	} else {
  
  		/* Advance callbacks. */
  		rcu_advance_cbs(rsp, rnp, rdp);
  
  		/* 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.  Of course, any quiescent
  		 * states we found for the old GP are now invalid.
  		 */
  		if (ULONG_CMP_LT(rdp->gpnum, rdp->completed)) {
  			rdp->gpnum = rdp->completed;
  			rdp->passed_quiesce = 0;
  		}
  
  		/*
  		 * 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);
  
  	/* Set state so that this CPU will detect the next quiescent state. */
  	__note_new_gpnum(rsp, rnp, rdp);
  }
  
  /*
   * Initialize a new grace period.
   */
  static int rcu_gp_init(struct rcu_state *rsp)
  {
  	struct rcu_data *rdp;
  	struct rcu_node *rnp = rcu_get_root(rsp);
  
  	raw_spin_lock_irq(&rnp->lock);
  	rsp->gp_flags = 0; /* Clear all flags: New grace period. */
  
  	if (rcu_gp_in_progress(rsp)) {
  		/* Grace period already in progress, don't start another.  */
  		raw_spin_unlock_irq(&rnp->lock);
  		return 0;
  	}
  
  	/* Advance to a new grace period and initialize state. */
  	rsp->gpnum++;
  	trace_rcu_grace_period(rsp->name, rsp->gpnum, "start");
  	record_gp_stall_check_time(rsp);
  	raw_spin_unlock_irq(&rnp->lock);
  
  	/* Exclude any concurrent CPU-hotplug operations. */
  	mutex_lock(&rsp->onoff_mutex);
  
  	/*
  	 * 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, relying on the layout
  	 * of the tree within the rsp->node[] array.  Note that other CPUs
  	 * will access only the leaves of the hierarchy, thus seeing 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.
  	 *
  	 * The grace period cannot complete until the initialization
  	 * process finishes, because this kthread handles both.
  	 */
  	rcu_for_each_node_breadth_first(rsp, rnp) {
  		raw_spin_lock_irq(&rnp->lock);
  		rdp = this_cpu_ptr(rsp->rda);
  		rcu_preempt_check_blocked_tasks(rnp);
  		rnp->qsmask = rnp->qsmaskinit;
  		ACCESS_ONCE(rnp->gpnum) = rsp->gpnum;
  		WARN_ON_ONCE(rnp->completed != rsp->completed);
  		ACCESS_ONCE(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_irq(&rnp->lock);
  #ifdef CONFIG_PROVE_RCU_DELAY
  		if ((prandom_u32() % (rcu_num_nodes + 1)) == 0 &&
  		    system_state == SYSTEM_RUNNING)
  			udelay(200);
  #endif /* #ifdef CONFIG_PROVE_RCU_DELAY */
  		cond_resched();
  	}
  
  	mutex_unlock(&rsp->onoff_mutex);
  	return 1;
  }
  
  /*
   * Do one round of quiescent-state forcing.
   */
  int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
  {
  	int fqs_state = fqs_state_in;
  	struct rcu_node *rnp = rcu_get_root(rsp);
  
  	rsp->n_force_qs++;
  	if (fqs_state == RCU_SAVE_DYNTICK) {
  		/* Collect dyntick-idle snapshots. */
  		force_qs_rnp(rsp, dyntick_save_progress_counter);
  		fqs_state = RCU_FORCE_QS;
  	} else {
  		/* Handle dyntick-idle and offline CPUs. */
  		force_qs_rnp(rsp, rcu_implicit_dynticks_qs);
  	}
  	/* Clear flag to prevent immediate re-entry. */
  	if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
  		raw_spin_lock_irq(&rnp->lock);
  		rsp->gp_flags &= ~RCU_GP_FLAG_FQS;
  		raw_spin_unlock_irq(&rnp->lock);
  	}
  	return fqs_state;
  }
  
  /*
   * Clean up after the old grace period.
   */
  static void rcu_gp_cleanup(struct rcu_state *rsp)
  {
  	unsigned long gp_duration;
  	int nocb = 0;
  	struct rcu_data *rdp;
  	struct rcu_node *rnp = rcu_get_root(rsp);
  
  	raw_spin_lock_irq(&rnp->lock);
  	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.
  	 */
  	raw_spin_unlock_irq(&rnp->lock);
  
  	/*
  	 * 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.  This also avoids
  	 * some nasty RCU grace-period initialization races by forcing
  	 * the end of the current grace period to be completely recorded in
  	 * all of the rcu_node structures before the beginning of the next
  	 * grace period is recorded in any of the rcu_node structures.
  	 */
  	rcu_for_each_node_breadth_first(rsp, rnp) {
  		raw_spin_lock_irq(&rnp->lock);
  		ACCESS_ONCE(rnp->completed) = rsp->gpnum;
  		rdp = this_cpu_ptr(rsp->rda);
  		if (rnp == rdp->mynode)
  			__rcu_process_gp_end(rsp, rnp, rdp);
  		nocb += rcu_future_gp_cleanup(rsp, rnp);
  		raw_spin_unlock_irq(&rnp->lock);
  		cond_resched();
  	}
  	rnp = rcu_get_root(rsp);
  	raw_spin_lock_irq(&rnp->lock);
  	rcu_nocb_gp_set(rnp, nocb);
  
  	rsp->completed = rsp->gpnum; /* Declare grace period done. */
  	trace_rcu_grace_period(rsp->name, rsp->completed, "end");
  	rsp->fqs_state = RCU_GP_IDLE;
  	rdp = this_cpu_ptr(rsp->rda);
  	rcu_advance_cbs(rsp, rnp, rdp);  /* Reduce false positives below. */
  	if (cpu_needs_another_gp(rsp, rdp))
  		rsp->gp_flags = 1;
  	raw_spin_unlock_irq(&rnp->lock);
  }
  
  /*
   * Body of kthread that handles grace periods.
   */
  static int __noreturn rcu_gp_kthread(void *arg)
  {
  	int fqs_state;
  	unsigned long j;
  	int ret;
  	struct rcu_state *rsp = arg;
  	struct rcu_node *rnp = rcu_get_root(rsp);
  
  	for (;;) {
  
  		/* Handle grace-period start. */
  		for (;;) {
  			wait_event_interruptible(rsp->gp_wq,
  						 rsp->gp_flags &
  						 RCU_GP_FLAG_INIT);
  			if ((rsp->gp_flags & RCU_GP_FLAG_INIT) &&
  			    rcu_gp_init(rsp))
  				break;
  			cond_resched();
  			flush_signals(current);
  		}
  
  		/* Handle quiescent-state forcing. */
  		fqs_state = RCU_SAVE_DYNTICK;
  		j = jiffies_till_first_fqs;
  		if (j > HZ) {
  			j = HZ;
  			jiffies_till_first_fqs = HZ;
  		}
  		for (;;) {
  			rsp->jiffies_force_qs = jiffies + j;
  			ret = wait_event_interruptible_timeout(rsp->gp_wq,
  					(rsp->gp_flags & RCU_GP_FLAG_FQS) ||
  					(!ACCESS_ONCE(rnp->qsmask) &&
  					 !rcu_preempt_blocked_readers_cgp(rnp)),
  					j);
  			/* If grace period done, leave loop. */
  			if (!ACCESS_ONCE(rnp->qsmask) &&
  			    !rcu_preempt_blocked_readers_cgp(rnp))
  				break;
  			/* If time for quiescent-state forcing, do it. */
  			if (ret == 0 || (rsp->gp_flags & RCU_GP_FLAG_FQS)) {
  				fqs_state = rcu_gp_fqs(rsp, fqs_state);
  				cond_resched();
  			} else {
  				/* Deal with stray signal. */
  				cond_resched();
  				flush_signals(current);
  			}
  			j = jiffies_till_next_fqs;
  			if (j > HZ) {
  				j = HZ;
  				jiffies_till_next_fqs = HZ;
  			} else if (j < 1) {
  				j = 1;
  				jiffies_till_next_fqs = 1;
  			}
  		}
  
  		/* Handle grace-period end. */
  		rcu_gp_cleanup(rsp);
  	}
  }
  
  static void rsp_wakeup(struct irq_work *work)
  {
  	struct rcu_state *rsp = container_of(work, struct rcu_state, wakeup_work);
  
  	/* Wake up rcu_gp_kthread() to start the grace period. */
  	wake_up(&rsp->gp_wq);
  }
  
  /*
   * 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 and hard irqs must be disabled.
   *
   * Note that it is legal for a dying CPU (which is marked as offline) to
   * invoke this function.  This can happen when the dying CPU reports its
   * quiescent state.
   */
  static void
  rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
  		      struct rcu_data *rdp)
  {
  	if (!rsp->gp_kthread || !cpu_needs_another_gp(rsp, rdp)) {
  		/*
  		 * Either we have not yet spawned the grace-period
  		 * task, this CPU does not need another grace period,
  		 * or a grace period is already in progress.
  		 * Either way, don't start a new grace period.
  		 */
  		return;
  	}
  	rsp->gp_flags = RCU_GP_FLAG_INIT;
  
  	/*
  	 * We can't do wakeups while holding the rnp->lock, as that
  	 * could cause possible deadlocks with the rq->lock. Deter
  	 * the wakeup to interrupt context.
  	 */
  	irq_work_queue(&rsp->wakeup_work);
  }
  
  /*
   * Similar to rcu_start_gp_advanced(), but also advance the calling CPU's
   * callbacks.  Note that rcu_start_gp_advanced() cannot do this because it
   * is invoked indirectly from rcu_advance_cbs(), which would result in
   * endless recursion -- or would do so if it wasn't for the self-deadlock
   * that is encountered beforehand.
   */
  static void
  rcu_start_gp(struct rcu_state *rsp)
  {
  	struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
  	struct rcu_node *rnp = rcu_get_root(rsp);
  
  	/*
  	 * If there is no grace period in progress right now, any
  	 * callbacks we have up to this point will be satisfied by the
  	 * next grace period.  Also, advancing the callbacks reduces the
  	 * probability of false positives from cpu_needs_another_gp()
  	 * resulting in pointless grace periods.  So, advance callbacks
  	 * then start the grace period!
  	 */
  	rcu_advance_cbs(rsp, rnp, rdp);
  	rcu_start_gp_advanced(rsp, rnp, rdp);
  }
  
  /*
   * 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, which
   * is released before return.
   */
  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));
  	raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags);
  	wake_up(&rsp->gp_wq);  /* Memory barrier implied by wake_up() path. */
  }
  
  /*
   * 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)
  {
  	unsigned long flags;
  	unsigned long mask;
  	struct rcu_node *rnp;
  
  	rnp = rdp->mynode;
  	raw_spin_lock_irqsave(&rnp->lock, flags);
  	if (rdp->passed_quiesce == 0 || rdp->gpnum != 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.
  		 */
  		rcu_accelerate_cbs(rsp, rnp, rdp);
  
  		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);
  }
  
  #ifdef CONFIG_HOTPLUG_CPU
  
  /*
   * Send the specified CPU's RCU callbacks to the orphanage.  The
   * specified CPU must be offline, and the caller must hold the
   * ->orphan_lock.
   */
  static void
  rcu_send_cbs_to_orphanage(int cpu, struct rcu_state *rsp,
  			  struct rcu_node *rnp, struct rcu_data *rdp)
  {
  	/* No-CBs CPUs do not have orphanable callbacks. */
  	if (rcu_is_nocb_cpu(rdp->cpu))
  		return;
  
  	/*
  	 * Orphan the callbacks.  First adjust the counts.  This is safe
  	 * because _rcu_barrier() excludes CPU-hotplug operations, so it
  	 * cannot be running now.  Thus no memory barrier is required.
  	 */
  	if (rdp->nxtlist != NULL) {
  		rsp->qlen_lazy += rdp->qlen_lazy;
  		rsp->qlen += rdp->qlen;
  		rdp->n_cbs_orphaned += rdp->qlen;
  		rdp->qlen_lazy = 0;
  		ACCESS_ONCE(rdp->qlen) = 0;
  	}
  
  	/*
  	 * Next, move those callbacks still needing a grace period to
  	 * the orphanage, where some other CPU will pick them up.
  	 * Some of the callbacks might have gone partway through a grace
  	 * period, but that is too bad.  They get to start over because we
  	 * cannot assume that grace periods are synchronized across CPUs.
  	 * We don't bother updating the ->nxttail[] array yet, instead
  	 * we just reset the whole thing later on.
  	 */
  	if (*rdp->nxttail[RCU_DONE_TAIL] != NULL) {
  		*rsp->orphan_nxttail = *rdp->nxttail[RCU_DONE_TAIL];
  		rsp->orphan_nxttail = rdp->nxttail[RCU_NEXT_TAIL];
  		*rdp->nxttail[RCU_DONE_TAIL] = NULL;
  	}
  
  	/*
  	 * Then move the ready-to-invoke callbacks to the orphanage,
  	 * where some other CPU will pick them up.  These will not be
  	 * required to pass though another grace period: They are done.
  	 */
  	if (rdp->nxtlist != NULL) {
  		*rsp->orphan_donetail = rdp->nxtlist;
  		rsp->orphan_donetail = rdp->nxttail[RCU_DONE_TAIL];
  	}
  
  	/* Finally, initialize the rcu_data structure's list to empty.  */
  	init_callback_list(rdp);
  }
  
  /*
   * Adopt the RCU callbacks from the specified rcu_state structure's
   * orphanage.  The caller must hold the ->orphan_lock.
   */
  static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
  {
  	int i;
  	struct rcu_data *rdp = __this_cpu_ptr(rsp->rda);
  
  	/* No-CBs CPUs are handled specially. */
  	if (rcu_nocb_adopt_orphan_cbs(rsp, rdp))
  		return;
  
  	/* Do the accounting first. */
  	rdp->qlen_lazy += rsp->qlen_lazy;
  	rdp->qlen += rsp->qlen;
  	rdp->n_cbs_adopted += rsp->qlen;
  	if (rsp->qlen_lazy != rsp->qlen)
  		rcu_idle_count_callbacks_posted();
  	rsp->qlen_lazy = 0;
  	rsp->qlen = 0;
  
  	/*
  	 * We do not need a memory barrier here because the only way we
  	 * can get here if there is an rcu_barrier() in flight is if
  	 * we are the task doing the rcu_barrier().
  	 */
  
  	/* First adopt the ready-to-invoke callbacks. */
  	if (rsp->orphan_donelist != NULL) {
  		*rsp->orphan_donetail = *rdp->nxttail[RCU_DONE_TAIL];
  		*rdp->nxttail[RCU_DONE_TAIL] = rsp->orphan_donelist;
  		for (i = RCU_NEXT_SIZE - 1; i >= RCU_DONE_TAIL; i--)
  			if (rdp->nxttail[i] == rdp->nxttail[RCU_DONE_TAIL])
  				rdp->nxttail[i] = rsp->orphan_donetail;
  		rsp->orphan_donelist = NULL;
  		rsp->orphan_donetail = &rsp->orphan_donelist;
  	}
  
  	/* And then adopt the callbacks that still need a grace period. */
  	if (rsp->orphan_nxtlist != NULL) {
  		*rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_nxtlist;
  		rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_nxttail;
  		rsp->orphan_nxtlist = NULL;
  		rsp->orphan_nxttail = &rsp->orphan_nxtlist;
  	}
  }
  
  /*
   * Trace the fact that this CPU is going offline.
   */
  static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
  {
  	RCU_TRACE(unsigned long mask);
  	RCU_TRACE(struct rcu_data *rdp = this_cpu_ptr(rsp->rda));
  	RCU_TRACE(struct rcu_node *rnp = rdp->mynode);
  
  	RCU_TRACE(mask = rdp->grpmask);
  	trace_rcu_grace_period(rsp->name,
  			       rnp->gpnum + 1 - !!(rnp->qsmask & mask),
  			       "cpuofl");
  }
  
  /*
   * The CPU has been completely removed, and some other CPU is reporting
   * this fact from process context.  Do the remainder of the cleanup,
   * including orphaning the outgoing CPU's RCU callbacks, and also
   * adopting them.  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_cleanup_dead_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 = rdp->mynode;  /* Outgoing CPU's rdp & rnp. */
  
  	/* Adjust any no-longer-needed kthreads. */
  	rcu_boost_kthread_setaffinity(rnp, -1);
  
  	/* Remove the dead CPU from the bitmasks in the rcu_node hierarchy. */
  
  	/* Exclude any attempts to start a new grace period. */
  	mutex_lock(&rsp->onoff_mutex);
  	raw_spin_lock_irqsave(&rsp->orphan_lock, flags);
  
  	/* Orphan the dead CPU's callbacks, and adopt them if appropriate. */
  	rcu_send_cbs_to_orphanage(cpu, rsp, rnp, rdp);
  	rcu_adopt_orphan_cbs(rsp);
  
  	/* Remove the outgoing CPU from the masks in the rcu_node hierarchy. */
  	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. */
  			break;
  		}
  		if (rnp == rdp->mynode)
  			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 ->orphan_lock
  	 * held leads to deadlock.
  	 */
  	raw_spin_unlock(&rsp->orphan_lock); /* 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);
  	WARN_ONCE(rdp->qlen != 0 || rdp->nxtlist != NULL,
  		  "rcu_cleanup_dead_cpu: Callbacks on offline CPU %d: qlen=%lu, nxtlist=%p\n",
  		  cpu, rdp->qlen, rdp->nxtlist);
  	init_callback_list(rdp);
  	/* Disallow further callbacks on this CPU. */
  	rdp->nxttail[RCU_NEXT_TAIL] = NULL;
  	mutex_unlock(&rsp->onoff_mutex);
  }
  
  #else /* #ifdef CONFIG_HOTPLUG_CPU */
  
  static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
  {
  }
  
  static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
  {
  }
  
  #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;
  	long bl, count, count_lazy;
  	int i;
  
  	/* If no callbacks are ready, just return. */
  	if (!cpu_has_callbacks_ready_to_invoke(rdp)) {
  		trace_rcu_batch_start(rsp->name, rdp->qlen_lazy, rdp->qlen, 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);
  	WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
  	bl = rdp->blimit;
  	trace_rcu_batch_start(rsp->name, rdp->qlen_lazy, 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 (i = RCU_NEXT_SIZE - 1; i >= 0; i--)
  		if (rdp->nxttail[i] == rdp->nxttail[RCU_DONE_TAIL])
  			rdp->nxttail[i] = &rdp->nxtlist;
  	local_irq_restore(flags);
  
  	/* Invoke callbacks. */
  	count = count_lazy = 0;
  	while (list) {
  		next = list->next;
  		prefetch(next);
  		debug_rcu_head_unqueue(list);
  		if (__rcu_reclaim(rsp->name, list))
  			count_lazy++;
  		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. */
  	if (list != NULL) {
  		*tail = rdp->nxtlist;
  		rdp->nxtlist = list;
  		for (i = 0; i < RCU_NEXT_SIZE; i++)
  			if (&rdp->nxtlist == rdp->nxttail[i])
  				rdp->nxttail[i] = tail;
  			else
  				break;
  	}
  	smp_mb(); /* List handling before counting for rcu_barrier(). */
  	rdp->qlen_lazy -= count_lazy;
  	ACCESS_ONCE(rdp->qlen) -= count;
  	rdp->n_cbs_invoked += count;
  
  	/* 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;
  	WARN_ON_ONCE((rdp->nxtlist == NULL) != (rdp->qlen == 0));
  
  	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");
  	increment_cpu_stall_ticks();
  	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");
  }
  
  /*
   * 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) {
  		cond_resched();
  		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)
  {
  	unsigned long flags;
  	bool ret;
  	struct rcu_node *rnp;
  	struct rcu_node *rnp_old = NULL;
  
  	/* Funnel through hierarchy to reduce memory contention. */
  	rnp = per_cpu_ptr(rsp->rda, raw_smp_processor_id())->mynode;
  	for (; rnp != NULL; rnp = rnp->parent) {
  		ret = (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) ||
  		      !raw_spin_trylock(&rnp->fqslock);
  		if (rnp_old != NULL)
  			raw_spin_unlock(&rnp_old->fqslock);
  		if (ret) {
  			rsp->n_force_qs_lh++;
  			return;
  		}
  		rnp_old = rnp;
  	}
  	/* rnp_old == rcu_get_root(rsp), rnp == NULL. */
  
  	/* Reached the root of the rcu_node tree, acquire lock. */
  	raw_spin_lock_irqsave(&rnp_old->lock, flags);
  	raw_spin_unlock(&rnp_old->fqslock);
  	if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
  		rsp->n_force_qs_lh++;
  		raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
  		return;  /* Someone beat us to it. */
  	}
  	rsp->gp_flags |= RCU_GP_FLAG_FQS;
  	raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
  	wake_up(&rsp->gp_wq);  /* Memory barrier implied by wake_up() path. */
  }
  
  /*
   * 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)
  {
  	unsigned long flags;
  	struct rcu_data *rdp = __this_cpu_ptr(rsp->rda);
  
  	WARN_ON_ONCE(rdp->beenonline == 0);
  
  	/* Handle the end of a 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? */
  	local_irq_save(flags);
  	if (cpu_needs_another_gp(rsp, rdp)) {
  		raw_spin_lock(&rcu_get_root(rsp)->lock); /* irqs disabled. */
  		rcu_start_gp(rsp);
  		raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags);
  	} else {
  		local_irq_restore(flags);
  	}
  
  	/* 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)
  {
  	struct rcu_state *rsp;
  
  	if (cpu_is_offline(smp_processor_id()))
  		return;
  	trace_rcu_utilization("Start RCU core");
  	for_each_rcu_flavor(rsp)
  		__rcu_process_callbacks(rsp);
  	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)
  {
  	if (cpu_online(smp_processor_id()))
  		raise_softirq(RCU_SOFTIRQ);
  }
  
  /*
   * Handle any core-RCU processing required by a call_rcu() invocation.
   */
  static void __call_rcu_core(struct rcu_state *rsp, struct rcu_data *rdp,
  			    struct rcu_head *head, unsigned long flags)
  {
  	/*
  	 * If called from an extended quiescent state, invoke the RCU
  	 * core in order to force a re-evaluation of RCU's idleness.
  	 */
  	if (rcu_is_cpu_idle() && cpu_online(smp_processor_id()))
  		invoke_rcu_core();
  
  	/* If interrupts were disabled or CPU offline, don't invoke RCU core. */
  	if (irqs_disabled_flags(flags) || cpu_is_offline(smp_processor_id()))
  		return;
  
  	/*
  	 * 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)) {
  
  		/* Are we ignoring a completed grace period? */
  		rcu_process_gp_end(rsp, rdp);
  		check_for_new_grace_period(rsp, rdp);
  
  		/* Start a new grace period if one not already started. */
  		if (!rcu_gp_in_progress(rsp)) {
  			struct rcu_node *rnp_root = rcu_get_root(rsp);
  
  			raw_spin_lock(&rnp_root->lock);
  			rcu_start_gp(rsp);
  			raw_spin_unlock(&rnp_root->lock);
  		} else {
  			/* Give the grace period a kick. */
  			rdp->blimit = LONG_MAX;
  			if (rsp->n_force_qs == rdp->n_force_qs_snap &&
  			    *rdp->nxttail[RCU_DONE_TAIL] != head)
  				force_quiescent_state(rsp);
  			rdp->n_force_qs_snap = rsp->n_force_qs;
  			rdp->qlen_last_fqs_check = rdp->qlen;
  		}
  	}
  }
  
  /*
   * Helper function for call_rcu() and friends.  The cpu argument will
   * normally be -1, indicating "currently running CPU".  It may specify
   * a CPU only if that CPU is a no-CBs CPU.  Currently, only _rcu_barrier()
   * is expected to specify a CPU.
   */
  static void
  __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
  	   struct rcu_state *rsp, int cpu, bool lazy)
  {
  	unsigned long flags;
  	struct rcu_data *rdp;
  
  	WARN_ON_ONCE((unsigned long)head & 0x3); /* Misaligned rcu_head! */
  	debug_rcu_head_queue(head);
  	head->func = func;
  	head->next = NULL;
  
  	/*
  	 * 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);
  
  	/* Add the callback to our list. */
  	if (unlikely(rdp->nxttail[RCU_NEXT_TAIL] == NULL) || cpu != -1) {
  		int offline;
  
  		if (cpu != -1)
  			rdp = per_cpu_ptr(rsp->rda, cpu);
  		offline = !__call_rcu_nocb(rdp, head, lazy);
  		WARN_ON_ONCE(offline);
  		/* _call_rcu() is illegal on offline CPU; leak the callback. */
  		local_irq_restore(flags);
  		return;
  	}
  	ACCESS_ONCE(rdp->qlen)++;
  	if (lazy)
  		rdp->qlen_lazy++;
  	else
  		rcu_idle_count_callbacks_posted();
  	smp_mb();  /* Count before adding callback for rcu_barrier(). */
  	*rdp->nxttail[RCU_NEXT_TAIL] = head;
  	rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
  
  	if (__is_kfree_rcu_offset((unsigned long)func))
  		trace_rcu_kfree_callback(rsp->name, head, (unsigned long)func,
  					 rdp->qlen_lazy, rdp->qlen);
  	else
  		trace_rcu_callback(rsp->name, head, rdp->qlen_lazy, rdp->qlen);
  
  	/* Go handle any RCU core processing required. */
  	__call_rcu_core(rsp, rdp, head, flags);
  	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, -1, 0);
  }
  EXPORT_SYMBOL_GPL(call_rcu_sched);
  
  /*
   * Queue an RCU callback 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, -1, 0);
  }
  EXPORT_SYMBOL_GPL(call_rcu_bh);
  
  /*
   * Because a context switch is a grace period for RCU-sched and RCU-bh,
   * any blocking grace-period wait automatically implies a grace period
   * if there is only one CPU online at any point time during execution
   * of either synchronize_sched() or synchronize_rcu_bh().  It is OK to
   * occasionally incorrectly indicate that there are multiple CPUs online
   * when there was in fact only one the whole time, as this just adds
   * some overhead: RCU still operates correctly.
   */
  static inline int rcu_blocking_is_gp(void)
  {
  	int ret;
  
  	might_sleep();  /* Check for RCU read-side critical section. */
  	preempt_disable();
  	ret = num_online_cpus() <= 1;
  	preempt_enable();
  	return ret;
  }
  
  /**
   * 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
   * non-threaded 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.
   *
   * Note that this guarantee implies further memory-ordering guarantees.
   * On systems with more than one CPU, when synchronize_sched() returns,
   * each CPU is guaranteed to have executed a full memory barrier since the
   * end of its last RCU-sched read-side critical section whose beginning
   * preceded the call to synchronize_sched().  In addition, each CPU having
   * an RCU read-side critical section that extends beyond the return from
   * synchronize_sched() is guaranteed to have executed a full memory barrier
   * after the beginning of synchronize_sched() and before the beginning of
   * that RCU read-side critical section.  Note that these guarantees include
   * CPUs that are offline, idle, or executing in user mode, as well as CPUs
   * that are executing in the kernel.
   *
   * Furthermore, if CPU A invoked synchronize_sched(), which returned
   * to its caller on CPU B, then both CPU A and CPU B are guaranteed
   * to have executed a full memory barrier during the execution of
   * synchronize_sched() -- even if CPU A and CPU B are the same CPU (but
   * again only if the system has more than one CPU).
   *
   * 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)
  {
  	rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map) &&
  			   !lock_is_held(&rcu_lock_map) &&
  			   !lock_is_held(&rcu_sched_lock_map),
  			   "Illegal synchronize_sched() in RCU-sched read-side critical section");
  	if (rcu_blocking_is_gp())
  		return;
  	if (rcu_expedited)
  		synchronize_sched_expedited();
  	else
  		wait_rcu_gp(call_rcu_sched);
  }
  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.
   *
   * See the description of synchronize_sched() for more detailed information
   * on memory ordering guarantees.
   */
  void synchronize_rcu_bh(void)
  {
  	rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map) &&
  			   !lock_is_held(&rcu_lock_map) &&
  			   !lock_is_held(&rcu_sched_lock_map),
  			   "Illegal synchronize_rcu_bh() in RCU-bh read-side critical section");
  	if (rcu_blocking_is_gp())
  		return;
  	if (rcu_expedited)
  		synchronize_rcu_bh_expedited();
  	else
  		wait_rcu_gp(call_rcu_bh);
  }
  EXPORT_SYMBOL_GPL(synchronize_rcu_bh);
  
  static int synchronize_sched_expedited_cpu_stop(void *data)
  {
  	/*
  	 * There must be a full memory barrier on each affected CPU
  	 * between the time that try_stop_cpus() is called and the
  	 * time that it returns.
  	 *
  	 * In the current initial implementation of cpu_stop, the
  	 * above condition is already met when the control reaches
  	 * this point and the following smp_mb() is not strictly
  	 * necessary.  Do smp_mb() anyway for documentation and
  	 * robustness against future implementation changes.
  	 */
  	smp_mb(); /* See above comment block. */
  	return 0;
  }
  
  /**
   * synchronize_sched_expedited - Brute-force RCU-sched grace period
   *
   * Wait for an RCU-sched grace period to elapse, but use a "big hammer"
   * approach to force the grace period to end quickly.  This consumes
   * significant time on all CPUs and is unfriendly to real-time workloads,
   * so is thus not recommended for any sort of common-case code.  In fact,
   * if you are using synchronize_sched_expedited() in a loop, please
   * restructure your code to batch your updates, and then use a single
   * synchronize_sched() instead.
   *
   * Note that it is illegal to call this function while holding any lock
   * that is acquired by a CPU-hotplug notifier.  And yes, it is also illegal
   * to call this function from a CPU-hotplug notifier.  Failing to observe
   * these restriction will result in deadlock.
   *
   * This implementation can be thought of as an application of ticket
   * locking to RCU, with sync_sched_expedited_started and
   * sync_sched_expedited_done taking on the roles of the halves
   * of the ticket-lock word.  Each task atomically increments
   * sync_sched_expedited_started upon entry, snapshotting the old value,
   * then attempts to stop all the CPUs.  If this succeeds, then each
   * CPU will have executed a context switch, resulting in an RCU-sched
   * grace period.  We are then done, so we use atomic_cmpxchg() to
   * update sync_sched_expedited_done to match our snapshot -- but
   * only if someone else has not already advanced past our snapshot.
   *
   * On the other hand, if try_stop_cpus() fails, we check the value
   * of sync_sched_expedited_done.  If it has advanced past our
   * initial snapshot, then someone else must have forced a grace period
   * some time after we took our snapshot.  In this case, our work is
   * done for us, and we can simply return.  Otherwise, we try again,
   * but keep our initial snapshot for purposes of checking for someone
   * doing our work for us.
   *
   * If we fail too many times in a row, we fall back to synchronize_sched().
   */
  void synchronize_sched_expedited(void)
  {
  	long firstsnap, s, snap;
  	int trycount = 0;
  	struct rcu_state *rsp = &rcu_sched_state;
  
  	/*
  	 * If we are in danger of counter wrap, just do synchronize_sched().
  	 * By allowing sync_sched_expedited_started to advance no more than
  	 * ULONG_MAX/8 ahead of sync_sched_expedited_done, we are ensuring
  	 * that more than 3.5 billion CPUs would be required to force a
  	 * counter wrap on a 32-bit system.  Quite a few more CPUs would of
  	 * course be required on a 64-bit system.
  	 */
  	if (ULONG_CMP_GE((ulong)atomic_long_read(&rsp->expedited_start),
  			 (ulong)atomic_long_read(&rsp->expedited_done) +
  			 ULONG_MAX / 8)) {
  		synchronize_sched();
  		atomic_long_inc(&rsp->expedited_wrap);
  		return;
  	}
  
  	/*
  	 * Take a ticket.  Note that atomic_inc_return() implies a
  	 * full memory barrier.
  	 */
  	snap = atomic_long_inc_return(&rsp->expedited_start);
  	firstsnap = snap;
  	get_online_cpus();
  	WARN_ON_ONCE(cpu_is_offline(raw_smp_processor_id()));
  
  	/*
  	 * Each pass through the following loop attempts to force a
  	 * context switch on each CPU.
  	 */
  	while (try_stop_cpus(cpu_online_mask,
  			     synchronize_sched_expedited_cpu_stop,
  			     NULL) == -EAGAIN) {
  		put_online_cpus();
  		atomic_long_inc(&rsp->expedited_tryfail);
  
  		/* Check to see if someone else did our work for us. */
  		s = atomic_long_read(&rsp->expedited_done);
  		if (ULONG_CMP_GE((ulong)s, (ulong)firstsnap)) {
  			/* ensure test happens before caller kfree */
  			smp_mb__before_atomic_inc(); /* ^^^ */
  			atomic_long_inc(&rsp->expedited_workdone1);
  			return;
  		}
  
  		/* No joy, try again later.  Or just synchronize_sched(). */
  		if (trycount++ < 10) {
  			udelay(trycount * num_online_cpus());
  		} else {
  			wait_rcu_gp(call_rcu_sched);
  			atomic_long_inc(&rsp->expedited_normal);
  			return;
  		}
  
  		/* Recheck to see if someone else did our work for us. */
  		s = atomic_long_read(&rsp->expedited_done);
  		if (ULONG_CMP_GE((ulong)s, (ulong)firstsnap)) {
  			/* ensure test happens before caller kfree */
  			smp_mb__before_atomic_inc(); /* ^^^ */
  			atomic_long_inc(&rsp->expedited_workdone2);
  			return;
  		}
  
  		/*
  		 * Refetching sync_sched_expedited_started allows later
  		 * callers to piggyback on our grace period.  We retry
  		 * after they started, so our grace period works for them,
  		 * and they started after our first try, so their grace
  		 * period works for us.
  		 */
  		get_online_cpus();
  		snap = atomic_long_read(&rsp->expedited_start);
  		smp_mb(); /* ensure read is before try_stop_cpus(). */
  	}
  	atomic_long_inc(&rsp->expedited_stoppedcpus);
  
  	/*
  	 * Everyone up to our most recent fetch is covered by our grace
  	 * period.  Update the counter, but only if our work is still
  	 * relevant -- which it won't be if someone who started later
  	 * than we did already did their update.
  	 */
  	do {
  		atomic_long_inc(&rsp->expedited_done_tries);
  		s = atomic_long_read(&rsp->expedited_done);
  		if (ULONG_CMP_GE((ulong)s, (ulong)snap)) {
  			/* ensure test happens before caller kfree */
  			smp_mb__before_atomic_inc(); /* ^^^ */
  			atomic_long_inc(&rsp->expedited_done_lost);
  			break;
  		}
  	} while (atomic_long_cmpxchg(&rsp->expedited_done, s, snap) != s);
  	atomic_long_inc(&rsp->expedited_done_exit);
  
  	put_online_cpus();
  }
  EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
  
  /*
   * 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 (rcu_scheduler_fully_active &&
  	    rdp->qs_pending && !rdp->passed_quiesce) {
  		rdp->n_rp_qs_pending++;
  	} else if (rdp->qs_pending && rdp->passed_quiesce) {
  		rdp->n_rp_report_qs++;
  		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;
  	}
  
  	/* 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)
  {
  	struct rcu_state *rsp;
  
  	for_each_rcu_flavor(rsp)
  		if (__rcu_pending(rsp, per_cpu_ptr(rsp->rda, cpu)))
  			return 1;
  	return 0;
  }
  
  /*
   * Return true if the specified CPU has any callback.  If all_lazy is
   * non-NULL, store an indication of whether all callbacks are lazy.
   * (If there are no callbacks, all of them are deemed to be lazy.)
   */
  static int rcu_cpu_has_callbacks(int cpu, bool *all_lazy)
  {
  	bool al = true;
  	bool hc = false;
  	struct rcu_data *rdp;
  	struct rcu_state *rsp;
  
  	for_each_rcu_flavor(rsp) {
  		rdp = per_cpu_ptr(rsp->rda, cpu);
  		if (rdp->qlen != rdp->qlen_lazy)
  			al = false;
  		if (rdp->nxtlist)
  			hc = true;
  	}
  	if (all_lazy)
  		*all_lazy = al;
  	return hc;
  }
  
  /*
   * Helper function for _rcu_barrier() tracing.  If tracing is disabled,
   * the compiler is expected to optimize this away.
   */
  static void _rcu_barrier_trace(struct rcu_state *rsp, char *s,
  			       int cpu, unsigned long done)
  {
  	trace_rcu_barrier(rsp->name, s, cpu,
  			  atomic_read(&rsp->barrier_cpu_count), done);
  }
  
  /*
   * RCU callback function for _rcu_barrier().  If we are last, wake
   * up the task executing _rcu_barrier().
   */
  static void rcu_barrier_callback(struct rcu_head *rhp)
  {
  	struct rcu_data *rdp = container_of(rhp, struct rcu_data, barrier_head);
  	struct rcu_state *rsp = rdp->rsp;
  
  	if (atomic_dec_and_test(&rsp->barrier_cpu_count)) {
  		_rcu_barrier_trace(rsp, "LastCB", -1, rsp->n_barrier_done);
  		complete(&rsp->barrier_completion);
  	} else {
  		_rcu_barrier_trace(rsp, "CB", -1, rsp->n_barrier_done);
  	}
  }
  
  /*
   * Called with preemption disabled, and from cross-cpu IRQ context.
   */
  static void rcu_barrier_func(void *type)
  {
  	struct rcu_state *rsp = type;
  	struct rcu_data *rdp = __this_cpu_ptr(rsp->rda);
  
  	_rcu_barrier_trace(rsp, "IRQ", -1, rsp->n_barrier_done);
  	atomic_inc(&rsp->barrier_cpu_count);
  	rsp->call(&rdp->barrier_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)
  {
  	int cpu;
  	struct rcu_data *rdp;
  	unsigned long snap = ACCESS_ONCE(rsp->n_barrier_done);
  	unsigned long snap_done;
  
  	_rcu_barrier_trace(rsp, "Begin", -1, snap);
  
  	/* Take mutex to serialize concurrent rcu_barrier() requests. */
  	mutex_lock(&rsp->barrier_mutex);
  
  	/*
  	 * Ensure that all prior references, including to ->n_barrier_done,
  	 * are ordered before the _rcu_barrier() machinery.
  	 */
  	smp_mb();  /* See above block comment. */
  
  	/*
  	 * Recheck ->n_barrier_done to see if others did our work for us.
  	 * This means checking ->n_barrier_done for an even-to-odd-to-even
  	 * transition.  The "if" expression below therefore rounds the old
  	 * value up to the next even number and adds two before comparing.
  	 */
  	snap_done = ACCESS_ONCE(rsp->n_barrier_done);
  	_rcu_barrier_trace(rsp, "Check", -1, snap_done);
  	if (ULONG_CMP_GE(snap_done, ((snap + 1) & ~0x1) + 2)) {
  		_rcu_barrier_trace(rsp, "EarlyExit", -1, snap_done);
  		smp_mb(); /* caller's subsequent code after above check. */
  		mutex_unlock(&rsp->barrier_mutex);
  		return;
  	}
  
  	/*
  	 * Increment ->n_barrier_done to avoid duplicate work.  Use
  	 * ACCESS_ONCE() to prevent the compiler from speculating
  	 * the increment to precede the early-exit check.
  	 */
  	ACCESS_ONCE(rsp->n_barrier_done)++;
  	WARN_ON_ONCE((rsp->n_barrier_done & 0x1) != 1);
  	_rcu_barrier_trace(rsp, "Inc1", -1, rsp->n_barrier_done);
  	smp_mb(); /* Order ->n_barrier_done increment with below mechanism. */
  
  	/*
  	 * Initialize the count to one rather than to zero in order to
  	 * avoid a too-soon return to zero in case of a short grace period
  	 * (or preemption of this task).  Exclude CPU-hotplug operations
  	 * to ensure that no offline CPU has callbacks queued.
  	 */
  	init_completion(&rsp->barrier_completion);
  	atomic_set(&rsp->barrier_cpu_count, 1);
  	get_online_cpus();
  
  	/*
  	 * Force each CPU with callbacks to register a new callback.
  	 * When that callback is invoked, we will know that all of the
  	 * corresponding CPU's preceding callbacks have been invoked.
  	 */
  	for_each_possible_cpu(cpu) {
  		if (!cpu_online(cpu) && !rcu_is_nocb_cpu(cpu))
  			continue;
  		rdp = per_cpu_ptr(rsp->rda, cpu);
  		if (rcu_is_nocb_cpu(cpu)) {
  			_rcu_barrier_trace(rsp, "OnlineNoCB", cpu,
  					   rsp->n_barrier_done);
  			atomic_inc(&rsp->barrier_cpu_count);
  			__call_rcu(&rdp->barrier_head, rcu_barrier_callback,
  				   rsp, cpu, 0);
  		} else if (ACCESS_ONCE(rdp->qlen)) {
  			_rcu_barrier_trace(rsp, "OnlineQ", cpu,
  					   rsp->n_barrier_done);
  			smp_call_function_single(cpu, rcu_barrier_func, rsp, 1);
  		} else {
  			_rcu_barrier_trace(rsp, "OnlineNQ", cpu,
  					   rsp->n_barrier_done);
  		}
  	}
  	put_online_cpus();
  
  	/*
  	 * Now that we have an rcu_barrier_callback() callback on each
  	 * CPU, and thus each counted, remove the initial count.
  	 */
  	if (atomic_dec_and_test(&rsp->barrier_cpu_count))
  		complete(&rsp->barrier_completion);
  
  	/* Increment ->n_barrier_done to prevent duplicate work. */
  	smp_mb(); /* Keep increment after above mechanism. */
  	ACCESS_ONCE(rsp->n_barrier_done)++;
  	WARN_ON_ONCE((rsp->n_barrier_done & 0x1) != 0);
  	_rcu_barrier_trace(rsp, "Inc2", -1, rsp->n_barrier_done);
  	smp_mb(); /* Keep increment before caller's subsequent code. */
  
  	/* Wait for all rcu_barrier_callback() callbacks to be invoked. */
  	wait_for_completion(&rsp->barrier_completion);
  
  	/* Other rcu_barrier() invocations can now safely proceed. */
  	mutex_unlock(&rsp->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);
  }
  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);
  }
  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;
  	struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
  	struct rcu_node *rnp = rcu_get_root(rsp);
  
  	/* Set up local state, ensuring consistent view of global state. */
  	raw_spin_lock_irqsave(&rnp->lock, flags);
  	rdp->grpmask = 1UL << (cpu - rdp->mynode->grplo);
  	init_callback_list(rdp);
  	rdp->qlen_lazy = 0;
  	ACCESS_ONCE(rdp->qlen) = 0;
  	rdp->dynticks = &per_cpu(rcu_dynticks, cpu);
  	WARN_ON_ONCE(rdp->dynticks->dynticks_nesting != DYNTICK_TASK_EXIT_IDLE);
  	WARN_ON_ONCE(atomic_read(&rdp->dynticks->dynticks) != 1);
  	rdp->cpu = cpu;
  	rdp->rsp = rsp;
  	rcu_boot_init_nocb_percpu_data(rdp);
  	raw_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 preemptible)
  {
  	unsigned long flags;
  	unsigned long mask;
  	struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
  	struct rcu_node *rnp = rcu_get_root(rsp);
  
  	/* Exclude new grace periods. */
  	mutex_lock(&rsp->onoff_mutex);
  
  	/* Set up local state, ensuring consistent view of global state. */
  	raw_spin_lock_irqsave(&rnp->lock, flags);
  	rdp->beenonline = 1;	 /* We have now been online. */
  	rdp->preemptible = preemptible;
  	rdp->qlen_last_fqs_check = 0;
  	rdp->n_force_qs_snap = rsp->n_force_qs;
  	rdp->blimit = blimit;
  	init_callback_list(rdp);  /* Re-enable callbacks on this CPU. */
  	rdp->dynticks->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
  	atomic_set(&rdp->dynticks->dynticks,
  		   (atomic_read(&rdp->dynticks->dynticks) & ~0x1) + 1);
  	raw_spin_unlock(&rnp->lock);		/* irqs remain 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. */
  		raw_spin_lock(&rnp->lock);	/* irqs already disabled. */
  		rnp->qsmaskinit |= mask;
  		mask = rnp->grpmask;
  		if (rnp == rdp->mynode) {
  			/*
  			 * If there is a grace period in progress, we will
  			 * set up to wait for it next time we run the
  			 * RCU core code.
  			 */
  			rdp->gpnum = rnp->completed;
  			rdp->completed = rnp->completed;
  			rdp->passed_quiesce = 0;
  			rdp->qs_pending = 0;
  			trace_rcu_grace_period(rsp->name, rdp->gpnum, "cpuonl");
  		}
  		raw_spin_unlock(&rnp->lock); /* irqs already disabled. */
  		rnp = rnp->parent;
  	} while (rnp != NULL && !(rnp->qsmaskinit & mask));
  	local_irq_restore(flags);
  
  	mutex_unlock(&rsp->onoff_mutex);
  }
  
  static void __cpuinit rcu_prepare_cpu(int cpu)
  {
  	struct rcu_state *rsp;
  
  	for_each_rcu_flavor(rsp)
  		rcu_init_percpu_data(cpu, rsp,
  				     strcmp(rsp->name, "rcu_preempt") == 0);
  }
  
  /*
   * 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;
  	struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);
  	struct rcu_node *rnp = rdp->mynode;
  	struct rcu_state *rsp;
  
  	trace_rcu_utilization("Start CPU hotplug");
  	switch (action) {
  	case CPU_UP_PREPARE:
  	case CPU_UP_PREPARE_FROZEN:
  		rcu_prepare_cpu(cpu);
  		rcu_prepare_kthreads(cpu);
  		break;
  	case CPU_ONLINE:
  	case CPU_DOWN_FAILED:
  		rcu_boost_kthread_setaffinity(rnp, -1);
  		break;
  	case CPU_DOWN_PREPARE:
  		rcu_boost_kthread_setaffinity(rnp, cpu);
  		break;
  	case CPU_DYING:
  	case CPU_DYING_FROZEN:
  		for_each_rcu_flavor(rsp)
  			rcu_cleanup_dying_cpu(rsp);
  		break;
  	case CPU_DEAD:
  	case CPU_DEAD_FROZEN:
  	case CPU_UP_CANCELED:
  	case CPU_UP_CANCELED_FROZEN:
  		for_each_rcu_flavor(rsp)
  			rcu_cleanup_dead_cpu(cpu, rsp);
  		break;
  	default:
  		break;
  	}
  	trace_rcu_utilization("End CPU hotplug");
  	return NOTIFY_OK;
  }
  
  /*
   * Spawn the kthread that handles this RCU flavor's grace periods.
   */
  static int __init rcu_spawn_gp_kthread(void)
  {
  	unsigned long flags;
  	struct rcu_node *rnp;
  	struct rcu_state *rsp;
  	struct task_struct *t;
  
  	for_each_rcu_flavor(rsp) {
  		t = kthread_run(rcu_gp_kthread, rsp, rsp->name);
  		BUG_ON(IS_ERR(t));
  		rnp = rcu_get_root(rsp);
  		raw_spin_lock_irqsave(&rnp->lock, flags);
  		rsp->gp_kthread = t;
  		raw_spin_unlock_irqrestore(&rnp->lock, flags);
  		rcu_spawn_nocb_kthreads(rsp);
  	}
  	return 0;
  }
  early_initcall(rcu_spawn_gp_kthread);
  
  /*
   * 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.  This function also enables RCU lockdep checking.
   */
  void rcu_scheduler_starting(void)
  {
  	WARN_ON(num_online_cpus() != 1);
  	WARN_ON(nr_context_switches() > 0);
  	rcu_scheduler_active = 1;
  }
  
  /*
   * 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 = rcu_num_lvls - 1; i > 0; i--)
  		rsp->levelspread[i] = CONFIG_RCU_FANOUT;
  	rsp->levelspread[0] = rcu_fanout_leaf;
  }
  #else /* #ifdef CONFIG_RCU_FANOUT_EXACT */
  static void __init rcu_init_levelspread(struct rcu_state *rsp)
  {
  	int ccur;
  	int cprv;
  	int i;
  
  	cprv = nr_cpu_ids;
  	for (i = rcu_num_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,
  		struct rcu_data __percpu *rda)
  {
  	static char *buf[] = { "rcu_node_0",
  			       "rcu_node_1",
  			       "rcu_node_2",
  			       "rcu_node_3" };  /* Match MAX_RCU_LVLS */
  	static char *fqs[] = { "rcu_node_fqs_0",
  			       "rcu_node_fqs_1",
  			       "rcu_node_fqs_2",
  			       "rcu_node_fqs_3" };  /* Match MAX_RCU_LVLS */
  	int cpustride = 1;
  	int i;
  	int j;
  	struct rcu_node *rnp;
  
  	BUILD_BUG_ON(MAX_RCU_LVLS > ARRAY_SIZE(buf));  /* Fix buf[] init! */
  
  	/* Silence gcc 4.8 warning about array index out of range. */
  	if (rcu_num_lvls > RCU_NUM_LVLS)
  		panic("rcu_init_one: rcu_num_lvls overflow");
  
  	/* Initialize the level-tracking arrays. */
  
  	for (i = 0; i < rcu_num_lvls; i++)
  		rsp->levelcnt[i] = num_rcu_lvl[i];
  	for (i = 1; i < rcu_num_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 = rcu_num_lvls - 1; i >= 0; i--) {
  		cpustride *= rsp->levelspread[i];
  		rnp = rsp->level[i];
  		for (j = 0; j < rsp->levelcnt[i]; j++, rnp++) {
  			raw_spin_lock_init(&rnp->lock);
  			lockdep_set_class_and_name(&rnp->lock,
  						   &rcu_node_class[i], buf[i]);
  			raw_spin_lock_init(&rnp->fqslock);
  			lockdep_set_class_and_name(&rnp->fqslock,
  						   &rcu_fqs_class[i], fqs[i]);
  			rnp->gpnum = rsp->gpnum;
  			rnp->completed = rsp->completed;
  			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->blkd_tasks);
  			rcu_init_one_nocb(rnp);
  		}
  	}
  
  	rsp->rda = rda;
  	init_waitqueue_head(&rsp->gp_wq);
  	init_irq_work(&rsp->wakeup_work, rsp_wakeup);
  	rnp = rsp->level[rcu_num_lvls - 1];
  	for_each_possible_cpu(i) {
  		while (i > rnp->grphi)
  			rnp++;
  		per_cpu_ptr(rsp->rda, i)->mynode = rnp;
  		rcu_boot_init_percpu_data(i, rsp);
  	}
  	list_add(&rsp->flavors, &rcu_struct_flavors);
  }
  
  /*
   * Compute the rcu_node tree geometry from kernel parameters.  This cannot
   * replace the definitions in rcutree.h because those are needed to size
   * the ->node array in the rcu_state structure.
   */
  static void __init rcu_init_geometry(void)
  {
  	int i;
  	int j;
  	int n = nr_cpu_ids;
  	int rcu_capacity[MAX_RCU_LVLS + 1];
  
  	/* If the compile-time values are accurate, just leave. */
  	if (rcu_fanout_leaf == CONFIG_RCU_FANOUT_LEAF &&
  	    nr_cpu_ids == NR_CPUS)
  		return;
  
  	/*
  	 * Compute number of nodes that can be handled an rcu_node tree
  	 * with the given number of levels.  Setting rcu_capacity[0] makes
  	 * some of the arithmetic easier.
  	 */
  	rcu_capacity[0] = 1;
  	rcu_capacity[1] = rcu_fanout_leaf;
  	for (i = 2; i <= MAX_RCU_LVLS; i++)
  		rcu_capacity[i] = rcu_capacity[i - 1] * CONFIG_RCU_FANOUT;
  
  	/*
  	 * The boot-time rcu_fanout_leaf parameter is only permitted
  	 * to increase the leaf-level fanout, not decrease it.  Of course,
  	 * the leaf-level fanout cannot exceed the number of bits in
  	 * the rcu_node masks.  Finally, the tree must be able to accommodate
  	 * the configured number of CPUs.  Complain and fall back to the
  	 * compile-time values if these limits are exceeded.
  	 */
  	if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
  	    rcu_fanout_leaf > sizeof(unsigned long) * 8 ||
  	    n > rcu_capacity[MAX_RCU_LVLS]) {
  		WARN_ON(1);
  		return;
  	}
  
  	/* Calculate the number of rcu_nodes at each level of the tree. */
  	for (i = 1; i <= MAX_RCU_LVLS; i++)
  		if (n <= rcu_capacity[i]) {
  			for (j = 0; j <= i; j++)
  				num_rcu_lvl[j] =
  					DIV_ROUND_UP(n, rcu_capacity[i - j]);
  			rcu_num_lvls = i;
  			for (j = i + 1; j <= MAX_RCU_LVLS; j++)
  				num_rcu_lvl[j] = 0;
  			break;
  		}
  
  	/* Calculate the total number of rcu_node structures. */
  	rcu_num_nodes = 0;
  	for (i = 0; i <= MAX_RCU_LVLS; i++)
  		rcu_num_nodes += num_rcu_lvl[i];
  	rcu_num_nodes -= n;
  }
  
  void __init rcu_init(void)
  {
  	int cpu;
  
  	rcu_bootup_announce();
  	rcu_init_geometry();
  	rcu_init_one(&rcu_sched_state, &rcu_sched_data);
  	rcu_init_one(&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(cpu)
  		rcu_cpu_notify(NULL, CPU_UP_PREPARE, (void *)(long)cpu);
  }
  
  #include "rcutree_plugin.h"
  