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, you can access it online at
   * http://www.gnu.org/licenses/gpl-2.0.html.
   *
   * 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 <linux/trace_events.h>
  #include <linux/suspend.h>
  
  #include "tree.h"
  #include "rcu.h"
  
  #ifdef MODULE_PARAM_PREFIX
  #undef MODULE_PARAM_PREFIX
  #endif
  #define MODULE_PARAM_PREFIX "rcutree."
  
  /* Data structures. */
  
  /*
   * In order to export the rcu_state name to the tracing tools, it
   * needs to be added in the __tracepoint_string section.
   * This requires defining a separate variable tp_<sname>_varname
   * that points to the string being used, and this will allow
   * the tracing userspace tools to be able to decipher the string
   * address to the matching string.
   */
  #ifdef CONFIG_TRACING
  # define DEFINE_RCU_TPS(sname) \
  static char sname##_varname[] = #sname; \
  static const char *tp_##sname##_varname __used __tracepoint_string = sname##_varname;
  # define RCU_STATE_NAME(sname) sname##_varname
  #else
  # define DEFINE_RCU_TPS(sname)
  # define RCU_STATE_NAME(sname) __stringify(sname)
  #endif
  
  #define RCU_STATE_INITIALIZER(sname, sabbr, cr) \
  DEFINE_RCU_TPS(sname) \
  static DEFINE_PER_CPU_SHARED_ALIGNED(struct rcu_data, sname##_data); \
  struct rcu_state sname##_state = { \
  	.level = { &sname##_state.node[0] }, \
  	.rda = &sname##_data, \
  	.call = cr, \
  	.gp_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), \
  	.name = RCU_STATE_NAME(sname), \
  	.abbr = sabbr, \
  	.exp_mutex = __MUTEX_INITIALIZER(sname##_state.exp_mutex), \
  	.exp_wake_mutex = __MUTEX_INITIALIZER(sname##_state.exp_wake_mutex), \
  }
  
  RCU_STATE_INITIALIZER(rcu_sched, 's', call_rcu_sched);
  RCU_STATE_INITIALIZER(rcu_bh, 'b', call_rcu_bh);
  
  static struct rcu_state *const rcu_state_p;
  LIST_HEAD(rcu_struct_flavors);
  
  /* Dump rcu_node combining tree at boot to verify correct setup. */
  static bool dump_tree;
  module_param(dump_tree, bool, 0444);
  /* Control rcu_node-tree auto-balancing at boot time. */
  static bool rcu_fanout_exact;
  module_param(rcu_fanout_exact, bool, 0444);
  /* Increase (but not decrease) the RCU_FANOUT_LEAF at boot time. */
  static int rcu_fanout_leaf = RCU_FANOUT_LEAF;
  module_param(rcu_fanout_leaf, int, 0444);
  int rcu_num_lvls __read_mostly = RCU_NUM_LVLS;
  /* Number of rcu_nodes at specified level. */
  static int num_rcu_lvl[] = NUM_RCU_LVL_INIT;
  int rcu_num_nodes __read_mostly = NUM_RCU_NODES; /* Total # rcu_nodes in use. */
  /* panic() on RCU Stall sysctl. */
  int sysctl_panic_on_rcu_stall __read_mostly;
  
  /*
   * The rcu_scheduler_active variable is initialized to the value
   * RCU_SCHEDULER_INACTIVE and transitions RCU_SCHEDULER_INIT just before the
   * first task is spawned.  So when this variable is RCU_SCHEDULER_INACTIVE,
   * RCU can assume that there is but one task, allowing RCU to (for example)
   * optimize synchronize_rcu() to a simple barrier().  When this variable
   * is RCU_SCHEDULER_INIT, 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.  Finally, it
   * transitions from RCU_SCHEDULER_INIT to RCU_SCHEDULER_RUNNING after RCU
   * is fully initialized, including all of its kthreads having been spawned.
   */
  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;
  
  static void rcu_init_new_rnp(struct rcu_node *rnp_leaf);
  static void rcu_cleanup_dead_rnp(struct rcu_node *rnp_leaf);
  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);
  static void rcu_report_exp_rdp(struct rcu_state *rsp,
  			       struct rcu_data *rdp, bool wake);
  static void sync_sched_exp_online_cleanup(int cpu);
  
  /* rcuc/rcub kthread realtime priority */
  #ifdef CONFIG_RCU_KTHREAD_PRIO
  static int kthread_prio = CONFIG_RCU_KTHREAD_PRIO;
  #else /* #ifdef CONFIG_RCU_KTHREAD_PRIO */
  static int kthread_prio = IS_ENABLED(CONFIG_RCU_BOOST) ? 1 : 0;
  #endif /* #else #ifdef CONFIG_RCU_KTHREAD_PRIO */
  module_param(kthread_prio, int, 0644);
  
  /* Delay in jiffies for grace-period initialization delays, debug only. */
  
  #ifdef CONFIG_RCU_TORTURE_TEST_SLOW_PREINIT
  static int gp_preinit_delay = CONFIG_RCU_TORTURE_TEST_SLOW_PREINIT_DELAY;
  module_param(gp_preinit_delay, int, 0644);
  #else /* #ifdef CONFIG_RCU_TORTURE_TEST_SLOW_PREINIT */
  static const int gp_preinit_delay;
  #endif /* #else #ifdef CONFIG_RCU_TORTURE_TEST_SLOW_PREINIT */
  
  #ifdef CONFIG_RCU_TORTURE_TEST_SLOW_INIT
  static int gp_init_delay = CONFIG_RCU_TORTURE_TEST_SLOW_INIT_DELAY;
  module_param(gp_init_delay, int, 0644);
  #else /* #ifdef CONFIG_RCU_TORTURE_TEST_SLOW_INIT */
  static const int gp_init_delay;
  #endif /* #else #ifdef CONFIG_RCU_TORTURE_TEST_SLOW_INIT */
  
  #ifdef CONFIG_RCU_TORTURE_TEST_SLOW_CLEANUP
  static int gp_cleanup_delay = CONFIG_RCU_TORTURE_TEST_SLOW_CLEANUP_DELAY;
  module_param(gp_cleanup_delay, int, 0644);
  #else /* #ifdef CONFIG_RCU_TORTURE_TEST_SLOW_CLEANUP */
  static const int gp_cleanup_delay;
  #endif /* #else #ifdef CONFIG_RCU_TORTURE_TEST_SLOW_CLEANUP */
  
  /*
   * Number of grace periods between delays, normalized by the duration of
   * the delay.  The longer the the delay, the more the grace periods between
   * each delay.  The reason for this normalization is that it means that,
   * for non-zero delays, the overall slowdown of grace periods is constant
   * regardless of the duration of the delay.  This arrangement balances
   * the need for long delays to increase some race probabilities with the
   * need for fast grace periods to increase other race probabilities.
   */
  #define PER_RCU_NODE_PERIOD 3	/* Number of grace periods between delays. */
  
  /*
   * 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;
  
  /*
   * Compute the mask of online CPUs for the specified rcu_node structure.
   * This will not be stable unless the rcu_node structure's ->lock is
   * held, but the bit corresponding to the current CPU will be stable
   * in most contexts.
   */
  unsigned long rcu_rnp_online_cpus(struct rcu_node *rnp)
  {
  	return READ_ONCE(rnp->qsmaskinitnext);
  }
  
  /*
   * Return true if an RCU grace period is in progress.  The READ_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 READ_ONCE(rsp->completed) != READ_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(void)
  {
  	if (!__this_cpu_read(rcu_sched_data.cpu_no_qs.s))
  		return;
  	trace_rcu_grace_period(TPS("rcu_sched"),
  			       __this_cpu_read(rcu_sched_data.gpnum),
  			       TPS("cpuqs"));
  	__this_cpu_write(rcu_sched_data.cpu_no_qs.b.norm, false);
  	if (!__this_cpu_read(rcu_sched_data.cpu_no_qs.b.exp))
  		return;
  	__this_cpu_write(rcu_sched_data.cpu_no_qs.b.exp, false);
  	rcu_report_exp_rdp(&rcu_sched_state,
  			   this_cpu_ptr(&rcu_sched_data), true);
  }
  
  void rcu_bh_qs(void)
  {
  	if (__this_cpu_read(rcu_bh_data.cpu_no_qs.s)) {
  		trace_rcu_grace_period(TPS("rcu_bh"),
  				       __this_cpu_read(rcu_bh_data.gpnum),
  				       TPS("cpuqs"));
  		__this_cpu_write(rcu_bh_data.cpu_no_qs.b.norm, false);
  	}
  }
  
  static DEFINE_PER_CPU(int, rcu_sched_qs_mask);
  
  static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
  	.dynticks_nesting = DYNTICK_TASK_EXIT_IDLE,
  	.dynticks = ATOMIC_INIT(1),
  #ifdef CONFIG_NO_HZ_FULL_SYSIDLE
  	.dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE,
  	.dynticks_idle = ATOMIC_INIT(1),
  #endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
  };
  
  DEFINE_PER_CPU_SHARED_ALIGNED(unsigned long, rcu_qs_ctr);
  EXPORT_PER_CPU_SYMBOL_GPL(rcu_qs_ctr);
  
  /*
   * Let the RCU core know that this CPU has gone through the scheduler,
   * which is a quiescent state.  This is called when the need for a
   * quiescent state is urgent, so we burn an atomic operation and full
   * memory barriers to let the RCU core know about it, regardless of what
   * this CPU might (or might not) do in the near future.
   *
   * We inform the RCU core by emulating a zero-duration dyntick-idle
   * period, which we in turn do by incrementing the ->dynticks counter
   * by two.
   *
   * The caller must have disabled interrupts.
   */
  static void rcu_momentary_dyntick_idle(void)
  {
  	struct rcu_data *rdp;
  	struct rcu_dynticks *rdtp;
  	int resched_mask;
  	struct rcu_state *rsp;
  
  	/*
  	 * Yes, we can lose flag-setting operations.  This is OK, because
  	 * the flag will be set again after some delay.
  	 */
  	resched_mask = raw_cpu_read(rcu_sched_qs_mask);
  	raw_cpu_write(rcu_sched_qs_mask, 0);
  
  	/* Find the flavor that needs a quiescent state. */
  	for_each_rcu_flavor(rsp) {
  		rdp = raw_cpu_ptr(rsp->rda);
  		if (!(resched_mask & rsp->flavor_mask))
  			continue;
  		smp_mb(); /* rcu_sched_qs_mask before cond_resched_completed. */
  		if (READ_ONCE(rdp->mynode->completed) !=
  		    READ_ONCE(rdp->cond_resched_completed))
  			continue;
  
  		/*
  		 * Pretend to be momentarily idle for the quiescent state.
  		 * This allows the grace-period kthread to record the
  		 * quiescent state, with no need for this CPU to do anything
  		 * further.
  		 */
  		rdtp = this_cpu_ptr(&rcu_dynticks);
  		smp_mb__before_atomic(); /* Earlier stuff before QS. */
  		atomic_add(2, &rdtp->dynticks);  /* QS. */
  		smp_mb__after_atomic(); /* Later stuff after QS. */
  		break;
  	}
  }
  
  /*
   * Note a context switch.  This is a quiescent state for RCU-sched,
   * and requires special handling for preemptible RCU.
   * The caller must have disabled interrupts.
   */
  void rcu_note_context_switch(void)
  {
  	barrier(); /* Avoid RCU read-side critical sections leaking down. */
  	trace_rcu_utilization(TPS("Start context switch"));
  	rcu_sched_qs();
  	rcu_preempt_note_context_switch();
  	if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
  		rcu_momentary_dyntick_idle();
  	trace_rcu_utilization(TPS("End context switch"));
  	barrier(); /* Avoid RCU read-side critical sections leaking up. */
  }
  EXPORT_SYMBOL_GPL(rcu_note_context_switch);
  
  /*
   * Register a quiescent state for all RCU flavors.  If there is an
   * emergency, invoke rcu_momentary_dyntick_idle() to do a heavy-weight
   * dyntick-idle quiescent state visible to other CPUs (but only for those
   * RCU flavors in desperate need of a quiescent state, which will normally
   * be none of them).  Either way, do a lightweight quiescent state for
   * all RCU flavors.
   *
   * The barrier() calls are redundant in the common case when this is
   * called externally, but just in case this is called from within this
   * file.
   *
   */
  void rcu_all_qs(void)
  {
  	unsigned long flags;
  
  	barrier(); /* Avoid RCU read-side critical sections leaking down. */
  	if (unlikely(raw_cpu_read(rcu_sched_qs_mask))) {
  		local_irq_save(flags);
  		rcu_momentary_dyntick_idle();
  		local_irq_restore(flags);
  	}
  	if (unlikely(raw_cpu_read(rcu_sched_data.cpu_no_qs.b.exp))) {
  		/*
  		 * Yes, we just checked a per-CPU variable with preemption
  		 * enabled, so we might be migrated to some other CPU at
  		 * this point.  That is OK because in that case, the
  		 * migration will supply the needed quiescent state.
  		 * We might end up needlessly disabling preemption and
  		 * invoking rcu_sched_qs() on the destination CPU, but
  		 * the probability and cost are both quite low, so this
  		 * should not be a problem in practice.
  		 */
  		preempt_disable();
  		rcu_sched_qs();
  		preempt_enable();
  	}
  	this_cpu_inc(rcu_qs_ctr);
  	barrier(); /* Avoid RCU read-side critical sections leaking up. */
  }
  EXPORT_SYMBOL_GPL(rcu_all_qs);
  
  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 = ULONG_MAX;
  static ulong jiffies_till_next_fqs = ULONG_MAX;
  static bool rcu_kick_kthreads;
  
  module_param(jiffies_till_first_fqs, ulong, 0644);
  module_param(jiffies_till_next_fqs, ulong, 0644);
  module_param(rcu_kick_kthreads, bool, 0644);
  
  /*
   * How long the grace period must be before we start recruiting
   * quiescent-state help from rcu_note_context_switch().
   */
  static ulong jiffies_till_sched_qs = HZ / 20;
  module_param(jiffies_till_sched_qs, ulong, 0644);
  
  static bool 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 *rsp, bool *isidle,
  				  unsigned long *maxj),
  			 bool *isidle, unsigned long *maxj);
  static void force_quiescent_state(struct rcu_state *rsp);
  static int rcu_pending(void);
  
  /*
   * Return the number of RCU batches started thus far for debug & stats.
   */
  unsigned long rcu_batches_started(void)
  {
  	return rcu_state_p->gpnum;
  }
  EXPORT_SYMBOL_GPL(rcu_batches_started);
  
  /*
   * Return the number of RCU-sched batches started thus far for debug & stats.
   */
  unsigned long rcu_batches_started_sched(void)
  {
  	return rcu_sched_state.gpnum;
  }
  EXPORT_SYMBOL_GPL(rcu_batches_started_sched);
  
  /*
   * Return the number of RCU BH batches started thus far for debug & stats.
   */
  unsigned long rcu_batches_started_bh(void)
  {
  	return rcu_bh_state.gpnum;
  }
  EXPORT_SYMBOL_GPL(rcu_batches_started_bh);
  
  /*
   * Return the number of RCU batches completed thus far for debug & stats.
   */
  unsigned long rcu_batches_completed(void)
  {
  	return rcu_state_p->completed;
  }
  EXPORT_SYMBOL_GPL(rcu_batches_completed);
  
  /*
   * Return the number of RCU-sched batches completed thus far for debug & stats.
   */
  unsigned 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 completed thus far for debug & stats.
   */
  unsigned long rcu_batches_completed_bh(void)
  {
  	return rcu_bh_state.completed;
  }
  EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);
  
  /*
   * Return the number of RCU expedited batches completed thus far for
   * debug & stats.  Odd numbers mean that a batch is in progress, even
   * numbers mean idle.  The value returned will thus be roughly double
   * the cumulative batches since boot.
   */
  unsigned long rcu_exp_batches_completed(void)
  {
  	return rcu_state_p->expedited_sequence;
  }
  EXPORT_SYMBOL_GPL(rcu_exp_batches_completed);
  
  /*
   * Return the number of RCU-sched expedited batches completed thus far
   * for debug & stats.  Similar to rcu_exp_batches_completed().
   */
  unsigned long rcu_exp_batches_completed_sched(void)
  {
  	return rcu_sched_state.expedited_sequence;
  }
  EXPORT_SYMBOL_GPL(rcu_exp_batches_completed_sched);
  
  /*
   * Force a quiescent state.
   */
  void rcu_force_quiescent_state(void)
  {
  	force_quiescent_state(rcu_state_p);
  }
  EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);
  
  /*
   * 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);
  
  /*
   * 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);
  
  /*
   * Show the state of the grace-period kthreads.
   */
  void show_rcu_gp_kthreads(void)
  {
  	struct rcu_state *rsp;
  
  	for_each_rcu_flavor(rsp) {
  		pr_info("%s: wait state: %d ->state: %#lx\n",
  			rsp->name, rsp->gp_state, rsp->gp_kthread->state);
  		/* sched_show_task(rsp->gp_kthread); */
  	}
  }
  EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);
  
  /*
   * 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);
  
  /*
   * Send along grace-period-related data for rcutorture diagnostics.
   */
  void rcutorture_get_gp_data(enum rcutorture_type test_type, int *flags,
  			    unsigned long *gpnum, unsigned long *completed)
  {
  	struct rcu_state *rsp = NULL;
  
  	switch (test_type) {
  	case RCU_FLAVOR:
  		rsp = rcu_state_p;
  		break;
  	case RCU_BH_FLAVOR:
  		rsp = &rcu_bh_state;
  		break;
  	case RCU_SCHED_FLAVOR:
  		rsp = &rcu_sched_state;
  		break;
  	default:
  		break;
  	}
  	if (rsp != NULL) {
  		*flags = READ_ONCE(rsp->gp_flags);
  		*gpnum = READ_ONCE(rsp->gpnum);
  		*completed = READ_ONCE(rsp->completed);
  		return;
  	}
  	*flags = 0;
  	*gpnum = 0;
  	*completed = 0;
  }
  EXPORT_SYMBOL_GPL(rcutorture_get_gp_data);
  
  /*
   * 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);
  
  /*
   * 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;
  }
  
  /*
   * 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];
  }
  
  /*
   * Is there any need for future grace periods?
   * Interrupts must be disabled.  If the caller does not hold the root
   * rnp_node structure's ->lock, the results are advisory only.
   */
  static int rcu_future_needs_gp(struct rcu_state *rsp)
  {
  	struct rcu_node *rnp = rcu_get_root(rsp);
  	int idx = (READ_ONCE(rnp->completed) + 1) & 0x1;
  	int *fp = &rnp->need_future_gp[idx];
  
  	return READ_ONCE(*fp);
  }
  
  /*
   * 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 bool
  cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
  {
  	int i;
  
  	if (rcu_gp_in_progress(rsp))
  		return false;  /* No, a grace period is already in progress. */
  	if (rcu_future_needs_gp(rsp))
  		return true;  /* Yes, a no-CBs CPU needs one. */
  	if (!rdp->nxttail[RCU_NEXT_TAIL])
  		return false;  /* No, this is a no-CBs (or offline) CPU. */
  	if (*rdp->nxttail[RCU_NEXT_READY_TAIL])
  		return true;  /* Yes, 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(READ_ONCE(rsp->completed),
  				 rdp->nxtcompleted[i]))
  			return true;  /* Yes, CBs for future grace period. */
  	return false; /* No grace period needed. */
  }
  
  /*
   * 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(long long oldval, bool user)
  {
  	struct rcu_state *rsp;
  	struct rcu_data *rdp;
  	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
  
  	trace_rcu_dyntick(TPS("Start"), oldval, rdtp->dynticks_nesting);
  	if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
  	    !user && !is_idle_task(current)) {
  		struct task_struct *idle __maybe_unused =
  			idle_task(smp_processor_id());
  
  		trace_rcu_dyntick(TPS("Error on entry: not idle task"), oldval, 0);
  		rcu_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! */
  	}
  	for_each_rcu_flavor(rsp) {
  		rdp = this_cpu_ptr(rsp->rda);
  		do_nocb_deferred_wakeup(rdp);
  	}
  	rcu_prepare_for_idle();
  	/* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */
  	smp_mb__before_atomic();  /* See above. */
  	atomic_inc(&rdtp->dynticks);
  	smp_mb__after_atomic();  /* Force ordering with next sojourn. */
  	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
  		     atomic_read(&rdtp->dynticks) & 0x1);
  	rcu_dynticks_task_enter();
  
  	/*
  	 * It is illegal to enter an extended quiescent state while
  	 * in an RCU read-side critical section.
  	 */
  	RCU_LOCKDEP_WARN(lock_is_held(&rcu_lock_map),
  			 "Illegal idle entry in RCU read-side critical section.");
  	RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map),
  			 "Illegal idle entry in RCU-bh read-side critical section.");
  	RCU_LOCKDEP_WARN(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 = this_cpu_ptr(&rcu_dynticks);
  	oldval = rdtp->dynticks_nesting;
  	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
  		     (oldval & DYNTICK_TASK_NEST_MASK) == 0);
  	if ((oldval & DYNTICK_TASK_NEST_MASK) == DYNTICK_TASK_NEST_VALUE) {
  		rdtp->dynticks_nesting = 0;
  		rcu_eqs_enter_common(oldval, user);
  	} else {
  		rdtp->dynticks_nesting -= DYNTICK_TASK_NEST_VALUE;
  	}
  }
  
  /**
   * 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);
  	rcu_sysidle_enter(0);
  	local_irq_restore(flags);
  }
  EXPORT_SYMBOL_GPL(rcu_idle_enter);
  
  #ifdef CONFIG_NO_HZ_FULL
  /**
   * 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);
  }
  #endif /* CONFIG_NO_HZ_FULL */
  
  /**
   * 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.  The caller must have disabled interrupts.
   *
   * 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)
  {
  	long long oldval;
  	struct rcu_dynticks *rdtp;
  
  	rdtp = this_cpu_ptr(&rcu_dynticks);
  
  	/* Page faults can happen in NMI handlers, so check... */
  	if (READ_ONCE(rdtp->dynticks_nmi_nesting))
  		return;
  
  	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_exit() invoked with irqs enabled!!!");
  	oldval = rdtp->dynticks_nesting;
  	rdtp->dynticks_nesting--;
  	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
  		     rdtp->dynticks_nesting < 0);
  	if (rdtp->dynticks_nesting)
  		trace_rcu_dyntick(TPS("--="), oldval, rdtp->dynticks_nesting);
  	else
  		rcu_eqs_enter_common(oldval, true);
  	rcu_sysidle_enter(1);
  }
  
  /*
   * Wrapper for rcu_irq_exit() where interrupts are enabled.
   */
  void rcu_irq_exit_irqson(void)
  {
  	unsigned long flags;
  
  	local_irq_save(flags);
  	rcu_irq_exit();
  	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(long long oldval, int user)
  {
  	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
  
  	rcu_dynticks_task_exit();
  	smp_mb__before_atomic();  /* 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();  /* See above. */
  	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
  		     !(atomic_read(&rdtp->dynticks) & 0x1));
  	rcu_cleanup_after_idle();
  	trace_rcu_dyntick(TPS("End"), oldval, rdtp->dynticks_nesting);
  	if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
  	    !user && !is_idle_task(current)) {
  		struct task_struct *idle __maybe_unused =
  			idle_task(smp_processor_id());
  
  		trace_rcu_dyntick(TPS("Error on exit: not idle task"),
  				  oldval, rdtp->dynticks_nesting);
  		rcu_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 = this_cpu_ptr(&rcu_dynticks);
  	oldval = rdtp->dynticks_nesting;
  	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && 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(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);
  	rcu_sysidle_exit(0);
  	local_irq_restore(flags);
  }
  EXPORT_SYMBOL_GPL(rcu_idle_exit);
  
  #ifdef CONFIG_NO_HZ_FULL
  /**
   * 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);
  }
  #endif /* CONFIG_NO_HZ_FULL */
  
  /**
   * 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.  The caller must have disabled interrupts.
   *
   * 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)
  {
  	struct rcu_dynticks *rdtp;
  	long long oldval;
  
  	rdtp = this_cpu_ptr(&rcu_dynticks);
  
  	/* Page faults can happen in NMI handlers, so check... */
  	if (READ_ONCE(rdtp->dynticks_nmi_nesting))
  		return;
  
  	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_enter() invoked with irqs enabled!!!");
  	oldval = rdtp->dynticks_nesting;
  	rdtp->dynticks_nesting++;
  	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
  		     rdtp->dynticks_nesting == 0);
  	if (oldval)
  		trace_rcu_dyntick(TPS("++="), oldval, rdtp->dynticks_nesting);
  	else
  		rcu_eqs_exit_common(oldval, true);
  	rcu_sysidle_exit(1);
  }
  
  /*
   * Wrapper for rcu_irq_enter() where interrupts are enabled.
   */
  void rcu_irq_enter_irqson(void)
  {
  	unsigned long flags;
  
  	local_irq_save(flags);
  	rcu_irq_enter();
  	local_irq_restore(flags);
  }
  
  /**
   * rcu_nmi_enter - inform RCU of entry to NMI context
   *
   * If the CPU was idle from RCU's viewpoint, update rdtp->dynticks and
   * rdtp->dynticks_nmi_nesting to let the RCU grace-period handling know
   * that the CPU is active.  This implementation permits nested NMIs, as
   * long as the nesting level does not overflow an int.  (You will probably
   * run out of stack space first.)
   */
  void rcu_nmi_enter(void)
  {
  	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
  	int incby = 2;
  
  	/* Complain about underflow. */
  	WARN_ON_ONCE(rdtp->dynticks_nmi_nesting < 0);
  
  	/*
  	 * If idle from RCU viewpoint, atomically increment ->dynticks
  	 * to mark non-idle and increment ->dynticks_nmi_nesting by one.
  	 * Otherwise, increment ->dynticks_nmi_nesting by two.  This means
  	 * if ->dynticks_nmi_nesting is equal to one, we are guaranteed
  	 * to be in the outermost NMI handler that interrupted an RCU-idle
  	 * period (observation due to Andy Lutomirski).
  	 */
  	if (!(atomic_read(&rdtp->dynticks) & 0x1)) {
  		smp_mb__before_atomic();  /* Force delay from prior write. */
  		atomic_inc(&rdtp->dynticks);
  		/* atomic_inc() before later RCU read-side crit sects */
  		smp_mb__after_atomic();  /* See above. */
  		WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
  		incby = 1;
  	}
  	rdtp->dynticks_nmi_nesting += incby;
  	barrier();
  }
  
  /**
   * rcu_nmi_exit - inform RCU of exit from NMI context
   *
   * If we are returning from the outermost NMI handler that interrupted an
   * RCU-idle period, update rdtp->dynticks and rdtp->dynticks_nmi_nesting
   * to let the RCU grace-period handling know that the CPU is back to
   * being RCU-idle.
   */
  void rcu_nmi_exit(void)
  {
  	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
  
  	/*
  	 * Check for ->dynticks_nmi_nesting underflow and bad ->dynticks.
  	 * (We are exiting an NMI handler, so RCU better be paying attention
  	 * to us!)
  	 */
  	WARN_ON_ONCE(rdtp->dynticks_nmi_nesting <= 0);
  	WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
  
  	/*
  	 * If the nesting level is not 1, the CPU wasn't RCU-idle, so
  	 * leave it in non-RCU-idle state.
  	 */
  	if (rdtp->dynticks_nmi_nesting != 1) {
  		rdtp->dynticks_nmi_nesting -= 2;
  		return;
  	}
  
  	/* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
  	rdtp->dynticks_nmi_nesting = 0;
  	/* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */
  	smp_mb__before_atomic();  /* See above. */
  	atomic_inc(&rdtp->dynticks);
  	smp_mb__after_atomic();  /* Force delay to next write. */
  	WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1);
  }
  
  /**
   * __rcu_is_watching - are RCU read-side critical sections safe?
   *
   * Return true if RCU is watching the running CPU, which means that
   * this CPU can safely enter RCU read-side critical sections.  Unlike
   * rcu_is_watching(), the caller of __rcu_is_watching() must have at
   * least disabled preemption.
   */
  bool notrace __rcu_is_watching(void)
  {
  	return atomic_read(this_cpu_ptr(&rcu_dynticks.dynticks)) & 0x1;
  }
  
  /**
   * rcu_is_watching - 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.
   */
  bool notrace rcu_is_watching(void)
  {
  	bool ret;
  
  	preempt_disable_notrace();
  	ret = __rcu_is_watching();
  	preempt_enable_notrace();
  	return ret;
  }
  EXPORT_SYMBOL_GPL(rcu_is_watching);
  
  #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 teardown
   * of the CPU.
   *
   * This is also why RCU internally marks CPUs online during in the
   * preparation phase and offline after the CPU has been taken down.
   *
   * 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 true;
  	preempt_disable();
  	rdp = this_cpu_ptr(&rcu_sched_data);
  	rnp = rdp->mynode;
  	ret = (rdp->grpmask & rcu_rnp_online_cpus(rnp)) ||
  	      !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 __this_cpu_read(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,
  					 bool *isidle, unsigned long *maxj)
  {
  	rdp->dynticks_snap = atomic_add_return(0, &rdp->dynticks->dynticks);
  	rcu_sysidle_check_cpu(rdp, isidle, maxj);
  	if ((rdp->dynticks_snap & 0x1) == 0) {
  		trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("dti"));
  		if (ULONG_CMP_LT(READ_ONCE(rdp->gpnum) + ULONG_MAX / 4,
  				 rdp->mynode->gpnum))
  			WRITE_ONCE(rdp->gpwrap, true);
  		return 1;
  	}
  	return 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,
  				    bool *isidle, unsigned long *maxj)
  {
  	unsigned int curr;
  	int *rcrmp;
  	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, TPS("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, TPS("ofl"));
  		rdp->offline_fqs++;
  		return 1;
  	}
  
  	/*
  	 * A CPU running for an extended time within the kernel can
  	 * delay RCU grace periods.  When the CPU is in NO_HZ_FULL mode,
  	 * even context-switching back and forth between a pair of
  	 * in-kernel CPU-bound tasks cannot advance grace periods.
  	 * So if the grace period is old enough, make the CPU pay attention.
  	 * Note that the unsynchronized assignments to the per-CPU
  	 * rcu_sched_qs_mask variable are safe.  Yes, setting of
  	 * bits can be lost, but they will be set again on the next
  	 * force-quiescent-state pass.  So lost bit sets do not result
  	 * in incorrect behavior, merely in a grace period lasting
  	 * a few jiffies longer than it might otherwise.  Because
  	 * there are at most four threads involved, and because the
  	 * updates are only once every few jiffies, the probability of
  	 * lossage (and thus of slight grace-period extension) is
  	 * quite low.
  	 *
  	 * Note that if the jiffies_till_sched_qs boot/sysfs parameter
  	 * is set too high, we override with half of the RCU CPU stall
  	 * warning delay.
  	 */
  	rcrmp = &per_cpu(rcu_sched_qs_mask, rdp->cpu);
  	if (ULONG_CMP_GE(jiffies,
  			 rdp->rsp->gp_start + jiffies_till_sched_qs) ||
  	    ULONG_CMP_GE(jiffies, rdp->rsp->jiffies_resched)) {
  		if (!(READ_ONCE(*rcrmp) & rdp->rsp->flavor_mask)) {
  			WRITE_ONCE(rdp->cond_resched_completed,
  				   READ_ONCE(rdp->mynode->completed));
  			smp_mb(); /* ->cond_resched_completed before *rcrmp. */
  			WRITE_ONCE(*rcrmp,
  				   READ_ONCE(*rcrmp) + rdp->rsp->flavor_mask);
  		}
  		rdp->rsp->jiffies_resched += 5; /* Re-enable beating. */
  	}
  
  	/* And if it has been a really long time, kick the CPU as well. */
  	if (ULONG_CMP_GE(jiffies,
  			 rdp->rsp->gp_start + 2 * jiffies_till_sched_qs) ||
  	    ULONG_CMP_GE(jiffies, rdp->rsp->gp_start + jiffies_till_sched_qs))
  		resched_cpu(rdp->cpu);  /* Force CPU into scheduler. */
  
  	return 0;
  }
  
  static void record_gp_stall_check_time(struct rcu_state *rsp)
  {
  	unsigned long j = jiffies;
  	unsigned long j1;
  
  	rsp->gp_start = j;
  	smp_wmb(); /* Record start time before stall time. */
  	j1 = rcu_jiffies_till_stall_check();
  	WRITE_ONCE(rsp->jiffies_stall, j + j1);
  	rsp->jiffies_resched = j + j1 / 2;
  	rsp->n_force_qs_gpstart = READ_ONCE(rsp->n_force_qs);
  }
  
  /*
   * Convert a ->gp_state value to a character string.
   */
  static const char *gp_state_getname(short gs)
  {
  	if (gs < 0 || gs >= ARRAY_SIZE(gp_state_names))
  		return "???";
  	return gp_state_names[gs];
  }
  
  /*
   * Complain about starvation of grace-period kthread.
   */
  static void rcu_check_gp_kthread_starvation(struct rcu_state *rsp)
  {
  	unsigned long gpa;
  	unsigned long j;
  
  	j = jiffies;
  	gpa = READ_ONCE(rsp->gp_activity);
  	if (j - gpa > 2 * HZ) {
  		pr_err("%s kthread starved for %ld jiffies! g%lu c%lu f%#x %s(%d) ->state=%#lx\n",
  		       rsp->name, j - gpa,
  		       rsp->gpnum, rsp->completed,
  		       rsp->gp_flags,
  		       gp_state_getname(rsp->gp_state), rsp->gp_state,
  		       rsp->gp_kthread ? rsp->gp_kthread->state : ~0);
  		if (rsp->gp_kthread) {
  			sched_show_task(rsp->gp_kthread);
  			wake_up_process(rsp->gp_kthread);
  		}
  	}
  }
  
  /*
   * Dump stacks of all tasks running on stalled CPUs.
   */
  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_rcu_node(rnp, flags);
  		if (rnp->qsmask != 0) {
  			for_each_leaf_node_possible_cpu(rnp, cpu)
  				if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu))
  					dump_cpu_task(cpu);
  		}
  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  	}
  }
  
  /*
   * If too much time has passed in the current grace period, and if
   * so configured, go kick the relevant kthreads.
   */
  static void rcu_stall_kick_kthreads(struct rcu_state *rsp)
  {
  	unsigned long j;
  
  	if (!rcu_kick_kthreads)
  		return;
  	j = READ_ONCE(rsp->jiffies_kick_kthreads);
  	if (time_after(jiffies, j) && rsp->gp_kthread) {
  		WARN_ONCE(1, "Kicking %s grace-period kthread\n", rsp->name);
  		rcu_ftrace_dump(DUMP_ALL);
  		wake_up_process(rsp->gp_kthread);
  		WRITE_ONCE(rsp->jiffies_kick_kthreads, j + HZ);
  	}
  }
  
  static inline void panic_on_rcu_stall(void)
  {
  	if (sysctl_panic_on_rcu_stall)
  		panic("RCU Stall\n");
  }
  
  static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
  {
  	int cpu;
  	long delta;
  	unsigned long flags;
  	unsigned long gpa;
  	unsigned long j;
  	int ndetected = 0;
  	struct rcu_node *rnp = rcu_get_root(rsp);
  	long totqlen = 0;
  
  	/* Kick and suppress, if so configured. */
  	rcu_stall_kick_kthreads(rsp);
  	if (rcu_cpu_stall_suppress)
  		return;
  
  	/* Only let one CPU complain about others per time interval. */
  
  	raw_spin_lock_irqsave_rcu_node(rnp, flags);
  	delta = jiffies - READ_ONCE(rsp->jiffies_stall);
  	if (delta < RCU_STALL_RAT_DELAY || !rcu_gp_in_progress(rsp)) {
  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  		return;
  	}
  	WRITE_ONCE(rsp->jiffies_stall,
  		   jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
  	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  
  	/*
  	 * OK, time to rat on our buddy...
  	 * See Documentation/RCU/stallwarn.txt for info on how to debug
  	 * RCU CPU stall warnings.
  	 */
  	pr_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_rcu_node(rnp, flags);
  		ndetected += rcu_print_task_stall(rnp);
  		if (rnp->qsmask != 0) {
  			for_each_leaf_node_possible_cpu(rnp, cpu)
  				if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
  					print_cpu_stall_info(rsp, cpu);
  					ndetected++;
  				}
  		}
  		raw_spin_unlock_irqrestore_rcu_node(rnp, 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=%ld, c=%ld, q=%lu)\n",
  	       smp_processor_id(), (long)(jiffies - rsp->gp_start),
  	       (long)rsp->gpnum, (long)rsp->completed, totqlen);
  	if (ndetected) {
  		rcu_dump_cpu_stacks(rsp);
  	} else {
  		if (READ_ONCE(rsp->gpnum) != gpnum ||
  		    READ_ONCE(rsp->completed) == gpnum) {
  			pr_err("INFO: Stall ended before state dump start\n");
  		} else {
  			j = jiffies;
  			gpa = READ_ONCE(rsp->gp_activity);
  			pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld, root ->qsmask %#lx\n",
  			       rsp->name, j - gpa, j, gpa,
  			       jiffies_till_next_fqs,
  			       rcu_get_root(rsp)->qsmask);
  			/* In this case, the current CPU might be at fault. */
  			sched_show_task(current);
  		}
  	}
  
  	/* Complain about tasks blocking the grace period. */
  	rcu_print_detail_task_stall(rsp);
  
  	rcu_check_gp_kthread_starvation(rsp);
  
  	panic_on_rcu_stall();
  
  	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;
  
  	/* Kick and suppress, if so configured. */
  	rcu_stall_kick_kthreads(rsp);
  	if (rcu_cpu_stall_suppress)
  		return;
  
  	/*
  	 * OK, time to rat on ourselves...
  	 * See Documentation/RCU/stallwarn.txt for info on how to debug
  	 * RCU CPU stall warnings.
  	 */
  	pr_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=%ld c=%ld q=%lu)\n",
  		jiffies - rsp->gp_start,
  		(long)rsp->gpnum, (long)rsp->completed, totqlen);
  
  	rcu_check_gp_kthread_starvation(rsp);
  
  	rcu_dump_cpu_stacks(rsp);
  
  	raw_spin_lock_irqsave_rcu_node(rnp, flags);
  	if (ULONG_CMP_GE(jiffies, READ_ONCE(rsp->jiffies_stall)))
  		WRITE_ONCE(rsp->jiffies_stall,
  			   jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
  	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  
  	panic_on_rcu_stall();
  
  	/*
  	 * Attempt to revive the RCU machinery by forcing a context switch.
  	 *
  	 * A context switch would normally allow the RCU state machine to make
  	 * progress and it could be we're stuck in kernel space without context
  	 * switches for an entirely unreasonable amount of time.
  	 */
  	resched_cpu(smp_processor_id());
  }
  
  static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
  {
  	unsigned long completed;
  	unsigned long gpnum;
  	unsigned long gps;
  	unsigned long j;
  	unsigned long js;
  	struct rcu_node *rnp;
  
  	if ((rcu_cpu_stall_suppress && !rcu_kick_kthreads) ||
  	    !rcu_gp_in_progress(rsp))
  		return;
  	rcu_stall_kick_kthreads(rsp);
  	j = jiffies;
  
  	/*
  	 * Lots of memory barriers to reject false positives.
  	 *
  	 * The idea is to pick up rsp->gpnum, then rsp->jiffies_stall,
  	 * then rsp->gp_start, and finally rsp->completed.  These values
  	 * are updated in the opposite order with memory barriers (or
  	 * equivalent) during grace-period initialization and cleanup.
  	 * Now, a false positive can occur if we get an new value of
  	 * rsp->gp_start and a old value of rsp->jiffies_stall.  But given
  	 * the memory barriers, the only way that this can happen is if one
  	 * grace period ends and another starts between these two fetches.
  	 * Detect this by comparing rsp->completed with the previous fetch
  	 * from rsp->gpnum.
  	 *
  	 * Given this check, comparisons of jiffies, rsp->jiffies_stall,
  	 * and rsp->gp_start suffice to forestall false positives.
  	 */
  	gpnum = READ_ONCE(rsp->gpnum);
  	smp_rmb(); /* Pick up ->gpnum first... */
  	js = READ_ONCE(rsp->jiffies_stall);
  	smp_rmb(); /* ...then ->jiffies_stall before the rest... */
  	gps = READ_ONCE(rsp->gp_start);
  	smp_rmb(); /* ...and finally ->gp_start before ->completed. */
  	completed = READ_ONCE(rsp->completed);
  	if (ULONG_CMP_GE(completed, gpnum) ||
  	    ULONG_CMP_LT(j, js) ||
  	    ULONG_CMP_GE(gps, js))
  		return; /* No stall or GP completed since entering function. */
  	rnp = rdp->mynode;
  	if (rcu_gp_in_progress(rsp) &&
  	    (READ_ONCE(rnp->qsmask) & rdp->grpmask)) {
  
  		/* 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, gpnum);
  	}
  }
  
  /**
   * 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)
  		WRITE_ONCE(rsp->jiffies_stall, jiffies + ULONG_MAX / 2);
  }
  
  /*
   * Initialize the specified rcu_data structure's default callback list
   * to empty.  The default callback list is the one that is not used by
   * no-callbacks CPUs.
   */
  static void init_default_callback_list(struct rcu_data *rdp)
  {
  	int i;
  
  	rdp->nxtlist = NULL;
  	for (i = 0; i < RCU_NEXT_SIZE; i++)
  		rdp->nxttail[i] = &rdp->nxtlist;
  }
  
  /*
   * Initialize the specified rcu_data structure's callback list to empty.
   */
  static void init_callback_list(struct rcu_data *rdp)
  {
  	if (init_nocb_callback_list(rdp))
  		return;
  	init_default_callback_list(rdp);
  }
  
  /*
   * 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, const 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.  Returns true if there
   * is reason to awaken the grace-period kthread.
   *
   * The caller must hold the specified rcu_node structure's ->lock.
   */
  static bool __maybe_unused
  rcu_start_future_gp(struct rcu_node *rnp, struct rcu_data *rdp,
  		    unsigned long *c_out)
  {
  	unsigned long c;
  	int i;
  	bool ret = false;
  	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, TPS("Startleaf"));
  	if (rnp->need_future_gp[c & 0x1]) {
  		trace_rcu_future_gp(rnp, rdp, c, TPS("Prestartleaf"));
  		goto out;
  	}
  
  	/*
  	 * 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.  We only do the lockless check
  	 * of rnp_root's fields if the current rcu_node structure thinks
  	 * there is no grace period in flight, and because we hold rnp->lock,
  	 * the only possible change is when rnp_root's two fields are
  	 * equal, in which case rnp_root->gpnum might be concurrently
  	 * incremented.  But that is OK, as it will just result in our
  	 * doing some extra useless work.
  	 */
  	if (rnp->gpnum != rnp->completed ||
  	    READ_ONCE(rnp_root->gpnum) != READ_ONCE(rnp_root->completed)) {
  		rnp->need_future_gp[c & 0x1]++;
  		trace_rcu_future_gp(rnp, rdp, c, TPS("Startedleaf"));
  		goto out;
  	}
  
  	/*
  	 * 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_rcu_node(rnp_root);
  
  	/*
  	 * 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, TPS("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, TPS("Startedleafroot"));
  	} else {
  		trace_rcu_future_gp(rnp, rdp, c, TPS("Startedroot"));
  		ret = rcu_start_gp_advanced(rdp->rsp, rnp_root, rdp);
  	}
  unlock_out:
  	if (rnp != rnp_root)
  		raw_spin_unlock_rcu_node(rnp_root);
  out:
  	if (c_out != NULL)
  		*c_out = c;
  	return ret;
  }
  
  /*
   * 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);
  
  	rnp->need_future_gp[c & 0x1] = 0;
  	needmore = rnp->need_future_gp[(c + 1) & 0x1];
  	trace_rcu_future_gp(rnp, rdp, c,
  			    needmore ? TPS("CleanupMore") : TPS("Cleanup"));
  	return needmore;
  }
  
  /*
   * Awaken the grace-period kthread for the specified flavor of RCU.
   * Don't do a self-awaken, and don't bother awakening when there is
   * nothing for the grace-period kthread to do (as in several CPUs
   * raced to awaken, and we lost), and finally don't try to awaken
   * a kthread that has not yet been created.
   */
  static void rcu_gp_kthread_wake(struct rcu_state *rsp)
  {
  	if (current == rsp->gp_kthread ||
  	    !READ_ONCE(rsp->gp_flags) ||
  	    !rsp->gp_kthread)
  		return;
  	swake_up(&rsp->gp_wq);
  }
  
  /*
   * 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.  Returns an flag saying that we should
   * awaken the RCU grace-period kthread.
   *
   * The caller must hold rnp->lock with interrupts disabled.
   */
  static bool rcu_accelerate_cbs(struct rcu_state *rsp, struct rcu_node *rnp,
  			       struct rcu_data *rdp)
  {
  	unsigned long c;
  	int i;
  	bool ret;
  
  	/* If the CPU has no callbacks, nothing to do. */
  	if (!rdp->nxttail[RCU_NEXT_TAIL] || !*rdp->nxttail[RCU_DONE_TAIL])
  		return false;
  
  	/*
  	 * 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 false;
  
  	/*
  	 * 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. */
  	ret = rcu_start_future_gp(rnp, rdp, NULL);
  
  	/* Trace depending on how much we were able to accelerate. */
  	if (!*rdp->nxttail[RCU_WAIT_TAIL])
  		trace_rcu_grace_period(rsp->name, rdp->gpnum, TPS("AccWaitCB"));
  	else
  		trace_rcu_grace_period(rsp->name, rdp->gpnum, TPS("AccReadyCB"));
  	return ret;
  }
  
  /*
   * 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...
   * Returns true if the RCU grace-period kthread needs to be awakened.
   *
   * The caller must hold rnp->lock with interrupts disabled.
   */
  static bool 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 false;
  
  	/*
  	 * 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. */
  	return rcu_accelerate_cbs(rsp, rnp, rdp);
  }
  
  /*
   * Update CPU-local rcu_data state to record the beginnings and ends of
   * grace periods.  The caller must hold the ->lock of the leaf rcu_node
   * structure corresponding to the current CPU, and must have irqs disabled.
   * Returns true if the grace-period kthread needs to be awakened.
   */
  static bool __note_gp_changes(struct rcu_state *rsp, struct rcu_node *rnp,
  			      struct rcu_data *rdp)
  {
  	bool ret;
  	bool need_gp;
  
  	/* Handle the ends of any preceding grace periods first. */
  	if (rdp->completed == rnp->completed &&
  	    !unlikely(READ_ONCE(rdp->gpwrap))) {
  
  		/* No grace period end, so just accelerate recent callbacks. */
  		ret = rcu_accelerate_cbs(rsp, rnp, rdp);
  
  	} else {
  
  		/* Advance callbacks. */
  		ret = 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, TPS("cpuend"));
  	}
  
  	if (rdp->gpnum != rnp->gpnum || unlikely(READ_ONCE(rdp->gpwrap))) {
  		/*
  		 * 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, TPS("cpustart"));
  		need_gp = !!(rnp->qsmask & rdp->grpmask);
  		rdp->cpu_no_qs.b.norm = need_gp;
  		rdp->rcu_qs_ctr_snap = __this_cpu_read(rcu_qs_ctr);
  		rdp->core_needs_qs = need_gp;
  		zero_cpu_stall_ticks(rdp);
  		WRITE_ONCE(rdp->gpwrap, false);
  	}
  	return ret;
  }
  
  static void note_gp_changes(struct rcu_state *rsp, struct rcu_data *rdp)
  {
  	unsigned long flags;
  	bool needwake;
  	struct rcu_node *rnp;
  
  	local_irq_save(flags);
  	rnp = rdp->mynode;
  	if ((rdp->gpnum == READ_ONCE(rnp->gpnum) &&
  	     rdp->completed == READ_ONCE(rnp->completed) &&
  	     !unlikely(READ_ONCE(rdp->gpwrap))) || /* w/out lock. */
  	    !raw_spin_trylock_rcu_node(rnp)) { /* irqs already off, so later. */
  		local_irq_restore(flags);
  		return;
  	}
  	needwake = __note_gp_changes(rsp, rnp, rdp);
  	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  	if (needwake)
  		rcu_gp_kthread_wake(rsp);
  }
  
  static void rcu_gp_slow(struct rcu_state *rsp, int delay)
  {
  	if (delay > 0 &&
  	    !(rsp->gpnum % (rcu_num_nodes * PER_RCU_NODE_PERIOD * delay)))
  		schedule_timeout_uninterruptible(delay);
  }
  
  /*
   * Initialize a new grace period.  Return false if no grace period required.
   */
  static bool rcu_gp_init(struct rcu_state *rsp)
  {
  	unsigned long oldmask;
  	struct rcu_data *rdp;
  	struct rcu_node *rnp = rcu_get_root(rsp);
  
  	WRITE_ONCE(rsp->gp_activity, jiffies);
  	raw_spin_lock_irq_rcu_node(rnp);
  	if (!READ_ONCE(rsp->gp_flags)) {
  		/* Spurious wakeup, tell caller to go back to sleep.  */
  		raw_spin_unlock_irq_rcu_node(rnp);
  		return false;
  	}
  	WRITE_ONCE(rsp->gp_flags, 0); /* Clear all flags: New grace period. */
  
  	if (WARN_ON_ONCE(rcu_gp_in_progress(rsp))) {
  		/*
  		 * Grace period already in progress, don't start another.
  		 * Not supposed to be able to happen.
  		 */
  		raw_spin_unlock_irq_rcu_node(rnp);
  		return false;
  	}
  
  	/* Advance to a new grace period and initialize state. */
  	record_gp_stall_check_time(rsp);
  	/* Record GP times before starting GP, hence smp_store_release(). */
  	smp_store_release(&rsp->gpnum, rsp->gpnum + 1);
  	trace_rcu_grace_period(rsp->name, rsp->gpnum, TPS("start"));
  	raw_spin_unlock_irq_rcu_node(rnp);
  
  	/*
  	 * Apply per-leaf buffered online and offline operations to the
  	 * rcu_node tree.  Note that this new grace period need not wait
  	 * for subsequent online CPUs, and that quiescent-state forcing
  	 * will handle subsequent offline CPUs.
  	 */
  	rcu_for_each_leaf_node(rsp, rnp) {
  		rcu_gp_slow(rsp, gp_preinit_delay);
  		raw_spin_lock_irq_rcu_node(rnp);
  		if (rnp->qsmaskinit == rnp->qsmaskinitnext &&
  		    !rnp->wait_blkd_tasks) {
  			/* Nothing to do on this leaf rcu_node structure. */
  			raw_spin_unlock_irq_rcu_node(rnp);
  			continue;
  		}
  
  		/* Record old state, apply changes to ->qsmaskinit field. */
  		oldmask = rnp->qsmaskinit;
  		rnp->qsmaskinit = rnp->qsmaskinitnext;
  
  		/* If zero-ness of ->qsmaskinit changed, propagate up tree. */
  		if (!oldmask != !rnp->qsmaskinit) {
  			if (!oldmask) /* First online CPU for this rcu_node. */
  				rcu_init_new_rnp(rnp);
  			else if (rcu_preempt_has_tasks(rnp)) /* blocked tasks */
  				rnp->wait_blkd_tasks = true;
  			else /* Last offline CPU and can propagate. */
  				rcu_cleanup_dead_rnp(rnp);
  		}
  
  		/*
  		 * If all waited-on tasks from prior grace period are
  		 * done, and if all this rcu_node structure's CPUs are
  		 * still offline, propagate up the rcu_node tree and
  		 * clear ->wait_blkd_tasks.  Otherwise, if one of this
  		 * rcu_node structure's CPUs has since come back online,
  		 * simply clear ->wait_blkd_tasks (but rcu_cleanup_dead_rnp()
  		 * checks for this, so just call it unconditionally).
  		 */
  		if (rnp->wait_blkd_tasks &&
  		    (!rcu_preempt_has_tasks(rnp) ||
  		     rnp->qsmaskinit)) {
  			rnp->wait_blkd_tasks = false;
  			rcu_cleanup_dead_rnp(rnp);
  		}
  
  		raw_spin_unlock_irq_rcu_node(rnp);
  	}
  
  	/*
  	 * 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.
  	 *
  	 * The grace period cannot complete until the initialization
  	 * process finishes, because this kthread handles both.
  	 */
  	rcu_for_each_node_breadth_first(rsp, rnp) {
  		rcu_gp_slow(rsp, gp_init_delay);
  		raw_spin_lock_irq_rcu_node(rnp);
  		rdp = this_cpu_ptr(rsp->rda);
  		rcu_preempt_check_blocked_tasks(rnp);
  		rnp->qsmask = rnp->qsmaskinit;
  		WRITE_ONCE(rnp->gpnum, rsp->gpnum);
  		if (WARN_ON_ONCE(rnp->completed != rsp->completed))
  			WRITE_ONCE(rnp->completed, rsp->completed);
  		if (rnp == rdp->mynode)
  			(void)__note_gp_changes(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_rcu_node(rnp);
  		cond_resched_rcu_qs();
  		WRITE_ONCE(rsp->gp_activity, jiffies);
  	}
  
  	return true;
  }
  
  /*
   * Helper function for wait_event_interruptible_timeout() wakeup
   * at force-quiescent-state time.
   */
  static bool rcu_gp_fqs_check_wake(struct rcu_state *rsp, int *gfp)
  {
  	struct rcu_node *rnp = rcu_get_root(rsp);
  
  	/* Someone like call_rcu() requested a force-quiescent-state scan. */
  	*gfp = READ_ONCE(rsp->gp_flags);
  	if (*gfp & RCU_GP_FLAG_FQS)
  		return true;
  
  	/* The current grace period has completed. */
  	if (!READ_ONCE(rnp->qsmask) && !rcu_preempt_blocked_readers_cgp(rnp))
  		return true;
  
  	return false;
  }
  
  /*
   * Do one round of quiescent-state forcing.
   */
  static void rcu_gp_fqs(struct rcu_state *rsp, bool first_time)
  {
  	bool isidle = false;
  	unsigned long maxj;
  	struct rcu_node *rnp = rcu_get_root(rsp);
  
  	WRITE_ONCE(rsp->gp_activity, jiffies);
  	rsp->n_force_qs++;
  	if (first_time) {
  		/* Collect dyntick-idle snapshots. */
  		if (is_sysidle_rcu_state(rsp)) {
  			isidle = true;
  			maxj = jiffies - ULONG_MAX / 4;
  		}
  		force_qs_rnp(rsp, dyntick_save_progress_counter,
  			     &isidle, &maxj);
  		rcu_sysidle_report_gp(rsp, isidle, maxj);
  	} else {
  		/* Handle dyntick-idle and offline CPUs. */
  		isidle = true;
  		force_qs_rnp(rsp, rcu_implicit_dynticks_qs, &isidle, &maxj);
  	}
  	/* Clear flag to prevent immediate re-entry. */
  	if (READ_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
  		raw_spin_lock_irq_rcu_node(rnp);
  		WRITE_ONCE(rsp->gp_flags,
  			   READ_ONCE(rsp->gp_flags) & ~RCU_GP_FLAG_FQS);
  		raw_spin_unlock_irq_rcu_node(rnp);
  	}
  }
  
  /*
   * Clean up after the old grace period.
   */
  static void rcu_gp_cleanup(struct rcu_state *rsp)
  {
  	unsigned long gp_duration;
  	bool needgp = false;
  	int nocb = 0;
  	struct rcu_data *rdp;
  	struct rcu_node *rnp = rcu_get_root(rsp);
  	struct swait_queue_head *sq;
  
  	WRITE_ONCE(rsp->gp_activity, jiffies);
  	raw_spin_lock_irq_rcu_node(rnp);
  	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_rcu_node(rnp);
  
  	/*
  	 * 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_rcu_node(rnp);
  		WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp));
  		WARN_ON_ONCE(rnp->qsmask);
  		WRITE_ONCE(rnp->completed, rsp->gpnum);
  		rdp = this_cpu_ptr(rsp->rda);
  		if (rnp == rdp->mynode)
  			needgp = __note_gp_changes(rsp, rnp, rdp) || needgp;
  		/* smp_mb() provided by prior unlock-lock pair. */
  		nocb += rcu_future_gp_cleanup(rsp, rnp);
  		sq = rcu_nocb_gp_get(rnp);
  		raw_spin_unlock_irq_rcu_node(rnp);
  		rcu_nocb_gp_cleanup(sq);
  		cond_resched_rcu_qs();
  		WRITE_ONCE(rsp->gp_activity, jiffies);
  		rcu_gp_slow(rsp, gp_cleanup_delay);
  	}
  	rnp = rcu_get_root(rsp);
  	raw_spin_lock_irq_rcu_node(rnp); /* Order GP before ->completed update. */
  	rcu_nocb_gp_set(rnp, nocb);
  
  	/* Declare grace period done. */
  	WRITE_ONCE(rsp->completed, rsp->gpnum);
  	trace_rcu_grace_period(rsp->name, rsp->completed, TPS("end"));
  	rsp->gp_state = RCU_GP_IDLE;
  	rdp = this_cpu_ptr(rsp->rda);
  	/* Advance CBs to reduce false positives below. */
  	needgp = rcu_advance_cbs(rsp, rnp, rdp) || needgp;
  	if (needgp || cpu_needs_another_gp(rsp, rdp)) {
  		WRITE_ONCE(rsp->gp_flags, RCU_GP_FLAG_INIT);
  		trace_rcu_grace_period(rsp->name,
  				       READ_ONCE(rsp->gpnum),
  				       TPS("newreq"));
  	}
  	raw_spin_unlock_irq_rcu_node(rnp);
  }
  
  /*
   * Body of kthread that handles grace periods.
   */
  static int __noreturn rcu_gp_kthread(void *arg)
  {
  	bool first_gp_fqs;
  	int gf;
  	unsigned long j;
  	int ret;
  	struct rcu_state *rsp = arg;
  	struct rcu_node *rnp = rcu_get_root(rsp);
  
  	rcu_bind_gp_kthread();
  	for (;;) {
  
  		/* Handle grace-period start. */
  		for (;;) {
  			trace_rcu_grace_period(rsp->name,
  					       READ_ONCE(rsp->gpnum),
  					       TPS("reqwait"));
  			rsp->gp_state = RCU_GP_WAIT_GPS;
  			swait_event_interruptible(rsp->gp_wq,
  						 READ_ONCE(rsp->gp_flags) &
  						 RCU_GP_FLAG_INIT);
  			rsp->gp_state = RCU_GP_DONE_GPS;
  			/* Locking provides needed memory barrier. */
  			if (rcu_gp_init(rsp))
  				break;
  			cond_resched_rcu_qs();
  			WRITE_ONCE(rsp->gp_activity, jiffies);
  			WARN_ON(signal_pending(current));
  			trace_rcu_grace_period(rsp->name,
  					       READ_ONCE(rsp->gpnum),
  					       TPS("reqwaitsig"));
  		}
  
  		/* Handle quiescent-state forcing. */
  		first_gp_fqs = true;
  		j = jiffies_till_first_fqs;
  		if (j > HZ) {
  			j = HZ;
  			jiffies_till_first_fqs = HZ;
  		}
  		ret = 0;
  		for (;;) {
  			if (!ret) {
  				rsp->jiffies_force_qs = jiffies + j;
  				WRITE_ONCE(rsp->jiffies_kick_kthreads,
  					   jiffies + 3 * j);
  			}
  			trace_rcu_grace_period(rsp->name,
  					       READ_ONCE(rsp->gpnum),
  					       TPS("fqswait"));
  			rsp->gp_state = RCU_GP_WAIT_FQS;
  			ret = swait_event_interruptible_timeout(rsp->gp_wq,
  					rcu_gp_fqs_check_wake(rsp, &gf), j);
  			rsp->gp_state = RCU_GP_DOING_FQS;
  			/* Locking provides needed memory barriers. */
  			/* If grace period done, leave loop. */
  			if (!READ_ONCE(rnp->qsmask) &&
  			    !rcu_preempt_blocked_readers_cgp(rnp))
  				break;
  			/* If time for quiescent-state forcing, do it. */
  			if (ULONG_CMP_GE(jiffies, rsp->jiffies_force_qs) ||
  			    (gf & RCU_GP_FLAG_FQS)) {
  				trace_rcu_grace_period(rsp->name,
  						       READ_ONCE(rsp->gpnum),
  						       TPS("fqsstart"));
  				rcu_gp_fqs(rsp, first_gp_fqs);
  				first_gp_fqs = false;
  				trace_rcu_grace_period(rsp->name,
  						       READ_ONCE(rsp->gpnum),
  						       TPS("fqsend"));
  				cond_resched_rcu_qs();
  				WRITE_ONCE(rsp->gp_activity, jiffies);
  				ret = 0; /* Force full wait till next FQS. */
  				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;
  				}
  			} else {
  				/* Deal with stray signal. */
  				cond_resched_rcu_qs();
  				WRITE_ONCE(rsp->gp_activity, jiffies);
  				WARN_ON(signal_pending(current));
  				trace_rcu_grace_period(rsp->name,
  						       READ_ONCE(rsp->gpnum),
  						       TPS("fqswaitsig"));
  				ret = 1; /* Keep old FQS timing. */
  				j = jiffies;
  				if (time_after(jiffies, rsp->jiffies_force_qs))
  					j = 1;
  				else
  					j = rsp->jiffies_force_qs - j;
  			}
  		}
  
  		/* Handle grace-period end. */
  		rsp->gp_state = RCU_GP_CLEANUP;
  		rcu_gp_cleanup(rsp);
  		rsp->gp_state = RCU_GP_CLEANED;
  	}
  }
  
  /*
   * 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.
   *
   * Returns true if the grace-period kthread must be awakened.
   */
  static bool
  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 false;
  	}
  	WRITE_ONCE(rsp->gp_flags, RCU_GP_FLAG_INIT);
  	trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gpnum),
  			       TPS("newreq"));
  
  	/*
  	 * We can't do wakeups while holding the rnp->lock, as that
  	 * could cause possible deadlocks with the rq->lock. Defer
  	 * the wakeup to our caller.
  	 */
  	return true;
  }
  
  /*
   * 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.
   *
   * Returns true if the grace-period kthread needs to be awakened.
   */
  static bool rcu_start_gp(struct rcu_state *rsp)
  {
  	struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
  	struct rcu_node *rnp = rcu_get_root(rsp);
  	bool ret = false;
  
  	/*
  	 * 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!
  	 */
  	ret = rcu_advance_cbs(rsp, rnp, rdp) || ret;
  	ret = rcu_start_gp_advanced(rsp, rnp, rdp) || ret;
  	return ret;
  }
  
  /*
   * Report a full set of quiescent states to the specified rcu_state data
   * structure.  Invoke rcu_gp_kthread_wake() to awaken the grace-period
   * kthread if another grace period is required.  Whether we wake
   * the grace-period kthread or it awakens itself for the next round
   * of quiescent-state forcing, that kthread will clean up after the
   * just-completed grace period.  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));
  	WRITE_ONCE(rsp->gp_flags, READ_ONCE(rsp->gp_flags) | RCU_GP_FLAG_FQS);
  	raw_spin_unlock_irqrestore_rcu_node(rcu_get_root(rsp), flags);
  	rcu_gp_kthread_wake(rsp);
  }
  
  /*
   * 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).  The gps parameter
   * is the grace-period snapshot, which means that the quiescent states
   * are valid only if rnp->gpnum is equal to gps.  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 gps, unsigned long flags)
  	__releases(rnp->lock)
  {
  	unsigned long oldmask = 0;
  	struct rcu_node *rnp_c;
  
  	/* Walk up the rcu_node hierarchy. */
  	for (;;) {
  		if (!(rnp->qsmask & mask) || rnp->gpnum != gps) {
  
  			/*
  			 * Our bit has already been cleared, or the
  			 * relevant grace period is already over, so done.
  			 */
  			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  			return;
  		}
  		WARN_ON_ONCE(oldmask); /* Any child must be all zeroed! */
  		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_rcu_node(rnp, flags);
  			return;
  		}
  		mask = rnp->grpmask;
  		if (rnp->parent == NULL) {
  
  			/* No more levels.  Exit loop holding root lock. */
  
  			break;
  		}
  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  		rnp_c = rnp;
  		rnp = rnp->parent;
  		raw_spin_lock_irqsave_rcu_node(rnp, flags);
  		oldmask = 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 all tasks that were previously queued
   * on the specified rcu_node structure and that were blocking the current
   * RCU grace period.  The caller must hold the specified rnp->lock with
   * irqs disabled, and this lock is released upon return, but irqs remain
   * disabled.
   */
  static void rcu_report_unblock_qs_rnp(struct rcu_state *rsp,
  				      struct rcu_node *rnp, unsigned long flags)
  	__releases(rnp->lock)
  {
  	unsigned long gps;
  	unsigned long mask;
  	struct rcu_node *rnp_p;
  
  	if (rcu_state_p == &rcu_sched_state || rsp != rcu_state_p ||
  	    rnp->qsmask != 0 || rcu_preempt_blocked_readers_cgp(rnp)) {
  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  		return;  /* Still need more quiescent states! */
  	}
  
  	rnp_p = rnp->parent;
  	if (rnp_p == NULL) {
  		/*
  		 * Only one rcu_node structure in the tree, so don't
  		 * try to report up to its nonexistent parent!
  		 */
  		rcu_report_qs_rsp(rsp, flags);
  		return;
  	}
  
  	/* Report up the rest of the hierarchy, tracking current ->gpnum. */
  	gps = rnp->gpnum;
  	mask = rnp->grpmask;
  	raw_spin_unlock_rcu_node(rnp);	/* irqs remain disabled. */
  	raw_spin_lock_rcu_node(rnp_p);	/* irqs already disabled. */
  	rcu_report_qs_rnp(mask, rsp, rnp_p, gps, flags);
  }
  
  /*
   * Record a quiescent state for the specified CPU to that CPU's rcu_data
   * structure.  This must be called from the specified CPU.
   */
  static void
  rcu_report_qs_rdp(int cpu, struct rcu_state *rsp, struct rcu_data *rdp)
  {
  	unsigned long flags;
  	unsigned long mask;
  	bool needwake;
  	struct rcu_node *rnp;
  
  	rnp = rdp->mynode;
  	raw_spin_lock_irqsave_rcu_node(rnp, flags);
  	if ((rdp->cpu_no_qs.b.norm &&
  	     rdp->rcu_qs_ctr_snap == __this_cpu_read(rcu_qs_ctr)) ||
  	    rdp->gpnum != rnp->gpnum || rnp->completed == rnp->gpnum ||
  	    rdp->gpwrap) {
  
  		/*
  		 * 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->cpu_no_qs.b.norm = true;	/* need qs for new gp. */
  		rdp->rcu_qs_ctr_snap = __this_cpu_read(rcu_qs_ctr);
  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  		return;
  	}
  	mask = rdp->grpmask;
  	if ((rnp->qsmask & mask) == 0) {
  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  	} else {
  		rdp->core_needs_qs = false;
  
  		/*
  		 * This GP can't end until cpu checks in, so all of our
  		 * callbacks can be processed during the next GP.
  		 */
  		needwake = rcu_accelerate_cbs(rsp, rnp, rdp);
  
  		rcu_report_qs_rnp(mask, rsp, rnp, rnp->gpnum, flags);
  		/* ^^^ Released rnp->lock */
  		if (needwake)
  			rcu_gp_kthread_wake(rsp);
  	}
  }
  
  /*
   * 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)
  {
  	/* Check for grace-period ends and beginnings. */
  	note_gp_changes(rsp, rdp);
  
  	/*
  	 * 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->core_needs_qs)
  		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->cpu_no_qs.b.norm &&
  	    rdp->rcu_qs_ctr_snap == __this_cpu_read(rcu_qs_ctr))
  		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);
  }
  
  /*
   * 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 (!IS_ENABLED(CONFIG_HOTPLUG_CPU) || 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;
  		WRITE_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 and
  	 * disallow further callbacks on this CPU.
  	 */
  	init_callback_list(rdp);
  	rdp->nxttail[RCU_NEXT_TAIL] = NULL;
  }
  
  /*
   * 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, unsigned long flags)
  {
  	int i;
  	struct rcu_data *rdp = raw_cpu_ptr(rsp->rda);
  
  	/* No-CBs CPUs are handled specially. */
  	if (!IS_ENABLED(CONFIG_HOTPLUG_CPU) ||
  	    rcu_nocb_adopt_orphan_cbs(rsp, rdp, flags))
  		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);
  
  	if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
  		return;
  
  	RCU_TRACE(mask = rdp->grpmask);
  	trace_rcu_grace_period(rsp->name,
  			       rnp->gpnum + 1 - !!(rnp->qsmask & mask),
  			       TPS("cpuofl"));
  }
  
  /*
   * All CPUs for the specified rcu_node structure have gone offline,
   * and all tasks that were preempted within an RCU read-side critical
   * section while running on one of those CPUs have since exited their RCU
   * read-side critical section.  Some other CPU is reporting this fact with
   * the specified rcu_node structure's ->lock held and interrupts disabled.
   * This function therefore goes up the tree of rcu_node structures,
   * clearing the corresponding bits in the ->qsmaskinit fields.  Note that
   * the leaf rcu_node structure's ->qsmaskinit field has already been
   * updated
   *
   * This function does check that the specified rcu_node structure has
   * all CPUs offline and no blocked tasks, so it is OK to invoke it
   * prematurely.  That said, invoking it after the fact will cost you
   * a needless lock acquisition.  So once it has done its work, don't
   * invoke it again.
   */
  static void rcu_cleanup_dead_rnp(struct rcu_node *rnp_leaf)
  {
  	long mask;
  	struct rcu_node *rnp = rnp_leaf;
  
  	if (!IS_ENABLED(CONFIG_HOTPLUG_CPU) ||
  	    rnp->qsmaskinit || rcu_preempt_has_tasks(rnp))
  		return;
  	for (;;) {
  		mask = rnp->grpmask;
  		rnp = rnp->parent;
  		if (!rnp)
  			break;
  		raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
  		rnp->qsmaskinit &= ~mask;
  		rnp->qsmask &= ~mask;
  		if (rnp->qsmaskinit) {
  			raw_spin_unlock_rcu_node(rnp);
  			/* irqs remain disabled. */
  			return;
  		}
  		raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
  	}
  }
  
  /*
   * 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;
  	struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
  	struct rcu_node *rnp = rdp->mynode;  /* Outgoing CPU's rdp & rnp. */
  
  	if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
  		return;
  
  	/* Adjust any no-longer-needed kthreads. */
  	rcu_boost_kthread_setaffinity(rnp, -1);
  
  	/* Orphan the dead CPU's callbacks, and adopt them if appropriate. */
  	raw_spin_lock_irqsave(&rsp->orphan_lock, flags);
  	rcu_send_cbs_to_orphanage(cpu, rsp, rnp, rdp);
  	rcu_adopt_orphan_cbs(rsp, flags);
  	raw_spin_unlock_irqrestore(&rsp->orphan_lock, flags);
  
  	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);
  }
  
  /*
   * 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, !!READ_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;
  	WRITE_ONCE(rdp->qlen, 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 user)
  {
  	trace_rcu_utilization(TPS("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();
  		rcu_bh_qs();
  
  	} 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();
  	}
  	rcu_preempt_check_callbacks();
  	if (rcu_pending())
  		invoke_rcu_core();
  	if (user)
  		rcu_note_voluntary_context_switch(current);
  	trace_rcu_utilization(TPS("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 *rsp, bool *isidle,
  				  unsigned long *maxj),
  			 bool *isidle, unsigned long *maxj)
  {
  	int cpu;
  	unsigned long flags;
  	unsigned long mask;
  	struct rcu_node *rnp;
  
  	rcu_for_each_leaf_node(rsp, rnp) {
  		cond_resched_rcu_qs();
  		mask = 0;
  		raw_spin_lock_irqsave_rcu_node(rnp, flags);
  		if (rnp->qsmask == 0) {
  			if (rcu_state_p == &rcu_sched_state ||
  			    rsp != rcu_state_p ||
  			    rcu_preempt_blocked_readers_cgp(rnp)) {
  				/*
  				 * No point in scanning bits because they
  				 * are all zero.  But we might need to
  				 * priority-boost blocked readers.
  				 */
  				rcu_initiate_boost(rnp, flags);
  				/* rcu_initiate_boost() releases rnp->lock */
  				continue;
  			}
  			if (rnp->parent &&
  			    (rnp->parent->qsmask & rnp->grpmask)) {
  				/*
  				 * Race between grace-period
  				 * initialization and task exiting RCU
  				 * read-side critical section: Report.
  				 */
  				rcu_report_unblock_qs_rnp(rsp, rnp, flags);
  				/* rcu_report_unblock_qs_rnp() rlses ->lock */
  				continue;
  			}
  		}
  		for_each_leaf_node_possible_cpu(rnp, cpu) {
  			unsigned long bit = leaf_node_cpu_bit(rnp, cpu);
  			if ((rnp->qsmask & bit) != 0) {
  				if (f(per_cpu_ptr(rsp->rda, cpu), isidle, maxj))
  					mask |= bit;
  			}
  		}
  		if (mask != 0) {
  			/* Idle/offline CPUs, report (releases rnp->lock. */
  			rcu_report_qs_rnp(mask, rsp, rnp, rnp->gpnum, flags);
  		} else {
  			/* Nothing to do here, so just drop the lock. */
  			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  		}
  	}
  }
  
  /*
   * Force quiescent states on reluctant CPUs, and also detect which
   * CPUs are in dyntick-idle mode.
   */
  static void force_quiescent_state(struct rcu_state *rsp)
  {
  	unsigned long flags;
  	bool ret;
  	struct rcu_node *rnp;
  	struct rcu_node *rnp_old = NULL;
  
  	/* Funnel through hierarchy to reduce memory contention. */
  	rnp = __this_cpu_read(rsp->rda->mynode);
  	for (; rnp != NULL; rnp = rnp->parent) {
  		ret = (READ_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_rcu_node(rnp_old, flags);
  	raw_spin_unlock(&rnp_old->fqslock);
  	if (READ_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
  		rsp->n_force_qs_lh++;
  		raw_spin_unlock_irqrestore_rcu_node(rnp_old, flags);
  		return;  /* Someone beat us to it. */
  	}
  	WRITE_ONCE(rsp->gp_flags, READ_ONCE(rsp->gp_flags) | RCU_GP_FLAG_FQS);
  	raw_spin_unlock_irqrestore_rcu_node(rnp_old, flags);
  	rcu_gp_kthread_wake(rsp);
  }
  
  /*
   * 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;
  	bool needwake;
  	struct rcu_data *rdp = raw_cpu_ptr(rsp->rda);
  
  	WARN_ON_ONCE(rdp->beenonline == 0);
  
  	/* 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_node(rcu_get_root(rsp)); /* irqs disabled. */
  		needwake = rcu_start_gp(rsp);
  		raw_spin_unlock_irqrestore_rcu_node(rcu_get_root(rsp), flags);
  		if (needwake)
  			rcu_gp_kthread_wake(rsp);
  	} 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 any needed deferred wakeups of rcuo kthreads. */
  	do_nocb_deferred_wakeup(rdp);
  }
  
  /*
   * Do RCU core processing for the current CPU.
   */
  static __latent_entropy void rcu_process_callbacks(struct softirq_action *unused)
  {
  	struct rcu_state *rsp;
  
  	if (cpu_is_offline(smp_processor_id()))
  		return;
  	trace_rcu_utilization(TPS("Start RCU core"));
  	for_each_rcu_flavor(rsp)
  		__rcu_process_callbacks(rsp);
  	trace_rcu_utilization(TPS("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 softirqs 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(!READ_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)
  {
  	bool needwake;
  
  	/*
  	 * 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_watching())
  		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? */
  		note_gp_changes(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_rcu_node(rnp_root);
  			needwake = rcu_start_gp(rsp);
  			raw_spin_unlock_rcu_node(rnp_root);
  			if (needwake)
  				rcu_gp_kthread_wake(rsp);
  		} 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;
  		}
  	}
  }
  
  /*
   * RCU callback function to leak a callback.
   */
  static void rcu_leak_callback(struct rcu_head *rhp)
  {
  }
  
  /*
   * 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, rcu_callback_t func,
  	   struct rcu_state *rsp, int cpu, bool lazy)
  {
  	unsigned long flags;
  	struct rcu_data *rdp;
  
  	WARN_ON_ONCE((unsigned long)head & 0x1); /* Misaligned rcu_head! */
  	if (debug_rcu_head_queue(head)) {
  		/* Probable double call_rcu(), so leak the callback. */
  		WRITE_ONCE(head->func, rcu_leak_callback);
  		WARN_ONCE(1, "__call_rcu(): Leaked duplicate callback\n");
  		return;
  	}
  	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);
  		if (likely(rdp->mynode)) {
  			/* Post-boot, so this should be for a no-CBs CPU. */
  			offline = !__call_rcu_nocb(rdp, head, lazy, flags);
  			WARN_ON_ONCE(offline);
  			/* Offline CPU, _call_rcu() illegal, leak callback.  */
  			local_irq_restore(flags);
  			return;
  		}
  		/*
  		 * Very early boot, before rcu_init().  Initialize if needed
  		 * and then drop through to queue the callback.
  		 */
  		BUG_ON(cpu != -1);
  		WARN_ON_ONCE(!rcu_is_watching());
  		if (!likely(rdp->nxtlist))
  			init_default_callback_list(rdp);
  	}
  	WRITE_ONCE(rdp->qlen, rdp->qlen + 1);
  	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, rcu_callback_t func)
  {
  	__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, rcu_callback_t func)
  {
  	__call_rcu(head, func, &rcu_bh_state, -1, 0);
  }
  EXPORT_SYMBOL_GPL(call_rcu_bh);
  
  /*
   * Queue an RCU callback for lazy invocation after a grace period.
   * This will likely be later named something like "call_rcu_lazy()",
   * but this change will require some way of tagging the lazy RCU
   * callbacks in the list of pending callbacks. Until then, this
   * function may only be called from __kfree_rcu().
   */
  void kfree_call_rcu(struct rcu_head *head,
  		    rcu_callback_t func)
  {
  	__call_rcu(head, func, rcu_state_p, -1, 1);
  }
  EXPORT_SYMBOL_GPL(kfree_call_rcu);
  
  /*
   * 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_WARN(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_gp_is_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_WARN(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_gp_is_expedited())
  		synchronize_rcu_bh_expedited();
  	else
  		wait_rcu_gp(call_rcu_bh);
  }
  EXPORT_SYMBOL_GPL(synchronize_rcu_bh);
  
  /**
   * get_state_synchronize_rcu - Snapshot current RCU state
   *
   * Returns a cookie that is used by a later call to cond_synchronize_rcu()
   * to determine whether or not a full grace period has elapsed in the
   * meantime.
   */
  unsigned long get_state_synchronize_rcu(void)
  {
  	/*
  	 * Any prior manipulation of RCU-protected data must happen
  	 * before the load from ->gpnum.
  	 */
  	smp_mb();  /* ^^^ */
  
  	/*
  	 * Make sure this load happens before the purportedly
  	 * time-consuming work between get_state_synchronize_rcu()
  	 * and cond_synchronize_rcu().
  	 */
  	return smp_load_acquire(&rcu_state_p->gpnum);
  }
  EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);
  
  /**
   * cond_synchronize_rcu - Conditionally wait for an RCU grace period
   *
   * @oldstate: return value from earlier call to get_state_synchronize_rcu()
   *
   * If a full RCU grace period has elapsed since the earlier call to
   * get_state_synchronize_rcu(), just return.  Otherwise, invoke
   * synchronize_rcu() to wait for a full grace period.
   *
   * Yes, this function does not take counter wrap into account.  But
   * counter wrap is harmless.  If the counter wraps, we have waited for
   * more than 2 billion grace periods (and way more on a 64-bit system!),
   * so waiting for one additional grace period should be just fine.
   */
  void cond_synchronize_rcu(unsigned long oldstate)
  {
  	unsigned long newstate;
  
  	/*
  	 * Ensure that this load happens before any RCU-destructive
  	 * actions the caller might carry out after we return.
  	 */
  	newstate = smp_load_acquire(&rcu_state_p->completed);
  	if (ULONG_CMP_GE(oldstate, newstate))
  		synchronize_rcu();
  }
  EXPORT_SYMBOL_GPL(cond_synchronize_rcu);
  
  /**
   * get_state_synchronize_sched - Snapshot current RCU-sched state
   *
   * Returns a cookie that is used by a later call to cond_synchronize_sched()
   * to determine whether or not a full grace period has elapsed in the
   * meantime.
   */
  unsigned long get_state_synchronize_sched(void)
  {
  	/*
  	 * Any prior manipulation of RCU-protected data must happen
  	 * before the load from ->gpnum.
  	 */
  	smp_mb();  /* ^^^ */
  
  	/*
  	 * Make sure this load happens before the purportedly
  	 * time-consuming work between get_state_synchronize_sched()
  	 * and cond_synchronize_sched().
  	 */
  	return smp_load_acquire(&rcu_sched_state.gpnum);
  }
  EXPORT_SYMBOL_GPL(get_state_synchronize_sched);
  
  /**
   * cond_synchronize_sched - Conditionally wait for an RCU-sched grace period
   *
   * @oldstate: return value from earlier call to get_state_synchronize_sched()
   *
   * If a full RCU-sched grace period has elapsed since the earlier call to
   * get_state_synchronize_sched(), just return.  Otherwise, invoke
   * synchronize_sched() to wait for a full grace period.
   *
   * Yes, this function does not take counter wrap into account.  But
   * counter wrap is harmless.  If the counter wraps, we have waited for
   * more than 2 billion grace periods (and way more on a 64-bit system!),
   * so waiting for one additional grace period should be just fine.
   */
  void cond_synchronize_sched(unsigned long oldstate)
  {
  	unsigned long newstate;
  
  	/*
  	 * Ensure that this load happens before any RCU-destructive
  	 * actions the caller might carry out after we return.
  	 */
  	newstate = smp_load_acquire(&rcu_sched_state.completed);
  	if (ULONG_CMP_GE(oldstate, newstate))
  		synchronize_sched();
  }
  EXPORT_SYMBOL_GPL(cond_synchronize_sched);
  
  /* Adjust sequence number for start of update-side operation. */
  static void rcu_seq_start(unsigned long *sp)
  {
  	WRITE_ONCE(*sp, *sp + 1);
  	smp_mb(); /* Ensure update-side operation after counter increment. */
  	WARN_ON_ONCE(!(*sp & 0x1));
  }
  
  /* Adjust sequence number for end of update-side operation. */
  static void rcu_seq_end(unsigned long *sp)
  {
  	smp_mb(); /* Ensure update-side operation before counter increment. */
  	WRITE_ONCE(*sp, *sp + 1);
  	WARN_ON_ONCE(*sp & 0x1);
  }
  
  /* Take a snapshot of the update side's sequence number. */
  static unsigned long rcu_seq_snap(unsigned long *sp)
  {
  	unsigned long s;
  
  	s = (READ_ONCE(*sp) + 3) & ~0x1;
  	smp_mb(); /* Above access must not bleed into critical section. */
  	return s;
  }
  
  /*
   * Given a snapshot from rcu_seq_snap(), determine whether or not a
   * full update-side operation has occurred.
   */
  static bool rcu_seq_done(unsigned long *sp, unsigned long s)
  {
  	return ULONG_CMP_GE(READ_ONCE(*sp), s);
  }
  
  /*
   * 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 this CPU a NO_HZ_FULL CPU that should ignore RCU? */
  	if (rcu_nohz_full_cpu(rsp))
  		return 0;
  
  	/* Is the RCU core waiting for a quiescent state from this CPU? */
  	if (rcu_scheduler_fully_active &&
  	    rdp->core_needs_qs && rdp->cpu_no_qs.b.norm &&
  	    rdp->rcu_qs_ctr_snap == __this_cpu_read(rcu_qs_ctr)) {
  		rdp->n_rp_core_needs_qs++;
  	} else if (rdp->core_needs_qs &&
  		   (!rdp->cpu_no_qs.b.norm ||
  		    rdp->rcu_qs_ctr_snap != __this_cpu_read(rcu_qs_ctr))) {
  		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 (READ_ONCE(rnp->completed) != rdp->completed) { /* outside lock */
  		rdp->n_rp_gp_completed++;
  		return 1;
  	}
  
  	/* Has a new RCU grace period started? */
  	if (READ_ONCE(rnp->gpnum) != rdp->gpnum ||
  	    unlikely(READ_ONCE(rdp->gpwrap))) { /* outside lock */
  		rdp->n_rp_gp_started++;
  		return 1;
  	}
  
  	/* Does this CPU need a deferred NOCB wakeup? */
  	if (rcu_nocb_need_deferred_wakeup(rdp)) {
  		rdp->n_rp_nocb_defer_wakeup++;
  		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(void)
  {
  	struct rcu_state *rsp;
  
  	for_each_rcu_flavor(rsp)
  		if (__rcu_pending(rsp, this_cpu_ptr(rsp->rda)))
  			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 bool __maybe_unused rcu_cpu_has_callbacks(bool *all_lazy)
  {
  	bool al = true;
  	bool hc = false;
  	struct rcu_data *rdp;
  	struct rcu_state *rsp;
  
  	for_each_rcu_flavor(rsp) {
  		rdp = this_cpu_ptr(rsp->rda);
  		if (!rdp->nxtlist)
  			continue;
  		hc = true;
  		if (rdp->qlen != rdp->qlen_lazy || !all_lazy) {
  			al = false;
  			break;
  		}
  	}
  	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, const 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->barrier_sequence);
  		complete(&rsp->barrier_completion);
  	} else {
  		_rcu_barrier_trace(rsp, "CB", -1, rsp->barrier_sequence);
  	}
  }
  
  /*
   * 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 = raw_cpu_ptr(rsp->rda);
  
  	_rcu_barrier_trace(rsp, "IRQ", -1, rsp->barrier_sequence);
  	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 s = rcu_seq_snap(&rsp->barrier_sequence);
  
  	_rcu_barrier_trace(rsp, "Begin", -1, s);
  
  	/* Take mutex to serialize concurrent rcu_barrier() requests. */
  	mutex_lock(&rsp->barrier_mutex);
  
  	/* Did someone else do our work for us? */
  	if (rcu_seq_done(&rsp->barrier_sequence, s)) {
  		_rcu_barrier_trace(rsp, "EarlyExit", -1, rsp->barrier_sequence);
  		smp_mb(); /* caller's subsequent code after above check. */
  		mutex_unlock(&rsp->barrier_mutex);
  		return;
  	}
  
  	/* Mark the start of the barrier operation. */
  	rcu_seq_start(&rsp->barrier_sequence);
  	_rcu_barrier_trace(rsp, "Inc1", -1, rsp->barrier_sequence);
  
  	/*
  	 * 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)) {
  			if (!rcu_nocb_cpu_needs_barrier(rsp, cpu)) {
  				_rcu_barrier_trace(rsp, "OfflineNoCB", cpu,
  						   rsp->barrier_sequence);
  			} else {
  				_rcu_barrier_trace(rsp, "OnlineNoCB", cpu,
  						   rsp->barrier_sequence);
  				smp_mb__before_atomic();
  				atomic_inc(&rsp->barrier_cpu_count);
  				__call_rcu(&rdp->barrier_head,
  					   rcu_barrier_callback, rsp, cpu, 0);
  			}
  		} else if (READ_ONCE(rdp->qlen)) {
  			_rcu_barrier_trace(rsp, "OnlineQ", cpu,
  					   rsp->barrier_sequence);
  			smp_call_function_single(cpu, rcu_barrier_func, rsp, 1);
  		} else {
  			_rcu_barrier_trace(rsp, "OnlineNQ", cpu,
  					   rsp->barrier_sequence);
  		}
  	}
  	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);
  
  	/* Wait for all rcu_barrier_callback() callbacks to be invoked. */
  	wait_for_completion(&rsp->barrier_completion);
  
  	/* Mark the end of the barrier operation. */
  	_rcu_barrier_trace(rsp, "Inc2", -1, rsp->barrier_sequence);
  	rcu_seq_end(&rsp->barrier_sequence);
  
  	/* 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);
  
  /*
   * Propagate ->qsinitmask bits up the rcu_node tree to account for the
   * first CPU in a given leaf rcu_node structure coming online.  The caller
   * must hold the corresponding leaf rcu_node ->lock with interrrupts
   * disabled.
   */
  static void rcu_init_new_rnp(struct rcu_node *rnp_leaf)
  {
  	long mask;
  	struct rcu_node *rnp = rnp_leaf;
  
  	for (;;) {
  		mask = rnp->grpmask;
  		rnp = rnp->parent;
  		if (rnp == NULL)
  			return;
  		raw_spin_lock_rcu_node(rnp); /* Interrupts already disabled. */
  		rnp->qsmaskinit |= mask;
  		raw_spin_unlock_rcu_node(rnp); /* Interrupts remain disabled. */
  	}
  }
  
  /*
   * 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_rcu_node(rnp, flags);
  	rdp->grpmask = leaf_node_cpu_bit(rdp->mynode, cpu);
  	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_rcu_node(rnp, 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
  rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
  {
  	unsigned long flags;
  	unsigned long mask;
  	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_rcu_node(rnp, flags);
  	rdp->qlen_last_fqs_check = 0;
  	rdp->n_force_qs_snap = rsp->n_force_qs;
  	rdp->blimit = blimit;
  	if (!rdp->nxtlist)
  		init_callback_list(rdp);  /* Re-enable callbacks on this CPU. */
  	rdp->dynticks->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
  	rcu_sysidle_init_percpu_data(rdp->dynticks);
  	atomic_set(&rdp->dynticks->dynticks,
  		   (atomic_read(&rdp->dynticks->dynticks) & ~0x1) + 1);
  	raw_spin_unlock_rcu_node(rnp);		/* irqs remain disabled. */
  
  	/*
  	 * Add CPU to leaf rcu_node pending-online bitmask.  Any needed
  	 * propagation up the rcu_node tree will happen at the beginning
  	 * of the next grace period.
  	 */
  	rnp = rdp->mynode;
  	mask = rdp->grpmask;
  	raw_spin_lock_rcu_node(rnp);		/* irqs already disabled. */
  	if (!rdp->beenonline)
  		WRITE_ONCE(rsp->ncpus, READ_ONCE(rsp->ncpus) + 1);
  	rdp->beenonline = true;	 /* We have now been online. */
  	rdp->gpnum = rnp->completed; /* Make CPU later note any new GP. */
  	rdp->completed = rnp->completed;
  	rdp->cpu_no_qs.b.norm = true;
  	rdp->rcu_qs_ctr_snap = per_cpu(rcu_qs_ctr, cpu);
  	rdp->core_needs_qs = false;
  	trace_rcu_grace_period(rsp->name, rdp->gpnum, TPS("cpuonl"));
  	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  }
  
  int rcutree_prepare_cpu(unsigned int cpu)
  {
  	struct rcu_state *rsp;
  
  	for_each_rcu_flavor(rsp)
  		rcu_init_percpu_data(cpu, rsp);
  
  	rcu_prepare_kthreads(cpu);
  	rcu_spawn_all_nocb_kthreads(cpu);
  
  	return 0;
  }
  
  static void rcutree_affinity_setting(unsigned int cpu, int outgoing)
  {
  	struct rcu_data *rdp = per_cpu_ptr(rcu_state_p->rda, cpu);
  
  	rcu_boost_kthread_setaffinity(rdp->mynode, outgoing);
  }
  
  int rcutree_online_cpu(unsigned int cpu)
  {
  	sync_sched_exp_online_cleanup(cpu);
  	rcutree_affinity_setting(cpu, -1);
  	return 0;
  }
  
  int rcutree_offline_cpu(unsigned int cpu)
  {
  	rcutree_affinity_setting(cpu, cpu);
  	return 0;
  }
  
  
  int rcutree_dying_cpu(unsigned int cpu)
  {
  	struct rcu_state *rsp;
  
  	for_each_rcu_flavor(rsp)
  		rcu_cleanup_dying_cpu(rsp);
  	return 0;
  }
  
  int rcutree_dead_cpu(unsigned int cpu)
  {
  	struct rcu_state *rsp;
  
  	for_each_rcu_flavor(rsp) {
  		rcu_cleanup_dead_cpu(cpu, rsp);
  		do_nocb_deferred_wakeup(per_cpu_ptr(rsp->rda, cpu));
  	}
  	return 0;
  }
  
  /*
   * Mark the specified CPU as being online so that subsequent grace periods
   * (both expedited and normal) will wait on it.  Note that this means that
   * incoming CPUs are not allowed to use RCU read-side critical sections
   * until this function is called.  Failing to observe this restriction
   * will result in lockdep splats.
   */
  void rcu_cpu_starting(unsigned int cpu)
  {
  	unsigned long flags;
  	unsigned long mask;
  	struct rcu_data *rdp;
  	struct rcu_node *rnp;
  	struct rcu_state *rsp;
  
  	for_each_rcu_flavor(rsp) {
  		rdp = this_cpu_ptr(rsp->rda);
  		rnp = rdp->mynode;
  		mask = rdp->grpmask;
  		raw_spin_lock_irqsave_rcu_node(rnp, flags);
  		rnp->qsmaskinitnext |= mask;
  		rnp->expmaskinitnext |= mask;
  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  	}
  }
  
  #ifdef CONFIG_HOTPLUG_CPU
  /*
   * The CPU is exiting the idle loop into the arch_cpu_idle_dead()
   * function.  We now remove it from the rcu_node tree's ->qsmaskinit
   * bit masks.
   * The CPU is exiting the idle loop into the arch_cpu_idle_dead()
   * function.  We now remove it from the rcu_node tree's ->qsmaskinit
   * bit masks.
   */
  static void rcu_cleanup_dying_idle_cpu(int cpu, struct rcu_state *rsp)
  {
  	unsigned long flags;
  	unsigned long mask;
  	struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
  	struct rcu_node *rnp = rdp->mynode;  /* Outgoing CPU's rdp & rnp. */
  
  	/* Remove outgoing CPU from mask in the leaf rcu_node structure. */
  	mask = rdp->grpmask;
  	raw_spin_lock_irqsave_rcu_node(rnp, flags); /* Enforce GP memory-order guarantee. */
  	rnp->qsmaskinitnext &= ~mask;
  	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  }
  
  void rcu_report_dead(unsigned int cpu)
  {
  	struct rcu_state *rsp;
  
  	/* QS for any half-done expedited RCU-sched GP. */
  	preempt_disable();
  	rcu_report_exp_rdp(&rcu_sched_state,
  			   this_cpu_ptr(rcu_sched_state.rda), true);
  	preempt_enable();
  	for_each_rcu_flavor(rsp)
  		rcu_cleanup_dying_idle_cpu(cpu, rsp);
  }
  #endif
  
  static int rcu_pm_notify(struct notifier_block *self,
  			 unsigned long action, void *hcpu)
  {
  	switch (action) {
  	case PM_HIBERNATION_PREPARE:
  	case PM_SUSPEND_PREPARE:
  		if (nr_cpu_ids <= 256) /* Expediting bad for large systems. */
  			rcu_expedite_gp();
  		break;
  	case PM_POST_HIBERNATION:
  	case PM_POST_SUSPEND:
  		if (nr_cpu_ids <= 256) /* Expediting bad for large systems. */
  			rcu_unexpedite_gp();
  		break;
  	default:
  		break;
  	}
  	return NOTIFY_OK;
  }
  
  /*
   * Spawn the kthreads that handle each RCU flavor's grace periods.
   */
  static int __init rcu_spawn_gp_kthread(void)
  {
  	unsigned long flags;
  	int kthread_prio_in = kthread_prio;
  	struct rcu_node *rnp;
  	struct rcu_state *rsp;
  	struct sched_param sp;
  	struct task_struct *t;
  
  	/* Force priority into range. */
  	if (IS_ENABLED(CONFIG_RCU_BOOST) && kthread_prio < 1)
  		kthread_prio = 1;
  	else if (kthread_prio < 0)
  		kthread_prio = 0;
  	else if (kthread_prio > 99)
  		kthread_prio = 99;
  	if (kthread_prio != kthread_prio_in)
  		pr_alert("rcu_spawn_gp_kthread(): Limited prio to %d from %d\n",
  			 kthread_prio, kthread_prio_in);
  
  	rcu_scheduler_fully_active = 1;
  	for_each_rcu_flavor(rsp) {
  		t = kthread_create(rcu_gp_kthread, rsp, "%s", rsp->name);
  		BUG_ON(IS_ERR(t));
  		rnp = rcu_get_root(rsp);
  		raw_spin_lock_irqsave_rcu_node(rnp, flags);
  		rsp->gp_kthread = t;
  		if (kthread_prio) {
  			sp.sched_priority = kthread_prio;
  			sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
  		}
  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
  		wake_up_process(t);
  	}
  	rcu_spawn_nocb_kthreads();
  	rcu_spawn_boost_kthreads();
  	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 synchronous grace-period primitives (during which time, this idle
   * task is booting the system, and such primitives are no-ops).  After this
   * function is called, any synchronous grace-period primitives are run as
   * expedited, with the requesting task driving the grace period forward.
   * A later core_initcall() rcu_exp_runtime_mode() will switch to full
   * runtime RCU functionality.
   */
  void rcu_scheduler_starting(void)
  {
  	WARN_ON(num_online_cpus() != 1);
  	WARN_ON(nr_context_switches() > 0);
  	rcu_test_sync_prims();
  	rcu_scheduler_active = RCU_SCHEDULER_INIT;
  	rcu_test_sync_prims();
  }
  
  /*
   * Compute the per-level fanout, either using the exact fanout specified
   * or balancing the tree, depending on the rcu_fanout_exact boot parameter.
   */
  static void __init rcu_init_levelspread(int *levelspread, const int *levelcnt)
  {
  	int i;
  
  	if (rcu_fanout_exact) {
  		levelspread[rcu_num_lvls - 1] = rcu_fanout_leaf;
  		for (i = rcu_num_lvls - 2; i >= 0; i--)
  			levelspread[i] = RCU_FANOUT;
  	} else {
  		int ccur;
  		int cprv;
  
  		cprv = nr_cpu_ids;
  		for (i = rcu_num_lvls - 1; i >= 0; i--) {
  			ccur = levelcnt[i];
  			levelspread[i] = (cprv + ccur - 1) / ccur;
  			cprv = ccur;
  		}
  	}
  }
  
  /*
   * Helper function for rcu_init() that initializes one rcu_state structure.
   */
  static void __init rcu_init_one(struct rcu_state *rsp)
  {
  	static const char * const buf[] = RCU_NODE_NAME_INIT;
  	static const char * const fqs[] = RCU_FQS_NAME_INIT;
  	static struct lock_class_key rcu_node_class[RCU_NUM_LVLS];
  	static struct lock_class_key rcu_fqs_class[RCU_NUM_LVLS];
  	static u8 fl_mask = 0x1;
  
  	int levelcnt[RCU_NUM_LVLS];		/* # nodes in each level. */
  	int levelspread[RCU_NUM_LVLS];		/* kids/node in each level. */
  	int cpustride = 1;
  	int i;
  	int j;
  	struct rcu_node *rnp;
  
  	BUILD_BUG_ON(RCU_NUM_LVLS > ARRAY_SIZE(buf));  /* Fix buf[] init! */
  
  	/* Silence gcc 4.8 false positive about array index out of range. */
  	if (rcu_num_lvls <= 0 || rcu_num_lvls > RCU_NUM_LVLS)
  		panic("rcu_init_one: rcu_num_lvls out of range");
  
  	/* Initialize the level-tracking arrays. */
  
  	for (i = 0; i < rcu_num_lvls; i++)
  		levelcnt[i] = num_rcu_lvl[i];
  	for (i = 1; i < rcu_num_lvls; i++)
  		rsp->level[i] = rsp->level[i - 1] + levelcnt[i - 1];
  	rcu_init_levelspread(levelspread, levelcnt);
  	rsp->flavor_mask = fl_mask;
  	fl_mask <<= 1;
  
  	/* Initialize the elements themselves, starting from the leaves. */
  
  	for (i = rcu_num_lvls - 1; i >= 0; i--) {
  		cpustride *= levelspread[i];
  		rnp = rsp->level[i];
  		for (j = 0; j < levelcnt[i]; j++, rnp++) {
  			raw_spin_lock_init(&ACCESS_PRIVATE(rnp, lock));
  			lockdep_set_class_and_name(&ACCESS_PRIVATE(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_cpu_ids)
  				rnp->grphi = nr_cpu_ids - 1;
  			if (i == 0) {
  				rnp->grpnum = 0;
  				rnp->grpmask = 0;
  				rnp->parent = NULL;
  			} else {
  				rnp->grpnum = j % levelspread[i - 1];
  				rnp->grpmask = 1UL << rnp->grpnum;
  				rnp->parent = rsp->level[i - 1] +
  					      j / levelspread[i - 1];
  			}
  			rnp->level = i;
  			INIT_LIST_HEAD(&rnp->blkd_tasks);
  			rcu_init_one_nocb(rnp);
  			init_waitqueue_head(&rnp->exp_wq[0]);
  			init_waitqueue_head(&rnp->exp_wq[1]);
  			init_waitqueue_head(&rnp->exp_wq[2]);
  			init_waitqueue_head(&rnp->exp_wq[3]);
  			spin_lock_init(&rnp->exp_lock);
  		}
  	}
  
  	init_swait_queue_head(&rsp->gp_wq);
  	init_swait_queue_head(&rsp->expedited_wq);
  	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 tree.h because those are needed to size
   * the ->node array in the rcu_state structure.
   */
  static void __init rcu_init_geometry(void)
  {
  	ulong d;
  	int i;
  	int rcu_capacity[RCU_NUM_LVLS];
  
  	/*
  	 * Initialize any unspecified boot parameters.
  	 * The default values of jiffies_till_first_fqs and
  	 * jiffies_till_next_fqs are set to the RCU_JIFFIES_TILL_FORCE_QS
  	 * value, which is a function of HZ, then adding one for each
  	 * RCU_JIFFIES_FQS_DIV CPUs that might be on the system.
  	 */
  	d = RCU_JIFFIES_TILL_FORCE_QS + nr_cpu_ids / RCU_JIFFIES_FQS_DIV;
  	if (jiffies_till_first_fqs == ULONG_MAX)
  		jiffies_till_first_fqs = d;
  	if (jiffies_till_next_fqs == ULONG_MAX)
  		jiffies_till_next_fqs = d;
  
  	/* If the compile-time values are accurate, just leave. */
  	if (rcu_fanout_leaf == RCU_FANOUT_LEAF &&
  	    nr_cpu_ids == NR_CPUS)
  		return;
  	pr_info("RCU: Adjusting geometry for rcu_fanout_leaf=%d, nr_cpu_ids=%d\n",
  		rcu_fanout_leaf, nr_cpu_ids);
  
  	/*
  	 * The boot-time rcu_fanout_leaf parameter must be at least two
  	 * and cannot exceed the number of bits in the rcu_node masks.
  	 * Complain and fall back to the compile-time values if this
  	 * limit is exceeded.
  	 */
  	if (rcu_fanout_leaf < 2 ||
  	    rcu_fanout_leaf > sizeof(unsigned long) * 8) {
  		rcu_fanout_leaf = RCU_FANOUT_LEAF;
  		WARN_ON(1);
  		return;
  	}
  
  	/*
  	 * Compute number of nodes that can be handled an rcu_node tree
  	 * with the given number of levels.
  	 */
  	rcu_capacity[0] = rcu_fanout_leaf;
  	for (i = 1; i < RCU_NUM_LVLS; i++)
  		rcu_capacity[i] = rcu_capacity[i - 1] * RCU_FANOUT;
  
  	/*
  	 * The tree must be able to accommodate the configured number of CPUs.
  	 * If this limit is exceeded, fall back to the compile-time values.
  	 */
  	if (nr_cpu_ids > rcu_capacity[RCU_NUM_LVLS - 1]) {
  		rcu_fanout_leaf = RCU_FANOUT_LEAF;
  		WARN_ON(1);
  		return;
  	}
  
  	/* Calculate the number of levels in the tree. */
  	for (i = 0; nr_cpu_ids > rcu_capacity[i]; i++) {
  	}
  	rcu_num_lvls = i + 1;
  
  	/* Calculate the number of rcu_nodes at each level of the tree. */
  	for (i = 0; i < rcu_num_lvls; i++) {
  		int cap = rcu_capacity[(rcu_num_lvls - 1) - i];
  		num_rcu_lvl[i] = DIV_ROUND_UP(nr_cpu_ids, cap);
  	}
  
  	/* Calculate the total number of rcu_node structures. */
  	rcu_num_nodes = 0;
  	for (i = 0; i < rcu_num_lvls; i++)
  		rcu_num_nodes += num_rcu_lvl[i];
  }
  
  /*
   * Dump out the structure of the rcu_node combining tree associated
   * with the rcu_state structure referenced by rsp.
   */
  static void __init rcu_dump_rcu_node_tree(struct rcu_state *rsp)
  {
  	int level = 0;
  	struct rcu_node *rnp;
  
  	pr_info("rcu_node tree layout dump\n");
  	pr_info(" ");
  	rcu_for_each_node_breadth_first(rsp, rnp) {
  		if (rnp->level != level) {
  			pr_cont("\n");
  			pr_info(" ");
  			level = rnp->level;
  		}
  		pr_cont("%d:%d ^%d  ", rnp->grplo, rnp->grphi, rnp->grpnum);
  	}
  	pr_cont("\n");
  }
  
  void __init rcu_init(void)
  {
  	int cpu;
  
  	rcu_early_boot_tests();
  
  	rcu_bootup_announce();
  	rcu_init_geometry();
  	rcu_init_one(&rcu_bh_state);
  	rcu_init_one(&rcu_sched_state);
  	if (dump_tree)
  		rcu_dump_rcu_node_tree(&rcu_sched_state);
  	__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.
  	 */
  	pm_notifier(rcu_pm_notify, 0);
  	for_each_online_cpu(cpu) {
  		rcutree_prepare_cpu(cpu);
  		rcu_cpu_starting(cpu);
  	}
  }
  
  #include "tree_exp.h"
  #include "tree_plugin.h"
  