/*
   * 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, 2001

   *
   * Authors: Dipankar Sarma <dipankar@in.ibm.com>
   *	    Manfred Spraul <manfred@colorfullife.com>

   *

   * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
   * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
   * Papers:
   * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
   * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
   *
   * For detailed explanation of Read-Copy Update mechanism see -

   *		http://lse.sourceforge.net/locking/rcupdate.html

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

  #include <linux/atomic.h>

  #include <linux/bitops.h>

  #include <linux/percpu.h>
  #include <linux/notifier.h>

  #include <linux/cpu.h>

  #include <linux/mutex.h>

  #include <linux/export.h>

  #include <linux/hardirq.h>

  #include <linux/delay.h>

  #include <linux/moduleparam.h>

  #include <linux/kthread.h>

  #include <linux/tick.h>

  #define CREATE_TRACE_POINTS

  
  #include "rcu.h"

  #ifdef MODULE_PARAM_PREFIX
  #undef MODULE_PARAM_PREFIX
  #endif
  #define MODULE_PARAM_PREFIX "rcupdate."

  #ifndef CONFIG_TINY_RCU

  module_param(rcu_expedited, int, 0);

  module_param(rcu_normal, int, 0);

  static int rcu_normal_after_boot;
  module_param(rcu_normal_after_boot, int, 0);

  #endif /* #ifndef CONFIG_TINY_RCU */

  #ifdef CONFIG_DEBUG_LOCK_ALLOC

  /**
   * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section?
   *
   * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an
   * RCU-sched read-side critical section.  In absence of
   * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side
   * critical section unless it can prove otherwise.  Note that disabling
   * of preemption (including disabling irqs) counts as an RCU-sched
   * read-side critical section.  This is useful for debug checks in functions
   * that required that they be called within an RCU-sched read-side
   * critical section.
   *
   * Check debug_lockdep_rcu_enabled() to prevent false positives during boot
   * and while lockdep is disabled.
   *
   * Note that if the CPU is in the idle loop from an RCU point of
   * view (ie: that we are in the section between rcu_idle_enter() and
   * rcu_idle_exit()) then rcu_read_lock_held() returns false even if the CPU
   * did an rcu_read_lock().  The reason for this is that RCU ignores CPUs
   * that are in such a section, considering these as in extended quiescent
   * state, so such a CPU is effectively never in an RCU read-side critical
   * section regardless of what RCU primitives it invokes.  This state of
   * affairs is required --- we need to keep an RCU-free window in idle
   * where the CPU may possibly enter into low power mode. This way we can
   * notice an extended quiescent state to other CPUs that started a grace
   * period. Otherwise we would delay any grace period as long as we run in
   * the idle task.
   *
   * Similarly, we avoid claiming an SRCU read lock held if the current
   * CPU is offline.
   */
  int rcu_read_lock_sched_held(void)
  {
  	int lockdep_opinion = 0;
  
  	if (!debug_lockdep_rcu_enabled())
  		return 1;
  	if (!rcu_is_watching())
  		return 0;
  	if (!rcu_lockdep_current_cpu_online())
  		return 0;
  	if (debug_locks)
  		lockdep_opinion = lock_is_held(&rcu_sched_lock_map);

  	return lockdep_opinion || !preemptible();

  }
  EXPORT_SYMBOL(rcu_read_lock_sched_held);
  #endif

  #ifndef CONFIG_TINY_RCU

  /*
   * Should expedited grace-period primitives always fall back to their
   * non-expedited counterparts?  Intended for use within RCU.  Note
   * that if the user specifies both rcu_expedited and rcu_normal, then

   * rcu_normal wins.  (Except during the time period during boot from
   * when the first task is spawned until the rcu_exp_runtime_mode()
   * core_initcall() is invoked, at which point everything is expedited.)

   */
  bool rcu_gp_is_normal(void)
  {

  	return READ_ONCE(rcu_normal) &&
  	       rcu_scheduler_active != RCU_SCHEDULER_INIT;

  }

  EXPORT_SYMBOL_GPL(rcu_gp_is_normal);

  static atomic_t rcu_expedited_nesting =
  	ATOMIC_INIT(IS_ENABLED(CONFIG_RCU_EXPEDITE_BOOT) ? 1 : 0);

  
  /*
   * Should normal grace-period primitives be expedited?  Intended for
   * use within RCU.  Note that this function takes the rcu_expedited

   * sysfs/boot variable and rcu_scheduler_active into account as well
   * as the rcu_expedite_gp() nesting.  So looping on rcu_unexpedite_gp()
   * until rcu_gp_is_expedited() returns false is a -really- bad idea.

   */
  bool rcu_gp_is_expedited(void)
  {

  	return rcu_expedited || atomic_read(&rcu_expedited_nesting) ||
  	       rcu_scheduler_active == RCU_SCHEDULER_INIT;

  }
  EXPORT_SYMBOL_GPL(rcu_gp_is_expedited);
  
  /**
   * rcu_expedite_gp - Expedite future RCU grace periods
   *
   * After a call to this function, future calls to synchronize_rcu() and
   * friends act as the corresponding synchronize_rcu_expedited() function
   * had instead been called.
   */
  void rcu_expedite_gp(void)
  {
  	atomic_inc(&rcu_expedited_nesting);
  }
  EXPORT_SYMBOL_GPL(rcu_expedite_gp);
  
  /**
   * rcu_unexpedite_gp - Cancel prior rcu_expedite_gp() invocation
   *
   * Undo a prior call to rcu_expedite_gp().  If all prior calls to
   * rcu_expedite_gp() are undone by a subsequent call to rcu_unexpedite_gp(),
   * and if the rcu_expedited sysfs/boot parameter is not set, then all
   * subsequent calls to synchronize_rcu() and friends will return to
   * their normal non-expedited behavior.
   */
  void rcu_unexpedite_gp(void)
  {
  	atomic_dec(&rcu_expedited_nesting);
  }
  EXPORT_SYMBOL_GPL(rcu_unexpedite_gp);

  /*
   * Inform RCU of the end of the in-kernel boot sequence.
   */
  void rcu_end_inkernel_boot(void)
  {
  	if (IS_ENABLED(CONFIG_RCU_EXPEDITE_BOOT))
  		rcu_unexpedite_gp();

  	if (rcu_normal_after_boot)
  		WRITE_ONCE(rcu_normal, 1);

  }

  #endif /* #ifndef CONFIG_TINY_RCU */

  #ifdef CONFIG_PREEMPT_RCU
  
  /*

   * Preemptible RCU implementation for rcu_read_lock().
   * Just increment ->rcu_read_lock_nesting, shared state will be updated
   * if we block.
   */
  void __rcu_read_lock(void)
  {
  	current->rcu_read_lock_nesting++;
  	barrier();  /* critical section after entry code. */
  }
  EXPORT_SYMBOL_GPL(__rcu_read_lock);
  
  /*
   * Preemptible RCU implementation for rcu_read_unlock().
   * Decrement ->rcu_read_lock_nesting.  If the result is zero (outermost
   * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
   * invoke rcu_read_unlock_special() to clean up after a context switch
   * in an RCU read-side critical section and other special cases.
   */
  void __rcu_read_unlock(void)
  {
  	struct task_struct *t = current;
  
  	if (t->rcu_read_lock_nesting != 1) {
  		--t->rcu_read_lock_nesting;
  	} else {
  		barrier();  /* critical section before exit code. */
  		t->rcu_read_lock_nesting = INT_MIN;
  		barrier();  /* assign before ->rcu_read_unlock_special load */

  		if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))

  			rcu_read_unlock_special(t);
  		barrier();  /* ->rcu_read_unlock_special load before assign */
  		t->rcu_read_lock_nesting = 0;
  	}
  #ifdef CONFIG_PROVE_LOCKING
  	{

  		int rrln = READ_ONCE(t->rcu_read_lock_nesting);

  
  		WARN_ON_ONCE(rrln < 0 && rrln > INT_MIN / 2);
  	}
  #endif /* #ifdef CONFIG_PROVE_LOCKING */
  }
  EXPORT_SYMBOL_GPL(__rcu_read_unlock);

  #endif /* #ifdef CONFIG_PREEMPT_RCU */

  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  static struct lock_class_key rcu_lock_key;
  struct lockdep_map rcu_lock_map =
  	STATIC_LOCKDEP_MAP_INIT("rcu_read_lock", &rcu_lock_key);
  EXPORT_SYMBOL_GPL(rcu_lock_map);

  
  static struct lock_class_key rcu_bh_lock_key;
  struct lockdep_map rcu_bh_lock_map =
  	STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_bh", &rcu_bh_lock_key);
  EXPORT_SYMBOL_GPL(rcu_bh_lock_map);
  
  static struct lock_class_key rcu_sched_lock_key;
  struct lockdep_map rcu_sched_lock_map =
  	STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_sched", &rcu_sched_lock_key);
  EXPORT_SYMBOL_GPL(rcu_sched_lock_map);

  static struct lock_class_key rcu_callback_key;
  struct lockdep_map rcu_callback_map =
  	STATIC_LOCKDEP_MAP_INIT("rcu_callback", &rcu_callback_key);
  EXPORT_SYMBOL_GPL(rcu_callback_map);

  int notrace debug_lockdep_rcu_enabled(void)

  {

  	return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks &&

  	       current->lockdep_recursion == 0;
  }
  EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled);

  /**

   * rcu_read_lock_held() - might we be in RCU read-side critical section?
   *
   * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an RCU
   * read-side critical section.  In absence of CONFIG_DEBUG_LOCK_ALLOC,
   * this assumes we are in an RCU read-side critical section unless it can
   * prove otherwise.  This is useful for debug checks in functions that
   * require that they be called within an RCU read-side critical section.
   *
   * Checks debug_lockdep_rcu_enabled() to prevent false positives during boot
   * and while lockdep is disabled.
   *
   * Note that rcu_read_lock() and the matching rcu_read_unlock() must
   * occur in the same context, for example, it is illegal to invoke
   * rcu_read_unlock() in process context if the matching rcu_read_lock()
   * was invoked from within an irq handler.
   *
   * Note that rcu_read_lock() is disallowed if the CPU is either idle or
   * offline from an RCU perspective, so check for those as well.
   */
  int rcu_read_lock_held(void)
  {
  	if (!debug_lockdep_rcu_enabled())
  		return 1;
  	if (!rcu_is_watching())
  		return 0;
  	if (!rcu_lockdep_current_cpu_online())
  		return 0;
  	return lock_is_held(&rcu_lock_map);
  }
  EXPORT_SYMBOL_GPL(rcu_read_lock_held);
  
  /**

   * rcu_read_lock_bh_held() - might we be in RCU-bh read-side critical section?

   *
   * Check for bottom half being disabled, which covers both the
   * CONFIG_PROVE_RCU and not cases.  Note that if someone uses
   * rcu_read_lock_bh(), but then later enables BH, lockdep (if enabled)

   * will show the situation.  This is useful for debug checks in functions
   * that require that they be called within an RCU read-side critical
   * section.

   *
   * Check debug_lockdep_rcu_enabled() to prevent false positives during boot.

   *
   * Note that rcu_read_lock() is disallowed if the CPU is either idle or
   * offline from an RCU perspective, so check for those as well.

   */
  int rcu_read_lock_bh_held(void)
  {
  	if (!debug_lockdep_rcu_enabled())
  		return 1;

  	if (!rcu_is_watching())

  		return 0;

  	if (!rcu_lockdep_current_cpu_online())
  		return 0;

  	return in_softirq() || irqs_disabled();

  }
  EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);
  
  #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */

  /**
   * wakeme_after_rcu() - Callback function to awaken a task after grace period
   * @head: Pointer to rcu_head member within rcu_synchronize structure
   *
   * Awaken the corresponding task now that a grace period has elapsed.

   */

  void wakeme_after_rcu(struct rcu_head *head)

  {

  	struct rcu_synchronize *rcu;
  
  	rcu = container_of(head, struct rcu_synchronize, head);
  	complete(&rcu->completion);

  }

  EXPORT_SYMBOL_GPL(wakeme_after_rcu);

  void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array,
  		   struct rcu_synchronize *rs_array)

  {

  	int i;
  
  	/* Initialize and register callbacks for each flavor specified. */
  	for (i = 0; i < n; i++) {
  		if (checktiny &&
  		    (crcu_array[i] == call_rcu ||
  		     crcu_array[i] == call_rcu_bh)) {
  			might_sleep();
  			continue;
  		}
  		init_rcu_head_on_stack(&rs_array[i].head);
  		init_completion(&rs_array[i].completion);
  		(crcu_array[i])(&rs_array[i].head, wakeme_after_rcu);
  	}
  
  	/* Wait for all callbacks to be invoked. */
  	for (i = 0; i < n; i++) {
  		if (checktiny &&
  		    (crcu_array[i] == call_rcu ||
  		     crcu_array[i] == call_rcu_bh))
  			continue;
  		wait_for_completion(&rs_array[i].completion);
  		destroy_rcu_head_on_stack(&rs_array[i].head);
  	}

  }

  EXPORT_SYMBOL_GPL(__wait_rcu_gp);

  #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD

  void init_rcu_head(struct rcu_head *head)

  {
  	debug_object_init(head, &rcuhead_debug_descr);
  }

  void destroy_rcu_head(struct rcu_head *head)

  {
  	debug_object_free(head, &rcuhead_debug_descr);
  }

  static bool rcuhead_is_static_object(void *addr)

  {

  	return true;

  }
  
  /**
   * init_rcu_head_on_stack() - initialize on-stack rcu_head for debugobjects
   * @head: pointer to rcu_head structure to be initialized
   *
   * This function informs debugobjects of a new rcu_head structure that
   * has been allocated as an auto variable on the stack.  This function
   * is not required for rcu_head structures that are statically defined or
   * that are dynamically allocated on the heap.  This function has no
   * effect for !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds.
   */
  void init_rcu_head_on_stack(struct rcu_head *head)
  {
  	debug_object_init_on_stack(head, &rcuhead_debug_descr);
  }
  EXPORT_SYMBOL_GPL(init_rcu_head_on_stack);
  
  /**
   * destroy_rcu_head_on_stack() - destroy on-stack rcu_head for debugobjects
   * @head: pointer to rcu_head structure to be initialized
   *
   * This function informs debugobjects that an on-stack rcu_head structure
   * is about to go out of scope.  As with init_rcu_head_on_stack(), this
   * function is not required for rcu_head structures that are statically
   * defined or that are dynamically allocated on the heap.  Also as with
   * init_rcu_head_on_stack(), this function has no effect for
   * !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds.
   */
  void destroy_rcu_head_on_stack(struct rcu_head *head)
  {
  	debug_object_free(head, &rcuhead_debug_descr);
  }
  EXPORT_SYMBOL_GPL(destroy_rcu_head_on_stack);
  
  struct debug_obj_descr rcuhead_debug_descr = {
  	.name = "rcu_head",

  	.is_static_object = rcuhead_is_static_object,

  };
  EXPORT_SYMBOL_GPL(rcuhead_debug_descr);
  #endif /* #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD */

  #if defined(CONFIG_TREE_RCU) || defined(CONFIG_PREEMPT_RCU) || defined(CONFIG_RCU_TRACE)

  void do_trace_rcu_torture_read(const char *rcutorturename, struct rcu_head *rhp,

  			       unsigned long secs,
  			       unsigned long c_old, unsigned long c)

  {

  	trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c);

  }
  EXPORT_SYMBOL_GPL(do_trace_rcu_torture_read);
  #else

  #define do_trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c) \
  	do { } while (0)

  #endif

  
  #ifdef CONFIG_RCU_STALL_COMMON
  
  #ifdef CONFIG_PROVE_RCU
  #define RCU_STALL_DELAY_DELTA	       (5 * HZ)
  #else
  #define RCU_STALL_DELAY_DELTA	       0
  #endif
  
  int rcu_cpu_stall_suppress __read_mostly; /* 1 = suppress stall warnings. */

  static int rcu_cpu_stall_timeout __read_mostly = CONFIG_RCU_CPU_STALL_TIMEOUT;

  
  module_param(rcu_cpu_stall_suppress, int, 0644);
  module_param(rcu_cpu_stall_timeout, int, 0644);
  
  int rcu_jiffies_till_stall_check(void)
  {

  	int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout);

  
  	/*
  	 * Limit check must be consistent with the Kconfig limits
  	 * for CONFIG_RCU_CPU_STALL_TIMEOUT.
  	 */
  	if (till_stall_check < 3) {

  		WRITE_ONCE(rcu_cpu_stall_timeout, 3);

  		till_stall_check = 3;
  	} else if (till_stall_check > 300) {

  		WRITE_ONCE(rcu_cpu_stall_timeout, 300);

  		till_stall_check = 300;
  	}
  	return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
  }

  void rcu_sysrq_start(void)
  {
  	if (!rcu_cpu_stall_suppress)
  		rcu_cpu_stall_suppress = 2;
  }
  
  void rcu_sysrq_end(void)
  {
  	if (rcu_cpu_stall_suppress == 2)
  		rcu_cpu_stall_suppress = 0;
  }

  static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
  {
  	rcu_cpu_stall_suppress = 1;
  	return NOTIFY_DONE;
  }
  
  static struct notifier_block rcu_panic_block = {
  	.notifier_call = rcu_panic,
  };
  
  static int __init check_cpu_stall_init(void)
  {
  	atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block);
  	return 0;
  }
  early_initcall(check_cpu_stall_init);
  
  #endif /* #ifdef CONFIG_RCU_STALL_COMMON */

  
  #ifdef CONFIG_TASKS_RCU
  
  /*
   * Simple variant of RCU whose quiescent states are voluntary context switch,
   * user-space execution, and idle.  As such, grace periods can take one good
   * long time.  There are no read-side primitives similar to rcu_read_lock()
   * and rcu_read_unlock() because this implementation is intended to get
   * the system into a safe state for some of the manipulations involved in
   * tracing and the like.  Finally, this implementation does not support
   * high call_rcu_tasks() rates from multiple CPUs.  If this is required,
   * per-CPU callback lists will be needed.
   */
  
  /* Global list of callbacks and associated lock. */
  static struct rcu_head *rcu_tasks_cbs_head;
  static struct rcu_head **rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;

  static DECLARE_WAIT_QUEUE_HEAD(rcu_tasks_cbs_wq);

  static DEFINE_RAW_SPINLOCK(rcu_tasks_cbs_lock);

  /* Track exiting tasks in order to allow them to be waited for. */
  DEFINE_SRCU(tasks_rcu_exit_srcu);
  
  /* Control stall timeouts.  Disable with <= 0, otherwise jiffies till stall. */

  static int rcu_task_stall_timeout __read_mostly = HZ * 60 * 10;

  module_param(rcu_task_stall_timeout, int, 0644);

  static void rcu_spawn_tasks_kthread(void);

  static struct task_struct *rcu_tasks_kthread_ptr;

  
  /*
   * Post an RCU-tasks callback.  First call must be from process context
   * after the scheduler if fully operational.
   */

  void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)

  {
  	unsigned long flags;

  	bool needwake;

  	bool havetask = READ_ONCE(rcu_tasks_kthread_ptr);

  
  	rhp->next = NULL;
  	rhp->func = func;
  	raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);

  	needwake = !rcu_tasks_cbs_head;

  	*rcu_tasks_cbs_tail = rhp;
  	rcu_tasks_cbs_tail = &rhp->next;
  	raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);

  	/* We can't create the thread unless interrupts are enabled. */
  	if ((needwake && havetask) ||
  	    (!havetask && !irqs_disabled_flags(flags))) {

  		rcu_spawn_tasks_kthread();

  		wake_up(&rcu_tasks_cbs_wq);

  	}

  }
  EXPORT_SYMBOL_GPL(call_rcu_tasks);

  /**
   * synchronize_rcu_tasks - wait until an rcu-tasks grace period has elapsed.
   *
   * Control will return to the caller some time after a full rcu-tasks
   * grace period has elapsed, in other words after all currently
   * executing rcu-tasks read-side critical sections have elapsed.  These
   * read-side critical sections are delimited by calls to schedule(),
   * cond_resched_rcu_qs(), idle execution, userspace execution, calls
   * to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched().
   *
   * This is a very specialized primitive, intended only for a few uses in
   * tracing and other situations requiring manipulation of function
   * preambles and profiling hooks.  The synchronize_rcu_tasks() function
   * is not (yet) intended for heavy use from multiple CPUs.
   *
   * Note that this guarantee implies further memory-ordering guarantees.
   * On systems with more than one CPU, when synchronize_rcu_tasks() returns,
   * each CPU is guaranteed to have executed a full memory barrier since the
   * end of its last RCU-tasks read-side critical section whose beginning
   * preceded the call to synchronize_rcu_tasks().  In addition, each CPU
   * having an RCU-tasks read-side critical section that extends beyond
   * the return from synchronize_rcu_tasks() is guaranteed to have executed
   * a full memory barrier after the beginning of synchronize_rcu_tasks()
   * and before the beginning of that RCU-tasks 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_rcu_tasks(), 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_rcu_tasks() -- even if CPU A and CPU B are the same CPU
   * (but again only if the system has more than one CPU).
   */
  void synchronize_rcu_tasks(void)
  {
  	/* Complain if the scheduler has not started.  */

  	RCU_LOCKDEP_WARN(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,

  			 "synchronize_rcu_tasks called too soon");

  
  	/* Wait for the grace period. */
  	wait_rcu_gp(call_rcu_tasks);
  }

  EXPORT_SYMBOL_GPL(synchronize_rcu_tasks);

  
  /**
   * rcu_barrier_tasks - Wait for in-flight call_rcu_tasks() callbacks.
   *
   * Although the current implementation is guaranteed to wait, it is not
   * obligated to, for example, if there are no pending callbacks.
   */
  void rcu_barrier_tasks(void)
  {
  	/* There is only one callback queue, so this is easy.  ;-) */
  	synchronize_rcu_tasks();
  }

  EXPORT_SYMBOL_GPL(rcu_barrier_tasks);

  /* See if tasks are still holding out, complain if so. */
  static void check_holdout_task(struct task_struct *t,
  			       bool needreport, bool *firstreport)

  {

  	int cpu;

  	if (!READ_ONCE(t->rcu_tasks_holdout) ||
  	    t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) ||
  	    !READ_ONCE(t->on_rq) ||

  	    (IS_ENABLED(CONFIG_NO_HZ_FULL) &&
  	     !is_idle_task(t) && t->rcu_tasks_idle_cpu >= 0)) {

  		WRITE_ONCE(t->rcu_tasks_holdout, false);

  		list_del_init(&t->rcu_tasks_holdout_list);

  		put_task_struct(t);

  		return;

  	}

  	if (!needreport)
  		return;
  	if (*firstreport) {
  		pr_err("INFO: rcu_tasks detected stalls on tasks:
  ");
  		*firstreport = false;
  	}

  	cpu = task_cpu(t);
  	pr_alert("%p: %c%c nvcsw: %lu/%lu holdout: %d idle_cpu: %d/%d
  ",
  		 t, ".I"[is_idle_task(t)],
  		 "N."[cpu < 0 || !tick_nohz_full_cpu(cpu)],
  		 t->rcu_tasks_nvcsw, t->nvcsw, t->rcu_tasks_holdout,
  		 t->rcu_tasks_idle_cpu, cpu);

  	sched_show_task(t);

  }
  
  /* RCU-tasks kthread that detects grace periods and invokes callbacks. */
  static int __noreturn rcu_tasks_kthread(void *arg)
  {
  	unsigned long flags;
  	struct task_struct *g, *t;

  	unsigned long lastreport;

  	struct rcu_head *list;
  	struct rcu_head *next;
  	LIST_HEAD(rcu_tasks_holdouts);

  	/* Run on housekeeping CPUs by default.  Sysadm can move if desired. */
  	housekeeping_affine(current);

  
  	/*
  	 * Each pass through the following loop makes one check for
  	 * newly arrived callbacks, and, if there are some, waits for
  	 * one RCU-tasks grace period and then invokes the callbacks.
  	 * This loop is terminated by the system going down.  ;-)
  	 */
  	for (;;) {
  
  		/* Pick up any new callbacks. */
  		raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
  		list = rcu_tasks_cbs_head;
  		rcu_tasks_cbs_head = NULL;
  		rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
  		raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
  
  		/* If there were none, wait a bit and start over. */
  		if (!list) {

  			wait_event_interruptible(rcu_tasks_cbs_wq,
  						 rcu_tasks_cbs_head);
  			if (!rcu_tasks_cbs_head) {
  				WARN_ON(signal_pending(current));
  				schedule_timeout_interruptible(HZ/10);
  			}

  			continue;
  		}
  
  		/*
  		 * Wait for all pre-existing t->on_rq and t->nvcsw
  		 * transitions to complete.  Invoking synchronize_sched()
  		 * suffices because all these transitions occur with
  		 * interrupts disabled.  Without this synchronize_sched(),
  		 * a read-side critical section that started before the
  		 * grace period might be incorrectly seen as having started
  		 * after the grace period.
  		 *
  		 * This synchronize_sched() also dispenses with the
  		 * need for a memory barrier on the first store to
  		 * ->rcu_tasks_holdout, as it forces the store to happen
  		 * after the beginning of the grace period.
  		 */
  		synchronize_sched();
  
  		/*
  		 * There were callbacks, so we need to wait for an
  		 * RCU-tasks grace period.  Start off by scanning
  		 * the task list for tasks that are not already
  		 * voluntarily blocked.  Mark these tasks and make
  		 * a list of them in rcu_tasks_holdouts.
  		 */
  		rcu_read_lock();
  		for_each_process_thread(g, t) {

  			if (t != current && READ_ONCE(t->on_rq) &&

  			    !is_idle_task(t)) {
  				get_task_struct(t);

  				t->rcu_tasks_nvcsw = READ_ONCE(t->nvcsw);
  				WRITE_ONCE(t->rcu_tasks_holdout, true);

  				list_add(&t->rcu_tasks_holdout_list,
  					 &rcu_tasks_holdouts);
  			}
  		}
  		rcu_read_unlock();
  
  		/*

  		 * Wait for tasks that are in the process of exiting.
  		 * This does only part of the job, ensuring that all
  		 * tasks that were previously exiting reach the point
  		 * where they have disabled preemption, allowing the
  		 * later synchronize_sched() to finish the job.
  		 */
  		synchronize_srcu(&tasks_rcu_exit_srcu);
  
  		/*

  		 * Each pass through the following loop scans the list
  		 * of holdout tasks, removing any that are no longer
  		 * holdouts.  When the list is empty, we are done.
  		 */

  		lastreport = jiffies;

  		while (!list_empty(&rcu_tasks_holdouts)) {

  			bool firstreport;
  			bool needreport;
  			int rtst;

  			struct task_struct *t1;

  			schedule_timeout_interruptible(HZ);

  			rtst = READ_ONCE(rcu_task_stall_timeout);

  			needreport = rtst > 0 &&
  				     time_after(jiffies, lastreport + rtst);
  			if (needreport)
  				lastreport = jiffies;
  			firstreport = true;

  			WARN_ON(signal_pending(current));

  			list_for_each_entry_safe(t, t1, &rcu_tasks_holdouts,
  						rcu_tasks_holdout_list) {

  				check_holdout_task(t, needreport, &firstreport);

  				cond_resched();
  			}

  		}
  
  		/*
  		 * Because ->on_rq and ->nvcsw are not guaranteed
  		 * to have a full memory barriers prior to them in the
  		 * schedule() path, memory reordering on other CPUs could
  		 * cause their RCU-tasks read-side critical sections to
  		 * extend past the end of the grace period.  However,
  		 * because these ->nvcsw updates are carried out with
  		 * interrupts disabled, we can use synchronize_sched()
  		 * to force the needed ordering on all such CPUs.
  		 *
  		 * This synchronize_sched() also confines all
  		 * ->rcu_tasks_holdout accesses to be within the grace
  		 * period, avoiding the need for memory barriers for
  		 * ->rcu_tasks_holdout accesses.

  		 *
  		 * In addition, this synchronize_sched() waits for exiting
  		 * tasks to complete their final preempt_disable() region
  		 * of execution, cleaning up after the synchronize_srcu()
  		 * above.

  		 */
  		synchronize_sched();
  
  		/* Invoke the callbacks. */
  		while (list) {
  			next = list->next;
  			local_bh_disable();
  			list->func(list);
  			local_bh_enable();
  			list = next;
  			cond_resched();
  		}

  		schedule_timeout_uninterruptible(HZ/10);

  	}
  }

  /* Spawn rcu_tasks_kthread() at first call to call_rcu_tasks(). */
  static void rcu_spawn_tasks_kthread(void)

  {

  	static DEFINE_MUTEX(rcu_tasks_kthread_mutex);

  	struct task_struct *t;

  	if (READ_ONCE(rcu_tasks_kthread_ptr)) {

  		smp_mb(); /* Ensure caller sees full kthread. */
  		return;
  	}
  	mutex_lock(&rcu_tasks_kthread_mutex);
  	if (rcu_tasks_kthread_ptr) {
  		mutex_unlock(&rcu_tasks_kthread_mutex);
  		return;
  	}

  	t = kthread_run(rcu_tasks_kthread, NULL, "rcu_tasks_kthread");
  	BUG_ON(IS_ERR(t));

  	smp_mb(); /* Ensure others see full kthread. */

  	WRITE_ONCE(rcu_tasks_kthread_ptr, t);

  	mutex_unlock(&rcu_tasks_kthread_mutex);

  }

  
  #endif /* #ifdef CONFIG_TASKS_RCU */

  /*
   * Test each non-SRCU synchronous grace-period wait API.  This is
   * useful just after a change in mode for these primitives, and
   * during early boot.
   */
  void rcu_test_sync_prims(void)
  {
  	if (!IS_ENABLED(CONFIG_PROVE_RCU))
  		return;
  	synchronize_rcu();
  	synchronize_rcu_bh();
  	synchronize_sched();
  	synchronize_rcu_expedited();
  	synchronize_rcu_bh_expedited();
  	synchronize_sched_expedited();
  }

  #ifdef CONFIG_PROVE_RCU
  
  /*
   * Early boot self test parameters, one for each flavor
   */
  static bool rcu_self_test;
  static bool rcu_self_test_bh;
  static bool rcu_self_test_sched;
  
  module_param(rcu_self_test, bool, 0444);
  module_param(rcu_self_test_bh, bool, 0444);
  module_param(rcu_self_test_sched, bool, 0444);
  
  static int rcu_self_test_counter;
  
  static void test_callback(struct rcu_head *r)
  {
  	rcu_self_test_counter++;
  	pr_info("RCU test callback executed %d
  ", rcu_self_test_counter);
  }
  
  static void early_boot_test_call_rcu(void)
  {
  	static struct rcu_head head;
  
  	call_rcu(&head, test_callback);
  }
  
  static void early_boot_test_call_rcu_bh(void)
  {
  	static struct rcu_head head;
  
  	call_rcu_bh(&head, test_callback);
  }
  
  static void early_boot_test_call_rcu_sched(void)
  {
  	static struct rcu_head head;
  
  	call_rcu_sched(&head, test_callback);
  }
  
  void rcu_early_boot_tests(void)
  {
  	pr_info("Running RCU self tests
  ");
  
  	if (rcu_self_test)
  		early_boot_test_call_rcu();
  	if (rcu_self_test_bh)
  		early_boot_test_call_rcu_bh();
  	if (rcu_self_test_sched)
  		early_boot_test_call_rcu_sched();

  	rcu_test_sync_prims();

  }
  
  static int rcu_verify_early_boot_tests(void)
  {
  	int ret = 0;
  	int early_boot_test_counter = 0;
  
  	if (rcu_self_test) {
  		early_boot_test_counter++;
  		rcu_barrier();
  	}
  	if (rcu_self_test_bh) {
  		early_boot_test_counter++;
  		rcu_barrier_bh();
  	}
  	if (rcu_self_test_sched) {
  		early_boot_test_counter++;
  		rcu_barrier_sched();
  	}
  
  	if (rcu_self_test_counter != early_boot_test_counter) {
  		WARN_ON(1);
  		ret = -1;
  	}
  
  	return ret;
  }
  late_initcall(rcu_verify_early_boot_tests);
  #else
  void rcu_early_boot_tests(void) {}
  #endif /* CONFIG_PROVE_RCU */