#define pr_fmt(fmt) "%s: " fmt "
  ", __func__
  
  #include <linux/kernel.h>

  #include <linux/sched.h>
  #include <linux/wait.h>

  #include <linux/percpu-refcount.h>
  
  /*
   * Initially, a percpu refcount is just a set of percpu counters. Initially, we
   * don't try to detect the ref hitting 0 - which means that get/put can just
   * increment or decrement the local counter. Note that the counter on a
   * particular cpu can (and will) wrap - this is fine, when we go to shutdown the
   * percpu counters will all sum to the correct value
   *
   * (More precisely: because moduler arithmatic is commutative the sum of all the

   * percpu_count vars will be equal to what it would have been if all the gets
   * and puts were done to a single integer, even if some of the percpu integers

   * overflow or underflow).
   *
   * The real trick to implementing percpu refcounts is shutdown. We can't detect
   * the ref hitting 0 on every put - this would require global synchronization
   * and defeat the whole purpose of using percpu refs.
   *
   * What we do is require the user to keep track of the initial refcount; we know
   * the ref can't hit 0 before the user drops the initial ref, so as long as we
   * convert to non percpu mode before the initial ref is dropped everything
   * works.
   *
   * Converting to non percpu mode is done with some RCUish stuff in

   * percpu_ref_kill. Additionally, we need a bias value so that the
   * atomic_long_t can't hit 0 before we've added up all the percpu refs.

   */

  #define PERCPU_COUNT_BIAS	(1LU << (BITS_PER_LONG - 1))

  static DECLARE_WAIT_QUEUE_HEAD(percpu_ref_switch_waitq);

  static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref)

  {

  	return (unsigned long __percpu *)

  		(ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC_DEAD);

  }

  /**
   * percpu_ref_init - initialize a percpu refcount

   * @ref: percpu_ref to initialize
   * @release: function which will be called when refcount hits 0

   * @flags: PERCPU_REF_INIT_* flags

   * @gfp: allocation mask to use

   *

   * Initializes @ref.  If @flags is zero, @ref starts in percpu mode with a
   * refcount of 1; analagous to atomic_long_set(ref, 1).  See the
   * definitions of PERCPU_REF_INIT_* flags for flag behaviors.

   *
   * Note that @release must not sleep - it may potentially be called from RCU
   * callback context by percpu_ref_kill().
   */

  int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,

  		    unsigned int flags, gfp_t gfp)

  {

  	size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS,
  			     __alignof__(unsigned long));

  	unsigned long start_count = 0;

  	ref->percpu_count_ptr = (unsigned long)
  		__alloc_percpu_gfp(sizeof(unsigned long), align, gfp);

  	if (!ref->percpu_count_ptr)

  		return -ENOMEM;

  	ref->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;

  	if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD))
  		ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
  	else
  		start_count += PERCPU_COUNT_BIAS;
  
  	if (flags & PERCPU_REF_INIT_DEAD)
  		ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
  	else
  		start_count++;
  
  	atomic_long_set(&ref->count, start_count);

  	ref->release = release;
  	return 0;
  }

  EXPORT_SYMBOL_GPL(percpu_ref_init);

  /**

   * percpu_ref_exit - undo percpu_ref_init()
   * @ref: percpu_ref to exit

   *

   * This function exits @ref.  The caller is responsible for ensuring that
   * @ref is no longer in active use.  The usual places to invoke this
   * function from are the @ref->release() callback or in init failure path
   * where percpu_ref_init() succeeded but other parts of the initialization
   * of the embedding object failed.

   */

  void percpu_ref_exit(struct percpu_ref *ref)

  {

  	unsigned long __percpu *percpu_count = percpu_count_ptr(ref);

  	if (percpu_count) {
  		free_percpu(percpu_count);

  		ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;

  	}
  }

  EXPORT_SYMBOL_GPL(percpu_ref_exit);

  static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
  {
  	struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
  
  	ref->confirm_switch(ref);
  	ref->confirm_switch = NULL;
  	wake_up_all(&percpu_ref_switch_waitq);
  
  	/* drop ref from percpu_ref_switch_to_atomic() */
  	percpu_ref_put(ref);
  }
  
  static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)

  {
  	struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);

  	unsigned long __percpu *percpu_count = percpu_count_ptr(ref);

  	unsigned long count = 0;

  	int cpu;

  	for_each_possible_cpu(cpu)

  		count += *per_cpu_ptr(percpu_count, cpu);

  	pr_debug("global %ld percpu %ld",

  		 atomic_long_read(&ref->count), (long)count);

  
  	/*
  	 * It's crucial that we sum the percpu counters _before_ adding the sum
  	 * to &ref->count; since gets could be happening on one cpu while puts
  	 * happen on another, adding a single cpu's count could cause
  	 * @ref->count to hit 0 before we've got a consistent value - but the
  	 * sum of all the counts will be consistent and correct.
  	 *
  	 * Subtracting the bias value then has to happen _after_ adding count to
  	 * &ref->count; we need the bias value to prevent &ref->count from
  	 * reaching 0 before we add the percpu counts. But doing it at the same
  	 * time is equivalent and saves us atomic operations:
  	 */

  	atomic_long_add((long)count - PERCPU_COUNT_BIAS, &ref->count);

  	WARN_ONCE(atomic_long_read(&ref->count) <= 0,

  		  "percpu ref (%pf) <= 0 (%ld) after switching to atomic",

  		  ref->release, atomic_long_read(&ref->count));

  	/* @ref is viewed as dead on all CPUs, send out switch confirmation */
  	percpu_ref_call_confirm_rcu(rcu);
  }

  static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref)
  {
  }
  
  static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
  					  percpu_ref_func_t *confirm_switch)
  {
  	if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC)) {
  		/* switching from percpu to atomic */
  		ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
  
  		/*
  		 * Non-NULL ->confirm_switch is used to indicate that
  		 * switching is in progress.  Use noop one if unspecified.
  		 */
  		WARN_ON_ONCE(ref->confirm_switch);
  		ref->confirm_switch =
  			confirm_switch ?: percpu_ref_noop_confirm_switch;
  
  		percpu_ref_get(ref);	/* put after confirmation */
  		call_rcu_sched(&ref->rcu, percpu_ref_switch_to_atomic_rcu);
  	} else if (confirm_switch) {
  		/*
  		 * Somebody already set ATOMIC.  Switching may still be in
  		 * progress.  @confirm_switch must be invoked after the
  		 * switching is complete and a full sched RCU grace period
  		 * has passed.  Wait synchronously for the previous
  		 * switching and schedule @confirm_switch invocation.
  		 */
  		wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
  		ref->confirm_switch = confirm_switch;
  
  		percpu_ref_get(ref);	/* put after confirmation */
  		call_rcu_sched(&ref->rcu, percpu_ref_call_confirm_rcu);
  	}

  }
  
  /**

   * percpu_ref_switch_to_atomic - switch a percpu_ref to atomic mode
   * @ref: percpu_ref to switch to atomic mode
   * @confirm_switch: optional confirmation callback

   *

   * There's no reason to use this function for the usual reference counting.
   * Use percpu_ref_kill[_and_confirm]().
   *
   * Schedule switching of @ref to atomic mode.  All its percpu counts will
   * be collected to the main atomic counter.  On completion, when all CPUs
   * are guaraneed to be in atomic mode, @confirm_switch, which may not
   * block, is invoked.  This function may be invoked concurrently with all
   * the get/put operations and can safely be mixed with kill and reinit

   * operations.  Note that @ref will stay in atomic mode across kill/reinit
   * cycles until percpu_ref_switch_to_percpu() is called.

   *

   * This function normally doesn't block and can be called from any context
   * but it may block if @confirm_kill is specified and @ref is already in
   * the process of switching to atomic mode.  In such cases, @confirm_switch
   * will be invoked after the switching is complete.
   *
   * Due to the way percpu_ref is implemented, @confirm_switch will be called
   * after at least one full sched RCU grace period has passed but this is an
   * implementation detail and must not be depended upon.

   */

  void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
  				 percpu_ref_func_t *confirm_switch)

  {

  	ref->force_atomic = true;

  	__percpu_ref_switch_to_atomic(ref, confirm_switch);

  }

  static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)

  {

  	unsigned long __percpu *percpu_count = percpu_count_ptr(ref);

  	int cpu;

  	BUG_ON(!percpu_count);

  	if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))
  		return;
  
  	wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
  
  	atomic_long_add(PERCPU_COUNT_BIAS, &ref->count);

  
  	/*
  	 * Restore per-cpu operation.  smp_store_release() is paired with

  	 * smp_read_barrier_depends() in __ref_is_percpu() and guarantees
  	 * that the zeroing is visible to all percpu accesses which can see

  	 * the following __PERCPU_REF_ATOMIC clearing.

  	 */
  	for_each_possible_cpu(cpu)

  		*per_cpu_ptr(percpu_count, cpu) = 0;

  	smp_store_release(&ref->percpu_count_ptr,

  			  ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);
  }
  
  /**
   * percpu_ref_switch_to_percpu - switch a percpu_ref to percpu mode
   * @ref: percpu_ref to switch to percpu mode
   *
   * There's no reason to use this function for the usual reference counting.
   * To re-use an expired ref, use percpu_ref_reinit().
   *
   * Switch @ref to percpu mode.  This function may be invoked concurrently
   * with all the get/put operations and can safely be mixed with kill and

   * reinit operations.  This function reverses the sticky atomic state set
   * by PERCPU_REF_INIT_ATOMIC or percpu_ref_switch_to_atomic().  If @ref is
   * dying or dead, the actual switching takes place on the following
   * percpu_ref_reinit().

   *
   * This function normally doesn't block and can be called from any context
   * but it may block if @ref is in the process of switching to atomic mode
   * by percpu_ref_switch_atomic().
   */
  void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
  {

  	ref->force_atomic = false;

  	/* a dying or dead ref can't be switched to percpu mode w/o reinit */
  	if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD))
  		__percpu_ref_switch_to_percpu(ref);

  }

  
  /**
   * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
   * @ref: percpu_ref to kill
   * @confirm_kill: optional confirmation callback
   *
   * Equivalent to percpu_ref_kill() but also schedules kill confirmation if
   * @confirm_kill is not NULL.  @confirm_kill, which may not block, will be
   * called after @ref is seen as dead from all CPUs at which point all
   * further invocations of percpu_ref_tryget_live() will fail.  See
   * percpu_ref_tryget_live() for details.
   *
   * This function normally doesn't block and can be called from any context

   * but it may block if @confirm_kill is specified and @ref is in the
   * process of switching to atomic mode by percpu_ref_switch_atomic().

   *
   * Due to the way percpu_ref is implemented, @confirm_switch will be called
   * after at least one full sched RCU grace period has passed but this is an
   * implementation detail and must not be depended upon.
   */
  void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
  				 percpu_ref_func_t *confirm_kill)
  {
  	WARN_ONCE(ref->percpu_count_ptr & __PERCPU_REF_DEAD,
  		  "%s called more than once on %pf!", __func__, ref->release);
  
  	ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
  	__percpu_ref_switch_to_atomic(ref, confirm_kill);
  	percpu_ref_put(ref);
  }
  EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);

  
  /**
   * percpu_ref_reinit - re-initialize a percpu refcount
   * @ref: perpcu_ref to re-initialize
   *
   * Re-initialize @ref so that it's in the same state as when it finished

   * percpu_ref_init() ignoring %PERCPU_REF_INIT_DEAD.  @ref must have been
   * initialized successfully and reached 0 but not exited.

   *
   * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
   * this function is in progress.
   */
  void percpu_ref_reinit(struct percpu_ref *ref)
  {
  	WARN_ON_ONCE(!percpu_ref_is_zero(ref));
  
  	ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
  	percpu_ref_get(ref);

  	if (!ref->force_atomic)
  		__percpu_ref_switch_to_percpu(ref);

  }
  EXPORT_SYMBOL_GPL(percpu_ref_reinit);