#define pr_fmt(fmt) "%s: " fmt "
  ", __func__
  
  #include <linux/kernel.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
   * pcpu_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_t
   * can't hit 0 before we've added up all the percpu refs.
   */
  
  #define PCPU_COUNT_BIAS		(1U << 31)
  
  /**
   * percpu_ref_init - initialize a percpu refcount

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

   *
   * Initializes the refcount in single atomic counter mode with a refcount of 1;
   * analagous to atomic_set(ref, 1).
   *
   * 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)

  {
  	atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS);
  
  	ref->pcpu_count = alloc_percpu(unsigned);
  	if (!ref->pcpu_count)
  		return -ENOMEM;
  
  	ref->release = release;
  	return 0;
  }

  /**
   * percpu_ref_cancel_init - cancel percpu_ref_init()
   * @ref: percpu_ref to cancel init for
   *
   * Once a percpu_ref is initialized, its destruction is initiated by
   * percpu_ref_kill() and completes asynchronously, which can be painful to
   * do when destroying a half-constructed object in init failure path.
   *
   * This function destroys @ref without invoking @ref->release and the
   * memory area containing it can be freed immediately on return.  To
   * prevent accidental misuse, it's required that @ref has finished
   * percpu_ref_init(), whether successful or not, but never used.
   *
   * The weird name and usage restriction are to prevent people from using
   * this function by mistake for normal shutdown instead of
   * percpu_ref_kill().
   */
  void percpu_ref_cancel_init(struct percpu_ref *ref)
  {
  	unsigned __percpu *pcpu_count = ref->pcpu_count;
  	int cpu;
  
  	WARN_ON_ONCE(atomic_read(&ref->count) != 1 + PCPU_COUNT_BIAS);
  
  	if (pcpu_count) {
  		for_each_possible_cpu(cpu)
  			WARN_ON_ONCE(*per_cpu_ptr(pcpu_count, cpu));
  		free_percpu(ref->pcpu_count);
  	}
  }

  static void percpu_ref_kill_rcu(struct rcu_head *rcu)
  {
  	struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);

  	unsigned __percpu *pcpu_count = ref->pcpu_count;

  	unsigned count = 0;
  	int cpu;

  	/* Mask out PCPU_REF_DEAD */
  	pcpu_count = (unsigned __percpu *)
  		(((unsigned long) pcpu_count) & ~PCPU_STATUS_MASK);
  
  	for_each_possible_cpu(cpu)
  		count += *per_cpu_ptr(pcpu_count, cpu);
  
  	free_percpu(pcpu_count);
  
  	pr_debug("global %i pcpu %i", atomic_read(&ref->count), (int) 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_add((int) count - PCPU_COUNT_BIAS, &ref->count);

  	/* @ref is viewed as dead on all CPUs, send out kill confirmation */
  	if (ref->confirm_kill)
  		ref->confirm_kill(ref);

  	/*
  	 * Now we're in single atomic_t mode with a consistent refcount, so it's
  	 * safe to drop our initial ref:
  	 */
  	percpu_ref_put(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 - all further
   * invocations of percpu_ref_tryget() will fail.  See percpu_ref_tryget()
   * for more details.

   *

   * Due to the way percpu_ref is implemented, @confirm_kill will be called
   * after at least one full RCU grace period has passed but this is an
   * implementation detail and callers must not depend on it.

   */

  void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
  				 percpu_ref_func_t *confirm_kill)

  {

  	WARN_ONCE(REF_STATUS(ref->pcpu_count) == PCPU_REF_DEAD,
  		  "percpu_ref_kill() called more than once!
  ");

  	ref->pcpu_count = (unsigned __percpu *)
  		(((unsigned long) ref->pcpu_count)|PCPU_REF_DEAD);

  	ref->confirm_kill = confirm_kill;

  	call_rcu_sched(&ref->rcu, percpu_ref_kill_rcu);

  }