/*
   * Percpu refcounts:
   * (C) 2012 Google, Inc.
   * Author: Kent Overstreet <koverstreet@google.com>
   *
   * This implements a refcount with similar semantics to atomic_t - atomic_inc(),
   * atomic_dec_and_test() - but percpu.
   *
   * There's one important difference between percpu refs and normal atomic_t
   * refcounts; you have to keep track of your initial refcount, and then when you
   * start shutting down you call percpu_ref_kill() _before_ dropping the initial
   * refcount.
   *
   * The refcount will have a range of 0 to ((1U << 31) - 1), i.e. one bit less
   * than an atomic_t - this is because of the way shutdown works, see

   * percpu_ref_kill()/PERCPU_COUNT_BIAS.

   *
   * Before you call percpu_ref_kill(), percpu_ref_put() does not check for the
   * refcount hitting 0 - it can't, if it was in percpu mode. percpu_ref_kill()
   * puts the ref back in single atomic_t mode, collecting the per cpu refs and
   * issuing the appropriate barriers, and then marks the ref as shutting down so
   * that percpu_ref_put() will check for the ref hitting 0.  After it returns,
   * it's safe to drop the initial ref.
   *
   * USAGE:
   *
   * See fs/aio.c for some example usage; it's used there for struct kioctx, which
   * is created when userspaces calls io_setup(), and destroyed when userspace
   * calls io_destroy() or the process exits.
   *
   * In the aio code, kill_ioctx() is called when we wish to destroy a kioctx; it

   * calls percpu_ref_kill(), then hlist_del_rcu() and synchronize_rcu() to remove

   * the kioctx from the proccess's list of kioctxs - after that, there can't be
   * any new users of the kioctx (from lookup_ioctx()) and it's then safe to drop
   * the initial ref with percpu_ref_put().
   *
   * Code that does a two stage shutdown like this often needs some kind of
   * explicit synchronization to ensure the initial refcount can only be dropped
   * once - percpu_ref_kill() does this for you, it returns true once and false if
   * someone else already called it. The aio code uses it this way, but it's not
   * necessary if the code has some other mechanism to synchronize teardown.
   * around.
   */
  
  #ifndef _LINUX_PERCPU_REFCOUNT_H
  #define _LINUX_PERCPU_REFCOUNT_H
  
  #include <linux/atomic.h>
  #include <linux/kernel.h>
  #include <linux/percpu.h>
  #include <linux/rcupdate.h>

  #include <linux/gfp.h>

  
  struct percpu_ref;

  typedef void (percpu_ref_func_t)(struct percpu_ref *);

  /* flags set in the lower bits of percpu_ref->percpu_count_ptr */
  enum {
  	__PERCPU_REF_ATOMIC	= 1LU << 0,	/* operating in atomic mode */

  	__PERCPU_REF_DEAD	= 1LU << 1,	/* (being) killed */
  	__PERCPU_REF_ATOMIC_DEAD = __PERCPU_REF_ATOMIC | __PERCPU_REF_DEAD,
  
  	__PERCPU_REF_FLAG_BITS	= 2,

  };

  /* @flags for percpu_ref_init() */
  enum {
  	/*
  	 * Start w/ ref == 1 in atomic mode.  Can be switched to percpu

  	 * operation using percpu_ref_switch_to_percpu().  If initialized
  	 * with this flag, the ref will stay in atomic mode until
  	 * percpu_ref_switch_to_percpu() is invoked on it.

  	 */
  	PERCPU_REF_INIT_ATOMIC	= 1 << 0,
  
  	/*
  	 * Start dead w/ ref == 0 in atomic mode.  Must be revived with
  	 * percpu_ref_reinit() before used.  Implies INIT_ATOMIC.
  	 */
  	PERCPU_REF_INIT_DEAD	= 1 << 1,
  };

  struct percpu_ref {

  	atomic_long_t		count;

  	/*
  	 * The low bit of the pointer indicates whether the ref is in percpu

  	 * mode; if set, then get/put will manipulate the atomic_t.

  	 */

  	unsigned long		percpu_count_ptr;

  	percpu_ref_func_t	*release;

  	percpu_ref_func_t	*confirm_switch;

  	bool			force_atomic:1;

  	struct rcu_head		rcu;
  };

  int __must_check percpu_ref_init(struct percpu_ref *ref,

  				 percpu_ref_func_t *release, unsigned int flags,
  				 gfp_t gfp);

  void percpu_ref_exit(struct percpu_ref *ref);

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

  void percpu_ref_switch_to_percpu(struct percpu_ref *ref);

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

  void percpu_ref_reinit(struct percpu_ref *ref);

  
  /**
   * percpu_ref_kill - drop the initial ref
   * @ref: percpu_ref to kill
   *
   * Must be used to drop the initial ref on a percpu refcount; must be called
   * precisely once before shutdown.
   *
   * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
   * percpu counters and dropping the initial ref.
   */
  static inline void percpu_ref_kill(struct percpu_ref *ref)
  {
  	return percpu_ref_kill_and_confirm(ref, NULL);
  }

  /*
   * Internal helper.  Don't use outside percpu-refcount proper.  The
   * function doesn't return the pointer and let the caller test it for NULL
   * because doing so forces the compiler to generate two conditional

   * branches as it can't assume that @ref->percpu_count is not NULL.

   */

  static inline bool __ref_is_percpu(struct percpu_ref *ref,
  					  unsigned long __percpu **percpu_countp)

  {

  	unsigned long percpu_ptr;
  
  	/*
  	 * The value of @ref->percpu_count_ptr is tested for
  	 * !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then
  	 * used as a pointer.  If the compiler generates a separate fetch
  	 * when using it as a pointer, __PERCPU_REF_ATOMIC may be set in
  	 * between contaminating the pointer value, meaning that
  	 * ACCESS_ONCE() is required when fetching it.
  	 *
  	 * Also, we need a data dependency barrier to be paired with
  	 * smp_store_release() in __percpu_ref_switch_to_percpu().
  	 *
  	 * Use lockless deref which contains both.
  	 */
  	percpu_ptr = lockless_dereference(ref->percpu_count_ptr);

  	/*
  	 * Theoretically, the following could test just ATOMIC; however,
  	 * then we'd have to mask off DEAD separately as DEAD may be
  	 * visible without ATOMIC if we race with percpu_ref_kill().  DEAD
  	 * implies ATOMIC anyway.  Test them together.
  	 */
  	if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD))

  		return false;

  	*percpu_countp = (unsigned long __percpu *)percpu_ptr;

  	return true;
  }

  
  /**

   * percpu_ref_get_many - increment a percpu refcount

   * @ref: percpu_ref to get

   * @nr: number of references to get

   *

   * Analogous to atomic_long_add().

   *
   * This function is safe to call as long as @ref is between init and exit.
   */

  static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr)

  {

  	unsigned long __percpu *percpu_count;

  	rcu_read_lock_sched();

  	if (__ref_is_percpu(ref, &percpu_count))

  		this_cpu_add(*percpu_count, nr);

  	else

  		atomic_long_add(nr, &ref->count);

  	rcu_read_unlock_sched();

  }
  
  /**

   * percpu_ref_get - increment a percpu refcount
   * @ref: percpu_ref to get
   *
   * Analagous to atomic_long_inc().
   *
   * This function is safe to call as long as @ref is between init and exit.
   */
  static inline void percpu_ref_get(struct percpu_ref *ref)
  {
  	percpu_ref_get_many(ref, 1);
  }
  
  /**

   * percpu_ref_tryget - try to increment a percpu refcount
   * @ref: percpu_ref to try-get
   *
   * Increment a percpu refcount unless its count already reached zero.
   * Returns %true on success; %false on failure.
   *

   * This function is safe to call as long as @ref is between init and exit.

   */
  static inline bool percpu_ref_tryget(struct percpu_ref *ref)
  {

  	unsigned long __percpu *percpu_count;

  	int ret;

  
  	rcu_read_lock_sched();

  	if (__ref_is_percpu(ref, &percpu_count)) {

  		this_cpu_inc(*percpu_count);

  		ret = true;
  	} else {

  		ret = atomic_long_inc_not_zero(&ref->count);

  	}
  
  	rcu_read_unlock_sched();
  
  	return ret;
  }
  
  /**

   * percpu_ref_tryget_live - try to increment a live percpu refcount

   * @ref: percpu_ref to try-get
   *
   * Increment a percpu refcount unless it has already been killed.  Returns
   * %true on success; %false on failure.
   *

   * Completion of percpu_ref_kill() in itself doesn't guarantee that this
   * function will fail.  For such guarantee, percpu_ref_kill_and_confirm()
   * should be used.  After the confirm_kill callback is invoked, it's
   * guaranteed that no new reference will be given out by
   * percpu_ref_tryget_live().

   *

   * This function is safe to call as long as @ref is between init and exit.

   */

  static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)

  {

  	unsigned long __percpu *percpu_count;

  	int ret = false;

  	rcu_read_lock_sched();

  	if (__ref_is_percpu(ref, &percpu_count)) {

  		this_cpu_inc(*percpu_count);

  		ret = true;

  	} else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {

  		ret = atomic_long_inc_not_zero(&ref->count);

  	}

  	rcu_read_unlock_sched();

  
  	return ret;
  }
  
  /**

   * percpu_ref_put_many - decrement a percpu refcount

   * @ref: percpu_ref to put

   * @nr: number of references to put

   *
   * Decrement the refcount, and if 0, call the release function (which was passed
   * to percpu_ref_init())

   *
   * This function is safe to call as long as @ref is between init and exit.

   */

  static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr)

  {

  	unsigned long __percpu *percpu_count;

  	rcu_read_lock_sched();

  	if (__ref_is_percpu(ref, &percpu_count))

  		this_cpu_sub(*percpu_count, nr);
  	else if (unlikely(atomic_long_sub_and_test(nr, &ref->count)))

  		ref->release(ref);

  	rcu_read_unlock_sched();

  }

  /**

   * percpu_ref_put - decrement a percpu refcount
   * @ref: percpu_ref to put
   *
   * Decrement the refcount, and if 0, call the release function (which was passed
   * to percpu_ref_init())
   *
   * This function is safe to call as long as @ref is between init and exit.
   */
  static inline void percpu_ref_put(struct percpu_ref *ref)
  {
  	percpu_ref_put_many(ref, 1);
  }
  
  /**

   * percpu_ref_is_dying - test whether a percpu refcount is dying or dead
   * @ref: percpu_ref to test
   *
   * Returns %true if @ref is dying or dead.
   *
   * This function is safe to call as long as @ref is between init and exit
   * and the caller is responsible for synchronizing against state changes.
   */
  static inline bool percpu_ref_is_dying(struct percpu_ref *ref)
  {
  	return ref->percpu_count_ptr & __PERCPU_REF_DEAD;
  }
  
  /**

   * percpu_ref_is_zero - test whether a percpu refcount reached zero
   * @ref: percpu_ref to test
   *
   * Returns %true if @ref reached zero.

   *
   * This function is safe to call as long as @ref is between init and exit.

   */
  static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
  {

  	unsigned long __percpu *percpu_count;

  	if (__ref_is_percpu(ref, &percpu_count))

  		return false;

  	return !atomic_long_read(&ref->count);

  }

  #endif