/*
   * kref.h - library routines for handling generic reference counted objects
   *
   * Copyright (C) 2004 Greg Kroah-Hartman <greg@kroah.com>
   * Copyright (C) 2004 IBM Corp.
   *
   * based on kobject.h which was:
   * Copyright (C) 2002-2003 Patrick Mochel <mochel@osdl.org>
   * Copyright (C) 2002-2003 Open Source Development Labs
   *
   * This file is released under the GPLv2.
   *
   */
  
  #ifndef _KREF_H_
  #define _KREF_H_
  
  #include <linux/bug.h>
  #include <linux/atomic.h>
  #include <linux/kernel.h>
  #include <linux/mutex.h>
  #include <linux/spinlock.h>
  
  struct kref {
  	atomic_t refcount;
  };
  
  /**
   * kref_init - initialize object.
   * @kref: object in question.
   */
  static inline void kref_init(struct kref *kref)
  {
  	atomic_set(&kref->refcount, 1);
  }
  
  /**
   * kref_get - increment refcount for object.
   * @kref: object.
   */
  static inline void kref_get(struct kref *kref)
  {
  	/* If refcount was 0 before incrementing then we have a race
  	 * condition when this kref is freeing by some other thread right now.
  	 * In this case one should use kref_get_unless_zero()
  	 */
  	WARN_ON_ONCE(atomic_inc_return(&kref->refcount) < 2);
  }
  
  /**
   * kref_sub - subtract a number of refcounts for object.
   * @kref: object.
   * @count: Number of recounts to subtract.
   * @release: pointer to the function that will clean up the object when the
   *	     last reference to the object is released.
   *	     This pointer is required, and it is not acceptable to pass kfree
   *	     in as this function.  If the caller does pass kfree to this
   *	     function, you will be publicly mocked mercilessly by the kref
   *	     maintainer, and anyone else who happens to notice it.  You have
   *	     been warned.
   *
   * Subtract @count from the refcount, and if 0, call release().
   * Return 1 if the object was removed, otherwise return 0.  Beware, if this
   * function returns 0, you still can not count on the kref from remaining in
   * memory.  Only use the return value if you want to see if the kref is now
   * gone, not present.
   */
  static inline int kref_sub(struct kref *kref, unsigned int count,
  	     void (*release)(struct kref *kref))
  {
  	WARN_ON(release == NULL);
  
  	if (atomic_sub_and_test((int) count, &kref->refcount)) {
  		release(kref);
  		return 1;
  	}
  	return 0;
  }
  
  /**
   * kref_put - decrement refcount for object.
   * @kref: object.
   * @release: pointer to the function that will clean up the object when the
   *	     last reference to the object is released.
   *	     This pointer is required, and it is not acceptable to pass kfree
   *	     in as this function.  If the caller does pass kfree to this
   *	     function, you will be publicly mocked mercilessly by the kref
   *	     maintainer, and anyone else who happens to notice it.  You have
   *	     been warned.
   *
   * Decrement the refcount, and if 0, call release().
   * Return 1 if the object was removed, otherwise return 0.  Beware, if this
   * function returns 0, you still can not count on the kref from remaining in
   * memory.  Only use the return value if you want to see if the kref is now
   * gone, not present.
   */
  static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref))
  {
  	return kref_sub(kref, 1, release);
  }
  
  /**
   * kref_put_spinlock_irqsave - decrement refcount for object.
   * @kref: object.
   * @release: pointer to the function that will clean up the object when the
   *	     last reference to the object is released.
   *	     This pointer is required, and it is not acceptable to pass kfree
   *	     in as this function.
   * @lock: lock to take in release case
   *
   * Behaves identical to kref_put with one exception.  If the reference count
   * drops to zero, the lock will be taken atomically wrt dropping the reference
   * count.  The release function has to call spin_unlock() without _irqrestore.
   */
  static inline int kref_put_spinlock_irqsave(struct kref *kref,
  		void (*release)(struct kref *kref),
  		spinlock_t *lock)
  {
  	unsigned long flags;
  
  	WARN_ON(release == NULL);
  	if (atomic_add_unless(&kref->refcount, -1, 1))
  		return 0;
  	spin_lock_irqsave(lock, flags);
  	if (atomic_dec_and_test(&kref->refcount)) {
  		release(kref);
  		local_irq_restore(flags);
  		return 1;
  	}
  	spin_unlock_irqrestore(lock, flags);
  	return 0;
  }
  
  static inline int kref_put_mutex(struct kref *kref,
  				 void (*release)(struct kref *kref),
  				 struct mutex *lock)
  {
  	WARN_ON(release == NULL);
  	if (unlikely(!atomic_add_unless(&kref->refcount, -1, 1))) {
  		mutex_lock(lock);
  		if (unlikely(!atomic_dec_and_test(&kref->refcount))) {
  			mutex_unlock(lock);
  			return 0;
  		}
  		release(kref);
  		return 1;
  	}
  	return 0;
  }
  
  /**
   * kref_get_unless_zero - Increment refcount for object unless it is zero.
   * @kref: object.
   *
   * Return non-zero if the increment succeeded. Otherwise return 0.
   *
   * This function is intended to simplify locking around refcounting for
   * objects that can be looked up from a lookup structure, and which are
   * removed from that lookup structure in the object destructor.
   * Operations on such objects require at least a read lock around
   * lookup + kref_get, and a write lock around kref_put + remove from lookup
   * structure. Furthermore, RCU implementations become extremely tricky.
   * With a lookup followed by a kref_get_unless_zero *with return value check*
   * locking in the kref_put path can be deferred to the actual removal from
   * the lookup structure and RCU lookups become trivial.
   */
  static inline int __must_check kref_get_unless_zero(struct kref *kref)
  {
  	return atomic_add_unless(&kref->refcount, 1, 0);
  }
  #endif /* _KREF_H_ */