/*
 * 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>

struct kref {
	atomic_t refcount;
};

static inline void kref_init(struct kref *kref)
{
	atomic_set(&kref->refcount, 1);
}

static inline void kref_get(struct kref *kref)
{
	WARN_ON(!atomic_read(&kref->refcount));
	atomic_inc(&kref->refcount);
}

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;
}

static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref))
{
	return kref_sub(kref, 1, release);
}


/**
 * 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_ */