/*	$NetBSD$	*/

/*-
 * Copyright (c) 2015 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This code is derived from software contributed to The NetBSD Foundation
 * by Taylor R. Campbell.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

#ifndef	_SYS_REFCOUNT_H
#define	_SYS_REFCOUNT_H

#include <sys/types.h>
#include <sys/atomic.h>
#include <sys/condvar.h>
#include <sys/errno.h>
#include <sys/mutex.h>
#include <sys/systm.h>

#include <machine/limits.h>

struct refcount {
	volatile unsigned	rc_value;
};

#if DIAGNOSTIC
static inline bool
refcount_referenced_p(const struct refcount *refcount)
{

	return 0 < refcount->rc_value;
}

static inline bool
refcount_exclusive_p(const struct refcount *refcount)
{

	KASSERT(refcount_referenced_p(refcount));
	return refcount->rc_value == 1;
}
#endif

static inline void
refcount_init(struct refcount *refcount)
{

	refcount->rc_value = 1;
}

static inline void
refcount_fini(struct refcount *refcount)
{

	KASSERT(!refcount_referenced_p(refcount));
	refcount->rc_value = UINT_MAX;
}

static inline int
refcount_inc(struct refcount *refcount)
{
	unsigned old, new;

	do {
		old = refcount->rc_value;
		if (old == UINT_MAX)
			return EBUSY;
		KASSERT(0 < old);
		new = old + 1;
	} while (atomic_cas_uint(&refcount->rc_value, old, new) != old);

	return 0;
}

static inline bool
refcount_dec_local(struct refcount *refcount)
{
	unsigned old, new;

	do {
		old = refcount->rc_value;
		KASSERT(0 < old);
		if (old == 1) {
			/*
			 * Avoid an atomic if we don't need it.  Caller
			 * guarantees that if the reference count is 1,
			 * nobody else can acquire new references.
			 */
			refcount->rc_value = 0;
			return true;
		}
		KASSERT(1 < old);
		new = old - 1;
	} while (atomic_cas_uint(&refcount->rc_value, old, new) != old);

	KASSERT(0 < new);
	return false;
}

static inline bool
refcount_dec_lock(struct refcount *refcount, kmutex_t *interlock)
{
	unsigned old, new;

	do {
		old = refcount->rc_value;
		KASSERT(0 < old);
		if (old == 1) {
			/*
			 * Transition from 1->0 is allowed only under
			 * the interlock.
			 */
			mutex_enter(interlock);
			new = atomic_dec_uint_nv(&refcount->rc_value);
			KASSERT(new != UINT_MAX);
			if (new == 0)
				return true;
			mutex_exit(interlock);
			return false;
		}
		KASSERT(1 < old);
		new = old - 1;
	} while (atomic_cas_uint(&refcount->rc_value, old, new) != old);

	KASSERT(0 < new);
	return false;
}

static inline void
refcount_dec_signal(struct refcount *refcount, kmutex_t *interlock,
    kcondvar_t *cv)
{

	if (refcount_dec_lock(refcount, interlock)) {
		cv_signal(cv);
		mutex_exit(interlock);
	}
}

static inline void
refcount_dec_broadcast(struct refcount *refcount, kmutex_t *interlock,
    kcondvar_t *cv)
{

	if (refcount_dec_lock(refcount, interlock)) {
		cv_broadcast(cv);
		mutex_exit(interlock);
	}
}

static inline void
refcount_dec_drain(struct refcount *refcount, kmutex_t *interlock,
    kcondvar_t *cv)
{

	KASSERT(mutex_owned(interlock));

	if (0 < atomic_dec_uint_nv(&refcount->rc_value))
		do cv_wait(cv, interlock); while (0 < refcount->rc_value);
}

#endif	/* _SYS_REFCOUNT_H */