/*	$NetBSD$	*/

/*-
 * 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_REFCNT_H
#define	_SYS_REFCNT_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 refcnt {
	volatile unsigned	rc_refcnt;
};

static inline void
refcnt_init(struct refcnt *refcnt)
{

	refcnt->rc_refcnt = 1;
}

static inline void
refcnt_fini(struct refcnt *refcnt)
{

	KASSERT(refcnt->rc_refcnt == 0);
	refcnt->rc_refcnt = UINT_MAX;
}

static inline int
refcnt_acquire(struct refcnt *refcnt)
{
	unsigned old, new;

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

	return 0;
}

static inline bool
refcnt_release_tryenter(struct refcnt *refcnt, kmutex_t *interlock)
{
	unsigned old, new;

	do {
		old = refcnt->rc_refcnt;
		KASSERT(0 < old);
		if (old == 1) {
			mutex_enter(interlock);
			new = atomic_dec_uint_nv(&refcnt->rc_refcnt);
			KASSERT(new != UINT_MAX);
			if (new == 0)
				return true;
			mutex_exit(interlock);
			return false;
		}
		new = old - 1;
	} while (atomic_cas_uint(&refcnt->rc_refcnt, old, new) != old);

	return false;
}

static inline void
refcnt_release_signal(struct refcnt *refcnt, kmutex_t *interlock,
    kcondvar_t *cv)
{

	if (refcnt_release_tryenter(refcnt, interlock)) {
		cv_signal(cv);
		mutex_exit(interlock);
	}
}

static inline void
refcnt_release_broadcast(struct refcnt *refcnt, kmutex_t *interlock,
    kcondvar_t *cv)
{

	if (refcnt_release_tryenter(refcnt, interlock)) {
		cv_broadcast(cv);
		mutex_exit(interlock);
	}
}

static inline void
refcnt_release_wait(struct refcnt *refcnt, kmutex_t *interlock, kcondvar_t *cv)
{
	unsigned old, new;

	ASSERT_SLEEPABLE();
	KASSERT(mutex_owned(interlock));

	refcnt = atomic_dec_uint_nv(&refcnt->rc_refcnt);
	if (0 < refcnt) {
		do cv_wait(cv, interlock) while (0 < refcnt);
	}
}

static inline void
refcnt_release_wait_unlocked(struct refcnt *refcnt, kmutex_t *interlock,
    kcondvar_t *cv)
{
	unsigned refcnt;

	ASSERT_SLEEPABLE();

	/*
	 * This is the only place the reference count can drop to zero
	 * without holding the interlock and without an atomic op.
	 * This is OK because if the reference count drops to zero, the
	 * caller promises -- by guaranteeing no new references can be
	 * made -- that it has exclusive access to the object.
	 */
	if (refcnt->rc_refcnt == 1) {
		refcnt->rc_refcnt = 0;
		return;
	}

	refcnt = atomic_dec_uint_nv(&refcnt->rc_refcnt);
	if (0 < refcnt) {
		mutex_enter(interlock);
		while (0 < refcnt->rc_refcnt)
			cv_wait(cv, interlock);
		mutex_exit(interlock);
	}
}

#if DIAGNOSTIC
static inline bool
refcnt_referenced_p(struct refcnt *refcnt)
{

	return 0 < refcnt->rc_refcnt;
}

static inline bool
refcnt_exclusive_p(struct refcnt *refcnt)
{

	KASSERT(refcnt_referenced_p(refcnt));
	return refcnt->rc_refcnt == 1;
}
#endif

#endif	/* _SYS_REFCNT_H */