/*

 * CDDL HEADER START

 *

 * The contents of this file are subject to the terms of the

 * Common Development and Distribution License (the "License").

 * You may not use this file except in compliance with the License.

 *

 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE

 * or http://www.opensolaris.org/os/licensing.

 * See the License for the specific language governing permissions

 * and limitations under the License.

 *

 * When distributing Covered Code, include this CDDL HEADER in each

 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.

 * If applicable, add the following below this CDDL HEADER, with the

 * fields enclosed by brackets "[]" replaced with your own identifying

 * information: Portions Copyright [yyyy] [name of copyright owner]

 *

 * CDDL HEADER END

 */

/*

 * Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved.

 */

/*

 * A Zero Reference Lock (ZRL) is a reference count that can lock out new

 * references only when the count is zero and only without waiting if the count

 * is not already zero. It is similar to a read-write lock in that it allows

 * multiple readers and only a single writer, but it does not allow a writer to

 * block while waiting for readers to exit, and therefore the question of

 * reader/writer priority is moot (no WRWANT bit). Since the equivalent of

 * rw_enter(&lock, RW_WRITER) is disallowed and only tryenter() is allowed, it

 * is perfectly safe for the same reader to acquire the same lock multiple

 * times. The fact that a ZRL is reentrant for readers (through multiple calls

 * to zrl_add()) makes it convenient for determining whether something is

 * actively referenced without the fuss of flagging lock ownership across

 * function calls.

 */

#include <sys/zrlock.h>

/*

 * A ZRL can be locked only while there are zero references, so ZRL_LOCKED is

 * treated as zero references.

 */

#define	ZRL_LOCKED	((uint32_t)-1)

#define	ZRL_DESTROYED	-2

void

zrl_init(zrlock_t *zrl)

{

	mutex_init(&zrl->zr_mtx, NULL, MUTEX_DEFAULT, NULL);

	zrl->zr_refcount = 0;

	cv_init(&zrl->zr_cv, NULL, CV_DEFAULT, NULL);

#ifdef	ZFS_DEBUG

	zrl->zr_owner = NULL;

	zrl->zr_caller = NULL;

#endif

}

void

zrl_destroy(zrlock_t *zrl)

{

	ASSERT(zrl->zr_refcount == 0);

	mutex_destroy(&zrl->zr_mtx);

	zrl->zr_refcount = ZRL_DESTROYED;

	cv_destroy(&zrl->zr_cv);

}

void

#ifdef	ZFS_DEBUG

zrl_add_debug(zrlock_t *zrl, const char *zc)

#else

zrl_add(zrlock_t *zrl)

#endif

{

	uint32_t n = (uint32_t)zrl->zr_refcount;

	while (n != ZRL_LOCKED) {

		uint32_t cas = atomic_cas_32(

		    (uint32_t *)&zrl->zr_refcount, n, n + 1);

		if (cas == n) {

			ASSERT((int32_t)n >= 0);

#ifdef	ZFS_DEBUG

			if (zrl->zr_owner == curthread) {

				DTRACE_PROBE2(zrlock__reentry,

				    zrlock_t *, zrl, uint32_t, n);

			}

			zrl->zr_owner = curthread;

			zrl->zr_caller = zc;

#endif

			return;

		}

		n = cas;

	}

	mutex_enter(&zrl->zr_mtx);

	while (zrl->zr_refcount == ZRL_LOCKED) {

		cv_wait(&zrl->zr_cv, &zrl->zr_mtx);

	}

	ASSERT(zrl->zr_refcount >= 0);

	zrl->zr_refcount++;

#ifdef	ZFS_DEBUG

	zrl->zr_owner = curthread;

	zrl->zr_caller = zc;

#endif

	mutex_exit(&zrl->zr_mtx);

}

void

zrl_remove(zrlock_t *zrl)

{

	uint32_t n;

	n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount);

	ASSERT((int32_t)n >= 0);

#ifdef	ZFS_DEBUG

	if (zrl->zr_owner == curthread) {

		zrl->zr_owner = NULL;

		zrl->zr_caller = NULL;

	}

#endif

}

int

zrl_tryenter(zrlock_t *zrl)

{

	uint32_t n = (uint32_t)zrl->zr_refcount;

	if (n == 0) {

		uint32_t cas = atomic_cas_32(

		    (uint32_t *)&zrl->zr_refcount, 0, ZRL_LOCKED);

		if (cas == 0) {

#ifdef	ZFS_DEBUG

			ASSERT(zrl->zr_owner == NULL);

			zrl->zr_owner = curthread;

#endif

			return (1);

		}

	}

	ASSERT((int32_t)n > ZRL_DESTROYED);

	return (0);

}

void

zrl_exit(zrlock_t *zrl)

{

	ASSERT(zrl->zr_refcount == ZRL_LOCKED);

	mutex_enter(&zrl->zr_mtx);

#ifdef	ZFS_DEBUG

	ASSERT(zrl->zr_owner == curthread);

	zrl->zr_owner = NULL;

	membar_producer();	/* make sure the owner store happens first */

#endif

	zrl->zr_refcount = 0;

	cv_broadcast(&zrl->zr_cv);

	mutex_exit(&zrl->zr_mtx);

}

int

zrl_refcount(zrlock_t *zrl)

{

	ASSERT(zrl->zr_refcount > ZRL_DESTROYED);

	int n = (int)zrl->zr_refcount;

	return (n <= 0 ? 0 : n);

}

int

zrl_is_zero(zrlock_t *zrl)

{

	ASSERT(zrl->zr_refcount > ZRL_DESTROYED);

	return (zrl->zr_refcount <= 0);

}

int

zrl_is_locked(zrlock_t *zrl)

{

	ASSERT(zrl->zr_refcount > ZRL_DESTROYED);

	return (zrl->zr_refcount == ZRL_LOCKED);

}

#ifdef	ZFS_DEBUG

kthread_t *

zrl_owner(zrlock_t *zrl)

{

	return (zrl->zr_owner);

}

#endif