fs/zfs/zrlock.c

/*
 * 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.
 * Copyright (c) 2014, 2015 by Delphix. All rights reserved.
 * Copyright 2016 The MathWorks, Inc. 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  -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)
{
    ASSERT0(zrl->zr_refcount);

    mutex_destroy(&zrl->zr_mtx);
    zrl->zr_refcount = ZRL_DESTROYED;
    cv_destroy(&zrl->zr_cv);
}

void
zrl_add_impl(zrlock_t *zrl, const char *zc)
{
    for (;;) {
        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) {
                ASSERT3S((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);
        }
        mutex_exit(&zrl->zr_mtx);
    }
}

void
zrl_remove(zrlock_t *zrl)
{
    uint32_t n;

#ifdef  ZFS_DEBUG
    if (zrl->zr_owner == curthread) {
        zrl->zr_owner = NULL;
        zrl->zr_caller = NULL;
    }
#endif
    n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount);
    ASSERT3S((int32_t)n, >=, 0);
}

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
            ASSERT3P(zrl->zr_owner, ==, NULL);
            zrl->zr_owner = curthread;
#endif
            return (1);
        }
    }

    ASSERT3S((int32_t)n, >, ZRL_DESTROYED);

    return (0);
}

void
zrl_exit(zrlock_t *zrl)
{
    ASSERT3S(zrl->zr_refcount, ==, ZRL_LOCKED);

    mutex_enter(&zrl->zr_mtx);
#ifdef  ZFS_DEBUG
    ASSERT3P(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)
{
    ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);

    int n = (int)zrl->zr_refcount;
    return (n <= 0 ? 0 : n);
}

int
zrl_is_zero(zrlock_t *zrl)
{
    ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);

    return (zrl->zr_refcount <= 0);
}

int
zrl_is_locked(zrlock_t *zrl)
{
    ASSERT3S(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