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      1 /*
      2  * CDDL HEADER START
      3  *
      4  * The contents of this file are subject to the terms of the
      5  * Common Development and Distribution License (the "License").
      6  * You may not use this file except in compliance with the License.
      7  *
      8  * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
      9  * or http://www.opensolaris.org/os/licensing.
     10  * See the License for the specific language governing permissions
     11  * and limitations under the License.
     12  *
     13  * When distributing Covered Code, include this CDDL HEADER in each
     14  * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
     15  * If applicable, add the following below this CDDL HEADER, with the
     16  * fields enclosed by brackets "[]" replaced with your own identifying
     17  * information: Portions Copyright [yyyy] [name of copyright owner]
     18  *
     19  * CDDL HEADER END
     20  */
     21 /*
     22  * Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved.
     23  * Copyright (c) 2014, 2015 by Delphix. All rights reserved.
     24  * Copyright 2016 The MathWorks, Inc. All rights reserved.
     25  */
     26 
     27 /*
     28  * A Zero Reference Lock (ZRL) is a reference count that can lock out new
     29  * references only when the count is zero and only without waiting if the count
     30  * is not already zero. It is similar to a read-write lock in that it allows
     31  * multiple readers and only a single writer, but it does not allow a writer to
     32  * block while waiting for readers to exit, and therefore the question of
     33  * reader/writer priority is moot (no WRWANT bit). Since the equivalent of
     34  * rw_enter(&lock, RW_WRITER) is disallowed and only tryenter() is allowed, it
     35  * is perfectly safe for the same reader to acquire the same lock multiple
     36  * times. The fact that a ZRL is reentrant for readers (through multiple calls
     37  * to zrl_add()) makes it convenient for determining whether something is
     38  * actively referenced without the fuss of flagging lock ownership across
     39  * function calls.
     40  */
     41 #include <sys/zrlock.h>
     42 
     43 /*
     44  * A ZRL can be locked only while there are zero references, so ZRL_LOCKED is
     45  * treated as zero references.
     46  */
     47 #define	ZRL_LOCKED	-1
     48 #define	ZRL_DESTROYED	-2
     49 
     50 void
     51 zrl_init(zrlock_t *zrl)
     52 {
     53 	mutex_init(&zrl->zr_mtx, NULL, MUTEX_DEFAULT, NULL);
     54 	zrl->zr_refcount = 0;
     55 	cv_init(&zrl->zr_cv, NULL, CV_DEFAULT, NULL);
     56 #ifdef	ZFS_DEBUG
     57 	zrl->zr_owner = NULL;
     58 	zrl->zr_caller = NULL;
     59 #endif
     60 }
     61 
     62 void
     63 zrl_destroy(zrlock_t *zrl)
     64 {
     65 	ASSERT0(zrl->zr_refcount);
     66 
     67 	mutex_destroy(&zrl->zr_mtx);
     68 	zrl->zr_refcount = ZRL_DESTROYED;
     69 	cv_destroy(&zrl->zr_cv);
     70 }
     71 
     72 void
     73 zrl_add_impl(zrlock_t *zrl, const char *zc)
     74 {
     75 	for (;;) {
     76 		uint32_t n = (uint32_t)zrl->zr_refcount;
     77 		while (n != ZRL_LOCKED) {
     78 			uint32_t cas = atomic_cas_32(
     79 			    (uint32_t *)&zrl->zr_refcount, n, n + 1);
     80 			if (cas == n) {
     81 				ASSERT3S((int32_t)n, >=, 0);
     82 #ifdef	ZFS_DEBUG
     83 				if (zrl->zr_owner == curthread) {
     84 					DTRACE_PROBE2(zrlock__reentry,
     85 					    zrlock_t *, zrl, uint32_t, n);
     86 				}
     87 				zrl->zr_owner = curthread;
     88 				zrl->zr_caller = zc;
     89 #endif
     90 				return;
     91 			}
     92 			n = cas;
     93 		}
     94 
     95 		mutex_enter(&zrl->zr_mtx);
     96 		while (zrl->zr_refcount == ZRL_LOCKED) {
     97 			cv_wait(&zrl->zr_cv, &zrl->zr_mtx);
     98 		}
     99 		mutex_exit(&zrl->zr_mtx);
    100 	}
    101 }
    102 
    103 void
    104 zrl_remove(zrlock_t *zrl)
    105 {
    106 	uint32_t n;
    107 
    108 #ifdef	ZFS_DEBUG
    109 	if (zrl->zr_owner == curthread) {
    110 		zrl->zr_owner = NULL;
    111 		zrl->zr_caller = NULL;
    112 	}
    113 #endif
    114 	n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount);
    115 	ASSERT3S((int32_t)n, >=, 0);
    116 }
    117 
    118 int
    119 zrl_tryenter(zrlock_t *zrl)
    120 {
    121 	uint32_t n = (uint32_t)zrl->zr_refcount;
    122 
    123 	if (n == 0) {
    124 		uint32_t cas = atomic_cas_32(
    125 		    (uint32_t *)&zrl->zr_refcount, 0, ZRL_LOCKED);
    126 		if (cas == 0) {
    127 #ifdef	ZFS_DEBUG
    128 			ASSERT3P(zrl->zr_owner, ==, NULL);
    129 			zrl->zr_owner = curthread;
    130 #endif
    131 			return (1);
    132 		}
    133 	}
    134 
    135 	ASSERT3S((int32_t)n, >, ZRL_DESTROYED);
    136 
    137 	return (0);
    138 }
    139 
    140 void
    141 zrl_exit(zrlock_t *zrl)
    142 {
    143 	ASSERT3S(zrl->zr_refcount, ==, ZRL_LOCKED);
    144 
    145 	mutex_enter(&zrl->zr_mtx);
    146 #ifdef	ZFS_DEBUG
    147 	ASSERT3P(zrl->zr_owner, ==, curthread);
    148 	zrl->zr_owner = NULL;
    149 	membar_producer();	/* make sure the owner store happens first */
    150 #endif
    151 	zrl->zr_refcount = 0;
    152 	cv_broadcast(&zrl->zr_cv);
    153 	mutex_exit(&zrl->zr_mtx);
    154 }
    155 
    156 int
    157 zrl_refcount(zrlock_t *zrl)
    158 {
    159 	ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
    160 
    161 	int n = (int)zrl->zr_refcount;
    162 	return (n <= 0 ? 0 : n);
    163 }
    164 
    165 int
    166 zrl_is_zero(zrlock_t *zrl)
    167 {
    168 	ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
    169 
    170 	return (zrl->zr_refcount <= 0);
    171 }
    172 
    173 int
    174 zrl_is_locked(zrlock_t *zrl)
    175 {
    176 	ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
    177 
    178 	return (zrl->zr_refcount == ZRL_LOCKED);
    179 }
    180 
    181 #ifdef	ZFS_DEBUG
    182 kthread_t *
    183 zrl_owner(zrlock_t *zrl)
    184 {
    185 	return (zrl->zr_owner);
    186 }
    187 #endif
    188