module/zfs/zrlock.c

   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 #include <sys/trace_zrlock.h>
  43
  44 /*
  45  * A ZRL can be locked only while there are zero references, so ZRL_LOCKED is
  46  * treated as zero references.
  47  */
  48 #define ZRL_LOCKED      -1
  49 #define ZRL_DESTROYED   -2
  50
  51 void
  52 zrl_init(zrlock_t *zrl)
  53 {
  54         mutex_init(&zrl->zr_mtx, NULL, MUTEX_DEFAULT, NULL);
  55         zrl->zr_refcount = 0;
  56         cv_init(&zrl->zr_cv, NULL, CV_DEFAULT, NULL);
  57 #ifdef  ZFS_DEBUG
  58         zrl->zr_owner = NULL;
  59         zrl->zr_caller = NULL;
  60 #endif
  61 }
  62
  63 void
  64 zrl_destroy(zrlock_t *zrl)
  65 {
  66         ASSERT0(zrl->zr_refcount);
  67
  68         mutex_destroy(&zrl->zr_mtx);
  69         zrl->zr_refcount = ZRL_DESTROYED;
  70         cv_destroy(&zrl->zr_cv);
  71 }
  72
  73 void
  74 zrl_add_impl(zrlock_t *zrl, const char *zc)
  75 {
  76         for (;;) {
  77                 uint32_t n = (uint32_t)zrl->zr_refcount;
  78                 while (n != ZRL_LOCKED) {
  79                         uint32_t cas = atomic_cas_32(
  80                             (uint32_t *)&zrl->zr_refcount, n, n + 1);
  81                         if (cas == n) {
  82                                 ASSERT3S((int32_t)n, >=, 0);
  83 #ifdef  ZFS_DEBUG
  84                                 if (zrl->zr_owner == curthread) {
  85                                         DTRACE_PROBE2(zrlock__reentry,
  86                                             zrlock_t *, zrl, uint32_t, n);
  87                                 }
  88                                 zrl->zr_owner = curthread;
  89                                 zrl->zr_caller = zc;
  90 #endif
  91                                 return;
  92                         }
  93                         n = cas;
  94                 }
  95
  96                 mutex_enter(&zrl->zr_mtx);
  97                 while (zrl->zr_refcount == ZRL_LOCKED) {
  98                         cv_wait(&zrl->zr_cv, &zrl->zr_mtx);
  99                 }
 100                 mutex_exit(&zrl->zr_mtx);
 101         }
 102 }
 103
 104 void
 105 zrl_remove(zrlock_t *zrl)
 106 {
 107         uint32_t n;
 108
 109 #ifdef  ZFS_DEBUG
 110         if (zrl->zr_owner == curthread) {
 111                 zrl->zr_owner = NULL;
 112                 zrl->zr_caller = NULL;
 113         }
 114 #endif
 115         n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount);
 116         ASSERT3S((int32_t)n, >=, 0);
 117 }
 118
 119 int
 120 zrl_tryenter(zrlock_t *zrl)
 121 {
 122         uint32_t n = (uint32_t)zrl->zr_refcount;
 123
 124         if (n == 0) {
 125                 uint32_t cas = atomic_cas_32(
 126                     (uint32_t *)&zrl->zr_refcount, 0, ZRL_LOCKED);
 127                 if (cas == 0) {
 128 #ifdef  ZFS_DEBUG
 129                         ASSERT3P(zrl->zr_owner, ==, NULL);
 130                         zrl->zr_owner = curthread;
 131 #endif
 132                         return (1);
 133                 }
 134         }
 135
 136         ASSERT3S((int32_t)n, >, ZRL_DESTROYED);
 137
 138         return (0);
 139 }
 140
 141 void
 142 zrl_exit(zrlock_t *zrl)
 143 {
 144         ASSERT3S(zrl->zr_refcount, ==, ZRL_LOCKED);
 145
 146         mutex_enter(&zrl->zr_mtx);
 147 #ifdef  ZFS_DEBUG
 148         ASSERT3P(zrl->zr_owner, ==, curthread);
 149         zrl->zr_owner = NULL;
 150         membar_producer();      /* make sure the owner store happens first */
 151 #endif
 152         zrl->zr_refcount = 0;
 153         cv_broadcast(&zrl->zr_cv);
 154         mutex_exit(&zrl->zr_mtx);
 155 }
 156
 157 int
 158 zrl_refcount(zrlock_t *zrl)
 159 {
 160         ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
 161
 162         int n = (int)zrl->zr_refcount;
 163         return (n <= 0 ? 0 : n);
 164 }
 165
 166 int
 167 zrl_is_zero(zrlock_t *zrl)
 168 {
 169         ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
 170
 171         return (zrl->zr_refcount <= 0);
 172 }
 173
 174 int
 175 zrl_is_locked(zrlock_t *zrl)
 176 {
 177         ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
 178
 179         return (zrl->zr_refcount == ZRL_LOCKED);
 180 }
 181
 182 #ifdef  ZFS_DEBUG
 183 kthread_t *
 184 zrl_owner(zrlock_t *zrl)
 185 {
 186         return (zrl->zr_owner);
 187 }
 188 #endif
 189
 190 #if defined(_KERNEL) && defined(HAVE_SPL)
 191
 192 EXPORT_SYMBOL(zrl_add_impl);
 193 EXPORT_SYMBOL(zrl_remove);
 194
 195 #endif