kern_sx.c revision 74912
1/* 2 * Copyright (C) 2001 Jason Evans <jasone@freebsd.org>. All rights reserved. 3 * 4 * Redistribution and use in source and binary forms, with or without 5 * modification, are permitted provided that the following conditions 6 * are met: 7 * 1. Redistributions of source code must retain the above copyright 8 * notice(s), this list of conditions and the following disclaimer as 9 * the first lines of this file unmodified other than the possible 10 * addition of one or more copyright notices. 11 * 2. Redistributions in binary form must reproduce the above copyright 12 * notice(s), this list of conditions and the following disclaimer in the 13 * documentation and/or other materials provided with the distribution. 14 * 15 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY 16 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED 17 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE 18 * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY 19 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES 20 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR 21 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER 22 * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 23 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 24 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH 25 * DAMAGE. 26 * 27 * $FreeBSD: head/sys/kern/kern_sx.c 74912 2001-03-28 09:03:24Z jhb $ 28 */ 29 30/* 31 * Shared/exclusive locks. This implementation assures deterministic lock 32 * granting behavior, so that slocks and xlocks are interleaved. 33 * 34 * Priority propagation will not generally raise the priority of lock holders, 35 * so should not be relied upon in combination with sx locks. 36 * 37 * The witness code can not detect lock cycles (yet). 38 * 39 * XXX: When witness is made to function with sx locks, it will need to 40 * XXX: be taught to deal with these situations, as they are more involved: 41 * slock --> xlock (deadlock) 42 * slock --> slock (slock recursion, not fatal) 43 */ 44 45#include <sys/param.h> 46#include <sys/systm.h> 47#include <sys/ktr.h> 48#include <sys/condvar.h> 49#include <sys/lock.h> 50#include <sys/mutex.h> 51#include <sys/sx.h> 52 53/* 54 * XXX: We don't implement the LO_RECURSED flag for this lock yet. 55 * We could do this by walking p_sleeplocks if we really wanted to. 56 */ 57struct lock_class lock_class_sx = { 58 "sx", 59 LC_SLEEPLOCK | LC_SLEEPABLE | LC_RECURSABLE 60}; 61 62void 63sx_init(struct sx *sx, const char *description) 64{ 65 struct lock_object *lock; 66 67 bzero(sx, sizeof(*sx)); 68 lock = &sx->sx_object; 69 lock->lo_class = &lock_class_sx; 70 lock->lo_name = description; 71 lock->lo_flags = LO_WITNESS | LO_SLEEPABLE; 72 mtx_init(&sx->sx_lock, "sx backing lock", 73 MTX_DEF | MTX_NOWITNESS | MTX_QUIET); 74 sx->sx_cnt = 0; 75 cv_init(&sx->sx_shrd_cv, description); 76 sx->sx_shrd_wcnt = 0; 77 cv_init(&sx->sx_excl_cv, description); 78 sx->sx_excl_wcnt = 0; 79 sx->sx_xholder = NULL; 80 81 LOCK_LOG_INIT(lock, 0); 82 83 WITNESS_INIT(lock); 84} 85 86void 87sx_destroy(struct sx *sx) 88{ 89 90 LOCK_LOG_DESTROY(&sx->sx_object, 0); 91 92 KASSERT((sx->sx_cnt == 0 && sx->sx_shrd_wcnt == 0 && sx->sx_excl_wcnt == 93 0), ("%s (%s): holders or waiters\n", __FUNCTION__, 94 sx->sx_object.lo_name)); 95 96 mtx_destroy(&sx->sx_lock); 97 cv_destroy(&sx->sx_shrd_cv); 98 cv_destroy(&sx->sx_excl_cv); 99 100 WITNESS_DESTROY(&sx->sx_object); 101} 102 103void 104_sx_slock(struct sx *sx, const char *file, int line) 105{ 106 107 mtx_lock(&sx->sx_lock); 108 KASSERT(sx->sx_xholder != curproc, 109 ("%s (%s): trying to get slock while xlock is held\n", __FUNCTION__, 110 sx->sx_object.lo_name)); 111 112 /* 113 * Loop in case we lose the race for lock acquisition. 114 */ 115 while (sx->sx_cnt < 0) { 116 sx->sx_shrd_wcnt++; 117 cv_wait(&sx->sx_shrd_cv, &sx->sx_lock); 118 sx->sx_shrd_wcnt--; 119 } 120 121 /* Acquire a shared lock. */ 122 sx->sx_cnt++; 123 124#ifdef WITNESS 125 sx->sx_object.lo_flags |= LO_LOCKED; 126#endif 127 LOCK_LOG_LOCK("SLOCK", &sx->sx_object, 0, 0, file, line); 128 WITNESS_LOCK(&sx->sx_object, 0, file, line); 129 130 mtx_unlock(&sx->sx_lock); 131} 132 133void 134_sx_xlock(struct sx *sx, const char *file, int line) 135{ 136 137 mtx_lock(&sx->sx_lock); 138 139 /* 140 * With sx locks, we're absolutely not permitted to recurse on 141 * xlocks, as it is fatal (deadlock). Normally, recursion is handled 142 * by WITNESS, but as it is not semantically correct to hold the 143 * xlock while in here, we consider it API abuse and put it under 144 * INVARIANTS. 145 */ 146 KASSERT(sx->sx_xholder != curproc, 147 ("%s (%s): xlock already held @ %s:%d", __FUNCTION__, 148 sx->sx_object.lo_name, file, line)); 149 150 /* Loop in case we lose the race for lock acquisition. */ 151 while (sx->sx_cnt != 0) { 152 sx->sx_excl_wcnt++; 153 cv_wait(&sx->sx_excl_cv, &sx->sx_lock); 154 sx->sx_excl_wcnt--; 155 } 156 157 MPASS(sx->sx_cnt == 0); 158 159 /* Acquire an exclusive lock. */ 160 sx->sx_cnt--; 161 sx->sx_xholder = curproc; 162 163#ifdef WITNESS 164 sx->sx_object.lo_flags |= LO_LOCKED; 165#endif 166 LOCK_LOG_LOCK("XLOCK", &sx->sx_object, 0, 0, file, line); 167 WITNESS_LOCK(&sx->sx_object, 0, file, line); 168 169 mtx_unlock(&sx->sx_lock); 170} 171 172void 173_sx_sunlock(struct sx *sx, const char *file, int line) 174{ 175 176 mtx_lock(&sx->sx_lock); 177 _SX_ASSERT_SLOCKED(sx); 178 179#ifdef WITNESS 180 if (sx->sx_cnt == 0) 181 sx->sx_object.lo_flags &= ~LO_LOCKED; 182#endif 183 WITNESS_UNLOCK(&sx->sx_object, 0, file, line); 184 185 /* Release. */ 186 sx->sx_cnt--; 187 188 /* 189 * If we just released the last shared lock, wake any waiters up, giving 190 * exclusive lockers precedence. In order to make sure that exclusive 191 * lockers won't be blocked forever, don't wake shared lock waiters if 192 * there are exclusive lock waiters. 193 */ 194 if (sx->sx_excl_wcnt > 0) { 195 if (sx->sx_cnt == 0) 196 cv_signal(&sx->sx_excl_cv); 197 } else if (sx->sx_shrd_wcnt > 0) 198 cv_broadcast(&sx->sx_shrd_cv); 199 200 LOCK_LOG_LOCK("SUNLOCK", &sx->sx_object, 0, 0, file, line); 201 202 mtx_unlock(&sx->sx_lock); 203} 204 205void 206_sx_xunlock(struct sx *sx, const char *file, int line) 207{ 208 209 mtx_lock(&sx->sx_lock); 210 _SX_ASSERT_XLOCKED(sx); 211 MPASS(sx->sx_cnt == -1); 212 213#ifdef WITNESS 214 sx->sx_object.lo_flags &= ~LO_LOCKED; 215#endif 216 WITNESS_UNLOCK(&sx->sx_object, 0, file, line); 217 218 /* Release. */ 219 sx->sx_cnt++; 220 sx->sx_xholder = NULL; 221 222 /* 223 * Wake up waiters if there are any. Give precedence to slock waiters. 224 */ 225 if (sx->sx_shrd_wcnt > 0) 226 cv_broadcast(&sx->sx_shrd_cv); 227 else if (sx->sx_excl_wcnt > 0) 228 cv_signal(&sx->sx_excl_cv); 229 230 LOCK_LOG_LOCK("XUNLOCK", &sx->sx_object, 0, 0, file, line); 231 232 mtx_unlock(&sx->sx_lock); 233} 234