1 /* $NetBSD: pthread_atfork.c,v 1.10 2015/01/20 18:31:25 christos Exp $ */ 2 3 /*- 4 * Copyright (c) 2002 The NetBSD Foundation, Inc. 5 * All rights reserved. 6 * 7 * This code is derived from software contributed to The NetBSD Foundation 8 * by Nathan J. Williams. 9 * 10 * Redistribution and use in source and binary forms, with or without 11 * modification, are permitted provided that the following conditions 12 * are met: 13 * 1. Redistributions of source code must retain the above copyright 14 * notice, this list of conditions and the following disclaimer. 15 * 2. Redistributions in binary form must reproduce the above copyright 16 * notice, this list of conditions and the following disclaimer in the 17 * documentation and/or other materials provided with the distribution. 18 * 19 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 20 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 21 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 22 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 23 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 24 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 25 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 26 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 27 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 28 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 29 * POSSIBILITY OF SUCH DAMAGE. 30 */ 31 32 #include <sys/cdefs.h> 33 #if defined(LIBC_SCCS) && !defined(lint) 34 __RCSID("$NetBSD: pthread_atfork.c,v 1.10 2015/01/20 18:31:25 christos Exp $"); 35 #endif /* LIBC_SCCS and not lint */ 36 37 #include "namespace.h" 38 39 #include <errno.h> 40 #include <stdlib.h> 41 #include <unistd.h> 42 #include <sys/queue.h> 43 #include "reentrant.h" 44 45 #ifdef __weak_alias 46 __weak_alias(pthread_atfork, _pthread_atfork) 47 __weak_alias(fork, _fork) 48 #endif /* __weak_alias */ 49 50 pid_t __fork(void); /* XXX */ 51 52 struct atfork_callback { 53 SIMPLEQ_ENTRY(atfork_callback) next; 54 void (*fn)(void); 55 }; 56 57 /* 58 * Hypothetically, we could protect the queues with a rwlock which is 59 * write-locked by pthread_atfork() and read-locked by fork(), but 60 * since the intended use of the functions is obtaining locks to hold 61 * across the fork, forking is going to be serialized anyway. 62 */ 63 static struct atfork_callback atfork_builtin; 64 #ifdef _REENTRANT 65 static mutex_t atfork_lock = MUTEX_INITIALIZER; 66 #endif 67 SIMPLEQ_HEAD(atfork_callback_q, atfork_callback); 68 69 static struct atfork_callback_q prepareq = SIMPLEQ_HEAD_INITIALIZER(prepareq); 70 static struct atfork_callback_q parentq = SIMPLEQ_HEAD_INITIALIZER(parentq); 71 static struct atfork_callback_q childq = SIMPLEQ_HEAD_INITIALIZER(childq); 72 73 static struct atfork_callback * 74 af_alloc(void) 75 { 76 77 if (atfork_builtin.fn == NULL) 78 return &atfork_builtin; 79 80 return malloc(sizeof(atfork_builtin)); 81 } 82 83 static void 84 af_free(struct atfork_callback *af) 85 { 86 87 if (af != &atfork_builtin) 88 free(af); 89 } 90 91 int 92 pthread_atfork(void (*prepare)(void), void (*parent)(void), 93 void (*child)(void)) 94 { 95 struct atfork_callback *newprepare, *newparent, *newchild; 96 97 newprepare = newparent = newchild = NULL; 98 99 mutex_lock(&atfork_lock); 100 if (prepare != NULL) { 101 newprepare = af_alloc(); 102 if (newprepare == NULL) { 103 mutex_unlock(&atfork_lock); 104 return ENOMEM; 105 } 106 newprepare->fn = prepare; 107 } 108 109 if (parent != NULL) { 110 newparent = af_alloc(); 111 if (newparent == NULL) { 112 if (newprepare != NULL) 113 af_free(newprepare); 114 mutex_unlock(&atfork_lock); 115 return ENOMEM; 116 } 117 newparent->fn = parent; 118 } 119 120 if (child != NULL) { 121 newchild = af_alloc(); 122 if (newchild == NULL) { 123 if (newprepare != NULL) 124 af_free(newprepare); 125 if (newparent != NULL) 126 af_free(newparent); 127 mutex_unlock(&atfork_lock); 128 return ENOMEM; 129 } 130 newchild->fn = child; 131 } 132 133 /* 134 * The order in which the functions are called is specified as 135 * LIFO for the prepare handler and FIFO for the others; insert 136 * at the head and tail as appropriate so that SIMPLEQ_FOREACH() 137 * produces the right order. 138 */ 139 if (prepare) 140 SIMPLEQ_INSERT_HEAD(&prepareq, newprepare, next); 141 if (parent) 142 SIMPLEQ_INSERT_TAIL(&parentq, newparent, next); 143 if (child) 144 SIMPLEQ_INSERT_TAIL(&childq, newchild, next); 145 mutex_unlock(&atfork_lock); 146 147 return 0; 148 } 149 150 pid_t 151 fork(void) 152 { 153 struct atfork_callback *iter; 154 pid_t ret; 155 156 mutex_lock(&atfork_lock); 157 SIMPLEQ_FOREACH(iter, &prepareq, next) 158 (*iter->fn)(); 159 160 ret = __fork(); 161 162 if (ret != 0) { 163 /* 164 * We are the parent. It doesn't matter here whether 165 * the fork call succeeded or failed. 166 */ 167 SIMPLEQ_FOREACH(iter, &parentq, next) 168 (*iter->fn)(); 169 mutex_unlock(&atfork_lock); 170 } else { 171 /* We are the child */ 172 SIMPLEQ_FOREACH(iter, &childq, next) 173 (*iter->fn)(); 174 /* 175 * Note: We are explicitly *not* unlocking 176 * atfork_lock. Unlocking atfork_lock is problematic, 177 * because if any threads in the parent blocked on it 178 * between the initial lock and the fork() syscall, 179 * unlocking in the child will try to schedule 180 * threads, and either the internal mutex interlock or 181 * the runqueue spinlock could have been held at the 182 * moment of fork(). Since the other threads do not 183 * exist in this process, the spinlock will never be 184 * unlocked, and we would wedge. 185 * Instead, we reinitialize atfork_lock, since we know 186 * that the state of the atfork lists is consistent here, 187 * and that there are no other threads to be affected by 188 * the forcible cleaning of the queue. 189 * This permits double-forking to work, although 190 * it requires knowing that it's "safe" to initialize 191 * a locked mutex in this context. 192 * 193 * The problem exists for users of this interface, 194 * too, since the intented use of pthread_atfork() is 195 * to acquire locks across the fork call to ensure 196 * that the child sees consistent state. There's not 197 * much that can usefully be done in a child handler, 198 * and conventional wisdom discourages using them, but 199 * they're part of the interface, so here we are... 200 */ 201 mutex_init(&atfork_lock, NULL); 202 } 203 204 return ret; 205 } 206