1 /* $NetBSD: pthread_tsd.c,v 1.3 2007/08/16 12:01:49 ad Exp $ */ 2 3 /*- 4 * Copyright (c) 2001, 2007 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 * 3. All advertising materials mentioning features or use of this software 19 * must display the following acknowledgement: 20 * This product includes software developed by the NetBSD 21 * Foundation, Inc. and its contributors. 22 * 4. Neither the name of The NetBSD Foundation nor the names of its 23 * contributors may be used to endorse or promote products derived 24 * from this software without specific prior written permission. 25 * 26 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 27 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 28 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 29 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 30 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 31 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 32 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 33 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 34 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 35 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 36 * POSSIBILITY OF SUCH DAMAGE. 37 */ 38 39 #include <sys/cdefs.h> 40 __RCSID("$NetBSD: pthread_tsd.c,v 1.3 2007/08/16 12:01:49 ad Exp $"); 41 42 /* Functions and structures dealing with thread-specific data */ 43 #include <errno.h> 44 45 #include "pthread.h" 46 #include "pthread_int.h" 47 48 static pthread_mutex_t tsd_mutex = PTHREAD_MUTEX_INITIALIZER; 49 static int nextkey; 50 int pthread__tsd_alloc[PTHREAD_KEYS_MAX]; 51 void (*pthread__tsd_destructors[PTHREAD_KEYS_MAX])(void *); 52 53 __strong_alias(__libc_thr_keycreate,pthread_key_create) 54 __strong_alias(__libc_thr_keydelete,pthread_key_delete) 55 56 int 57 pthread_key_create(pthread_key_t *key, void (*destructor)(void *)) 58 { 59 int i; 60 61 /* Get a lock on the allocation list */ 62 pthread_mutex_lock(&tsd_mutex); 63 64 /* Find an available slot */ 65 /* 1. Search from "nextkey" to the end of the list. */ 66 for (i = nextkey; i < PTHREAD_KEYS_MAX; i++) 67 if (pthread__tsd_alloc[i] == 0) 68 break; 69 70 if (i == PTHREAD_KEYS_MAX) { 71 /* 2. If that didn't work, search from the start 72 * of the list back to "nextkey". 73 */ 74 for (i = 0; i < nextkey; i++) 75 if (pthread__tsd_alloc[i] == 0) 76 break; 77 78 if (i == nextkey) { 79 /* If we didn't find one here, there isn't one 80 * to be found. 81 */ 82 pthread_mutex_unlock(&tsd_mutex); 83 return EAGAIN; 84 } 85 } 86 87 /* Got one. */ 88 pthread__tsd_alloc[i] = 1; 89 nextkey = (i + 1) % PTHREAD_KEYS_MAX; 90 pthread__tsd_destructors[i] = destructor; 91 pthread_mutex_unlock(&tsd_mutex); 92 *key = i; 93 94 return 0; 95 } 96 97 int 98 pthread_key_delete(pthread_key_t key) 99 { 100 101 /* 102 * This is tricky. The standard says of pthread_key_create() 103 * that new keys have the value NULL associated with them in 104 * all threads. According to people who were present at the 105 * standardization meeting, that requirement was written 106 * before pthread_key_delete() was introduced, and not 107 * reconsidered when it was. 108 * 109 * See David Butenhof's article in comp.programming.threads: 110 * Subject: Re: TSD key reusing issue 111 * Message-ID: <u97d8.29$fL6.200@news.cpqcorp.net> 112 * Date: Thu, 21 Feb 2002 09:06:17 -0500 113 * http://groups.google.com/groups?hl=en&selm=u97d8.29%24fL6.200%40news.cpqcorp.net 114 * 115 * Given: 116 * 117 * 1: Applications are not required to clear keys in all 118 * threads before calling pthread_key_delete(). 119 * 2: Clearing pointers without running destructors is a 120 * memory leak. 121 * 3: The pthread_key_delete() function is expressly forbidden 122 * to run any destructors. 123 * 124 * Option 1: Make this function effectively a no-op and 125 * prohibit key reuse. This is a possible resource-exhaustion 126 * problem given that we have a static storage area for keys, 127 * but having a non-static storage area would make 128 * pthread_setspecific() expensive (might need to realloc the 129 * TSD array). 130 * 131 * Option 2: Ignore the specified behavior of 132 * pthread_key_create() and leave the old values. If an 133 * application deletes a key that still has non-NULL values in 134 * some threads... it's probably a memory leak and hence 135 * incorrect anyway, and we're within our rights to let the 136 * application lose. However, it's possible (if unlikely) that 137 * the application is storing pointers to non-heap data, or 138 * non-pointers that have been wedged into a void pointer, so 139 * we can't entirely write off such applications as incorrect. 140 * This could also lead to running (new) destructors on old 141 * data that was never supposed to be associated with that 142 * destructor. 143 * 144 * Option 3: Follow the specified behavior of 145 * pthread_key_create(). Either pthread_key_create() or 146 * pthread_key_delete() would then have to clear the values in 147 * every thread's slot for that key. In order to guarantee the 148 * visibility of the NULL value in other threads, there would 149 * have to be synchronization operations in both the clearer 150 * and pthread_getspecific(). Putting synchronization in 151 * pthread_getspecific() is a big performance lose. But in 152 * reality, only (buggy) reuse of an old key would require 153 * this synchronization; for a new key, there has to be a 154 * memory-visibility propagating event between the call to 155 * pthread_key_create() and pthread_getspecific() with that 156 * key, so setting the entries to NULL without synchronization 157 * will work, subject to problem (2) above. However, it's kind 158 * of slow. 159 * 160 * Note that the argument in option 3 only applies because we 161 * keep TSD in ordinary memory which follows the pthreads 162 * visibility rules. The visibility rules are not required by 163 * the standard to apply to TSD, so the argument doesn't 164 * apply in general, just to this implementation. 165 */ 166 167 /* For the momemt, we're going with option 1. */ 168 pthread_mutex_lock(&tsd_mutex); 169 pthread__tsd_destructors[key] = NULL; 170 pthread_mutex_unlock(&tsd_mutex); 171 172 return 0; 173 } 174 175 /* Perform thread-exit-time destruction of thread-specific data. */ 176 void 177 pthread__destroy_tsd(pthread_t self) 178 { 179 int i, done, iterations; 180 void *val; 181 void (*destructor)(void *); 182 183 if (!self->pt_havespecific) 184 return; 185 186 /* Butenhof, section 5.4.2 (page 167): 187 * 188 * ``Also, Pthreads sets the thread-specific data value for a 189 * key to NULL before calling that key's destructor (passing 190 * the previous value of the key) when a thread terminates [*]. 191 * ... 192 * [*] That is, unfortunately, not what the standard 193 * says. This is one of the problems with formal standards - 194 * they say what they say, not what they were intended to 195 * say. Somehow, an error crept in, and the sentence 196 * specifying that "the implementation clears the 197 * thread-specific data value before calling the destructor" 198 * was deleted. Nobody noticed, and the standard was approved 199 * with the error. So the standard says (by omission) that if 200 * you want to write a portable application using 201 * thread-specific data, that will not hang on thread 202 * termination, you must call pthread_setspecific within your 203 * destructor function to change the value to NULL. This would 204 * be silly, and any serious implementation of Pthreads will 205 * violate the standard in this respect. Of course, the 206 * standard will be fixed, probably by the 1003.1n amendment 207 * (assorted corrections to 1003.1c-1995), but that will take 208 * a while.'' 209 */ 210 211 iterations = 4; /* We're not required to try very hard */ 212 do { 213 done = 1; 214 for (i = 0; i < PTHREAD_KEYS_MAX; i++) { 215 if (self->pt_specific[i] != NULL) { 216 pthread_mutex_lock(&tsd_mutex); 217 destructor = pthread__tsd_destructors[i]; 218 pthread_mutex_unlock(&tsd_mutex); 219 if (destructor != NULL) { 220 done = 0; 221 val = self->pt_specific[i]; 222 self->pt_specific[i] = NULL; /* see above */ 223 (*destructor)(val); 224 } 225 } 226 } 227 } while (!done && iterations--); 228 229 self->pt_havespecific = 0; 230 } 231