1 /* $NetBSD: pthread_tsd.c,v 1.11 2013/03/21 16:49:12 christos 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, by Andrew Doran, and by Christos Zoulas. 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 __RCSID("$NetBSD: pthread_tsd.c,v 1.11 2013/03/21 16:49:12 christos Exp $"); 34 35 /* Functions and structures dealing with thread-specific data */ 36 #include <errno.h> 37 38 #include "pthread.h" 39 #include "pthread_int.h" 40 #include "reentrant.h" 41 42 43 static pthread_mutex_t tsd_mutex = PTHREAD_MUTEX_INITIALIZER; 44 static int nextkey; 45 46 PTQ_HEAD(pthread__tsd_list, pt_specific) 47 pthread__tsd_list[PTHREAD_KEYS_MAX]; 48 void (*pthread__tsd_destructors[PTHREAD_KEYS_MAX])(void *); 49 50 __strong_alias(__libc_thr_keycreate,pthread_key_create) 51 __strong_alias(__libc_thr_keydelete,pthread_key_delete) 52 53 static void 54 /*ARGSUSED*/ 55 null_destructor(void *p) 56 { 57 } 58 59 #include <err.h> 60 #include <stdlib.h> 61 int 62 pthread_key_create(pthread_key_t *key, void (*destructor)(void *)) 63 { 64 int i; 65 66 if (__predict_false(__uselibcstub)) 67 return __libc_thr_keycreate_stub(key, destructor); 68 69 /* Get a lock on the allocation list */ 70 pthread_mutex_lock(&tsd_mutex); 71 72 /* Find an available slot: 73 * The condition for an available slot is one with the destructor 74 * not being NULL. If the desired destructor is NULL we set it to 75 * our own internal destructor to satisfy the non NULL condition. 76 */ 77 /* 1. Search from "nextkey" to the end of the list. */ 78 for (i = nextkey; i < PTHREAD_KEYS_MAX; i++) 79 if (pthread__tsd_destructors[i] == NULL) 80 break; 81 82 if (i == PTHREAD_KEYS_MAX) { 83 /* 2. If that didn't work, search from the start 84 * of the list back to "nextkey". 85 */ 86 for (i = 0; i < nextkey; i++) 87 if (pthread__tsd_destructors[i] == NULL) 88 break; 89 90 if (i == nextkey) { 91 /* If we didn't find one here, there isn't one 92 * to be found. 93 */ 94 pthread_mutex_unlock(&tsd_mutex); 95 return EAGAIN; 96 } 97 } 98 99 /* Got one. */ 100 pthread__assert(PTQ_EMPTY(&pthread__tsd_list[i])); 101 pthread__tsd_destructors[i] = destructor ? destructor : null_destructor; 102 103 nextkey = (i + 1) % PTHREAD_KEYS_MAX; 104 pthread_mutex_unlock(&tsd_mutex); 105 *key = i; 106 107 return 0; 108 } 109 110 /* 111 * Each thread holds an array of PTHREAD_KEYS_MAX pt_specific list 112 * elements. When an element is used it is inserted into the appropriate 113 * key bucket of pthread__tsd_list. This means that ptqe_prev == NULL, 114 * means that the element is not threaded, ptqe_prev != NULL it is 115 * already part of the list. When we set to a NULL value we delete from the 116 * list if it was in the list, and when we set to non-NULL value, we insert 117 * in the list if it was not already there. 118 * 119 * We keep this global array of lists of threads that have called 120 * pthread_set_specific with non-null values, for each key so that 121 * we don't have to check all threads for non-NULL values in 122 * pthread_key_destroy 123 * 124 * We could keep an accounting of the number of specific used 125 * entries per thread, so that we can update pt_havespecific when we delete 126 * the last one, but we don't bother for now 127 */ 128 int 129 pthread__add_specific(pthread_t self, pthread_key_t key, const void *value) 130 { 131 struct pt_specific *pt; 132 133 pthread__assert(key >= 0 && key < PTHREAD_KEYS_MAX); 134 135 pthread_mutex_lock(&tsd_mutex); 136 pthread__assert(pthread__tsd_destructors[key] != NULL); 137 pt = &self->pt_specific[key]; 138 self->pt_havespecific = 1; 139 if (value) { 140 if (pt->pts_next.ptqe_prev == NULL) 141 PTQ_INSERT_HEAD(&pthread__tsd_list[key], pt, pts_next); 142 } else { 143 if (pt->pts_next.ptqe_prev != NULL) { 144 PTQ_REMOVE(&pthread__tsd_list[key], pt, pts_next); 145 pt->pts_next.ptqe_prev = NULL; 146 } 147 } 148 pt->pts_value = __UNCONST(value); 149 pthread_mutex_unlock(&tsd_mutex); 150 151 return 0; 152 } 153 154 int 155 pthread_key_delete(pthread_key_t key) 156 { 157 /* 158 * This is tricky. The standard says of pthread_key_create() 159 * that new keys have the value NULL associated with them in 160 * all threads. According to people who were present at the 161 * standardization meeting, that requirement was written 162 * before pthread_key_delete() was introduced, and not 163 * reconsidered when it was. 164 * 165 * See David Butenhof's article in comp.programming.threads: 166 * Subject: Re: TSD key reusing issue 167 * Message-ID: <u97d8.29$fL6.200@news.cpqcorp.net> 168 * Date: Thu, 21 Feb 2002 09:06:17 -0500 169 * http://groups.google.com/groups?\ 170 * hl=en&selm=u97d8.29%24fL6.200%40news.cpqcorp.net 171 * 172 * Given: 173 * 174 * 1: Applications are not required to clear keys in all 175 * threads before calling pthread_key_delete(). 176 * 2: Clearing pointers without running destructors is a 177 * memory leak. 178 * 3: The pthread_key_delete() function is expressly forbidden 179 * to run any destructors. 180 * 181 * Option 1: Make this function effectively a no-op and 182 * prohibit key reuse. This is a possible resource-exhaustion 183 * problem given that we have a static storage area for keys, 184 * but having a non-static storage area would make 185 * pthread_setspecific() expensive (might need to realloc the 186 * TSD array). 187 * 188 * Option 2: Ignore the specified behavior of 189 * pthread_key_create() and leave the old values. If an 190 * application deletes a key that still has non-NULL values in 191 * some threads... it's probably a memory leak and hence 192 * incorrect anyway, and we're within our rights to let the 193 * application lose. However, it's possible (if unlikely) that 194 * the application is storing pointers to non-heap data, or 195 * non-pointers that have been wedged into a void pointer, so 196 * we can't entirely write off such applications as incorrect. 197 * This could also lead to running (new) destructors on old 198 * data that was never supposed to be associated with that 199 * destructor. 200 * 201 * Option 3: Follow the specified behavior of 202 * pthread_key_create(). Either pthread_key_create() or 203 * pthread_key_delete() would then have to clear the values in 204 * every thread's slot for that key. In order to guarantee the 205 * visibility of the NULL value in other threads, there would 206 * have to be synchronization operations in both the clearer 207 * and pthread_getspecific(). Putting synchronization in 208 * pthread_getspecific() is a big performance lose. But in 209 * reality, only (buggy) reuse of an old key would require 210 * this synchronization; for a new key, there has to be a 211 * memory-visibility propagating event between the call to 212 * pthread_key_create() and pthread_getspecific() with that 213 * key, so setting the entries to NULL without synchronization 214 * will work, subject to problem (2) above. However, it's kind 215 * of slow. 216 * 217 * Note that the argument in option 3 only applies because we 218 * keep TSD in ordinary memory which follows the pthreads 219 * visibility rules. The visibility rules are not required by 220 * the standard to apply to TSD, so the argument doesn't 221 * apply in general, just to this implementation. 222 */ 223 224 /* 225 * We do option 3; we find the list of all pt_specific structures 226 * threaded on the key we are deleting, unthread them, and set the 227 * pointer to NULL. Finally we unthread the entry, freeing it for 228 * further use. 229 * 230 * We don't call the destructor here, it is the responsibility 231 * of the application to cleanup the storage: 232 * http://pubs.opengroup.org/onlinepubs/9699919799/functions/\ 233 * pthread_key_delete.html 234 */ 235 struct pt_specific *pt; 236 237 if (__predict_false(__uselibcstub)) 238 return __libc_thr_keydelete_stub(key); 239 240 pthread__assert(key >= 0 && key < PTHREAD_KEYS_MAX); 241 242 pthread_mutex_lock(&tsd_mutex); 243 244 pthread__assert(pthread__tsd_destructors[key] != NULL); 245 246 while ((pt = PTQ_FIRST(&pthread__tsd_list[key])) != NULL) { 247 PTQ_REMOVE(&pthread__tsd_list[key], pt, pts_next); 248 pt->pts_value = NULL; 249 pt->pts_next.ptqe_prev = NULL; 250 } 251 252 pthread__tsd_destructors[key] = NULL; 253 pthread_mutex_unlock(&tsd_mutex); 254 255 return 0; 256 } 257 258 /* Perform thread-exit-time destruction of thread-specific data. */ 259 void 260 pthread__destroy_tsd(pthread_t self) 261 { 262 int i, done, iterations; 263 void *val; 264 void (*destructor)(void *); 265 266 if (!self->pt_havespecific) 267 return; 268 pthread_mutex_unlock(&self->pt_lock); 269 270 /* Butenhof, section 5.4.2 (page 167): 271 * 272 * ``Also, Pthreads sets the thread-specific data value for a 273 * key to NULL before calling that key's destructor (passing 274 * the previous value of the key) when a thread terminates [*]. 275 * ... 276 * [*] That is, unfortunately, not what the standard 277 * says. This is one of the problems with formal standards - 278 * they say what they say, not what they were intended to 279 * say. Somehow, an error crept in, and the sentence 280 * specifying that "the implementation clears the 281 * thread-specific data value before calling the destructor" 282 * was deleted. Nobody noticed, and the standard was approved 283 * with the error. So the standard says (by omission) that if 284 * you want to write a portable application using 285 * thread-specific data, that will not hang on thread 286 * termination, you must call pthread_setspecific within your 287 * destructor function to change the value to NULL. This would 288 * be silly, and any serious implementation of Pthreads will 289 * violate the standard in this respect. Of course, the 290 * standard will be fixed, probably by the 1003.1n amendment 291 * (assorted corrections to 1003.1c-1995), but that will take 292 * a while.'' 293 */ 294 295 iterations = 4; /* We're not required to try very hard */ 296 do { 297 done = 1; 298 for (i = 0; i < PTHREAD_KEYS_MAX; i++) { 299 struct pt_specific *pt = &self->pt_specific[i]; 300 if (pt->pts_next.ptqe_prev == NULL) 301 continue; 302 pthread_mutex_lock(&tsd_mutex); 303 304 if (pt->pts_next.ptqe_prev != NULL) { 305 PTQ_REMOVE(&pthread__tsd_list[i], pt, pts_next); 306 val = pt->pts_value; 307 pt->pts_value = NULL; 308 pt->pts_next.ptqe_prev = NULL; 309 destructor = pthread__tsd_destructors[i]; 310 } else 311 destructor = NULL; 312 313 pthread_mutex_unlock(&tsd_mutex); 314 if (destructor != NULL) { 315 done = 0; 316 (*destructor)(val); 317 } 318 } 319 } while (!done && iterations--); 320 321 self->pt_havespecific = 0; 322 pthread_mutex_lock(&self->pt_lock); 323 } 324