1 /* $NetBSD: pthread_tsd.c,v 1.16 2017/07/09 20:21:08 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 1.16 2017/07/09 20:21:08 christos Exp $"); 34 35 /* Functions and structures dealing with thread-specific data */ 36 #include <errno.h> 37 #include <sys/mman.h> 38 39 #include "pthread.h" 40 #include "pthread_int.h" 41 #include "reentrant.h" 42 43 int pthread_keys_max; 44 static pthread_mutex_t tsd_mutex = PTHREAD_MUTEX_INITIALIZER; 45 static int nextkey; 46 47 PTQ_HEAD(pthread__tsd_list, pt_specific) *pthread__tsd_list = NULL; 48 void (**pthread__tsd_destructors)(void *) = NULL; 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 #include <stdio.h> 62 63 void * 64 pthread_tsd_init(size_t *tlen) 65 { 66 char *pkm; 67 size_t alen; 68 char *arena; 69 70 if ((pkm = pthread__getenv("PTHREAD_KEYS_MAX")) != NULL) { 71 pthread_keys_max = (int)strtol(pkm, NULL, 0); 72 if (pthread_keys_max < _POSIX_THREAD_KEYS_MAX) 73 pthread_keys_max = _POSIX_THREAD_KEYS_MAX; 74 } else { 75 pthread_keys_max = PTHREAD_KEYS_MAX; 76 } 77 78 /* 79 * Can't use malloc here yet, because malloc will use the fake 80 * libc thread functions to initialize itself, so mmap the space. 81 */ 82 *tlen = sizeof(struct __pthread_st) 83 + pthread_keys_max * sizeof(struct pt_specific); 84 alen = *tlen 85 + sizeof(*pthread__tsd_list) * pthread_keys_max 86 + sizeof(*pthread__tsd_destructors) * pthread_keys_max; 87 88 arena = mmap(NULL, alen, PROT_READ|PROT_WRITE, MAP_ANON, -1, 0); 89 if (arena == MAP_FAILED) { 90 pthread_keys_max = 0; 91 return NULL; 92 } 93 94 pthread__tsd_list = (void *)arena; 95 arena += sizeof(*pthread__tsd_list) * pthread_keys_max; 96 pthread__tsd_destructors = (void *)arena; 97 arena += sizeof(*pthread__tsd_destructors) * pthread_keys_max; 98 return arena; 99 } 100 101 int 102 pthread_key_create(pthread_key_t *key, void (*destructor)(void *)) 103 { 104 int i; 105 106 if (__predict_false(__uselibcstub)) 107 return __libc_thr_keycreate_stub(key, destructor); 108 109 /* Get a lock on the allocation list */ 110 pthread_mutex_lock(&tsd_mutex); 111 112 /* Find an available slot: 113 * The condition for an available slot is one with the destructor 114 * not being NULL. If the desired destructor is NULL we set it to 115 * our own internal destructor to satisfy the non NULL condition. 116 */ 117 /* 1. Search from "nextkey" to the end of the list. */ 118 for (i = nextkey; i < pthread_keys_max; i++) 119 if (pthread__tsd_destructors[i] == NULL) 120 break; 121 122 if (i == pthread_keys_max) { 123 /* 2. If that didn't work, search from the start 124 * of the list back to "nextkey". 125 */ 126 for (i = 0; i < nextkey; i++) 127 if (pthread__tsd_destructors[i] == NULL) 128 break; 129 130 if (i == nextkey) { 131 /* If we didn't find one here, there isn't one 132 * to be found. 133 */ 134 pthread_mutex_unlock(&tsd_mutex); 135 return EAGAIN; 136 } 137 } 138 139 /* Got one. */ 140 pthread__assert(PTQ_EMPTY(&pthread__tsd_list[i])); 141 pthread__tsd_destructors[i] = destructor ? destructor : null_destructor; 142 143 nextkey = (i + 1) % pthread_keys_max; 144 pthread_mutex_unlock(&tsd_mutex); 145 *key = i; 146 147 return 0; 148 } 149 150 /* 151 * Each thread holds an array of pthread_keys_max pt_specific list 152 * elements. When an element is used it is inserted into the appropriate 153 * key bucket of pthread__tsd_list. This means that ptqe_prev == NULL, 154 * means that the element is not threaded, ptqe_prev != NULL it is 155 * already part of the list. When we set to a NULL value we delete from the 156 * list if it was in the list, and when we set to non-NULL value, we insert 157 * in the list if it was not already there. 158 * 159 * We keep this global array of lists of threads that have called 160 * pthread_set_specific with non-null values, for each key so that 161 * we don't have to check all threads for non-NULL values in 162 * pthread_key_destroy 163 * 164 * We could keep an accounting of the number of specific used 165 * entries per thread, so that we can update pt_havespecific when we delete 166 * the last one, but we don't bother for now 167 */ 168 int 169 pthread__add_specific(pthread_t self, pthread_key_t key, const void *value) 170 { 171 struct pt_specific *pt; 172 173 pthread__assert(key >= 0 && key < pthread_keys_max); 174 175 pthread_mutex_lock(&tsd_mutex); 176 pthread__assert(pthread__tsd_destructors[key] != NULL); 177 pt = &self->pt_specific[key]; 178 self->pt_havespecific = 1; 179 if (value) { 180 if (pt->pts_next.ptqe_prev == NULL) 181 PTQ_INSERT_HEAD(&pthread__tsd_list[key], pt, pts_next); 182 } else { 183 if (pt->pts_next.ptqe_prev != NULL) { 184 PTQ_REMOVE(&pthread__tsd_list[key], pt, pts_next); 185 pt->pts_next.ptqe_prev = NULL; 186 } 187 } 188 pt->pts_value = __UNCONST(value); 189 pthread_mutex_unlock(&tsd_mutex); 190 191 return 0; 192 } 193 194 int 195 pthread_key_delete(pthread_key_t key) 196 { 197 /* 198 * This is tricky. The standard says of pthread_key_create() 199 * that new keys have the value NULL associated with them in 200 * all threads. According to people who were present at the 201 * standardization meeting, that requirement was written 202 * before pthread_key_delete() was introduced, and not 203 * reconsidered when it was. 204 * 205 * See David Butenhof's article in comp.programming.threads: 206 * Subject: Re: TSD key reusing issue 207 * Message-ID: <u97d8.29$fL6.200@news.cpqcorp.net> 208 * Date: Thu, 21 Feb 2002 09:06:17 -0500 209 * http://groups.google.com/groups?\ 210 * hl=en&selm=u97d8.29%24fL6.200%40news.cpqcorp.net 211 * 212 * Given: 213 * 214 * 1: Applications are not required to clear keys in all 215 * threads before calling pthread_key_delete(). 216 * 2: Clearing pointers without running destructors is a 217 * memory leak. 218 * 3: The pthread_key_delete() function is expressly forbidden 219 * to run any destructors. 220 * 221 * Option 1: Make this function effectively a no-op and 222 * prohibit key reuse. This is a possible resource-exhaustion 223 * problem given that we have a static storage area for keys, 224 * but having a non-static storage area would make 225 * pthread_setspecific() expensive (might need to realloc the 226 * TSD array). 227 * 228 * Option 2: Ignore the specified behavior of 229 * pthread_key_create() and leave the old values. If an 230 * application deletes a key that still has non-NULL values in 231 * some threads... it's probably a memory leak and hence 232 * incorrect anyway, and we're within our rights to let the 233 * application lose. However, it's possible (if unlikely) that 234 * the application is storing pointers to non-heap data, or 235 * non-pointers that have been wedged into a void pointer, so 236 * we can't entirely write off such applications as incorrect. 237 * This could also lead to running (new) destructors on old 238 * data that was never supposed to be associated with that 239 * destructor. 240 * 241 * Option 3: Follow the specified behavior of 242 * pthread_key_create(). Either pthread_key_create() or 243 * pthread_key_delete() would then have to clear the values in 244 * every thread's slot for that key. In order to guarantee the 245 * visibility of the NULL value in other threads, there would 246 * have to be synchronization operations in both the clearer 247 * and pthread_getspecific(). Putting synchronization in 248 * pthread_getspecific() is a big performance lose. But in 249 * reality, only (buggy) reuse of an old key would require 250 * this synchronization; for a new key, there has to be a 251 * memory-visibility propagating event between the call to 252 * pthread_key_create() and pthread_getspecific() with that 253 * key, so setting the entries to NULL without synchronization 254 * will work, subject to problem (2) above. However, it's kind 255 * of slow. 256 * 257 * Note that the argument in option 3 only applies because we 258 * keep TSD in ordinary memory which follows the pthreads 259 * visibility rules. The visibility rules are not required by 260 * the standard to apply to TSD, so the argument doesn't 261 * apply in general, just to this implementation. 262 */ 263 264 /* 265 * We do option 3; we find the list of all pt_specific structures 266 * threaded on the key we are deleting, unthread them, and set the 267 * pointer to NULL. Finally we unthread the entry, freeing it for 268 * further use. 269 * 270 * We don't call the destructor here, it is the responsibility 271 * of the application to cleanup the storage: 272 * http://pubs.opengroup.org/onlinepubs/9699919799/functions/\ 273 * pthread_key_delete.html 274 */ 275 struct pt_specific *pt; 276 277 if (__predict_false(__uselibcstub)) 278 return __libc_thr_keydelete_stub(key); 279 280 pthread__assert(key >= 0 && key < pthread_keys_max); 281 282 pthread_mutex_lock(&tsd_mutex); 283 284 pthread__assert(pthread__tsd_destructors[key] != NULL); 285 286 while ((pt = PTQ_FIRST(&pthread__tsd_list[key])) != NULL) { 287 PTQ_REMOVE(&pthread__tsd_list[key], pt, pts_next); 288 pt->pts_value = NULL; 289 pt->pts_next.ptqe_prev = NULL; 290 } 291 292 pthread__tsd_destructors[key] = NULL; 293 pthread_mutex_unlock(&tsd_mutex); 294 295 return 0; 296 } 297 298 /* Perform thread-exit-time destruction of thread-specific data. */ 299 void 300 pthread__destroy_tsd(pthread_t self) 301 { 302 int i, done, iterations; 303 void *val; 304 void (*destructor)(void *); 305 306 if (!self->pt_havespecific) 307 return; 308 pthread_mutex_unlock(&self->pt_lock); 309 310 /* Butenhof, section 5.4.2 (page 167): 311 * 312 * ``Also, Pthreads sets the thread-specific data value for a 313 * key to NULL before calling that key's destructor (passing 314 * the previous value of the key) when a thread terminates [*]. 315 * ... 316 * [*] That is, unfortunately, not what the standard 317 * says. This is one of the problems with formal standards - 318 * they say what they say, not what they were intended to 319 * say. Somehow, an error crept in, and the sentence 320 * specifying that "the implementation clears the 321 * thread-specific data value before calling the destructor" 322 * was deleted. Nobody noticed, and the standard was approved 323 * with the error. So the standard says (by omission) that if 324 * you want to write a portable application using 325 * thread-specific data, that will not hang on thread 326 * termination, you must call pthread_setspecific within your 327 * destructor function to change the value to NULL. This would 328 * be silly, and any serious implementation of Pthreads will 329 * violate the standard in this respect. Of course, the 330 * standard will be fixed, probably by the 1003.1n amendment 331 * (assorted corrections to 1003.1c-1995), but that will take 332 * a while.'' 333 */ 334 335 /* We're not required to try very hard */ 336 iterations = PTHREAD_DESTRUCTOR_ITERATIONS; 337 do { 338 done = 1; 339 for (i = 0; i < pthread_keys_max; i++) { 340 struct pt_specific *pt = &self->pt_specific[i]; 341 if (pt->pts_next.ptqe_prev == NULL) 342 continue; 343 pthread_mutex_lock(&tsd_mutex); 344 345 if (pt->pts_next.ptqe_prev != NULL) { 346 PTQ_REMOVE(&pthread__tsd_list[i], pt, pts_next); 347 val = pt->pts_value; 348 pt->pts_value = NULL; 349 pt->pts_next.ptqe_prev = NULL; 350 destructor = pthread__tsd_destructors[i]; 351 } else 352 destructor = NULL; 353 354 pthread_mutex_unlock(&tsd_mutex); 355 if (destructor != NULL) { 356 done = 0; 357 (*destructor)(val); 358 } 359 } 360 } while (!done && --iterations); 361 362 self->pt_havespecific = 0; 363 pthread_mutex_lock(&self->pt_lock); 364 } 365