1 /* $NetBSD: pthread_tsd.c,v 1.23 2020/06/11 18:42:02 ad Exp $ */ 2 3 /*- 4 * Copyright (c) 2001, 2007, 2020 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 1.23 2020/06/11 18:42:02 ad 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 #include "tsd.h" 43 44 int pthread_keys_max; 45 static pthread_mutex_t tsd_mutex = PTHREAD_MUTEX_INITIALIZER; 46 static int nextkey; 47 48 PTQ_HEAD(pthread__tsd_list, pt_specific) *pthread__tsd_list = NULL; 49 void (**pthread__tsd_destructors)(void *) = NULL; 50 51 __strong_alias(__libc_thr_keycreate,pthread_key_create) 52 __strong_alias(__libc_thr_keydelete,pthread_key_delete) 53 54 static void 55 /*ARGSUSED*/ 56 null_destructor(void *p) 57 { 58 } 59 60 #include <err.h> 61 #include <stdlib.h> 62 #include <stdio.h> 63 64 static void 65 pthread_tsd_prefork(void) 66 { 67 pthread_mutex_lock(&tsd_mutex); 68 } 69 70 static void 71 pthread_tsd_postfork(void) 72 { 73 pthread_mutex_unlock(&tsd_mutex); 74 } 75 76 static void 77 pthread_tsd_postfork_child(void) 78 { 79 pthread_mutex_init(&tsd_mutex, NULL); 80 } 81 82 void * 83 pthread_tsd_init(size_t *tlen) 84 { 85 char *pkm; 86 size_t alen; 87 char *arena; 88 89 pthread_atfork(pthread_tsd_prefork, pthread_tsd_postfork, pthread_tsd_postfork_child); 90 91 if ((pkm = pthread__getenv("PTHREAD_KEYS_MAX")) != NULL) { 92 pthread_keys_max = (int)strtol(pkm, NULL, 0); 93 if (pthread_keys_max < _POSIX_THREAD_KEYS_MAX) 94 pthread_keys_max = _POSIX_THREAD_KEYS_MAX; 95 } else { 96 pthread_keys_max = PTHREAD_KEYS_MAX; 97 } 98 99 /* 100 * Can't use malloc here yet, because malloc will use the fake 101 * libc thread functions to initialize itself, so mmap the space. 102 */ 103 *tlen = sizeof(struct __pthread_st) 104 + pthread_keys_max * sizeof(struct pt_specific); 105 alen = *tlen 106 + sizeof(*pthread__tsd_list) * pthread_keys_max 107 + sizeof(*pthread__tsd_destructors) * pthread_keys_max; 108 109 arena = mmap(NULL, alen, PROT_READ|PROT_WRITE, MAP_ANON, -1, 0); 110 if (arena == MAP_FAILED) { 111 pthread_keys_max = 0; 112 return NULL; 113 } 114 115 pthread__tsd_list = (void *)arena; 116 arena += sizeof(*pthread__tsd_list) * pthread_keys_max; 117 pthread__tsd_destructors = (void *)arena; 118 arena += sizeof(*pthread__tsd_destructors) * pthread_keys_max; 119 return arena; 120 } 121 122 int 123 pthread_key_create(pthread_key_t *key, void (*destructor)(void *)) 124 { 125 int i; 126 127 if (__predict_false(__uselibcstub)) 128 return __libc_thr_keycreate_stub(key, destructor); 129 130 /* Get a lock on the allocation list */ 131 pthread_mutex_lock(&tsd_mutex); 132 133 /* Find an available slot: 134 * The condition for an available slot is one with the destructor 135 * not being NULL. If the desired destructor is NULL we set it to 136 * our own internal destructor to satisfy the non NULL condition. 137 */ 138 /* 1. Search from "nextkey" to the end of the list. */ 139 for (i = nextkey; i < pthread_keys_max; i++) 140 if (pthread__tsd_destructors[i] == NULL) 141 break; 142 143 if (i == pthread_keys_max) { 144 /* 2. If that didn't work, search from the start 145 * of the list back to "nextkey". 146 */ 147 for (i = 0; i < nextkey; i++) 148 if (pthread__tsd_destructors[i] == NULL) 149 break; 150 151 if (i == nextkey) { 152 /* If we didn't find one here, there isn't one 153 * to be found. 154 */ 155 pthread_mutex_unlock(&tsd_mutex); 156 return EAGAIN; 157 } 158 } 159 160 /* Got one. */ 161 pthread__assert(PTQ_EMPTY(&pthread__tsd_list[i])); 162 pthread__tsd_destructors[i] = destructor ? destructor : null_destructor; 163 164 nextkey = (i + 1) % pthread_keys_max; 165 pthread_mutex_unlock(&tsd_mutex); 166 *key = i; 167 168 return 0; 169 } 170 171 /* 172 * Each thread holds an array of pthread_keys_max pt_specific list 173 * elements. When an element is used it is inserted into the appropriate 174 * key bucket of pthread__tsd_list. This means that ptqe_prev == NULL, 175 * means that the element is not threaded, ptqe_prev != NULL it is 176 * already part of the list. If a key is set to a non-NULL value for the 177 * first time, it is added to the list. 178 * 179 * We keep this global array of lists of threads that have called 180 * pthread_set_specific with non-null values, for each key so that 181 * we don't have to check all threads for non-NULL values in 182 * pthread_key_destroy. 183 * 184 * The assumption here is that a concurrent pthread_key_delete is already 185 * undefined behavior. The mutex is taken only once per thread/key 186 * combination. 187 * 188 * We could keep an accounting of the number of specific used 189 * entries per thread, so that we can update pt_havespecific when we delete 190 * the last one, but we don't bother for now 191 */ 192 int 193 pthread__add_specific(pthread_t self, pthread_key_t key, const void *value) 194 { 195 struct pt_specific *pt; 196 197 pthread__assert(key >= 0 && key < pthread_keys_max); 198 199 pthread__assert(pthread__tsd_destructors[key] != NULL); 200 pt = &self->pt_specific[key]; 201 self->pt_havespecific = 1; 202 if (value && !pt->pts_next.ptqe_prev) { 203 pthread_mutex_lock(&tsd_mutex); 204 PTQ_INSERT_HEAD(&pthread__tsd_list[key], pt, pts_next); 205 pthread_mutex_unlock(&tsd_mutex); 206 } 207 pt->pts_value = __UNCONST(value); 208 209 return 0; 210 } 211 212 int 213 pthread_key_delete(pthread_key_t key) 214 { 215 /* 216 * This is tricky. The standard says of pthread_key_create() 217 * that new keys have the value NULL associated with them in 218 * all threads. According to people who were present at the 219 * standardization meeting, that requirement was written 220 * before pthread_key_delete() was introduced, and not 221 * reconsidered when it was. 222 * 223 * See David Butenhof's article in comp.programming.threads: 224 * Subject: Re: TSD key reusing issue 225 * Message-ID: <u97d8.29$fL6.200@news.cpqcorp.net> 226 * Date: Thu, 21 Feb 2002 09:06:17 -0500 227 * http://groups.google.com/groups?\ 228 * hl=en&selm=u97d8.29%24fL6.200%40news.cpqcorp.net 229 * 230 * Given: 231 * 232 * 1: Applications are not required to clear keys in all 233 * threads before calling pthread_key_delete(). 234 * 2: Clearing pointers without running destructors is a 235 * memory leak. 236 * 3: The pthread_key_delete() function is expressly forbidden 237 * to run any destructors. 238 * 239 * Option 1: Make this function effectively a no-op and 240 * prohibit key reuse. This is a possible resource-exhaustion 241 * problem given that we have a static storage area for keys, 242 * but having a non-static storage area would make 243 * pthread_setspecific() expensive (might need to realloc the 244 * TSD array). 245 * 246 * Option 2: Ignore the specified behavior of 247 * pthread_key_create() and leave the old values. If an 248 * application deletes a key that still has non-NULL values in 249 * some threads... it's probably a memory leak and hence 250 * incorrect anyway, and we're within our rights to let the 251 * application lose. However, it's possible (if unlikely) that 252 * the application is storing pointers to non-heap data, or 253 * non-pointers that have been wedged into a void pointer, so 254 * we can't entirely write off such applications as incorrect. 255 * This could also lead to running (new) destructors on old 256 * data that was never supposed to be associated with that 257 * destructor. 258 * 259 * Option 3: Follow the specified behavior of 260 * pthread_key_create(). Either pthread_key_create() or 261 * pthread_key_delete() would then have to clear the values in 262 * every thread's slot for that key. In order to guarantee the 263 * visibility of the NULL value in other threads, there would 264 * have to be synchronization operations in both the clearer 265 * and pthread_getspecific(). Putting synchronization in 266 * pthread_getspecific() is a big performance lose. But in 267 * reality, only (buggy) reuse of an old key would require 268 * this synchronization; for a new key, there has to be a 269 * memory-visibility propagating event between the call to 270 * pthread_key_create() and pthread_getspecific() with that 271 * key, so setting the entries to NULL without synchronization 272 * will work, subject to problem (2) above. However, it's kind 273 * of slow. 274 * 275 * Note that the argument in option 3 only applies because we 276 * keep TSD in ordinary memory which follows the pthreads 277 * visibility rules. The visibility rules are not required by 278 * the standard to apply to TSD, so the argument doesn't 279 * apply in general, just to this implementation. 280 */ 281 282 /* 283 * We do option 3; we find the list of all pt_specific structures 284 * threaded on the key we are deleting, unthread them, and set the 285 * pointer to NULL. Finally we unthread the entry, freeing it for 286 * further use. 287 * 288 * We don't call the destructor here, it is the responsibility 289 * of the application to cleanup the storage: 290 * http://pubs.opengroup.org/onlinepubs/9699919799/functions/\ 291 * pthread_key_delete.html 292 */ 293 struct pt_specific *pt; 294 295 if (__predict_false(__uselibcstub)) 296 return __libc_thr_keydelete_stub(key); 297 298 pthread__assert(key >= 0 && key < pthread_keys_max); 299 300 pthread_mutex_lock(&tsd_mutex); 301 302 pthread__assert(pthread__tsd_destructors[key] != NULL); 303 304 while ((pt = PTQ_FIRST(&pthread__tsd_list[key])) != NULL) { 305 PTQ_REMOVE(&pthread__tsd_list[key], pt, pts_next); 306 pt->pts_value = NULL; 307 pt->pts_next.ptqe_prev = NULL; 308 } 309 310 pthread__tsd_destructors[key] = NULL; 311 pthread_mutex_unlock(&tsd_mutex); 312 313 return 0; 314 } 315 316 /* Perform thread-exit-time destruction of thread-specific data. */ 317 void 318 pthread__destroy_tsd(pthread_t self) 319 { 320 int i, done, iterations; 321 void *val; 322 void (*destructor)(void *); 323 324 if (!self->pt_havespecific) 325 return; 326 327 /* Butenhof, section 5.4.2 (page 167): 328 * 329 * ``Also, Pthreads sets the thread-specific data value for a 330 * key to NULL before calling that key's destructor (passing 331 * the previous value of the key) when a thread terminates [*]. 332 * ... 333 * [*] That is, unfortunately, not what the standard 334 * says. This is one of the problems with formal standards - 335 * they say what they say, not what they were intended to 336 * say. Somehow, an error crept in, and the sentence 337 * specifying that "the implementation clears the 338 * thread-specific data value before calling the destructor" 339 * was deleted. Nobody noticed, and the standard was approved 340 * with the error. So the standard says (by omission) that if 341 * you want to write a portable application using 342 * thread-specific data, that will not hang on thread 343 * termination, you must call pthread_setspecific within your 344 * destructor function to change the value to NULL. This would 345 * be silly, and any serious implementation of Pthreads will 346 * violate the standard in this respect. Of course, the 347 * standard will be fixed, probably by the 1003.1n amendment 348 * (assorted corrections to 1003.1c-1995), but that will take 349 * a while.'' 350 */ 351 352 /* We're not required to try very hard */ 353 iterations = PTHREAD_DESTRUCTOR_ITERATIONS; 354 do { 355 done = 1; 356 for (i = 0; i < pthread_keys_max; i++) { 357 struct pt_specific *pt = &self->pt_specific[i]; 358 if (pt->pts_next.ptqe_prev == NULL) 359 continue; 360 pthread_mutex_lock(&tsd_mutex); 361 362 if (pt->pts_next.ptqe_prev != NULL) { 363 PTQ_REMOVE(&pthread__tsd_list[i], pt, pts_next); 364 val = pt->pts_value; 365 pt->pts_value = NULL; 366 pt->pts_next.ptqe_prev = NULL; 367 destructor = pthread__tsd_destructors[i]; 368 } else 369 destructor = NULL; 370 371 pthread_mutex_unlock(&tsd_mutex); 372 if (destructor != NULL && val != NULL) { 373 done = 0; 374 (*destructor)(val); 375 } 376 } 377 } while (!done && --iterations); 378 379 self->pt_havespecific = 0; 380 } 381 382 void 383 pthread__copy_tsd(pthread_t self) 384 { 385 for (size_t key = 0; key < TSD_KEYS_MAX; key++) { 386 387 if (__libc_tsd[key].tsd_inuse == 0) 388 continue; 389 390 pthread__assert(pthread__tsd_destructors[key] == NULL); 391 pthread__tsd_destructors[key] = __libc_tsd[key].tsd_dtor ? 392 __libc_tsd[key].tsd_dtor : null_destructor; 393 nextkey = (key + 1) % pthread_keys_max; 394 395 self->pt_havespecific = 1; 396 struct pt_specific *pt = &self->pt_specific[key]; 397 pt->pts_value = __libc_tsd[key].tsd_val; 398 __libc_tsd[key].tsd_inuse = 0; 399 } 400 } 401