1/* $NetBSD: pthread_tsd.c,v 1.25 2022/04/10 10:38:33 riastradh 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 1.25 2022/04/10 10:38:33 riastradh Exp $"); 34 35/* Need to use libc-private names for atomic operations. */ 36#include "../../common/lib/libc/atomic/atomic_op_namespace.h" 37 38/* Functions and structures dealing with thread-specific data */ 39#include <errno.h> 40#include <sys/mman.h> 41 42#include "pthread.h" 43#include "pthread_int.h" 44#include "reentrant.h" 45#include "tsd.h" 46 47int pthread_keys_max; 48static pthread_mutex_t tsd_mutex = PTHREAD_MUTEX_INITIALIZER; 49static int nextkey; 50 51PTQ_HEAD(pthread__tsd_list, pt_specific) *pthread__tsd_list = NULL; 52void (**pthread__tsd_destructors)(void *) = NULL; 53 54__strong_alias(__libc_thr_keycreate,pthread_key_create) 55__strong_alias(__libc_thr_keydelete,pthread_key_delete) 56 57static void 58/*ARGSUSED*/ 59null_destructor(void *p) 60{ 61} 62 63#include <err.h> 64#include <stdlib.h> 65#include <stdio.h> 66 67static void 68pthread_tsd_prefork(void) 69{ 70 pthread_mutex_lock(&tsd_mutex); 71} 72 73static void 74pthread_tsd_postfork(void) 75{ 76 pthread_mutex_unlock(&tsd_mutex); 77} 78 79static void 80pthread_tsd_postfork_child(void) 81{ 82 pthread_mutex_init(&tsd_mutex, NULL); 83} 84 85void * 86pthread_tsd_init(size_t *tlen) 87{ 88 char *pkm; 89 size_t alen; 90 char *arena; 91 92 pthread_atfork(pthread_tsd_prefork, pthread_tsd_postfork, pthread_tsd_postfork_child); 93 94 if ((pkm = pthread__getenv("PTHREAD_KEYS_MAX")) != NULL) { 95 pthread_keys_max = (int)strtol(pkm, NULL, 0); 96 if (pthread_keys_max < _POSIX_THREAD_KEYS_MAX) 97 pthread_keys_max = _POSIX_THREAD_KEYS_MAX; 98 } else { 99 pthread_keys_max = PTHREAD_KEYS_MAX; 100 } 101 102 /* 103 * Can't use malloc here yet, because malloc will use the fake 104 * libc thread functions to initialize itself, so mmap the space. 105 */ 106 *tlen = sizeof(struct __pthread_st) 107 + pthread_keys_max * sizeof(struct pt_specific); 108 alen = *tlen 109 + sizeof(*pthread__tsd_list) * pthread_keys_max 110 + sizeof(*pthread__tsd_destructors) * pthread_keys_max; 111 112 arena = mmap(NULL, alen, PROT_READ|PROT_WRITE, MAP_ANON, -1, 0); 113 if (arena == MAP_FAILED) { 114 pthread_keys_max = 0; 115 return NULL; 116 } 117 118 pthread__tsd_list = (void *)arena; 119 arena += sizeof(*pthread__tsd_list) * pthread_keys_max; 120 pthread__tsd_destructors = (void *)arena; 121 arena += sizeof(*pthread__tsd_destructors) * pthread_keys_max; 122 return arena; 123} 124 125int 126pthread_key_create(pthread_key_t *key, void (*destructor)(void *)) 127{ 128 int i; 129 130 if (__predict_false(__uselibcstub)) 131 return __libc_thr_keycreate_stub(key, destructor); 132 133 /* Get a lock on the allocation list */ 134 pthread_mutex_lock(&tsd_mutex); 135 136 /* Find an available slot: 137 * The condition for an available slot is one with the destructor 138 * not being NULL. If the desired destructor is NULL we set it to 139 * our own internal destructor to satisfy the non NULL condition. 140 */ 141 /* 1. Search from "nextkey" to the end of the list. */ 142 for (i = nextkey; i < pthread_keys_max; i++) 143 if (pthread__tsd_destructors[i] == NULL) 144 break; 145 146 if (i == pthread_keys_max) { 147 /* 2. If that didn't work, search from the start 148 * of the list back to "nextkey". 149 */ 150 for (i = 0; i < nextkey; i++) 151 if (pthread__tsd_destructors[i] == NULL) 152 break; 153 154 if (i == nextkey) { 155 /* If we didn't find one here, there isn't one 156 * to be found. 157 */ 158 pthread_mutex_unlock(&tsd_mutex); 159 return EAGAIN; 160 } 161 } 162 163 /* Got one. */ 164 pthread__assert(PTQ_EMPTY(&pthread__tsd_list[i])); 165 pthread__tsd_destructors[i] = destructor ? destructor : null_destructor; 166 167 nextkey = (i + 1) % pthread_keys_max; 168 pthread_mutex_unlock(&tsd_mutex); 169 *key = i; 170 171 return 0; 172} 173 174/* 175 * Each thread holds an array of pthread_keys_max pt_specific list 176 * elements. When an element is used it is inserted into the appropriate 177 * key bucket of pthread__tsd_list. This means that ptqe_prev == NULL, 178 * means that the element is not threaded, ptqe_prev != NULL it is 179 * already part of the list. If a key is set to a non-NULL value for the 180 * first time, it is added to the list. 181 * 182 * We keep this global array of lists of threads that have called 183 * pthread_set_specific with non-null values, for each key so that 184 * we don't have to check all threads for non-NULL values in 185 * pthread_key_destroy. 186 * 187 * The assumption here is that a concurrent pthread_key_delete is already 188 * undefined behavior. The mutex is taken only once per thread/key 189 * combination. 190 * 191 * We could keep an accounting of the number of specific used 192 * entries per thread, so that we can update pt_havespecific when we delete 193 * the last one, but we don't bother for now 194 */ 195int 196pthread__add_specific(pthread_t self, pthread_key_t key, const void *value) 197{ 198 struct pt_specific *pt; 199 200 pthread__assert(key >= 0 && key < pthread_keys_max); 201 202 pthread__assert(pthread__tsd_destructors[key] != NULL); 203 pt = &self->pt_specific[key]; 204 self->pt_havespecific = 1; 205 if (value && !pt->pts_next.ptqe_prev) { 206 pthread_mutex_lock(&tsd_mutex); 207 PTQ_INSERT_HEAD(&pthread__tsd_list[key], pt, pts_next); 208 pthread_mutex_unlock(&tsd_mutex); 209 } 210 pt->pts_value = __UNCONST(value); 211 212 return 0; 213} 214 215int 216pthread_key_delete(pthread_key_t key) 217{ 218 /* 219 * This is tricky. The standard says of pthread_key_create() 220 * that new keys have the value NULL associated with them in 221 * all threads. According to people who were present at the 222 * standardization meeting, that requirement was written 223 * before pthread_key_delete() was introduced, and not 224 * reconsidered when it was. 225 * 226 * See David Butenhof's article in comp.programming.threads: 227 * Subject: Re: TSD key reusing issue 228 * Message-ID: <u97d8.29$fL6.200@news.cpqcorp.net> 229 * Date: Thu, 21 Feb 2002 09:06:17 -0500 230 * http://groups.google.com/groups?\ 231 * hl=en&selm=u97d8.29%24fL6.200%40news.cpqcorp.net 232 * 233 * Given: 234 * 235 * 1: Applications are not required to clear keys in all 236 * threads before calling pthread_key_delete(). 237 * 2: Clearing pointers without running destructors is a 238 * memory leak. 239 * 3: The pthread_key_delete() function is expressly forbidden 240 * to run any destructors. 241 * 242 * Option 1: Make this function effectively a no-op and 243 * prohibit key reuse. This is a possible resource-exhaustion 244 * problem given that we have a static storage area for keys, 245 * but having a non-static storage area would make 246 * pthread_setspecific() expensive (might need to realloc the 247 * TSD array). 248 * 249 * Option 2: Ignore the specified behavior of 250 * pthread_key_create() and leave the old values. If an 251 * application deletes a key that still has non-NULL values in 252 * some threads... it's probably a memory leak and hence 253 * incorrect anyway, and we're within our rights to let the 254 * application lose. However, it's possible (if unlikely) that 255 * the application is storing pointers to non-heap data, or 256 * non-pointers that have been wedged into a void pointer, so 257 * we can't entirely write off such applications as incorrect. 258 * This could also lead to running (new) destructors on old 259 * data that was never supposed to be associated with that 260 * destructor. 261 * 262 * Option 3: Follow the specified behavior of 263 * pthread_key_create(). Either pthread_key_create() or 264 * pthread_key_delete() would then have to clear the values in 265 * every thread's slot for that key. In order to guarantee the 266 * visibility of the NULL value in other threads, there would 267 * have to be synchronization operations in both the clearer 268 * and pthread_getspecific(). Putting synchronization in 269 * pthread_getspecific() is a big performance lose. But in 270 * reality, only (buggy) reuse of an old key would require 271 * this synchronization; for a new key, there has to be a 272 * memory-visibility propagating event between the call to 273 * pthread_key_create() and pthread_getspecific() with that 274 * key, so setting the entries to NULL without synchronization 275 * will work, subject to problem (2) above. However, it's kind 276 * of slow. 277 * 278 * Note that the argument in option 3 only applies because we 279 * keep TSD in ordinary memory which follows the pthreads 280 * visibility rules. The visibility rules are not required by 281 * the standard to apply to TSD, so the argument doesn't 282 * apply in general, just to this implementation. 283 */ 284 285 /* 286 * We do option 3; we find the list of all pt_specific structures 287 * threaded on the key we are deleting, unthread them, and set the 288 * pointer to NULL. Finally we unthread the entry, freeing it for 289 * further use. 290 * 291 * We don't call the destructor here, it is the responsibility 292 * of the application to cleanup the storage: 293 * http://pubs.opengroup.org/onlinepubs/9699919799/functions/\ 294 * pthread_key_delete.html 295 */ 296 struct pt_specific *pt; 297 298 if (__predict_false(__uselibcstub)) 299 return __libc_thr_keydelete_stub(key); 300 301 pthread__assert(key >= 0 && key < pthread_keys_max); 302 303 pthread_mutex_lock(&tsd_mutex); 304 305 pthread__assert(pthread__tsd_destructors[key] != NULL); 306 307 while ((pt = PTQ_FIRST(&pthread__tsd_list[key])) != NULL) { 308 PTQ_REMOVE(&pthread__tsd_list[key], pt, pts_next); 309 pt->pts_value = NULL; 310 pt->pts_next.ptqe_prev = NULL; 311 } 312 313 pthread__tsd_destructors[key] = NULL; 314 pthread_mutex_unlock(&tsd_mutex); 315 316 return 0; 317} 318 319/* Perform thread-exit-time destruction of thread-specific data. */ 320void 321pthread__destroy_tsd(pthread_t self) 322{ 323 int i, done, iterations; 324 void *val; 325 void (*destructor)(void *); 326 327 if (!self->pt_havespecific) 328 return; 329 330 /* Butenhof, section 5.4.2 (page 167): 331 * 332 * ``Also, Pthreads sets the thread-specific data value for a 333 * key to NULL before calling that key's destructor (passing 334 * the previous value of the key) when a thread terminates [*]. 335 * ... 336 * [*] That is, unfortunately, not what the standard 337 * says. This is one of the problems with formal standards - 338 * they say what they say, not what they were intended to 339 * say. Somehow, an error crept in, and the sentence 340 * specifying that "the implementation clears the 341 * thread-specific data value before calling the destructor" 342 * was deleted. Nobody noticed, and the standard was approved 343 * with the error. So the standard says (by omission) that if 344 * you want to write a portable application using 345 * thread-specific data, that will not hang on thread 346 * termination, you must call pthread_setspecific within your 347 * destructor function to change the value to NULL. This would 348 * be silly, and any serious implementation of Pthreads will 349 * violate the standard in this respect. Of course, the 350 * standard will be fixed, probably by the 1003.1n amendment 351 * (assorted corrections to 1003.1c-1995), but that will take 352 * a while.'' 353 */ 354 355 /* We're not required to try very hard */ 356 iterations = PTHREAD_DESTRUCTOR_ITERATIONS; 357 do { 358 done = 1; 359 for (i = 0; i < pthread_keys_max; i++) { 360 struct pt_specific *pt = &self->pt_specific[i]; 361 if (pt->pts_next.ptqe_prev == NULL) 362 continue; 363 pthread_mutex_lock(&tsd_mutex); 364 365 if (pt->pts_next.ptqe_prev != NULL) { 366 PTQ_REMOVE(&pthread__tsd_list[i], pt, pts_next); 367 val = pt->pts_value; 368 pt->pts_value = NULL; 369 pt->pts_next.ptqe_prev = NULL; 370 destructor = pthread__tsd_destructors[i]; 371 } else 372 destructor = NULL; 373 374 pthread_mutex_unlock(&tsd_mutex); 375 if (destructor != NULL && val != NULL) { 376 done = 0; 377 (*destructor)(val); 378 } 379 } 380 } while (!done && --iterations); 381 382 self->pt_havespecific = 0; 383} 384 385void 386pthread__copy_tsd(pthread_t self) 387{ 388 for (size_t key = 0; key < TSD_KEYS_MAX; key++) { 389 390 if (__libc_tsd[key].tsd_inuse == 0) 391 continue; 392 393 pthread__assert(pthread__tsd_destructors[key] == NULL); 394 pthread__tsd_destructors[key] = __libc_tsd[key].tsd_dtor ? 395 __libc_tsd[key].tsd_dtor : null_destructor; 396 nextkey = (key + 1) % pthread_keys_max; 397 398 self->pt_havespecific = 1; 399 struct pt_specific *pt = &self->pt_specific[key]; 400 pt->pts_value = __libc_tsd[key].tsd_val; 401 __libc_tsd[key].tsd_inuse = 0; 402 } 403} 404