| 1 | /* |
| 2 | * urcu.c |
| 3 | * |
| 4 | * Userspace RCU library |
| 5 | * |
| 6 | * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> |
| 7 | * Copyright (c) 2009 Paul E. McKenney, IBM Corporation. |
| 8 | * |
| 9 | * This library is free software; you can redistribute it and/or |
| 10 | * modify it under the terms of the GNU Lesser General Public |
| 11 | * License as published by the Free Software Foundation; either |
| 12 | * version 2.1 of the License, or (at your option) any later version. |
| 13 | * |
| 14 | * This library is distributed in the hope that it will be useful, |
| 15 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 16 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
| 17 | * Lesser General Public License for more details. |
| 18 | * |
| 19 | * You should have received a copy of the GNU Lesser General Public |
| 20 | * License along with this library; if not, write to the Free Software |
| 21 | * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA |
| 22 | * |
| 23 | * IBM's contributions to this file may be relicensed under LGPLv2 or later. |
| 24 | */ |
| 25 | |
| 26 | #include <stdio.h> |
| 27 | #include <pthread.h> |
| 28 | #include <signal.h> |
| 29 | #include <assert.h> |
| 30 | #include <stdlib.h> |
| 31 | #include <string.h> |
| 32 | #include <errno.h> |
| 33 | #include <poll.h> |
| 34 | |
| 35 | #include "urcu-static.h" |
| 36 | /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */ |
| 37 | #include "urcu.h" |
| 38 | |
| 39 | #ifndef URCU_MB |
| 40 | static int init_done; |
| 41 | |
| 42 | void __attribute__((constructor)) urcu_init(void); |
| 43 | void __attribute__((destructor)) urcu_exit(void); |
| 44 | #else |
| 45 | void urcu_init(void) |
| 46 | { |
| 47 | } |
| 48 | #endif |
| 49 | |
| 50 | static pthread_mutex_t urcu_mutex = PTHREAD_MUTEX_INITIALIZER; |
| 51 | |
| 52 | int gp_futex; |
| 53 | |
| 54 | /* |
| 55 | * Global grace period counter. |
| 56 | * Contains the current RCU_GP_CTR_BIT. |
| 57 | * Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path. |
| 58 | * Written to only by writer with mutex taken. Read by both writer and readers. |
| 59 | */ |
| 60 | long urcu_gp_ctr = RCU_GP_COUNT; |
| 61 | |
| 62 | /* |
| 63 | * Written to only by each individual reader. Read by both the reader and the |
| 64 | * writers. |
| 65 | */ |
| 66 | struct urcu_reader __thread urcu_reader; |
| 67 | |
| 68 | #ifdef DEBUG_YIELD |
| 69 | unsigned int yield_active; |
| 70 | unsigned int __thread rand_yield; |
| 71 | #endif |
| 72 | |
| 73 | static LIST_HEAD(registry); |
| 74 | |
| 75 | static void internal_urcu_lock(void) |
| 76 | { |
| 77 | int ret; |
| 78 | |
| 79 | #ifndef DISTRUST_SIGNALS_EXTREME |
| 80 | ret = pthread_mutex_lock(&urcu_mutex); |
| 81 | if (ret) { |
| 82 | perror("Error in pthread mutex lock"); |
| 83 | exit(-1); |
| 84 | } |
| 85 | #else /* #ifndef DISTRUST_SIGNALS_EXTREME */ |
| 86 | while ((ret = pthread_mutex_trylock(&urcu_mutex)) != 0) { |
| 87 | if (ret != EBUSY && ret != EINTR) { |
| 88 | printf("ret = %d, errno = %d\n", ret, errno); |
| 89 | perror("Error in pthread mutex lock"); |
| 90 | exit(-1); |
| 91 | } |
| 92 | if (urcu_reader.need_mb) { |
| 93 | smp_mb(); |
| 94 | urcu_reader.need_mb = 0; |
| 95 | smp_mb(); |
| 96 | } |
| 97 | poll(NULL,0,10); |
| 98 | } |
| 99 | #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */ |
| 100 | } |
| 101 | |
| 102 | static void internal_urcu_unlock(void) |
| 103 | { |
| 104 | int ret; |
| 105 | |
| 106 | ret = pthread_mutex_unlock(&urcu_mutex); |
| 107 | if (ret) { |
| 108 | perror("Error in pthread mutex unlock"); |
| 109 | exit(-1); |
| 110 | } |
| 111 | } |
| 112 | |
| 113 | /* |
| 114 | * called with urcu_mutex held. |
| 115 | */ |
| 116 | static void switch_next_urcu_qparity(void) |
| 117 | { |
| 118 | STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_CTR_BIT); |
| 119 | } |
| 120 | |
| 121 | #ifdef URCU_MB |
| 122 | #if 0 /* unused */ |
| 123 | static void force_mb_single_thread(struct urcu_reader *index) |
| 124 | { |
| 125 | smp_mb(); |
| 126 | } |
| 127 | #endif //0 |
| 128 | |
| 129 | static void force_mb_all_threads(void) |
| 130 | { |
| 131 | smp_mb(); |
| 132 | } |
| 133 | #else /* #ifdef URCU_MB */ |
| 134 | #if 0 /* unused */ |
| 135 | static void force_mb_single_thread(struct urcu_reader *index) |
| 136 | { |
| 137 | assert(!list_empty(®istry)); |
| 138 | /* |
| 139 | * pthread_kill has a smp_mb(). But beware, we assume it performs |
| 140 | * a cache flush on architectures with non-coherent cache. Let's play |
| 141 | * safe and don't assume anything : we use smp_mc() to make sure the |
| 142 | * cache flush is enforced. |
| 143 | */ |
| 144 | index->need_mb = 1; |
| 145 | smp_mc(); /* write ->need_mb before sending the signals */ |
| 146 | pthread_kill(index->tid, SIGURCU); |
| 147 | smp_mb(); |
| 148 | /* |
| 149 | * Wait for sighandler (and thus mb()) to execute on every thread. |
| 150 | * BUSY-LOOP. |
| 151 | */ |
| 152 | while (index->need_mb) { |
| 153 | poll(NULL, 0, 1); |
| 154 | } |
| 155 | smp_mb(); /* read ->need_mb before ending the barrier */ |
| 156 | } |
| 157 | #endif //0 |
| 158 | |
| 159 | static void force_mb_all_threads(void) |
| 160 | { |
| 161 | struct urcu_reader *index; |
| 162 | |
| 163 | /* |
| 164 | * Ask for each threads to execute a smp_mb() so we can consider the |
| 165 | * compiler barriers around rcu read lock as real memory barriers. |
| 166 | */ |
| 167 | if (list_empty(®istry)) |
| 168 | return; |
| 169 | /* |
| 170 | * pthread_kill has a smp_mb(). But beware, we assume it performs |
| 171 | * a cache flush on architectures with non-coherent cache. Let's play |
| 172 | * safe and don't assume anything : we use smp_mc() to make sure the |
| 173 | * cache flush is enforced. |
| 174 | */ |
| 175 | list_for_each_entry(index, ®istry, head) { |
| 176 | index->need_mb = 1; |
| 177 | smp_mc(); /* write need_mb before sending the signal */ |
| 178 | pthread_kill(index->tid, SIGURCU); |
| 179 | } |
| 180 | /* |
| 181 | * Wait for sighandler (and thus mb()) to execute on every thread. |
| 182 | * |
| 183 | * Note that the pthread_kill() will never be executed on systems |
| 184 | * that correctly deliver signals in a timely manner. However, it |
| 185 | * is not uncommon for kernels to have bugs that can result in |
| 186 | * lost or unduly delayed signals. |
| 187 | * |
| 188 | * If you are seeing the below pthread_kill() executing much at |
| 189 | * all, we suggest testing the underlying kernel and filing the |
| 190 | * relevant bug report. For Linux kernels, we recommend getting |
| 191 | * the Linux Test Project (LTP). |
| 192 | */ |
| 193 | list_for_each_entry(index, ®istry, head) { |
| 194 | while (index->need_mb) { |
| 195 | pthread_kill(index->tid, SIGURCU); |
| 196 | poll(NULL, 0, 1); |
| 197 | } |
| 198 | } |
| 199 | smp_mb(); /* read ->need_mb before ending the barrier */ |
| 200 | } |
| 201 | #endif /* #else #ifdef URCU_MB */ |
| 202 | |
| 203 | /* |
| 204 | * synchronize_rcu() waiting. Single thread. |
| 205 | */ |
| 206 | static void wait_gp(void) |
| 207 | { |
| 208 | /* Read reader_gp before read futex */ |
| 209 | force_mb_all_threads(); |
| 210 | if (uatomic_read(&gp_futex) == -1) |
| 211 | futex(&gp_futex, FUTEX_WAIT, -1, |
| 212 | NULL, NULL, 0); |
| 213 | } |
| 214 | |
| 215 | void wait_for_quiescent_state(void) |
| 216 | { |
| 217 | LIST_HEAD(qsreaders); |
| 218 | int wait_loops = 0; |
| 219 | struct urcu_reader *index, *tmp; |
| 220 | |
| 221 | if (list_empty(®istry)) |
| 222 | return; |
| 223 | /* |
| 224 | * Wait for each thread urcu_reader.ctr count to become 0. |
| 225 | */ |
| 226 | for (;;) { |
| 227 | wait_loops++; |
| 228 | if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) { |
| 229 | uatomic_dec(&gp_futex); |
| 230 | /* Write futex before read reader_gp */ |
| 231 | force_mb_all_threads(); |
| 232 | } |
| 233 | |
| 234 | list_for_each_entry_safe(index, tmp, ®istry, head) { |
| 235 | if (!rcu_old_gp_ongoing(&index->ctr)) |
| 236 | list_move(&index->head, &qsreaders); |
| 237 | } |
| 238 | |
| 239 | #ifndef HAS_INCOHERENT_CACHES |
| 240 | if (list_empty(®istry)) { |
| 241 | if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) { |
| 242 | /* Read reader_gp before write futex */ |
| 243 | force_mb_all_threads(); |
| 244 | uatomic_set(&gp_futex, 0); |
| 245 | } |
| 246 | break; |
| 247 | } else { |
| 248 | if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) |
| 249 | wait_gp(); |
| 250 | else |
| 251 | cpu_relax(); |
| 252 | } |
| 253 | #else /* #ifndef HAS_INCOHERENT_CACHES */ |
| 254 | /* |
| 255 | * BUSY-LOOP. Force the reader thread to commit its |
| 256 | * urcu_reader.ctr update to memory if we wait for too long. |
| 257 | */ |
| 258 | if (list_empty(®istry)) { |
| 259 | if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) { |
| 260 | /* Read reader_gp before write futex */ |
| 261 | force_mb_all_threads(); |
| 262 | uatomic_set(&gp_futex, 0); |
| 263 | } |
| 264 | break; |
| 265 | } else { |
| 266 | switch (wait_loops) { |
| 267 | case RCU_QS_ACTIVE_ATTEMPTS: |
| 268 | wait_gp(); |
| 269 | break; /* only escape switch */ |
| 270 | case KICK_READER_LOOPS: |
| 271 | force_mb_all_threads(); |
| 272 | wait_loops = 0; |
| 273 | break; /* only escape switch */ |
| 274 | default: |
| 275 | cpu_relax(); |
| 276 | } |
| 277 | } |
| 278 | #endif /* #else #ifndef HAS_INCOHERENT_CACHES */ |
| 279 | } |
| 280 | /* put back the reader list in the registry */ |
| 281 | list_splice(&qsreaders, ®istry); |
| 282 | } |
| 283 | |
| 284 | void synchronize_rcu(void) |
| 285 | { |
| 286 | internal_urcu_lock(); |
| 287 | |
| 288 | /* All threads should read qparity before accessing data structure |
| 289 | * where new ptr points to. Must be done within internal_urcu_lock |
| 290 | * because it iterates on reader threads.*/ |
| 291 | /* Write new ptr before changing the qparity */ |
| 292 | force_mb_all_threads(); |
| 293 | |
| 294 | switch_next_urcu_qparity(); /* 0 -> 1 */ |
| 295 | |
| 296 | /* |
| 297 | * Must commit qparity update to memory before waiting for parity |
| 298 | * 0 quiescent state. Failure to do so could result in the writer |
| 299 | * waiting forever while new readers are always accessing data (no |
| 300 | * progress). |
| 301 | * Ensured by STORE_SHARED and LOAD_SHARED. |
| 302 | */ |
| 303 | |
| 304 | /* |
| 305 | * Adding a smp_mb() which is _not_ formally required, but makes the |
| 306 | * model easier to understand. It does not have a big performance impact |
| 307 | * anyway, given this is the write-side. |
| 308 | */ |
| 309 | smp_mb(); |
| 310 | |
| 311 | /* |
| 312 | * Wait for previous parity to be empty of readers. |
| 313 | */ |
| 314 | wait_for_quiescent_state(); /* Wait readers in parity 0 */ |
| 315 | |
| 316 | /* |
| 317 | * Must finish waiting for quiescent state for parity 0 before |
| 318 | * committing qparity update to memory. Failure to do so could result in |
| 319 | * the writer waiting forever while new readers are always accessing |
| 320 | * data (no progress). |
| 321 | * Ensured by STORE_SHARED and LOAD_SHARED. |
| 322 | */ |
| 323 | |
| 324 | /* |
| 325 | * Adding a smp_mb() which is _not_ formally required, but makes the |
| 326 | * model easier to understand. It does not have a big performance impact |
| 327 | * anyway, given this is the write-side. |
| 328 | */ |
| 329 | smp_mb(); |
| 330 | |
| 331 | switch_next_urcu_qparity(); /* 1 -> 0 */ |
| 332 | |
| 333 | /* |
| 334 | * Must commit qparity update to memory before waiting for parity |
| 335 | * 1 quiescent state. Failure to do so could result in the writer |
| 336 | * waiting forever while new readers are always accessing data (no |
| 337 | * progress). |
| 338 | * Ensured by STORE_SHARED and LOAD_SHARED. |
| 339 | */ |
| 340 | |
| 341 | /* |
| 342 | * Adding a smp_mb() which is _not_ formally required, but makes the |
| 343 | * model easier to understand. It does not have a big performance impact |
| 344 | * anyway, given this is the write-side. |
| 345 | */ |
| 346 | smp_mb(); |
| 347 | |
| 348 | /* |
| 349 | * Wait for previous parity to be empty of readers. |
| 350 | */ |
| 351 | wait_for_quiescent_state(); /* Wait readers in parity 1 */ |
| 352 | |
| 353 | /* Finish waiting for reader threads before letting the old ptr being |
| 354 | * freed. Must be done within internal_urcu_lock because it iterates on |
| 355 | * reader threads. */ |
| 356 | force_mb_all_threads(); |
| 357 | |
| 358 | internal_urcu_unlock(); |
| 359 | } |
| 360 | |
| 361 | /* |
| 362 | * library wrappers to be used by non-LGPL compatible source code. |
| 363 | */ |
| 364 | |
| 365 | void rcu_read_lock(void) |
| 366 | { |
| 367 | _rcu_read_lock(); |
| 368 | } |
| 369 | |
| 370 | void rcu_read_unlock(void) |
| 371 | { |
| 372 | _rcu_read_unlock(); |
| 373 | } |
| 374 | |
| 375 | void rcu_register_thread(void) |
| 376 | { |
| 377 | urcu_reader.tid = pthread_self(); |
| 378 | assert(urcu_reader.need_mb == 0); |
| 379 | assert(urcu_reader.ctr == 0); |
| 380 | |
| 381 | internal_urcu_lock(); |
| 382 | urcu_init(); /* In case gcc does not support constructor attribute */ |
| 383 | list_add(&urcu_reader.head, ®istry); |
| 384 | internal_urcu_unlock(); |
| 385 | } |
| 386 | |
| 387 | void rcu_unregister_thread(void) |
| 388 | { |
| 389 | internal_urcu_lock(); |
| 390 | list_del(&urcu_reader.head); |
| 391 | internal_urcu_unlock(); |
| 392 | } |
| 393 | |
| 394 | #ifndef URCU_MB |
| 395 | static void sigurcu_handler(int signo, siginfo_t *siginfo, void *context) |
| 396 | { |
| 397 | /* |
| 398 | * Executing this smp_mb() is the only purpose of this signal handler. |
| 399 | * It punctually promotes barrier() into smp_mb() on every thread it is |
| 400 | * executed on. |
| 401 | */ |
| 402 | smp_mb(); |
| 403 | urcu_reader.need_mb = 0; |
| 404 | smp_mb(); |
| 405 | } |
| 406 | |
| 407 | /* |
| 408 | * urcu_init constructor. Called when the library is linked, but also when |
| 409 | * reader threads are calling rcu_register_thread(). |
| 410 | * Should only be called by a single thread at a given time. This is ensured by |
| 411 | * holing the internal_urcu_lock() from rcu_register_thread() or by running at |
| 412 | * library load time, which should not be executed by multiple threads nor |
| 413 | * concurrently with rcu_register_thread() anyway. |
| 414 | */ |
| 415 | void urcu_init(void) |
| 416 | { |
| 417 | struct sigaction act; |
| 418 | int ret; |
| 419 | |
| 420 | if (init_done) |
| 421 | return; |
| 422 | init_done = 1; |
| 423 | |
| 424 | act.sa_sigaction = sigurcu_handler; |
| 425 | act.sa_flags = SA_SIGINFO | SA_RESTART; |
| 426 | sigemptyset(&act.sa_mask); |
| 427 | ret = sigaction(SIGURCU, &act, NULL); |
| 428 | if (ret) { |
| 429 | perror("Error in sigaction"); |
| 430 | exit(-1); |
| 431 | } |
| 432 | } |
| 433 | |
| 434 | void urcu_exit(void) |
| 435 | { |
| 436 | struct sigaction act; |
| 437 | int ret; |
| 438 | |
| 439 | ret = sigaction(SIGURCU, NULL, &act); |
| 440 | if (ret) { |
| 441 | perror("Error in sigaction"); |
| 442 | exit(-1); |
| 443 | } |
| 444 | assert(act.sa_sigaction == sigurcu_handler); |
| 445 | assert(list_empty(®istry)); |
| 446 | } |
| 447 | #endif /* #ifndef URCU_MB */ |