urcu-defer.c

   1 /*
   2  * urcu-defer.c
   3  *
   4  * Userspace RCU library - batch memory reclamation
   5  *
   6  * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
   7  *
   8  * This library is free software; you can redistribute it and/or
   9  * modify it under the terms of the GNU Lesser General Public
  10  * License as published by the Free Software Foundation; either
  11  * version 2.1 of the License, or (at your option) any later version.
  12  *
  13  * This library is distributed in the hope that it will be useful,
  14  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  16  * Lesser General Public License for more details.
  17  *
  18  * You should have received a copy of the GNU Lesser General Public
  19  * License along with this library; if not, write to the Free Software
  20  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  21  */
  22
  23 #include <stdio.h>
  24 #include <pthread.h>
  25 #include <signal.h>
  26 #include <assert.h>
  27 #include <stdlib.h>
  28 #include <string.h>
  29 #include <errno.h>
  30 #include <poll.h>
  31 #include <sys/time.h>
  32 #include <syscall.h>
  33 #include <unistd.h>
  34
  35 #include "urcu-defer-static.h"
  36 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
  37 #include "urcu-defer.h"
  38
  39 #define futex(...)      syscall(__NR_futex, __VA_ARGS__)
  40 #define FUTEX_WAIT              0
  41 #define FUTEX_WAKE              1
  42
  43 void __attribute__((destructor)) urcu_defer_exit(void);
  44
  45 extern void synchronize_rcu(void);
  46
  47 /*
  48  * urcu_defer_mutex nests inside defer_thread_mutex.
  49  */
  50 static pthread_mutex_t urcu_defer_mutex = PTHREAD_MUTEX_INITIALIZER;
  51 static pthread_mutex_t defer_thread_mutex = PTHREAD_MUTEX_INITIALIZER;
  52
  53 static int defer_thread_futex;
  54
  55 /*
  56  * Written to only by each individual deferer. Read by both the deferer and
  57  * the reclamation tread.
  58  */
  59 static struct defer_queue __thread defer_queue;
  60
  61 /* Thread IDs of registered deferers */
  62 #define INIT_NUM_THREADS 4
  63
  64 struct deferer_registry {
  65         pthread_t tid;
  66         struct defer_queue *defer_queue;
  67         unsigned long last_head;
  68 };
  69
  70 static struct deferer_registry *registry;
  71 static int num_deferers, alloc_deferers;
  72
  73 static pthread_t tid_defer;
  74
  75 static void internal_urcu_lock(pthread_mutex_t *mutex)
  76 {
  77         int ret;
  78
  79 #ifndef DISTRUST_SIGNALS_EXTREME
  80         ret = pthread_mutex_lock(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(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                 pthread_testcancel();
  93                 poll(NULL,0,10);
  94         }
  95 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
  96 }
  97
  98 static void internal_urcu_unlock(pthread_mutex_t *mutex)
  99 {
 100         int ret;
 101
 102         ret = pthread_mutex_unlock(mutex);
 103         if (ret) {
 104                 perror("Error in pthread mutex unlock");
 105                 exit(-1);
 106         }
 107 }
 108
 109 /*
 110  * Wake-up any waiting defer thread. Called from many concurrent threads.
 111  */
 112 static void wake_up_defer(void)
 113 {
 114         if (unlikely(atomic_read(&defer_thread_futex) == -1)) {
 115                 atomic_set(&defer_thread_futex, 0);
 116                 futex(&defer_thread_futex, FUTEX_WAKE, 1,
 117                       NULL, NULL, 0);
 118         }
 119 }
 120
 121 static unsigned long rcu_defer_num_callbacks(void)
 122 {
 123         unsigned long num_items = 0, head;
 124         struct deferer_registry *index;
 125
 126         internal_urcu_lock(&urcu_defer_mutex);
 127         for (index = registry; index < registry + num_deferers; index++) {
 128                 head = LOAD_SHARED(index->defer_queue->head);
 129                 num_items += head - index->defer_queue->tail;
 130         }
 131         internal_urcu_unlock(&urcu_defer_mutex);
 132         return num_items;
 133 }
 134
 135 /*
 136  * Defer thread waiting. Single thread.
 137  */
 138 static void wait_defer(void)
 139 {
 140         atomic_dec(&defer_thread_futex);
 141         smp_mb();       /* Write futex before read queue */
 142         if (rcu_defer_num_callbacks()) {
 143                 smp_mb();       /* Read queue before write futex */
 144                 /* Callbacks are queued, don't wait. */
 145                 atomic_set(&defer_thread_futex, 0);
 146         } else {
 147                 smp_rmb();      /* Read queue before read futex */
 148                 if (atomic_read(&defer_thread_futex) == -1)
 149                         futex(&defer_thread_futex, FUTEX_WAIT, -1,
 150                               NULL, NULL, 0);
 151         }
 152 }
 153
 154 /*
 155  * Must be called after Q.S. is reached.
 156  */
 157 static void rcu_defer_barrier_queue(struct defer_queue *queue,
 158                                     unsigned long head)
 159 {
 160         unsigned long i;
 161         void (*fct)(void *p);
 162         void *p;
 163
 164         /*
 165          * Tail is only modified when lock is held.
 166          * Head is only modified by owner thread.
 167          */
 168
 169         for (i = queue->tail; i != head;) {
 170                 smp_rmb();       /* read head before q[]. */
 171                 p = LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 172                 if (unlikely(DQ_IS_FCT_BIT(p))) {
 173                         DQ_CLEAR_FCT_BIT(p);
 174                         queue->last_fct_out = p;
 175                         p = LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 176                 } else if (unlikely(p == DQ_FCT_MARK)) {
 177                         p = LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 178                         queue->last_fct_out = p;
 179                         p = LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 180                 }
 181                 fct = queue->last_fct_out;
 182                 fct(p);
 183         }
 184         smp_mb();       /* push tail after having used q[] */
 185         STORE_SHARED(queue->tail, i);
 186 }
 187
 188 static void _rcu_defer_barrier_thread(void)
 189 {
 190         unsigned long head, num_items;
 191
 192         head = defer_queue.head;
 193         num_items = head - defer_queue.tail;
 194         if (unlikely(!num_items))
 195                 return;
 196         synchronize_rcu();
 197         rcu_defer_barrier_queue(&defer_queue, head);
 198 }
 199
 200
 201 void rcu_defer_barrier_thread(void)
 202 {
 203         internal_urcu_lock(&urcu_defer_mutex);
 204         _rcu_defer_barrier_thread();
 205         internal_urcu_unlock(&urcu_defer_mutex);
 206 }
 207
 208 /*
 209  * rcu_defer_barrier - Execute all queued rcu callbacks.
 210  *
 211  * Execute all RCU callbacks queued before rcu_defer_barrier() execution.
 212  * All callbacks queued on the local thread prior to a rcu_defer_barrier() call
 213  * are guaranteed to be executed.
 214  * Callbacks queued by other threads concurrently with rcu_defer_barrier()
 215  * execution are not guaranteed to be executed in the current batch (could
 216  * be left for the next batch). These callbacks queued by other threads are only
 217  * guaranteed to be executed if there is explicit synchronization between
 218  * the thread adding to the queue and the thread issuing the defer_barrier call.
 219  */
 220
 221 void rcu_defer_barrier(void)
 222 {
 223         struct deferer_registry *index;
 224         unsigned long num_items = 0;
 225
 226         if (!registry)
 227                 return;
 228
 229         internal_urcu_lock(&urcu_defer_mutex);
 230         for (index = registry; index < registry + num_deferers; index++) {
 231                 index->last_head = LOAD_SHARED(index->defer_queue->head);
 232                 num_items += index->last_head - index->defer_queue->tail;
 233         }
 234         if (likely(!num_items)) {
 235                 /*
 236                  * We skip the grace period because there are no queued
 237                  * callbacks to execute.
 238                  */
 239                 goto end;
 240         }
 241         synchronize_rcu();
 242         for (index = registry; index < registry + num_deferers; index++)
 243                 rcu_defer_barrier_queue(index->defer_queue,
 244                                           index->last_head);
 245 end:
 246         internal_urcu_unlock(&urcu_defer_mutex);
 247 }
 248
 249 /*
 250  * _rcu_defer_queue - Queue a RCU callback.
 251  */
 252 void _rcu_defer_queue(void (*fct)(void *p), void *p)
 253 {
 254         unsigned long head, tail;
 255
 256         /*
 257          * Head is only modified by ourself. Tail can be modified by reclamation
 258          * thread.
 259          */
 260         head = defer_queue.head;
 261         tail = LOAD_SHARED(defer_queue.tail);
 262
 263         /*
 264          * If queue is full, empty it ourself.
 265          * Worse-case: must allow 2 supplementary entries for fct pointer.
 266          */
 267         if (unlikely(head - tail >= DEFER_QUEUE_SIZE - 2)) {
 268                 assert(head - tail <= DEFER_QUEUE_SIZE);
 269                 rcu_defer_barrier_thread();
 270                 assert(head - LOAD_SHARED(defer_queue.tail) == 0);
 271         }
 272
 273         if (unlikely(defer_queue.last_fct_in != fct)) {
 274                 defer_queue.last_fct_in = fct;
 275                 if (unlikely(DQ_IS_FCT_BIT(fct) || fct == DQ_FCT_MARK)) {
 276                         /*
 277                          * If the function to encode is not aligned or the
 278                          * marker, write DQ_FCT_MARK followed by the function
 279                          * pointer.
 280                          */
 281                         _STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
 282                                       DQ_FCT_MARK);
 283                         _STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
 284                                       fct);
 285                 } else {
 286                         DQ_SET_FCT_BIT(fct);
 287                         _STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
 288                                       fct);
 289                 }
 290         } else {
 291                 if (unlikely(DQ_IS_FCT_BIT(p) || p == DQ_FCT_MARK)) {
 292                         /*
 293                          * If the data to encode is not aligned or the marker,
 294                          * write DQ_FCT_MARK followed by the function pointer.
 295                          */
 296                         _STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
 297                                       DQ_FCT_MARK);
 298                         _STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
 299                                       fct);
 300                 }
 301         }
 302         _STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK], p);
 303         smp_wmb();      /* Publish new pointer before head */
 304                         /* Write q[] before head. */
 305         STORE_SHARED(defer_queue.head, head);
 306         smp_mb();       /* Write queue head before read futex */
 307         /*
 308          * Wake-up any waiting defer thread.
 309          */
 310         wake_up_defer();
 311 }
 312
 313 void *thr_defer(void *args)
 314 {
 315         for (;;) {
 316                 pthread_testcancel();
 317                 /*
 318                  * "Be green". Don't wake up the CPU if there is no RCU work
 319                  * to perform whatsoever. Aims at saving laptop battery life by
 320                  * leaving the processor in sleep state when idle.
 321                  */
 322                 wait_defer();
 323                 /* Sleeping after wait_defer to let many callbacks enqueue */
 324                 poll(NULL,0,100);       /* wait for 100ms */
 325                 rcu_defer_barrier();
 326         }
 327
 328         return NULL;
 329 }
 330
 331 /*
 332  * library wrappers to be used by non-LGPL compatible source code.
 333  */
 334
 335 void rcu_defer_queue(void (*fct)(void *p), void *p)
 336 {
 337         _rcu_defer_queue(fct, p);
 338 }
 339
 340 static void rcu_add_deferer(pthread_t id)
 341 {
 342         struct deferer_registry *oldarray;
 343
 344         if (!registry) {
 345                 alloc_deferers = INIT_NUM_THREADS;
 346                 num_deferers = 0;
 347                 registry =
 348                         malloc(sizeof(struct deferer_registry) * alloc_deferers);
 349         }
 350         if (alloc_deferers < num_deferers + 1) {
 351                 oldarray = registry;
 352                 registry = malloc(sizeof(struct deferer_registry)
 353                                 * (alloc_deferers << 1));
 354                 memcpy(registry, oldarray,
 355                         sizeof(struct deferer_registry) * alloc_deferers);
 356                 alloc_deferers <<= 1;
 357                 free(oldarray);
 358         }
 359         registry[num_deferers].tid = id;
 360         /* reference to the TLS of _this_ deferer thread. */
 361         registry[num_deferers].defer_queue = &defer_queue;
 362         registry[num_deferers].last_head = 0;
 363         num_deferers++;
 364 }
 365
 366 /*
 367  * Never shrink (implementation limitation).
 368  * This is O(nb threads). Eventually use a hash table.
 369  */
 370 static void rcu_remove_deferer(pthread_t id)
 371 {
 372         struct deferer_registry *index;
 373
 374         assert(registry != NULL);
 375         for (index = registry; index < registry + num_deferers; index++) {
 376                 if (pthread_equal(index->tid, id)) {
 377                         memcpy(index, &registry[num_deferers - 1],
 378                                 sizeof(struct deferer_registry));
 379                         registry[num_deferers - 1].tid = 0;
 380                         registry[num_deferers - 1].defer_queue = NULL;
 381                         registry[num_deferers - 1].last_head = 0;
 382                         num_deferers--;
 383                         return;
 384                 }
 385         }
 386         /* Hrm not found, forgot to register ? */
 387         assert(0);
 388 }
 389
 390 static void start_defer_thread(void)
 391 {
 392         int ret;
 393
 394         ret = pthread_create(&tid_defer, NULL, thr_defer,
 395                 NULL);
 396         assert(!ret);
 397 }
 398
 399 static void stop_defer_thread(void)
 400 {
 401         int ret;
 402         void *tret;
 403
 404         pthread_cancel(tid_defer);
 405         wake_up_defer();
 406         ret = pthread_join(tid_defer, &tret);
 407         assert(!ret);
 408 }
 409
 410 void rcu_defer_register_thread(void)
 411 {
 412         int deferers;
 413
 414         internal_urcu_lock(&defer_thread_mutex);
 415         internal_urcu_lock(&urcu_defer_mutex);
 416         defer_queue.q = malloc(sizeof(void *) * DEFER_QUEUE_SIZE);
 417         rcu_add_deferer(pthread_self());
 418         deferers = num_deferers;
 419         internal_urcu_unlock(&urcu_defer_mutex);
 420
 421         if (deferers == 1)
 422                 start_defer_thread();
 423         internal_urcu_unlock(&defer_thread_mutex);
 424 }
 425
 426 void rcu_defer_unregister_thread(void)
 427 {
 428         int deferers;
 429
 430         internal_urcu_lock(&defer_thread_mutex);
 431         internal_urcu_lock(&urcu_defer_mutex);
 432         rcu_remove_deferer(pthread_self());
 433         _rcu_defer_barrier_thread();
 434         free(defer_queue.q);
 435         defer_queue.q = NULL;
 436         deferers = num_deferers;
 437         internal_urcu_unlock(&urcu_defer_mutex);
 438
 439         if (deferers == 0)
 440                 stop_defer_thread();
 441         internal_urcu_unlock(&defer_thread_mutex);
 442 }
 443
 444 void urcu_defer_exit(void)
 445 {
 446         free(registry);
 447 }