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 <linux/futex.h>
  32 #include <sys/time.h>
  33 #include <syscall.h>
  34 #include <unistd.h>
  35
  36 #include "urcu-defer-static.h"
  37 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
  38 #include "urcu-defer.h"
  39
  40 #define futex(...)      syscall(__NR_futex, __VA_ARGS__)
  41
  42 void __attribute__((destructor)) urcu_defer_exit(void);
  43
  44 extern void synchronize_rcu(void);
  45
  46 /*
  47  * urcu_defer_mutex nests inside defer_thread_mutex.
  48  */
  49 static pthread_mutex_t urcu_defer_mutex = PTHREAD_MUTEX_INITIALIZER;
  50 static pthread_mutex_t defer_thread_mutex = PTHREAD_MUTEX_INITIALIZER;
  51
  52 int defer_thread_futex;
  53
  54 /*
  55  * Written to only by each individual deferer. Read by both the deferer and
  56  * the reclamation tread.
  57  */
  58 struct defer_queue __thread defer_queue;
  59
  60 /* Thread IDs of registered deferers */
  61 #define INIT_NUM_THREADS 4
  62
  63 struct deferer_registry {
  64         pthread_t tid;
  65         struct defer_queue *defer_queue;
  66         unsigned long last_head;
  67 };
  68
  69 static struct deferer_registry *registry;
  70 static int num_deferers, alloc_deferers;
  71
  72 static pthread_t tid_defer;
  73
  74 /*
  75  * Wake-up any waiting defer thread. Called from many concurrent threads.
  76  */
  77 static void wake_up_defer(void)
  78 {
  79         if (unlikely(atomic_read(&defer_thread_futex) == -1))
  80                 atomic_set(&defer_thread_futex, 0);
  81                 futex(&defer_thread_futex, FUTEX_WAKE,
  82                       0, NULL, NULL, 0);
  83 }
  84
  85 /*
  86  * Defer thread waiting. Single thread.
  87  */
  88 static void wait_defer(void)
  89 {
  90         atomic_dec(&defer_thread_futex);
  91         if (atomic_read(&defer_thread_futex) == -1)
  92                 futex(&defer_thread_futex, FUTEX_WAIT, -1,
  93                       NULL, NULL, 0);
  94 }
  95
  96 static void internal_urcu_lock(pthread_mutex_t *mutex)
  97 {
  98         int ret;
  99
 100 #ifndef DISTRUST_SIGNALS_EXTREME
 101         ret = pthread_mutex_lock(mutex);
 102         if (ret) {
 103                 perror("Error in pthread mutex lock");
 104                 exit(-1);
 105         }
 106 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
 107         while ((ret = pthread_mutex_trylock(mutex)) != 0) {
 108                 if (ret != EBUSY && ret != EINTR) {
 109                         printf("ret = %d, errno = %d\n", ret, errno);
 110                         perror("Error in pthread mutex lock");
 111                         exit(-1);
 112                 }
 113                 pthread_testcancel();
 114                 poll(NULL,0,10);
 115         }
 116 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
 117 }
 118
 119 static void internal_urcu_unlock(pthread_mutex_t *mutex)
 120 {
 121         int ret;
 122
 123         ret = pthread_mutex_unlock(mutex);
 124         if (ret) {
 125                 perror("Error in pthread mutex unlock");
 126                 exit(-1);
 127         }
 128 }
 129
 130 /*
 131  * Must be called after Q.S. is reached.
 132  */
 133 static void rcu_defer_barrier_queue(struct defer_queue *queue,
 134                                     unsigned long head)
 135 {
 136         unsigned long i;
 137         void (*fct)(void *p);
 138         void *p;
 139
 140         /*
 141          * Tail is only modified when lock is held.
 142          * Head is only modified by owner thread.
 143          */
 144
 145         for (i = queue->tail; i != head;) {
 146                 smp_rmb();       /* read head before q[]. */
 147                 p = LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 148                 if (unlikely(DQ_IS_FCT_BIT(p))) {
 149                         DQ_CLEAR_FCT_BIT(p);
 150                         queue->last_fct_out = p;
 151                         p = LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 152                 } else if (unlikely(p == DQ_FCT_MARK)) {
 153                         p = LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 154                         queue->last_fct_out = p;
 155                         p = LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 156                 }
 157                 fct = queue->last_fct_out;
 158                 fct(p);
 159         }
 160         smp_mb();       /* push tail after having used q[] */
 161         STORE_SHARED(queue->tail, i);
 162 }
 163
 164 static void _rcu_defer_barrier_thread(void)
 165 {
 166         unsigned long head, num_items;
 167
 168         head = defer_queue.head;
 169         num_items = head - defer_queue.tail;
 170         if (unlikely(!num_items))
 171                 return;
 172         synchronize_rcu();
 173         rcu_defer_barrier_queue(&defer_queue, head);
 174 }
 175
 176
 177 void rcu_defer_barrier_thread(void)
 178 {
 179         internal_urcu_lock(&urcu_defer_mutex);
 180         _rcu_defer_barrier_thread();
 181         internal_urcu_unlock(&urcu_defer_mutex);
 182 }
 183
 184 /*
 185  * rcu_defer_barrier - Execute all queued rcu callbacks.
 186  *
 187  * Execute all RCU callbacks queued before rcu_defer_barrier() execution.
 188  * All callbacks queued on the local thread prior to a rcu_defer_barrier() call
 189  * are guaranteed to be executed.
 190  * Callbacks queued by other threads concurrently with rcu_defer_barrier()
 191  * execution are not guaranteed to be executed in the current batch (could
 192  * be left for the next batch). These callbacks queued by other threads are only
 193  * guaranteed to be executed if there is explicit synchronization between
 194  * the thread adding to the queue and the thread issuing the defer_barrier call.
 195  */
 196
 197 void rcu_defer_barrier(void)
 198 {
 199         struct deferer_registry *index;
 200         unsigned long num_items = 0;
 201
 202         if (!registry)
 203                 return;
 204
 205         internal_urcu_lock(&urcu_defer_mutex);
 206         for (index = registry; index < registry + num_deferers; index++) {
 207                 index->last_head = LOAD_SHARED(index->defer_queue->head);
 208                 num_items += index->last_head - index->defer_queue->tail;
 209         }
 210         if (likely(!num_items)) {
 211                 /*
 212                  * We skip the grace period because there are no queued
 213                  * callbacks to execute.
 214                  */
 215                 goto end;
 216         }
 217         synchronize_rcu();
 218         for (index = registry; index < registry + num_deferers; index++)
 219                 rcu_defer_barrier_queue(index->defer_queue,
 220                                           index->last_head);
 221 end:
 222         internal_urcu_unlock(&urcu_defer_mutex);
 223 }
 224
 225 /*
 226  * _rcu_defer_queue - Queue a RCU callback.
 227  */
 228 void _rcu_defer_queue(void (*fct)(void *p), void *p)
 229 {
 230         unsigned long head, tail;
 231
 232         /*
 233          * Head is only modified by ourself. Tail can be modified by reclamation
 234          * thread.
 235          */
 236         head = defer_queue.head;
 237         tail = LOAD_SHARED(defer_queue.tail);
 238
 239         /*
 240          * If queue is full, empty it ourself.
 241          * Worse-case: must allow 2 supplementary entries for fct pointer.
 242          */
 243         if (unlikely(head - tail >= DEFER_QUEUE_SIZE - 2)) {
 244                 assert(head - tail <= DEFER_QUEUE_SIZE);
 245                 rcu_defer_barrier_thread();
 246                 assert(head - LOAD_SHARED(defer_queue.tail) == 0);
 247         }
 248
 249         if (unlikely(defer_queue.last_fct_in != fct)) {
 250                 defer_queue.last_fct_in = fct;
 251                 if (unlikely(DQ_IS_FCT_BIT(fct) || fct == DQ_FCT_MARK)) {
 252                         /*
 253                          * If the function to encode is not aligned or the
 254                          * marker, write DQ_FCT_MARK followed by the function
 255                          * pointer.
 256                          */
 257                         _STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
 258                                       DQ_FCT_MARK);
 259                         _STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
 260                                       fct);
 261                 } else {
 262                         DQ_SET_FCT_BIT(fct);
 263                         _STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
 264                                       fct);
 265                 }
 266         } else {
 267                 if (unlikely(DQ_IS_FCT_BIT(p) || p == DQ_FCT_MARK)) {
 268                         /*
 269                          * If the data to encode is not aligned or the marker,
 270                          * write DQ_FCT_MARK followed by the function pointer.
 271                          */
 272                         _STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
 273                                       DQ_FCT_MARK);
 274                         _STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
 275                                       fct);
 276                 }
 277         }
 278         _STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK], p);
 279         smp_wmb();      /* Publish new pointer before head */
 280                         /* Write q[] before head. */
 281         STORE_SHARED(defer_queue.head, head);
 282         /*
 283          * Wake-up any waiting defer thread.
 284          */
 285         wake_up_defer();
 286 }
 287
 288 void *thr_defer(void *args)
 289 {
 290         for (;;) {
 291                 pthread_testcancel();
 292                 printf("a\n");
 293                 /*
 294                  * "Be green". Don't wake up the CPU if there is no RCU work
 295                  * to perform whatsoever. Aims at saving laptop battery life by
 296                  * leaving the processor in sleep state when idle.
 297                  */
 298                 printf("b\n");
 299                 wait_defer();
 300                 printf("e\n");
 301                 /* Sleeping after wait_defer to let many callbacks enqueue */
 302                 //TEST poll(NULL,0,100);        /* wait for 100ms */
 303                 printf("f\n");
 304                 rcu_defer_barrier();
 305                 printf("perform deferred call_rcu() from worker thread %lu.\n",
 306                         time(NULL));
 307         }
 308
 309         return NULL;
 310 }
 311
 312 /*
 313  * library wrappers to be used by non-LGPL compatible source code.
 314  */
 315
 316 void rcu_defer_queue(void (*fct)(void *p), void *p)
 317 {
 318         _rcu_defer_queue(fct, p);
 319 }
 320
 321 static void rcu_add_deferer(pthread_t id)
 322 {
 323         struct deferer_registry *oldarray;
 324
 325         if (!registry) {
 326                 alloc_deferers = INIT_NUM_THREADS;
 327                 num_deferers = 0;
 328                 registry =
 329                         malloc(sizeof(struct deferer_registry) * alloc_deferers);
 330         }
 331         if (alloc_deferers < num_deferers + 1) {
 332                 oldarray = registry;
 333                 registry = malloc(sizeof(struct deferer_registry)
 334                                 * (alloc_deferers << 1));
 335                 memcpy(registry, oldarray,
 336                         sizeof(struct deferer_registry) * alloc_deferers);
 337                 alloc_deferers <<= 1;
 338                 free(oldarray);
 339         }
 340         registry[num_deferers].tid = id;
 341         /* reference to the TLS of _this_ deferer thread. */
 342         registry[num_deferers].defer_queue = &defer_queue;
 343         registry[num_deferers].last_head = 0;
 344         num_deferers++;
 345 }
 346
 347 /*
 348  * Never shrink (implementation limitation).
 349  * This is O(nb threads). Eventually use a hash table.
 350  */
 351 static void rcu_remove_deferer(pthread_t id)
 352 {
 353         struct deferer_registry *index;
 354
 355         assert(registry != NULL);
 356         for (index = registry; index < registry + num_deferers; index++) {
 357                 if (pthread_equal(index->tid, id)) {
 358                         memcpy(index, &registry[num_deferers - 1],
 359                                 sizeof(struct deferer_registry));
 360                         registry[num_deferers - 1].tid = 0;
 361                         registry[num_deferers - 1].defer_queue = NULL;
 362                         registry[num_deferers - 1].last_head = 0;
 363                         num_deferers--;
 364                         return;
 365                 }
 366         }
 367         /* Hrm not found, forgot to register ? */
 368         assert(0);
 369 }
 370
 371 static void start_defer_thread(void)
 372 {
 373         int ret;
 374
 375         ret = pthread_create(&tid_defer, NULL, thr_defer,
 376                 NULL);
 377         assert(!ret);
 378 }
 379
 380 static void stop_defer_thread(void)
 381 {
 382         int ret;
 383         void *tret;
 384
 385         pthread_cancel(tid_defer);
 386         wake_up_defer();
 387         ret = pthread_join(tid_defer, &tret);
 388         assert(!ret);
 389 }
 390
 391 void rcu_defer_register_thread(void)
 392 {
 393         int deferers;
 394
 395         internal_urcu_lock(&defer_thread_mutex);
 396         internal_urcu_lock(&urcu_defer_mutex);
 397         defer_queue.q = malloc(sizeof(void *) * DEFER_QUEUE_SIZE);
 398         rcu_add_deferer(pthread_self());
 399         deferers = num_deferers;
 400         internal_urcu_unlock(&urcu_defer_mutex);
 401
 402         if (deferers == 1)
 403                 start_defer_thread();
 404         internal_urcu_unlock(&defer_thread_mutex);
 405 }
 406
 407 void rcu_defer_unregister_thread(void)
 408 {
 409         int deferers;
 410
 411         internal_urcu_lock(&defer_thread_mutex);
 412         internal_urcu_lock(&urcu_defer_mutex);
 413         rcu_remove_deferer(pthread_self());
 414         _rcu_defer_barrier_thread();
 415         free(defer_queue.q);
 416         defer_queue.q = NULL;
 417         deferers = num_deferers;
 418         internal_urcu_unlock(&urcu_defer_mutex);
 419
 420         if (deferers == 0)
 421                 stop_defer_thread();
 422         internal_urcu_unlock(&defer_thread_mutex);
 423 }
 424
 425 void urcu_defer_exit(void)
 426 {
 427         free(registry);
 428 }