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