src/urcu-defer-impl.h

   1 #ifndef _URCU_DEFER_IMPL_H
   2 #define _URCU_DEFER_IMPL_H
   3
   4 /*
   5  * urcu-defer-impl.h
   6  *
   7  * Userspace RCU header - memory reclamation.
   8  *
   9  * TO BE INCLUDED ONLY FROM URCU LIBRARY CODE. See urcu-defer.h for linking
  10  * dynamically with the userspace rcu reclamation library.
  11  *
  12  * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  13  * Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
  14  *
  15  * This library is free software; you can redistribute it and/or
  16  * modify it under the terms of the GNU Lesser General Public
  17  * License as published by the Free Software Foundation; either
  18  * version 2.1 of the License, or (at your option) any later version.
  19  *
  20  * This library is distributed in the hope that it will be useful,
  21  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  22  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  23  * Lesser General Public License for more details.
  24  *
  25  * You should have received a copy of the GNU Lesser General Public
  26  * License along with this library; if not, write to the Free Software
  27  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  28  *
  29  * IBM's contributions to this file may be relicensed under LGPLv2 or later.
  30  */
  31
  32 #include <stdlib.h>
  33 #include <pthread.h>
  34 #include <stdio.h>
  35 #include <signal.h>
  36 #include <assert.h>
  37 #include <string.h>
  38 #include <errno.h>
  39 #include <poll.h>
  40 #include <sys/time.h>
  41 #include <unistd.h>
  42 #include <stdint.h>
  43
  44 #include "urcu/futex.h"
  45
  46 #include <urcu/compiler.h>
  47 #include <urcu/arch.h>
  48 #include <urcu/uatomic.h>
  49 #include <urcu/list.h>
  50 #include <urcu/system.h>
  51 #include <urcu/tls-compat.h>
  52 #include "urcu-die.h"
  53 #include "urcu-utils.h"
  54
  55 /*
  56  * Number of entries in the per-thread defer queue. Must be power of 2.
  57  */
  58 #define DEFER_QUEUE_SIZE        (1 << 12)
  59 #define DEFER_QUEUE_MASK        (DEFER_QUEUE_SIZE - 1)
  60
  61 /*
  62  * Typically, data is aligned at least on the architecture size.
  63  * Use lowest bit to indicate that the current callback is changing.
  64  * Assumes that (void *)-2L is not used often. Used to encode non-aligned
  65  * functions and non-aligned data using extra space.
  66  * We encode the (void *)-2L fct as: -2L, fct, data.
  67  * We encode the (void *)-2L data as either:
  68  *   fct | DQ_FCT_BIT, data (if fct is aligned), or
  69  *   -2L, fct, data (if fct is not aligned).
  70  * Here, DQ_FCT_MARK == ~DQ_FCT_BIT. Required for the test order.
  71  */
  72 #define DQ_FCT_BIT              (1 << 0)
  73 #define DQ_IS_FCT_BIT(x)        ((unsigned long)(x) & DQ_FCT_BIT)
  74 #define DQ_SET_FCT_BIT(x)       \
  75         (x = (void *)((unsigned long)(x) | DQ_FCT_BIT))
  76 #define DQ_CLEAR_FCT_BIT(x)     \
  77         (x = (void *)((unsigned long)(x) & ~DQ_FCT_BIT))
  78 #define DQ_FCT_MARK             ((void *)(~DQ_FCT_BIT))
  79
  80 /*
  81  * This code section can only be included in LGPL 2.1 compatible source code.
  82  * See below for the function call wrappers which can be used in code meant to
  83  * be only linked with the Userspace RCU library. This comes with a small
  84  * performance degradation on the read-side due to the added function calls.
  85  * This is required to permit relinking with newer versions of the library.
  86  */
  87
  88 /*
  89  * defer queue.
  90  * Contains pointers. Encoded to save space when same callback is often used.
  91  * When looking up the next item:
  92  * - if DQ_FCT_BIT is set, set the current callback to DQ_CLEAR_FCT_BIT(ptr)
  93  *   - next element contains pointer to data.
  94  * - else if item == DQ_FCT_MARK
  95  *   - set the current callback to next element ptr
  96  *   - following next element contains pointer to data.
  97  * - else current element contains data
  98  */
  99 struct defer_queue {
 100         unsigned long head;     /* add element at head */
 101         void *last_fct_in;      /* last fct pointer encoded */
 102         unsigned long tail;     /* next element to remove at tail */
 103         void *last_fct_out;     /* last fct pointer encoded */
 104         void **q;
 105         /* registry information */
 106         unsigned long last_head;
 107         struct cds_list_head list;      /* list of thread queues */
 108 };
 109
 110 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
 111 #include <urcu/defer.h>
 112
 113 void __attribute__((destructor)) rcu_defer_exit(void);
 114
 115 extern void synchronize_rcu(void);
 116
 117 /*
 118  * rcu_defer_mutex nests inside defer_thread_mutex.
 119  */
 120 static pthread_mutex_t rcu_defer_mutex = PTHREAD_MUTEX_INITIALIZER;
 121 static pthread_mutex_t defer_thread_mutex = PTHREAD_MUTEX_INITIALIZER;
 122
 123 static int32_t defer_thread_futex;
 124 static int32_t defer_thread_stop;
 125
 126 /*
 127  * Written to only by each individual deferer. Read by both the deferer and
 128  * the reclamation tread.
 129  */
 130 static DEFINE_URCU_TLS(struct defer_queue, defer_queue);
 131 static CDS_LIST_HEAD(registry_defer);
 132 static pthread_t tid_defer;
 133
 134 static void mutex_lock_defer(pthread_mutex_t *mutex)
 135 {
 136         int ret;
 137
 138 #ifndef DISTRUST_SIGNALS_EXTREME
 139         ret = pthread_mutex_lock(mutex);
 140         if (ret)
 141                 urcu_die(ret);
 142 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
 143         while ((ret = pthread_mutex_trylock(mutex)) != 0) {
 144                 if (ret != EBUSY && ret != EINTR)
 145                         urcu_die(ret);
 146                 (void) poll(NULL,0,10);
 147         }
 148 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
 149 }
 150
 151 /*
 152  * Wake-up any waiting defer thread. Called from many concurrent threads.
 153  */
 154 static void wake_up_defer(void)
 155 {
 156         if (caa_unlikely(uatomic_read(&defer_thread_futex) == -1)) {
 157                 uatomic_set(&defer_thread_futex, 0);
 158                 if (futex_noasync(&defer_thread_futex, FUTEX_WAKE, 1,
 159                                 NULL, NULL, 0) < 0)
 160                         urcu_die(errno);
 161         }
 162 }
 163
 164 static unsigned long rcu_defer_num_callbacks(void)
 165 {
 166         unsigned long num_items = 0, head;
 167         struct defer_queue *index;
 168
 169         mutex_lock_defer(&rcu_defer_mutex);
 170         cds_list_for_each_entry(index, &registry_defer, list) {
 171                 head = CMM_LOAD_SHARED(index->head);
 172                 num_items += head - index->tail;
 173         }
 174         mutex_unlock(&rcu_defer_mutex);
 175         return num_items;
 176 }
 177
 178 /*
 179  * Defer thread waiting. Single thread.
 180  */
 181 static void wait_defer(void)
 182 {
 183         uatomic_dec(&defer_thread_futex);
 184         /* Write futex before read queue */
 185         /* Write futex before read defer_thread_stop */
 186         cmm_smp_mb();
 187         if (_CMM_LOAD_SHARED(defer_thread_stop)) {
 188                 uatomic_set(&defer_thread_futex, 0);
 189                 pthread_exit(0);
 190         }
 191         if (rcu_defer_num_callbacks()) {
 192                 cmm_smp_mb();   /* Read queue before write futex */
 193                 /* Callbacks are queued, don't wait. */
 194                 uatomic_set(&defer_thread_futex, 0);
 195         } else {
 196                 cmm_smp_rmb();  /* Read queue before read futex */
 197                 while (uatomic_read(&defer_thread_futex) == -1) {
 198                         if (!futex_noasync(&defer_thread_futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
 199                                 /*
 200                                  * Prior queued wakeups queued by unrelated code
 201                                  * using the same address can cause futex wait to
 202                                  * return 0 even through the futex value is still
 203                                  * -1 (spurious wakeups). Check the value again
 204                                  * in user-space to validate whether it really
 205                                  * differs from -1.
 206                                  */
 207                                 continue;
 208                         }
 209                         switch (errno) {
 210                         case EAGAIN:
 211                                 /* Value already changed. */
 212                                 return;
 213                         case EINTR:
 214                                 /* Retry if interrupted by signal. */
 215                                 break;  /* Get out of switch. Check again. */
 216                         default:
 217                                 /* Unexpected error. */
 218                                 urcu_die(errno);
 219                         }
 220                 }
 221         }
 222 }
 223
 224 /*
 225  * Must be called after Q.S. is reached.
 226  */
 227 static void rcu_defer_barrier_queue(struct defer_queue *queue,
 228                                     unsigned long head)
 229 {
 230         unsigned long i;
 231         void (*fct)(void *p);
 232         void *p;
 233
 234         /*
 235          * Tail is only modified when lock is held.
 236          * Head is only modified by owner thread.
 237          */
 238
 239         for (i = queue->tail; i != head;) {
 240                 cmm_smp_rmb();       /* read head before q[]. */
 241                 p = CMM_LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 242                 if (caa_unlikely(DQ_IS_FCT_BIT(p))) {
 243                         DQ_CLEAR_FCT_BIT(p);
 244                         queue->last_fct_out = p;
 245                         p = CMM_LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 246                 } else if (caa_unlikely(p == DQ_FCT_MARK)) {
 247                         p = CMM_LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 248                         queue->last_fct_out = p;
 249                         p = CMM_LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 250                 }
 251                 fct = queue->last_fct_out;
 252                 fct(p);
 253         }
 254         cmm_smp_mb();   /* push tail after having used q[] */
 255         CMM_STORE_SHARED(queue->tail, i);
 256 }
 257
 258 static void _rcu_defer_barrier_thread(void)
 259 {
 260         unsigned long head, num_items;
 261
 262         head = URCU_TLS(defer_queue).head;
 263         num_items = head - URCU_TLS(defer_queue).tail;
 264         if (caa_unlikely(!num_items))
 265                 return;
 266         synchronize_rcu();
 267         rcu_defer_barrier_queue(&URCU_TLS(defer_queue), head);
 268 }
 269
 270 void rcu_defer_barrier_thread(void)
 271 {
 272         mutex_lock_defer(&rcu_defer_mutex);
 273         _rcu_defer_barrier_thread();
 274         mutex_unlock(&rcu_defer_mutex);
 275 }
 276 URCU_ATTR_ALIAS(urcu_stringify(rcu_defer_barrier_thread))
 277 void alias_rcu_defer_barrier_thread();
 278
 279 /*
 280  * rcu_defer_barrier - Execute all queued rcu callbacks.
 281  *
 282  * Execute all RCU callbacks queued before rcu_defer_barrier() execution.
 283  * All callbacks queued on the local thread prior to a rcu_defer_barrier() call
 284  * are guaranteed to be executed.
 285  * Callbacks queued by other threads concurrently with rcu_defer_barrier()
 286  * execution are not guaranteed to be executed in the current batch (could
 287  * be left for the next batch). These callbacks queued by other threads are only
 288  * guaranteed to be executed if there is explicit synchronization between
 289  * the thread adding to the queue and the thread issuing the defer_barrier call.
 290  */
 291
 292 void rcu_defer_barrier(void)
 293 {
 294         struct defer_queue *index;
 295         unsigned long num_items = 0;
 296
 297         if (cds_list_empty(&registry_defer))
 298                 return;
 299
 300         mutex_lock_defer(&rcu_defer_mutex);
 301         cds_list_for_each_entry(index, &registry_defer, list) {
 302                 index->last_head = CMM_LOAD_SHARED(index->head);
 303                 num_items += index->last_head - index->tail;
 304         }
 305         if (caa_likely(!num_items)) {
 306                 /*
 307                  * We skip the grace period because there are no queued
 308                  * callbacks to execute.
 309                  */
 310                 goto end;
 311         }
 312         synchronize_rcu();
 313         cds_list_for_each_entry(index, &registry_defer, list)
 314                 rcu_defer_barrier_queue(index, index->last_head);
 315 end:
 316         mutex_unlock(&rcu_defer_mutex);
 317 }
 318 URCU_ATTR_ALIAS(urcu_stringify(rcu_defer_barrier))
 319 void alias_rcu_defer_barrier();
 320
 321 /*
 322  * _defer_rcu - Queue a RCU callback.
 323  */
 324 static void _defer_rcu(void (*fct)(void *p), void *p)
 325 {
 326         unsigned long head, tail;
 327
 328         /*
 329          * Head is only modified by ourself. Tail can be modified by reclamation
 330          * thread.
 331          */
 332         head = URCU_TLS(defer_queue).head;
 333         tail = CMM_LOAD_SHARED(URCU_TLS(defer_queue).tail);
 334
 335         /*
 336          * If queue is full, or reached threshold. Empty queue ourself.
 337          * Worse-case: must allow 2 supplementary entries for fct pointer.
 338          */
 339         if (caa_unlikely(head - tail >= DEFER_QUEUE_SIZE - 2)) {
 340                 assert(head - tail <= DEFER_QUEUE_SIZE);
 341                 rcu_defer_barrier_thread();
 342                 assert(head - CMM_LOAD_SHARED(URCU_TLS(defer_queue).tail) == 0);
 343         }
 344
 345         /*
 346          * Encode:
 347          * if the function is not changed and the data is aligned and it is
 348          * not the marker:
 349          *      store the data
 350          * otherwise if the function is aligned and its not the marker:
 351          *      store the function with DQ_FCT_BIT
 352          *      store the data
 353          * otherwise:
 354          *      store the marker (DQ_FCT_MARK)
 355          *      store the function
 356          *      store the data
 357          *
 358          * Decode: see the comments before 'struct defer_queue'
 359          *         or the code in rcu_defer_barrier_queue().
 360          */
 361         if (caa_unlikely(URCU_TLS(defer_queue).last_fct_in != fct
 362                         || DQ_IS_FCT_BIT(p)
 363                         || p == DQ_FCT_MARK)) {
 364                 URCU_TLS(defer_queue).last_fct_in = fct;
 365                 if (caa_unlikely(DQ_IS_FCT_BIT(fct) || fct == DQ_FCT_MARK)) {
 366                         _CMM_STORE_SHARED(URCU_TLS(defer_queue).q[head++ & DEFER_QUEUE_MASK],
 367                                       DQ_FCT_MARK);
 368                         _CMM_STORE_SHARED(URCU_TLS(defer_queue).q[head++ & DEFER_QUEUE_MASK],
 369                                       fct);
 370                 } else {
 371                         DQ_SET_FCT_BIT(fct);
 372                         _CMM_STORE_SHARED(URCU_TLS(defer_queue).q[head++ & DEFER_QUEUE_MASK],
 373                                       fct);
 374                 }
 375         }
 376         _CMM_STORE_SHARED(URCU_TLS(defer_queue).q[head++ & DEFER_QUEUE_MASK], p);
 377         cmm_smp_wmb();  /* Publish new pointer before head */
 378                         /* Write q[] before head. */
 379         CMM_STORE_SHARED(URCU_TLS(defer_queue).head, head);
 380         cmm_smp_mb();   /* Write queue head before read futex */
 381         /*
 382          * Wake-up any waiting defer thread.
 383          */
 384         wake_up_defer();
 385 }
 386
 387 static void *thr_defer(void *args __attribute__((unused)))
 388 {
 389         for (;;) {
 390                 /*
 391                  * "Be green". Don't wake up the CPU if there is no RCU work
 392                  * to perform whatsoever. Aims at saving laptop battery life by
 393                  * leaving the processor in sleep state when idle.
 394                  */
 395                 wait_defer();
 396                 /* Sleeping after wait_defer to let many callbacks enqueue */
 397                 (void) poll(NULL,0,100);        /* wait for 100ms */
 398                 rcu_defer_barrier();
 399         }
 400
 401         return NULL;
 402 }
 403
 404 /*
 405  * library wrappers to be used by non-LGPL compatible source code.
 406  */
 407
 408 void defer_rcu(void (*fct)(void *p), void *p)
 409 {
 410         _defer_rcu(fct, p);
 411 }
 412 URCU_ATTR_ALIAS(urcu_stringify(defer_rcu)) void alias_defer_rcu();
 413
 414 static void start_defer_thread(void)
 415 {
 416         int ret;
 417
 418         ret = pthread_create(&tid_defer, NULL, thr_defer, NULL);
 419         if (ret)
 420                 urcu_die(ret);
 421 }
 422
 423 static void stop_defer_thread(void)
 424 {
 425         int ret;
 426         void *tret;
 427
 428         _CMM_STORE_SHARED(defer_thread_stop, 1);
 429         /* Store defer_thread_stop before testing futex */
 430         cmm_smp_mb();
 431         wake_up_defer();
 432
 433         ret = pthread_join(tid_defer, &tret);
 434         assert(!ret);
 435
 436         CMM_STORE_SHARED(defer_thread_stop, 0);
 437         /* defer thread should always exit when futex value is 0 */
 438         assert(uatomic_read(&defer_thread_futex) == 0);
 439 }
 440
 441 int rcu_defer_register_thread(void)
 442 {
 443         int was_empty;
 444
 445         assert(URCU_TLS(defer_queue).last_head == 0);
 446         assert(URCU_TLS(defer_queue).q == NULL);
 447         URCU_TLS(defer_queue).q = malloc(sizeof(void *) * DEFER_QUEUE_SIZE);
 448         if (!URCU_TLS(defer_queue).q)
 449                 return -ENOMEM;
 450
 451         mutex_lock_defer(&defer_thread_mutex);
 452         mutex_lock_defer(&rcu_defer_mutex);
 453         was_empty = cds_list_empty(&registry_defer);
 454         cds_list_add(&URCU_TLS(defer_queue).list, &registry_defer);
 455         mutex_unlock(&rcu_defer_mutex);
 456
 457         if (was_empty)
 458                 start_defer_thread();
 459         mutex_unlock(&defer_thread_mutex);
 460         return 0;
 461 }
 462 URCU_ATTR_ALIAS(urcu_stringify(rcu_defer_register_thread))
 463 int alias_rcu_defer_register_thread();
 464
 465 void rcu_defer_unregister_thread(void)
 466 {
 467         int is_empty;
 468
 469         mutex_lock_defer(&defer_thread_mutex);
 470         mutex_lock_defer(&rcu_defer_mutex);
 471         cds_list_del(&URCU_TLS(defer_queue).list);
 472         _rcu_defer_barrier_thread();
 473         free(URCU_TLS(defer_queue).q);
 474         URCU_TLS(defer_queue).q = NULL;
 475         is_empty = cds_list_empty(&registry_defer);
 476         mutex_unlock(&rcu_defer_mutex);
 477
 478         if (is_empty)
 479                 stop_defer_thread();
 480         mutex_unlock(&defer_thread_mutex);
 481 }
 482 URCU_ATTR_ALIAS(urcu_stringify(rcu_defer_unregister_thread))
 483 void alias_rcu_defer_unregister_thread();
 484
 485 void rcu_defer_exit(void)
 486 {
 487         assert(cds_list_empty(&registry_defer));
 488 }
 489 URCU_ATTR_ALIAS(urcu_stringify(rcu_defer_exit))
 490 void alias_rcu_defer_exit();
 491
 492 #endif /* _URCU_DEFER_IMPL_H */