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                 if (uatomic_read(&defer_thread_futex) != -1)
 198                         return;
 199                 while (futex_noasync(&defer_thread_futex, FUTEX_WAIT, -1,
 200                                 NULL, NULL, 0)) {
 201                         switch (errno) {
 202                         case EWOULDBLOCK:
 203                                 /* Value already changed. */
 204                                 return;
 205                         case EINTR:
 206                                 /* Retry if interrupted by signal. */
 207                                 break;  /* Get out of switch. */
 208                         default:
 209                                 /* Unexpected error. */
 210                                 urcu_die(errno);
 211                         }
 212                 }
 213         }
 214 }
 215
 216 /*
 217  * Must be called after Q.S. is reached.
 218  */
 219 static void rcu_defer_barrier_queue(struct defer_queue *queue,
 220                                     unsigned long head)
 221 {
 222         unsigned long i;
 223         void (*fct)(void *p);
 224         void *p;
 225
 226         /*
 227          * Tail is only modified when lock is held.
 228          * Head is only modified by owner thread.
 229          */
 230
 231         for (i = queue->tail; i != head;) {
 232                 cmm_smp_rmb();       /* read head before q[]. */
 233                 p = CMM_LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 234                 if (caa_unlikely(DQ_IS_FCT_BIT(p))) {
 235                         DQ_CLEAR_FCT_BIT(p);
 236                         queue->last_fct_out = p;
 237                         p = CMM_LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 238                 } else if (caa_unlikely(p == DQ_FCT_MARK)) {
 239                         p = CMM_LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 240                         queue->last_fct_out = p;
 241                         p = CMM_LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 242                 }
 243                 fct = queue->last_fct_out;
 244                 fct(p);
 245         }
 246         cmm_smp_mb();   /* push tail after having used q[] */
 247         CMM_STORE_SHARED(queue->tail, i);
 248 }
 249
 250 static void _rcu_defer_barrier_thread(void)
 251 {
 252         unsigned long head, num_items;
 253
 254         head = URCU_TLS(defer_queue).head;
 255         num_items = head - URCU_TLS(defer_queue).tail;
 256         if (caa_unlikely(!num_items))
 257                 return;
 258         synchronize_rcu();
 259         rcu_defer_barrier_queue(&URCU_TLS(defer_queue), head);
 260 }
 261
 262 void rcu_defer_barrier_thread(void)
 263 {
 264         mutex_lock_defer(&rcu_defer_mutex);
 265         _rcu_defer_barrier_thread();
 266         mutex_unlock(&rcu_defer_mutex);
 267 }
 268
 269 /*
 270  * rcu_defer_barrier - Execute all queued rcu callbacks.
 271  *
 272  * Execute all RCU callbacks queued before rcu_defer_barrier() execution.
 273  * All callbacks queued on the local thread prior to a rcu_defer_barrier() call
 274  * are guaranteed to be executed.
 275  * Callbacks queued by other threads concurrently with rcu_defer_barrier()
 276  * execution are not guaranteed to be executed in the current batch (could
 277  * be left for the next batch). These callbacks queued by other threads are only
 278  * guaranteed to be executed if there is explicit synchronization between
 279  * the thread adding to the queue and the thread issuing the defer_barrier call.
 280  */
 281
 282 void rcu_defer_barrier(void)
 283 {
 284         struct defer_queue *index;
 285         unsigned long num_items = 0;
 286
 287         if (cds_list_empty(&registry_defer))
 288                 return;
 289
 290         mutex_lock_defer(&rcu_defer_mutex);
 291         cds_list_for_each_entry(index, &registry_defer, list) {
 292                 index->last_head = CMM_LOAD_SHARED(index->head);
 293                 num_items += index->last_head - index->tail;
 294         }
 295         if (caa_likely(!num_items)) {
 296                 /*
 297                  * We skip the grace period because there are no queued
 298                  * callbacks to execute.
 299                  */
 300                 goto end;
 301         }
 302         synchronize_rcu();
 303         cds_list_for_each_entry(index, &registry_defer, list)
 304                 rcu_defer_barrier_queue(index, index->last_head);
 305 end:
 306         mutex_unlock(&rcu_defer_mutex);
 307 }
 308
 309 /*
 310  * _defer_rcu - Queue a RCU callback.
 311  */
 312 static void _defer_rcu(void (*fct)(void *p), void *p)
 313 {
 314         unsigned long head, tail;
 315
 316         /*
 317          * Head is only modified by ourself. Tail can be modified by reclamation
 318          * thread.
 319          */
 320         head = URCU_TLS(defer_queue).head;
 321         tail = CMM_LOAD_SHARED(URCU_TLS(defer_queue).tail);
 322
 323         /*
 324          * If queue is full, or reached threshold. Empty queue ourself.
 325          * Worse-case: must allow 2 supplementary entries for fct pointer.
 326          */
 327         if (caa_unlikely(head - tail >= DEFER_QUEUE_SIZE - 2)) {
 328                 assert(head - tail <= DEFER_QUEUE_SIZE);
 329                 rcu_defer_barrier_thread();
 330                 assert(head - CMM_LOAD_SHARED(URCU_TLS(defer_queue).tail) == 0);
 331         }
 332
 333         /*
 334          * Encode:
 335          * if the function is not changed and the data is aligned and it is
 336          * not the marker:
 337          *      store the data
 338          * otherwise if the function is aligned and its not the marker:
 339          *      store the function with DQ_FCT_BIT
 340          *      store the data
 341          * otherwise:
 342          *      store the marker (DQ_FCT_MARK)
 343          *      store the function
 344          *      store the data
 345          *
 346          * Decode: see the comments before 'struct defer_queue'
 347          *         or the code in rcu_defer_barrier_queue().
 348          */
 349         if (caa_unlikely(URCU_TLS(defer_queue).last_fct_in != fct
 350                         || DQ_IS_FCT_BIT(p)
 351                         || p == DQ_FCT_MARK)) {
 352                 URCU_TLS(defer_queue).last_fct_in = fct;
 353                 if (caa_unlikely(DQ_IS_FCT_BIT(fct) || fct == DQ_FCT_MARK)) {
 354                         _CMM_STORE_SHARED(URCU_TLS(defer_queue).q[head++ & DEFER_QUEUE_MASK],
 355                                       DQ_FCT_MARK);
 356                         _CMM_STORE_SHARED(URCU_TLS(defer_queue).q[head++ & DEFER_QUEUE_MASK],
 357                                       fct);
 358                 } else {
 359                         DQ_SET_FCT_BIT(fct);
 360                         _CMM_STORE_SHARED(URCU_TLS(defer_queue).q[head++ & DEFER_QUEUE_MASK],
 361                                       fct);
 362                 }
 363         }
 364         _CMM_STORE_SHARED(URCU_TLS(defer_queue).q[head++ & DEFER_QUEUE_MASK], p);
 365         cmm_smp_wmb();  /* Publish new pointer before head */
 366                         /* Write q[] before head. */
 367         CMM_STORE_SHARED(URCU_TLS(defer_queue).head, head);
 368         cmm_smp_mb();   /* Write queue head before read futex */
 369         /*
 370          * Wake-up any waiting defer thread.
 371          */
 372         wake_up_defer();
 373 }
 374
 375 static void *thr_defer(void *args __attribute__((unused)))
 376 {
 377         for (;;) {
 378                 /*
 379                  * "Be green". Don't wake up the CPU if there is no RCU work
 380                  * to perform whatsoever. Aims at saving laptop battery life by
 381                  * leaving the processor in sleep state when idle.
 382                  */
 383                 wait_defer();
 384                 /* Sleeping after wait_defer to let many callbacks enqueue */
 385                 (void) poll(NULL,0,100);        /* wait for 100ms */
 386                 rcu_defer_barrier();
 387         }
 388
 389         return NULL;
 390 }
 391
 392 /*
 393  * library wrappers to be used by non-LGPL compatible source code.
 394  */
 395
 396 void defer_rcu(void (*fct)(void *p), void *p)
 397 {
 398         _defer_rcu(fct, p);
 399 }
 400
 401 static void start_defer_thread(void)
 402 {
 403         int ret;
 404
 405         ret = pthread_create(&tid_defer, NULL, thr_defer, NULL);
 406         assert(!ret);
 407 }
 408
 409 static void stop_defer_thread(void)
 410 {
 411         int ret;
 412         void *tret;
 413
 414         _CMM_STORE_SHARED(defer_thread_stop, 1);
 415         /* Store defer_thread_stop before testing futex */
 416         cmm_smp_mb();
 417         wake_up_defer();
 418
 419         ret = pthread_join(tid_defer, &tret);
 420         assert(!ret);
 421
 422         CMM_STORE_SHARED(defer_thread_stop, 0);
 423         /* defer thread should always exit when futex value is 0 */
 424         assert(uatomic_read(&defer_thread_futex) == 0);
 425 }
 426
 427 int rcu_defer_register_thread(void)
 428 {
 429         int was_empty;
 430
 431         assert(URCU_TLS(defer_queue).last_head == 0);
 432         assert(URCU_TLS(defer_queue).q == NULL);
 433         URCU_TLS(defer_queue).q = malloc(sizeof(void *) * DEFER_QUEUE_SIZE);
 434         if (!URCU_TLS(defer_queue).q)
 435                 return -ENOMEM;
 436
 437         mutex_lock_defer(&defer_thread_mutex);
 438         mutex_lock_defer(&rcu_defer_mutex);
 439         was_empty = cds_list_empty(&registry_defer);
 440         cds_list_add(&URCU_TLS(defer_queue).list, &registry_defer);
 441         mutex_unlock(&rcu_defer_mutex);
 442
 443         if (was_empty)
 444                 start_defer_thread();
 445         mutex_unlock(&defer_thread_mutex);
 446         return 0;
 447 }
 448
 449 void rcu_defer_unregister_thread(void)
 450 {
 451         int is_empty;
 452
 453         mutex_lock_defer(&defer_thread_mutex);
 454         mutex_lock_defer(&rcu_defer_mutex);
 455         cds_list_del(&URCU_TLS(defer_queue).list);
 456         _rcu_defer_barrier_thread();
 457         free(URCU_TLS(defer_queue).q);
 458         URCU_TLS(defer_queue).q = NULL;
 459         is_empty = cds_list_empty(&registry_defer);
 460         mutex_unlock(&rcu_defer_mutex);
 461
 462         if (is_empty)
 463                 stop_defer_thread();
 464         mutex_unlock(&defer_thread_mutex);
 465 }
 466
 467 void rcu_defer_exit(void)
 468 {
 469         assert(cds_list_empty(&registry_defer));
 470 }
 471
 472 #endif /* _URCU_DEFER_IMPL_H */