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