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 <syscall.h>
  42 #include <unistd.h>
  43 #include <stdint.h>
  44
  45 #include "urcu/futex.h"
  46
  47 #include <urcu/compiler.h>
  48 #include <urcu/arch.h>
  49 #include <urcu/uatomic.h>
  50 #include <urcu/list.h>
  51 #include <urcu/system.h>
  52
  53 /*
  54  * Number of entries in the per-thread defer queue. Must be power of 2.
  55  */
  56 #define DEFER_QUEUE_SIZE        (1 << 12)
  57 #define DEFER_QUEUE_MASK        (DEFER_QUEUE_SIZE - 1)
  58
  59 /*
  60  * Typically, data is aligned at least on the architecture size.
  61  * Use lowest bit to indicate that the current callback is changing.
  62  * Assumes that (void *)-2L is not used often. Used to encode non-aligned
  63  * functions and non-aligned data using extra space.
  64  * We encode the (void *)-2L fct as: -2L, fct, data.
  65  * We encode the (void *)-2L data as: -2L, fct, data.
  66  * Here, DQ_FCT_MARK == ~DQ_FCT_BIT. Required for the test order.
  67  */
  68 #define DQ_FCT_BIT              (1 << 0)
  69 #define DQ_IS_FCT_BIT(x)        ((unsigned long)(x) & DQ_FCT_BIT)
  70 #define DQ_SET_FCT_BIT(x)       \
  71         (x = (void *)((unsigned long)(x) | DQ_FCT_BIT))
  72 #define DQ_CLEAR_FCT_BIT(x)     \
  73         (x = (void *)((unsigned long)(x) & ~DQ_FCT_BIT))
  74 #define DQ_FCT_MARK             ((void *)(~DQ_FCT_BIT))
  75
  76 /*
  77  * This code section can only be included in LGPL 2.1 compatible source code.
  78  * See below for the function call wrappers which can be used in code meant to
  79  * be only linked with the Userspace RCU library. This comes with a small
  80  * performance degradation on the read-side due to the added function calls.
  81  * This is required to permit relinking with newer versions of the library.
  82  */
  83
  84 #ifdef DEBUG_RCU
  85 #define rcu_assert(args...)     assert(args)
  86 #else
  87 #define rcu_assert(args...)
  88 #endif
  89
  90 /*
  91  * defer queue.
  92  * Contains pointers. Encoded to save space when same callback is often used.
  93  * When looking up the next item:
  94  * - if DQ_FCT_BIT is set, set the current callback to DQ_CLEAR_FCT_BIT(ptr)
  95  *   - next element contains pointer to data.
  96  * - else if item == DQ_FCT_MARK
  97  *   - set the current callback to next element ptr
  98  *   - following next element contains pointer to data.
  99  * - else current element contains data
 100  */
 101 struct defer_queue {
 102         unsigned long head;     /* add element at head */
 103         void *last_fct_in;      /* last fct pointer encoded */
 104         unsigned long tail;     /* next element to remove at tail */
 105         void *last_fct_out;     /* last fct pointer encoded */
 106         void **q;
 107         /* registry information */
 108         unsigned long last_head;
 109         struct cds_list_head list;      /* list of thread queues */
 110 };
 111
 112 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
 113 #include "urcu-defer.h"
 114
 115 void __attribute__((destructor)) rcu_defer_exit(void);
 116
 117 extern void synchronize_rcu(void);
 118
 119 /*
 120  * rcu_defer_mutex nests inside defer_thread_mutex.
 121  */
 122 static pthread_mutex_t rcu_defer_mutex = PTHREAD_MUTEX_INITIALIZER;
 123 static pthread_mutex_t defer_thread_mutex = PTHREAD_MUTEX_INITIALIZER;
 124
 125 static int32_t defer_thread_futex;
 126
 127 /*
 128  * Written to only by each individual deferer. Read by both the deferer and
 129  * the reclamation tread.
 130  */
 131 static struct defer_queue __thread defer_queue;
 132 static CDS_LIST_HEAD(registry_defer);
 133 static pthread_t tid_defer;
 134
 135 static void mutex_lock_defer(pthread_mutex_t *mutex)
 136 {
 137         int ret;
 138
 139 #ifndef DISTRUST_SIGNALS_EXTREME
 140         ret = pthread_mutex_lock(mutex);
 141         if (ret) {
 142                 perror("Error in pthread mutex lock");
 143                 exit(-1);
 144         }
 145 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
 146         while ((ret = pthread_mutex_trylock(mutex)) != 0) {
 147                 if (ret != EBUSY && ret != EINTR) {
 148                         printf("ret = %d, errno = %d\n", ret, errno);
 149                         perror("Error in pthread mutex lock");
 150                         exit(-1);
 151                 }
 152                 pthread_testcancel();
 153                 poll(NULL,0,10);
 154         }
 155 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
 156 }
 157
 158 /*
 159  * Wake-up any waiting defer thread. Called from many concurrent threads.
 160  */
 161 static void wake_up_defer(void)
 162 {
 163         if (unlikely(uatomic_read(&defer_thread_futex) == -1)) {
 164                 uatomic_set(&defer_thread_futex, 0);
 165                 futex_noasync(&defer_thread_futex, FUTEX_WAKE, 1,
 166                       NULL, NULL, 0);
 167         }
 168 }
 169
 170 static unsigned long rcu_defer_num_callbacks(void)
 171 {
 172         unsigned long num_items = 0, head;
 173         struct defer_queue *index;
 174
 175         mutex_lock_defer(&rcu_defer_mutex);
 176         cds_list_for_each_entry(index, &registry_defer, list) {
 177                 head = CMM_LOAD_SHARED(index->head);
 178                 num_items += head - index->tail;
 179         }
 180         mutex_unlock(&rcu_defer_mutex);
 181         return num_items;
 182 }
 183
 184 /*
 185  * Defer thread waiting. Single thread.
 186  */
 187 static void wait_defer(void)
 188 {
 189         uatomic_dec(&defer_thread_futex);
 190         cmm_smp_mb();   /* Write futex before read queue */
 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                         futex_noasync(&defer_thread_futex, FUTEX_WAIT, -1,
 199                               NULL, NULL, 0);
 200         }
 201 }
 202
 203 /*
 204  * Must be called after Q.S. is reached.
 205  */
 206 static void rcu_defer_barrier_queue(struct defer_queue *queue,
 207                                     unsigned long head)
 208 {
 209         unsigned long i;
 210         void (*fct)(void *p);
 211         void *p;
 212
 213         /*
 214          * Tail is only modified when lock is held.
 215          * Head is only modified by owner thread.
 216          */
 217
 218         for (i = queue->tail; i != head;) {
 219                 cmm_smp_rmb();       /* read head before q[]. */
 220                 p = CMM_LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 221                 if (unlikely(DQ_IS_FCT_BIT(p))) {
 222                         DQ_CLEAR_FCT_BIT(p);
 223                         queue->last_fct_out = p;
 224                         p = CMM_LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 225                 } else if (unlikely(p == DQ_FCT_MARK)) {
 226                         p = CMM_LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 227                         queue->last_fct_out = p;
 228                         p = CMM_LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
 229                 }
 230                 fct = queue->last_fct_out;
 231                 fct(p);
 232         }
 233         cmm_smp_mb();   /* push tail after having used q[] */
 234         CMM_STORE_SHARED(queue->tail, i);
 235 }
 236
 237 static void _rcu_defer_barrier_thread(void)
 238 {
 239         unsigned long head, num_items;
 240
 241         head = defer_queue.head;
 242         num_items = head - defer_queue.tail;
 243         if (unlikely(!num_items))
 244                 return;
 245         synchronize_rcu();
 246         rcu_defer_barrier_queue(&defer_queue, head);
 247 }
 248
 249 void rcu_defer_barrier_thread(void)
 250 {
 251         mutex_lock_defer(&rcu_defer_mutex);
 252         _rcu_defer_barrier_thread();
 253         mutex_unlock(&rcu_defer_mutex);
 254 }
 255
 256 /*
 257  * rcu_defer_barrier - Execute all queued rcu callbacks.
 258  *
 259  * Execute all RCU callbacks queued before rcu_defer_barrier() execution.
 260  * All callbacks queued on the local thread prior to a rcu_defer_barrier() call
 261  * are guaranteed to be executed.
 262  * Callbacks queued by other threads concurrently with rcu_defer_barrier()
 263  * execution are not guaranteed to be executed in the current batch (could
 264  * be left for the next batch). These callbacks queued by other threads are only
 265  * guaranteed to be executed if there is explicit synchronization between
 266  * the thread adding to the queue and the thread issuing the defer_barrier call.
 267  */
 268
 269 void rcu_defer_barrier(void)
 270 {
 271         struct defer_queue *index;
 272         unsigned long num_items = 0;
 273
 274         if (cds_list_empty(&registry_defer))
 275                 return;
 276
 277         mutex_lock_defer(&rcu_defer_mutex);
 278         cds_list_for_each_entry(index, &registry_defer, list) {
 279                 index->last_head = CMM_LOAD_SHARED(index->head);
 280                 num_items += index->last_head - index->tail;
 281         }
 282         if (likely(!num_items)) {
 283                 /*
 284                  * We skip the grace period because there are no queued
 285                  * callbacks to execute.
 286                  */
 287                 goto end;
 288         }
 289         synchronize_rcu();
 290         cds_list_for_each_entry(index, &registry_defer, list)
 291                 rcu_defer_barrier_queue(index, index->last_head);
 292 end:
 293         mutex_unlock(&rcu_defer_mutex);
 294 }
 295
 296 /*
 297  * _defer_rcu - Queue a RCU callback.
 298  */
 299 void _defer_rcu(void (*fct)(void *p), void *p)
 300 {
 301         unsigned long head, tail;
 302
 303         /*
 304          * Head is only modified by ourself. Tail can be modified by reclamation
 305          * thread.
 306          */
 307         head = defer_queue.head;
 308         tail = CMM_LOAD_SHARED(defer_queue.tail);
 309
 310         /*
 311          * If queue is full, or reached threshold. Empty queue ourself.
 312          * Worse-case: must allow 2 supplementary entries for fct pointer.
 313          */
 314         if (unlikely(head - tail >= DEFER_QUEUE_SIZE - 2)) {
 315                 assert(head - tail <= DEFER_QUEUE_SIZE);
 316                 rcu_defer_barrier_thread();
 317                 assert(head - CMM_LOAD_SHARED(defer_queue.tail) == 0);
 318         }
 319
 320         if (unlikely(defer_queue.last_fct_in != fct)) {
 321                 defer_queue.last_fct_in = fct;
 322                 if (unlikely(DQ_IS_FCT_BIT(fct) || fct == DQ_FCT_MARK)) {
 323                         /*
 324                          * If the function to encode is not aligned or the
 325                          * marker, write DQ_FCT_MARK followed by the function
 326                          * pointer.
 327                          */
 328                         _CMM_STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
 329                                       DQ_FCT_MARK);
 330                         _CMM_STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
 331                                       fct);
 332                 } else {
 333                         DQ_SET_FCT_BIT(fct);
 334                         _CMM_STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
 335                                       fct);
 336                 }
 337         } else {
 338                 if (unlikely(DQ_IS_FCT_BIT(p) || p == DQ_FCT_MARK)) {
 339                         /*
 340                          * If the data to encode is not aligned or the marker,
 341                          * write DQ_FCT_MARK followed by the function pointer.
 342                          */
 343                         _CMM_STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
 344                                       DQ_FCT_MARK);
 345                         _CMM_STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
 346                                       fct);
 347                 }
 348         }
 349         _CMM_STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK], p);
 350         cmm_smp_wmb();  /* Publish new pointer before head */
 351                         /* Write q[] before head. */
 352         CMM_STORE_SHARED(defer_queue.head, head);
 353         cmm_smp_mb();   /* Write queue head before read futex */
 354         /*
 355          * Wake-up any waiting defer thread.
 356          */
 357         wake_up_defer();
 358 }
 359
 360 void *thr_defer(void *args)
 361 {
 362         for (;;) {
 363                 pthread_testcancel();
 364                 /*
 365                  * "Be green". Don't wake up the CPU if there is no RCU work
 366                  * to perform whatsoever. Aims at saving laptop battery life by
 367                  * leaving the processor in sleep state when idle.
 368                  */
 369                 wait_defer();
 370                 /* Sleeping after wait_defer to let many callbacks enqueue */
 371                 poll(NULL,0,100);       /* wait for 100ms */
 372                 rcu_defer_barrier();
 373         }
 374
 375         return NULL;
 376 }
 377
 378 /*
 379  * library wrappers to be used by non-LGPL compatible source code.
 380  */
 381
 382 void defer_rcu(void (*fct)(void *p), void *p)
 383 {
 384         _defer_rcu(fct, p);
 385 }
 386
 387 static void start_defer_thread(void)
 388 {
 389         int ret;
 390
 391         ret = pthread_create(&tid_defer, NULL, thr_defer, NULL);
 392         assert(!ret);
 393 }
 394
 395 static void stop_defer_thread(void)
 396 {
 397         int ret;
 398         void *tret;
 399
 400         pthread_cancel(tid_defer);
 401         wake_up_defer();
 402         ret = pthread_join(tid_defer, &tret);
 403         assert(!ret);
 404 }
 405
 406 int rcu_defer_register_thread(void)
 407 {
 408         int was_empty;
 409
 410         assert(defer_queue.last_head == 0);
 411         assert(defer_queue.q == NULL);
 412         defer_queue.q = malloc(sizeof(void *) * DEFER_QUEUE_SIZE);
 413         if (!defer_queue.q)
 414                 return -ENOMEM;
 415
 416         mutex_lock_defer(&defer_thread_mutex);
 417         mutex_lock_defer(&rcu_defer_mutex);
 418         was_empty = cds_list_empty(&registry_defer);
 419         cds_list_add(&defer_queue.list, &registry_defer);
 420         mutex_unlock(&rcu_defer_mutex);
 421
 422         if (was_empty)
 423                 start_defer_thread();
 424         mutex_unlock(&defer_thread_mutex);
 425         return 0;
 426 }
 427
 428 void rcu_defer_unregister_thread(void)
 429 {
 430         int is_empty;
 431
 432         mutex_lock_defer(&defer_thread_mutex);
 433         mutex_lock_defer(&rcu_defer_mutex);
 434         cds_list_del(&defer_queue.list);
 435         _rcu_defer_barrier_thread();
 436         free(defer_queue.q);
 437         defer_queue.q = NULL;
 438         is_empty = cds_list_empty(&registry_defer);
 439         mutex_unlock(&rcu_defer_mutex);
 440
 441         if (is_empty)
 442                 stop_defer_thread();
 443         mutex_unlock(&defer_thread_mutex);
 444 }
 445
 446 void rcu_defer_exit(void)
 447 {
 448         assert(cds_list_empty(&registry_defer));
 449 }
 450
 451 #endif /* _URCU_DEFER_IMPL_H */