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