src/urcu-defer-impl.h

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