urcu-bp.c

   1 /*
   2  * urcu-bp.c
   3  *
   4  * Userspace RCU library, "bulletproof" version.
   5  *
   6  * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
   7  * Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
   8  *
   9  * This library is free software; you can redistribute it and/or
  10  * modify it under the terms of the GNU Lesser General Public
  11  * License as published by the Free Software Foundation; either
  12  * version 2.1 of the License, or (at your option) any later version.
  13  *
  14  * This library is distributed in the hope that it will be useful,
  15  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  16  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  17  * Lesser General Public License for more details.
  18  *
  19  * You should have received a copy of the GNU Lesser General Public
  20  * License along with this library; if not, write to the Free Software
  21  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  22  *
  23  * IBM's contributions to this file may be relicensed under LGPLv2 or later.
  24  */
  25
  26 #define _GNU_SOURCE
  27 #include <stdio.h>
  28 #include <pthread.h>
  29 #include <signal.h>
  30 #include <assert.h>
  31 #include <stdlib.h>
  32 #include <string.h>
  33 #include <errno.h>
  34 #include <poll.h>
  35 #include <unistd.h>
  36 #include <sys/mman.h>
  37
  38 #include "urcu-bp-static.h"
  39 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
  40 #include "urcu-bp.h"
  41
  42 /* Sleep delay in us */
  43 #define RCU_SLEEP_DELAY         1000
  44 #define ARENA_INIT_ALLOC        16
  45
  46 void __attribute__((destructor)) urcu_bp_exit(void);
  47
  48 static pthread_mutex_t urcu_mutex = PTHREAD_MUTEX_INITIALIZER;
  49
  50 #ifdef DEBUG_YIELD
  51 unsigned int yield_active;
  52 unsigned int __thread rand_yield;
  53 #endif
  54
  55 /*
  56  * Global grace period counter.
  57  * Contains the current RCU_GP_CTR_BIT.
  58  * Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path.
  59  * Written to only by writer with mutex taken. Read by both writer and readers.
  60  */
  61 long urcu_gp_ctr = RCU_GP_COUNT;
  62
  63 /*
  64  * Pointer to registry elements. Written to only by each individual reader. Read
  65  * by both the reader and the writers.
  66  */
  67 struct urcu_reader __thread *urcu_reader;
  68
  69 static LIST_HEAD(registry);
  70
  71 struct registry_arena {
  72         void *p;
  73         size_t len;
  74         size_t used;
  75 };
  76
  77 static struct registry_arena registry_arena;
  78
  79 static void rcu_gc_registry(void);
  80
  81 static void internal_urcu_lock(void)
  82 {
  83         int ret;
  84
  85 #ifndef DISTRUST_SIGNALS_EXTREME
  86         ret = pthread_mutex_lock(&urcu_mutex);
  87         if (ret) {
  88                 perror("Error in pthread mutex lock");
  89                 exit(-1);
  90         }
  91 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
  92         while ((ret = pthread_mutex_trylock(&urcu_mutex)) != 0) {
  93                 if (ret != EBUSY && ret != EINTR) {
  94                         printf("ret = %d, errno = %d\n", ret, errno);
  95                         perror("Error in pthread mutex lock");
  96                         exit(-1);
  97                 }
  98                 if (urcu_reader.need_mb) {
  99                         smp_mb();
 100                         urcu_reader.need_mb = 0;
 101                         smp_mb();
 102                 }
 103                 poll(NULL,0,10);
 104         }
 105 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
 106 }
 107
 108 static void internal_urcu_unlock(void)
 109 {
 110         int ret;
 111
 112         ret = pthread_mutex_unlock(&urcu_mutex);
 113         if (ret) {
 114                 perror("Error in pthread mutex unlock");
 115                 exit(-1);
 116         }
 117 }
 118
 119 /*
 120  * called with urcu_mutex held.
 121  */
 122 static void switch_next_urcu_qparity(void)
 123 {
 124         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_CTR_BIT);
 125 }
 126
 127 void wait_for_quiescent_state(void)
 128 {
 129         LIST_HEAD(qsreaders);
 130         int wait_loops = 0;
 131         struct urcu_reader *index, *tmp;
 132
 133         if (list_empty(&registry))
 134                 return;
 135         /*
 136          * Wait for each thread urcu_reader.ctr count to become 0.
 137          */
 138         for (;;) {
 139                 wait_loops++;
 140                 list_for_each_entry_safe(index, tmp, &registry, head) {
 141                         if (!rcu_old_gp_ongoing(&index->ctr))
 142                                 list_move(&index->head, &qsreaders);
 143                 }
 144
 145                 if (list_empty(&registry)) {
 146                         break;
 147                 } else {
 148                         if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS)
 149                                 usleep(RCU_SLEEP_DELAY);
 150                         else
 151                                 cpu_relax();
 152                 }
 153         }
 154         /* put back the reader list in the registry */
 155         list_splice(&qsreaders, &registry);
 156 }
 157
 158 void synchronize_rcu(void)
 159 {
 160         sigset_t newmask, oldmask;
 161         int ret;
 162
 163         ret = sigemptyset(&newmask);
 164         assert(!ret);
 165         ret = pthread_sigmask(SIG_SETMASK, &newmask, &oldmask);
 166         assert(!ret);
 167
 168         internal_urcu_lock();
 169
 170         /* Remove old registry elements */
 171         rcu_gc_registry();
 172
 173         /* All threads should read qparity before accessing data structure
 174          * where new ptr points to. Must be done within internal_urcu_lock
 175          * because it iterates on reader threads.*/
 176         /* Write new ptr before changing the qparity */
 177         smp_mb();
 178
 179         switch_next_urcu_qparity();     /* 0 -> 1 */
 180
 181         /*
 182          * Must commit qparity update to memory before waiting for parity
 183          * 0 quiescent state. Failure to do so could result in the writer
 184          * waiting forever while new readers are always accessing data (no
 185          * progress).
 186          * Ensured by STORE_SHARED and LOAD_SHARED.
 187          */
 188
 189         /*
 190          * Adding a smp_mb() which is _not_ formally required, but makes the
 191          * model easier to understand. It does not have a big performance impact
 192          * anyway, given this is the write-side.
 193          */
 194         smp_mb();
 195
 196         /*
 197          * Wait for previous parity to be empty of readers.
 198          */
 199         wait_for_quiescent_state();     /* Wait readers in parity 0 */
 200
 201         /*
 202          * Must finish waiting for quiescent state for parity 0 before
 203          * committing qparity update to memory. Failure to do so could result in
 204          * the writer waiting forever while new readers are always accessing
 205          * data (no progress).
 206          * Ensured by STORE_SHARED and LOAD_SHARED.
 207          */
 208
 209         /*
 210          * Adding a smp_mb() which is _not_ formally required, but makes the
 211          * model easier to understand. It does not have a big performance impact
 212          * anyway, given this is the write-side.
 213          */
 214         smp_mb();
 215
 216         switch_next_urcu_qparity();     /* 1 -> 0 */
 217
 218         /*
 219          * Must commit qparity update to memory before waiting for parity
 220          * 1 quiescent state. Failure to do so could result in the writer
 221          * waiting forever while new readers are always accessing data (no
 222          * progress).
 223          * Ensured by STORE_SHARED and LOAD_SHARED.
 224          */
 225
 226         /*
 227          * Adding a smp_mb() which is _not_ formally required, but makes the
 228          * model easier to understand. It does not have a big performance impact
 229          * anyway, given this is the write-side.
 230          */
 231         smp_mb();
 232
 233         /*
 234          * Wait for previous parity to be empty of readers.
 235          */
 236         wait_for_quiescent_state();     /* Wait readers in parity 1 */
 237
 238         /* Finish waiting for reader threads before letting the old ptr being
 239          * freed. Must be done within internal_urcu_lock because it iterates on
 240          * reader threads. */
 241         smp_mb();
 242
 243         internal_urcu_unlock();
 244         ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
 245         assert(!ret);
 246 }
 247
 248 /*
 249  * library wrappers to be used by non-LGPL compatible source code.
 250  */
 251
 252 void rcu_read_lock(void)
 253 {
 254         _rcu_read_lock();
 255 }
 256
 257 void rcu_read_unlock(void)
 258 {
 259         _rcu_read_unlock();
 260 }
 261
 262 /*
 263  * only grow for now.
 264  */
 265 static void resize_arena(struct registry_arena *arena, size_t len)
 266 {
 267         void *new_arena;
 268
 269         if (!arena->p)
 270                 new_arena = mmap(arena->p, len,
 271                                  PROT_READ | PROT_WRITE,
 272                                  MAP_ANONYMOUS | MAP_PRIVATE,
 273                                  -1, 0);
 274         else
 275                 new_arena = mremap(arena->p, arena->len,
 276                                    len, MREMAP_MAYMOVE);
 277         assert(new_arena != MAP_FAILED);
 278
 279         /*
 280          * re-used the same region ?
 281          */
 282         if (new_arena == arena->p)
 283                 return;
 284
 285         memcpy(new_arena, arena->p, arena->len);
 286         bzero(new_arena + arena->len, len - arena->len);
 287         arena->p = new_arena;
 288 }
 289
 290 /* Called with signals off and mutex locked */
 291 static void add_thread(void)
 292 {
 293         struct urcu_reader *urcu_reader_reg;
 294
 295         if (registry_arena.len
 296             < registry_arena.used + sizeof(struct urcu_reader))
 297                 resize_arena(&registry_arena,
 298                 max(registry_arena.len << 1, ARENA_INIT_ALLOC));
 299         /*
 300          * Find a free spot.
 301          */
 302         for (urcu_reader_reg = registry_arena.p;
 303              (void *)urcu_reader_reg < registry_arena.p + registry_arena.len;
 304              urcu_reader_reg++) {
 305                 if (!urcu_reader_reg->alloc)
 306                         break;
 307         }
 308         urcu_reader_reg->alloc = 1;
 309         registry_arena.used += sizeof(struct urcu_reader);
 310
 311         /* Add to registry */
 312         urcu_reader_reg->tid = pthread_self();
 313         assert(urcu_reader_reg->ctr == 0);
 314         list_add(&urcu_reader_reg->head, &registry);
 315         urcu_reader = urcu_reader_reg;
 316 }
 317
 318 /* Called with signals off and mutex locked */
 319 static void rcu_gc_registry(void)
 320 {
 321         struct urcu_reader *urcu_reader_reg;
 322         pthread_t tid;
 323         int ret;
 324
 325         for (urcu_reader_reg = registry_arena.p;
 326              (void *)urcu_reader_reg < registry_arena.p + registry_arena.len;
 327              urcu_reader_reg++) {
 328                 if (!urcu_reader_reg->alloc)
 329                         continue;
 330                 tid = urcu_reader_reg->tid;
 331                 ret = pthread_kill(tid, 0);
 332                 assert(ret != EINVAL);
 333                 if (ret == ESRCH) {
 334                         list_del(&urcu_reader_reg->head);
 335                         urcu_reader_reg->alloc = 0;
 336                         registry_arena.used -= sizeof(struct urcu_reader);
 337                 }
 338         }
 339 }
 340
 341 /* Disable signals, take mutex, add to registry */
 342 void rcu_bp_register(void)
 343 {
 344         sigset_t newmask, oldmask;
 345         int ret;
 346
 347         ret = sigemptyset(&newmask);
 348         assert(!ret);
 349         ret = pthread_sigmask(SIG_SETMASK, &newmask, &oldmask);
 350         assert(!ret);
 351
 352         /*
 353          * Check if a signal concurrently registered our thread since
 354          * the check in rcu_read_lock(). */
 355         if (urcu_reader)
 356                 goto end;
 357
 358         internal_urcu_lock();
 359         add_thread();
 360         internal_urcu_unlock();
 361 end:
 362         ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
 363         assert(!ret);
 364 }
 365
 366 void urcu_bp_exit()
 367 {
 368         munmap(registry_arena.p, registry_arena.len);
 369 }