libringbuffer/ring_buffer_backend.c

   1 /*
   2  * ring_buffer_backend.c
   3  *
   4  * Copyright (C) 2005-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   5  *
   6  * This library is free software; you can redistribute it and/or
   7  * modify it under the terms of the GNU Lesser General Public
   8  * License as published by the Free Software Foundation; only
   9  * version 2.1 of the License.
  10  *
  11  * This library is distributed in the hope that it will be useful,
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  14  * Lesser General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU Lesser General Public
  17  * License along with this library; if not, write to the Free Software
  18  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  19  */
  20
  21 #define _GNU_SOURCE
  22 #define _LGPL_SOURCE
  23 #include <unistd.h>
  24 #include <urcu/arch.h>
  25 #include <limits.h>
  26
  27 #include <lttng/ringbuffer-config.h>
  28 #include "vatomic.h"
  29 #include "backend.h"
  30 #include "frontend.h"
  31 #include "smp.h"
  32 #include "shm.h"
  33
  34 /**
  35  * lib_ring_buffer_backend_allocate - allocate a channel buffer
  36  * @config: ring buffer instance configuration
  37  * @buf: the buffer struct
  38  * @size: total size of the buffer
  39  * @num_subbuf: number of subbuffers
  40  * @extra_reader_sb: need extra subbuffer for reader
  41  */
  42 static
  43 int lib_ring_buffer_backend_allocate(const struct lttng_ust_lib_ring_buffer_config *config,
  44                                      struct lttng_ust_lib_ring_buffer_backend *bufb,
  45                                      size_t size, size_t num_subbuf,
  46                                      int extra_reader_sb,
  47                                      struct lttng_ust_shm_handle *handle,
  48                                      struct shm_object *shmobj)
  49 {
  50         struct channel_backend *chanb;
  51         unsigned long subbuf_size, mmap_offset = 0;
  52         unsigned long num_subbuf_alloc;
  53         unsigned long i;
  54         long page_size;
  55
  56         chanb = &shmp(handle, bufb->chan)->backend;
  57         if (!chanb)
  58                 return -EINVAL;
  59
  60         subbuf_size = chanb->subbuf_size;
  61         num_subbuf_alloc = num_subbuf;
  62
  63         if (extra_reader_sb)
  64                 num_subbuf_alloc++;
  65
  66         page_size = sysconf(_SC_PAGE_SIZE);
  67         if (page_size <= 0) {
  68                 goto page_size_error;
  69         }
  70
  71         align_shm(shmobj, __alignof__(struct lttng_ust_lib_ring_buffer_backend_pages_shmp));
  72         set_shmp(bufb->array, zalloc_shm(shmobj,
  73                         sizeof(struct lttng_ust_lib_ring_buffer_backend_pages_shmp) * num_subbuf_alloc));
  74         if (caa_unlikely(!shmp(handle, bufb->array)))
  75                 goto array_error;
  76
  77         /*
  78          * This is the largest element (the buffer pages) which needs to
  79          * be aligned on page size.
  80          */
  81         align_shm(shmobj, page_size);
  82         set_shmp(bufb->memory_map, zalloc_shm(shmobj,
  83                         subbuf_size * num_subbuf_alloc));
  84         if (caa_unlikely(!shmp(handle, bufb->memory_map)))
  85                 goto memory_map_error;
  86
  87         /* Allocate backend pages array elements */
  88         for (i = 0; i < num_subbuf_alloc; i++) {
  89                 align_shm(shmobj, __alignof__(struct lttng_ust_lib_ring_buffer_backend_pages));
  90                 set_shmp(shmp_index(handle, bufb->array, i)->shmp,
  91                         zalloc_shm(shmobj,
  92                                 sizeof(struct lttng_ust_lib_ring_buffer_backend_pages)));
  93                 if (!shmp(handle, shmp_index(handle, bufb->array, i)->shmp))
  94                         goto free_array;
  95         }
  96
  97         /* Allocate write-side subbuffer table */
  98         align_shm(shmobj, __alignof__(struct lttng_ust_lib_ring_buffer_backend_subbuffer));
  99         set_shmp(bufb->buf_wsb, zalloc_shm(shmobj,
 100                                 sizeof(struct lttng_ust_lib_ring_buffer_backend_subbuffer)
 101                                 * num_subbuf));
 102         if (caa_unlikely(!shmp(handle, bufb->buf_wsb)))
 103                 goto free_array;
 104
 105         for (i = 0; i < num_subbuf; i++) {
 106                 struct lttng_ust_lib_ring_buffer_backend_subbuffer *sb;
 107
 108                 sb = shmp_index(handle, bufb->buf_wsb, i);
 109                 if (!sb)
 110                         goto free_array;
 111                 sb->id = subbuffer_id(config, 0, 1, i);
 112         }
 113
 114         /* Assign read-side subbuffer table */
 115         if (extra_reader_sb)
 116                 bufb->buf_rsb.id = subbuffer_id(config, 0, 1,
 117                                                 num_subbuf_alloc - 1);
 118         else
 119                 bufb->buf_rsb.id = subbuffer_id(config, 0, 1, 0);
 120
 121         /* Allocate subbuffer packet counter table */
 122         align_shm(shmobj, __alignof__(struct lttng_ust_lib_ring_buffer_backend_subbuffer));
 123         set_shmp(bufb->buf_cnt, zalloc_shm(shmobj,
 124                                 sizeof(struct lttng_ust_lib_ring_buffer_backend_counts)
 125                                 * num_subbuf));
 126         if (caa_unlikely(!shmp(handle, bufb->buf_cnt)))
 127                 goto free_wsb;
 128
 129         /* Assign pages to page index */
 130         for (i = 0; i < num_subbuf_alloc; i++) {
 131                 struct lttng_ust_lib_ring_buffer_backend_pages_shmp *sbp;
 132                 struct lttng_ust_lib_ring_buffer_backend_pages *pages;
 133                 struct shm_ref ref;
 134
 135                 ref.index = bufb->memory_map._ref.index;
 136                 ref.offset = bufb->memory_map._ref.offset;
 137                 ref.offset += i * subbuf_size;
 138
 139                 sbp = shmp_index(handle, bufb->array, i);
 140                 if (!sbp)
 141                         goto free_array;
 142                 pages = shmp(handle, sbp->shmp);
 143                 if (!pages)
 144                         goto free_array;
 145                 set_shmp(pages->p, ref);
 146                 if (config->output == RING_BUFFER_MMAP) {
 147                         pages->mmap_offset = mmap_offset;
 148                         mmap_offset += subbuf_size;
 149                 }
 150         }
 151         return 0;
 152
 153 free_wsb:
 154         /* bufb->buf_wsb will be freed by shm teardown */
 155 free_array:
 156         /* bufb->array[i] will be freed by shm teardown */
 157 memory_map_error:
 158         /* bufb->array will be freed by shm teardown */
 159 array_error:
 160 page_size_error:
 161         return -ENOMEM;
 162 }
 163
 164 int lib_ring_buffer_backend_create(struct lttng_ust_lib_ring_buffer_backend *bufb,
 165                                    struct channel_backend *chanb, int cpu,
 166                                    struct lttng_ust_shm_handle *handle,
 167                                    struct shm_object *shmobj)
 168 {
 169         const struct lttng_ust_lib_ring_buffer_config *config = &chanb->config;
 170
 171         set_shmp(bufb->chan, handle->chan._ref);
 172         bufb->cpu = cpu;
 173
 174         return lib_ring_buffer_backend_allocate(config, bufb, chanb->buf_size,
 175                                                 chanb->num_subbuf,
 176                                                 chanb->extra_reader_sb,
 177                                                 handle, shmobj);
 178 }
 179
 180 void lib_ring_buffer_backend_reset(struct lttng_ust_lib_ring_buffer_backend *bufb,
 181                                    struct lttng_ust_shm_handle *handle)
 182 {
 183         struct channel_backend *chanb;
 184         const struct lttng_ust_lib_ring_buffer_config *config;
 185         unsigned long num_subbuf_alloc;
 186         unsigned int i;
 187
 188         chanb = &shmp(handle, bufb->chan)->backend;
 189         if (!chanb)
 190                 abort();
 191         config = &chanb->config;
 192
 193         num_subbuf_alloc = chanb->num_subbuf;
 194         if (chanb->extra_reader_sb)
 195                 num_subbuf_alloc++;
 196
 197         for (i = 0; i < chanb->num_subbuf; i++) {
 198                 struct lttng_ust_lib_ring_buffer_backend_subbuffer *sb;
 199
 200                 sb = shmp_index(handle, bufb->buf_wsb, i);
 201                 if (!sb)
 202                         abort();
 203                 sb->id = subbuffer_id(config, 0, 1, i);
 204         }
 205         if (chanb->extra_reader_sb)
 206                 bufb->buf_rsb.id = subbuffer_id(config, 0, 1,
 207                                                 num_subbuf_alloc - 1);
 208         else
 209                 bufb->buf_rsb.id = subbuffer_id(config, 0, 1, 0);
 210
 211         for (i = 0; i < num_subbuf_alloc; i++) {
 212                 struct lttng_ust_lib_ring_buffer_backend_pages_shmp *sbp;
 213                 struct lttng_ust_lib_ring_buffer_backend_pages *pages;
 214
 215                 sbp = shmp_index(handle, bufb->array, i);
 216                 if (!sbp)
 217                         abort();
 218                 pages = shmp(handle, sbp->shmp);
 219                 if (!pages)
 220                         abort();
 221                 /* Don't reset mmap_offset */
 222                 v_set(config, &pages->records_commit, 0);
 223                 v_set(config, &pages->records_unread, 0);
 224                 pages->data_size = 0;
 225                 /* Don't reset backend page and virt addresses */
 226         }
 227         /* Don't reset num_pages_per_subbuf, cpu, allocated */
 228         v_set(config, &bufb->records_read, 0);
 229 }
 230
 231 /*
 232  * The frontend is responsible for also calling ring_buffer_backend_reset for
 233  * each buffer when calling channel_backend_reset.
 234  */
 235 void channel_backend_reset(struct channel_backend *chanb)
 236 {
 237         struct channel *chan = caa_container_of(chanb, struct channel, backend);
 238         const struct lttng_ust_lib_ring_buffer_config *config = &chanb->config;
 239
 240         /*
 241          * Don't reset buf_size, subbuf_size, subbuf_size_order,
 242          * num_subbuf_order, buf_size_order, extra_reader_sb, num_subbuf,
 243          * priv, notifiers, config, cpumask and name.
 244          */
 245         chanb->start_tsc = config->cb.ring_buffer_clock_read(chan);
 246 }
 247
 248 /**
 249  * channel_backend_init - initialize a channel backend
 250  * @chanb: channel backend
 251  * @name: channel name
 252  * @config: client ring buffer configuration
 253  * @parent: dentry of parent directory, %NULL for root directory
 254  * @subbuf_size: size of sub-buffers (> page size, power of 2)
 255  * @num_subbuf: number of sub-buffers (power of 2)
 256  * @lttng_ust_shm_handle: shared memory handle
 257  * @stream_fds: stream file descriptors.
 258  *
 259  * Returns channel pointer if successful, %NULL otherwise.
 260  *
 261  * Creates per-cpu channel buffers using the sizes and attributes
 262  * specified.  The created channel buffer files will be named
 263  * name_0...name_N-1.  File permissions will be %S_IRUSR.
 264  *
 265  * Called with CPU hotplug disabled.
 266  */
 267 int channel_backend_init(struct channel_backend *chanb,
 268                          const char *name,
 269                          const struct lttng_ust_lib_ring_buffer_config *config,
 270                          size_t subbuf_size, size_t num_subbuf,
 271                          struct lttng_ust_shm_handle *handle,
 272                          const int *stream_fds)
 273 {
 274         struct channel *chan = caa_container_of(chanb, struct channel, backend);
 275         unsigned int i;
 276         int ret;
 277         size_t shmsize = 0, num_subbuf_alloc;
 278         long page_size;
 279
 280         if (!name)
 281                 return -EPERM;
 282
 283         page_size = sysconf(_SC_PAGE_SIZE);
 284         if (page_size <= 0) {
 285                 return -ENOMEM;
 286         }
 287         /* Check that the subbuffer size is larger than a page. */
 288         if (subbuf_size < page_size)
 289                 return -EINVAL;
 290
 291         /*
 292          * Make sure the number of subbuffers and subbuffer size are
 293          * power of 2, and nonzero.
 294          */
 295         if (!subbuf_size || (subbuf_size & (subbuf_size - 1)))
 296                 return -EINVAL;
 297         if (!num_subbuf || (num_subbuf & (num_subbuf - 1)))
 298                 return -EINVAL;
 299         /*
 300          * Overwrite mode buffers require at least 2 subbuffers per
 301          * buffer.
 302          */
 303         if (config->mode == RING_BUFFER_OVERWRITE && num_subbuf < 2)
 304                 return -EINVAL;
 305
 306         ret = subbuffer_id_check_index(config, num_subbuf);
 307         if (ret)
 308                 return ret;
 309
 310         chanb->buf_size = num_subbuf * subbuf_size;
 311         chanb->subbuf_size = subbuf_size;
 312         chanb->buf_size_order = get_count_order(chanb->buf_size);
 313         chanb->subbuf_size_order = get_count_order(subbuf_size);
 314         chanb->num_subbuf_order = get_count_order(num_subbuf);
 315         chanb->extra_reader_sb =
 316                         (config->mode == RING_BUFFER_OVERWRITE) ? 1 : 0;
 317         chanb->num_subbuf = num_subbuf;
 318         strncpy(chanb->name, name, NAME_MAX);
 319         chanb->name[NAME_MAX - 1] = '\0';
 320         memcpy(&chanb->config, config, sizeof(*config));
 321
 322         /* Per-cpu buffer size: control (prior to backend) */
 323         shmsize = offset_align(shmsize, __alignof__(struct lttng_ust_lib_ring_buffer));
 324         shmsize += sizeof(struct lttng_ust_lib_ring_buffer);
 325
 326         /* Per-cpu buffer size: backend */
 327         /* num_subbuf + 1 is the worse case */
 328         num_subbuf_alloc = num_subbuf + 1;
 329         shmsize += offset_align(shmsize, __alignof__(struct lttng_ust_lib_ring_buffer_backend_pages_shmp));
 330         shmsize += sizeof(struct lttng_ust_lib_ring_buffer_backend_pages_shmp) * num_subbuf_alloc;
 331         shmsize += offset_align(shmsize, page_size);
 332         shmsize += subbuf_size * num_subbuf_alloc;
 333         shmsize += offset_align(shmsize, __alignof__(struct lttng_ust_lib_ring_buffer_backend_pages));
 334         shmsize += sizeof(struct lttng_ust_lib_ring_buffer_backend_pages) * num_subbuf_alloc;
 335         shmsize += offset_align(shmsize, __alignof__(struct lttng_ust_lib_ring_buffer_backend_subbuffer));
 336         shmsize += sizeof(struct lttng_ust_lib_ring_buffer_backend_subbuffer) * num_subbuf;
 337         /* Per-cpu buffer size: control (after backend) */
 338         shmsize += offset_align(shmsize, __alignof__(struct commit_counters_hot));
 339         shmsize += sizeof(struct commit_counters_hot) * num_subbuf;
 340         shmsize += offset_align(shmsize, __alignof__(struct commit_counters_cold));
 341         shmsize += sizeof(struct commit_counters_cold) * num_subbuf;
 342
 343         if (config->alloc == RING_BUFFER_ALLOC_PER_CPU) {
 344                 struct lttng_ust_lib_ring_buffer *buf;
 345                 /*
 346                  * We need to allocate for all possible cpus.
 347                  */
 348                 for_each_possible_cpu(i) {
 349                         struct shm_object *shmobj;
 350
 351                         shmobj = shm_object_table_alloc(handle->table, shmsize,
 352                                         SHM_OBJECT_SHM, stream_fds[i]);
 353                         if (!shmobj)
 354                                 goto end;
 355                         align_shm(shmobj, __alignof__(struct lttng_ust_lib_ring_buffer));
 356                         set_shmp(chanb->buf[i].shmp, zalloc_shm(shmobj, sizeof(struct lttng_ust_lib_ring_buffer)));
 357                         buf = shmp(handle, chanb->buf[i].shmp);
 358                         if (!buf)
 359                                 goto end;
 360                         set_shmp(buf->self, chanb->buf[i].shmp._ref);
 361                         ret = lib_ring_buffer_create(buf, chanb, i,
 362                                         handle, shmobj);
 363                         if (ret)
 364                                 goto free_bufs; /* cpu hotplug locked */
 365                 }
 366         } else {
 367                 struct shm_object *shmobj;
 368                 struct lttng_ust_lib_ring_buffer *buf;
 369
 370                 shmobj = shm_object_table_alloc(handle->table, shmsize,
 371                                         SHM_OBJECT_SHM, stream_fds[0]);
 372                 if (!shmobj)
 373                         goto end;
 374                 align_shm(shmobj, __alignof__(struct lttng_ust_lib_ring_buffer));
 375                 set_shmp(chanb->buf[0].shmp, zalloc_shm(shmobj, sizeof(struct lttng_ust_lib_ring_buffer)));
 376                 buf = shmp(handle, chanb->buf[0].shmp);
 377                 if (!buf)
 378                         goto end;
 379                 set_shmp(buf->self, chanb->buf[0].shmp._ref);
 380                 ret = lib_ring_buffer_create(buf, chanb, -1,
 381                                         handle, shmobj);
 382                 if (ret)
 383                         goto free_bufs;
 384         }
 385         chanb->start_tsc = config->cb.ring_buffer_clock_read(chan);
 386
 387         return 0;
 388
 389 free_bufs:
 390         /* We only free the buffer data upon shm teardown */
 391 end:
 392         return -ENOMEM;
 393 }
 394
 395 /**
 396  * channel_backend_free - destroy the channel
 397  * @chan: the channel
 398  *
 399  * Destroy all channel buffers and frees the channel.
 400  */
 401 void channel_backend_free(struct channel_backend *chanb,
 402                           struct lttng_ust_shm_handle *handle)
 403 {
 404         /* SHM teardown takes care of everything */
 405 }
 406
 407 /**
 408  * lib_ring_buffer_read - read data from ring_buffer_buffer.
 409  * @bufb : buffer backend
 410  * @offset : offset within the buffer
 411  * @dest : destination address
 412  * @len : length to copy to destination
 413  *
 414  * Should be protected by get_subbuf/put_subbuf.
 415  * Returns the length copied.
 416  */
 417 size_t lib_ring_buffer_read(struct lttng_ust_lib_ring_buffer_backend *bufb, size_t offset,
 418                             void *dest, size_t len, struct lttng_ust_shm_handle *handle)
 419 {
 420         struct channel_backend *chanb;
 421         const struct lttng_ust_lib_ring_buffer_config *config;
 422         ssize_t orig_len;
 423         struct lttng_ust_lib_ring_buffer_backend_pages_shmp *rpages;
 424         unsigned long sb_bindex, id;
 425         void *src;
 426
 427         chanb = &shmp(handle, bufb->chan)->backend;
 428         if (!chanb)
 429                 return 0;
 430         config = &chanb->config;
 431         orig_len = len;
 432         offset &= chanb->buf_size - 1;
 433
 434         if (caa_unlikely(!len))
 435                 return 0;
 436         id = bufb->buf_rsb.id;
 437         sb_bindex = subbuffer_id_get_index(config, id);
 438         rpages = shmp_index(handle, bufb->array, sb_bindex);
 439         /*
 440          * Underlying layer should never ask for reads across
 441          * subbuffers.
 442          */
 443         CHAN_WARN_ON(chanb, offset >= chanb->buf_size);
 444         CHAN_WARN_ON(chanb, config->mode == RING_BUFFER_OVERWRITE
 445                      && subbuffer_id_is_noref(config, id));
 446         src = shmp_index(handle, shmp(handle, rpages->shmp)->p,
 447                         offset & (chanb->subbuf_size - 1));
 448         if (caa_unlikely(!src))
 449                 return 0;
 450         memcpy(dest, src, len);
 451         return orig_len;
 452 }
 453
 454 /**
 455  * lib_ring_buffer_read_cstr - read a C-style string from ring_buffer.
 456  * @bufb : buffer backend
 457  * @offset : offset within the buffer
 458  * @dest : destination address
 459  * @len : destination's length
 460  *
 461  * Return string's length, or -EINVAL on error.
 462  * Should be protected by get_subbuf/put_subbuf.
 463  * Destination length should be at least 1 to hold '\0'.
 464  */
 465 int lib_ring_buffer_read_cstr(struct lttng_ust_lib_ring_buffer_backend *bufb, size_t offset,
 466                               void *dest, size_t len, struct lttng_ust_shm_handle *handle)
 467 {
 468         struct channel_backend *chanb;
 469         const struct lttng_ust_lib_ring_buffer_config *config;
 470         ssize_t string_len, orig_offset;
 471         char *str;
 472         struct lttng_ust_lib_ring_buffer_backend_pages_shmp *rpages;
 473         unsigned long sb_bindex, id;
 474
 475         chanb = &shmp(handle, bufb->chan)->backend;
 476         if (!chanb)
 477                 return -EINVAL;
 478         config = &chanb->config;
 479         if (caa_unlikely(!len))
 480                 return -EINVAL;
 481         offset &= chanb->buf_size - 1;
 482         orig_offset = offset;
 483         id = bufb->buf_rsb.id;
 484         sb_bindex = subbuffer_id_get_index(config, id);
 485         rpages = shmp_index(handle, bufb->array, sb_bindex);
 486         /*
 487          * Underlying layer should never ask for reads across
 488          * subbuffers.
 489          */
 490         CHAN_WARN_ON(chanb, offset >= chanb->buf_size);
 491         CHAN_WARN_ON(chanb, config->mode == RING_BUFFER_OVERWRITE
 492                      && subbuffer_id_is_noref(config, id));
 493         str = shmp_index(handle, shmp(handle, rpages->shmp)->p, offset & (chanb->subbuf_size - 1));
 494         if (caa_unlikely(!str))
 495                 return -EINVAL;
 496         string_len = strnlen(str, len);
 497         if (dest && len) {
 498                 memcpy(dest, str, string_len);
 499                 ((char *)dest)[0] = 0;
 500         }
 501         return offset - orig_offset;
 502 }
 503
 504 /**
 505  * lib_ring_buffer_read_offset_address - get address of a buffer location
 506  * @bufb : buffer backend
 507  * @offset : offset within the buffer.
 508  *
 509  * Return the address where a given offset is located (for read).
 510  * Should be used to get the current subbuffer header pointer. Given we know
 511  * it's never on a page boundary, it's safe to read/write directly
 512  * from/to this address, as long as the read/write is never bigger than
 513  * a page size.
 514  */
 515 void *lib_ring_buffer_read_offset_address(struct lttng_ust_lib_ring_buffer_backend *bufb,
 516                                           size_t offset,
 517                                           struct lttng_ust_shm_handle *handle)
 518 {
 519         struct lttng_ust_lib_ring_buffer_backend_pages_shmp *rpages;
 520         struct channel_backend *chanb;
 521         const struct lttng_ust_lib_ring_buffer_config *config;
 522         unsigned long sb_bindex, id;
 523
 524         chanb = &shmp(handle, bufb->chan)->backend;
 525         if (!chanb)
 526                 return NULL;
 527         config = &chanb->config;
 528         offset &= chanb->buf_size - 1;
 529         id = bufb->buf_rsb.id;
 530         sb_bindex = subbuffer_id_get_index(config, id);
 531         rpages = shmp_index(handle, bufb->array, sb_bindex);
 532         CHAN_WARN_ON(chanb, config->mode == RING_BUFFER_OVERWRITE
 533                      && subbuffer_id_is_noref(config, id));
 534         return shmp_index(handle, shmp(handle, rpages->shmp)->p, offset & (chanb->subbuf_size - 1));
 535 }
 536
 537 /**
 538  * lib_ring_buffer_offset_address - get address of a location within the buffer
 539  * @bufb : buffer backend
 540  * @offset : offset within the buffer.
 541  *
 542  * Return the address where a given offset is located.
 543  * Should be used to get the current subbuffer header pointer. Given we know
 544  * it's always at the beginning of a page, it's safe to write directly to this
 545  * address, as long as the write is never bigger than a page size.
 546  */
 547 void *lib_ring_buffer_offset_address(struct lttng_ust_lib_ring_buffer_backend *bufb,
 548                                      size_t offset,
 549                                      struct lttng_ust_shm_handle *handle)
 550 {
 551         size_t sbidx;
 552         struct lttng_ust_lib_ring_buffer_backend_pages_shmp *rpages;
 553         struct channel_backend *chanb;
 554         const struct lttng_ust_lib_ring_buffer_config *config;
 555         unsigned long sb_bindex, id;
 556         struct lttng_ust_lib_ring_buffer_backend_subbuffer *sb;
 557
 558         chanb = &shmp(handle, bufb->chan)->backend;
 559         if (!chanb)
 560                 return NULL;
 561         config = &chanb->config;
 562         offset &= chanb->buf_size - 1;
 563         sbidx = offset >> chanb->subbuf_size_order;
 564         sb = shmp_index(handle, bufb->buf_wsb, sbidx);
 565         if (!sb)
 566                 return NULL;
 567         id = sb->id;
 568         sb_bindex = subbuffer_id_get_index(config, id);
 569         rpages = shmp_index(handle, bufb->array, sb_bindex);
 570         CHAN_WARN_ON(chanb, config->mode == RING_BUFFER_OVERWRITE
 571                      && subbuffer_id_is_noref(config, id));
 572         return shmp_index(handle, shmp(handle, rpages->shmp)->p, offset & (chanb->subbuf_size - 1));
 573 }