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