libringbuffer/backend_internal.h

   1 #ifndef _LTTNG_RING_BUFFER_BACKEND_INTERNAL_H
   2 #define _LTTNG_RING_BUFFER_BACKEND_INTERNAL_H
   3
   4 /*
   5  * libringbuffer/backend_internal.h
   6  *
   7  * Ring buffer backend (internal helpers).
   8  *
   9  * Copyright (C) 2005-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  10  *
  11  * This library is free software; you can redistribute it and/or
  12  * modify it under the terms of the GNU Lesser General Public
  13  * License as published by the Free Software Foundation; only
  14  * version 2.1 of the License.
  15  *
  16  * This library is distributed in the hope that it will be useful,
  17  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  18  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  19  * Lesser General Public License for more details.
  20  *
  21  * You should have received a copy of the GNU Lesser General Public
  22  * License along with this library; if not, write to the Free Software
  23  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  24  */
  25
  26 #include <unistd.h>
  27 #include <urcu/compiler.h>
  28
  29 #include <lttng/ringbuffer-config.h>
  30 #include "backend_types.h"
  31 #include "frontend_types.h"
  32 #include "shm.h"
  33
  34 /* Ring buffer backend API presented to the frontend */
  35
  36 /* Ring buffer and channel backend create/free */
  37
  38 int lib_ring_buffer_backend_create(struct lttng_ust_lib_ring_buffer_backend *bufb,
  39                                    struct channel_backend *chan, int cpu,
  40                                    struct lttng_ust_shm_handle *handle,
  41                                    struct shm_object *shmobj);
  42 void channel_backend_unregister_notifiers(struct channel_backend *chanb);
  43 void lib_ring_buffer_backend_free(struct lttng_ust_lib_ring_buffer_backend *bufb);
  44 int channel_backend_init(struct channel_backend *chanb,
  45                          const char *name,
  46                          const struct lttng_ust_lib_ring_buffer_config *config,
  47                          size_t subbuf_size,
  48                          size_t num_subbuf, struct lttng_ust_shm_handle *handle,
  49                          const int *stream_fds);
  50 void channel_backend_free(struct channel_backend *chanb,
  51                           struct lttng_ust_shm_handle *handle);
  52
  53 void lib_ring_buffer_backend_reset(struct lttng_ust_lib_ring_buffer_backend *bufb,
  54                                    struct lttng_ust_shm_handle *handle);
  55 void channel_backend_reset(struct channel_backend *chanb);
  56
  57 int lib_ring_buffer_backend_init(void);
  58 void lib_ring_buffer_backend_exit(void);
  59
  60 extern void _lib_ring_buffer_write(struct lttng_ust_lib_ring_buffer_backend *bufb,
  61                                    size_t offset, const void *src, size_t len,
  62                                    ssize_t pagecpy);
  63
  64 /*
  65  * Subbuffer ID bits for overwrite mode. Need to fit within a single word to be
  66  * exchanged atomically.
  67  *
  68  * Top half word, except lowest bit, belongs to "offset", which is used to keep
  69  * to count the produced buffers.  For overwrite mode, this provides the
  70  * consumer with the capacity to read subbuffers in order, handling the
  71  * situation where producers would write up to 2^15 buffers (or 2^31 for 64-bit
  72  * systems) concurrently with a single execution of get_subbuf (between offset
  73  * sampling and subbuffer ID exchange).
  74  */
  75
  76 #define HALF_ULONG_BITS         (CAA_BITS_PER_LONG >> 1)
  77
  78 #define SB_ID_OFFSET_SHIFT      (HALF_ULONG_BITS + 1)
  79 #define SB_ID_OFFSET_COUNT      (1UL << SB_ID_OFFSET_SHIFT)
  80 #define SB_ID_OFFSET_MASK       (~(SB_ID_OFFSET_COUNT - 1))
  81 /*
  82  * Lowest bit of top word half belongs to noref. Used only for overwrite mode.
  83  */
  84 #define SB_ID_NOREF_SHIFT       (SB_ID_OFFSET_SHIFT - 1)
  85 #define SB_ID_NOREF_COUNT       (1UL << SB_ID_NOREF_SHIFT)
  86 #define SB_ID_NOREF_MASK        SB_ID_NOREF_COUNT
  87 /*
  88  * In overwrite mode: lowest half of word is used for index.
  89  * Limit of 2^16 subbuffers per buffer on 32-bit, 2^32 on 64-bit.
  90  * In producer-consumer mode: whole word used for index.
  91  */
  92 #define SB_ID_INDEX_SHIFT       0
  93 #define SB_ID_INDEX_COUNT       (1UL << SB_ID_INDEX_SHIFT)
  94 #define SB_ID_INDEX_MASK        (SB_ID_NOREF_COUNT - 1)
  95
  96 /*
  97  * Construct the subbuffer id from offset, index and noref. Use only the index
  98  * for producer-consumer mode (offset and noref are only used in overwrite
  99  * mode).
 100  */
 101 static inline
 102 unsigned long subbuffer_id(const struct lttng_ust_lib_ring_buffer_config *config,
 103                            unsigned long offset, unsigned long noref,
 104                            unsigned long index)
 105 {
 106         if (config->mode == RING_BUFFER_OVERWRITE)
 107                 return (offset << SB_ID_OFFSET_SHIFT)
 108                        | (noref << SB_ID_NOREF_SHIFT)
 109                        | index;
 110         else
 111                 return index;
 112 }
 113
 114 /*
 115  * Compare offset with the offset contained within id. Return 1 if the offset
 116  * bits are identical, else 0.
 117  */
 118 static inline
 119 int subbuffer_id_compare_offset(const struct lttng_ust_lib_ring_buffer_config *config,
 120                                 unsigned long id, unsigned long offset)
 121 {
 122         return (id & SB_ID_OFFSET_MASK) == (offset << SB_ID_OFFSET_SHIFT);
 123 }
 124
 125 static inline
 126 unsigned long subbuffer_id_get_index(const struct lttng_ust_lib_ring_buffer_config *config,
 127                                      unsigned long id)
 128 {
 129         if (config->mode == RING_BUFFER_OVERWRITE)
 130                 return id & SB_ID_INDEX_MASK;
 131         else
 132                 return id;
 133 }
 134
 135 static inline
 136 unsigned long subbuffer_id_is_noref(const struct lttng_ust_lib_ring_buffer_config *config,
 137                                     unsigned long id)
 138 {
 139         if (config->mode == RING_BUFFER_OVERWRITE)
 140                 return !!(id & SB_ID_NOREF_MASK);
 141         else
 142                 return 1;
 143 }
 144
 145 /*
 146  * Only used by reader on subbuffer ID it has exclusive access to. No volatile
 147  * needed.
 148  */
 149 static inline
 150 void subbuffer_id_set_noref(const struct lttng_ust_lib_ring_buffer_config *config,
 151                             unsigned long *id)
 152 {
 153         if (config->mode == RING_BUFFER_OVERWRITE)
 154                 *id |= SB_ID_NOREF_MASK;
 155 }
 156
 157 static inline
 158 void subbuffer_id_set_noref_offset(const struct lttng_ust_lib_ring_buffer_config *config,
 159                                    unsigned long *id, unsigned long offset)
 160 {
 161         unsigned long tmp;
 162
 163         if (config->mode == RING_BUFFER_OVERWRITE) {
 164                 tmp = *id;
 165                 tmp &= ~SB_ID_OFFSET_MASK;
 166                 tmp |= offset << SB_ID_OFFSET_SHIFT;
 167                 tmp |= SB_ID_NOREF_MASK;
 168                 /* Volatile store, read concurrently by readers. */
 169                 CMM_ACCESS_ONCE(*id) = tmp;
 170         }
 171 }
 172
 173 /* No volatile access, since already used locally */
 174 static inline
 175 void subbuffer_id_clear_noref(const struct lttng_ust_lib_ring_buffer_config *config,
 176                               unsigned long *id)
 177 {
 178         if (config->mode == RING_BUFFER_OVERWRITE)
 179                 *id &= ~SB_ID_NOREF_MASK;
 180 }
 181
 182 /*
 183  * For overwrite mode, cap the number of subbuffers per buffer to:
 184  * 2^16 on 32-bit architectures
 185  * 2^32 on 64-bit architectures
 186  * This is required to fit in the index part of the ID. Return 0 on success,
 187  * -EPERM on failure.
 188  */
 189 static inline
 190 int subbuffer_id_check_index(const struct lttng_ust_lib_ring_buffer_config *config,
 191                              unsigned long num_subbuf)
 192 {
 193         if (config->mode == RING_BUFFER_OVERWRITE)
 194                 return (num_subbuf > (1UL << HALF_ULONG_BITS)) ? -EPERM : 0;
 195         else
 196                 return 0;
 197 }
 198
 199 static inline
 200 int lib_ring_buffer_backend_get_pages(const struct lttng_ust_lib_ring_buffer_config *config,
 201                         struct lttng_ust_lib_ring_buffer_ctx *ctx,
 202                         struct lttng_ust_lib_ring_buffer_backend_pages **backend_pages)
 203 {
 204         struct lttng_ust_lib_ring_buffer_backend *bufb = &ctx->buf->backend;
 205         struct channel_backend *chanb = &ctx->chan->backend;
 206         struct lttng_ust_shm_handle *handle = ctx->handle;
 207         size_t sbidx;
 208         size_t offset = ctx->buf_offset;
 209         struct lttng_ust_lib_ring_buffer_backend_subbuffer *wsb;
 210         struct lttng_ust_lib_ring_buffer_backend_pages_shmp *rpages;
 211         unsigned long sb_bindex, id;
 212         struct lttng_ust_lib_ring_buffer_backend_pages *_backend_pages;
 213
 214         offset &= chanb->buf_size - 1;
 215         sbidx = offset >> chanb->subbuf_size_order;
 216         wsb = shmp_index(handle, bufb->buf_wsb, sbidx);
 217         if (caa_unlikely(!wsb))
 218                 return -1;
 219         id = wsb->id;
 220         sb_bindex = subbuffer_id_get_index(config, id);
 221         rpages = shmp_index(handle, bufb->array, sb_bindex);
 222         if (caa_unlikely(!rpages))
 223                 return -1;
 224         CHAN_WARN_ON(ctx->chan,
 225                      config->mode == RING_BUFFER_OVERWRITE
 226                      && subbuffer_id_is_noref(config, id));
 227         _backend_pages = shmp(handle, rpages->shmp);
 228         if (caa_unlikely(!_backend_pages))
 229                 return -1;
 230         *backend_pages = _backend_pages;
 231         return 0;
 232 }
 233
 234 /* Get backend pages from cache. */
 235 static inline
 236 struct lttng_ust_lib_ring_buffer_backend_pages *
 237         lib_ring_buffer_get_backend_pages_from_ctx(const struct lttng_ust_lib_ring_buffer_config *config,
 238                 struct lttng_ust_lib_ring_buffer_ctx *ctx)
 239 {
 240         if (caa_unlikely(ctx->ctx_len
 241                         < sizeof(struct lttng_ust_lib_ring_buffer_ctx)))
 242                 return NULL;
 243         return ctx->backend_pages;
 244 }
 245
 246 /*
 247  * The ring buffer can count events recorded and overwritten per buffer,
 248  * but it is disabled by default due to its performance overhead.
 249  */
 250 #ifdef LTTNG_RING_BUFFER_COUNT_EVENTS
 251 static inline
 252 void subbuffer_count_record(const struct lttng_ust_lib_ring_buffer_config *config,
 253                             const struct lttng_ust_lib_ring_buffer_ctx *ctx,
 254                             struct lttng_ust_lib_ring_buffer_backend *bufb,
 255                             unsigned long idx, struct lttng_ust_shm_handle *handle)
 256 {
 257         struct lttng_ust_lib_ring_buffer_backend_pages *backend_pages;
 258
 259         backend_pages = lib_ring_buffer_get_backend_pages_from_ctx(config, ctx);
 260         if (caa_unlikely(!backend_pages)) {
 261                 if (lib_ring_buffer_backend_get_pages(config, ctx, &backend_pages))
 262                         return;
 263         }
 264         v_inc(config, &backend_pages->records_commit);
 265 }
 266 #else /* LTTNG_RING_BUFFER_COUNT_EVENTS */
 267 static inline
 268 void subbuffer_count_record(const struct lttng_ust_lib_ring_buffer_config *config,
 269                             const struct lttng_ust_lib_ring_buffer_ctx *ctx,
 270                             struct lttng_ust_lib_ring_buffer_backend *bufb,
 271                             unsigned long idx, struct lttng_ust_shm_handle *handle)
 272 {
 273 }
 274 #endif /* #else LTTNG_RING_BUFFER_COUNT_EVENTS */
 275
 276 /*
 277  * Reader has exclusive subbuffer access for record consumption. No need to
 278  * perform the decrement atomically.
 279  */
 280 static inline
 281 void subbuffer_consume_record(const struct lttng_ust_lib_ring_buffer_config *config,
 282                               struct lttng_ust_lib_ring_buffer_backend *bufb,
 283                               struct lttng_ust_shm_handle *handle)
 284 {
 285         unsigned long sb_bindex;
 286         struct channel *chan;
 287         struct lttng_ust_lib_ring_buffer_backend_pages_shmp *pages_shmp;
 288         struct lttng_ust_lib_ring_buffer_backend_pages *backend_pages;
 289
 290         sb_bindex = subbuffer_id_get_index(config, bufb->buf_rsb.id);
 291         chan = shmp(handle, bufb->chan);
 292         if (!chan)
 293                 return;
 294         pages_shmp = shmp_index(handle, bufb->array, sb_bindex);
 295         if (!pages_shmp)
 296                 return;
 297         backend_pages = shmp(handle, pages_shmp->shmp);
 298         if (!backend_pages)
 299                 return;
 300         CHAN_WARN_ON(chan, !v_read(config, &backend_pages->records_unread));
 301         /* Non-atomic decrement protected by exclusive subbuffer access */
 302         _v_dec(config, &backend_pages->records_unread);
 303         v_inc(config, &bufb->records_read);
 304 }
 305
 306 static inline
 307 unsigned long subbuffer_get_records_count(
 308                                 const struct lttng_ust_lib_ring_buffer_config *config,
 309                                 struct lttng_ust_lib_ring_buffer_backend *bufb,
 310                                 unsigned long idx,
 311                                 struct lttng_ust_shm_handle *handle)
 312 {
 313         unsigned long sb_bindex;
 314         struct lttng_ust_lib_ring_buffer_backend_subbuffer *wsb;
 315         struct lttng_ust_lib_ring_buffer_backend_pages_shmp *rpages;
 316         struct lttng_ust_lib_ring_buffer_backend_pages *backend_pages;
 317
 318         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 319         if (!wsb)
 320                 return 0;
 321         sb_bindex = subbuffer_id_get_index(config, wsb->id);
 322         rpages = shmp_index(handle, bufb->array, sb_bindex);
 323         if (!rpages)
 324                 return 0;
 325         backend_pages = shmp(handle, rpages->shmp);
 326         if (!backend_pages)
 327                 return 0;
 328         return v_read(config, &backend_pages->records_commit);
 329 }
 330
 331 /*
 332  * Must be executed at subbuffer delivery when the writer has _exclusive_
 333  * subbuffer access. See lib_ring_buffer_check_deliver() for details.
 334  * lib_ring_buffer_get_records_count() must be called to get the records
 335  * count before this function, because it resets the records_commit
 336  * count.
 337  */
 338 static inline
 339 unsigned long subbuffer_count_records_overrun(
 340                                 const struct lttng_ust_lib_ring_buffer_config *config,
 341                                 struct lttng_ust_lib_ring_buffer_backend *bufb,
 342                                 unsigned long idx,
 343                                 struct lttng_ust_shm_handle *handle)
 344 {
 345         unsigned long overruns, sb_bindex;
 346         struct lttng_ust_lib_ring_buffer_backend_subbuffer *wsb;
 347         struct lttng_ust_lib_ring_buffer_backend_pages_shmp *rpages;
 348         struct lttng_ust_lib_ring_buffer_backend_pages *backend_pages;
 349
 350         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 351         if (!wsb)
 352                 return 0;
 353         sb_bindex = subbuffer_id_get_index(config, wsb->id);
 354         rpages = shmp_index(handle, bufb->array, sb_bindex);
 355         if (!rpages)
 356                 return 0;
 357         backend_pages = shmp(handle, rpages->shmp);
 358         if (!backend_pages)
 359                 return 0;
 360         overruns = v_read(config, &backend_pages->records_unread);
 361         v_set(config, &backend_pages->records_unread,
 362               v_read(config, &backend_pages->records_commit));
 363         v_set(config, &backend_pages->records_commit, 0);
 364
 365         return overruns;
 366 }
 367
 368 static inline
 369 void subbuffer_set_data_size(const struct lttng_ust_lib_ring_buffer_config *config,
 370                              struct lttng_ust_lib_ring_buffer_backend *bufb,
 371                              unsigned long idx,
 372                              unsigned long data_size,
 373                              struct lttng_ust_shm_handle *handle)
 374 {
 375         unsigned long sb_bindex;
 376         struct lttng_ust_lib_ring_buffer_backend_subbuffer *wsb;
 377         struct lttng_ust_lib_ring_buffer_backend_pages_shmp *rpages;
 378         struct lttng_ust_lib_ring_buffer_backend_pages *backend_pages;
 379
 380         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 381         if (!wsb)
 382                 return;
 383         sb_bindex = subbuffer_id_get_index(config, wsb->id);
 384         rpages = shmp_index(handle, bufb->array, sb_bindex);
 385         if (!rpages)
 386                 return;
 387         backend_pages = shmp(handle, rpages->shmp);
 388         if (!backend_pages)
 389                 return;
 390         backend_pages->data_size = data_size;
 391 }
 392
 393 static inline
 394 unsigned long subbuffer_get_read_data_size(
 395                                 const struct lttng_ust_lib_ring_buffer_config *config,
 396                                 struct lttng_ust_lib_ring_buffer_backend *bufb,
 397                                 struct lttng_ust_shm_handle *handle)
 398 {
 399         unsigned long sb_bindex;
 400         struct lttng_ust_lib_ring_buffer_backend_pages_shmp *pages_shmp;
 401         struct lttng_ust_lib_ring_buffer_backend_pages *backend_pages;
 402
 403         sb_bindex = subbuffer_id_get_index(config, bufb->buf_rsb.id);
 404         pages_shmp = shmp_index(handle, bufb->array, sb_bindex);
 405         if (!pages_shmp)
 406                 return 0;
 407         backend_pages = shmp(handle, pages_shmp->shmp);
 408         if (!backend_pages)
 409                 return 0;
 410         return backend_pages->data_size;
 411 }
 412
 413 static inline
 414 unsigned long subbuffer_get_data_size(
 415                                 const struct lttng_ust_lib_ring_buffer_config *config,
 416                                 struct lttng_ust_lib_ring_buffer_backend *bufb,
 417                                 unsigned long idx,
 418                                 struct lttng_ust_shm_handle *handle)
 419 {
 420         unsigned long sb_bindex;
 421         struct lttng_ust_lib_ring_buffer_backend_subbuffer *wsb;
 422         struct lttng_ust_lib_ring_buffer_backend_pages_shmp *rpages;
 423         struct lttng_ust_lib_ring_buffer_backend_pages *backend_pages;
 424
 425         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 426         if (!wsb)
 427                 return 0;
 428         sb_bindex = subbuffer_id_get_index(config, wsb->id);
 429         rpages = shmp_index(handle, bufb->array, sb_bindex);
 430         if (!rpages)
 431                 return 0;
 432         backend_pages = shmp(handle, rpages->shmp);
 433         if (!backend_pages)
 434                 return 0;
 435         return backend_pages->data_size;
 436 }
 437
 438 static inline
 439 void subbuffer_inc_packet_count(const struct lttng_ust_lib_ring_buffer_config *config,
 440                 struct lttng_ust_lib_ring_buffer_backend *bufb,
 441                 unsigned long idx, struct lttng_ust_shm_handle *handle)
 442 {
 443         struct lttng_ust_lib_ring_buffer_backend_counts *counts;
 444
 445         counts = shmp_index(handle, bufb->buf_cnt, idx);
 446         if (!counts)
 447                 return;
 448         counts->seq_cnt++;
 449 }
 450
 451 /**
 452  * lib_ring_buffer_clear_noref - Clear the noref subbuffer flag, called by
 453  *                               writer.
 454  */
 455 static inline
 456 void lib_ring_buffer_clear_noref(const struct lttng_ust_lib_ring_buffer_config *config,
 457                                  struct lttng_ust_lib_ring_buffer_backend *bufb,
 458                                  unsigned long idx,
 459                                  struct lttng_ust_shm_handle *handle)
 460 {
 461         unsigned long id, new_id;
 462         struct lttng_ust_lib_ring_buffer_backend_subbuffer *wsb;
 463
 464         if (config->mode != RING_BUFFER_OVERWRITE)
 465                 return;
 466
 467         /*
 468          * Performing a volatile access to read the sb_pages, because we want to
 469          * read a coherent version of the pointer and the associated noref flag.
 470          */
 471         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 472         if (!wsb)
 473                 return;
 474         id = CMM_ACCESS_ONCE(wsb->id);
 475         for (;;) {
 476                 /* This check is called on the fast path for each record. */
 477                 if (caa_likely(!subbuffer_id_is_noref(config, id))) {
 478                         /*
 479                          * Store after load dependency ordering the writes to
 480                          * the subbuffer after load and test of the noref flag
 481                          * matches the memory barrier implied by the cmpxchg()
 482                          * in update_read_sb_index().
 483                          */
 484                         return; /* Already writing to this buffer */
 485                 }
 486                 new_id = id;
 487                 subbuffer_id_clear_noref(config, &new_id);
 488                 new_id = uatomic_cmpxchg(&wsb->id, id, new_id);
 489                 if (caa_likely(new_id == id))
 490                         break;
 491                 id = new_id;
 492         }
 493 }
 494
 495 /**
 496  * lib_ring_buffer_set_noref_offset - Set the noref subbuffer flag and offset,
 497  *                                    called by writer.
 498  */
 499 static inline
 500 void lib_ring_buffer_set_noref_offset(const struct lttng_ust_lib_ring_buffer_config *config,
 501                                       struct lttng_ust_lib_ring_buffer_backend *bufb,
 502                                       unsigned long idx, unsigned long offset,
 503                                       struct lttng_ust_shm_handle *handle)
 504 {
 505         struct lttng_ust_lib_ring_buffer_backend_subbuffer *wsb;
 506         struct channel *chan;
 507
 508         if (config->mode != RING_BUFFER_OVERWRITE)
 509                 return;
 510
 511         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 512         if (!wsb)
 513                 return;
 514         /*
 515          * Because ring_buffer_set_noref() is only called by a single thread
 516          * (the one which updated the cc_sb value), there are no concurrent
 517          * updates to take care of: other writers have not updated cc_sb, so
 518          * they cannot set the noref flag, and concurrent readers cannot modify
 519          * the pointer because the noref flag is not set yet.
 520          * The smp_wmb() in ring_buffer_commit() takes care of ordering writes
 521          * to the subbuffer before this set noref operation.
 522          * subbuffer_set_noref() uses a volatile store to deal with concurrent
 523          * readers of the noref flag.
 524          */
 525         chan = shmp(handle, bufb->chan);
 526         if (!chan)
 527                 return;
 528         CHAN_WARN_ON(chan, subbuffer_id_is_noref(config, wsb->id));
 529         /*
 530          * Memory barrier that ensures counter stores are ordered before set
 531          * noref and offset.
 532          */
 533         cmm_smp_mb();
 534         subbuffer_id_set_noref_offset(config, &wsb->id, offset);
 535 }
 536
 537 /**
 538  * update_read_sb_index - Read-side subbuffer index update.
 539  */
 540 static inline
 541 int update_read_sb_index(const struct lttng_ust_lib_ring_buffer_config *config,
 542                          struct lttng_ust_lib_ring_buffer_backend *bufb,
 543                          struct channel_backend *chanb,
 544                          unsigned long consumed_idx,
 545                          unsigned long consumed_count,
 546                          struct lttng_ust_shm_handle *handle)
 547 {
 548         struct lttng_ust_lib_ring_buffer_backend_subbuffer *wsb;
 549         unsigned long old_id, new_id;
 550
 551         wsb = shmp_index(handle, bufb->buf_wsb, consumed_idx);
 552         if (caa_unlikely(!wsb))
 553                 return -EPERM;
 554
 555         if (config->mode == RING_BUFFER_OVERWRITE) {
 556                 struct channel *chan;
 557
 558                 /*
 559                  * Exchange the target writer subbuffer with our own unused
 560                  * subbuffer. No need to use CMM_ACCESS_ONCE() here to read the
 561                  * old_wpage, because the value read will be confirmed by the
 562                  * following cmpxchg().
 563                  */
 564                 old_id = wsb->id;
 565                 if (caa_unlikely(!subbuffer_id_is_noref(config, old_id)))
 566                         return -EAGAIN;
 567                 /*
 568                  * Make sure the offset count we are expecting matches the one
 569                  * indicated by the writer.
 570                  */
 571                 if (caa_unlikely(!subbuffer_id_compare_offset(config, old_id,
 572                                                           consumed_count)))
 573                         return -EAGAIN;
 574                 chan = shmp(handle, bufb->chan);
 575                 if (caa_unlikely(!chan))
 576                         return -EPERM;
 577                 CHAN_WARN_ON(chan, !subbuffer_id_is_noref(config, bufb->buf_rsb.id));
 578                 subbuffer_id_set_noref_offset(config, &bufb->buf_rsb.id,
 579                                               consumed_count);
 580                 new_id = uatomic_cmpxchg(&wsb->id, old_id, bufb->buf_rsb.id);
 581                 if (caa_unlikely(old_id != new_id))
 582                         return -EAGAIN;
 583                 bufb->buf_rsb.id = new_id;
 584         } else {
 585                 /* No page exchange, use the writer page directly */
 586                 bufb->buf_rsb.id = wsb->id;
 587         }
 588         return 0;
 589 }
 590
 591 #ifndef inline_memcpy
 592 #define inline_memcpy(dest, src, n)     memcpy(dest, src, n)
 593 #endif
 594
 595 static inline __attribute__((always_inline))
 596 void lttng_inline_memcpy(void *dest, const void *src,
 597                 unsigned long len)
 598 {
 599         switch (len) {
 600         case 1:
 601                 *(uint8_t *) dest = *(const uint8_t *) src;
 602                 break;
 603         case 2:
 604                 *(uint16_t *) dest = *(const uint16_t *) src;
 605                 break;
 606         case 4:
 607                 *(uint32_t *) dest = *(const uint32_t *) src;
 608                 break;
 609         case 8:
 610                 *(uint64_t *) dest = *(const uint64_t *) src;
 611                 break;
 612         default:
 613                 inline_memcpy(dest, src, len);
 614         }
 615 }
 616
 617 /*
 618  * Use the architecture-specific memcpy implementation for constant-sized
 619  * inputs, but rely on an inline memcpy for length statically unknown.
 620  * The function call to memcpy is just way too expensive for a fast path.
 621  */
 622 #define lib_ring_buffer_do_copy(config, dest, src, len)         \
 623 do {                                                            \
 624         size_t __len = (len);                                   \
 625         if (__builtin_constant_p(len))                          \
 626                 memcpy(dest, src, __len);                       \
 627         else                                                    \
 628                 lttng_inline_memcpy(dest, src, __len);          \
 629 } while (0)
 630
 631 /*
 632  * write len bytes to dest with c
 633  */
 634 static inline
 635 void lib_ring_buffer_do_memset(char *dest, int c, unsigned long len)
 636 {
 637         unsigned long i;
 638
 639         for (i = 0; i < len; i++)
 640                 dest[i] = c;
 641 }
 642
 643 /* arch-agnostic implementation */
 644
 645 static inline int lttng_ust_fls(unsigned int x)
 646 {
 647         int r = 32;
 648
 649         if (!x)
 650                 return 0;
 651         if (!(x & 0xFFFF0000U)) {
 652                 x <<= 16;
 653                 r -= 16;
 654         }
 655         if (!(x & 0xFF000000U)) {
 656                 x <<= 8;
 657                 r -= 8;
 658         }
 659         if (!(x & 0xF0000000U)) {
 660                 x <<= 4;
 661                 r -= 4;
 662         }
 663         if (!(x & 0xC0000000U)) {
 664                 x <<= 2;
 665                 r -= 2;
 666         }
 667         if (!(x & 0x80000000U)) {
 668                 /* No need to bit shift on last operation */
 669                 r -= 1;
 670         }
 671         return r;
 672 }
 673
 674 static inline int get_count_order(unsigned int count)
 675 {
 676         int order;
 677
 678         order = lttng_ust_fls(count) - 1;
 679         if (count & (count - 1))
 680                 order++;
 681         return order;
 682 }
 683
 684 #endif /* _LTTNG_RING_BUFFER_BACKEND_INTERNAL_H */