src/common/ringbuffer/backend_internal.h

   1 /*
   2  * SPDX-License-Identifier: LGPL-2.1-only
   3  *
   4  * Copyright (C) 2005-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   5  *
   6  * Ring buffer backend (internal helpers).
   7  */
   8
   9 #ifndef _LTTNG_RING_BUFFER_BACKEND_INTERNAL_H
  10 #define _LTTNG_RING_BUFFER_BACKEND_INTERNAL_H
  11
  12 #include <stddef.h>
  13 #include <stdint.h>
  14 #include <unistd.h>
  15 #include <urcu/compiler.h>
  16
  17 #include <lttng/ust-ringbuffer-context.h>
  18 #include "ringbuffer-config.h"
  19 #include "backend_types.h"
  20 #include "frontend_types.h"
  21 #include "shm.h"
  22
  23 /* Ring buffer backend API presented to the frontend */
  24
  25 /* Ring buffer and channel backend create/free */
  26
  27 int lib_ring_buffer_backend_create(struct lttng_ust_ring_buffer_backend *bufb,
  28                                    struct channel_backend *chan,
  29                                    int cpu,
  30                                    struct lttng_ust_shm_handle *handle,
  31                                    struct shm_object *shmobj)
  32         __attribute__((visibility("hidden")));
  33
  34 void channel_backend_unregister_notifiers(struct channel_backend *chanb)
  35         __attribute__((visibility("hidden")));
  36
  37 void lib_ring_buffer_backend_free(struct lttng_ust_ring_buffer_backend *bufb)
  38         __attribute__((visibility("hidden")));
  39
  40 int channel_backend_init(struct channel_backend *chanb,
  41                          const char *name,
  42                          const struct lttng_ust_ring_buffer_config *config,
  43                          size_t subbuf_size,
  44                          size_t num_subbuf, struct lttng_ust_shm_handle *handle,
  45                          const int *stream_fds)
  46         __attribute__((visibility("hidden")));
  47
  48 void channel_backend_free(struct channel_backend *chanb,
  49                           struct lttng_ust_shm_handle *handle)
  50         __attribute__((visibility("hidden")));
  51
  52 void lib_ring_buffer_backend_reset(struct lttng_ust_ring_buffer_backend *bufb,
  53                                    struct lttng_ust_shm_handle *handle)
  54         __attribute__((visibility("hidden")));
  55
  56 void channel_backend_reset(struct channel_backend *chanb)
  57         __attribute__((visibility("hidden")));
  58
  59 int lib_ring_buffer_backend_init(void)
  60         __attribute__((visibility("hidden")));
  61
  62 void lib_ring_buffer_backend_exit(void)
  63         __attribute__((visibility("hidden")));
  64
  65 extern void _lib_ring_buffer_write(struct lttng_ust_ring_buffer_backend *bufb,
  66                                    size_t offset, const void *src, size_t len,
  67                                    ssize_t pagecpy)
  68         __attribute__((visibility("hidden")));
  69
  70 /*
  71  * Subbuffer ID bits for overwrite mode. Need to fit within a single word to be
  72  * exchanged atomically.
  73  *
  74  * Top half word, except lowest bit, belongs to "offset", which is used to keep
  75  * to count the produced buffers.  For overwrite mode, this provides the
  76  * consumer with the capacity to read subbuffers in order, handling the
  77  * situation where producers would write up to 2^15 buffers (or 2^31 for 64-bit
  78  * systems) concurrently with a single execution of get_subbuf (between offset
  79  * sampling and subbuffer ID exchange).
  80  */
  81
  82 #define HALF_ULONG_BITS         (CAA_BITS_PER_LONG >> 1)
  83
  84 #define SB_ID_OFFSET_SHIFT      (HALF_ULONG_BITS + 1)
  85 #define SB_ID_OFFSET_COUNT      (1UL << SB_ID_OFFSET_SHIFT)
  86 #define SB_ID_OFFSET_MASK       (~(SB_ID_OFFSET_COUNT - 1))
  87 /*
  88  * Lowest bit of top word half belongs to noref. Used only for overwrite mode.
  89  */
  90 #define SB_ID_NOREF_SHIFT       (SB_ID_OFFSET_SHIFT - 1)
  91 #define SB_ID_NOREF_COUNT       (1UL << SB_ID_NOREF_SHIFT)
  92 #define SB_ID_NOREF_MASK        SB_ID_NOREF_COUNT
  93 /*
  94  * In overwrite mode: lowest half of word is used for index.
  95  * Limit of 2^16 subbuffers per buffer on 32-bit, 2^32 on 64-bit.
  96  * In producer-consumer mode: whole word used for index.
  97  */
  98 #define SB_ID_INDEX_SHIFT       0
  99 #define SB_ID_INDEX_COUNT       (1UL << SB_ID_INDEX_SHIFT)
 100 #define SB_ID_INDEX_MASK        (SB_ID_NOREF_COUNT - 1)
 101
 102 /*
 103  * Construct the subbuffer id from offset, index and noref. Use only the index
 104  * for producer-consumer mode (offset and noref are only used in overwrite
 105  * mode).
 106  */
 107 static inline
 108 unsigned long subbuffer_id(const struct lttng_ust_ring_buffer_config *config,
 109                            unsigned long offset, unsigned long noref,
 110                            unsigned long index)
 111 {
 112         if (config->mode == RING_BUFFER_OVERWRITE)
 113                 return (offset << SB_ID_OFFSET_SHIFT)
 114                        | (noref << SB_ID_NOREF_SHIFT)
 115                        | index;
 116         else
 117                 return index;
 118 }
 119
 120 /*
 121  * Compare offset with the offset contained within id. Return 1 if the offset
 122  * bits are identical, else 0.
 123  */
 124 static inline
 125 int subbuffer_id_compare_offset(
 126                 const struct lttng_ust_ring_buffer_config *config __attribute__((unused)),
 127                 unsigned long id, unsigned long offset)
 128 {
 129         return (id & SB_ID_OFFSET_MASK) == (offset << SB_ID_OFFSET_SHIFT);
 130 }
 131
 132 static inline
 133 unsigned long subbuffer_id_get_index(const struct lttng_ust_ring_buffer_config *config,
 134                                      unsigned long id)
 135 {
 136         if (config->mode == RING_BUFFER_OVERWRITE)
 137                 return id & SB_ID_INDEX_MASK;
 138         else
 139                 return id;
 140 }
 141
 142 static inline
 143 unsigned long subbuffer_id_is_noref(const struct lttng_ust_ring_buffer_config *config,
 144                                     unsigned long id)
 145 {
 146         if (config->mode == RING_BUFFER_OVERWRITE)
 147                 return !!(id & SB_ID_NOREF_MASK);
 148         else
 149                 return 1;
 150 }
 151
 152 /*
 153  * Only used by reader on subbuffer ID it has exclusive access to. No volatile
 154  * needed.
 155  */
 156 static inline
 157 void subbuffer_id_set_noref(const struct lttng_ust_ring_buffer_config *config,
 158                             unsigned long *id)
 159 {
 160         if (config->mode == RING_BUFFER_OVERWRITE)
 161                 *id |= SB_ID_NOREF_MASK;
 162 }
 163
 164 static inline
 165 void subbuffer_id_set_noref_offset(const struct lttng_ust_ring_buffer_config *config,
 166                                    unsigned long *id, unsigned long offset)
 167 {
 168         unsigned long tmp;
 169
 170         if (config->mode == RING_BUFFER_OVERWRITE) {
 171                 tmp = *id;
 172                 tmp &= ~SB_ID_OFFSET_MASK;
 173                 tmp |= offset << SB_ID_OFFSET_SHIFT;
 174                 tmp |= SB_ID_NOREF_MASK;
 175                 /* Volatile store, read concurrently by readers. */
 176                 CMM_ACCESS_ONCE(*id) = tmp;
 177         }
 178 }
 179
 180 /* No volatile access, since already used locally */
 181 static inline
 182 void subbuffer_id_clear_noref(const struct lttng_ust_ring_buffer_config *config,
 183                               unsigned long *id)
 184 {
 185         if (config->mode == RING_BUFFER_OVERWRITE)
 186                 *id &= ~SB_ID_NOREF_MASK;
 187 }
 188
 189 /*
 190  * For overwrite mode, cap the number of subbuffers per buffer to:
 191  * 2^16 on 32-bit architectures
 192  * 2^32 on 64-bit architectures
 193  * This is required to fit in the index part of the ID. Return 0 on success,
 194  * -EPERM on failure.
 195  */
 196 static inline
 197 int subbuffer_id_check_index(const struct lttng_ust_ring_buffer_config *config,
 198                              unsigned long num_subbuf)
 199 {
 200         if (config->mode == RING_BUFFER_OVERWRITE)
 201                 return (num_subbuf > (1UL << HALF_ULONG_BITS)) ? -EPERM : 0;
 202         else
 203                 return 0;
 204 }
 205
 206 static inline
 207 int lib_ring_buffer_backend_get_pages(const struct lttng_ust_ring_buffer_config *config,
 208                         const struct lttng_ust_ring_buffer_ctx *ctx,
 209                         struct lttng_ust_ring_buffer_backend_pages **backend_pages)
 210 {
 211         struct lttng_ust_ring_buffer_ctx_private *ctx_private = ctx->priv;
 212         struct lttng_ust_ring_buffer_backend *bufb = &ctx_private->buf->backend;
 213         struct channel_backend *chanb = &ctx_private->chan->backend;
 214         struct lttng_ust_shm_handle *handle = ctx_private->chan->handle;
 215         size_t sbidx;
 216         size_t offset = ctx_private->buf_offset;
 217         struct lttng_ust_ring_buffer_backend_subbuffer *wsb;
 218         struct lttng_ust_ring_buffer_backend_pages_shmp *rpages;
 219         unsigned long sb_bindex, id;
 220         struct lttng_ust_ring_buffer_backend_pages *_backend_pages;
 221
 222         offset &= chanb->buf_size - 1;
 223         sbidx = offset >> chanb->subbuf_size_order;
 224         wsb = shmp_index(handle, bufb->buf_wsb, sbidx);
 225         if (caa_unlikely(!wsb))
 226                 return -1;
 227         id = wsb->id;
 228         sb_bindex = subbuffer_id_get_index(config, id);
 229         rpages = shmp_index(handle, bufb->array, sb_bindex);
 230         if (caa_unlikely(!rpages))
 231                 return -1;
 232         CHAN_WARN_ON(ctx_private->chan,
 233                      config->mode == RING_BUFFER_OVERWRITE
 234                      && subbuffer_id_is_noref(config, id));
 235         _backend_pages = shmp(handle, rpages->shmp);
 236         if (caa_unlikely(!_backend_pages))
 237                 return -1;
 238         *backend_pages = _backend_pages;
 239         return 0;
 240 }
 241
 242 /* Get backend pages from cache. */
 243 static inline
 244 struct lttng_ust_ring_buffer_backend_pages *
 245         lib_ring_buffer_get_backend_pages_from_ctx(
 246                 const struct lttng_ust_ring_buffer_config *config __attribute__((unused)),
 247                 const struct lttng_ust_ring_buffer_ctx *ctx)
 248 {
 249         return ctx->priv->backend_pages;
 250 }
 251
 252 /*
 253  * The ring buffer can count events recorded and overwritten per buffer,
 254  * but it is disabled by default due to its performance overhead.
 255  */
 256 #ifdef LTTNG_RING_BUFFER_COUNT_EVENTS
 257 static inline
 258 void subbuffer_count_record(const struct lttng_ust_ring_buffer_config *config,
 259                             const struct lttng_ust_ring_buffer_ctx *ctx)
 260 {
 261         struct lttng_ust_ring_buffer_backend_pages *backend_pages;
 262
 263         backend_pages = lib_ring_buffer_get_backend_pages_from_ctx(config, ctx);
 264         if (caa_unlikely(!backend_pages)) {
 265                 if (lib_ring_buffer_backend_get_pages(config, ctx, &backend_pages))
 266                         return;
 267         }
 268         v_inc(config, &backend_pages->records_commit);
 269 }
 270 #else /* LTTNG_RING_BUFFER_COUNT_EVENTS */
 271 static inline
 272 void subbuffer_count_record(const struct lttng_ust_ring_buffer_config *config __attribute__((unused)),
 273                 const struct lttng_ust_ring_buffer_ctx *ctx __attribute__((unused)))
 274 {
 275 }
 276 #endif /* #else LTTNG_RING_BUFFER_COUNT_EVENTS */
 277
 278 /*
 279  * Reader has exclusive subbuffer access for record consumption. No need to
 280  * perform the decrement atomically.
 281  */
 282 static inline
 283 void subbuffer_consume_record(const struct lttng_ust_ring_buffer_config *config,
 284                               struct lttng_ust_ring_buffer_backend *bufb,
 285                               struct lttng_ust_shm_handle *handle)
 286 {
 287         unsigned long sb_bindex;
 288         struct lttng_ust_ring_buffer_channel *chan;
 289         struct lttng_ust_ring_buffer_backend_pages_shmp *pages_shmp;
 290         struct lttng_ust_ring_buffer_backend_pages *backend_pages;
 291
 292         sb_bindex = subbuffer_id_get_index(config, bufb->buf_rsb.id);
 293         chan = shmp(handle, bufb->chan);
 294         if (!chan)
 295                 return;
 296         pages_shmp = shmp_index(handle, bufb->array, sb_bindex);
 297         if (!pages_shmp)
 298                 return;
 299         backend_pages = shmp(handle, pages_shmp->shmp);
 300         if (!backend_pages)
 301                 return;
 302         CHAN_WARN_ON(chan, !v_read(config, &backend_pages->records_unread));
 303         /* Non-atomic decrement protected by exclusive subbuffer access */
 304         _v_dec(config, &backend_pages->records_unread);
 305         v_inc(config, &bufb->records_read);
 306 }
 307
 308 static inline
 309 unsigned long subbuffer_get_records_count(
 310                                 const struct lttng_ust_ring_buffer_config *config,
 311                                 struct lttng_ust_ring_buffer_backend *bufb,
 312                                 unsigned long idx,
 313                                 struct lttng_ust_shm_handle *handle)
 314 {
 315         unsigned long sb_bindex;
 316         struct lttng_ust_ring_buffer_backend_subbuffer *wsb;
 317         struct lttng_ust_ring_buffer_backend_pages_shmp *rpages;
 318         struct lttng_ust_ring_buffer_backend_pages *backend_pages;
 319
 320         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 321         if (!wsb)
 322                 return 0;
 323         sb_bindex = subbuffer_id_get_index(config, wsb->id);
 324         rpages = shmp_index(handle, bufb->array, sb_bindex);
 325         if (!rpages)
 326                 return 0;
 327         backend_pages = shmp(handle, rpages->shmp);
 328         if (!backend_pages)
 329                 return 0;
 330         return v_read(config, &backend_pages->records_commit);
 331 }
 332
 333 /*
 334  * Must be executed at subbuffer delivery when the writer has _exclusive_
 335  * subbuffer access. See lib_ring_buffer_check_deliver() for details.
 336  * lib_ring_buffer_get_records_count() must be called to get the records
 337  * count before this function, because it resets the records_commit
 338  * count.
 339  */
 340 static inline
 341 unsigned long subbuffer_count_records_overrun(
 342                                 const struct lttng_ust_ring_buffer_config *config,
 343                                 struct lttng_ust_ring_buffer_backend *bufb,
 344                                 unsigned long idx,
 345                                 struct lttng_ust_shm_handle *handle)
 346 {
 347         unsigned long overruns, sb_bindex;
 348         struct lttng_ust_ring_buffer_backend_subbuffer *wsb;
 349         struct lttng_ust_ring_buffer_backend_pages_shmp *rpages;
 350         struct lttng_ust_ring_buffer_backend_pages *backend_pages;
 351
 352         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 353         if (!wsb)
 354                 return 0;
 355         sb_bindex = subbuffer_id_get_index(config, wsb->id);
 356         rpages = shmp_index(handle, bufb->array, sb_bindex);
 357         if (!rpages)
 358                 return 0;
 359         backend_pages = shmp(handle, rpages->shmp);
 360         if (!backend_pages)
 361                 return 0;
 362         overruns = v_read(config, &backend_pages->records_unread);
 363         v_set(config, &backend_pages->records_unread,
 364               v_read(config, &backend_pages->records_commit));
 365         v_set(config, &backend_pages->records_commit, 0);
 366
 367         return overruns;
 368 }
 369
 370 static inline
 371 void subbuffer_set_data_size(const struct lttng_ust_ring_buffer_config *config,
 372                              struct lttng_ust_ring_buffer_backend *bufb,
 373                              unsigned long idx,
 374                              unsigned long data_size,
 375                              struct lttng_ust_shm_handle *handle)
 376 {
 377         unsigned long sb_bindex;
 378         struct lttng_ust_ring_buffer_backend_subbuffer *wsb;
 379         struct lttng_ust_ring_buffer_backend_pages_shmp *rpages;
 380         struct lttng_ust_ring_buffer_backend_pages *backend_pages;
 381
 382         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 383         if (!wsb)
 384                 return;
 385         sb_bindex = subbuffer_id_get_index(config, wsb->id);
 386         rpages = shmp_index(handle, bufb->array, sb_bindex);
 387         if (!rpages)
 388                 return;
 389         backend_pages = shmp(handle, rpages->shmp);
 390         if (!backend_pages)
 391                 return;
 392         backend_pages->data_size = data_size;
 393 }
 394
 395 static inline
 396 unsigned long subbuffer_get_read_data_size(
 397                                 const struct lttng_ust_ring_buffer_config *config,
 398                                 struct lttng_ust_ring_buffer_backend *bufb,
 399                                 struct lttng_ust_shm_handle *handle)
 400 {
 401         unsigned long sb_bindex;
 402         struct lttng_ust_ring_buffer_backend_pages_shmp *pages_shmp;
 403         struct lttng_ust_ring_buffer_backend_pages *backend_pages;
 404
 405         sb_bindex = subbuffer_id_get_index(config, bufb->buf_rsb.id);
 406         pages_shmp = shmp_index(handle, bufb->array, sb_bindex);
 407         if (!pages_shmp)
 408                 return 0;
 409         backend_pages = shmp(handle, pages_shmp->shmp);
 410         if (!backend_pages)
 411                 return 0;
 412         return backend_pages->data_size;
 413 }
 414
 415 static inline
 416 unsigned long subbuffer_get_data_size(
 417                                 const struct lttng_ust_ring_buffer_config *config,
 418                                 struct lttng_ust_ring_buffer_backend *bufb,
 419                                 unsigned long idx,
 420                                 struct lttng_ust_shm_handle *handle)
 421 {
 422         unsigned long sb_bindex;
 423         struct lttng_ust_ring_buffer_backend_subbuffer *wsb;
 424         struct lttng_ust_ring_buffer_backend_pages_shmp *rpages;
 425         struct lttng_ust_ring_buffer_backend_pages *backend_pages;
 426
 427         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 428         if (!wsb)
 429                 return 0;
 430         sb_bindex = subbuffer_id_get_index(config, wsb->id);
 431         rpages = shmp_index(handle, bufb->array, sb_bindex);
 432         if (!rpages)
 433                 return 0;
 434         backend_pages = shmp(handle, rpages->shmp);
 435         if (!backend_pages)
 436                 return 0;
 437         return backend_pages->data_size;
 438 }
 439
 440 static inline
 441 void subbuffer_inc_packet_count(
 442                 const struct lttng_ust_ring_buffer_config *config __attribute__((unused)),
 443                 struct lttng_ust_ring_buffer_backend *bufb,
 444                 unsigned long idx, struct lttng_ust_shm_handle *handle)
 445 {
 446         struct lttng_ust_ring_buffer_backend_counts *counts;
 447
 448         counts = shmp_index(handle, bufb->buf_cnt, idx);
 449         if (!counts)
 450                 return;
 451         counts->seq_cnt++;
 452 }
 453
 454 /**
 455  * lib_ring_buffer_clear_noref - Clear the noref subbuffer flag, called by
 456  *                               writer.
 457  */
 458 static inline
 459 void lib_ring_buffer_clear_noref(const struct lttng_ust_ring_buffer_config *config,
 460                                  struct lttng_ust_ring_buffer_backend *bufb,
 461                                  unsigned long idx,
 462                                  struct lttng_ust_shm_handle *handle)
 463 {
 464         unsigned long id, new_id;
 465         struct lttng_ust_ring_buffer_backend_subbuffer *wsb;
 466
 467         if (config->mode != RING_BUFFER_OVERWRITE)
 468                 return;
 469
 470         /*
 471          * Performing a volatile access to read the sb_pages, because we want to
 472          * read a coherent version of the pointer and the associated noref flag.
 473          */
 474         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 475         if (!wsb)
 476                 return;
 477         id = CMM_ACCESS_ONCE(wsb->id);
 478         for (;;) {
 479                 /* This check is called on the fast path for each record. */
 480                 if (caa_likely(!subbuffer_id_is_noref(config, id))) {
 481                         /*
 482                          * Store after load dependency ordering the writes to
 483                          * the subbuffer after load and test of the noref flag
 484                          * matches the memory barrier implied by the cmpxchg()
 485                          * in update_read_sb_index().
 486                          */
 487                         return; /* Already writing to this buffer */
 488                 }
 489                 new_id = id;
 490                 subbuffer_id_clear_noref(config, &new_id);
 491                 new_id = uatomic_cmpxchg(&wsb->id, id, new_id);
 492                 if (caa_likely(new_id == id))
 493                         break;
 494                 id = new_id;
 495         }
 496 }
 497
 498 /**
 499  * lib_ring_buffer_set_noref_offset - Set the noref subbuffer flag and offset,
 500  *                                    called by writer.
 501  */
 502 static inline
 503 void lib_ring_buffer_set_noref_offset(const struct lttng_ust_ring_buffer_config *config,
 504                                       struct lttng_ust_ring_buffer_backend *bufb,
 505                                       unsigned long idx, unsigned long offset,
 506                                       struct lttng_ust_shm_handle *handle)
 507 {
 508         struct lttng_ust_ring_buffer_backend_subbuffer *wsb;
 509         struct lttng_ust_ring_buffer_channel *chan;
 510
 511         if (config->mode != RING_BUFFER_OVERWRITE)
 512                 return;
 513
 514         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 515         if (!wsb)
 516                 return;
 517         /*
 518          * Because ring_buffer_set_noref() is only called by a single thread
 519          * (the one which updated the cc_sb value), there are no concurrent
 520          * updates to take care of: other writers have not updated cc_sb, so
 521          * they cannot set the noref flag, and concurrent readers cannot modify
 522          * the pointer because the noref flag is not set yet.
 523          * The smp_wmb() in ring_buffer_commit() takes care of ordering writes
 524          * to the subbuffer before this set noref operation.
 525          * subbuffer_set_noref() uses a volatile store to deal with concurrent
 526          * readers of the noref flag.
 527          */
 528         chan = shmp(handle, bufb->chan);
 529         if (!chan)
 530                 return;
 531         CHAN_WARN_ON(chan, subbuffer_id_is_noref(config, wsb->id));
 532         /*
 533          * Memory barrier that ensures counter stores are ordered before set
 534          * noref and offset.
 535          */
 536         cmm_smp_mb();
 537         subbuffer_id_set_noref_offset(config, &wsb->id, offset);
 538 }
 539
 540 /**
 541  * update_read_sb_index - Read-side subbuffer index update.
 542  */
 543 static inline
 544 int update_read_sb_index(const struct lttng_ust_ring_buffer_config *config,
 545                          struct lttng_ust_ring_buffer_backend *bufb,
 546                          struct channel_backend *chanb __attribute__((unused)),
 547                          unsigned long consumed_idx,
 548                          unsigned long consumed_count,
 549                          struct lttng_ust_shm_handle *handle)
 550 {
 551         struct lttng_ust_ring_buffer_backend_subbuffer *wsb;
 552         unsigned long old_id, new_id;
 553
 554         wsb = shmp_index(handle, bufb->buf_wsb, consumed_idx);
 555         if (caa_unlikely(!wsb))
 556                 return -EPERM;
 557
 558         if (config->mode == RING_BUFFER_OVERWRITE) {
 559                 struct lttng_ust_ring_buffer_channel *chan;
 560
 561                 /*
 562                  * Exchange the target writer subbuffer with our own unused
 563                  * subbuffer. No need to use CMM_ACCESS_ONCE() here to read the
 564                  * old_wpage, because the value read will be confirmed by the
 565                  * following cmpxchg().
 566                  */
 567                 old_id = wsb->id;
 568                 if (caa_unlikely(!subbuffer_id_is_noref(config, old_id)))
 569                         return -EAGAIN;
 570                 /*
 571                  * Make sure the offset count we are expecting matches the one
 572                  * indicated by the writer.
 573                  */
 574                 if (caa_unlikely(!subbuffer_id_compare_offset(config, old_id,
 575                                                           consumed_count)))
 576                         return -EAGAIN;
 577                 chan = shmp(handle, bufb->chan);
 578                 if (caa_unlikely(!chan))
 579                         return -EPERM;
 580                 CHAN_WARN_ON(chan, !subbuffer_id_is_noref(config, bufb->buf_rsb.id));
 581                 subbuffer_id_set_noref_offset(config, &bufb->buf_rsb.id,
 582                                               consumed_count);
 583                 new_id = uatomic_cmpxchg(&wsb->id, old_id, bufb->buf_rsb.id);
 584                 if (caa_unlikely(old_id != new_id))
 585                         return -EAGAIN;
 586                 bufb->buf_rsb.id = new_id;
 587         } else {
 588                 /* No page exchange, use the writer page directly */
 589                 bufb->buf_rsb.id = wsb->id;
 590         }
 591         return 0;
 592 }
 593
 594 #ifndef inline_memcpy
 595 #define inline_memcpy(dest, src, n)     memcpy(dest, src, n)
 596 #endif
 597
 598 static inline
 599 void lttng_inline_memcpy(void *dest, const void *src,
 600                 unsigned long len)
 601         __attribute__((always_inline));
 602 static inline
 603 void lttng_inline_memcpy(void *dest, const void *src,
 604                 unsigned long len)
 605 {
 606         switch (len) {
 607         case 1:
 608                 *(uint8_t *) dest = *(const uint8_t *) src;
 609                 break;
 610         case 2:
 611                 *(uint16_t *) dest = *(const uint16_t *) src;
 612                 break;
 613         case 4:
 614                 *(uint32_t *) dest = *(const uint32_t *) src;
 615                 break;
 616         case 8:
 617                 *(uint64_t *) dest = *(const uint64_t *) src;
 618                 break;
 619         default:
 620                 inline_memcpy(dest, src, len);
 621         }
 622 }
 623
 624 /*
 625  * Use the architecture-specific memcpy implementation for constant-sized
 626  * inputs, but rely on an inline memcpy for length statically unknown.
 627  * The function call to memcpy is just way too expensive for a fast path.
 628  */
 629 #define lib_ring_buffer_do_copy(config, dest, src, len)         \
 630 do {                                                            \
 631         size_t __len = (len);                                   \
 632         if (__builtin_constant_p(len))                          \
 633                 memcpy(dest, src, __len);                       \
 634         else                                                    \
 635                 lttng_inline_memcpy(dest, src, __len);          \
 636 } while (0)
 637
 638 /*
 639  * write len bytes to dest with c
 640  */
 641 static inline
 642 void lib_ring_buffer_do_memset(char *dest, char c, unsigned long len)
 643 {
 644         unsigned long i;
 645
 646         for (i = 0; i < len; i++)
 647                 dest[i] = c;
 648 }
 649
 650 /* arch-agnostic implementation */
 651
 652 static inline int lttng_ust_fls(unsigned int x)
 653 {
 654         int r = 32;
 655
 656         if (!x)
 657                 return 0;
 658         if (!(x & 0xFFFF0000U)) {
 659                 x <<= 16;
 660                 r -= 16;
 661         }
 662         if (!(x & 0xFF000000U)) {
 663                 x <<= 8;
 664                 r -= 8;
 665         }
 666         if (!(x & 0xF0000000U)) {
 667                 x <<= 4;
 668                 r -= 4;
 669         }
 670         if (!(x & 0xC0000000U)) {
 671                 x <<= 2;
 672                 r -= 2;
 673         }
 674         if (!(x & 0x80000000U)) {
 675                 /* No need to bit shift on last operation */
 676                 r -= 1;
 677         }
 678         return r;
 679 }
 680
 681 static inline int get_count_order(unsigned int count)
 682 {
 683         int order;
 684
 685         order = lttng_ust_fls(count) - 1;
 686         if (count & (count - 1))
 687                 order++;
 688         return order;
 689 }
 690
 691 #endif /* _LTTNG_RING_BUFFER_BACKEND_INTERNAL_H */