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