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