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                         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                 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                             struct lttng_ust_ring_buffer_backend *bufb,
 261                             unsigned long idx, struct lttng_ust_shm_handle *handle)
 262 {
 263         struct lttng_ust_ring_buffer_backend_pages *backend_pages;
 264
 265         backend_pages = lib_ring_buffer_get_backend_pages_from_ctx(config, ctx);
 266         if (caa_unlikely(!backend_pages)) {
 267                 if (lib_ring_buffer_backend_get_pages(config, ctx, &backend_pages))
 268                         return;
 269         }
 270         v_inc(config, &backend_pages->records_commit);
 271 }
 272 #else /* LTTNG_RING_BUFFER_COUNT_EVENTS */
 273 static inline
 274 void subbuffer_count_record(const struct lttng_ust_ring_buffer_config *config __attribute__((unused)),
 275                 const struct lttng_ust_ring_buffer_ctx *ctx __attribute__((unused)),
 276                 struct lttng_ust_ring_buffer_backend *bufb __attribute__((unused)),
 277                 unsigned long idx __attribute__((unused)),
 278                 struct lttng_ust_shm_handle *handle __attribute__((unused)))
 279 {
 280 }
 281 #endif /* #else LTTNG_RING_BUFFER_COUNT_EVENTS */
 282
 283 /*
 284  * Reader has exclusive subbuffer access for record consumption. No need to
 285  * perform the decrement atomically.
 286  */
 287 static inline
 288 void subbuffer_consume_record(const struct lttng_ust_ring_buffer_config *config,
 289                               struct lttng_ust_ring_buffer_backend *bufb,
 290                               struct lttng_ust_shm_handle *handle)
 291 {
 292         unsigned long sb_bindex;
 293         struct lttng_ust_ring_buffer_channel *chan;
 294         struct lttng_ust_ring_buffer_backend_pages_shmp *pages_shmp;
 295         struct lttng_ust_ring_buffer_backend_pages *backend_pages;
 296
 297         sb_bindex = subbuffer_id_get_index(config, bufb->buf_rsb.id);
 298         chan = shmp(handle, bufb->chan);
 299         if (!chan)
 300                 return;
 301         pages_shmp = shmp_index(handle, bufb->array, sb_bindex);
 302         if (!pages_shmp)
 303                 return;
 304         backend_pages = shmp(handle, pages_shmp->shmp);
 305         if (!backend_pages)
 306                 return;
 307         CHAN_WARN_ON(chan, !v_read(config, &backend_pages->records_unread));
 308         /* Non-atomic decrement protected by exclusive subbuffer access */
 309         _v_dec(config, &backend_pages->records_unread);
 310         v_inc(config, &bufb->records_read);
 311 }
 312
 313 static inline
 314 unsigned long subbuffer_get_records_count(
 315                                 const struct lttng_ust_ring_buffer_config *config,
 316                                 struct lttng_ust_ring_buffer_backend *bufb,
 317                                 unsigned long idx,
 318                                 struct lttng_ust_shm_handle *handle)
 319 {
 320         unsigned long sb_bindex;
 321         struct lttng_ust_ring_buffer_backend_subbuffer *wsb;
 322         struct lttng_ust_ring_buffer_backend_pages_shmp *rpages;
 323         struct lttng_ust_ring_buffer_backend_pages *backend_pages;
 324
 325         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 326         if (!wsb)
 327                 return 0;
 328         sb_bindex = subbuffer_id_get_index(config, wsb->id);
 329         rpages = shmp_index(handle, bufb->array, sb_bindex);
 330         if (!rpages)
 331                 return 0;
 332         backend_pages = shmp(handle, rpages->shmp);
 333         if (!backend_pages)
 334                 return 0;
 335         return v_read(config, &backend_pages->records_commit);
 336 }
 337
 338 /*
 339  * Must be executed at subbuffer delivery when the writer has _exclusive_
 340  * subbuffer access. See lib_ring_buffer_check_deliver() for details.
 341  * lib_ring_buffer_get_records_count() must be called to get the records
 342  * count before this function, because it resets the records_commit
 343  * count.
 344  */
 345 static inline
 346 unsigned long subbuffer_count_records_overrun(
 347                                 const struct lttng_ust_ring_buffer_config *config,
 348                                 struct lttng_ust_ring_buffer_backend *bufb,
 349                                 unsigned long idx,
 350                                 struct lttng_ust_shm_handle *handle)
 351 {
 352         unsigned long overruns, sb_bindex;
 353         struct lttng_ust_ring_buffer_backend_subbuffer *wsb;
 354         struct lttng_ust_ring_buffer_backend_pages_shmp *rpages;
 355         struct lttng_ust_ring_buffer_backend_pages *backend_pages;
 356
 357         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 358         if (!wsb)
 359                 return 0;
 360         sb_bindex = subbuffer_id_get_index(config, wsb->id);
 361         rpages = shmp_index(handle, bufb->array, sb_bindex);
 362         if (!rpages)
 363                 return 0;
 364         backend_pages = shmp(handle, rpages->shmp);
 365         if (!backend_pages)
 366                 return 0;
 367         overruns = v_read(config, &backend_pages->records_unread);
 368         v_set(config, &backend_pages->records_unread,
 369               v_read(config, &backend_pages->records_commit));
 370         v_set(config, &backend_pages->records_commit, 0);
 371
 372         return overruns;
 373 }
 374
 375 static inline
 376 void subbuffer_set_data_size(const struct lttng_ust_ring_buffer_config *config,
 377                              struct lttng_ust_ring_buffer_backend *bufb,
 378                              unsigned long idx,
 379                              unsigned long data_size,
 380                              struct lttng_ust_shm_handle *handle)
 381 {
 382         unsigned long sb_bindex;
 383         struct lttng_ust_ring_buffer_backend_subbuffer *wsb;
 384         struct lttng_ust_ring_buffer_backend_pages_shmp *rpages;
 385         struct lttng_ust_ring_buffer_backend_pages *backend_pages;
 386
 387         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 388         if (!wsb)
 389                 return;
 390         sb_bindex = subbuffer_id_get_index(config, wsb->id);
 391         rpages = shmp_index(handle, bufb->array, sb_bindex);
 392         if (!rpages)
 393                 return;
 394         backend_pages = shmp(handle, rpages->shmp);
 395         if (!backend_pages)
 396                 return;
 397         backend_pages->data_size = data_size;
 398 }
 399
 400 static inline
 401 unsigned long subbuffer_get_read_data_size(
 402                                 const struct lttng_ust_ring_buffer_config *config,
 403                                 struct lttng_ust_ring_buffer_backend *bufb,
 404                                 struct lttng_ust_shm_handle *handle)
 405 {
 406         unsigned long sb_bindex;
 407         struct lttng_ust_ring_buffer_backend_pages_shmp *pages_shmp;
 408         struct lttng_ust_ring_buffer_backend_pages *backend_pages;
 409
 410         sb_bindex = subbuffer_id_get_index(config, bufb->buf_rsb.id);
 411         pages_shmp = shmp_index(handle, bufb->array, sb_bindex);
 412         if (!pages_shmp)
 413                 return 0;
 414         backend_pages = shmp(handle, pages_shmp->shmp);
 415         if (!backend_pages)
 416                 return 0;
 417         return backend_pages->data_size;
 418 }
 419
 420 static inline
 421 unsigned long subbuffer_get_data_size(
 422                                 const struct lttng_ust_ring_buffer_config *config,
 423                                 struct lttng_ust_ring_buffer_backend *bufb,
 424                                 unsigned long idx,
 425                                 struct lttng_ust_shm_handle *handle)
 426 {
 427         unsigned long sb_bindex;
 428         struct lttng_ust_ring_buffer_backend_subbuffer *wsb;
 429         struct lttng_ust_ring_buffer_backend_pages_shmp *rpages;
 430         struct lttng_ust_ring_buffer_backend_pages *backend_pages;
 431
 432         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 433         if (!wsb)
 434                 return 0;
 435         sb_bindex = subbuffer_id_get_index(config, wsb->id);
 436         rpages = shmp_index(handle, bufb->array, sb_bindex);
 437         if (!rpages)
 438                 return 0;
 439         backend_pages = shmp(handle, rpages->shmp);
 440         if (!backend_pages)
 441                 return 0;
 442         return backend_pages->data_size;
 443 }
 444
 445 static inline
 446 void subbuffer_inc_packet_count(
 447                 const struct lttng_ust_ring_buffer_config *config __attribute__((unused)),
 448                 struct lttng_ust_ring_buffer_backend *bufb,
 449                 unsigned long idx, struct lttng_ust_shm_handle *handle)
 450 {
 451         struct lttng_ust_ring_buffer_backend_counts *counts;
 452
 453         counts = shmp_index(handle, bufb->buf_cnt, idx);
 454         if (!counts)
 455                 return;
 456         counts->seq_cnt++;
 457 }
 458
 459 /**
 460  * lib_ring_buffer_clear_noref - Clear the noref subbuffer flag, called by
 461  *                               writer.
 462  */
 463 static inline
 464 void lib_ring_buffer_clear_noref(const struct lttng_ust_ring_buffer_config *config,
 465                                  struct lttng_ust_ring_buffer_backend *bufb,
 466                                  unsigned long idx,
 467                                  struct lttng_ust_shm_handle *handle)
 468 {
 469         unsigned long id, new_id;
 470         struct lttng_ust_ring_buffer_backend_subbuffer *wsb;
 471
 472         if (config->mode != RING_BUFFER_OVERWRITE)
 473                 return;
 474
 475         /*
 476          * Performing a volatile access to read the sb_pages, because we want to
 477          * read a coherent version of the pointer and the associated noref flag.
 478          */
 479         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 480         if (!wsb)
 481                 return;
 482         id = CMM_ACCESS_ONCE(wsb->id);
 483         for (;;) {
 484                 /* This check is called on the fast path for each record. */
 485                 if (caa_likely(!subbuffer_id_is_noref(config, id))) {
 486                         /*
 487                          * Store after load dependency ordering the writes to
 488                          * the subbuffer after load and test of the noref flag
 489                          * matches the memory barrier implied by the cmpxchg()
 490                          * in update_read_sb_index().
 491                          */
 492                         return; /* Already writing to this buffer */
 493                 }
 494                 new_id = id;
 495                 subbuffer_id_clear_noref(config, &new_id);
 496                 new_id = uatomic_cmpxchg(&wsb->id, id, new_id);
 497                 if (caa_likely(new_id == id))
 498                         break;
 499                 id = new_id;
 500         }
 501 }
 502
 503 /**
 504  * lib_ring_buffer_set_noref_offset - Set the noref subbuffer flag and offset,
 505  *                                    called by writer.
 506  */
 507 static inline
 508 void lib_ring_buffer_set_noref_offset(const struct lttng_ust_ring_buffer_config *config,
 509                                       struct lttng_ust_ring_buffer_backend *bufb,
 510                                       unsigned long idx, unsigned long offset,
 511                                       struct lttng_ust_shm_handle *handle)
 512 {
 513         struct lttng_ust_ring_buffer_backend_subbuffer *wsb;
 514         struct lttng_ust_ring_buffer_channel *chan;
 515
 516         if (config->mode != RING_BUFFER_OVERWRITE)
 517                 return;
 518
 519         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 520         if (!wsb)
 521                 return;
 522         /*
 523          * Because ring_buffer_set_noref() is only called by a single thread
 524          * (the one which updated the cc_sb value), there are no concurrent
 525          * updates to take care of: other writers have not updated cc_sb, so
 526          * they cannot set the noref flag, and concurrent readers cannot modify
 527          * the pointer because the noref flag is not set yet.
 528          * The smp_wmb() in ring_buffer_commit() takes care of ordering writes
 529          * to the subbuffer before this set noref operation.
 530          * subbuffer_set_noref() uses a volatile store to deal with concurrent
 531          * readers of the noref flag.
 532          */
 533         chan = shmp(handle, bufb->chan);
 534         if (!chan)
 535                 return;
 536         CHAN_WARN_ON(chan, subbuffer_id_is_noref(config, wsb->id));
 537         /*
 538          * Memory barrier that ensures counter stores are ordered before set
 539          * noref and offset.
 540          */
 541         cmm_smp_mb();
 542         subbuffer_id_set_noref_offset(config, &wsb->id, offset);
 543 }
 544
 545 /**
 546  * update_read_sb_index - Read-side subbuffer index update.
 547  */
 548 static inline
 549 int update_read_sb_index(const struct lttng_ust_ring_buffer_config *config,
 550                          struct lttng_ust_ring_buffer_backend *bufb,
 551                          struct channel_backend *chanb __attribute__((unused)),
 552                          unsigned long consumed_idx,
 553                          unsigned long consumed_count,
 554                          struct lttng_ust_shm_handle *handle)
 555 {
 556         struct lttng_ust_ring_buffer_backend_subbuffer *wsb;
 557         unsigned long old_id, new_id;
 558
 559         wsb = shmp_index(handle, bufb->buf_wsb, consumed_idx);
 560         if (caa_unlikely(!wsb))
 561                 return -EPERM;
 562
 563         if (config->mode == RING_BUFFER_OVERWRITE) {
 564                 struct lttng_ust_ring_buffer_channel *chan;
 565
 566                 /*
 567                  * Exchange the target writer subbuffer with our own unused
 568                  * subbuffer. No need to use CMM_ACCESS_ONCE() here to read the
 569                  * old_wpage, because the value read will be confirmed by the
 570                  * following cmpxchg().
 571                  */
 572                 old_id = wsb->id;
 573                 if (caa_unlikely(!subbuffer_id_is_noref(config, old_id)))
 574                         return -EAGAIN;
 575                 /*
 576                  * Make sure the offset count we are expecting matches the one
 577                  * indicated by the writer.
 578                  */
 579                 if (caa_unlikely(!subbuffer_id_compare_offset(config, old_id,
 580                                                           consumed_count)))
 581                         return -EAGAIN;
 582                 chan = shmp(handle, bufb->chan);
 583                 if (caa_unlikely(!chan))
 584                         return -EPERM;
 585                 CHAN_WARN_ON(chan, !subbuffer_id_is_noref(config, bufb->buf_rsb.id));
 586                 subbuffer_id_set_noref_offset(config, &bufb->buf_rsb.id,
 587                                               consumed_count);
 588                 new_id = uatomic_cmpxchg(&wsb->id, old_id, bufb->buf_rsb.id);
 589                 if (caa_unlikely(old_id != new_id))
 590                         return -EAGAIN;
 591                 bufb->buf_rsb.id = new_id;
 592         } else {
 593                 /* No page exchange, use the writer page directly */
 594                 bufb->buf_rsb.id = wsb->id;
 595         }
 596         return 0;
 597 }
 598
 599 #ifndef inline_memcpy
 600 #define inline_memcpy(dest, src, n)     memcpy(dest, src, n)
 601 #endif
 602
 603 static inline
 604 void lttng_inline_memcpy(void *dest, const void *src,
 605                 unsigned long len)
 606         __attribute__((always_inline));
 607 static inline
 608 void lttng_inline_memcpy(void *dest, const void *src,
 609                 unsigned long len)
 610 {
 611         switch (len) {
 612         case 1:
 613                 *(uint8_t *) dest = *(const uint8_t *) src;
 614                 break;
 615         case 2:
 616                 *(uint16_t *) dest = *(const uint16_t *) src;
 617                 break;
 618         case 4:
 619                 *(uint32_t *) dest = *(const uint32_t *) src;
 620                 break;
 621         case 8:
 622                 *(uint64_t *) dest = *(const uint64_t *) src;
 623                 break;
 624         default:
 625                 inline_memcpy(dest, src, len);
 626         }
 627 }
 628
 629 /*
 630  * Use the architecture-specific memcpy implementation for constant-sized
 631  * inputs, but rely on an inline memcpy for length statically unknown.
 632  * The function call to memcpy is just way too expensive for a fast path.
 633  */
 634 #define lib_ring_buffer_do_copy(config, dest, src, len)         \
 635 do {                                                            \
 636         size_t __len = (len);                                   \
 637         if (__builtin_constant_p(len))                          \
 638                 memcpy(dest, src, __len);                       \
 639         else                                                    \
 640                 lttng_inline_memcpy(dest, src, __len);          \
 641 } while (0)
 642
 643 /*
 644  * write len bytes to dest with c
 645  */
 646 static inline
 647 void lib_ring_buffer_do_memset(char *dest, char c, unsigned long len)
 648 {
 649         unsigned long i;
 650
 651         for (i = 0; i < len; i++)
 652                 dest[i] = c;
 653 }
 654
 655 /* arch-agnostic implementation */
 656
 657 static inline int lttng_ust_fls(unsigned int x)
 658 {
 659         int r = 32;
 660
 661         if (!x)
 662                 return 0;
 663         if (!(x & 0xFFFF0000U)) {
 664                 x <<= 16;
 665                 r -= 16;
 666         }
 667         if (!(x & 0xFF000000U)) {
 668                 x <<= 8;
 669                 r -= 8;
 670         }
 671         if (!(x & 0xF0000000U)) {
 672                 x <<= 4;
 673                 r -= 4;
 674         }
 675         if (!(x & 0xC0000000U)) {
 676                 x <<= 2;
 677                 r -= 2;
 678         }
 679         if (!(x & 0x80000000U)) {
 680                 /* No need to bit shift on last operation */
 681                 r -= 1;
 682         }
 683         return r;
 684 }
 685
 686 static inline int get_count_order(unsigned int count)
 687 {
 688         int order;
 689
 690         order = lttng_ust_fls(count) - 1;
 691         if (count & (count - 1))
 692                 order++;
 693         return order;
 694 }
 695
 696 #endif /* _LTTNG_RING_BUFFER_BACKEND_INTERNAL_H */