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