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