libringbuffer/backend_internal.h

   1 #ifndef _LTTNG_RING_BUFFER_BACKEND_INTERNAL_H
   2 #define _LTTNG_RING_BUFFER_BACKEND_INTERNAL_H
   3
   4 /*
   5  * libringbuffer/backend_internal.h
   6  *
   7  * Ring buffer backend (internal helpers).
   8  *
   9  * Copyright (C) 2005-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  10  *
  11  * This library is free software; you can redistribute it and/or
  12  * modify it under the terms of the GNU Lesser General Public
  13  * License as published by the Free Software Foundation; only
  14  * version 2.1 of the License.
  15  *
  16  * This library is distributed in the hope that it will be useful,
  17  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  18  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  19  * Lesser General Public License for more details.
  20  *
  21  * You should have received a copy of the GNU Lesser General Public
  22  * License along with this library; if not, write to the Free Software
  23  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  24  */
  25
  26 #include <stddef.h>
  27 #include <stdint.h>
  28 #include <unistd.h>
  29 #include <urcu/compiler.h>
  30
  31 #include <lttng/ringbuffer-config.h>
  32 #include "backend_types.h"
  33 #include "frontend_types.h"
  34 #include "shm.h"
  35
  36 /* Ring buffer backend API presented to the frontend */
  37
  38 /* Ring buffer and channel backend create/free */
  39
  40 int lib_ring_buffer_backend_create(struct lttng_ust_lib_ring_buffer_backend *bufb,
  41                                    struct channel_backend *chan, int cpu,
  42                                    struct lttng_ust_shm_handle *handle,
  43                                    struct shm_object *shmobj);
  44 void channel_backend_unregister_notifiers(struct channel_backend *chanb);
  45 void lib_ring_buffer_backend_free(struct lttng_ust_lib_ring_buffer_backend *bufb);
  46 int channel_backend_init(struct channel_backend *chanb,
  47                          const char *name,
  48                          const struct lttng_ust_lib_ring_buffer_config *config,
  49                          size_t subbuf_size,
  50                          size_t num_subbuf, struct lttng_ust_shm_handle *handle,
  51                          const int *stream_fds);
  52 void channel_backend_free(struct channel_backend *chanb,
  53                           struct lttng_ust_shm_handle *handle);
  54
  55 void lib_ring_buffer_backend_reset(struct lttng_ust_lib_ring_buffer_backend *bufb,
  56                                    struct lttng_ust_shm_handle *handle);
  57 void channel_backend_reset(struct channel_backend *chanb);
  58
  59 int lib_ring_buffer_backend_init(void);
  60 void lib_ring_buffer_backend_exit(void);
  61
  62 extern void _lib_ring_buffer_write(struct lttng_ust_lib_ring_buffer_backend *bufb,
  63                                    size_t offset, const void *src, size_t len,
  64                                    ssize_t pagecpy);
  65
  66 /*
  67  * Subbuffer ID bits for overwrite mode. Need to fit within a single word to be
  68  * exchanged atomically.
  69  *
  70  * Top half word, except lowest bit, belongs to "offset", which is used to keep
  71  * to count the produced buffers.  For overwrite mode, this provides the
  72  * consumer with the capacity to read subbuffers in order, handling the
  73  * situation where producers would write up to 2^15 buffers (or 2^31 for 64-bit
  74  * systems) concurrently with a single execution of get_subbuf (between offset
  75  * sampling and subbuffer ID exchange).
  76  */
  77
  78 #define HALF_ULONG_BITS         (CAA_BITS_PER_LONG >> 1)
  79
  80 #define SB_ID_OFFSET_SHIFT      (HALF_ULONG_BITS + 1)
  81 #define SB_ID_OFFSET_COUNT      (1UL << SB_ID_OFFSET_SHIFT)
  82 #define SB_ID_OFFSET_MASK       (~(SB_ID_OFFSET_COUNT - 1))
  83 /*
  84  * Lowest bit of top word half belongs to noref. Used only for overwrite mode.
  85  */
  86 #define SB_ID_NOREF_SHIFT       (SB_ID_OFFSET_SHIFT - 1)
  87 #define SB_ID_NOREF_COUNT       (1UL << SB_ID_NOREF_SHIFT)
  88 #define SB_ID_NOREF_MASK        SB_ID_NOREF_COUNT
  89 /*
  90  * In overwrite mode: lowest half of word is used for index.
  91  * Limit of 2^16 subbuffers per buffer on 32-bit, 2^32 on 64-bit.
  92  * In producer-consumer mode: whole word used for index.
  93  */
  94 #define SB_ID_INDEX_SHIFT       0
  95 #define SB_ID_INDEX_COUNT       (1UL << SB_ID_INDEX_SHIFT)
  96 #define SB_ID_INDEX_MASK        (SB_ID_NOREF_COUNT - 1)
  97
  98 /*
  99  * Construct the subbuffer id from offset, index and noref. Use only the index
 100  * for producer-consumer mode (offset and noref are only used in overwrite
 101  * mode).
 102  */
 103 static inline
 104 unsigned long subbuffer_id(const struct lttng_ust_lib_ring_buffer_config *config,
 105                            unsigned long offset, unsigned long noref,
 106                            unsigned long index)
 107 {
 108         if (config->mode == RING_BUFFER_OVERWRITE)
 109                 return (offset << SB_ID_OFFSET_SHIFT)
 110                        | (noref << SB_ID_NOREF_SHIFT)
 111                        | index;
 112         else
 113                 return index;
 114 }
 115
 116 /*
 117  * Compare offset with the offset contained within id. Return 1 if the offset
 118  * bits are identical, else 0.
 119  */
 120 static inline
 121 int subbuffer_id_compare_offset(const struct lttng_ust_lib_ring_buffer_config *config,
 122                                 unsigned long id, unsigned long offset)
 123 {
 124         return (id & SB_ID_OFFSET_MASK) == (offset << SB_ID_OFFSET_SHIFT);
 125 }
 126
 127 static inline
 128 unsigned long subbuffer_id_get_index(const struct lttng_ust_lib_ring_buffer_config *config,
 129                                      unsigned long id)
 130 {
 131         if (config->mode == RING_BUFFER_OVERWRITE)
 132                 return id & SB_ID_INDEX_MASK;
 133         else
 134                 return id;
 135 }
 136
 137 static inline
 138 unsigned long subbuffer_id_is_noref(const struct lttng_ust_lib_ring_buffer_config *config,
 139                                     unsigned long id)
 140 {
 141         if (config->mode == RING_BUFFER_OVERWRITE)
 142                 return !!(id & SB_ID_NOREF_MASK);
 143         else
 144                 return 1;
 145 }
 146
 147 /*
 148  * Only used by reader on subbuffer ID it has exclusive access to. No volatile
 149  * needed.
 150  */
 151 static inline
 152 void subbuffer_id_set_noref(const struct lttng_ust_lib_ring_buffer_config *config,
 153                             unsigned long *id)
 154 {
 155         if (config->mode == RING_BUFFER_OVERWRITE)
 156                 *id |= SB_ID_NOREF_MASK;
 157 }
 158
 159 static inline
 160 void subbuffer_id_set_noref_offset(const struct lttng_ust_lib_ring_buffer_config *config,
 161                                    unsigned long *id, unsigned long offset)
 162 {
 163         unsigned long tmp;
 164
 165         if (config->mode == RING_BUFFER_OVERWRITE) {
 166                 tmp = *id;
 167                 tmp &= ~SB_ID_OFFSET_MASK;
 168                 tmp |= offset << SB_ID_OFFSET_SHIFT;
 169                 tmp |= SB_ID_NOREF_MASK;
 170                 /* Volatile store, read concurrently by readers. */
 171                 CMM_ACCESS_ONCE(*id) = tmp;
 172         }
 173 }
 174
 175 /* No volatile access, since already used locally */
 176 static inline
 177 void subbuffer_id_clear_noref(const struct lttng_ust_lib_ring_buffer_config *config,
 178                               unsigned long *id)
 179 {
 180         if (config->mode == RING_BUFFER_OVERWRITE)
 181                 *id &= ~SB_ID_NOREF_MASK;
 182 }
 183
 184 /*
 185  * For overwrite mode, cap the number of subbuffers per buffer to:
 186  * 2^16 on 32-bit architectures
 187  * 2^32 on 64-bit architectures
 188  * This is required to fit in the index part of the ID. Return 0 on success,
 189  * -EPERM on failure.
 190  */
 191 static inline
 192 int subbuffer_id_check_index(const struct lttng_ust_lib_ring_buffer_config *config,
 193                              unsigned long num_subbuf)
 194 {
 195         if (config->mode == RING_BUFFER_OVERWRITE)
 196                 return (num_subbuf > (1UL << HALF_ULONG_BITS)) ? -EPERM : 0;
 197         else
 198                 return 0;
 199 }
 200
 201 static inline
 202 int lib_ring_buffer_backend_get_pages(const struct lttng_ust_lib_ring_buffer_config *config,
 203                         struct lttng_ust_lib_ring_buffer_ctx *ctx,
 204                         struct lttng_ust_lib_ring_buffer_backend_pages **backend_pages)
 205 {
 206         struct lttng_ust_lib_ring_buffer_backend *bufb = &ctx->buf->backend;
 207         struct channel_backend *chanb = &ctx->chan->backend;
 208         struct lttng_ust_shm_handle *handle = ctx->handle;
 209         size_t sbidx;
 210         size_t offset = ctx->buf_offset;
 211         struct lttng_ust_lib_ring_buffer_backend_subbuffer *wsb;
 212         struct lttng_ust_lib_ring_buffer_backend_pages_shmp *rpages;
 213         unsigned long sb_bindex, id;
 214         struct lttng_ust_lib_ring_buffer_backend_pages *_backend_pages;
 215
 216         offset &= chanb->buf_size - 1;
 217         sbidx = offset >> chanb->subbuf_size_order;
 218         wsb = shmp_index(handle, bufb->buf_wsb, sbidx);
 219         if (caa_unlikely(!wsb))
 220                 return -1;
 221         id = wsb->id;
 222         sb_bindex = subbuffer_id_get_index(config, id);
 223         rpages = shmp_index(handle, bufb->array, sb_bindex);
 224         if (caa_unlikely(!rpages))
 225                 return -1;
 226         CHAN_WARN_ON(ctx->chan,
 227                      config->mode == RING_BUFFER_OVERWRITE
 228                      && subbuffer_id_is_noref(config, id));
 229         _backend_pages = shmp(handle, rpages->shmp);
 230         if (caa_unlikely(!_backend_pages))
 231                 return -1;
 232         *backend_pages = _backend_pages;
 233         return 0;
 234 }
 235
 236 /* Get backend pages from cache. */
 237 static inline
 238 struct lttng_ust_lib_ring_buffer_backend_pages *
 239         lib_ring_buffer_get_backend_pages_from_ctx(const struct lttng_ust_lib_ring_buffer_config *config,
 240                 struct lttng_ust_lib_ring_buffer_ctx *ctx)
 241 {
 242         if (caa_unlikely(ctx->ctx_len
 243                         < sizeof(struct lttng_ust_lib_ring_buffer_ctx)))
 244                 return NULL;
 245         return ctx->backend_pages;
 246 }
 247
 248 /*
 249  * The ring buffer can count events recorded and overwritten per buffer,
 250  * but it is disabled by default due to its performance overhead.
 251  */
 252 #ifdef LTTNG_RING_BUFFER_COUNT_EVENTS
 253 static inline
 254 void subbuffer_count_record(const struct lttng_ust_lib_ring_buffer_config *config,
 255                             const struct lttng_ust_lib_ring_buffer_ctx *ctx,
 256                             struct lttng_ust_lib_ring_buffer_backend *bufb,
 257                             unsigned long idx, struct lttng_ust_shm_handle *handle)
 258 {
 259         struct lttng_ust_lib_ring_buffer_backend_pages *backend_pages;
 260
 261         backend_pages = lib_ring_buffer_get_backend_pages_from_ctx(config, ctx);
 262         if (caa_unlikely(!backend_pages)) {
 263                 if (lib_ring_buffer_backend_get_pages(config, ctx, &backend_pages))
 264                         return;
 265         }
 266         v_inc(config, &backend_pages->records_commit);
 267 }
 268 #else /* LTTNG_RING_BUFFER_COUNT_EVENTS */
 269 static inline
 270 void subbuffer_count_record(const struct lttng_ust_lib_ring_buffer_config *config,
 271                             const struct lttng_ust_lib_ring_buffer_ctx *ctx,
 272                             struct lttng_ust_lib_ring_buffer_backend *bufb,
 273                             unsigned long idx, struct lttng_ust_shm_handle *handle)
 274 {
 275 }
 276 #endif /* #else LTTNG_RING_BUFFER_COUNT_EVENTS */
 277
 278 /*
 279  * Reader has exclusive subbuffer access for record consumption. No need to
 280  * perform the decrement atomically.
 281  */
 282 static inline
 283 void subbuffer_consume_record(const struct lttng_ust_lib_ring_buffer_config *config,
 284                               struct lttng_ust_lib_ring_buffer_backend *bufb,
 285                               struct lttng_ust_shm_handle *handle)
 286 {
 287         unsigned long sb_bindex;
 288         struct channel *chan;
 289         struct lttng_ust_lib_ring_buffer_backend_pages_shmp *pages_shmp;
 290         struct lttng_ust_lib_ring_buffer_backend_pages *backend_pages;
 291
 292         sb_bindex = subbuffer_id_get_index(config, bufb->buf_rsb.id);
 293         chan = shmp(handle, bufb->chan);
 294         if (!chan)
 295                 return;
 296         pages_shmp = shmp_index(handle, bufb->array, sb_bindex);
 297         if (!pages_shmp)
 298                 return;
 299         backend_pages = shmp(handle, pages_shmp->shmp);
 300         if (!backend_pages)
 301                 return;
 302         CHAN_WARN_ON(chan, !v_read(config, &backend_pages->records_unread));
 303         /* Non-atomic decrement protected by exclusive subbuffer access */
 304         _v_dec(config, &backend_pages->records_unread);
 305         v_inc(config, &bufb->records_read);
 306 }
 307
 308 static inline
 309 unsigned long subbuffer_get_records_count(
 310                                 const struct lttng_ust_lib_ring_buffer_config *config,
 311                                 struct lttng_ust_lib_ring_buffer_backend *bufb,
 312                                 unsigned long idx,
 313                                 struct lttng_ust_shm_handle *handle)
 314 {
 315         unsigned long sb_bindex;
 316         struct lttng_ust_lib_ring_buffer_backend_subbuffer *wsb;
 317         struct lttng_ust_lib_ring_buffer_backend_pages_shmp *rpages;
 318         struct lttng_ust_lib_ring_buffer_backend_pages *backend_pages;
 319
 320         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 321         if (!wsb)
 322                 return 0;
 323         sb_bindex = subbuffer_id_get_index(config, wsb->id);
 324         rpages = shmp_index(handle, bufb->array, sb_bindex);
 325         if (!rpages)
 326                 return 0;
 327         backend_pages = shmp(handle, rpages->shmp);
 328         if (!backend_pages)
 329                 return 0;
 330         return v_read(config, &backend_pages->records_commit);
 331 }
 332
 333 /*
 334  * Must be executed at subbuffer delivery when the writer has _exclusive_
 335  * subbuffer access. See lib_ring_buffer_check_deliver() for details.
 336  * lib_ring_buffer_get_records_count() must be called to get the records
 337  * count before this function, because it resets the records_commit
 338  * count.
 339  */
 340 static inline
 341 unsigned long subbuffer_count_records_overrun(
 342                                 const struct lttng_ust_lib_ring_buffer_config *config,
 343                                 struct lttng_ust_lib_ring_buffer_backend *bufb,
 344                                 unsigned long idx,
 345                                 struct lttng_ust_shm_handle *handle)
 346 {
 347         unsigned long overruns, sb_bindex;
 348         struct lttng_ust_lib_ring_buffer_backend_subbuffer *wsb;
 349         struct lttng_ust_lib_ring_buffer_backend_pages_shmp *rpages;
 350         struct lttng_ust_lib_ring_buffer_backend_pages *backend_pages;
 351
 352         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 353         if (!wsb)
 354                 return 0;
 355         sb_bindex = subbuffer_id_get_index(config, wsb->id);
 356         rpages = shmp_index(handle, bufb->array, sb_bindex);
 357         if (!rpages)
 358                 return 0;
 359         backend_pages = shmp(handle, rpages->shmp);
 360         if (!backend_pages)
 361                 return 0;
 362         overruns = v_read(config, &backend_pages->records_unread);
 363         v_set(config, &backend_pages->records_unread,
 364               v_read(config, &backend_pages->records_commit));
 365         v_set(config, &backend_pages->records_commit, 0);
 366
 367         return overruns;
 368 }
 369
 370 static inline
 371 void subbuffer_set_data_size(const struct lttng_ust_lib_ring_buffer_config *config,
 372                              struct lttng_ust_lib_ring_buffer_backend *bufb,
 373                              unsigned long idx,
 374                              unsigned long data_size,
 375                              struct lttng_ust_shm_handle *handle)
 376 {
 377         unsigned long sb_bindex;
 378         struct lttng_ust_lib_ring_buffer_backend_subbuffer *wsb;
 379         struct lttng_ust_lib_ring_buffer_backend_pages_shmp *rpages;
 380         struct lttng_ust_lib_ring_buffer_backend_pages *backend_pages;
 381
 382         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 383         if (!wsb)
 384                 return;
 385         sb_bindex = subbuffer_id_get_index(config, wsb->id);
 386         rpages = shmp_index(handle, bufb->array, sb_bindex);
 387         if (!rpages)
 388                 return;
 389         backend_pages = shmp(handle, rpages->shmp);
 390         if (!backend_pages)
 391                 return;
 392         backend_pages->data_size = data_size;
 393 }
 394
 395 static inline
 396 unsigned long subbuffer_get_read_data_size(
 397                                 const struct lttng_ust_lib_ring_buffer_config *config,
 398                                 struct lttng_ust_lib_ring_buffer_backend *bufb,
 399                                 struct lttng_ust_shm_handle *handle)
 400 {
 401         unsigned long sb_bindex;
 402         struct lttng_ust_lib_ring_buffer_backend_pages_shmp *pages_shmp;
 403         struct lttng_ust_lib_ring_buffer_backend_pages *backend_pages;
 404
 405         sb_bindex = subbuffer_id_get_index(config, bufb->buf_rsb.id);
 406         pages_shmp = shmp_index(handle, bufb->array, sb_bindex);
 407         if (!pages_shmp)
 408                 return 0;
 409         backend_pages = shmp(handle, pages_shmp->shmp);
 410         if (!backend_pages)
 411                 return 0;
 412         return backend_pages->data_size;
 413 }
 414
 415 static inline
 416 unsigned long subbuffer_get_data_size(
 417                                 const struct lttng_ust_lib_ring_buffer_config *config,
 418                                 struct lttng_ust_lib_ring_buffer_backend *bufb,
 419                                 unsigned long idx,
 420                                 struct lttng_ust_shm_handle *handle)
 421 {
 422         unsigned long sb_bindex;
 423         struct lttng_ust_lib_ring_buffer_backend_subbuffer *wsb;
 424         struct lttng_ust_lib_ring_buffer_backend_pages_shmp *rpages;
 425         struct lttng_ust_lib_ring_buffer_backend_pages *backend_pages;
 426
 427         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 428         if (!wsb)
 429                 return 0;
 430         sb_bindex = subbuffer_id_get_index(config, wsb->id);
 431         rpages = shmp_index(handle, bufb->array, sb_bindex);
 432         if (!rpages)
 433                 return 0;
 434         backend_pages = shmp(handle, rpages->shmp);
 435         if (!backend_pages)
 436                 return 0;
 437         return backend_pages->data_size;
 438 }
 439
 440 static inline
 441 void subbuffer_inc_packet_count(const struct lttng_ust_lib_ring_buffer_config *config,
 442                 struct lttng_ust_lib_ring_buffer_backend *bufb,
 443                 unsigned long idx, struct lttng_ust_shm_handle *handle)
 444 {
 445         struct lttng_ust_lib_ring_buffer_backend_counts *counts;
 446
 447         counts = shmp_index(handle, bufb->buf_cnt, idx);
 448         if (!counts)
 449                 return;
 450         counts->seq_cnt++;
 451 }
 452
 453 /**
 454  * lib_ring_buffer_clear_noref - Clear the noref subbuffer flag, called by
 455  *                               writer.
 456  */
 457 static inline
 458 void lib_ring_buffer_clear_noref(const struct lttng_ust_lib_ring_buffer_config *config,
 459                                  struct lttng_ust_lib_ring_buffer_backend *bufb,
 460                                  unsigned long idx,
 461                                  struct lttng_ust_shm_handle *handle)
 462 {
 463         unsigned long id, new_id;
 464         struct lttng_ust_lib_ring_buffer_backend_subbuffer *wsb;
 465
 466         if (config->mode != RING_BUFFER_OVERWRITE)
 467                 return;
 468
 469         /*
 470          * Performing a volatile access to read the sb_pages, because we want to
 471          * read a coherent version of the pointer and the associated noref flag.
 472          */
 473         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 474         if (!wsb)
 475                 return;
 476         id = CMM_ACCESS_ONCE(wsb->id);
 477         for (;;) {
 478                 /* This check is called on the fast path for each record. */
 479                 if (caa_likely(!subbuffer_id_is_noref(config, id))) {
 480                         /*
 481                          * Store after load dependency ordering the writes to
 482                          * the subbuffer after load and test of the noref flag
 483                          * matches the memory barrier implied by the cmpxchg()
 484                          * in update_read_sb_index().
 485                          */
 486                         return; /* Already writing to this buffer */
 487                 }
 488                 new_id = id;
 489                 subbuffer_id_clear_noref(config, &new_id);
 490                 new_id = uatomic_cmpxchg(&wsb->id, id, new_id);
 491                 if (caa_likely(new_id == id))
 492                         break;
 493                 id = new_id;
 494         }
 495 }
 496
 497 /**
 498  * lib_ring_buffer_set_noref_offset - Set the noref subbuffer flag and offset,
 499  *                                    called by writer.
 500  */
 501 static inline
 502 void lib_ring_buffer_set_noref_offset(const struct lttng_ust_lib_ring_buffer_config *config,
 503                                       struct lttng_ust_lib_ring_buffer_backend *bufb,
 504                                       unsigned long idx, unsigned long offset,
 505                                       struct lttng_ust_shm_handle *handle)
 506 {
 507         struct lttng_ust_lib_ring_buffer_backend_subbuffer *wsb;
 508         struct channel *chan;
 509
 510         if (config->mode != RING_BUFFER_OVERWRITE)
 511                 return;
 512
 513         wsb = shmp_index(handle, bufb->buf_wsb, idx);
 514         if (!wsb)
 515                 return;
 516         /*
 517          * Because ring_buffer_set_noref() is only called by a single thread
 518          * (the one which updated the cc_sb value), there are no concurrent
 519          * updates to take care of: other writers have not updated cc_sb, so
 520          * they cannot set the noref flag, and concurrent readers cannot modify
 521          * the pointer because the noref flag is not set yet.
 522          * The smp_wmb() in ring_buffer_commit() takes care of ordering writes
 523          * to the subbuffer before this set noref operation.
 524          * subbuffer_set_noref() uses a volatile store to deal with concurrent
 525          * readers of the noref flag.
 526          */
 527         chan = shmp(handle, bufb->chan);
 528         if (!chan)
 529                 return;
 530         CHAN_WARN_ON(chan, subbuffer_id_is_noref(config, wsb->id));
 531         /*
 532          * Memory barrier that ensures counter stores are ordered before set
 533          * noref and offset.
 534          */
 535         cmm_smp_mb();
 536         subbuffer_id_set_noref_offset(config, &wsb->id, offset);
 537 }
 538
 539 /**
 540  * update_read_sb_index - Read-side subbuffer index update.
 541  */
 542 static inline
 543 int update_read_sb_index(const struct lttng_ust_lib_ring_buffer_config *config,
 544                          struct lttng_ust_lib_ring_buffer_backend *bufb,
 545                          struct channel_backend *chanb,
 546                          unsigned long consumed_idx,
 547                          unsigned long consumed_count,
 548                          struct lttng_ust_shm_handle *handle)
 549 {
 550         struct lttng_ust_lib_ring_buffer_backend_subbuffer *wsb;
 551         unsigned long old_id, new_id;
 552
 553         wsb = shmp_index(handle, bufb->buf_wsb, consumed_idx);
 554         if (caa_unlikely(!wsb))
 555                 return -EPERM;
 556
 557         if (config->mode == RING_BUFFER_OVERWRITE) {
 558                 struct channel *chan;
 559
 560                 /*
 561                  * Exchange the target writer subbuffer with our own unused
 562                  * subbuffer. No need to use CMM_ACCESS_ONCE() here to read the
 563                  * old_wpage, because the value read will be confirmed by the
 564                  * following cmpxchg().
 565                  */
 566                 old_id = wsb->id;
 567                 if (caa_unlikely(!subbuffer_id_is_noref(config, old_id)))
 568                         return -EAGAIN;
 569                 /*
 570                  * Make sure the offset count we are expecting matches the one
 571                  * indicated by the writer.
 572                  */
 573                 if (caa_unlikely(!subbuffer_id_compare_offset(config, old_id,
 574                                                           consumed_count)))
 575                         return -EAGAIN;
 576                 chan = shmp(handle, bufb->chan);
 577                 if (caa_unlikely(!chan))
 578                         return -EPERM;
 579                 CHAN_WARN_ON(chan, !subbuffer_id_is_noref(config, bufb->buf_rsb.id));
 580                 subbuffer_id_set_noref_offset(config, &bufb->buf_rsb.id,
 581                                               consumed_count);
 582                 new_id = uatomic_cmpxchg(&wsb->id, old_id, bufb->buf_rsb.id);
 583                 if (caa_unlikely(old_id != new_id))
 584                         return -EAGAIN;
 585                 bufb->buf_rsb.id = new_id;
 586         } else {
 587                 /* No page exchange, use the writer page directly */
 588                 bufb->buf_rsb.id = wsb->id;
 589         }
 590         return 0;
 591 }
 592
 593 #ifndef inline_memcpy
 594 #define inline_memcpy(dest, src, n)     memcpy(dest, src, n)
 595 #endif
 596
 597 static inline __attribute__((always_inline))
 598 void lttng_inline_memcpy(void *dest, const void *src,
 599                 unsigned long len)
 600 {
 601         switch (len) {
 602         case 1:
 603                 *(uint8_t *) dest = *(const uint8_t *) src;
 604                 break;
 605         case 2:
 606                 *(uint16_t *) dest = *(const uint16_t *) src;
 607                 break;
 608         case 4:
 609                 *(uint32_t *) dest = *(const uint32_t *) src;
 610                 break;
 611         case 8:
 612                 *(uint64_t *) dest = *(const uint64_t *) src;
 613                 break;
 614         default:
 615                 inline_memcpy(dest, src, len);
 616         }
 617 }
 618
 619 /*
 620  * Use the architecture-specific memcpy implementation for constant-sized
 621  * inputs, but rely on an inline memcpy for length statically unknown.
 622  * The function call to memcpy is just way too expensive for a fast path.
 623  */
 624 #define lib_ring_buffer_do_copy(config, dest, src, len)         \
 625 do {                                                            \
 626         size_t __len = (len);                                   \
 627         if (__builtin_constant_p(len))                          \
 628                 memcpy(dest, src, __len);                       \
 629         else                                                    \
 630                 lttng_inline_memcpy(dest, src, __len);          \
 631 } while (0)
 632
 633 /*
 634  * write len bytes to dest with c
 635  */
 636 static inline
 637 void lib_ring_buffer_do_memset(char *dest, int c, unsigned long len)
 638 {
 639         unsigned long i;
 640
 641         for (i = 0; i < len; i++)
 642                 dest[i] = c;
 643 }
 644
 645 /* arch-agnostic implementation */
 646
 647 static inline int lttng_ust_fls(unsigned int x)
 648 {
 649         int r = 32;
 650
 651         if (!x)
 652                 return 0;
 653         if (!(x & 0xFFFF0000U)) {
 654                 x <<= 16;
 655                 r -= 16;
 656         }
 657         if (!(x & 0xFF000000U)) {
 658                 x <<= 8;
 659                 r -= 8;
 660         }
 661         if (!(x & 0xF0000000U)) {
 662                 x <<= 4;
 663                 r -= 4;
 664         }
 665         if (!(x & 0xC0000000U)) {
 666                 x <<= 2;
 667                 r -= 2;
 668         }
 669         if (!(x & 0x80000000U)) {
 670                 /* No need to bit shift on last operation */
 671                 r -= 1;
 672         }
 673         return r;
 674 }
 675
 676 static inline int get_count_order(unsigned int count)
 677 {
 678         int order;
 679
 680         order = lttng_ust_fls(count) - 1;
 681         if (count & (count - 1))
 682                 order++;
 683         return order;
 684 }
 685
 686 #endif /* _LTTNG_RING_BUFFER_BACKEND_INTERNAL_H */