libringbuffer/backend_internal.h

   1 #ifndef _LINUX_RING_BUFFER_BACKEND_INTERNAL_H
   2 #define _LINUX_RING_BUFFER_BACKEND_INTERNAL_H
   3
   4 /*
   5  * linux/ringbuffer/backend_internal.h
   6  *
   7  * Copyright (C) 2008-2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   8  *
   9  * Ring buffer backend (internal helpers).
  10  *
  11  * Dual LGPL v2.1/GPL v2 license.
  12  */
  13
  14 #include <unistd.h>
  15 #include <urcu/compiler.h>
  16
  17 #include <ust/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 int lib_ring_buffer_backend_create(struct lib_ring_buffer_backend *bufb,
  27                                    struct channel_backend *chan, int cpu,
  28                                    struct shm_handle *handle,
  29                                    struct shm_object *shmobj);
  30 void channel_backend_unregister_notifiers(struct channel_backend *chanb);
  31 void lib_ring_buffer_backend_free(struct lib_ring_buffer_backend *bufb);
  32 int channel_backend_init(struct channel_backend *chanb,
  33                          const char *name,
  34                          const struct lib_ring_buffer_config *config,
  35                          void *priv, size_t subbuf_size,
  36                          size_t num_subbuf, struct shm_handle *handle);
  37 void channel_backend_free(struct channel_backend *chanb,
  38                           struct shm_handle *handle);
  39
  40 void lib_ring_buffer_backend_reset(struct lib_ring_buffer_backend *bufb,
  41                                    struct shm_handle *handle);
  42 void channel_backend_reset(struct channel_backend *chanb);
  43
  44 int lib_ring_buffer_backend_init(void);
  45 void lib_ring_buffer_backend_exit(void);
  46
  47 extern void _lib_ring_buffer_write(struct lib_ring_buffer_backend *bufb,
  48                                    size_t offset, const void *src, size_t len,
  49                                    ssize_t pagecpy);
  50
  51 /*
  52  * Subbuffer ID bits for overwrite mode. Need to fit within a single word to be
  53  * exchanged atomically.
  54  *
  55  * Top half word, except lowest bit, belongs to "offset", which is used to keep
  56  * to count the produced buffers.  For overwrite mode, this provides the
  57  * consumer with the capacity to read subbuffers in order, handling the
  58  * situation where producers would write up to 2^15 buffers (or 2^31 for 64-bit
  59  * systems) concurrently with a single execution of get_subbuf (between offset
  60  * sampling and subbuffer ID exchange).
  61  */
  62
  63 #define HALF_ULONG_BITS         (CAA_BITS_PER_LONG >> 1)
  64
  65 #define SB_ID_OFFSET_SHIFT      (HALF_ULONG_BITS + 1)
  66 #define SB_ID_OFFSET_COUNT      (1UL << SB_ID_OFFSET_SHIFT)
  67 #define SB_ID_OFFSET_MASK       (~(SB_ID_OFFSET_COUNT - 1))
  68 /*
  69  * Lowest bit of top word half belongs to noref. Used only for overwrite mode.
  70  */
  71 #define SB_ID_NOREF_SHIFT       (SB_ID_OFFSET_SHIFT - 1)
  72 #define SB_ID_NOREF_COUNT       (1UL << SB_ID_NOREF_SHIFT)
  73 #define SB_ID_NOREF_MASK        SB_ID_NOREF_COUNT
  74 /*
  75  * In overwrite mode: lowest half of word is used for index.
  76  * Limit of 2^16 subbuffers per buffer on 32-bit, 2^32 on 64-bit.
  77  * In producer-consumer mode: whole word used for index.
  78  */
  79 #define SB_ID_INDEX_SHIFT       0
  80 #define SB_ID_INDEX_COUNT       (1UL << SB_ID_INDEX_SHIFT)
  81 #define SB_ID_INDEX_MASK        (SB_ID_NOREF_COUNT - 1)
  82
  83 /*
  84  * Construct the subbuffer id from offset, index and noref. Use only the index
  85  * for producer-consumer mode (offset and noref are only used in overwrite
  86  * mode).
  87  */
  88 static inline
  89 unsigned long subbuffer_id(const struct lib_ring_buffer_config *config,
  90                            unsigned long offset, unsigned long noref,
  91                            unsigned long index)
  92 {
  93         if (config->mode == RING_BUFFER_OVERWRITE)
  94                 return (offset << SB_ID_OFFSET_SHIFT)
  95                        | (noref << SB_ID_NOREF_SHIFT)
  96                        | index;
  97         else
  98                 return index;
  99 }
 100
 101 /*
 102  * Compare offset with the offset contained within id. Return 1 if the offset
 103  * bits are identical, else 0.
 104  */
 105 static inline
 106 int subbuffer_id_compare_offset(const struct lib_ring_buffer_config *config,
 107                                 unsigned long id, unsigned long offset)
 108 {
 109         return (id & SB_ID_OFFSET_MASK) == (offset << SB_ID_OFFSET_SHIFT);
 110 }
 111
 112 static inline
 113 unsigned long subbuffer_id_get_index(const struct lib_ring_buffer_config *config,
 114                                      unsigned long id)
 115 {
 116         if (config->mode == RING_BUFFER_OVERWRITE)
 117                 return id & SB_ID_INDEX_MASK;
 118         else
 119                 return id;
 120 }
 121
 122 static inline
 123 unsigned long subbuffer_id_is_noref(const struct lib_ring_buffer_config *config,
 124                                     unsigned long id)
 125 {
 126         if (config->mode == RING_BUFFER_OVERWRITE)
 127                 return !!(id & SB_ID_NOREF_MASK);
 128         else
 129                 return 1;
 130 }
 131
 132 /*
 133  * Only used by reader on subbuffer ID it has exclusive access to. No volatile
 134  * needed.
 135  */
 136 static inline
 137 void subbuffer_id_set_noref(const struct lib_ring_buffer_config *config,
 138                             unsigned long *id)
 139 {
 140         if (config->mode == RING_BUFFER_OVERWRITE)
 141                 *id |= SB_ID_NOREF_MASK;
 142 }
 143
 144 static inline
 145 void subbuffer_id_set_noref_offset(const struct lib_ring_buffer_config *config,
 146                                    unsigned long *id, unsigned long offset)
 147 {
 148         unsigned long tmp;
 149
 150         if (config->mode == RING_BUFFER_OVERWRITE) {
 151                 tmp = *id;
 152                 tmp &= ~SB_ID_OFFSET_MASK;
 153                 tmp |= offset << SB_ID_OFFSET_SHIFT;
 154                 tmp |= SB_ID_NOREF_MASK;
 155                 /* Volatile store, read concurrently by readers. */
 156                 CMM_ACCESS_ONCE(*id) = tmp;
 157         }
 158 }
 159
 160 /* No volatile access, since already used locally */
 161 static inline
 162 void subbuffer_id_clear_noref(const struct lib_ring_buffer_config *config,
 163                               unsigned long *id)
 164 {
 165         if (config->mode == RING_BUFFER_OVERWRITE)
 166                 *id &= ~SB_ID_NOREF_MASK;
 167 }
 168
 169 /*
 170  * For overwrite mode, cap the number of subbuffers per buffer to:
 171  * 2^16 on 32-bit architectures
 172  * 2^32 on 64-bit architectures
 173  * This is required to fit in the index part of the ID. Return 0 on success,
 174  * -EPERM on failure.
 175  */
 176 static inline
 177 int subbuffer_id_check_index(const struct lib_ring_buffer_config *config,
 178                              unsigned long num_subbuf)
 179 {
 180         if (config->mode == RING_BUFFER_OVERWRITE)
 181                 return (num_subbuf > (1UL << HALF_ULONG_BITS)) ? -EPERM : 0;
 182         else
 183                 return 0;
 184 }
 185
 186 static inline
 187 void subbuffer_count_record(const struct lib_ring_buffer_config *config,
 188                             struct lib_ring_buffer_backend *bufb,
 189                             unsigned long idx, struct shm_handle *handle)
 190 {
 191         unsigned long sb_bindex;
 192
 193         sb_bindex = subbuffer_id_get_index(config, shmp(handle, bufb->buf_wsb)[idx].id);
 194         v_inc(config, &shmp(handle, (shmp(handle, bufb->array)[sb_bindex]).shmp)->records_commit);
 195 }
 196
 197 /*
 198  * Reader has exclusive subbuffer access for record consumption. No need to
 199  * perform the decrement atomically.
 200  */
 201 static inline
 202 void subbuffer_consume_record(const struct lib_ring_buffer_config *config,
 203                               struct lib_ring_buffer_backend *bufb,
 204                               struct shm_handle *handle)
 205 {
 206         unsigned long sb_bindex;
 207
 208         sb_bindex = subbuffer_id_get_index(config, bufb->buf_rsb.id);
 209         CHAN_WARN_ON(shmp(handle, bufb->chan),
 210                      !v_read(config, &shmp(handle, (shmp(handle, bufb->array)[sb_bindex]).shmp)->records_unread));
 211         /* Non-atomic decrement protected by exclusive subbuffer access */
 212         _v_dec(config, &shmp(handle, (shmp(handle, bufb->array)[sb_bindex]).shmp)->records_unread);
 213         v_inc(config, &bufb->records_read);
 214 }
 215
 216 static inline
 217 unsigned long subbuffer_get_records_count(
 218                                 const struct lib_ring_buffer_config *config,
 219                                 struct lib_ring_buffer_backend *bufb,
 220                                 unsigned long idx,
 221                                 struct shm_handle *handle)
 222 {
 223         unsigned long sb_bindex;
 224
 225         sb_bindex = subbuffer_id_get_index(config, shmp(handle, bufb->buf_wsb)[idx].id);
 226         return v_read(config, &shmp(handle, (shmp(handle, bufb->array)[sb_bindex]).shmp)->records_commit);
 227 }
 228
 229 /*
 230  * Must be executed at subbuffer delivery when the writer has _exclusive_
 231  * subbuffer access. See ring_buffer_check_deliver() for details.
 232  * ring_buffer_get_records_count() must be called to get the records count
 233  * before this function, because it resets the records_commit count.
 234  */
 235 static inline
 236 unsigned long subbuffer_count_records_overrun(
 237                                 const struct lib_ring_buffer_config *config,
 238                                 struct lib_ring_buffer_backend *bufb,
 239                                 unsigned long idx,
 240                                 struct shm_handle *handle)
 241 {
 242         struct lib_ring_buffer_backend_pages_shmp *pages;
 243         unsigned long overruns, sb_bindex;
 244
 245         sb_bindex = subbuffer_id_get_index(config, shmp(handle, bufb->buf_wsb)[idx].id);
 246         pages = &shmp(handle, bufb->array)[sb_bindex];
 247         overruns = v_read(config, &shmp(handle, pages->shmp)->records_unread);
 248         v_set(config, &shmp(handle, pages->shmp)->records_unread,
 249               v_read(config, &shmp(handle, pages->shmp)->records_commit));
 250         v_set(config, &shmp(handle, pages->shmp)->records_commit, 0);
 251
 252         return overruns;
 253 }
 254
 255 static inline
 256 void subbuffer_set_data_size(const struct lib_ring_buffer_config *config,
 257                              struct lib_ring_buffer_backend *bufb,
 258                              unsigned long idx,
 259                              unsigned long data_size,
 260                              struct shm_handle *handle)
 261 {
 262         struct lib_ring_buffer_backend_pages_shmp *pages;
 263         unsigned long sb_bindex;
 264
 265         sb_bindex = subbuffer_id_get_index(config, shmp(handle, bufb->buf_wsb)[idx].id);
 266         pages = &shmp(handle, bufb->array)[sb_bindex];
 267         shmp(handle, pages->shmp)->data_size = data_size;
 268 }
 269
 270 static inline
 271 unsigned long subbuffer_get_read_data_size(
 272                                 const struct lib_ring_buffer_config *config,
 273                                 struct lib_ring_buffer_backend *bufb,
 274                                 struct shm_handle *handle)
 275 {
 276         struct lib_ring_buffer_backend_pages_shmp *pages;
 277         unsigned long sb_bindex;
 278
 279         sb_bindex = subbuffer_id_get_index(config, bufb->buf_rsb.id);
 280         pages = &shmp(handle, bufb->array)[sb_bindex];
 281         return shmp(handle, pages->shmp)->data_size;
 282 }
 283
 284 static inline
 285 unsigned long subbuffer_get_data_size(
 286                                 const struct lib_ring_buffer_config *config,
 287                                 struct lib_ring_buffer_backend *bufb,
 288                                 unsigned long idx,
 289                                 struct shm_handle *handle)
 290 {
 291         struct lib_ring_buffer_backend_pages_shmp *pages;
 292         unsigned long sb_bindex;
 293
 294         sb_bindex = subbuffer_id_get_index(config, shmp(handle, bufb->buf_wsb)[idx].id);
 295         pages = &shmp(handle, bufb->array)[sb_bindex];
 296         return shmp(handle, pages->shmp)->data_size;
 297 }
 298
 299 /**
 300  * lib_ring_buffer_clear_noref - Clear the noref subbuffer flag, called by
 301  *                               writer.
 302  */
 303 static inline
 304 void lib_ring_buffer_clear_noref(const struct lib_ring_buffer_config *config,
 305                                  struct lib_ring_buffer_backend *bufb,
 306                                  unsigned long idx,
 307                                  struct shm_handle *handle)
 308 {
 309         unsigned long id, new_id;
 310
 311         if (config->mode != RING_BUFFER_OVERWRITE)
 312                 return;
 313
 314         /*
 315          * Performing a volatile access to read the sb_pages, because we want to
 316          * read a coherent version of the pointer and the associated noref flag.
 317          */
 318         id = CMM_ACCESS_ONCE(shmp(handle, bufb->buf_wsb)[idx].id);
 319         for (;;) {
 320                 /* This check is called on the fast path for each record. */
 321                 if (likely(!subbuffer_id_is_noref(config, id))) {
 322                         /*
 323                          * Store after load dependency ordering the writes to
 324                          * the subbuffer after load and test of the noref flag
 325                          * matches the memory barrier implied by the cmpxchg()
 326                          * in update_read_sb_index().
 327                          */
 328                         return; /* Already writing to this buffer */
 329                 }
 330                 new_id = id;
 331                 subbuffer_id_clear_noref(config, &new_id);
 332                 new_id = uatomic_cmpxchg(&shmp(handle, bufb->buf_wsb)[idx].id, id, new_id);
 333                 if (likely(new_id == id))
 334                         break;
 335                 id = new_id;
 336         }
 337 }
 338
 339 /**
 340  * lib_ring_buffer_set_noref_offset - Set the noref subbuffer flag and offset,
 341  *                                    called by writer.
 342  */
 343 static inline
 344 void lib_ring_buffer_set_noref_offset(const struct lib_ring_buffer_config *config,
 345                                       struct lib_ring_buffer_backend *bufb,
 346                                       unsigned long idx, unsigned long offset,
 347                                       struct shm_handle *handle)
 348 {
 349         if (config->mode != RING_BUFFER_OVERWRITE)
 350                 return;
 351
 352         /*
 353          * Because ring_buffer_set_noref() is only called by a single thread
 354          * (the one which updated the cc_sb value), there are no concurrent
 355          * updates to take care of: other writers have not updated cc_sb, so
 356          * they cannot set the noref flag, and concurrent readers cannot modify
 357          * the pointer because the noref flag is not set yet.
 358          * The smp_wmb() in ring_buffer_commit() takes care of ordering writes
 359          * to the subbuffer before this set noref operation.
 360          * subbuffer_set_noref() uses a volatile store to deal with concurrent
 361          * readers of the noref flag.
 362          */
 363         CHAN_WARN_ON(shmp(handle, bufb->chan),
 364                      subbuffer_id_is_noref(config, shmp(handle, bufb->buf_wsb)[idx].id));
 365         /*
 366          * Memory barrier that ensures counter stores are ordered before set
 367          * noref and offset.
 368          */
 369         cmm_smp_mb();
 370         subbuffer_id_set_noref_offset(config, &shmp(handle, bufb->buf_wsb)[idx].id, offset);
 371 }
 372
 373 /**
 374  * update_read_sb_index - Read-side subbuffer index update.
 375  */
 376 static inline
 377 int update_read_sb_index(const struct lib_ring_buffer_config *config,
 378                          struct lib_ring_buffer_backend *bufb,
 379                          struct channel_backend *chanb,
 380                          unsigned long consumed_idx,
 381                          unsigned long consumed_count,
 382                          struct shm_handle *handle)
 383 {
 384         unsigned long old_id, new_id;
 385
 386         if (config->mode == RING_BUFFER_OVERWRITE) {
 387                 /*
 388                  * Exchange the target writer subbuffer with our own unused
 389                  * subbuffer. No need to use CMM_ACCESS_ONCE() here to read the
 390                  * old_wpage, because the value read will be confirmed by the
 391                  * following cmpxchg().
 392                  */
 393                 old_id = shmp(handle, bufb->buf_wsb)[consumed_idx].id;
 394                 if (unlikely(!subbuffer_id_is_noref(config, old_id)))
 395                         return -EAGAIN;
 396                 /*
 397                  * Make sure the offset count we are expecting matches the one
 398                  * indicated by the writer.
 399                  */
 400                 if (unlikely(!subbuffer_id_compare_offset(config, old_id,
 401                                                           consumed_count)))
 402                         return -EAGAIN;
 403                 CHAN_WARN_ON(shmp(handle, bufb->chan),
 404                              !subbuffer_id_is_noref(config, bufb->buf_rsb.id));
 405                 subbuffer_id_set_noref_offset(config, &bufb->buf_rsb.id,
 406                                               consumed_count);
 407                 new_id = uatomic_cmpxchg(&shmp(handle, bufb->buf_wsb)[consumed_idx].id, old_id,
 408                                  bufb->buf_rsb.id);
 409                 if (unlikely(old_id != new_id))
 410                         return -EAGAIN;
 411                 bufb->buf_rsb.id = new_id;
 412         } else {
 413                 /* No page exchange, use the writer page directly */
 414                 bufb->buf_rsb.id = shmp(handle, bufb->buf_wsb)[consumed_idx].id;
 415         }
 416         return 0;
 417 }
 418
 419 /*
 420  * Use the architecture-specific memcpy implementation for constant-sized
 421  * inputs, but rely on an inline memcpy for length statically unknown.
 422  * The function call to memcpy is just way too expensive for a fast path.
 423  */
 424 #define lib_ring_buffer_do_copy(config, dest, src, len)         \
 425 do {                                                            \
 426         size_t __len = (len);                                   \
 427         if (__builtin_constant_p(len))                          \
 428                 memcpy(dest, src, __len);                       \
 429         else                                                    \
 430                 inline_memcpy(dest, src, __len);                \
 431 } while (0)
 432
 433 #endif /* _LINUX_RING_BUFFER_BACKEND_INTERNAL_H */