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