lib/ringbuffer/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 "../../wrapper/ringbuffer/config.h"
  15 #include "../../wrapper/ringbuffer/backend_types.h"
  16 #include "../../wrapper/ringbuffer/frontend_types.h"
  17 #include <linux/string.h>
  18
  19 /* Ring buffer backend API presented to the frontend */
  20
  21 /* Ring buffer and channel backend create/free */
  22
  23 int lib_ring_buffer_backend_create(struct lib_ring_buffer_backend *bufb,
  24                                    struct channel_backend *chan, int cpu);
  25 void channel_backend_unregister_notifiers(struct channel_backend *chanb);
  26 void lib_ring_buffer_backend_free(struct lib_ring_buffer_backend *bufb);
  27 int channel_backend_init(struct channel_backend *chanb,
  28                          const char *name,
  29                          const struct lib_ring_buffer_config *config,
  30                          void *priv, size_t subbuf_size,
  31                          size_t num_subbuf);
  32 void channel_backend_free(struct channel_backend *chanb);
  33
  34 void lib_ring_buffer_backend_reset(struct lib_ring_buffer_backend *bufb);
  35 void channel_backend_reset(struct channel_backend *chanb);
  36
  37 int lib_ring_buffer_backend_init(void);
  38 void lib_ring_buffer_backend_exit(void);
  39
  40 extern void _lib_ring_buffer_write(struct lib_ring_buffer_backend *bufb,
  41                                    size_t offset, const void *src, size_t len,
  42                                    ssize_t pagecpy);
  43
  44 /*
  45  * Subbuffer ID bits for overwrite mode. Need to fit within a single word to be
  46  * exchanged atomically.
  47  *
  48  * Top half word, except lowest bit, belongs to "offset", which is used to keep
  49  * to count the produced buffers.  For overwrite mode, this provides the
  50  * consumer with the capacity to read subbuffers in order, handling the
  51  * situation where producers would write up to 2^15 buffers (or 2^31 for 64-bit
  52  * systems) concurrently with a single execution of get_subbuf (between offset
  53  * sampling and subbuffer ID exchange).
  54  */
  55
  56 #define HALF_ULONG_BITS         (BITS_PER_LONG >> 1)
  57
  58 #define SB_ID_OFFSET_SHIFT      (HALF_ULONG_BITS + 1)
  59 #define SB_ID_OFFSET_COUNT      (1UL << SB_ID_OFFSET_SHIFT)
  60 #define SB_ID_OFFSET_MASK       (~(SB_ID_OFFSET_COUNT - 1))
  61 /*
  62  * Lowest bit of top word half belongs to noref. Used only for overwrite mode.
  63  */
  64 #define SB_ID_NOREF_SHIFT       (SB_ID_OFFSET_SHIFT - 1)
  65 #define SB_ID_NOREF_COUNT       (1UL << SB_ID_NOREF_SHIFT)
  66 #define SB_ID_NOREF_MASK        SB_ID_NOREF_COUNT
  67 /*
  68  * In overwrite mode: lowest half of word is used for index.
  69  * Limit of 2^16 subbuffers per buffer on 32-bit, 2^32 on 64-bit.
  70  * In producer-consumer mode: whole word used for index.
  71  */
  72 #define SB_ID_INDEX_SHIFT       0
  73 #define SB_ID_INDEX_COUNT       (1UL << SB_ID_INDEX_SHIFT)
  74 #define SB_ID_INDEX_MASK        (SB_ID_NOREF_COUNT - 1)
  75
  76 /*
  77  * Construct the subbuffer id from offset, index and noref. Use only the index
  78  * for producer-consumer mode (offset and noref are only used in overwrite
  79  * mode).
  80  */
  81 static inline
  82 unsigned long subbuffer_id(const struct lib_ring_buffer_config *config,
  83                            unsigned long offset, unsigned long noref,
  84                            unsigned long index)
  85 {
  86         if (config->mode == RING_BUFFER_OVERWRITE)
  87                 return (offset << SB_ID_OFFSET_SHIFT)
  88                        | (noref << SB_ID_NOREF_SHIFT)
  89                        | index;
  90         else
  91                 return index;
  92 }
  93
  94 /*
  95  * Compare offset with the offset contained within id. Return 1 if the offset
  96  * bits are identical, else 0.
  97  */
  98 static inline
  99 int subbuffer_id_compare_offset(const struct lib_ring_buffer_config *config,
 100                                 unsigned long id, unsigned long offset)
 101 {
 102         return (id & SB_ID_OFFSET_MASK) == (offset << SB_ID_OFFSET_SHIFT);
 103 }
 104
 105 static inline
 106 unsigned long subbuffer_id_get_index(const struct lib_ring_buffer_config *config,
 107                                      unsigned long id)
 108 {
 109         if (config->mode == RING_BUFFER_OVERWRITE)
 110                 return id & SB_ID_INDEX_MASK;
 111         else
 112                 return id;
 113 }
 114
 115 static inline
 116 unsigned long subbuffer_id_is_noref(const struct lib_ring_buffer_config *config,
 117                                     unsigned long id)
 118 {
 119         if (config->mode == RING_BUFFER_OVERWRITE)
 120                 return !!(id & SB_ID_NOREF_MASK);
 121         else
 122                 return 1;
 123 }
 124
 125 /*
 126  * Only used by reader on subbuffer ID it has exclusive access to. No volatile
 127  * needed.
 128  */
 129 static inline
 130 void subbuffer_id_set_noref(const struct lib_ring_buffer_config *config,
 131                             unsigned long *id)
 132 {
 133         if (config->mode == RING_BUFFER_OVERWRITE)
 134                 *id |= SB_ID_NOREF_MASK;
 135 }
 136
 137 static inline
 138 void subbuffer_id_set_noref_offset(const struct lib_ring_buffer_config *config,
 139                                    unsigned long *id, unsigned long offset)
 140 {
 141         unsigned long tmp;
 142
 143         if (config->mode == RING_BUFFER_OVERWRITE) {
 144                 tmp = *id;
 145                 tmp &= ~SB_ID_OFFSET_MASK;
 146                 tmp |= offset << SB_ID_OFFSET_SHIFT;
 147                 tmp |= SB_ID_NOREF_MASK;
 148                 /* Volatile store, read concurrently by readers. */
 149                 ACCESS_ONCE(*id) = tmp;
 150         }
 151 }
 152
 153 /* No volatile access, since already used locally */
 154 static inline
 155 void subbuffer_id_clear_noref(const struct lib_ring_buffer_config *config,
 156                               unsigned long *id)
 157 {
 158         if (config->mode == RING_BUFFER_OVERWRITE)
 159                 *id &= ~SB_ID_NOREF_MASK;
 160 }
 161
 162 /*
 163  * For overwrite mode, cap the number of subbuffers per buffer to:
 164  * 2^16 on 32-bit architectures
 165  * 2^32 on 64-bit architectures
 166  * This is required to fit in the index part of the ID. Return 0 on success,
 167  * -EPERM on failure.
 168  */
 169 static inline
 170 int subbuffer_id_check_index(const struct lib_ring_buffer_config *config,
 171                              unsigned long num_subbuf)
 172 {
 173         if (config->mode == RING_BUFFER_OVERWRITE)
 174                 return (num_subbuf > (1UL << HALF_ULONG_BITS)) ? -EPERM : 0;
 175         else
 176                 return 0;
 177 }
 178
 179 static inline
 180 void subbuffer_count_record(const struct lib_ring_buffer_config *config,
 181                             struct lib_ring_buffer_backend *bufb,
 182                             unsigned long idx)
 183 {
 184         unsigned long sb_bindex;
 185
 186         sb_bindex = subbuffer_id_get_index(config, bufb->buf_wsb[idx].id);
 187         v_inc(config, &bufb->array[sb_bindex]->records_commit);
 188 }
 189
 190 /*
 191  * Reader has exclusive subbuffer access for record consumption. No need to
 192  * perform the decrement atomically.
 193  */
 194 static inline
 195 void subbuffer_consume_record(const struct lib_ring_buffer_config *config,
 196                               struct lib_ring_buffer_backend *bufb)
 197 {
 198         unsigned long sb_bindex;
 199
 200         sb_bindex = subbuffer_id_get_index(config, bufb->buf_rsb.id);
 201         CHAN_WARN_ON(bufb->chan,
 202                      !v_read(config, &bufb->array[sb_bindex]->records_unread));
 203         /* Non-atomic decrement protected by exclusive subbuffer access */
 204         _v_dec(config, &bufb->array[sb_bindex]->records_unread);
 205         v_inc(config, &bufb->records_read);
 206 }
 207
 208 static inline
 209 unsigned long subbuffer_get_records_count(
 210                                 const struct lib_ring_buffer_config *config,
 211                                 struct lib_ring_buffer_backend *bufb,
 212                                 unsigned long idx)
 213 {
 214         unsigned long sb_bindex;
 215
 216         sb_bindex = subbuffer_id_get_index(config, bufb->buf_wsb[idx].id);
 217         return v_read(config, &bufb->array[sb_bindex]->records_commit);
 218 }
 219
 220 /*
 221  * Must be executed at subbuffer delivery when the writer has _exclusive_
 222  * subbuffer access. See ring_buffer_check_deliver() for details.
 223  * ring_buffer_get_records_count() must be called to get the records count
 224  * before this function, because it resets the records_commit count.
 225  */
 226 static inline
 227 unsigned long subbuffer_count_records_overrun(
 228                                 const struct lib_ring_buffer_config *config,
 229                                 struct lib_ring_buffer_backend *bufb,
 230                                 unsigned long idx)
 231 {
 232         struct lib_ring_buffer_backend_pages *pages;
 233         unsigned long overruns, sb_bindex;
 234
 235         sb_bindex = subbuffer_id_get_index(config, bufb->buf_wsb[idx].id);
 236         pages = bufb->array[sb_bindex];
 237         overruns = v_read(config, &pages->records_unread);
 238         v_set(config, &pages->records_unread,
 239               v_read(config, &pages->records_commit));
 240         v_set(config, &pages->records_commit, 0);
 241
 242         return overruns;
 243 }
 244
 245 static inline
 246 void subbuffer_set_data_size(const struct lib_ring_buffer_config *config,
 247                              struct lib_ring_buffer_backend *bufb,
 248                              unsigned long idx,
 249                              unsigned long data_size)
 250 {
 251         struct lib_ring_buffer_backend_pages *pages;
 252         unsigned long sb_bindex;
 253
 254         sb_bindex = subbuffer_id_get_index(config, bufb->buf_wsb[idx].id);
 255         pages = bufb->array[sb_bindex];
 256         pages->data_size = data_size;
 257 }
 258
 259 static inline
 260 unsigned long subbuffer_get_read_data_size(
 261                                 const struct lib_ring_buffer_config *config,
 262                                 struct lib_ring_buffer_backend *bufb)
 263 {
 264         struct lib_ring_buffer_backend_pages *pages;
 265         unsigned long sb_bindex;
 266
 267         sb_bindex = subbuffer_id_get_index(config, bufb->buf_rsb.id);
 268         pages = bufb->array[sb_bindex];
 269         return pages->data_size;
 270 }
 271
 272 static inline
 273 unsigned long subbuffer_get_data_size(
 274                                 const struct lib_ring_buffer_config *config,
 275                                 struct lib_ring_buffer_backend *bufb,
 276                                 unsigned long idx)
 277 {
 278         struct lib_ring_buffer_backend_pages *pages;
 279         unsigned long sb_bindex;
 280
 281         sb_bindex = subbuffer_id_get_index(config, bufb->buf_wsb[idx].id);
 282         pages = bufb->array[sb_bindex];
 283         return pages->data_size;
 284 }
 285
 286 /**
 287  * lib_ring_buffer_clear_noref - Clear the noref subbuffer flag, called by
 288  *                               writer.
 289  */
 290 static inline
 291 void lib_ring_buffer_clear_noref(const struct lib_ring_buffer_config *config,
 292                                  struct lib_ring_buffer_backend *bufb,
 293                                  unsigned long idx)
 294 {
 295         unsigned long id, new_id;
 296
 297         if (config->mode != RING_BUFFER_OVERWRITE)
 298                 return;
 299
 300         /*
 301          * Performing a volatile access to read the sb_pages, because we want to
 302          * read a coherent version of the pointer and the associated noref flag.
 303          */
 304         id = ACCESS_ONCE(bufb->buf_wsb[idx].id);
 305         for (;;) {
 306                 /* This check is called on the fast path for each record. */
 307                 if (likely(!subbuffer_id_is_noref(config, id))) {
 308                         /*
 309                          * Store after load dependency ordering the writes to
 310                          * the subbuffer after load and test of the noref flag
 311                          * matches the memory barrier implied by the cmpxchg()
 312                          * in update_read_sb_index().
 313                          */
 314                         return; /* Already writing to this buffer */
 315                 }
 316                 new_id = id;
 317                 subbuffer_id_clear_noref(config, &new_id);
 318                 new_id = cmpxchg(&bufb->buf_wsb[idx].id, id, new_id);
 319                 if (likely(new_id == id))
 320                         break;
 321                 id = new_id;
 322         }
 323 }
 324
 325 /**
 326  * lib_ring_buffer_set_noref_offset - Set the noref subbuffer flag and offset,
 327  *                                    called by writer.
 328  */
 329 static inline
 330 void lib_ring_buffer_set_noref_offset(const struct lib_ring_buffer_config *config,
 331                                       struct lib_ring_buffer_backend *bufb,
 332                                       unsigned long idx, unsigned long offset)
 333 {
 334         if (config->mode != RING_BUFFER_OVERWRITE)
 335                 return;
 336
 337         /*
 338          * Because ring_buffer_set_noref() is only called by a single thread
 339          * (the one which updated the cc_sb value), there are no concurrent
 340          * updates to take care of: other writers have not updated cc_sb, so
 341          * they cannot set the noref flag, and concurrent readers cannot modify
 342          * the pointer because the noref flag is not set yet.
 343          * The smp_wmb() in ring_buffer_commit() takes care of ordering writes
 344          * to the subbuffer before this set noref operation.
 345          * subbuffer_set_noref() uses a volatile store to deal with concurrent
 346          * readers of the noref flag.
 347          */
 348         CHAN_WARN_ON(bufb->chan,
 349                      subbuffer_id_is_noref(config, bufb->buf_wsb[idx].id));
 350         /*
 351          * Memory barrier that ensures counter stores are ordered before set
 352          * noref and offset.
 353          */
 354         smp_mb();
 355         subbuffer_id_set_noref_offset(config, &bufb->buf_wsb[idx].id, offset);
 356 }
 357
 358 /**
 359  * update_read_sb_index - Read-side subbuffer index update.
 360  */
 361 static inline
 362 int update_read_sb_index(const struct lib_ring_buffer_config *config,
 363                          struct lib_ring_buffer_backend *bufb,
 364                          struct channel_backend *chanb,
 365                          unsigned long consumed_idx,
 366                          unsigned long consumed_count)
 367 {
 368         unsigned long old_id, new_id;
 369
 370         if (config->mode == RING_BUFFER_OVERWRITE) {
 371                 /*
 372                  * Exchange the target writer subbuffer with our own unused
 373                  * subbuffer. No need to use ACCESS_ONCE() here to read the
 374                  * old_wpage, because the value read will be confirmed by the
 375                  * following cmpxchg().
 376                  */
 377                 old_id = bufb->buf_wsb[consumed_idx].id;
 378                 if (unlikely(!subbuffer_id_is_noref(config, old_id)))
 379                         return -EAGAIN;
 380                 /*
 381                  * Make sure the offset count we are expecting matches the one
 382                  * indicated by the writer.
 383                  */
 384                 if (unlikely(!subbuffer_id_compare_offset(config, old_id,
 385                                                           consumed_count)))
 386                         return -EAGAIN;
 387                 CHAN_WARN_ON(bufb->chan,
 388                              !subbuffer_id_is_noref(config, bufb->buf_rsb.id));
 389                 subbuffer_id_set_noref_offset(config, &bufb->buf_rsb.id,
 390                                               consumed_count);
 391                 new_id = cmpxchg(&bufb->buf_wsb[consumed_idx].id, old_id,
 392                                  bufb->buf_rsb.id);
 393                 if (unlikely(old_id != new_id))
 394                         return -EAGAIN;
 395                 bufb->buf_rsb.id = new_id;
 396         } else {
 397                 /* No page exchange, use the writer page directly */
 398                 bufb->buf_rsb.id = bufb->buf_wsb[consumed_idx].id;
 399         }
 400         return 0;
 401 }
 402
 403 /*
 404  * Use the architecture-specific memcpy implementation for constant-sized
 405  * inputs, but rely on an inline memcpy for length statically unknown.
 406  * The function call to memcpy is just way too expensive for a fast path.
 407  */
 408 #define lib_ring_buffer_do_copy(config, dest, src, len)         \
 409 do {                                                            \
 410         size_t __len = (len);                                   \
 411         if (__builtin_constant_p(len))                          \
 412                 memcpy(dest, src, __len);                       \
 413         else                                                    \
 414                 inline_memcpy(dest, src, __len);                \
 415 } while (0)
 416
 417 #endif /* _LINUX_RING_BUFFER_BACKEND_INTERNAL_H */