lib/ringbuffer/frontend_internal.h

   1 #ifndef _LIB_RING_BUFFER_FRONTEND_INTERNAL_H
   2 #define _LIB_RING_BUFFER_FRONTEND_INTERNAL_H
   3
   4 /*
   5  * linux/ringbuffer/frontend_internal.h
   6  *
   7  * Ring Buffer Library Synchronization Header (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  * Author:
  26  *      Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  27  *
  28  * See ring_buffer_frontend.c for more information on wait-free algorithms.
  29  */
  30
  31 #include "../../wrapper/ringbuffer/config.h"
  32 #include "../../wrapper/ringbuffer/backend_types.h"
  33 #include "../../wrapper/ringbuffer/frontend_types.h"
  34 #include "../../lib/prio_heap/lttng_prio_heap.h"        /* For per-CPU read-side iterator */
  35
  36 /* Buffer offset macros */
  37
  38 /* buf_trunc mask selects only the buffer number. */
  39 static inline
  40 unsigned long buf_trunc(unsigned long offset, struct channel *chan)
  41 {
  42         return offset & ~(chan->backend.buf_size - 1);
  43
  44 }
  45
  46 /* Select the buffer number value (counter). */
  47 static inline
  48 unsigned long buf_trunc_val(unsigned long offset, struct channel *chan)
  49 {
  50         return buf_trunc(offset, chan) >> chan->backend.buf_size_order;
  51 }
  52
  53 /* buf_offset mask selects only the offset within the current buffer. */
  54 static inline
  55 unsigned long buf_offset(unsigned long offset, struct channel *chan)
  56 {
  57         return offset & (chan->backend.buf_size - 1);
  58 }
  59
  60 /* subbuf_offset mask selects the offset within the current subbuffer. */
  61 static inline
  62 unsigned long subbuf_offset(unsigned long offset, struct channel *chan)
  63 {
  64         return offset & (chan->backend.subbuf_size - 1);
  65 }
  66
  67 /* subbuf_trunc mask selects the subbuffer number. */
  68 static inline
  69 unsigned long subbuf_trunc(unsigned long offset, struct channel *chan)
  70 {
  71         return offset & ~(chan->backend.subbuf_size - 1);
  72 }
  73
  74 /* subbuf_align aligns the offset to the next subbuffer. */
  75 static inline
  76 unsigned long subbuf_align(unsigned long offset, struct channel *chan)
  77 {
  78         return (offset + chan->backend.subbuf_size)
  79                & ~(chan->backend.subbuf_size - 1);
  80 }
  81
  82 /* subbuf_index returns the index of the current subbuffer within the buffer. */
  83 static inline
  84 unsigned long subbuf_index(unsigned long offset, struct channel *chan)
  85 {
  86         return buf_offset(offset, chan) >> chan->backend.subbuf_size_order;
  87 }
  88
  89 /*
  90  * Last TSC comparison functions. Check if the current TSC overflows tsc_bits
  91  * bits from the last TSC read. When overflows are detected, the full 64-bit
  92  * timestamp counter should be written in the record header. Reads and writes
  93  * last_tsc atomically.
  94  */
  95
  96 #if (BITS_PER_LONG == 32)
  97 static inline
  98 void save_last_tsc(const struct lib_ring_buffer_config *config,
  99                    struct lib_ring_buffer *buf, u64 tsc)
 100 {
 101         if (config->tsc_bits == 0 || config->tsc_bits == 64)
 102                 return;
 103
 104         /*
 105          * Ensure the compiler performs this update in a single instruction.
 106          */
 107         v_set(config, &buf->last_tsc, (unsigned long)(tsc >> config->tsc_bits));
 108 }
 109
 110 static inline
 111 int last_tsc_overflow(const struct lib_ring_buffer_config *config,
 112                       struct lib_ring_buffer *buf, u64 tsc)
 113 {
 114         unsigned long tsc_shifted;
 115
 116         if (config->tsc_bits == 0 || config->tsc_bits == 64)
 117                 return 0;
 118
 119         tsc_shifted = (unsigned long)(tsc >> config->tsc_bits);
 120         if (unlikely(tsc_shifted
 121                      - (unsigned long)v_read(config, &buf->last_tsc)))
 122                 return 1;
 123         else
 124                 return 0;
 125 }
 126 #else
 127 static inline
 128 void save_last_tsc(const struct lib_ring_buffer_config *config,
 129                    struct lib_ring_buffer *buf, u64 tsc)
 130 {
 131         if (config->tsc_bits == 0 || config->tsc_bits == 64)
 132                 return;
 133
 134         v_set(config, &buf->last_tsc, (unsigned long)tsc);
 135 }
 136
 137 static inline
 138 int last_tsc_overflow(const struct lib_ring_buffer_config *config,
 139                       struct lib_ring_buffer *buf, u64 tsc)
 140 {
 141         if (config->tsc_bits == 0 || config->tsc_bits == 64)
 142                 return 0;
 143
 144         if (unlikely((tsc - v_read(config, &buf->last_tsc))
 145                      >> config->tsc_bits))
 146                 return 1;
 147         else
 148                 return 0;
 149 }
 150 #endif
 151
 152 extern
 153 int lib_ring_buffer_reserve_slow(struct lib_ring_buffer_ctx *ctx);
 154
 155 extern
 156 void lib_ring_buffer_switch_slow(struct lib_ring_buffer *buf,
 157                                  enum switch_mode mode);
 158
 159 extern
 160 void lib_ring_buffer_switch_remote(struct lib_ring_buffer *buf);
 161
 162 /* Buffer write helpers */
 163
 164 static inline
 165 void lib_ring_buffer_reserve_push_reader(struct lib_ring_buffer *buf,
 166                                          struct channel *chan,
 167                                          unsigned long offset)
 168 {
 169         unsigned long consumed_old, consumed_new;
 170
 171         do {
 172                 consumed_old = atomic_long_read(&buf->consumed);
 173                 /*
 174                  * If buffer is in overwrite mode, push the reader consumed
 175                  * count if the write position has reached it and we are not
 176                  * at the first iteration (don't push the reader farther than
 177                  * the writer). This operation can be done concurrently by many
 178                  * writers in the same buffer, the writer being at the farthest
 179                  * write position sub-buffer index in the buffer being the one
 180                  * which will win this loop.
 181                  */
 182                 if (unlikely(subbuf_trunc(offset, chan)
 183                               - subbuf_trunc(consumed_old, chan)
 184                              >= chan->backend.buf_size))
 185                         consumed_new = subbuf_align(consumed_old, chan);
 186                 else
 187                         return;
 188         } while (unlikely(atomic_long_cmpxchg(&buf->consumed, consumed_old,
 189                                               consumed_new) != consumed_old));
 190 }
 191
 192 static inline
 193 void lib_ring_buffer_vmcore_check_deliver(const struct lib_ring_buffer_config *config,
 194                                           struct lib_ring_buffer *buf,
 195                                           unsigned long commit_count,
 196                                           unsigned long idx)
 197 {
 198         if (config->oops == RING_BUFFER_OOPS_CONSISTENCY)
 199                 v_set(config, &buf->commit_hot[idx].seq, commit_count);
 200 }
 201
 202 static inline
 203 int lib_ring_buffer_poll_deliver(const struct lib_ring_buffer_config *config,
 204                                  struct lib_ring_buffer *buf,
 205                                  struct channel *chan)
 206 {
 207         unsigned long consumed_old, consumed_idx, commit_count, write_offset;
 208
 209         consumed_old = atomic_long_read(&buf->consumed);
 210         consumed_idx = subbuf_index(consumed_old, chan);
 211         commit_count = v_read(config, &buf->commit_cold[consumed_idx].cc_sb);
 212         /*
 213          * No memory barrier here, since we are only interested
 214          * in a statistically correct polling result. The next poll will
 215          * get the data is we are racing. The mb() that ensures correct
 216          * memory order is in get_subbuf.
 217          */
 218         write_offset = v_read(config, &buf->offset);
 219
 220         /*
 221          * Check that the subbuffer we are trying to consume has been
 222          * already fully committed.
 223          */
 224
 225         if (((commit_count - chan->backend.subbuf_size)
 226              & chan->commit_count_mask)
 227             - (buf_trunc(consumed_old, chan)
 228                >> chan->backend.num_subbuf_order)
 229             != 0)
 230                 return 0;
 231
 232         /*
 233          * Check that we are not about to read the same subbuffer in
 234          * which the writer head is.
 235          */
 236         if (subbuf_trunc(write_offset, chan) - subbuf_trunc(consumed_old, chan)
 237             == 0)
 238                 return 0;
 239
 240         return 1;
 241
 242 }
 243
 244 static inline
 245 int lib_ring_buffer_pending_data(const struct lib_ring_buffer_config *config,
 246                                  struct lib_ring_buffer *buf,
 247                                  struct channel *chan)
 248 {
 249         return !!subbuf_offset(v_read(config, &buf->offset), chan);
 250 }
 251
 252 static inline
 253 unsigned long lib_ring_buffer_get_data_size(const struct lib_ring_buffer_config *config,
 254                                             struct lib_ring_buffer *buf,
 255                                             unsigned long idx)
 256 {
 257         return subbuffer_get_data_size(config, &buf->backend, idx);
 258 }
 259
 260 /*
 261  * Check if all space reservation in a buffer have been committed. This helps
 262  * knowing if an execution context is nested (for per-cpu buffers only).
 263  * This is a very specific ftrace use-case, so we keep this as "internal" API.
 264  */
 265 static inline
 266 int lib_ring_buffer_reserve_committed(const struct lib_ring_buffer_config *config,
 267                                       struct lib_ring_buffer *buf,
 268                                       struct channel *chan)
 269 {
 270         unsigned long offset, idx, commit_count;
 271
 272         CHAN_WARN_ON(chan, config->alloc != RING_BUFFER_ALLOC_PER_CPU);
 273         CHAN_WARN_ON(chan, config->sync != RING_BUFFER_SYNC_PER_CPU);
 274
 275         /*
 276          * Read offset and commit count in a loop so they are both read
 277          * atomically wrt interrupts. By deal with interrupt concurrency by
 278          * restarting both reads if the offset has been pushed. Note that given
 279          * we only have to deal with interrupt concurrency here, an interrupt
 280          * modifying the commit count will also modify "offset", so it is safe
 281          * to only check for offset modifications.
 282          */
 283         do {
 284                 offset = v_read(config, &buf->offset);
 285                 idx = subbuf_index(offset, chan);
 286                 commit_count = v_read(config, &buf->commit_hot[idx].cc);
 287         } while (offset != v_read(config, &buf->offset));
 288
 289         return ((buf_trunc(offset, chan) >> chan->backend.num_subbuf_order)
 290                      - (commit_count & chan->commit_count_mask) == 0);
 291 }
 292
 293 static inline
 294 void lib_ring_buffer_check_deliver(const struct lib_ring_buffer_config *config,
 295                                    struct lib_ring_buffer *buf,
 296                                    struct channel *chan,
 297                                    unsigned long offset,
 298                                    unsigned long commit_count,
 299                                    unsigned long idx)
 300 {
 301         unsigned long old_commit_count = commit_count
 302                                          - chan->backend.subbuf_size;
 303         u64 tsc;
 304
 305         /* Check if all commits have been done */
 306         if (unlikely((buf_trunc(offset, chan) >> chan->backend.num_subbuf_order)
 307                      - (old_commit_count & chan->commit_count_mask) == 0)) {
 308                 /*
 309                  * If we succeeded at updating cc_sb below, we are the subbuffer
 310                  * writer delivering the subbuffer. Deals with concurrent
 311                  * updates of the "cc" value without adding a add_return atomic
 312                  * operation to the fast path.
 313                  *
 314                  * We are doing the delivery in two steps:
 315                  * - First, we cmpxchg() cc_sb to the new value
 316                  *   old_commit_count + 1. This ensures that we are the only
 317                  *   subbuffer user successfully filling the subbuffer, but we
 318                  *   do _not_ set the cc_sb value to "commit_count" yet.
 319                  *   Therefore, other writers that would wrap around the ring
 320                  *   buffer and try to start writing to our subbuffer would
 321                  *   have to drop records, because it would appear as
 322                  *   non-filled.
 323                  *   We therefore have exclusive access to the subbuffer control
 324                  *   structures.  This mutual exclusion with other writers is
 325                  *   crucially important to perform record overruns count in
 326                  *   flight recorder mode locklessly.
 327                  * - When we are ready to release the subbuffer (either for
 328                  *   reading or for overrun by other writers), we simply set the
 329                  *   cc_sb value to "commit_count" and perform delivery.
 330                  *
 331                  * The subbuffer size is least 2 bytes (minimum size: 1 page).
 332                  * This guarantees that old_commit_count + 1 != commit_count.
 333                  */
 334
 335                 /*
 336                  * Order prior updates to reserve count prior to the
 337                  * commit_cold cc_sb update.
 338                  */
 339                 smp_wmb();
 340                 if (likely(v_cmpxchg(config, &buf->commit_cold[idx].cc_sb,
 341                                          old_commit_count, old_commit_count + 1)
 342                            == old_commit_count)) {
 343                         /*
 344                          * Start of exclusive subbuffer access. We are
 345                          * guaranteed to be the last writer in this subbuffer
 346                          * and any other writer trying to access this subbuffer
 347                          * in this state is required to drop records.
 348                          */
 349                         tsc = config->cb.ring_buffer_clock_read(chan);
 350                         v_add(config,
 351                               subbuffer_get_records_count(config,
 352                                                           &buf->backend, idx),
 353                               &buf->records_count);
 354                         v_add(config,
 355                               subbuffer_count_records_overrun(config,
 356                                                               &buf->backend,
 357                                                               idx),
 358                               &buf->records_overrun);
 359                         config->cb.buffer_end(buf, tsc, idx,
 360                                               lib_ring_buffer_get_data_size(config,
 361                                                                         buf,
 362                                                                         idx));
 363
 364                         /*
 365                          * Set noref flag and offset for this subbuffer id.
 366                          * Contains a memory barrier that ensures counter stores
 367                          * are ordered before set noref and offset.
 368                          */
 369                         lib_ring_buffer_set_noref_offset(config, &buf->backend, idx,
 370                                                          buf_trunc_val(offset, chan));
 371
 372                         /*
 373                          * Order set_noref and record counter updates before the
 374                          * end of subbuffer exclusive access. Orders with
 375                          * respect to writers coming into the subbuffer after
 376                          * wrap around, and also order wrt concurrent readers.
 377                          */
 378                         smp_mb();
 379                         /* End of exclusive subbuffer access */
 380                         v_set(config, &buf->commit_cold[idx].cc_sb,
 381                               commit_count);
 382                         /*
 383                          * Order later updates to reserve count after
 384                          * the commit_cold cc_sb update.
 385                          */
 386                         smp_wmb();
 387                         lib_ring_buffer_vmcore_check_deliver(config, buf,
 388                                                          commit_count, idx);
 389
 390                         /*
 391                          * RING_BUFFER_WAKEUP_BY_WRITER wakeup is not lock-free.
 392                          */
 393                         if (config->wakeup == RING_BUFFER_WAKEUP_BY_WRITER
 394                             && atomic_long_read(&buf->active_readers)
 395                             && lib_ring_buffer_poll_deliver(config, buf, chan)) {
 396                                 wake_up_interruptible(&buf->read_wait);
 397                                 wake_up_interruptible(&chan->read_wait);
 398                         }
 399
 400                 }
 401         }
 402 }
 403
 404 /*
 405  * lib_ring_buffer_write_commit_counter
 406  *
 407  * For flight recording. must be called after commit.
 408  * This function increments the subbuffer's commit_seq counter each time the
 409  * commit count reaches back the reserve offset (modulo subbuffer size). It is
 410  * useful for crash dump.
 411  */
 412 static inline
 413 void lib_ring_buffer_write_commit_counter(const struct lib_ring_buffer_config *config,
 414                                           struct lib_ring_buffer *buf,
 415                                           struct channel *chan,
 416                                           unsigned long idx,
 417                                           unsigned long buf_offset,
 418                                           unsigned long commit_count,
 419                                           size_t slot_size)
 420 {
 421         unsigned long offset, commit_seq_old;
 422
 423         if (config->oops != RING_BUFFER_OOPS_CONSISTENCY)
 424                 return;
 425
 426         offset = buf_offset + slot_size;
 427
 428         /*
 429          * subbuf_offset includes commit_count_mask. We can simply
 430          * compare the offsets within the subbuffer without caring about
 431          * buffer full/empty mismatch because offset is never zero here
 432          * (subbuffer header and record headers have non-zero length).
 433          */
 434         if (unlikely(subbuf_offset(offset - commit_count, chan)))
 435                 return;
 436
 437         commit_seq_old = v_read(config, &buf->commit_hot[idx].seq);
 438         while ((long) (commit_seq_old - commit_count) < 0)
 439                 commit_seq_old = v_cmpxchg(config, &buf->commit_hot[idx].seq,
 440                                            commit_seq_old, commit_count);
 441 }
 442
 443 extern int lib_ring_buffer_create(struct lib_ring_buffer *buf,
 444                                   struct channel_backend *chanb, int cpu);
 445 extern void lib_ring_buffer_free(struct lib_ring_buffer *buf);
 446
 447 /* Keep track of trap nesting inside ring buffer code */
 448 DECLARE_PER_CPU(unsigned int, lib_ring_buffer_nesting);
 449
 450 #endif /* _LIB_RING_BUFFER_FRONTEND_INTERNAL_H */