libringbuffer/frontend_api.h

   1 /*
   2  * SPDX-License-Identifier: LGPL-2.1-only
   3  *
   4  * Copyright (C) 2005-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   5  *
   6  * See ring_buffer_frontend.c for more information on wait-free
   7  * algorithms.
   8  * See frontend.h for channel allocation and read-side API.
   9  */
  10
  11 #ifndef _LTTNG_RING_BUFFER_FRONTEND_API_H
  12 #define _LTTNG_RING_BUFFER_FRONTEND_API_H
  13
  14 #include <stddef.h>
  15
  16 #include <urcu/compiler.h>
  17
  18 #include "frontend.h"
  19
  20 /**
  21  * lib_ring_buffer_get_cpu - Precedes ring buffer reserve/commit.
  22  *
  23  * Keeps a ring buffer nesting count as supplementary safety net to
  24  * ensure tracer client code will never trigger an endless recursion.
  25  * Returns the processor ID on success, -EPERM on failure (nesting count
  26  * too high).
  27  *
  28  * asm volatile and "memory" clobber prevent the compiler from moving
  29  * instructions out of the ring buffer nesting count. This is required to ensure
  30  * that probe side-effects which can cause recursion (e.g. unforeseen traps,
  31  * divisions by 0, ...) are triggered within the incremented nesting count
  32  * section.
  33  */
  34 static inline
  35 int lib_ring_buffer_get_cpu(const struct lttng_ust_lib_ring_buffer_config *config)
  36 {
  37         int cpu, nesting;
  38
  39         cpu = lttng_ust_get_cpu();
  40         nesting = ++URCU_TLS(lib_ring_buffer_nesting);
  41         cmm_barrier();
  42
  43         if (caa_unlikely(nesting > 4)) {
  44                 WARN_ON_ONCE(1);
  45                 URCU_TLS(lib_ring_buffer_nesting)--;
  46                 return -EPERM;
  47         } else
  48                 return cpu;
  49 }
  50
  51 /**
  52  * lib_ring_buffer_put_cpu - Follows ring buffer reserve/commit.
  53  */
  54 static inline
  55 void lib_ring_buffer_put_cpu(const struct lttng_ust_lib_ring_buffer_config *config)
  56 {
  57         cmm_barrier();
  58         URCU_TLS(lib_ring_buffer_nesting)--;            /* TLS */
  59 }
  60
  61 /*
  62  * lib_ring_buffer_try_reserve is called by lib_ring_buffer_reserve(). It is not
  63  * part of the API per se.
  64  *
  65  * returns 0 if reserve ok, or 1 if the slow path must be taken.
  66  */
  67 static inline
  68 int lib_ring_buffer_try_reserve(const struct lttng_ust_lib_ring_buffer_config *config,
  69                                 struct lttng_ust_lib_ring_buffer_ctx *ctx,
  70                                 void *client_ctx,
  71                                 unsigned long *o_begin, unsigned long *o_end,
  72                                 unsigned long *o_old, size_t *before_hdr_pad)
  73 {
  74         struct channel *chan = ctx->chan;
  75         struct lttng_ust_lib_ring_buffer *buf = ctx->buf;
  76         *o_begin = v_read(config, &buf->offset);
  77         *o_old = *o_begin;
  78
  79         ctx->tsc = lib_ring_buffer_clock_read(chan);
  80         if ((int64_t) ctx->tsc == -EIO)
  81                 return 1;
  82
  83         /*
  84          * Prefetch cacheline for read because we have to read the previous
  85          * commit counter to increment it and commit seq value to compare it to
  86          * the commit counter.
  87          */
  88         //prefetch(&buf->commit_hot[subbuf_index(*o_begin, chan)]);
  89
  90         if (last_tsc_overflow(config, buf, ctx->tsc))
  91                 ctx->rflags |= RING_BUFFER_RFLAG_FULL_TSC;
  92
  93         if (caa_unlikely(subbuf_offset(*o_begin, chan) == 0))
  94                 return 1;
  95
  96         ctx->slot_size = record_header_size(config, chan, *o_begin,
  97                                             before_hdr_pad, ctx, client_ctx);
  98         ctx->slot_size +=
  99                 lib_ring_buffer_align(*o_begin + ctx->slot_size,
 100                                       ctx->largest_align) + ctx->data_size;
 101         if (caa_unlikely((subbuf_offset(*o_begin, chan) + ctx->slot_size)
 102                      > chan->backend.subbuf_size))
 103                 return 1;
 104
 105         /*
 106          * Record fits in the current buffer and we are not on a switch
 107          * boundary. It's safe to write.
 108          */
 109         *o_end = *o_begin + ctx->slot_size;
 110
 111         if (caa_unlikely((subbuf_offset(*o_end, chan)) == 0))
 112                 /*
 113                  * The offset_end will fall at the very beginning of the next
 114                  * subbuffer.
 115                  */
 116                 return 1;
 117
 118         return 0;
 119 }
 120
 121 /**
 122  * lib_ring_buffer_reserve - Reserve space in a ring buffer.
 123  * @config: ring buffer instance configuration.
 124  * @ctx: ring buffer context. (input and output) Must be already initialized.
 125  *
 126  * Atomic wait-free slot reservation. The reserved space starts at the context
 127  * "pre_offset". Its length is "slot_size". The associated time-stamp is "tsc".
 128  *
 129  * Return :
 130  *  0 on success.
 131  * -EAGAIN if channel is disabled.
 132  * -ENOSPC if event size is too large for packet.
 133  * -ENOBUFS if there is currently not enough space in buffer for the event.
 134  * -EIO if data cannot be written into the buffer for any other reason.
 135  */
 136
 137 static inline
 138 int lib_ring_buffer_reserve(const struct lttng_ust_lib_ring_buffer_config *config,
 139                             struct lttng_ust_lib_ring_buffer_ctx *ctx,
 140                             void *client_ctx)
 141 {
 142         struct channel *chan = ctx->chan;
 143         struct lttng_ust_shm_handle *handle = ctx->handle;
 144         struct lttng_ust_lib_ring_buffer *buf;
 145         unsigned long o_begin, o_end, o_old;
 146         size_t before_hdr_pad = 0;
 147
 148         if (caa_unlikely(uatomic_read(&chan->record_disabled)))
 149                 return -EAGAIN;
 150
 151         if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
 152                 buf = shmp(handle, chan->backend.buf[ctx->cpu].shmp);
 153         else
 154                 buf = shmp(handle, chan->backend.buf[0].shmp);
 155         if (caa_unlikely(!buf))
 156                 return -EIO;
 157         if (caa_unlikely(uatomic_read(&buf->record_disabled)))
 158                 return -EAGAIN;
 159         ctx->buf = buf;
 160
 161         /*
 162          * Perform retryable operations.
 163          */
 164         if (caa_unlikely(lib_ring_buffer_try_reserve(config, ctx, client_ctx, &o_begin,
 165                                                  &o_end, &o_old, &before_hdr_pad)))
 166                 goto slow_path;
 167
 168         if (caa_unlikely(v_cmpxchg(config, &ctx->buf->offset, o_old, o_end)
 169                      != o_old))
 170                 goto slow_path;
 171
 172         /*
 173          * Atomically update last_tsc. This update races against concurrent
 174          * atomic updates, but the race will always cause supplementary full TSC
 175          * record headers, never the opposite (missing a full TSC record header
 176          * when it would be needed).
 177          */
 178         save_last_tsc(config, ctx->buf, ctx->tsc);
 179
 180         /*
 181          * Push the reader if necessary
 182          */
 183         lib_ring_buffer_reserve_push_reader(ctx->buf, chan, o_end - 1);
 184
 185         /*
 186          * Clear noref flag for this subbuffer.
 187          */
 188         lib_ring_buffer_clear_noref(config, &ctx->buf->backend,
 189                                 subbuf_index(o_end - 1, chan), handle);
 190
 191         ctx->pre_offset = o_begin;
 192         ctx->buf_offset = o_begin + before_hdr_pad;
 193         return 0;
 194 slow_path:
 195         return lib_ring_buffer_reserve_slow(ctx, client_ctx);
 196 }
 197
 198 /**
 199  * lib_ring_buffer_switch - Perform a sub-buffer switch for a per-cpu buffer.
 200  * @config: ring buffer instance configuration.
 201  * @buf: buffer
 202  * @mode: buffer switch mode (SWITCH_ACTIVE or SWITCH_FLUSH)
 203  *
 204  * This operation is completely reentrant : can be called while tracing is
 205  * active with absolutely no lock held.
 206  *
 207  * Note, however, that as a v_cmpxchg is used for some atomic operations and
 208  * requires to be executed locally for per-CPU buffers, this function must be
 209  * called from the CPU which owns the buffer for a ACTIVE flush, with preemption
 210  * disabled, for RING_BUFFER_SYNC_PER_CPU configuration.
 211  */
 212 static inline
 213 void lib_ring_buffer_switch(const struct lttng_ust_lib_ring_buffer_config *config,
 214                             struct lttng_ust_lib_ring_buffer *buf, enum switch_mode mode,
 215                             struct lttng_ust_shm_handle *handle)
 216 {
 217         lib_ring_buffer_switch_slow(buf, mode, handle);
 218 }
 219
 220 /* See ring_buffer_frontend_api.h for lib_ring_buffer_reserve(). */
 221
 222 /**
 223  * lib_ring_buffer_commit - Commit an record.
 224  * @config: ring buffer instance configuration.
 225  * @ctx: ring buffer context. (input arguments only)
 226  *
 227  * Atomic unordered slot commit. Increments the commit count in the
 228  * specified sub-buffer, and delivers it if necessary.
 229  */
 230 static inline
 231 void lib_ring_buffer_commit(const struct lttng_ust_lib_ring_buffer_config *config,
 232                             const struct lttng_ust_lib_ring_buffer_ctx *ctx)
 233 {
 234         struct channel *chan = ctx->chan;
 235         struct lttng_ust_shm_handle *handle = ctx->handle;
 236         struct lttng_ust_lib_ring_buffer *buf = ctx->buf;
 237         unsigned long offset_end = ctx->buf_offset;
 238         unsigned long endidx = subbuf_index(offset_end - 1, chan);
 239         unsigned long commit_count;
 240         struct commit_counters_hot *cc_hot = shmp_index(handle,
 241                                                 buf->commit_hot, endidx);
 242
 243         if (caa_unlikely(!cc_hot))
 244                 return;
 245
 246         /*
 247          * Must count record before incrementing the commit count.
 248          */
 249         subbuffer_count_record(config, ctx, &buf->backend, endidx, handle);
 250
 251         /*
 252          * Order all writes to buffer before the commit count update that will
 253          * determine that the subbuffer is full.
 254          */
 255         cmm_smp_wmb();
 256
 257         v_add(config, ctx->slot_size, &cc_hot->cc);
 258
 259         /*
 260          * commit count read can race with concurrent OOO commit count updates.
 261          * This is only needed for lib_ring_buffer_check_deliver (for
 262          * non-polling delivery only) and for
 263          * lib_ring_buffer_write_commit_counter.  The race can only cause the
 264          * counter to be read with the same value more than once, which could
 265          * cause :
 266          * - Multiple delivery for the same sub-buffer (which is handled
 267          *   gracefully by the reader code) if the value is for a full
 268          *   sub-buffer. It's important that we can never miss a sub-buffer
 269          *   delivery. Re-reading the value after the v_add ensures this.
 270          * - Reading a commit_count with a higher value that what was actually
 271          *   added to it for the lib_ring_buffer_write_commit_counter call
 272          *   (again caused by a concurrent committer). It does not matter,
 273          *   because this function is interested in the fact that the commit
 274          *   count reaches back the reserve offset for a specific sub-buffer,
 275          *   which is completely independent of the order.
 276          */
 277         commit_count = v_read(config, &cc_hot->cc);
 278
 279         lib_ring_buffer_check_deliver(config, buf, chan, offset_end - 1,
 280                                       commit_count, endidx, handle, ctx->tsc);
 281         /*
 282          * Update used size at each commit. It's needed only for extracting
 283          * ring_buffer buffers from vmcore, after crash.
 284          */
 285         lib_ring_buffer_write_commit_counter(config, buf, chan,
 286                         offset_end, commit_count, handle, cc_hot);
 287 }
 288
 289 /**
 290  * lib_ring_buffer_try_discard_reserve - Try discarding a record.
 291  * @config: ring buffer instance configuration.
 292  * @ctx: ring buffer context. (input arguments only)
 293  *
 294  * Only succeeds if no other record has been written after the record to
 295  * discard. If discard fails, the record must be committed to the buffer.
 296  *
 297  * Returns 0 upon success, -EPERM if the record cannot be discarded.
 298  */
 299 static inline
 300 int lib_ring_buffer_try_discard_reserve(const struct lttng_ust_lib_ring_buffer_config *config,
 301                                         const struct lttng_ust_lib_ring_buffer_ctx *ctx)
 302 {
 303         struct lttng_ust_lib_ring_buffer *buf = ctx->buf;
 304         unsigned long end_offset = ctx->pre_offset + ctx->slot_size;
 305
 306         /*
 307          * We need to ensure that if the cmpxchg succeeds and discards the
 308          * record, the next record will record a full TSC, because it cannot
 309          * rely on the last_tsc associated with the discarded record to detect
 310          * overflows. The only way to ensure this is to set the last_tsc to 0
 311          * (assuming no 64-bit TSC overflow), which forces to write a 64-bit
 312          * timestamp in the next record.
 313          *
 314          * Note: if discard fails, we must leave the TSC in the record header.
 315          * It is needed to keep track of TSC overflows for the following
 316          * records.
 317          */
 318         save_last_tsc(config, buf, 0ULL);
 319
 320         if (caa_likely(v_cmpxchg(config, &buf->offset, end_offset, ctx->pre_offset)
 321                    != end_offset))
 322                 return -EPERM;
 323         else
 324                 return 0;
 325 }
 326
 327 static inline
 328 void channel_record_disable(const struct lttng_ust_lib_ring_buffer_config *config,
 329                             struct channel *chan)
 330 {
 331         uatomic_inc(&chan->record_disabled);
 332 }
 333
 334 static inline
 335 void channel_record_enable(const struct lttng_ust_lib_ring_buffer_config *config,
 336                            struct channel *chan)
 337 {
 338         uatomic_dec(&chan->record_disabled);
 339 }
 340
 341 static inline
 342 void lib_ring_buffer_record_disable(const struct lttng_ust_lib_ring_buffer_config *config,
 343                                     struct lttng_ust_lib_ring_buffer *buf)
 344 {
 345         uatomic_inc(&buf->record_disabled);
 346 }
 347
 348 static inline
 349 void lib_ring_buffer_record_enable(const struct lttng_ust_lib_ring_buffer_config *config,
 350                                    struct lttng_ust_lib_ring_buffer *buf)
 351 {
 352         uatomic_dec(&buf->record_disabled);
 353 }
 354
 355 #endif /* _LTTNG_RING_BUFFER_FRONTEND_API_H */