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