libringbuffer/frontend_api.h

   1 #ifndef _LTTNG_RING_BUFFER_FRONTEND_API_H
   2 #define _LTTNG_RING_BUFFER_FRONTEND_API_H
   3
   4 /*
   5  * libringbuffer/frontend_api.h
   6  *
   7  * Copyright (C) 2005-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   8  *
   9  * This library is free software; you can redistribute it and/or
  10  * modify it under the terms of the GNU Lesser General Public
  11  * License as published by the Free Software Foundation; only
  12  * version 2.1 of the License.
  13  *
  14  * This library is distributed in the hope that it will be useful,
  15  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  16  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  17  * Lesser General Public License for more details.
  18  *
  19  * You should have received a copy of the GNU Lesser General Public
  20  * License along with this library; if not, write to the Free Software
  21  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  22  *
  23  * Ring Buffer Library Synchronization Header (buffer write API).
  24  *
  25  * Author:
  26  *      Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  27  *
  28  * See ring_buffer_frontend.c for more information on wait-free
  29  * algorithms.
  30  * See frontend.h for channel allocation and read-side API.
  31  */
  32
  33 #include "frontend.h"
  34 #include <urcu-bp.h>
  35 #include <urcu/compiler.h>
  36
  37 /**
  38  * lib_ring_buffer_get_cpu - Precedes ring buffer reserve/commit.
  39  *
  40  * Keeps a ring buffer nesting count as supplementary safety net to
  41  * ensure tracer client code will never trigger an endless recursion.
  42  * Returns the processor ID on success, -EPERM on failure (nesting count
  43  * too high).
  44  *
  45  * asm volatile and "memory" clobber prevent the compiler from moving
  46  * instructions out of the ring buffer nesting count. This is required to ensure
  47  * that probe side-effects which can cause recursion (e.g. unforeseen traps,
  48  * divisions by 0, ...) are triggered within the incremented nesting count
  49  * section.
  50  */
  51 static inline
  52 int lib_ring_buffer_get_cpu(const struct lttng_ust_lib_ring_buffer_config *config)
  53 {
  54         int cpu, nesting;
  55
  56         cpu = lttng_ust_get_cpu();
  57         nesting = ++URCU_TLS(lib_ring_buffer_nesting);
  58         cmm_barrier();
  59
  60         if (caa_unlikely(nesting > 4)) {
  61                 WARN_ON_ONCE(1);
  62                 URCU_TLS(lib_ring_buffer_nesting)--;
  63                 rcu_read_unlock();
  64                 return -EPERM;
  65         } else
  66                 return cpu;
  67 }
  68
  69 /**
  70  * lib_ring_buffer_put_cpu - Follows ring buffer reserve/commit.
  71  */
  72 static inline
  73 void lib_ring_buffer_put_cpu(const struct lttng_ust_lib_ring_buffer_config *config)
  74 {
  75         cmm_barrier();
  76         URCU_TLS(lib_ring_buffer_nesting)--;            /* TLS */
  77 }
  78
  79 /*
  80  * lib_ring_buffer_try_reserve is called by lib_ring_buffer_reserve(). It is not
  81  * part of the API per se.
  82  *
  83  * returns 0 if reserve ok, or 1 if the slow path must be taken.
  84  */
  85 static inline
  86 int lib_ring_buffer_try_reserve(const struct lttng_ust_lib_ring_buffer_config *config,
  87                                 struct lttng_ust_lib_ring_buffer_ctx *ctx,
  88                                 unsigned long *o_begin, unsigned long *o_end,
  89                                 unsigned long *o_old, size_t *before_hdr_pad)
  90 {
  91         struct channel *chan = ctx->chan;
  92         struct lttng_ust_lib_ring_buffer *buf = ctx->buf;
  93         *o_begin = v_read(config, &buf->offset);
  94         *o_old = *o_begin;
  95
  96         ctx->tsc = lib_ring_buffer_clock_read(chan);
  97         if ((int64_t) ctx->tsc == -EIO)
  98                 return 1;
  99
 100         /*
 101          * Prefetch cacheline for read because we have to read the previous
 102          * commit counter to increment it and commit seq value to compare it to
 103          * the commit counter.
 104          */
 105         //prefetch(&buf->commit_hot[subbuf_index(*o_begin, chan)]);
 106
 107         if (last_tsc_overflow(config, buf, ctx->tsc))
 108                 ctx->rflags |= RING_BUFFER_RFLAG_FULL_TSC;
 109
 110         if (caa_unlikely(subbuf_offset(*o_begin, chan) == 0))
 111                 return 1;
 112
 113         ctx->slot_size = record_header_size(config, chan, *o_begin,
 114                                             before_hdr_pad, ctx);
 115         ctx->slot_size +=
 116                 lib_ring_buffer_align(*o_begin + ctx->slot_size,
 117                                       ctx->largest_align) + ctx->data_size;
 118         if (caa_unlikely((subbuf_offset(*o_begin, chan) + ctx->slot_size)
 119                      > chan->backend.subbuf_size))
 120                 return 1;
 121
 122         /*
 123          * Record fits in the current buffer and we are not on a switch
 124          * boundary. It's safe to write.
 125          */
 126         *o_end = *o_begin + ctx->slot_size;
 127
 128         if (caa_unlikely((subbuf_offset(*o_end, chan)) == 0))
 129                 /*
 130                  * The offset_end will fall at the very beginning of the next
 131                  * subbuffer.
 132                  */
 133                 return 1;
 134
 135         return 0;
 136 }
 137
 138 /**
 139  * lib_ring_buffer_reserve - Reserve space in a ring buffer.
 140  * @config: ring buffer instance configuration.
 141  * @ctx: ring buffer context. (input and output) Must be already initialized.
 142  *
 143  * Atomic wait-free slot reservation. The reserved space starts at the context
 144  * "pre_offset". Its length is "slot_size". The associated time-stamp is "tsc".
 145  *
 146  * Return :
 147  *  0 on success.
 148  * -EAGAIN if channel is disabled.
 149  * -ENOSPC if event size is too large for packet.
 150  * -ENOBUFS if there is currently not enough space in buffer for the event.
 151  * -EIO if data cannot be written into the buffer for any other reason.
 152  */
 153
 154 static inline
 155 int lib_ring_buffer_reserve(const struct lttng_ust_lib_ring_buffer_config *config,
 156                             struct lttng_ust_lib_ring_buffer_ctx *ctx)
 157 {
 158         struct channel *chan = ctx->chan;
 159         struct lttng_ust_shm_handle *handle = ctx->handle;
 160         struct lttng_ust_lib_ring_buffer *buf;
 161         unsigned long o_begin, o_end, o_old;
 162         size_t before_hdr_pad = 0;
 163
 164         if (uatomic_read(&chan->record_disabled))
 165                 return -EAGAIN;
 166
 167         if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
 168                 buf = shmp(handle, chan->backend.buf[ctx->cpu].shmp);
 169         else
 170                 buf = shmp(handle, chan->backend.buf[0].shmp);
 171         if (uatomic_read(&buf->record_disabled))
 172                 return -EAGAIN;
 173         ctx->buf = buf;
 174
 175         /*
 176          * Perform retryable operations.
 177          */
 178         if (caa_unlikely(lib_ring_buffer_try_reserve(config, ctx, &o_begin,
 179                                                  &o_end, &o_old, &before_hdr_pad)))
 180                 goto slow_path;
 181
 182         if (caa_unlikely(v_cmpxchg(config, &ctx->buf->offset, o_old, o_end)
 183                      != o_old))
 184                 goto slow_path;
 185
 186         /*
 187          * Atomically update last_tsc. This update races against concurrent
 188          * atomic updates, but the race will always cause supplementary full TSC
 189          * record headers, never the opposite (missing a full TSC record header
 190          * when it would be needed).
 191          */
 192         save_last_tsc(config, ctx->buf, ctx->tsc);
 193
 194         /*
 195          * Push the reader if necessary
 196          */
 197         lib_ring_buffer_reserve_push_reader(ctx->buf, chan, o_end - 1);
 198
 199         /*
 200          * Clear noref flag for this subbuffer.
 201          */
 202         lib_ring_buffer_clear_noref(config, &ctx->buf->backend,
 203                                 subbuf_index(o_end - 1, chan), handle);
 204
 205         ctx->pre_offset = o_begin;
 206         ctx->buf_offset = o_begin + before_hdr_pad;
 207         return 0;
 208 slow_path:
 209         return lib_ring_buffer_reserve_slow(ctx);
 210 }
 211
 212 /**
 213  * lib_ring_buffer_switch - Perform a sub-buffer switch for a per-cpu buffer.
 214  * @config: ring buffer instance configuration.
 215  * @buf: buffer
 216  * @mode: buffer switch mode (SWITCH_ACTIVE or SWITCH_FLUSH)
 217  *
 218  * This operation is completely reentrant : can be called while tracing is
 219  * active with absolutely no lock held.
 220  *
 221  * Note, however, that as a v_cmpxchg is used for some atomic operations and
 222  * requires to be executed locally for per-CPU buffers, this function must be
 223  * called from the CPU which owns the buffer for a ACTIVE flush, with preemption
 224  * disabled, for RING_BUFFER_SYNC_PER_CPU configuration.
 225  */
 226 static inline
 227 void lib_ring_buffer_switch(const struct lttng_ust_lib_ring_buffer_config *config,
 228                             struct lttng_ust_lib_ring_buffer *buf, enum switch_mode mode,
 229                             struct lttng_ust_shm_handle *handle)
 230 {
 231         lib_ring_buffer_switch_slow(buf, mode, handle);
 232 }
 233
 234 /* See ring_buffer_frontend_api.h for lib_ring_buffer_reserve(). */
 235
 236 /**
 237  * lib_ring_buffer_commit - Commit an record.
 238  * @config: ring buffer instance configuration.
 239  * @ctx: ring buffer context. (input arguments only)
 240  *
 241  * Atomic unordered slot commit. Increments the commit count in the
 242  * specified sub-buffer, and delivers it if necessary.
 243  */
 244 static inline
 245 void lib_ring_buffer_commit(const struct lttng_ust_lib_ring_buffer_config *config,
 246                             const struct lttng_ust_lib_ring_buffer_ctx *ctx)
 247 {
 248         struct channel *chan = ctx->chan;
 249         struct lttng_ust_shm_handle *handle = ctx->handle;
 250         struct lttng_ust_lib_ring_buffer *buf = ctx->buf;
 251         unsigned long offset_end = ctx->buf_offset;
 252         unsigned long endidx = subbuf_index(offset_end - 1, chan);
 253         unsigned long commit_count;
 254
 255         /*
 256          * Must count record before incrementing the commit count.
 257          */
 258         subbuffer_count_record(config, &buf->backend, endidx, handle);
 259
 260         /*
 261          * Order all writes to buffer before the commit count update that will
 262          * determine that the subbuffer is full.
 263          */
 264         cmm_smp_wmb();
 265
 266         v_add(config, ctx->slot_size, &shmp_index(handle, buf->commit_hot, endidx)->cc);
 267
 268         /*
 269          * commit count read can race with concurrent OOO commit count updates.
 270          * This is only needed for lib_ring_buffer_check_deliver (for
 271          * non-polling delivery only) and for
 272          * lib_ring_buffer_write_commit_counter.  The race can only cause the
 273          * counter to be read with the same value more than once, which could
 274          * cause :
 275          * - Multiple delivery for the same sub-buffer (which is handled
 276          *   gracefully by the reader code) if the value is for a full
 277          *   sub-buffer. It's important that we can never miss a sub-buffer
 278          *   delivery. Re-reading the value after the v_add ensures this.
 279          * - Reading a commit_count with a higher value that what was actually
 280          *   added to it for the lib_ring_buffer_write_commit_counter call
 281          *   (again caused by a concurrent committer). It does not matter,
 282          *   because this function is interested in the fact that the commit
 283          *   count reaches back the reserve offset for a specific sub-buffer,
 284          *   which is completely independent of the order.
 285          */
 286         commit_count = v_read(config, &shmp_index(handle, buf->commit_hot, endidx)->cc);
 287
 288         lib_ring_buffer_check_deliver(config, buf, chan, offset_end - 1,
 289                                       commit_count, endidx, handle, ctx->tsc);
 290         /*
 291          * Update used size at each commit. It's needed only for extracting
 292          * ring_buffer buffers from vmcore, after crash.
 293          */
 294         lib_ring_buffer_write_commit_counter(config, buf, chan, endidx,
 295                         offset_end, commit_count, handle);
 296 }
 297
 298 /**
 299  * lib_ring_buffer_try_discard_reserve - Try discarding a record.
 300  * @config: ring buffer instance configuration.
 301  * @ctx: ring buffer context. (input arguments only)
 302  *
 303  * Only succeeds if no other record has been written after the record to
 304  * discard. If discard fails, the record must be committed to the buffer.
 305  *
 306  * Returns 0 upon success, -EPERM if the record cannot be discarded.
 307  */
 308 static inline
 309 int lib_ring_buffer_try_discard_reserve(const struct lttng_ust_lib_ring_buffer_config *config,
 310                                         const struct lttng_ust_lib_ring_buffer_ctx *ctx)
 311 {
 312         struct lttng_ust_lib_ring_buffer *buf = ctx->buf;
 313         unsigned long end_offset = ctx->pre_offset + ctx->slot_size;
 314
 315         /*
 316          * We need to ensure that if the cmpxchg succeeds and discards the
 317          * record, the next record will record a full TSC, because it cannot
 318          * rely on the last_tsc associated with the discarded record to detect
 319          * overflows. The only way to ensure this is to set the last_tsc to 0
 320          * (assuming no 64-bit TSC overflow), which forces to write a 64-bit
 321          * timestamp in the next record.
 322          *
 323          * Note: if discard fails, we must leave the TSC in the record header.
 324          * It is needed to keep track of TSC overflows for the following
 325          * records.
 326          */
 327         save_last_tsc(config, buf, 0ULL);
 328
 329         if (caa_likely(v_cmpxchg(config, &buf->offset, end_offset, ctx->pre_offset)
 330                    != end_offset))
 331                 return -EPERM;
 332         else
 333                 return 0;
 334 }
 335
 336 static inline
 337 void channel_record_disable(const struct lttng_ust_lib_ring_buffer_config *config,
 338                             struct channel *chan)
 339 {
 340         uatomic_inc(&chan->record_disabled);
 341 }
 342
 343 static inline
 344 void channel_record_enable(const struct lttng_ust_lib_ring_buffer_config *config,
 345                            struct channel *chan)
 346 {
 347         uatomic_dec(&chan->record_disabled);
 348 }
 349
 350 static inline
 351 void lib_ring_buffer_record_disable(const struct lttng_ust_lib_ring_buffer_config *config,
 352                                     struct lttng_ust_lib_ring_buffer *buf)
 353 {
 354         uatomic_inc(&buf->record_disabled);
 355 }
 356
 357 static inline
 358 void lib_ring_buffer_record_enable(const struct lttng_ust_lib_ring_buffer_config *config,
 359                                    struct lttng_ust_lib_ring_buffer *buf)
 360 {
 361         uatomic_dec(&buf->record_disabled);
 362 }
 363
 364 #endif /* _LTTNG_RING_BUFFER_FRONTEND_API_H */