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 #include "rseq.h"
  37
  38 static inline
  39 int lib_ring_buffer_get_cpu(const struct lttng_ust_lib_ring_buffer_config *config)
  40 {
  41         return lttng_ust_get_cpu();
  42 }
  43
  44 /**
  45  * lib_ring_buffer_begin - Precedes ring buffer reserve/commit.
  46  *
  47  * Keeps a ring buffer nesting count as supplementary safety net to
  48  * ensure tracer client code will never trigger an endless recursion.
  49  * Returns the processor ID on success, -EPERM on failure (nesting count
  50  * too high).
  51  *
  52  * asm volatile and "memory" clobber prevent the compiler from moving
  53  * instructions out of the ring buffer nesting count. This is required to ensure
  54  * that probe side-effects which can cause recursion (e.g. unforeseen traps,
  55  * divisions by 0, ...) are triggered within the incremented nesting count
  56  * section.
  57  */
  58 static inline
  59 int lib_ring_buffer_begin(const struct lttng_ust_lib_ring_buffer_config *config)
  60 {
  61         int nesting;
  62
  63         nesting = ++URCU_TLS(lib_ring_buffer_nesting);
  64         cmm_barrier();
  65
  66         if (caa_unlikely(nesting > 4)) {
  67                 WARN_ON_ONCE(1);
  68                 URCU_TLS(lib_ring_buffer_nesting)--;
  69                 return -EPERM;
  70         }
  71         return 0;
  72 }
  73
  74 /**
  75  * lib_ring_buffer_end - Follows ring buffer reserve/commit.
  76  */
  77 static inline
  78 void lib_ring_buffer_end(const struct lttng_ust_lib_ring_buffer_config *config)
  79 {
  80         cmm_barrier();
  81         URCU_TLS(lib_ring_buffer_nesting)--;            /* TLS */
  82 }
  83
  84 /*
  85  * lib_ring_buffer_try_reserve is called by lib_ring_buffer_reserve(). It is not
  86  * part of the API per se.
  87  *
  88  * returns 0 if reserve ok, or 1 if the slow path must be taken.
  89  */
  90 static inline
  91 int lib_ring_buffer_try_reserve(const struct lttng_ust_lib_ring_buffer_config *config,
  92                                 struct lttng_ust_lib_ring_buffer_ctx *ctx,
  93                                 unsigned long *o_begin, unsigned long *o_end,
  94                                 unsigned long *o_old, size_t *before_hdr_pad)
  95 {
  96         struct channel *chan = ctx->chan;
  97         struct lttng_ust_lib_ring_buffer *buf = ctx->buf;
  98         *o_begin = v_read(config, &buf->offset);
  99         *o_old = *o_begin;
 100
 101         ctx->tsc = lib_ring_buffer_clock_read(chan);
 102         if ((int64_t) ctx->tsc == -EIO)
 103                 return 1;
 104
 105         /*
 106          * Prefetch cacheline for read because we have to read the previous
 107          * commit counter to increment it and commit seq value to compare it to
 108          * the commit counter.
 109          */
 110         //prefetch(&buf->commit_hot[subbuf_index(*o_begin, chan)]);
 111
 112         if (last_tsc_overflow(config, buf, ctx->tsc))
 113                 ctx->rflags |= RING_BUFFER_RFLAG_FULL_TSC;
 114
 115         if (caa_unlikely(subbuf_offset(*o_begin, chan) == 0))
 116                 return 1;
 117
 118         ctx->slot_size = record_header_size(config, chan, *o_begin,
 119                                             before_hdr_pad, ctx);
 120         ctx->slot_size +=
 121                 lib_ring_buffer_align(*o_begin + ctx->slot_size,
 122                                       ctx->largest_align) + ctx->data_size;
 123         if (caa_unlikely((subbuf_offset(*o_begin, chan) + ctx->slot_size)
 124                      > chan->backend.subbuf_size))
 125                 return 1;
 126
 127         /*
 128          * Record fits in the current buffer and we are not on a switch
 129          * boundary. It's safe to write.
 130          */
 131         *o_end = *o_begin + ctx->slot_size;
 132
 133         if (caa_unlikely((subbuf_offset(*o_end, chan)) == 0))
 134                 /*
 135                  * The offset_end will fall at the very beginning of the next
 136                  * subbuffer.
 137                  */
 138                 return 1;
 139
 140         return 0;
 141 }
 142
 143 /**
 144  * lib_ring_buffer_reserve - Reserve space in a ring buffer.
 145  * @config: ring buffer instance configuration.
 146  * @ctx: ring buffer context. (input and output) Must be already initialized.
 147  *
 148  * Atomic wait-free slot reservation. The reserved space starts at the context
 149  * "pre_offset". Its length is "slot_size". The associated time-stamp is "tsc".
 150  *
 151  * Return :
 152  *  0 on success.
 153  * -EAGAIN if channel is disabled.
 154  * -ENOSPC if event size is too large for packet.
 155  * -ENOBUFS if there is currently not enough space in buffer for the event.
 156  * -EIO if data cannot be written into the buffer for any other reason.
 157  */
 158
 159 static inline
 160 int lib_ring_buffer_reserve(const struct lttng_ust_lib_ring_buffer_config *config,
 161                             struct lttng_ust_lib_ring_buffer_ctx *ctx)
 162 {
 163         struct channel *chan = ctx->chan;
 164         struct lttng_ust_shm_handle *handle = ctx->handle;
 165         struct lttng_ust_lib_ring_buffer *buf;
 166         unsigned long o_begin, o_end, o_old;
 167         size_t before_hdr_pad = 0;
 168
 169         if (caa_unlikely(uatomic_read(&chan->record_disabled)))
 170                 return -EAGAIN;
 171
 172         if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
 173                 buf = shmp(handle, chan->backend.buf[ctx->cpu].shmp);
 174         else
 175                 buf = shmp(handle, chan->backend.buf[0].shmp);
 176         if (caa_unlikely(!buf))
 177                 return -EIO;
 178         if (caa_unlikely(uatomic_read(&buf->record_disabled)))
 179                 return -EAGAIN;
 180         ctx->buf = buf;
 181
 182         /*
 183          * Perform retryable operations.
 184          */
 185         if (caa_unlikely(lib_ring_buffer_try_reserve(config, ctx, &o_begin,
 186                                                  &o_end, &o_old, &before_hdr_pad)))
 187                 goto slow_path;
 188
 189         if (caa_unlikely(v_cmpxchg(config, &ctx->buf->offset, o_old, o_end)
 190                      != o_old))
 191                 goto slow_path;
 192
 193         /*
 194          * Atomically update last_tsc. This update races against concurrent
 195          * atomic updates, but the race will always cause supplementary full TSC
 196          * record headers, never the opposite (missing a full TSC record header
 197          * when it would be needed).
 198          */
 199         save_last_tsc(config, ctx->buf, ctx->tsc);
 200
 201         /*
 202          * Push the reader if necessary
 203          */
 204         lib_ring_buffer_reserve_push_reader(ctx->buf, chan, o_end - 1);
 205
 206         /*
 207          * Clear noref flag for this subbuffer.
 208          */
 209         lib_ring_buffer_clear_noref(config, &ctx->buf->backend,
 210                                 subbuf_index(o_end - 1, chan), handle);
 211
 212         ctx->pre_offset = o_begin;
 213         ctx->buf_offset = o_begin + before_hdr_pad;
 214         return 0;
 215 slow_path:
 216         return lib_ring_buffer_reserve_slow(ctx);
 217 }
 218
 219 /**
 220  * lib_ring_buffer_switch - Perform a sub-buffer switch for a per-cpu buffer.
 221  * @config: ring buffer instance configuration.
 222  * @buf: buffer
 223  * @mode: buffer switch mode (SWITCH_ACTIVE or SWITCH_FLUSH)
 224  *
 225  * This operation is completely reentrant : can be called while tracing is
 226  * active with absolutely no lock held.
 227  *
 228  * Note, however, that as a v_cmpxchg is used for some atomic operations and
 229  * requires to be executed locally for per-CPU buffers, this function must be
 230  * called from the CPU which owns the buffer for a ACTIVE flush, with preemption
 231  * disabled, for RING_BUFFER_SYNC_PER_CPU configuration.
 232  */
 233 static inline
 234 void lib_ring_buffer_switch(const struct lttng_ust_lib_ring_buffer_config *config,
 235                             struct lttng_ust_lib_ring_buffer *buf, enum switch_mode mode,
 236                             struct lttng_ust_shm_handle *handle)
 237 {
 238         lib_ring_buffer_switch_slow(buf, mode, handle);
 239 }
 240
 241 /* See ring_buffer_frontend_api.h for lib_ring_buffer_reserve(). */
 242
 243 /**
 244  * lib_ring_buffer_commit - Commit an record.
 245  * @config: ring buffer instance configuration.
 246  * @ctx: ring buffer context. (input arguments only)
 247  *
 248  * Atomic unordered slot commit. Increments the commit count in the
 249  * specified sub-buffer, and delivers it if necessary.
 250  */
 251 static inline
 252 void lib_ring_buffer_commit(const struct lttng_ust_lib_ring_buffer_config *config,
 253                             const struct lttng_ust_lib_ring_buffer_ctx *ctx)
 254 {
 255         struct channel *chan = ctx->chan;
 256         struct lttng_ust_shm_handle *handle = ctx->handle;
 257         struct lttng_ust_lib_ring_buffer *buf = ctx->buf;
 258         unsigned long offset_end = ctx->buf_offset;
 259         unsigned long endidx = subbuf_index(offset_end - 1, chan);
 260         unsigned long commit_count;
 261         struct commit_counters_hot *cc_hot = shmp_index(handle,
 262                                                 buf->commit_hot, endidx);
 263         struct lttng_rseq_state rseq_state;
 264
 265         if (caa_likely(ctx->ctx_len
 266                         >= sizeof(struct lttng_ust_lib_ring_buffer_ctx))) {
 267                 rseq_state = ctx->rseq_state;
 268         } else {
 269                 rseq_state.cpu_id = -2;
 270         }
 271
 272         if (caa_unlikely(!cc_hot))
 273                 return;
 274
 275         /*
 276          * Must count record before incrementing the commit count.
 277          */
 278         subbuffer_count_record(config, ctx, &buf->backend, endidx, handle);
 279
 280         /*
 281          * Order all writes to buffer before the commit count update that will
 282          * determine that the subbuffer is full.
 283          */
 284         cmm_smp_wmb();
 285
 286         if (caa_likely(rseq_state.cpu_id >= 0)) {
 287                 unsigned long newv;
 288
 289                 newv = cc_hot->cc_rseq + ctx->slot_size;
 290                 if (caa_likely(__rseq_finish(NULL, 0, NULL, NULL, 0,
 291                                 (intptr_t *)&cc_hot->cc_rseq,
 292                                 (intptr_t) newv,
 293                                 rseq_state, RSEQ_FINISH_SINGLE, false)))
 294                         goto add_done;
 295         }
 296         v_add(config, ctx->slot_size, &cc_hot->cc);
 297 add_done:
 298
 299         /*
 300          * commit count read can race with concurrent OOO commit count updates.
 301          * This is only needed for lib_ring_buffer_check_deliver (for
 302          * non-polling delivery only) and for
 303          * lib_ring_buffer_write_commit_counter.  The race can only cause the
 304          * counter to be read with the same value more than once, which could
 305          * cause :
 306          * - Multiple delivery for the same sub-buffer (which is handled
 307          *   gracefully by the reader code) if the value is for a full
 308          *   sub-buffer. It's important that we can never miss a sub-buffer
 309          *   delivery. Re-reading the value after the v_add ensures this.
 310          * - Reading a commit_count with a higher value that what was actually
 311          *   added to it for the lib_ring_buffer_write_commit_counter call
 312          *   (again caused by a concurrent committer). It does not matter,
 313          *   because this function is interested in the fact that the commit
 314          *   count reaches back the reserve offset for a specific sub-buffer,
 315          *   which is completely independent of the order.
 316          */
 317         commit_count = v_read(config, &cc_hot->cc);
 318         commit_count += cc_hot->cc_rseq;
 319
 320         lib_ring_buffer_check_deliver(config, buf, chan, offset_end - 1,
 321                                       commit_count, endidx, handle, ctx->tsc);
 322         /*
 323          * Update used size at each commit. It's needed only for extracting
 324          * ring_buffer buffers from vmcore, after crash.
 325          */
 326         lib_ring_buffer_write_commit_counter(config, buf, chan,
 327                         offset_end, commit_count, handle, cc_hot);
 328 }
 329
 330 /**
 331  * lib_ring_buffer_try_discard_reserve - Try discarding a record.
 332  * @config: ring buffer instance configuration.
 333  * @ctx: ring buffer context. (input arguments only)
 334  *
 335  * Only succeeds if no other record has been written after the record to
 336  * discard. If discard fails, the record must be committed to the buffer.
 337  *
 338  * Returns 0 upon success, -EPERM if the record cannot be discarded.
 339  */
 340 static inline
 341 int lib_ring_buffer_try_discard_reserve(const struct lttng_ust_lib_ring_buffer_config *config,
 342                                         const struct lttng_ust_lib_ring_buffer_ctx *ctx)
 343 {
 344         struct lttng_ust_lib_ring_buffer *buf = ctx->buf;
 345         unsigned long end_offset = ctx->pre_offset + ctx->slot_size;
 346
 347         /*
 348          * We need to ensure that if the cmpxchg succeeds and discards the
 349          * record, the next record will record a full TSC, because it cannot
 350          * rely on the last_tsc associated with the discarded record to detect
 351          * overflows. The only way to ensure this is to set the last_tsc to 0
 352          * (assuming no 64-bit TSC overflow), which forces to write a 64-bit
 353          * timestamp in the next record.
 354          *
 355          * Note: if discard fails, we must leave the TSC in the record header.
 356          * It is needed to keep track of TSC overflows for the following
 357          * records.
 358          */
 359         save_last_tsc(config, buf, 0ULL);
 360
 361         if (caa_likely(v_cmpxchg(config, &buf->offset, end_offset, ctx->pre_offset)
 362                    != end_offset))
 363                 return -EPERM;
 364         else
 365                 return 0;
 366 }
 367
 368 static inline
 369 void channel_record_disable(const struct lttng_ust_lib_ring_buffer_config *config,
 370                             struct channel *chan)
 371 {
 372         uatomic_inc(&chan->record_disabled);
 373 }
 374
 375 static inline
 376 void channel_record_enable(const struct lttng_ust_lib_ring_buffer_config *config,
 377                            struct channel *chan)
 378 {
 379         uatomic_dec(&chan->record_disabled);
 380 }
 381
 382 static inline
 383 void lib_ring_buffer_record_disable(const struct lttng_ust_lib_ring_buffer_config *config,
 384                                     struct lttng_ust_lib_ring_buffer *buf)
 385 {
 386         uatomic_inc(&buf->record_disabled);
 387 }
 388
 389 static inline
 390 void lib_ring_buffer_record_enable(const struct lttng_ust_lib_ring_buffer_config *config,
 391                                    struct lttng_ust_lib_ring_buffer *buf)
 392 {
 393         uatomic_dec(&buf->record_disabled);
 394 }
 395
 396 #endif /* _LTTNG_RING_BUFFER_FRONTEND_API_H */