lib/ringbuffer/frontend_api.h

   1 #ifndef _LIB_RING_BUFFER_FRONTEND_API_H
   2 #define _LIB_RING_BUFFER_FRONTEND_API_H
   3
   4 /*
   5  * lib/ringbuffer/frontend_api.h
   6  *
   7  * Ring Buffer Library Synchronization Header (buffer write API).
   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  * See linux/ringbuffer/frontend.h for channel allocation and read-side API.
  30  */
  31
  32 #include "../../wrapper/ringbuffer/frontend.h"
  33 #include <linux/errno.h>
  34 #include <linux/prefetch.h>
  35
  36 /**
  37  * lib_ring_buffer_get_cpu - Precedes ring buffer reserve/commit.
  38  *
  39  * Disables preemption (acts as a RCU read-side critical section) and keeps a
  40  * ring buffer nesting count as supplementary safety net to ensure tracer client
  41  * code will never trigger an endless recursion. Returns the processor ID on
  42  * success, -EPERM on failure (nesting count too high).
  43  *
  44  * asm volatile and "memory" clobber prevent the compiler from moving
  45  * instructions out of the ring buffer nesting count. This is required to ensure
  46  * that probe side-effects which can cause recursion (e.g. unforeseen traps,
  47  * divisions by 0, ...) are triggered within the incremented nesting count
  48  * section.
  49  */
  50 static inline
  51 int lib_ring_buffer_get_cpu(const struct lib_ring_buffer_config *config)
  52 {
  53         int cpu, nesting;
  54
  55         rcu_read_lock_sched_notrace();
  56         cpu = smp_processor_id();
  57         nesting = ++per_cpu(lib_ring_buffer_nesting, cpu);
  58         barrier();
  59
  60         if (unlikely(nesting > 4)) {
  61                 WARN_ON_ONCE(1);
  62                 per_cpu(lib_ring_buffer_nesting, cpu)--;
  63                 rcu_read_unlock_sched_notrace();
  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 lib_ring_buffer_config *config)
  74 {
  75         barrier();
  76         __get_cpu_var(lib_ring_buffer_nesting)--;
  77         rcu_read_unlock_sched_notrace();
  78 }
  79
  80 /*
  81  * lib_ring_buffer_try_reserve is called by lib_ring_buffer_reserve(). It is not
  82  * part of the API per se.
  83  *
  84  * returns 0 if reserve ok, or 1 if the slow path must be taken.
  85  */
  86 static inline
  87 int lib_ring_buffer_try_reserve(const struct lib_ring_buffer_config *config,
  88                                 struct lib_ring_buffer_ctx *ctx,
  89                                 unsigned long *o_begin, unsigned long *o_end,
  90                                 unsigned long *o_old, size_t *before_hdr_pad)
  91 {
  92         struct channel *chan = ctx->chan;
  93         struct lib_ring_buffer *buf = ctx->buf;
  94         *o_begin = v_read(config, &buf->offset);
  95         *o_old = *o_begin;
  96
  97         ctx->tsc = lib_ring_buffer_clock_read(chan);
  98         if ((int64_t) ctx->tsc == -EIO)
  99                 return 1;
 100
 101         /*
 102          * Prefetch cacheline for read because we have to read the previous
 103          * commit counter to increment it and commit seq value to compare it to
 104          * the commit counter.
 105          */
 106         prefetch(&buf->commit_hot[subbuf_index(*o_begin, chan)]);
 107
 108         if (last_tsc_overflow(config, buf, ctx->tsc))
 109                 ctx->rflags |= RING_BUFFER_RFLAG_FULL_TSC;
 110
 111         if (unlikely(subbuf_offset(*o_begin, chan) == 0))
 112                 return 1;
 113
 114         ctx->slot_size = record_header_size(config, chan, *o_begin,
 115                                             before_hdr_pad, ctx);
 116         ctx->slot_size +=
 117                 lib_ring_buffer_align(*o_begin + ctx->slot_size,
 118                                       ctx->largest_align) + ctx->data_size;
 119         if (unlikely((subbuf_offset(*o_begin, chan) + ctx->slot_size)
 120                      > chan->backend.subbuf_size))
 121                 return 1;
 122
 123         /*
 124          * Record fits in the current buffer and we are not on a switch
 125          * boundary. It's safe to write.
 126          */
 127         *o_end = *o_begin + ctx->slot_size;
 128
 129         if (unlikely((subbuf_offset(*o_end, chan)) == 0))
 130                 /*
 131                  * The offset_end will fall at the very beginning of the next
 132                  * subbuffer.
 133                  */
 134                 return 1;
 135
 136         return 0;
 137 }
 138
 139 /**
 140  * lib_ring_buffer_reserve - Reserve space in a ring buffer.
 141  * @config: ring buffer instance configuration.
 142  * @ctx: ring buffer context. (input and output) Must be already initialized.
 143  *
 144  * Atomic wait-free slot reservation. The reserved space starts at the context
 145  * "pre_offset". Its length is "slot_size". The associated time-stamp is "tsc".
 146  *
 147  * Return :
 148  *  0 on success.
 149  * -EAGAIN if channel is disabled.
 150  * -ENOSPC if event size is too large for packet.
 151  * -ENOBUFS if there is currently not enough space in buffer for the event.
 152  * -EIO if data cannot be written into the buffer for any other reason.
 153  */
 154
 155 static inline
 156 int lib_ring_buffer_reserve(const struct lib_ring_buffer_config *config,
 157                             struct lib_ring_buffer_ctx *ctx)
 158 {
 159         struct channel *chan = ctx->chan;
 160         struct lib_ring_buffer *buf;
 161         unsigned long o_begin, o_end, o_old;
 162         size_t before_hdr_pad = 0;
 163
 164         if (atomic_read(&chan->record_disabled))
 165                 return -EAGAIN;
 166
 167         if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
 168                 buf = per_cpu_ptr(chan->backend.buf, ctx->cpu);
 169         else
 170                 buf = chan->backend.buf;
 171         if (atomic_read(&buf->record_disabled))
 172                 return -EAGAIN;
 173         ctx->buf = buf;
 174
 175         /*
 176          * Perform retryable operations.
 177          */
 178         if (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 (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));
 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 lib_ring_buffer_config *config,
 228                             struct lib_ring_buffer *buf, enum switch_mode mode)
 229 {
 230         lib_ring_buffer_switch_slow(buf, mode);
 231 }
 232
 233 /* See ring_buffer_frontend_api.h for lib_ring_buffer_reserve(). */
 234
 235 /**
 236  * lib_ring_buffer_commit - Commit an record.
 237  * @config: ring buffer instance configuration.
 238  * @ctx: ring buffer context. (input arguments only)
 239  *
 240  * Atomic unordered slot commit. Increments the commit count in the
 241  * specified sub-buffer, and delivers it if necessary.
 242  */
 243 static inline
 244 void lib_ring_buffer_commit(const struct lib_ring_buffer_config *config,
 245                             const struct lib_ring_buffer_ctx *ctx)
 246 {
 247         struct channel *chan = ctx->chan;
 248         struct lib_ring_buffer *buf = ctx->buf;
 249         unsigned long offset_end = ctx->buf_offset;
 250         unsigned long endidx = subbuf_index(offset_end - 1, chan);
 251         unsigned long commit_count;
 252
 253         /*
 254          * Must count record before incrementing the commit count.
 255          */
 256         subbuffer_count_record(config, &buf->backend, endidx);
 257
 258         /*
 259          * Order all writes to buffer before the commit count update that will
 260          * determine that the subbuffer is full.
 261          */
 262         if (config->ipi == RING_BUFFER_IPI_BARRIER) {
 263                 /*
 264                  * Must write slot data before incrementing commit count.  This
 265                  * compiler barrier is upgraded into a smp_mb() by the IPI sent
 266                  * by get_subbuf().
 267                  */
 268                 barrier();
 269         } else
 270                 smp_wmb();
 271
 272         v_add(config, ctx->slot_size, &buf->commit_hot[endidx].cc);
 273
 274         /*
 275          * commit count read can race with concurrent OOO commit count updates.
 276          * This is only needed for lib_ring_buffer_check_deliver (for
 277          * non-polling delivery only) and for
 278          * lib_ring_buffer_write_commit_counter.  The race can only cause the
 279          * counter to be read with the same value more than once, which could
 280          * cause :
 281          * - Multiple delivery for the same sub-buffer (which is handled
 282          *   gracefully by the reader code) if the value is for a full
 283          *   sub-buffer. It's important that we can never miss a sub-buffer
 284          *   delivery. Re-reading the value after the v_add ensures this.
 285          * - Reading a commit_count with a higher value that what was actually
 286          *   added to it for the lib_ring_buffer_write_commit_counter call
 287          *   (again caused by a concurrent committer). It does not matter,
 288          *   because this function is interested in the fact that the commit
 289          *   count reaches back the reserve offset for a specific sub-buffer,
 290          *   which is completely independent of the order.
 291          */
 292         commit_count = v_read(config, &buf->commit_hot[endidx].cc);
 293
 294         lib_ring_buffer_check_deliver(config, buf, chan, offset_end - 1,
 295                                       commit_count, endidx);
 296         /*
 297          * Update used size at each commit. It's needed only for extracting
 298          * ring_buffer buffers from vmcore, after crash.
 299          */
 300         lib_ring_buffer_write_commit_counter(config, buf, chan, endidx,
 301                                              ctx->buf_offset, commit_count,
 302                                          ctx->slot_size);
 303 }
 304
 305 /**
 306  * lib_ring_buffer_try_discard_reserve - Try discarding a record.
 307  * @config: ring buffer instance configuration.
 308  * @ctx: ring buffer context. (input arguments only)
 309  *
 310  * Only succeeds if no other record has been written after the record to
 311  * discard. If discard fails, the record must be committed to the buffer.
 312  *
 313  * Returns 0 upon success, -EPERM if the record cannot be discarded.
 314  */
 315 static inline
 316 int lib_ring_buffer_try_discard_reserve(const struct lib_ring_buffer_config *config,
 317                                         const struct lib_ring_buffer_ctx *ctx)
 318 {
 319         struct lib_ring_buffer *buf = ctx->buf;
 320         unsigned long end_offset = ctx->pre_offset + ctx->slot_size;
 321
 322         /*
 323          * We need to ensure that if the cmpxchg succeeds and discards the
 324          * record, the next record will record a full TSC, because it cannot
 325          * rely on the last_tsc associated with the discarded record to detect
 326          * overflows. The only way to ensure this is to set the last_tsc to 0
 327          * (assuming no 64-bit TSC overflow), which forces to write a 64-bit
 328          * timestamp in the next record.
 329          *
 330          * Note: if discard fails, we must leave the TSC in the record header.
 331          * It is needed to keep track of TSC overflows for the following
 332          * records.
 333          */
 334         save_last_tsc(config, buf, 0ULL);
 335
 336         if (likely(v_cmpxchg(config, &buf->offset, end_offset, ctx->pre_offset)
 337                    != end_offset))
 338                 return -EPERM;
 339         else
 340                 return 0;
 341 }
 342
 343 static inline
 344 void channel_record_disable(const struct lib_ring_buffer_config *config,
 345                             struct channel *chan)
 346 {
 347         atomic_inc(&chan->record_disabled);
 348 }
 349
 350 static inline
 351 void channel_record_enable(const struct lib_ring_buffer_config *config,
 352                            struct channel *chan)
 353 {
 354         atomic_dec(&chan->record_disabled);
 355 }
 356
 357 static inline
 358 void lib_ring_buffer_record_disable(const struct lib_ring_buffer_config *config,
 359                                     struct lib_ring_buffer *buf)
 360 {
 361         atomic_inc(&buf->record_disabled);
 362 }
 363
 364 static inline
 365 void lib_ring_buffer_record_enable(const struct lib_ring_buffer_config *config,
 366                                    struct lib_ring_buffer *buf)
 367 {
 368         atomic_dec(&buf->record_disabled);
 369 }
 370
 371 #endif /* _LIB_RING_BUFFER_FRONTEND_API_H */