libringbuffer/frontend_api.h

   1 #ifndef _LINUX_RING_BUFFER_FRONTEND_API_H
   2 #define _LINUX_RING_BUFFER_FRONTEND_API_H
   3
   4 /*
   5  * linux/ringbuffer/frontend_api.h
   6  *
   7  * (C) Copyright 2005-2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   8  *
   9  * Ring Buffer Library Synchronization Header (buffer write API).
  10  *
  11  * Author:
  12  *      Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  13  *
  14  * See ring_buffer_frontend.c for more information on wait-free algorithms.
  15  * See linux/ringbuffer/frontend.h for channel allocation and read-side API.
  16  *
  17  * Dual LGPL v2.1/GPL v2 license.
  18  */
  19
  20 #include "../../wrapper/ringbuffer/frontend.h"
  21 #include <linux/errno.h>
  22
  23 /**
  24  * lib_ring_buffer_get_cpu - Precedes ring buffer reserve/commit.
  25  *
  26  * Disables preemption (acts as a RCU read-side critical section) and keeps a
  27  * ring buffer nesting count as supplementary safety net to ensure tracer client
  28  * code will never trigger an endless recursion. Returns the processor ID on
  29  * success, -EPERM on failure (nesting count too high).
  30  *
  31  * asm volatile and "memory" clobber prevent the compiler from moving
  32  * instructions out of the ring buffer nesting count. This is required to ensure
  33  * that probe side-effects which can cause recursion (e.g. unforeseen traps,
  34  * divisions by 0, ...) are triggered within the incremented nesting count
  35  * section.
  36  */
  37 static inline
  38 int lib_ring_buffer_get_cpu(const struct lib_ring_buffer_config *config)
  39 {
  40         int cpu, nesting;
  41
  42         rcu_read_lock_sched_notrace();
  43         cpu = smp_processor_id();
  44         nesting = ++per_cpu(lib_ring_buffer_nesting, cpu);
  45         barrier();
  46
  47         if (unlikely(nesting > 4)) {
  48                 WARN_ON_ONCE(1);
  49                 per_cpu(lib_ring_buffer_nesting, cpu)--;
  50                 rcu_read_unlock_sched_notrace();
  51                 return -EPERM;
  52         } else
  53                 return cpu;
  54 }
  55
  56 /**
  57  * lib_ring_buffer_put_cpu - Follows ring buffer reserve/commit.
  58  */
  59 static inline
  60 void lib_ring_buffer_put_cpu(const struct lib_ring_buffer_config *config)
  61 {
  62         barrier();
  63         __get_cpu_var(lib_ring_buffer_nesting)--;
  64         rcu_read_unlock_sched_notrace();
  65 }
  66
  67 /*
  68  * lib_ring_buffer_try_reserve is called by lib_ring_buffer_reserve(). It is not
  69  * part of the API per se.
  70  *
  71  * returns 0 if reserve ok, or 1 if the slow path must be taken.
  72  */
  73 static inline
  74 int lib_ring_buffer_try_reserve(const struct lib_ring_buffer_config *config,
  75                                 struct lib_ring_buffer_ctx *ctx,
  76                                 unsigned long *o_begin, unsigned long *o_end,
  77                                 unsigned long *o_old, size_t *before_hdr_pad)
  78 {
  79         struct channel *chan = ctx->chan;
  80         struct lib_ring_buffer *buf = ctx->buf;
  81         *o_begin = v_read(config, &buf->offset);
  82         *o_old = *o_begin;
  83
  84         ctx->tsc = lib_ring_buffer_clock_read(chan);
  85         if ((int64_t) ctx->tsc == -EIO)
  86                 return 1;
  87
  88         /*
  89          * Prefetch cacheline for read because we have to read the previous
  90          * commit counter to increment it and commit seq value to compare it to
  91          * the commit counter.
  92          */
  93         prefetch(&buf->commit_hot[subbuf_index(*o_begin, chan)]);
  94
  95         if (last_tsc_overflow(config, buf, ctx->tsc))
  96                 ctx->rflags |= RING_BUFFER_RFLAG_FULL_TSC;
  97
  98         if (unlikely(subbuf_offset(*o_begin, chan) == 0))
  99                 return 1;
 100
 101         ctx->slot_size = record_header_size(config, chan, *o_begin,
 102                                             before_hdr_pad, ctx);
 103         ctx->slot_size +=
 104                 lib_ring_buffer_align(*o_begin + ctx->slot_size,
 105                                       ctx->largest_align) + ctx->data_size;
 106         if (unlikely((subbuf_offset(*o_begin, chan) + ctx->slot_size)
 107                      > chan->backend.subbuf_size))
 108                 return 1;
 109
 110         /*
 111          * Record fits in the current buffer and we are not on a switch
 112          * boundary. It's safe to write.
 113          */
 114         *o_end = *o_begin + ctx->slot_size;
 115
 116         if (unlikely((subbuf_offset(*o_end, chan)) == 0))
 117                 /*
 118                  * The offset_end will fall at the very beginning of the next
 119                  * subbuffer.
 120                  */
 121                 return 1;
 122
 123         return 0;
 124 }
 125
 126 /**
 127  * lib_ring_buffer_reserve - Reserve space in a ring buffer.
 128  * @config: ring buffer instance configuration.
 129  * @ctx: ring buffer context. (input and output) Must be already initialized.
 130  *
 131  * Atomic wait-free slot reservation. The reserved space starts at the context
 132  * "pre_offset". Its length is "slot_size". The associated time-stamp is "tsc".
 133  *
 134  * Return :
 135  *  0 on success.
 136  * -EAGAIN if channel is disabled.
 137  * -ENOSPC if event size is too large for packet.
 138  * -ENOBUFS if there is currently not enough space in buffer for the event.
 139  * -EIO if data cannot be written into the buffer for any other reason.
 140  */
 141
 142 static inline
 143 int lib_ring_buffer_reserve(const struct lib_ring_buffer_config *config,
 144                             struct lib_ring_buffer_ctx *ctx)
 145 {
 146         struct channel *chan = ctx->chan;
 147         struct lib_ring_buffer *buf;
 148         unsigned long o_begin, o_end, o_old;
 149         size_t before_hdr_pad = 0;
 150
 151         if (atomic_read(&chan->record_disabled))
 152                 return -EAGAIN;
 153
 154         if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
 155                 buf = per_cpu_ptr(chan->backend.buf, ctx->cpu);
 156         else
 157                 buf = chan->backend.buf;
 158         if (atomic_read(&buf->record_disabled))
 159                 return -EAGAIN;
 160         ctx->buf = buf;
 161
 162         /*
 163          * Perform retryable operations.
 164          */
 165         if (unlikely(lib_ring_buffer_try_reserve(config, ctx, &o_begin,
 166                                                  &o_end, &o_old, &before_hdr_pad)))
 167                 goto slow_path;
 168
 169         if (unlikely(v_cmpxchg(config, &ctx->buf->offset, o_old, o_end)
 170                      != o_old))
 171                 goto slow_path;
 172
 173         /*
 174          * Atomically update last_tsc. This update races against concurrent
 175          * atomic updates, but the race will always cause supplementary full TSC
 176          * record headers, never the opposite (missing a full TSC record header
 177          * when it would be needed).
 178          */
 179         save_last_tsc(config, ctx->buf, ctx->tsc);
 180
 181         /*
 182          * Push the reader if necessary
 183          */
 184         lib_ring_buffer_reserve_push_reader(ctx->buf, chan, o_end - 1);
 185
 186         /*
 187          * Clear noref flag for this subbuffer.
 188          */
 189         lib_ring_buffer_clear_noref(config, &ctx->buf->backend,
 190                                 subbuf_index(o_end - 1, chan));
 191
 192         ctx->pre_offset = o_begin;
 193         ctx->buf_offset = o_begin + before_hdr_pad;
 194         return 0;
 195 slow_path:
 196         return lib_ring_buffer_reserve_slow(ctx);
 197 }
 198
 199 /**
 200  * lib_ring_buffer_switch - Perform a sub-buffer switch for a per-cpu buffer.
 201  * @config: ring buffer instance configuration.
 202  * @buf: buffer
 203  * @mode: buffer switch mode (SWITCH_ACTIVE or SWITCH_FLUSH)
 204  *
 205  * This operation is completely reentrant : can be called while tracing is
 206  * active with absolutely no lock held.
 207  *
 208  * Note, however, that as a v_cmpxchg is used for some atomic operations and
 209  * requires to be executed locally for per-CPU buffers, this function must be
 210  * called from the CPU which owns the buffer for a ACTIVE flush, with preemption
 211  * disabled, for RING_BUFFER_SYNC_PER_CPU configuration.
 212  */
 213 static inline
 214 void lib_ring_buffer_switch(const struct lib_ring_buffer_config *config,
 215                             struct lib_ring_buffer *buf, enum switch_mode mode)
 216 {
 217         lib_ring_buffer_switch_slow(buf, mode);
 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 lib_ring_buffer_config *config,
 232                             const struct lib_ring_buffer_ctx *ctx)
 233 {
 234         struct channel *chan = ctx->chan;
 235         struct lib_ring_buffer *buf = ctx->buf;
 236         unsigned long offset_end = ctx->buf_offset;
 237         unsigned long endidx = subbuf_index(offset_end - 1, chan);
 238         unsigned long commit_count;
 239
 240         /*
 241          * Must count record before incrementing the commit count.
 242          */
 243         subbuffer_count_record(config, &buf->backend, endidx);
 244
 245         /*
 246          * Order all writes to buffer before the commit count update that will
 247          * determine that the subbuffer is full.
 248          */
 249         if (config->ipi == RING_BUFFER_IPI_BARRIER) {
 250                 /*
 251                  * Must write slot data before incrementing commit count.  This
 252                  * compiler barrier is upgraded into a smp_mb() by the IPI sent
 253                  * by get_subbuf().
 254                  */
 255                 barrier();
 256         } else
 257                 smp_wmb();
 258
 259         v_add(config, ctx->slot_size, &buf->commit_hot[endidx].cc);
 260
 261         /*
 262          * commit count read can race with concurrent OOO commit count updates.
 263          * This is only needed for lib_ring_buffer_check_deliver (for
 264          * non-polling delivery only) and for
 265          * lib_ring_buffer_write_commit_counter.  The race can only cause the
 266          * counter to be read with the same value more than once, which could
 267          * cause :
 268          * - Multiple delivery for the same sub-buffer (which is handled
 269          *   gracefully by the reader code) if the value is for a full
 270          *   sub-buffer. It's important that we can never miss a sub-buffer
 271          *   delivery. Re-reading the value after the v_add ensures this.
 272          * - Reading a commit_count with a higher value that what was actually
 273          *   added to it for the lib_ring_buffer_write_commit_counter call
 274          *   (again caused by a concurrent committer). It does not matter,
 275          *   because this function is interested in the fact that the commit
 276          *   count reaches back the reserve offset for a specific sub-buffer,
 277          *   which is completely independent of the order.
 278          */
 279         commit_count = v_read(config, &buf->commit_hot[endidx].cc);
 280
 281         lib_ring_buffer_check_deliver(config, buf, chan, offset_end - 1,
 282                                       commit_count, endidx);
 283         /*
 284          * Update used size at each commit. It's needed only for extracting
 285          * ring_buffer buffers from vmcore, after crash.
 286          */
 287         lib_ring_buffer_write_commit_counter(config, buf, chan, endidx,
 288                                              ctx->buf_offset, commit_count,
 289                                          ctx->slot_size);
 290 }
 291
 292 /**
 293  * lib_ring_buffer_try_discard_reserve - Try discarding a record.
 294  * @config: ring buffer instance configuration.
 295  * @ctx: ring buffer context. (input arguments only)
 296  *
 297  * Only succeeds if no other record has been written after the record to
 298  * discard. If discard fails, the record must be committed to the buffer.
 299  *
 300  * Returns 0 upon success, -EPERM if the record cannot be discarded.
 301  */
 302 static inline
 303 int lib_ring_buffer_try_discard_reserve(const struct lib_ring_buffer_config *config,
 304                                         const struct lib_ring_buffer_ctx *ctx)
 305 {
 306         struct lib_ring_buffer *buf = ctx->buf;
 307         unsigned long end_offset = ctx->pre_offset + ctx->slot_size;
 308
 309         /*
 310          * We need to ensure that if the cmpxchg succeeds and discards the
 311          * record, the next record will record a full TSC, because it cannot
 312          * rely on the last_tsc associated with the discarded record to detect
 313          * overflows. The only way to ensure this is to set the last_tsc to 0
 314          * (assuming no 64-bit TSC overflow), which forces to write a 64-bit
 315          * timestamp in the next record.
 316          *
 317          * Note: if discard fails, we must leave the TSC in the record header.
 318          * It is needed to keep track of TSC overflows for the following
 319          * records.
 320          */
 321         save_last_tsc(config, buf, 0ULL);
 322
 323         if (likely(v_cmpxchg(config, &buf->offset, end_offset, ctx->pre_offset)
 324                    != end_offset))
 325                 return -EPERM;
 326         else
 327                 return 0;
 328 }
 329
 330 static inline
 331 void channel_record_disable(const struct lib_ring_buffer_config *config,
 332                             struct channel *chan)
 333 {
 334         atomic_inc(&chan->record_disabled);
 335 }
 336
 337 static inline
 338 void channel_record_enable(const struct lib_ring_buffer_config *config,
 339                            struct channel *chan)
 340 {
 341         atomic_dec(&chan->record_disabled);
 342 }
 343
 344 static inline
 345 void lib_ring_buffer_record_disable(const struct lib_ring_buffer_config *config,
 346                                     struct lib_ring_buffer *buf)
 347 {
 348         atomic_inc(&buf->record_disabled);
 349 }
 350
 351 static inline
 352 void lib_ring_buffer_record_enable(const struct lib_ring_buffer_config *config,
 353                                    struct lib_ring_buffer *buf)
 354 {
 355         atomic_dec(&buf->record_disabled);
 356 }
 357
 358 #endif /* _LINUX_RING_BUFFER_FRONTEND_API_H */