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