include/ringbuffer/frontend_api.h

   1 /* SPDX-License-Identifier: (GPL-2.0-only OR LGPL-2.1-only)
   2  *
   3  * ringbuffer/frontend_api.h
   4  *
   5  * Ring Buffer Library Synchronization Header (buffer write API).
   6  *
   7  * Copyright (C) 2005-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   8  *
   9  * See ring_buffer_frontend.c for more information on wait-free algorithms.
  10  * See linux/ringbuffer/frontend.h for channel allocation and read-side API.
  11  */
  12
  13 #ifndef _LIB_RING_BUFFER_FRONTEND_API_H
  14 #define _LIB_RING_BUFFER_FRONTEND_API_H
  15
  16 #include <ringbuffer/frontend.h>
  17 #include <linux/percpu-defs.h>
  18 #include <linux/errno.h>
  19 #include <linux/prefetch.h>
  20
  21 /**
  22  * lib_ring_buffer_get_cpu - Precedes ring buffer reserve/commit.
  23  *
  24  * Disables preemption (acts as a RCU read-side critical section) and keeps a
  25  * ring buffer nesting count as supplementary safety net to ensure tracer client
  26  * code will never trigger an endless recursion. Returns the processor ID on
  27  * success, -EPERM on failure (nesting count too high).
  28  *
  29  * asm volatile and "memory" clobber prevent the compiler from moving
  30  * instructions out of the ring buffer nesting count. This is required to ensure
  31  * that probe side-effects which can cause recursion (e.g. unforeseen traps,
  32  * divisions by 0, ...) are triggered within the incremented nesting count
  33  * section.
  34  */
  35 static inline
  36 int lib_ring_buffer_get_cpu(const struct lttng_kernel_ring_buffer_config *config)
  37 {
  38         int cpu, nesting;
  39
  40         rcu_read_lock_sched_notrace();
  41         cpu = smp_processor_id();
  42         nesting = ++per_cpu(lib_ring_buffer_nesting, cpu);
  43         barrier();
  44
  45         if (unlikely(nesting > RING_BUFFER_MAX_NESTING)) {
  46                 WARN_ON_ONCE(1);
  47                 per_cpu(lib_ring_buffer_nesting, cpu)--;
  48                 rcu_read_unlock_sched_notrace();
  49                 return -EPERM;
  50         } else
  51                 return cpu;
  52 }
  53
  54 /**
  55  * lib_ring_buffer_put_cpu - Follows ring buffer reserve/commit.
  56  */
  57 static inline
  58 void lib_ring_buffer_put_cpu(const struct lttng_kernel_ring_buffer_config *config)
  59 {
  60         barrier();
  61         (*this_cpu_ptr(&lib_ring_buffer_nesting))--;
  62         rcu_read_unlock_sched_notrace();
  63 }
  64
  65 /*
  66  * lib_ring_buffer_try_reserve is called by lib_ring_buffer_reserve(). It is not
  67  * part of the API per se.
  68  *
  69  * returns 0 if reserve ok, or 1 if the slow path must be taken.
  70  */
  71 static inline
  72 int lib_ring_buffer_try_reserve(const struct lttng_kernel_ring_buffer_config *config,
  73                                 struct lttng_kernel_ring_buffer_ctx *ctx,
  74                                 void *client_ctx,
  75                                 unsigned long *o_begin, unsigned long *o_end,
  76                                 unsigned long *o_old, size_t *before_hdr_pad)
  77 {
  78         struct lttng_kernel_ring_buffer_channel *chan = ctx->priv.chan;
  79         struct lttng_kernel_ring_buffer *buf = ctx->priv.buf;
  80         *o_begin = v_read(config, &buf->offset);
  81         *o_old = *o_begin;
  82
  83         ctx->priv.timestamp = lib_ring_buffer_clock_read(chan);
  84         if ((int64_t) ctx->priv.timestamp == -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_timestamp_overflow(config, buf, ctx->priv.timestamp))
  95                 ctx->priv.rflags |= RING_BUFFER_RFLAG_FULL_TIMESTAMP;
  96
  97         if (unlikely(subbuf_offset(*o_begin, chan) == 0))
  98                 return 1;
  99
 100         ctx->priv.slot_size = record_header_size(config, chan, *o_begin,
 101                                             before_hdr_pad, ctx, client_ctx);
 102         ctx->priv.slot_size +=
 103                 lib_ring_buffer_align(*o_begin + ctx->priv.slot_size,
 104                                       ctx->largest_align) + ctx->data_size;
 105         if (unlikely((subbuf_offset(*o_begin, chan) + ctx->priv.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->priv.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
 132  * "timestamp".
 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 lttng_kernel_ring_buffer_config *config,
 144                             struct lttng_kernel_ring_buffer_ctx *ctx,
 145                             void *client_ctx)
 146 {
 147         struct lttng_kernel_ring_buffer_channel *chan = ctx->priv.chan;
 148         struct lttng_kernel_ring_buffer *buf;
 149         unsigned long o_begin, o_end, o_old;
 150         size_t before_hdr_pad = 0;
 151
 152         if (unlikely(atomic_read(&chan->record_disabled)))
 153                 return -EAGAIN;
 154
 155         if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
 156                 buf = per_cpu_ptr(chan->backend.buf, ctx->priv.reserve_cpu);
 157         else
 158                 buf = chan->backend.buf;
 159         if (unlikely(atomic_read(&buf->record_disabled)))
 160                 return -EAGAIN;
 161         ctx->priv.buf = buf;
 162
 163         /*
 164          * Perform retryable operations.
 165          */
 166         if (unlikely(lib_ring_buffer_try_reserve(config, ctx, client_ctx, &o_begin,
 167                                                  &o_end, &o_old, &before_hdr_pad)))
 168                 goto slow_path;
 169
 170         if (unlikely(v_cmpxchg(config, &ctx->priv.buf->offset, o_old, o_end)
 171                      != o_old))
 172                 goto slow_path;
 173
 174         /*
 175          * Atomically update last_timestamp. This update races against concurrent
 176          * atomic updates, but the race will always cause supplementary
 177          * full timestamp record headers, never the opposite (missing a
 178          * full timestamp record header when it would be needed).
 179          */
 180         save_last_timestamp(config, ctx->priv.buf, ctx->priv.timestamp);
 181
 182         /*
 183          * Push the reader if necessary
 184          */
 185         lib_ring_buffer_reserve_push_reader(ctx->priv.buf, chan, o_end - 1);
 186
 187         /*
 188          * Clear noref flag for this subbuffer.
 189          */
 190         lib_ring_buffer_clear_noref(config, &ctx->priv.buf->backend,
 191                                 subbuf_index(o_end - 1, chan));
 192
 193         ctx->priv.pre_offset = o_begin;
 194         ctx->priv.buf_offset = o_begin + before_hdr_pad;
 195         return 0;
 196 slow_path:
 197         return lib_ring_buffer_reserve_slow(ctx, client_ctx);
 198 }
 199
 200 /**
 201  * lib_ring_buffer_switch - Perform a sub-buffer switch for a per-cpu buffer.
 202  * @config: ring buffer instance configuration.
 203  * @buf: buffer
 204  * @mode: buffer switch mode (SWITCH_ACTIVE or SWITCH_FLUSH)
 205  *
 206  * This operation is completely reentrant : can be called while tracing is
 207  * active with absolutely no lock held.
 208  *
 209  * Note, however, that as a v_cmpxchg is used for some atomic operations and
 210  * requires to be executed locally for per-CPU buffers, this function must be
 211  * called from the CPU which owns the buffer for a ACTIVE flush, with preemption
 212  * disabled, for RING_BUFFER_SYNC_PER_CPU configuration.
 213  */
 214 static inline
 215 void lib_ring_buffer_switch(const struct lttng_kernel_ring_buffer_config *config,
 216                             struct lttng_kernel_ring_buffer *buf, enum switch_mode mode)
 217 {
 218         lib_ring_buffer_switch_slow(buf, mode);
 219 }
 220
 221 /* See ring_buffer_frontend_api.h for lib_ring_buffer_reserve(). */
 222
 223 /**
 224  * lib_ring_buffer_commit - Commit an record.
 225  * @config: ring buffer instance configuration.
 226  * @ctx: ring buffer context. (input arguments only)
 227  *
 228  * Atomic unordered slot commit. Increments the commit count in the
 229  * specified sub-buffer, and delivers it if necessary.
 230  */
 231 static inline
 232 void lib_ring_buffer_commit(const struct lttng_kernel_ring_buffer_config *config,
 233                             const struct lttng_kernel_ring_buffer_ctx *ctx)
 234 {
 235         struct lttng_kernel_ring_buffer_channel *chan = ctx->priv.chan;
 236         struct lttng_kernel_ring_buffer *buf = ctx->priv.buf;
 237         unsigned long offset_end = ctx->priv.buf_offset;
 238         unsigned long endidx = subbuf_index(offset_end - 1, chan);
 239         unsigned long commit_count;
 240         struct commit_counters_hot *cc_hot = &buf->commit_hot[endidx];
 241
 242         /*
 243          * Must count record before incrementing the commit count.
 244          */
 245         subbuffer_count_record(config, &buf->backend, endidx);
 246
 247         /*
 248          * Order all writes to buffer before the commit count update that will
 249          * determine that the subbuffer is full.
 250          */
 251         if (config->ipi == RING_BUFFER_IPI_BARRIER) {
 252                 /*
 253                  * Must write slot data before incrementing commit count.  This
 254                  * compiler barrier is upgraded into a smp_mb() by the IPI sent
 255                  * by get_subbuf().
 256                  */
 257                 barrier();
 258         } else
 259                 smp_wmb();
 260
 261         v_add(config, ctx->priv.slot_size, &cc_hot->cc);
 262
 263         /*
 264          * commit count read can race with concurrent OOO commit count updates.
 265          * This is only needed for lib_ring_buffer_check_deliver (for
 266          * non-polling delivery only) and for
 267          * lib_ring_buffer_write_commit_counter.  The race can only cause the
 268          * counter to be read with the same value more than once, which could
 269          * cause :
 270          * - Multiple delivery for the same sub-buffer (which is handled
 271          *   gracefully by the reader code) if the value is for a full
 272          *   sub-buffer. It's important that we can never miss a sub-buffer
 273          *   delivery. Re-reading the value after the v_add ensures this.
 274          * - Reading a commit_count with a higher value that what was actually
 275          *   added to it for the lib_ring_buffer_write_commit_counter call
 276          *   (again caused by a concurrent committer). It does not matter,
 277          *   because this function is interested in the fact that the commit
 278          *   count reaches back the reserve offset for a specific sub-buffer,
 279          *   which is completely independent of the order.
 280          */
 281         commit_count = v_read(config, &cc_hot->cc);
 282
 283         lib_ring_buffer_check_deliver(config, buf, chan, offset_end - 1,
 284                                       commit_count, endidx, ctx);
 285         /*
 286          * Update used size at each commit. It's needed only for extracting
 287          * ring_buffer buffers from vmcore, after crash.
 288          */
 289         lib_ring_buffer_write_commit_counter(config, buf, chan,
 290                         offset_end, commit_count, cc_hot);
 291 }
 292
 293 /**
 294  * lib_ring_buffer_try_discard_reserve - Try discarding a record.
 295  * @config: ring buffer instance configuration.
 296  * @ctx: ring buffer context. (input arguments only)
 297  *
 298  * Only succeeds if no other record has been written after the record to
 299  * discard. If discard fails, the record must be committed to the buffer.
 300  *
 301  * Returns 0 upon success, -EPERM if the record cannot be discarded.
 302  */
 303 static inline
 304 int lib_ring_buffer_try_discard_reserve(const struct lttng_kernel_ring_buffer_config *config,
 305                                         const struct lttng_kernel_ring_buffer_ctx *ctx)
 306 {
 307         struct lttng_kernel_ring_buffer *buf = ctx->priv.buf;
 308         unsigned long end_offset = ctx->priv.pre_offset + ctx->priv.slot_size;
 309
 310         /*
 311          * We need to ensure that if the cmpxchg succeeds and discards the
 312          * record, the next record will record a full timestamp, because
 313          * it cannot rely on the last_timestamp associated with the
 314          * discarded record to detect overflows. The only way to ensure
 315          * this is to set the last_timestamp to 0 (assuming no 64-bit
 316          * timestamp overflow), which forces to write a 64-bit timestamp in
 317          * the next record.
 318          *
 319          * Note: if discard fails, we must leave the timestamp in the
 320          * record header. It is needed to keep track of timestamp
 321          * overflows for the following records.
 322          */
 323         save_last_timestamp(config, buf, 0ULL);
 324
 325         if (likely(v_cmpxchg(config, &buf->offset, end_offset, ctx->priv.pre_offset)
 326                    != end_offset))
 327                 return -EPERM;
 328         else
 329                 return 0;
 330 }
 331
 332 static inline
 333 void channel_record_disable(const struct lttng_kernel_ring_buffer_config *config,
 334                             struct lttng_kernel_ring_buffer_channel *chan)
 335 {
 336         atomic_inc(&chan->record_disabled);
 337 }
 338
 339 static inline
 340 void channel_record_enable(const struct lttng_kernel_ring_buffer_config *config,
 341                            struct lttng_kernel_ring_buffer_channel *chan)
 342 {
 343         atomic_dec(&chan->record_disabled);
 344 }
 345
 346 static inline
 347 void lib_ring_buffer_record_disable(const struct lttng_kernel_ring_buffer_config *config,
 348                                     struct lttng_kernel_ring_buffer *buf)
 349 {
 350         atomic_inc(&buf->record_disabled);
 351 }
 352
 353 static inline
 354 void lib_ring_buffer_record_enable(const struct lttng_kernel_ring_buffer_config *config,
 355                                    struct lttng_kernel_ring_buffer *buf)
 356 {
 357         atomic_dec(&buf->record_disabled);
 358 }
 359
 360 #endif /* _LIB_RING_BUFFER_FRONTEND_API_H */