1 #ifndef _LTTNG_RING_BUFFER_FRONTEND_API_H
2 #define _LTTNG_RING_BUFFER_FRONTEND_API_H
5 * libringbuffer/frontend_api.h
7 * Copyright (C) 2005-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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.
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.
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
23 * Ring Buffer Library Synchronization Header (buffer write API).
26 * Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
28 * See ring_buffer_frontend.c for more information on wait-free
30 * See frontend.h for channel allocation and read-side API.
35 #include <urcu/compiler.h>
38 * lib_ring_buffer_get_cpu - Precedes ring buffer reserve/commit.
40 * Grabs RCU read-side lock and keeps a ring buffer nesting count as
41 * supplementary safety net to ensure tracer client code will never
42 * trigger an endless recursion. Returns the processor ID on success,
43 * -EPERM on failure (nesting count too high).
45 * asm volatile and "memory" clobber prevent the compiler from moving
46 * instructions out of the ring buffer nesting count. This is required to ensure
47 * that probe side-effects which can cause recursion (e.g. unforeseen traps,
48 * divisions by 0, ...) are triggered within the incremented nesting count
52 int lib_ring_buffer_get_cpu(const struct lttng_ust_lib_ring_buffer_config
*config
)
57 cpu
= lttng_ust_get_cpu();
58 nesting
= ++lib_ring_buffer_nesting
; /* TLS */
61 if (caa_unlikely(nesting
> 4)) {
63 lib_ring_buffer_nesting
--; /* TLS */
71 * lib_ring_buffer_put_cpu - Follows ring buffer reserve/commit.
74 void lib_ring_buffer_put_cpu(const struct lttng_ust_lib_ring_buffer_config
*config
)
77 lib_ring_buffer_nesting
--; /* TLS */
82 * lib_ring_buffer_try_reserve is called by lib_ring_buffer_reserve(). It is not
83 * part of the API per se.
85 * returns 0 if reserve ok, or 1 if the slow path must be taken.
88 int lib_ring_buffer_try_reserve(const struct lttng_ust_lib_ring_buffer_config
*config
,
89 struct lttng_ust_lib_ring_buffer_ctx
*ctx
,
90 unsigned long *o_begin
, unsigned long *o_end
,
91 unsigned long *o_old
, size_t *before_hdr_pad
)
93 struct channel
*chan
= ctx
->chan
;
94 struct lttng_ust_lib_ring_buffer
*buf
= ctx
->buf
;
95 *o_begin
= v_read(config
, &buf
->offset
);
98 ctx
->tsc
= lib_ring_buffer_clock_read(chan
);
99 if ((int64_t) ctx
->tsc
== -EIO
)
103 * Prefetch cacheline for read because we have to read the previous
104 * commit counter to increment it and commit seq value to compare it to
105 * the commit counter.
107 //prefetch(&buf->commit_hot[subbuf_index(*o_begin, chan)]);
109 if (last_tsc_overflow(config
, buf
, ctx
->tsc
))
110 ctx
->rflags
|= RING_BUFFER_RFLAG_FULL_TSC
;
112 if (caa_unlikely(subbuf_offset(*o_begin
, chan
) == 0))
115 ctx
->slot_size
= record_header_size(config
, chan
, *o_begin
,
116 before_hdr_pad
, ctx
);
118 lib_ring_buffer_align(*o_begin
+ ctx
->slot_size
,
119 ctx
->largest_align
) + ctx
->data_size
;
120 if (caa_unlikely((subbuf_offset(*o_begin
, chan
) + ctx
->slot_size
)
121 > chan
->backend
.subbuf_size
))
125 * Record fits in the current buffer and we are not on a switch
126 * boundary. It's safe to write.
128 *o_end
= *o_begin
+ ctx
->slot_size
;
130 if (caa_unlikely((subbuf_offset(*o_end
, chan
)) == 0))
132 * The offset_end will fall at the very beginning of the next
141 * lib_ring_buffer_reserve - Reserve space in a ring buffer.
142 * @config: ring buffer instance configuration.
143 * @ctx: ring buffer context. (input and output) Must be already initialized.
145 * Atomic wait-free slot reservation. The reserved space starts at the context
146 * "pre_offset". Its length is "slot_size". The associated time-stamp is "tsc".
150 * -EAGAIN if channel is disabled.
151 * -ENOSPC if event size is too large for packet.
152 * -ENOBUFS if there is currently not enough space in buffer for the event.
153 * -EIO if data cannot be written into the buffer for any other reason.
157 int lib_ring_buffer_reserve(const struct lttng_ust_lib_ring_buffer_config
*config
,
158 struct lttng_ust_lib_ring_buffer_ctx
*ctx
)
160 struct channel
*chan
= ctx
->chan
;
161 struct lttng_ust_shm_handle
*handle
= ctx
->handle
;
162 struct lttng_ust_lib_ring_buffer
*buf
;
163 unsigned long o_begin
, o_end
, o_old
;
164 size_t before_hdr_pad
= 0;
166 if (uatomic_read(&chan
->record_disabled
))
169 if (config
->alloc
== RING_BUFFER_ALLOC_PER_CPU
)
170 buf
= shmp(handle
, chan
->backend
.buf
[ctx
->cpu
].shmp
);
172 buf
= shmp(handle
, chan
->backend
.buf
[0].shmp
);
173 if (uatomic_read(&buf
->record_disabled
))
178 * Perform retryable operations.
180 if (caa_unlikely(lib_ring_buffer_try_reserve(config
, ctx
, &o_begin
,
181 &o_end
, &o_old
, &before_hdr_pad
)))
184 if (caa_unlikely(v_cmpxchg(config
, &ctx
->buf
->offset
, o_old
, o_end
)
189 * Atomically update last_tsc. This update races against concurrent
190 * atomic updates, but the race will always cause supplementary full TSC
191 * record headers, never the opposite (missing a full TSC record header
192 * when it would be needed).
194 save_last_tsc(config
, ctx
->buf
, ctx
->tsc
);
197 * Push the reader if necessary
199 lib_ring_buffer_reserve_push_reader(ctx
->buf
, chan
, o_end
- 1);
202 * Clear noref flag for this subbuffer.
204 lib_ring_buffer_clear_noref(config
, &ctx
->buf
->backend
,
205 subbuf_index(o_end
- 1, chan
), handle
);
207 ctx
->pre_offset
= o_begin
;
208 ctx
->buf_offset
= o_begin
+ before_hdr_pad
;
211 return lib_ring_buffer_reserve_slow(ctx
);
215 * lib_ring_buffer_switch - Perform a sub-buffer switch for a per-cpu buffer.
216 * @config: ring buffer instance configuration.
218 * @mode: buffer switch mode (SWITCH_ACTIVE or SWITCH_FLUSH)
220 * This operation is completely reentrant : can be called while tracing is
221 * active with absolutely no lock held.
223 * Note, however, that as a v_cmpxchg is used for some atomic operations and
224 * requires to be executed locally for per-CPU buffers, this function must be
225 * called from the CPU which owns the buffer for a ACTIVE flush, with preemption
226 * disabled, for RING_BUFFER_SYNC_PER_CPU configuration.
229 void lib_ring_buffer_switch(const struct lttng_ust_lib_ring_buffer_config
*config
,
230 struct lttng_ust_lib_ring_buffer
*buf
, enum switch_mode mode
,
231 struct lttng_ust_shm_handle
*handle
)
233 lib_ring_buffer_switch_slow(buf
, mode
, handle
);
236 /* See ring_buffer_frontend_api.h for lib_ring_buffer_reserve(). */
239 * lib_ring_buffer_commit - Commit an record.
240 * @config: ring buffer instance configuration.
241 * @ctx: ring buffer context. (input arguments only)
243 * Atomic unordered slot commit. Increments the commit count in the
244 * specified sub-buffer, and delivers it if necessary.
247 void lib_ring_buffer_commit(const struct lttng_ust_lib_ring_buffer_config
*config
,
248 const struct lttng_ust_lib_ring_buffer_ctx
*ctx
)
250 struct channel
*chan
= ctx
->chan
;
251 struct lttng_ust_shm_handle
*handle
= ctx
->handle
;
252 struct lttng_ust_lib_ring_buffer
*buf
= ctx
->buf
;
253 unsigned long offset_end
= ctx
->buf_offset
;
254 unsigned long endidx
= subbuf_index(offset_end
- 1, chan
);
255 unsigned long commit_count
;
258 * Must count record before incrementing the commit count.
260 subbuffer_count_record(config
, &buf
->backend
, endidx
, handle
);
263 * Order all writes to buffer before the commit count update that will
264 * determine that the subbuffer is full.
268 v_add(config
, ctx
->slot_size
, &shmp_index(handle
, buf
->commit_hot
, endidx
)->cc
);
271 * commit count read can race with concurrent OOO commit count updates.
272 * This is only needed for lib_ring_buffer_check_deliver (for
273 * non-polling delivery only) and for
274 * lib_ring_buffer_write_commit_counter. The race can only cause the
275 * counter to be read with the same value more than once, which could
277 * - Multiple delivery for the same sub-buffer (which is handled
278 * gracefully by the reader code) if the value is for a full
279 * sub-buffer. It's important that we can never miss a sub-buffer
280 * delivery. Re-reading the value after the v_add ensures this.
281 * - Reading a commit_count with a higher value that what was actually
282 * added to it for the lib_ring_buffer_write_commit_counter call
283 * (again caused by a concurrent committer). It does not matter,
284 * because this function is interested in the fact that the commit
285 * count reaches back the reserve offset for a specific sub-buffer,
286 * which is completely independent of the order.
288 commit_count
= v_read(config
, &shmp_index(handle
, buf
->commit_hot
, endidx
)->cc
);
290 lib_ring_buffer_check_deliver(config
, buf
, chan
, offset_end
- 1,
291 commit_count
, endidx
, handle
);
293 * Update used size at each commit. It's needed only for extracting
294 * ring_buffer buffers from vmcore, after crash.
296 lib_ring_buffer_write_commit_counter(config
, buf
, chan
, endidx
,
297 ctx
->buf_offset
, commit_count
,
298 ctx
->slot_size
, handle
);
302 * lib_ring_buffer_try_discard_reserve - Try discarding a record.
303 * @config: ring buffer instance configuration.
304 * @ctx: ring buffer context. (input arguments only)
306 * Only succeeds if no other record has been written after the record to
307 * discard. If discard fails, the record must be committed to the buffer.
309 * Returns 0 upon success, -EPERM if the record cannot be discarded.
312 int lib_ring_buffer_try_discard_reserve(const struct lttng_ust_lib_ring_buffer_config
*config
,
313 const struct lttng_ust_lib_ring_buffer_ctx
*ctx
)
315 struct lttng_ust_lib_ring_buffer
*buf
= ctx
->buf
;
316 unsigned long end_offset
= ctx
->pre_offset
+ ctx
->slot_size
;
319 * We need to ensure that if the cmpxchg succeeds and discards the
320 * record, the next record will record a full TSC, because it cannot
321 * rely on the last_tsc associated with the discarded record to detect
322 * overflows. The only way to ensure this is to set the last_tsc to 0
323 * (assuming no 64-bit TSC overflow), which forces to write a 64-bit
324 * timestamp in the next record.
326 * Note: if discard fails, we must leave the TSC in the record header.
327 * It is needed to keep track of TSC overflows for the following
330 save_last_tsc(config
, buf
, 0ULL);
332 if (caa_likely(v_cmpxchg(config
, &buf
->offset
, end_offset
, ctx
->pre_offset
)
340 void channel_record_disable(const struct lttng_ust_lib_ring_buffer_config
*config
,
341 struct channel
*chan
)
343 uatomic_inc(&chan
->record_disabled
);
347 void channel_record_enable(const struct lttng_ust_lib_ring_buffer_config
*config
,
348 struct channel
*chan
)
350 uatomic_dec(&chan
->record_disabled
);
354 void lib_ring_buffer_record_disable(const struct lttng_ust_lib_ring_buffer_config
*config
,
355 struct lttng_ust_lib_ring_buffer
*buf
)
357 uatomic_inc(&buf
->record_disabled
);
361 void lib_ring_buffer_record_enable(const struct lttng_ust_lib_ring_buffer_config
*config
,
362 struct lttng_ust_lib_ring_buffer
*buf
)
364 uatomic_dec(&buf
->record_disabled
);
367 #endif /* _LTTNG_RING_BUFFER_FRONTEND_API_H */