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
= ++URCU_TLS(lib_ring_buffer_nesting
);
61 if (caa_unlikely(nesting
> 4)) {
63 URCU_TLS(lib_ring_buffer_nesting
)--;
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 URCU_TLS(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
;
133 * lib_ring_buffer_reserve - Reserve space in a ring buffer.
134 * @config: ring buffer instance configuration.
135 * @ctx: ring buffer context. (input and output) Must be already initialized.
137 * Atomic wait-free slot reservation. The reserved space starts at the context
138 * "pre_offset". Its length is "slot_size". The associated time-stamp is "tsc".
142 * -EAGAIN if channel is disabled.
143 * -ENOSPC if event size is too large for packet.
144 * -ENOBUFS if there is currently not enough space in buffer for the event.
145 * -EIO if data cannot be written into the buffer for any other reason.
149 int lib_ring_buffer_reserve(const struct lttng_ust_lib_ring_buffer_config
*config
,
150 struct lttng_ust_lib_ring_buffer_ctx
*ctx
)
152 struct channel
*chan
= ctx
->chan
;
153 struct lttng_ust_shm_handle
*handle
= ctx
->handle
;
154 struct lttng_ust_lib_ring_buffer
*buf
;
155 unsigned long o_begin
, o_end
, o_old
;
156 size_t before_hdr_pad
= 0;
158 if (uatomic_read(&chan
->record_disabled
))
161 if (config
->alloc
== RING_BUFFER_ALLOC_PER_CPU
)
162 buf
= shmp(handle
, chan
->backend
.buf
[ctx
->cpu
].shmp
);
164 buf
= shmp(handle
, chan
->backend
.buf
[0].shmp
);
165 if (uatomic_read(&buf
->record_disabled
))
170 * Perform retryable operations.
172 if (caa_unlikely(lib_ring_buffer_try_reserve(config
, ctx
, &o_begin
,
173 &o_end
, &o_old
, &before_hdr_pad
)))
176 if (caa_unlikely(v_cmpxchg(config
, &ctx
->buf
->offset
, o_old
, o_end
)
181 * Atomically update last_tsc. This update races against concurrent
182 * atomic updates, but the race will always cause supplementary full TSC
183 * record headers, never the opposite (missing a full TSC record header
184 * when it would be needed).
186 save_last_tsc(config
, ctx
->buf
, ctx
->tsc
);
189 * Push the reader if necessary
191 lib_ring_buffer_reserve_push_reader(ctx
->buf
, chan
, o_end
- 1);
194 * Clear noref flag for this subbuffer.
196 lib_ring_buffer_clear_noref(config
, &ctx
->buf
->backend
,
197 subbuf_index(o_end
- 1, chan
), handle
);
199 ctx
->pre_offset
= o_begin
;
200 ctx
->buf_offset
= o_begin
+ before_hdr_pad
;
203 return lib_ring_buffer_reserve_slow(ctx
);
207 * lib_ring_buffer_switch - Perform a sub-buffer switch for a per-cpu buffer.
208 * @config: ring buffer instance configuration.
210 * @mode: buffer switch mode (SWITCH_ACTIVE or SWITCH_FLUSH)
212 * This operation is completely reentrant : can be called while tracing is
213 * active with absolutely no lock held.
215 * Note, however, that as a v_cmpxchg is used for some atomic operations and
216 * requires to be executed locally for per-CPU buffers, this function must be
217 * called from the CPU which owns the buffer for a ACTIVE flush, with preemption
218 * disabled, for RING_BUFFER_SYNC_PER_CPU configuration.
221 void lib_ring_buffer_switch(const struct lttng_ust_lib_ring_buffer_config
*config
,
222 struct lttng_ust_lib_ring_buffer
*buf
, enum switch_mode mode
,
223 struct lttng_ust_shm_handle
*handle
)
225 lib_ring_buffer_switch_slow(buf
, mode
, handle
);
228 /* See ring_buffer_frontend_api.h for lib_ring_buffer_reserve(). */
231 * lib_ring_buffer_commit - Commit an record.
232 * @config: ring buffer instance configuration.
233 * @ctx: ring buffer context. (input arguments only)
235 * Atomic unordered slot commit. Increments the commit count in the
236 * specified sub-buffer, and delivers it if necessary.
239 void lib_ring_buffer_commit(const struct lttng_ust_lib_ring_buffer_config
*config
,
240 const struct lttng_ust_lib_ring_buffer_ctx
*ctx
)
242 struct channel
*chan
= ctx
->chan
;
243 struct lttng_ust_shm_handle
*handle
= ctx
->handle
;
244 struct lttng_ust_lib_ring_buffer
*buf
= ctx
->buf
;
245 unsigned long offset_end
= ctx
->buf_offset
;
246 unsigned long endidx
= subbuf_index(offset_end
- 1, chan
);
247 unsigned long commit_count
;
250 * Must count record before incrementing the commit count.
252 subbuffer_count_record(config
, &buf
->backend
, endidx
, handle
);
255 * Order all writes to buffer before the commit count update that will
256 * determine that the subbuffer is full.
260 v_add(config
, ctx
->slot_size
, &shmp_index(handle
, buf
->commit_hot
, endidx
)->cc
);
263 * commit count read can race with concurrent OOO commit count updates.
264 * This is only needed for lib_ring_buffer_check_deliver (for
265 * non-polling delivery only) and for
266 * lib_ring_buffer_write_commit_counter. The race can only cause the
267 * counter to be read with the same value more than once, which could
269 * - Multiple delivery for the same sub-buffer (which is handled
270 * gracefully by the reader code) if the value is for a full
271 * sub-buffer. It's important that we can never miss a sub-buffer
272 * delivery. Re-reading the value after the v_add ensures this.
273 * - Reading a commit_count with a higher value that what was actually
274 * added to it for the lib_ring_buffer_write_commit_counter call
275 * (again caused by a concurrent committer). It does not matter,
276 * because this function is interested in the fact that the commit
277 * count reaches back the reserve offset for a specific sub-buffer,
278 * which is completely independent of the order.
280 commit_count
= v_read(config
, &shmp_index(handle
, buf
->commit_hot
, endidx
)->cc
);
282 lib_ring_buffer_check_deliver(config
, buf
, chan
, offset_end
- 1,
283 commit_count
, endidx
, handle
);
285 * Update used size at each commit. It's needed only for extracting
286 * ring_buffer buffers from vmcore, after crash.
288 lib_ring_buffer_write_commit_counter(config
, buf
, chan
, endidx
,
289 ctx
->buf_offset
, commit_count
,
290 ctx
->slot_size
, handle
);
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)
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.
301 * Returns 0 upon success, -EPERM if the record cannot be discarded.
304 int lib_ring_buffer_try_discard_reserve(const struct lttng_ust_lib_ring_buffer_config
*config
,
305 const struct lttng_ust_lib_ring_buffer_ctx
*ctx
)
307 struct lttng_ust_lib_ring_buffer
*buf
= ctx
->buf
;
308 unsigned long end_offset
= ctx
->pre_offset
+ ctx
->slot_size
;
311 * We need to ensure that if the cmpxchg succeeds and discards the
312 * record, the next record will record a full TSC, because it cannot
313 * rely on the last_tsc associated with the discarded record to detect
314 * overflows. The only way to ensure this is to set the last_tsc to 0
315 * (assuming no 64-bit TSC overflow), which forces to write a 64-bit
316 * timestamp in the next record.
318 * Note: if discard fails, we must leave the TSC in the record header.
319 * It is needed to keep track of TSC overflows for the following
322 save_last_tsc(config
, buf
, 0ULL);
324 if (caa_likely(v_cmpxchg(config
, &buf
->offset
, end_offset
, ctx
->pre_offset
)
332 void channel_record_disable(const struct lttng_ust_lib_ring_buffer_config
*config
,
333 struct channel
*chan
)
335 uatomic_inc(&chan
->record_disabled
);
339 void channel_record_enable(const struct lttng_ust_lib_ring_buffer_config
*config
,
340 struct channel
*chan
)
342 uatomic_dec(&chan
->record_disabled
);
346 void lib_ring_buffer_record_disable(const struct lttng_ust_lib_ring_buffer_config
*config
,
347 struct lttng_ust_lib_ring_buffer
*buf
)
349 uatomic_inc(&buf
->record_disabled
);
353 void lib_ring_buffer_record_enable(const struct lttng_ust_lib_ring_buffer_config
*config
,
354 struct lttng_ust_lib_ring_buffer
*buf
)
356 uatomic_dec(&buf
->record_disabled
);
359 #endif /* _LTTNG_RING_BUFFER_FRONTEND_API_H */