2 * SPDX-License-Identifier: LGPL-2.1-only
4 * Copyright (C) 2005-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
6 * See ring_buffer_frontend.c for more information on wait-free
8 * See frontend.h for channel allocation and read-side API.
11 #ifndef _LTTNG_RING_BUFFER_FRONTEND_API_H
12 #define _LTTNG_RING_BUFFER_FRONTEND_API_H
16 #include <urcu/compiler.h>
21 * lib_ring_buffer_nesting_inc - Ring buffer recursive use protection.
23 * The rint buffer buffer nesting count is a safety net to ensure tracer
24 * client code will never trigger an endless recursion.
25 * Returns 0 on success, -EPERM on failure (nesting count too high).
27 * asm volatile and "memory" clobber prevent the compiler from moving
28 * instructions out of the ring buffer nesting count. This is required to ensure
29 * that probe side-effects which can cause recursion (e.g. unforeseen traps,
30 * divisions by 0, ...) are triggered within the incremented nesting count
34 int lib_ring_buffer_nesting_inc(const struct lttng_ust_lib_ring_buffer_config
*config
)
38 nesting
= ++URCU_TLS(lib_ring_buffer_nesting
);
40 if (caa_unlikely(nesting
> 4)) {
42 URCU_TLS(lib_ring_buffer_nesting
)--;
49 void lib_ring_buffer_nesting_dec(const struct lttng_ust_lib_ring_buffer_config
*config
)
52 URCU_TLS(lib_ring_buffer_nesting
)--; /* TLS */
56 * lib_ring_buffer_try_reserve is called by lib_ring_buffer_reserve(). It is not
57 * part of the API per se.
59 * returns 0 if reserve ok, or 1 if the slow path must be taken.
62 int lib_ring_buffer_try_reserve(const struct lttng_ust_lib_ring_buffer_config
*config
,
63 struct lttng_ust_lib_ring_buffer_ctx
*ctx
,
65 unsigned long *o_begin
, unsigned long *o_end
,
66 unsigned long *o_old
, size_t *before_hdr_pad
)
68 struct lttng_ust_lib_ring_buffer_channel
*chan
= ctx
->chan
;
69 struct lttng_ust_lib_ring_buffer
*buf
= ctx
->buf
;
70 *o_begin
= v_read(config
, &buf
->offset
);
73 ctx
->tsc
= lib_ring_buffer_clock_read(chan
);
74 if ((int64_t) ctx
->tsc
== -EIO
)
78 * Prefetch cacheline for read because we have to read the previous
79 * commit counter to increment it and commit seq value to compare it to
82 //prefetch(&buf->commit_hot[subbuf_index(*o_begin, chan)]);
84 if (last_tsc_overflow(config
, buf
, ctx
->tsc
))
85 ctx
->rflags
|= RING_BUFFER_RFLAG_FULL_TSC
;
87 if (caa_unlikely(subbuf_offset(*o_begin
, chan
) == 0))
90 ctx
->slot_size
= record_header_size(config
, chan
, *o_begin
,
91 before_hdr_pad
, ctx
, client_ctx
);
93 lttng_ust_lib_ring_buffer_align(*o_begin
+ ctx
->slot_size
,
94 ctx
->largest_align
) + ctx
->data_size
;
95 if (caa_unlikely((subbuf_offset(*o_begin
, chan
) + ctx
->slot_size
)
96 > chan
->backend
.subbuf_size
))
100 * Record fits in the current buffer and we are not on a switch
101 * boundary. It's safe to write.
103 *o_end
= *o_begin
+ ctx
->slot_size
;
105 if (caa_unlikely((subbuf_offset(*o_end
, chan
)) == 0))
107 * The offset_end will fall at the very beginning of the next
116 * lib_ring_buffer_reserve - Reserve space in a ring buffer.
117 * @config: ring buffer instance configuration.
118 * @ctx: ring buffer context. (input and output) Must be already initialized.
120 * Atomic wait-free slot reservation. The reserved space starts at the context
121 * "pre_offset". Its length is "slot_size". The associated time-stamp is "tsc".
125 * -EAGAIN if channel is disabled.
126 * -ENOSPC if event size is too large for packet.
127 * -ENOBUFS if there is currently not enough space in buffer for the event.
128 * -EIO if data cannot be written into the buffer for any other reason.
132 int lib_ring_buffer_reserve(const struct lttng_ust_lib_ring_buffer_config
*config
,
133 struct lttng_ust_lib_ring_buffer_ctx
*ctx
,
136 struct lttng_ust_lib_ring_buffer_channel
*chan
= ctx
->chan
;
137 struct lttng_ust_shm_handle
*handle
= ctx
->chan
->handle
;
138 struct lttng_ust_lib_ring_buffer
*buf
;
139 unsigned long o_begin
, o_end
, o_old
;
140 size_t before_hdr_pad
= 0;
142 if (caa_unlikely(uatomic_read(&chan
->record_disabled
)))
145 if (config
->alloc
== RING_BUFFER_ALLOC_PER_CPU
) {
146 ctx
->reserve_cpu
= lttng_ust_get_cpu();
147 buf
= shmp(handle
, chan
->backend
.buf
[ctx
->reserve_cpu
].shmp
);
149 buf
= shmp(handle
, chan
->backend
.buf
[0].shmp
);
151 if (caa_unlikely(!buf
))
153 if (caa_unlikely(uatomic_read(&buf
->record_disabled
)))
158 * Perform retryable operations.
160 if (caa_unlikely(lib_ring_buffer_try_reserve(config
, ctx
, client_ctx
, &o_begin
,
161 &o_end
, &o_old
, &before_hdr_pad
)))
164 if (caa_unlikely(v_cmpxchg(config
, &ctx
->buf
->offset
, o_old
, o_end
)
169 * Atomically update last_tsc. This update races against concurrent
170 * atomic updates, but the race will always cause supplementary full TSC
171 * record headers, never the opposite (missing a full TSC record header
172 * when it would be needed).
174 save_last_tsc(config
, ctx
->buf
, ctx
->tsc
);
177 * Push the reader if necessary
179 lib_ring_buffer_reserve_push_reader(ctx
->buf
, chan
, o_end
- 1);
182 * Clear noref flag for this subbuffer.
184 lib_ring_buffer_clear_noref(config
, &ctx
->buf
->backend
,
185 subbuf_index(o_end
- 1, chan
), handle
);
187 ctx
->pre_offset
= o_begin
;
188 ctx
->buf_offset
= o_begin
+ before_hdr_pad
;
191 return lib_ring_buffer_reserve_slow(ctx
, client_ctx
);
195 * lib_ring_buffer_switch - Perform a sub-buffer switch for a per-cpu buffer.
196 * @config: ring buffer instance configuration.
198 * @mode: buffer switch mode (SWITCH_ACTIVE or SWITCH_FLUSH)
200 * This operation is completely reentrant : can be called while tracing is
201 * active with absolutely no lock held.
203 * Note, however, that as a v_cmpxchg is used for some atomic operations and
204 * requires to be executed locally for per-CPU buffers, this function must be
205 * called from the CPU which owns the buffer for a ACTIVE flush, with preemption
206 * disabled, for RING_BUFFER_SYNC_PER_CPU configuration.
209 void lib_ring_buffer_switch(const struct lttng_ust_lib_ring_buffer_config
*config
,
210 struct lttng_ust_lib_ring_buffer
*buf
, enum switch_mode mode
,
211 struct lttng_ust_shm_handle
*handle
)
213 lib_ring_buffer_switch_slow(buf
, mode
, handle
);
216 /* See ring_buffer_frontend_api.h for lib_ring_buffer_reserve(). */
219 * lib_ring_buffer_commit - Commit an record.
220 * @config: ring buffer instance configuration.
221 * @ctx: ring buffer context. (input arguments only)
223 * Atomic unordered slot commit. Increments the commit count in the
224 * specified sub-buffer, and delivers it if necessary.
227 void lib_ring_buffer_commit(const struct lttng_ust_lib_ring_buffer_config
*config
,
228 const struct lttng_ust_lib_ring_buffer_ctx
*ctx
)
230 struct lttng_ust_lib_ring_buffer_channel
*chan
= ctx
->chan
;
231 struct lttng_ust_shm_handle
*handle
= ctx
->chan
->handle
;
232 struct lttng_ust_lib_ring_buffer
*buf
= ctx
->buf
;
233 unsigned long offset_end
= ctx
->buf_offset
;
234 unsigned long endidx
= subbuf_index(offset_end
- 1, chan
);
235 unsigned long commit_count
;
236 struct commit_counters_hot
*cc_hot
= shmp_index(handle
,
237 buf
->commit_hot
, endidx
);
239 if (caa_unlikely(!cc_hot
))
243 * Must count record before incrementing the commit count.
245 subbuffer_count_record(config
, ctx
, &buf
->backend
, endidx
, handle
);
248 * Order all writes to buffer before the commit count update that will
249 * determine that the subbuffer is full.
253 v_add(config
, ctx
->slot_size
, &cc_hot
->cc
);
256 * commit count read can race with concurrent OOO commit count updates.
257 * This is only needed for lib_ring_buffer_check_deliver (for
258 * non-polling delivery only) and for
259 * lib_ring_buffer_write_commit_counter. The race can only cause the
260 * counter to be read with the same value more than once, which could
262 * - Multiple delivery for the same sub-buffer (which is handled
263 * gracefully by the reader code) if the value is for a full
264 * sub-buffer. It's important that we can never miss a sub-buffer
265 * delivery. Re-reading the value after the v_add ensures this.
266 * - Reading a commit_count with a higher value that what was actually
267 * added to it for the lib_ring_buffer_write_commit_counter call
268 * (again caused by a concurrent committer). It does not matter,
269 * because this function is interested in the fact that the commit
270 * count reaches back the reserve offset for a specific sub-buffer,
271 * which is completely independent of the order.
273 commit_count
= v_read(config
, &cc_hot
->cc
);
275 lib_ring_buffer_check_deliver(config
, buf
, chan
, offset_end
- 1,
276 commit_count
, endidx
, handle
, ctx
->tsc
);
278 * Update used size at each commit. It's needed only for extracting
279 * ring_buffer buffers from vmcore, after crash.
281 lib_ring_buffer_write_commit_counter(config
, buf
, chan
,
282 offset_end
, commit_count
, handle
, cc_hot
);
286 * lib_ring_buffer_try_discard_reserve - Try discarding a record.
287 * @config: ring buffer instance configuration.
288 * @ctx: ring buffer context. (input arguments only)
290 * Only succeeds if no other record has been written after the record to
291 * discard. If discard fails, the record must be committed to the buffer.
293 * Returns 0 upon success, -EPERM if the record cannot be discarded.
296 int lib_ring_buffer_try_discard_reserve(const struct lttng_ust_lib_ring_buffer_config
*config
,
297 const struct lttng_ust_lib_ring_buffer_ctx
*ctx
)
299 struct lttng_ust_lib_ring_buffer
*buf
= ctx
->buf
;
300 unsigned long end_offset
= ctx
->pre_offset
+ ctx
->slot_size
;
303 * We need to ensure that if the cmpxchg succeeds and discards the
304 * record, the next record will record a full TSC, because it cannot
305 * rely on the last_tsc associated with the discarded record to detect
306 * overflows. The only way to ensure this is to set the last_tsc to 0
307 * (assuming no 64-bit TSC overflow), which forces to write a 64-bit
308 * timestamp in the next record.
310 * Note: if discard fails, we must leave the TSC in the record header.
311 * It is needed to keep track of TSC overflows for the following
314 save_last_tsc(config
, buf
, 0ULL);
316 if (caa_likely(v_cmpxchg(config
, &buf
->offset
, end_offset
, ctx
->pre_offset
)
324 void channel_record_disable(const struct lttng_ust_lib_ring_buffer_config
*config
,
325 struct lttng_ust_lib_ring_buffer_channel
*chan
)
327 uatomic_inc(&chan
->record_disabled
);
331 void channel_record_enable(const struct lttng_ust_lib_ring_buffer_config
*config
,
332 struct lttng_ust_lib_ring_buffer_channel
*chan
)
334 uatomic_dec(&chan
->record_disabled
);
338 void lib_ring_buffer_record_disable(const struct lttng_ust_lib_ring_buffer_config
*config
,
339 struct lttng_ust_lib_ring_buffer
*buf
)
341 uatomic_inc(&buf
->record_disabled
);
345 void lib_ring_buffer_record_enable(const struct lttng_ust_lib_ring_buffer_config
*config
,
346 struct lttng_ust_lib_ring_buffer
*buf
)
348 uatomic_dec(&buf
->record_disabled
);
351 #endif /* _LTTNG_RING_BUFFER_FRONTEND_API_H */