2 * ring_buffer_frontend.c
4 * (C) Copyright 2005-2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
6 * Ring buffer wait-free buffer synchronization. Producer-consumer and flight
7 * recorder (overwrite) modes. See thesis:
9 * Desnoyers, Mathieu (2009), "Low-Impact Operating System Tracing", Ph.D.
10 * dissertation, Ecole Polytechnique de Montreal.
11 * http://www.lttng.org/pub/thesis/desnoyers-dissertation-2009-12.pdf
13 * - Algorithm presentation in Chapter 5:
14 * "Lockless Multi-Core High-Throughput Buffering".
15 * - Algorithm formal verification in Section 8.6:
16 * "Formal verification of LTTng"
19 * Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
21 * Inspired from LTT and RelayFS:
22 * Karim Yaghmour <karim@opersys.com>
23 * Tom Zanussi <zanussi@us.ibm.com>
24 * Bob Wisniewski <bob@watson.ibm.com>
26 * Bob Wisniewski <bob@watson.ibm.com>
28 * Buffer reader semantic :
31 * while buffer is not finalized and empty
33 * - if return value != 0, continue
34 * - splice one subbuffer worth of data to a pipe
35 * - splice the data from pipe to disk/network
38 * Dual LGPL v2.1/GPL v2 license.
41 #include <sys/types.h>
45 #include <urcu/compiler.h>
49 #include <ust/ringbuffer-config.h>
55 #define max(a, b) ((a) > (b) ? (a) : (b))
59 * Use POSIX SHM: shm_open(3) and shm_unlink(3).
60 * close(2) to close the fd returned by shm_open.
61 * shm_unlink releases the shared memory object name.
62 * ftruncate(2) sets the size of the memory object.
63 * mmap/munmap maps the shared memory obj to a virtual address in the
64 * calling proceess (should be done both in libust and consumer).
65 * See shm_overview(7) for details.
66 * Pass file descriptor returned by shm_open(3) to ltt-sessiond through
69 * Since we don't need to access the object using its name, we can
70 * immediately shm_unlink(3) it, and only keep the handle with its file
75 * Internal structure representing offsets to use at a sub-buffer switch.
77 struct switch_offsets
{
78 unsigned long begin
, end
, old
;
79 size_t pre_header_padding
, size
;
80 unsigned int switch_new_start
:1, switch_new_end
:1, switch_old_start
:1,
84 __thread
unsigned int lib_ring_buffer_nesting
;
87 void lib_ring_buffer_print_errors(struct channel
*chan
,
88 struct lib_ring_buffer
*buf
, int cpu
,
89 struct shm_handle
*handle
);
92 * Must be called under cpu hotplug protection.
94 void lib_ring_buffer_free(struct lib_ring_buffer
*buf
,
95 struct shm_handle
*handle
)
97 struct channel
*chan
= shmp(handle
, buf
->backend
.chan
);
99 lib_ring_buffer_print_errors(chan
, buf
, buf
->backend
.cpu
, handle
);
100 /* buf->commit_hot will be freed by shm teardown */
101 /* buf->commit_cold will be freed by shm teardown */
103 lib_ring_buffer_backend_free(&buf
->backend
);
107 * lib_ring_buffer_reset - Reset ring buffer to initial values.
110 * Effectively empty the ring buffer. Should be called when the buffer is not
111 * used for writing. The ring buffer can be opened for reading, but the reader
112 * should not be using the iterator concurrently with reset. The previous
113 * current iterator record is reset.
115 void lib_ring_buffer_reset(struct lib_ring_buffer
*buf
,
116 struct shm_handle
*handle
)
118 struct channel
*chan
= shmp(handle
, buf
->backend
.chan
);
119 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
123 * Reset iterator first. It will put the subbuffer if it currently holds
126 v_set(config
, &buf
->offset
, 0);
127 for (i
= 0; i
< chan
->backend
.num_subbuf
; i
++) {
128 v_set(config
, &shmp_index(handle
, buf
->commit_hot
, i
)->cc
, 0);
129 v_set(config
, &shmp_index(handle
, buf
->commit_hot
, i
)->seq
, 0);
130 v_set(config
, &shmp_index(handle
, buf
->commit_cold
, i
)->cc_sb
, 0);
132 uatomic_set(&buf
->consumed
, 0);
133 uatomic_set(&buf
->record_disabled
, 0);
134 v_set(config
, &buf
->last_tsc
, 0);
135 lib_ring_buffer_backend_reset(&buf
->backend
, handle
);
136 /* Don't reset number of active readers */
137 v_set(config
, &buf
->records_lost_full
, 0);
138 v_set(config
, &buf
->records_lost_wrap
, 0);
139 v_set(config
, &buf
->records_lost_big
, 0);
140 v_set(config
, &buf
->records_count
, 0);
141 v_set(config
, &buf
->records_overrun
, 0);
146 * channel_reset - Reset channel to initial values.
149 * Effectively empty the channel. Should be called when the channel is not used
150 * for writing. The channel can be opened for reading, but the reader should not
151 * be using the iterator concurrently with reset. The previous current iterator
154 void channel_reset(struct channel
*chan
)
157 * Reset iterators first. Will put the subbuffer if held for reading.
159 uatomic_set(&chan
->record_disabled
, 0);
160 /* Don't reset commit_count_mask, still valid */
161 channel_backend_reset(&chan
->backend
);
162 /* Don't reset switch/read timer interval */
163 /* Don't reset notifiers and notifier enable bits */
164 /* Don't reset reader reference count */
168 * Must be called under cpu hotplug protection.
170 int lib_ring_buffer_create(struct lib_ring_buffer
*buf
,
171 struct channel_backend
*chanb
, int cpu
,
172 struct shm_handle
*handle
,
173 struct shm_object
*shmobj
)
175 const struct lib_ring_buffer_config
*config
= &chanb
->config
;
176 struct channel
*chan
= caa_container_of(chanb
, struct channel
, backend
);
177 void *priv
= chanb
->priv
;
178 unsigned int num_subbuf
;
179 size_t subbuf_header_size
;
183 /* Test for cpu hotplug */
184 if (buf
->backend
.allocated
)
187 ret
= lib_ring_buffer_backend_create(&buf
->backend
, &chan
->backend
,
188 cpu
, handle
, shmobj
);
192 align_shm(shmobj
, __alignof__(struct commit_counters_hot
));
193 set_shmp(buf
->commit_hot
,
195 sizeof(struct commit_counters_hot
) * chan
->backend
.num_subbuf
));
196 if (!shmp(handle
, buf
->commit_hot
)) {
201 align_shm(shmobj
, __alignof__(struct commit_counters_cold
));
202 set_shmp(buf
->commit_cold
,
204 sizeof(struct commit_counters_cold
) * chan
->backend
.num_subbuf
));
205 if (!shmp(handle
, buf
->commit_cold
)) {
210 num_subbuf
= chan
->backend
.num_subbuf
;
211 //init_waitqueue_head(&buf->read_wait);
214 * Write the subbuffer header for first subbuffer so we know the total
215 * duration of data gathering.
217 subbuf_header_size
= config
->cb
.subbuffer_header_size();
218 v_set(config
, &buf
->offset
, subbuf_header_size
);
219 subbuffer_id_clear_noref(config
, &shmp_index(handle
, buf
->backend
.buf_wsb
, 0)->id
);
220 tsc
= config
->cb
.ring_buffer_clock_read(shmp(handle
, buf
->backend
.chan
));
221 config
->cb
.buffer_begin(buf
, tsc
, 0, handle
);
222 v_add(config
, subbuf_header_size
, &shmp_index(handle
, buf
->commit_hot
, 0)->cc
);
224 if (config
->cb
.buffer_create
) {
225 ret
= config
->cb
.buffer_create(buf
, priv
, cpu
, chanb
->name
, handle
);
229 buf
->backend
.allocated
= 1;
234 /* commit_cold will be freed by shm teardown */
236 /* commit_hot will be freed by shm teardown */
238 lib_ring_buffer_backend_free(&buf
->backend
);
243 static void switch_buffer_timer(unsigned long data
)
245 struct lib_ring_buffer
*buf
= (struct lib_ring_buffer
*)data
;
246 struct channel
*chan
= shmp(handle
, buf
->backend
.chan
);
247 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
250 * Only flush buffers periodically if readers are active.
252 if (uatomic_read(&buf
->active_readers
) || uatomic_read(&buf
->active_shadow_readers
))
253 lib_ring_buffer_switch_slow(buf
, SWITCH_ACTIVE
, handle
);
256 //if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
257 // mod_timer_pinned(&buf->switch_timer,
258 // jiffies + chan->switch_timer_interval);
260 // mod_timer(&buf->switch_timer,
261 // jiffies + chan->switch_timer_interval);
265 static void lib_ring_buffer_start_switch_timer(struct lib_ring_buffer
*buf
,
266 struct shm_handle
*handle
)
268 struct channel
*chan
= shmp(handle
, buf
->backend
.chan
);
269 //const struct lib_ring_buffer_config *config = &chan->backend.config;
271 if (!chan
->switch_timer_interval
|| buf
->switch_timer_enabled
)
274 //init_timer(&buf->switch_timer);
275 //buf->switch_timer.function = switch_buffer_timer;
276 //buf->switch_timer.expires = jiffies + chan->switch_timer_interval;
277 //buf->switch_timer.data = (unsigned long)buf;
278 //if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
279 // add_timer_on(&buf->switch_timer, buf->backend.cpu);
281 // add_timer(&buf->switch_timer);
282 buf
->switch_timer_enabled
= 1;
285 static void lib_ring_buffer_stop_switch_timer(struct lib_ring_buffer
*buf
,
286 struct shm_handle
*handle
)
288 struct channel
*chan
= shmp(handle
, buf
->backend
.chan
);
290 if (!chan
->switch_timer_interval
|| !buf
->switch_timer_enabled
)
294 //del_timer_sync(&buf->switch_timer);
295 buf
->switch_timer_enabled
= 0;
300 * Polling timer to check the channels for data.
302 static void read_buffer_timer(unsigned long data
)
304 struct lib_ring_buffer
*buf
= (struct lib_ring_buffer
*)data
;
305 struct channel
*chan
= shmp(handle
, buf
->backend
.chan
);
306 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
308 CHAN_WARN_ON(chan
, !buf
->backend
.allocated
);
310 if (uatomic_read(&buf
->active_readers
) || uatomic_read(&buf
->active_shadow_readers
))
311 && lib_ring_buffer_poll_deliver(config
, buf
, chan
)) {
313 //wake_up_interruptible(&buf->read_wait);
314 //wake_up_interruptible(&chan->read_wait);
318 //if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
319 // mod_timer_pinned(&buf->read_timer,
320 // jiffies + chan->read_timer_interval);
322 // mod_timer(&buf->read_timer,
323 // jiffies + chan->read_timer_interval);
327 static void lib_ring_buffer_start_read_timer(struct lib_ring_buffer
*buf
,
328 struct shm_handle
*handle
)
330 struct channel
*chan
= shmp(handle
, buf
->backend
.chan
);
331 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
333 if (config
->wakeup
!= RING_BUFFER_WAKEUP_BY_TIMER
334 || !chan
->read_timer_interval
335 || buf
->read_timer_enabled
)
339 //init_timer(&buf->read_timer);
340 //buf->read_timer.function = read_buffer_timer;
341 //buf->read_timer.expires = jiffies + chan->read_timer_interval;
342 //buf->read_timer.data = (unsigned long)buf;
344 //if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
345 // add_timer_on(&buf->read_timer, buf->backend.cpu);
347 // add_timer(&buf->read_timer);
348 buf
->read_timer_enabled
= 1;
351 static void lib_ring_buffer_stop_read_timer(struct lib_ring_buffer
*buf
,
352 struct shm_handle
*handle
)
354 struct channel
*chan
= shmp(handle
, buf
->backend
.chan
);
355 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
357 if (config
->wakeup
!= RING_BUFFER_WAKEUP_BY_TIMER
358 || !chan
->read_timer_interval
359 || !buf
->read_timer_enabled
)
363 //del_timer_sync(&buf->read_timer);
365 * do one more check to catch data that has been written in the last
368 if (lib_ring_buffer_poll_deliver(config
, buf
, chan
, handle
)) {
370 //wake_up_interruptible(&buf->read_wait);
371 //wake_up_interruptible(&chan->read_wait);
373 buf
->read_timer_enabled
= 0;
376 static void channel_unregister_notifiers(struct channel
*chan
,
377 struct shm_handle
*handle
)
379 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
382 if (config
->alloc
== RING_BUFFER_ALLOC_PER_CPU
) {
383 for_each_possible_cpu(cpu
) {
384 struct lib_ring_buffer
*buf
= shmp(handle
, chan
->backend
.buf
[cpu
].shmp
);
386 lib_ring_buffer_stop_switch_timer(buf
, handle
);
387 lib_ring_buffer_stop_read_timer(buf
, handle
);
390 struct lib_ring_buffer
*buf
= shmp(handle
, chan
->backend
.buf
[0].shmp
);
392 lib_ring_buffer_stop_switch_timer(buf
, handle
);
393 lib_ring_buffer_stop_read_timer(buf
, handle
);
395 //channel_backend_unregister_notifiers(&chan->backend);
398 static void channel_free(struct channel
*chan
, struct shm_handle
*handle
,
402 channel_backend_free(&chan
->backend
, handle
);
403 /* chan is freed by shm teardown */
404 shm_object_table_destroy(handle
->table
);
409 * channel_create - Create channel.
410 * @config: ring buffer instance configuration
411 * @name: name of the channel
412 * @priv: ring buffer client private data
413 * @buf_addr: pointer the the beginning of the preallocated buffer contiguous
414 * address mapping. It is used only by RING_BUFFER_STATIC
415 * configuration. It can be set to NULL for other backends.
416 * @subbuf_size: subbuffer size
417 * @num_subbuf: number of subbuffers
418 * @switch_timer_interval: Time interval (in us) to fill sub-buffers with
419 * padding to let readers get those sub-buffers.
420 * Used for live streaming.
421 * @read_timer_interval: Time interval (in us) to wake up pending readers.
424 * Returns NULL on failure.
426 struct shm_handle
*channel_create(const struct lib_ring_buffer_config
*config
,
427 const char *name
, void *priv
, void *buf_addr
,
429 size_t num_subbuf
, unsigned int switch_timer_interval
,
430 unsigned int read_timer_interval
,
431 int *shm_fd
, int *wait_fd
, uint64_t *memory_map_size
)
435 struct channel
*chan
;
436 struct shm_handle
*handle
;
437 struct shm_object
*shmobj
;
440 if (lib_ring_buffer_check_config(config
, switch_timer_interval
,
441 read_timer_interval
))
444 handle
= zmalloc(sizeof(struct shm_handle
));
448 /* Allocate table for channel + per-cpu buffers */
449 handle
->table
= shm_object_table_create(1 + num_possible_cpus());
451 goto error_table_alloc
;
453 /* Calculate the shm allocation layout */
454 shmsize
= sizeof(struct channel
);
455 if (config
->alloc
== RING_BUFFER_ALLOC_PER_CPU
)
456 shmsize
+= sizeof(struct lib_ring_buffer_shmp
) * num_possible_cpus();
458 shmsize
+= sizeof(struct lib_ring_buffer_shmp
);
460 shmobj
= shm_object_table_append(handle
->table
, shmsize
);
463 set_shmp(handle
->chan
, zalloc_shm(shmobj
, sizeof(struct channel
)));
464 chan
= shmp(handle
, handle
->chan
);
468 ret
= channel_backend_init(&chan
->backend
, name
, config
, priv
,
469 subbuf_size
, num_subbuf
, handle
);
471 goto error_backend_init
;
473 chan
->commit_count_mask
= (~0UL >> chan
->backend
.num_subbuf_order
);
475 //chan->switch_timer_interval = usecs_to_jiffies(switch_timer_interval);
476 //chan->read_timer_interval = usecs_to_jiffies(read_timer_interval);
478 //init_waitqueue_head(&chan->read_wait);
479 //init_waitqueue_head(&chan->hp_wait);
481 if (config
->alloc
== RING_BUFFER_ALLOC_PER_CPU
) {
483 * In case of non-hotplug cpu, if the ring-buffer is allocated
484 * in early initcall, it will not be notified of secondary cpus.
485 * In that off case, we need to allocate for all possible cpus.
487 for_each_possible_cpu(cpu
) {
488 struct lib_ring_buffer
*buf
= shmp(handle
, chan
->backend
.buf
[cpu
].shmp
);
489 lib_ring_buffer_start_switch_timer(buf
, handle
);
490 lib_ring_buffer_start_read_timer(buf
, handle
);
493 struct lib_ring_buffer
*buf
= shmp(handle
, chan
->backend
.buf
[0].shmp
);
495 lib_ring_buffer_start_switch_timer(buf
, handle
);
496 lib_ring_buffer_start_read_timer(buf
, handle
);
498 ref
= &handle
->chan
._ref
;
499 shm_get_object_data(handle
, ref
, shm_fd
, wait_fd
, memory_map_size
);
504 shm_object_table_destroy(handle
->table
);
510 struct shm_handle
*channel_handle_create(int shm_fd
, int wait_fd
,
511 uint64_t memory_map_size
)
513 struct shm_handle
*handle
;
514 struct shm_object
*object
;
516 handle
= zmalloc(sizeof(struct shm_handle
));
520 /* Allocate table for channel + per-cpu buffers */
521 handle
->table
= shm_object_table_create(1 + num_possible_cpus());
523 goto error_table_alloc
;
524 /* Add channel object */
525 object
= shm_object_table_append_shadow(handle
->table
,
526 shm_fd
, wait_fd
, memory_map_size
);
528 goto error_table_object
;
533 shm_object_table_destroy(handle
->table
);
539 int channel_handle_add_stream(struct shm_handle
*handle
,
540 int shm_fd
, int wait_fd
, uint64_t memory_map_size
)
542 struct shm_object
*object
;
544 /* Add stream object */
545 object
= shm_object_table_append_shadow(handle
->table
,
546 shm_fd
, wait_fd
, memory_map_size
);
553 void channel_release(struct channel
*chan
, struct shm_handle
*handle
,
556 channel_free(chan
, handle
, shadow
);
560 * channel_destroy - Finalize, wait for q.s. and destroy channel.
561 * @chan: channel to destroy
564 * Call "destroy" callback, finalize channels, decrement the channel
565 * reference count. Note that when readers have completed data
566 * consumption of finalized channels, get_subbuf() will return -ENODATA.
567 * They should release their handle at that point. Returns the private
570 void *channel_destroy(struct channel
*chan
, struct shm_handle
*handle
,
573 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
578 channel_release(chan
, handle
, shadow
);
582 channel_unregister_notifiers(chan
, handle
);
584 if (config
->alloc
== RING_BUFFER_ALLOC_PER_CPU
) {
585 for_each_channel_cpu(cpu
, chan
) {
586 struct lib_ring_buffer
*buf
= shmp(handle
, chan
->backend
.buf
[cpu
].shmp
);
588 if (config
->cb
.buffer_finalize
)
589 config
->cb
.buffer_finalize(buf
,
592 if (buf
->backend
.allocated
)
593 lib_ring_buffer_switch_slow(buf
, SWITCH_FLUSH
,
596 * Perform flush before writing to finalized.
599 CMM_ACCESS_ONCE(buf
->finalized
) = 1;
600 //wake_up_interruptible(&buf->read_wait);
603 struct lib_ring_buffer
*buf
= shmp(handle
, chan
->backend
.buf
[0].shmp
);
605 if (config
->cb
.buffer_finalize
)
606 config
->cb
.buffer_finalize(buf
, chan
->backend
.priv
, -1, handle
);
607 if (buf
->backend
.allocated
)
608 lib_ring_buffer_switch_slow(buf
, SWITCH_FLUSH
,
611 * Perform flush before writing to finalized.
614 CMM_ACCESS_ONCE(buf
->finalized
) = 1;
615 //wake_up_interruptible(&buf->read_wait);
617 CMM_ACCESS_ONCE(chan
->finalized
) = 1;
618 //wake_up_interruptible(&chan->hp_wait);
619 //wake_up_interruptible(&chan->read_wait);
621 * sessiond/consumer are keeping a reference on the shm file
622 * descriptor directly. No need to refcount.
624 priv
= chan
->backend
.priv
;
625 channel_release(chan
, handle
, shadow
);
629 struct lib_ring_buffer
*channel_get_ring_buffer(
630 const struct lib_ring_buffer_config
*config
,
631 struct channel
*chan
, int cpu
,
632 struct shm_handle
*handle
,
633 int *shm_fd
, int *wait_fd
,
634 uint64_t *memory_map_size
)
638 if (config
->alloc
== RING_BUFFER_ALLOC_GLOBAL
) {
639 ref
= &chan
->backend
.buf
[0].shmp
._ref
;
640 shm_get_object_data(handle
, ref
, shm_fd
, wait_fd
,
642 return shmp(handle
, chan
->backend
.buf
[0].shmp
);
644 if (cpu
>= num_possible_cpus())
646 ref
= &chan
->backend
.buf
[cpu
].shmp
._ref
;
647 shm_get_object_data(handle
, ref
, shm_fd
, wait_fd
,
649 return shmp(handle
, chan
->backend
.buf
[cpu
].shmp
);
653 int lib_ring_buffer_open_read(struct lib_ring_buffer
*buf
,
654 struct shm_handle
*handle
,
658 if (uatomic_cmpxchg(&buf
->active_shadow_readers
, 0, 1) != 0)
663 if (uatomic_cmpxchg(&buf
->active_readers
, 0, 1) != 0)
669 void lib_ring_buffer_release_read(struct lib_ring_buffer
*buf
,
670 struct shm_handle
*handle
,
673 struct channel
*chan
= shmp(handle
, buf
->backend
.chan
);
676 CHAN_WARN_ON(chan
, uatomic_read(&buf
->active_shadow_readers
) != 1);
678 uatomic_dec(&buf
->active_shadow_readers
);
681 CHAN_WARN_ON(chan
, uatomic_read(&buf
->active_readers
) != 1);
683 uatomic_dec(&buf
->active_readers
);
687 * lib_ring_buffer_snapshot - save subbuffer position snapshot (for read)
689 * @consumed: consumed count indicating the position where to read
690 * @produced: produced count, indicates position when to stop reading
692 * Returns -ENODATA if buffer is finalized, -EAGAIN if there is currently no
693 * data to read at consumed position, or 0 if the get operation succeeds.
696 int lib_ring_buffer_snapshot(struct lib_ring_buffer
*buf
,
697 unsigned long *consumed
, unsigned long *produced
,
698 struct shm_handle
*handle
)
700 struct channel
*chan
= shmp(handle
, buf
->backend
.chan
);
701 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
702 unsigned long consumed_cur
, write_offset
;
705 finalized
= CMM_ACCESS_ONCE(buf
->finalized
);
707 * Read finalized before counters.
710 consumed_cur
= uatomic_read(&buf
->consumed
);
712 * No need to issue a memory barrier between consumed count read and
713 * write offset read, because consumed count can only change
714 * concurrently in overwrite mode, and we keep a sequence counter
715 * identifier derived from the write offset to check we are getting
716 * the same sub-buffer we are expecting (the sub-buffers are atomically
717 * "tagged" upon writes, tags are checked upon read).
719 write_offset
= v_read(config
, &buf
->offset
);
722 * Check that we are not about to read the same subbuffer in
723 * which the writer head is.
725 if (subbuf_trunc(write_offset
, chan
) - subbuf_trunc(consumed_cur
, chan
)
729 *consumed
= consumed_cur
;
730 *produced
= subbuf_trunc(write_offset
, chan
);
736 * The memory barriers __wait_event()/wake_up_interruptible() take care
737 * of "raw_spin_is_locked" memory ordering.
746 * lib_ring_buffer_put_snapshot - move consumed counter forward
748 * @consumed_new: new consumed count value
750 void lib_ring_buffer_move_consumer(struct lib_ring_buffer
*buf
,
751 unsigned long consumed_new
,
752 struct shm_handle
*handle
)
754 struct lib_ring_buffer_backend
*bufb
= &buf
->backend
;
755 struct channel
*chan
= shmp(handle
, bufb
->chan
);
756 unsigned long consumed
;
758 CHAN_WARN_ON(chan
, uatomic_read(&buf
->active_readers
) != 1
759 && uatomic_read(&buf
->active_shadow_readers
) != 1);
762 * Only push the consumed value forward.
763 * If the consumed cmpxchg fails, this is because we have been pushed by
764 * the writer in flight recorder mode.
766 consumed
= uatomic_read(&buf
->consumed
);
767 while ((long) consumed
- (long) consumed_new
< 0)
768 consumed
= uatomic_cmpxchg(&buf
->consumed
, consumed
,
773 * lib_ring_buffer_get_subbuf - get exclusive access to subbuffer for reading
775 * @consumed: consumed count indicating the position where to read
777 * Returns -ENODATA if buffer is finalized, -EAGAIN if there is currently no
778 * data to read at consumed position, or 0 if the get operation succeeds.
780 int lib_ring_buffer_get_subbuf(struct lib_ring_buffer
*buf
,
781 unsigned long consumed
,
782 struct shm_handle
*handle
)
784 struct channel
*chan
= shmp(handle
, buf
->backend
.chan
);
785 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
786 unsigned long consumed_cur
, consumed_idx
, commit_count
, write_offset
;
791 finalized
= CMM_ACCESS_ONCE(buf
->finalized
);
793 * Read finalized before counters.
796 consumed_cur
= uatomic_read(&buf
->consumed
);
797 consumed_idx
= subbuf_index(consumed
, chan
);
798 commit_count
= v_read(config
, &shmp_index(handle
, buf
->commit_cold
, consumed_idx
)->cc_sb
);
800 * Make sure we read the commit count before reading the buffer
801 * data and the write offset. Correct consumed offset ordering
802 * wrt commit count is insured by the use of cmpxchg to update
803 * the consumed offset.
806 * Local rmb to match the remote wmb to read the commit count
807 * before the buffer data and the write offset.
811 write_offset
= v_read(config
, &buf
->offset
);
814 * Check that the buffer we are getting is after or at consumed_cur
817 if ((long) subbuf_trunc(consumed
, chan
)
818 - (long) subbuf_trunc(consumed_cur
, chan
) < 0)
822 * Check that the subbuffer we are trying to consume has been
823 * already fully committed.
825 if (((commit_count
- chan
->backend
.subbuf_size
)
826 & chan
->commit_count_mask
)
827 - (buf_trunc(consumed_cur
, chan
)
828 >> chan
->backend
.num_subbuf_order
)
833 * Check that we are not about to read the same subbuffer in
834 * which the writer head is.
836 if (subbuf_trunc(write_offset
, chan
) - subbuf_trunc(consumed_cur
, chan
)
841 * Failure to get the subbuffer causes a busy-loop retry without going
842 * to a wait queue. These are caused by short-lived race windows where
843 * the writer is getting access to a subbuffer we were trying to get
844 * access to. Also checks that the "consumed" buffer count we are
845 * looking for matches the one contained in the subbuffer id.
847 ret
= update_read_sb_index(config
, &buf
->backend
, &chan
->backend
,
848 consumed_idx
, buf_trunc_val(consumed
, chan
),
852 subbuffer_id_clear_noref(config
, &buf
->backend
.buf_rsb
.id
);
854 buf
->get_subbuf_consumed
= consumed
;
861 * The memory barriers __wait_event()/wake_up_interruptible() take care
862 * of "raw_spin_is_locked" memory ordering.
871 * lib_ring_buffer_put_subbuf - release exclusive subbuffer access
874 void lib_ring_buffer_put_subbuf(struct lib_ring_buffer
*buf
,
875 struct shm_handle
*handle
)
877 struct lib_ring_buffer_backend
*bufb
= &buf
->backend
;
878 struct channel
*chan
= shmp(handle
, bufb
->chan
);
879 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
880 unsigned long read_sb_bindex
, consumed_idx
, consumed
;
882 CHAN_WARN_ON(chan
, uatomic_read(&buf
->active_readers
) != 1
883 && uatomic_read(&buf
->active_shadow_readers
) != 1);
885 if (!buf
->get_subbuf
) {
887 * Reader puts a subbuffer it did not get.
889 CHAN_WARN_ON(chan
, 1);
892 consumed
= buf
->get_subbuf_consumed
;
896 * Clear the records_unread counter. (overruns counter)
897 * Can still be non-zero if a file reader simply grabbed the data
898 * without using iterators.
899 * Can be below zero if an iterator is used on a snapshot more than
902 read_sb_bindex
= subbuffer_id_get_index(config
, bufb
->buf_rsb
.id
);
903 v_add(config
, v_read(config
,
904 &shmp(handle
, shmp_index(handle
, bufb
->array
, read_sb_bindex
)->shmp
)->records_unread
),
905 &bufb
->records_read
);
906 v_set(config
, &shmp(handle
, shmp_index(handle
, bufb
->array
, read_sb_bindex
)->shmp
)->records_unread
, 0);
907 CHAN_WARN_ON(chan
, config
->mode
== RING_BUFFER_OVERWRITE
908 && subbuffer_id_is_noref(config
, bufb
->buf_rsb
.id
));
909 subbuffer_id_set_noref(config
, &bufb
->buf_rsb
.id
);
912 * Exchange the reader subbuffer with the one we put in its place in the
913 * writer subbuffer table. Expect the original consumed count. If
914 * update_read_sb_index fails, this is because the writer updated the
915 * subbuffer concurrently. We should therefore keep the subbuffer we
916 * currently have: it has become invalid to try reading this sub-buffer
917 * consumed count value anyway.
919 consumed_idx
= subbuf_index(consumed
, chan
);
920 update_read_sb_index(config
, &buf
->backend
, &chan
->backend
,
921 consumed_idx
, buf_trunc_val(consumed
, chan
),
924 * update_read_sb_index return value ignored. Don't exchange sub-buffer
925 * if the writer concurrently updated it.
930 * cons_offset is an iterator on all subbuffer offsets between the reader
931 * position and the writer position. (inclusive)
934 void lib_ring_buffer_print_subbuffer_errors(struct lib_ring_buffer
*buf
,
935 struct channel
*chan
,
936 unsigned long cons_offset
,
938 struct shm_handle
*handle
)
940 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
941 unsigned long cons_idx
, commit_count
, commit_count_sb
;
943 cons_idx
= subbuf_index(cons_offset
, chan
);
944 commit_count
= v_read(config
, &shmp_index(handle
, buf
->commit_hot
, cons_idx
)->cc
);
945 commit_count_sb
= v_read(config
, &shmp_index(handle
, buf
->commit_cold
, cons_idx
)->cc_sb
);
947 if (subbuf_offset(commit_count
, chan
) != 0)
948 ERRMSG("ring buffer %s, cpu %d: "
949 "commit count in subbuffer %lu,\n"
950 "expecting multiples of %lu bytes\n"
951 " [ %lu bytes committed, %lu bytes reader-visible ]\n",
952 chan
->backend
.name
, cpu
, cons_idx
,
953 chan
->backend
.subbuf_size
,
954 commit_count
, commit_count_sb
);
956 ERRMSG("ring buffer: %s, cpu %d: %lu bytes committed\n",
957 chan
->backend
.name
, cpu
, commit_count
);
961 void lib_ring_buffer_print_buffer_errors(struct lib_ring_buffer
*buf
,
962 struct channel
*chan
,
964 struct shm_handle
*handle
)
966 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
967 unsigned long write_offset
, cons_offset
;
970 * Can be called in the error path of allocation when
971 * trans_channel_data is not yet set.
976 * No need to order commit_count, write_offset and cons_offset reads
977 * because we execute at teardown when no more writer nor reader
978 * references are left.
980 write_offset
= v_read(config
, &buf
->offset
);
981 cons_offset
= uatomic_read(&buf
->consumed
);
982 if (write_offset
!= cons_offset
)
983 ERRMSG("ring buffer %s, cpu %d: "
984 "non-consumed data\n"
985 " [ %lu bytes written, %lu bytes read ]\n",
986 chan
->backend
.name
, cpu
, write_offset
, cons_offset
);
988 for (cons_offset
= uatomic_read(&buf
->consumed
);
989 (long) (subbuf_trunc((unsigned long) v_read(config
, &buf
->offset
),
992 cons_offset
= subbuf_align(cons_offset
, chan
))
993 lib_ring_buffer_print_subbuffer_errors(buf
, chan
, cons_offset
,
998 void lib_ring_buffer_print_errors(struct channel
*chan
,
999 struct lib_ring_buffer
*buf
, int cpu
,
1000 struct shm_handle
*handle
)
1002 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
1003 void *priv
= chan
->backend
.priv
;
1005 ERRMSG("ring buffer %s, cpu %d: %lu records written, "
1006 "%lu records overrun\n",
1007 chan
->backend
.name
, cpu
,
1008 v_read(config
, &buf
->records_count
),
1009 v_read(config
, &buf
->records_overrun
));
1011 if (v_read(config
, &buf
->records_lost_full
)
1012 || v_read(config
, &buf
->records_lost_wrap
)
1013 || v_read(config
, &buf
->records_lost_big
))
1014 ERRMSG("ring buffer %s, cpu %d: records were lost. Caused by:\n"
1015 " [ %lu buffer full, %lu nest buffer wrap-around, "
1016 "%lu event too big ]\n",
1017 chan
->backend
.name
, cpu
,
1018 v_read(config
, &buf
->records_lost_full
),
1019 v_read(config
, &buf
->records_lost_wrap
),
1020 v_read(config
, &buf
->records_lost_big
));
1022 lib_ring_buffer_print_buffer_errors(buf
, chan
, priv
, cpu
, handle
);
1026 * lib_ring_buffer_switch_old_start: Populate old subbuffer header.
1028 * Only executed when the buffer is finalized, in SWITCH_FLUSH.
1031 void lib_ring_buffer_switch_old_start(struct lib_ring_buffer
*buf
,
1032 struct channel
*chan
,
1033 struct switch_offsets
*offsets
,
1035 struct shm_handle
*handle
)
1037 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
1038 unsigned long oldidx
= subbuf_index(offsets
->old
, chan
);
1039 unsigned long commit_count
;
1041 config
->cb
.buffer_begin(buf
, tsc
, oldidx
, handle
);
1044 * Order all writes to buffer before the commit count update that will
1045 * determine that the subbuffer is full.
1048 v_add(config
, config
->cb
.subbuffer_header_size(),
1049 &shmp_index(handle
, buf
->commit_hot
, oldidx
)->cc
);
1050 commit_count
= v_read(config
, &shmp_index(handle
, buf
->commit_hot
, oldidx
)->cc
);
1051 /* Check if the written buffer has to be delivered */
1052 lib_ring_buffer_check_deliver(config
, buf
, chan
, offsets
->old
,
1053 commit_count
, oldidx
, handle
);
1054 lib_ring_buffer_write_commit_counter(config
, buf
, chan
, oldidx
,
1055 offsets
->old
, commit_count
,
1056 config
->cb
.subbuffer_header_size(),
1061 * lib_ring_buffer_switch_old_end: switch old subbuffer
1063 * Note : offset_old should never be 0 here. It is ok, because we never perform
1064 * buffer switch on an empty subbuffer in SWITCH_ACTIVE mode. The caller
1065 * increments the offset_old value when doing a SWITCH_FLUSH on an empty
1069 void lib_ring_buffer_switch_old_end(struct lib_ring_buffer
*buf
,
1070 struct channel
*chan
,
1071 struct switch_offsets
*offsets
,
1073 struct shm_handle
*handle
)
1075 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
1076 unsigned long oldidx
= subbuf_index(offsets
->old
- 1, chan
);
1077 unsigned long commit_count
, padding_size
, data_size
;
1079 data_size
= subbuf_offset(offsets
->old
- 1, chan
) + 1;
1080 padding_size
= chan
->backend
.subbuf_size
- data_size
;
1081 subbuffer_set_data_size(config
, &buf
->backend
, oldidx
, data_size
,
1085 * Order all writes to buffer before the commit count update that will
1086 * determine that the subbuffer is full.
1089 v_add(config
, padding_size
, &shmp_index(handle
, buf
->commit_hot
, oldidx
)->cc
);
1090 commit_count
= v_read(config
, &shmp_index(handle
, buf
->commit_hot
, oldidx
)->cc
);
1091 lib_ring_buffer_check_deliver(config
, buf
, chan
, offsets
->old
- 1,
1092 commit_count
, oldidx
, handle
);
1093 lib_ring_buffer_write_commit_counter(config
, buf
, chan
, oldidx
,
1094 offsets
->old
, commit_count
,
1095 padding_size
, handle
);
1099 * lib_ring_buffer_switch_new_start: Populate new subbuffer.
1101 * This code can be executed unordered : writers may already have written to the
1102 * sub-buffer before this code gets executed, caution. The commit makes sure
1103 * that this code is executed before the deliver of this sub-buffer.
1106 void lib_ring_buffer_switch_new_start(struct lib_ring_buffer
*buf
,
1107 struct channel
*chan
,
1108 struct switch_offsets
*offsets
,
1110 struct shm_handle
*handle
)
1112 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
1113 unsigned long beginidx
= subbuf_index(offsets
->begin
, chan
);
1114 unsigned long commit_count
;
1116 config
->cb
.buffer_begin(buf
, tsc
, beginidx
, handle
);
1119 * Order all writes to buffer before the commit count update that will
1120 * determine that the subbuffer is full.
1123 v_add(config
, config
->cb
.subbuffer_header_size(),
1124 &shmp_index(handle
, buf
->commit_hot
, beginidx
)->cc
);
1125 commit_count
= v_read(config
, &shmp_index(handle
, buf
->commit_hot
, beginidx
)->cc
);
1126 /* Check if the written buffer has to be delivered */
1127 lib_ring_buffer_check_deliver(config
, buf
, chan
, offsets
->begin
,
1128 commit_count
, beginidx
, handle
);
1129 lib_ring_buffer_write_commit_counter(config
, buf
, chan
, beginidx
,
1130 offsets
->begin
, commit_count
,
1131 config
->cb
.subbuffer_header_size(),
1136 * lib_ring_buffer_switch_new_end: finish switching current subbuffer
1138 * The only remaining threads could be the ones with pending commits. They will
1139 * have to do the deliver themselves.
1142 void lib_ring_buffer_switch_new_end(struct lib_ring_buffer
*buf
,
1143 struct channel
*chan
,
1144 struct switch_offsets
*offsets
,
1146 struct shm_handle
*handle
)
1148 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
1149 unsigned long endidx
= subbuf_index(offsets
->end
- 1, chan
);
1150 unsigned long commit_count
, padding_size
, data_size
;
1152 data_size
= subbuf_offset(offsets
->end
- 1, chan
) + 1;
1153 padding_size
= chan
->backend
.subbuf_size
- data_size
;
1154 subbuffer_set_data_size(config
, &buf
->backend
, endidx
, data_size
,
1158 * Order all writes to buffer before the commit count update that will
1159 * determine that the subbuffer is full.
1162 v_add(config
, padding_size
, &shmp_index(handle
, buf
->commit_hot
, endidx
)->cc
);
1163 commit_count
= v_read(config
, &shmp_index(handle
, buf
->commit_hot
, endidx
)->cc
);
1164 lib_ring_buffer_check_deliver(config
, buf
, chan
, offsets
->end
- 1,
1165 commit_count
, endidx
, handle
);
1166 lib_ring_buffer_write_commit_counter(config
, buf
, chan
, endidx
,
1167 offsets
->end
, commit_count
,
1168 padding_size
, handle
);
1174 * !0 if execution must be aborted.
1177 int lib_ring_buffer_try_switch_slow(enum switch_mode mode
,
1178 struct lib_ring_buffer
*buf
,
1179 struct channel
*chan
,
1180 struct switch_offsets
*offsets
,
1183 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
1186 offsets
->begin
= v_read(config
, &buf
->offset
);
1187 offsets
->old
= offsets
->begin
;
1188 offsets
->switch_old_start
= 0;
1189 off
= subbuf_offset(offsets
->begin
, chan
);
1191 *tsc
= config
->cb
.ring_buffer_clock_read(chan
);
1194 * Ensure we flush the header of an empty subbuffer when doing the
1195 * finalize (SWITCH_FLUSH). This ensures that we end up knowing the
1196 * total data gathering duration even if there were no records saved
1197 * after the last buffer switch.
1198 * In SWITCH_ACTIVE mode, switch the buffer when it contains events.
1199 * SWITCH_ACTIVE only flushes the current subbuffer, dealing with end of
1200 * subbuffer header as appropriate.
1201 * The next record that reserves space will be responsible for
1202 * populating the following subbuffer header. We choose not to populate
1203 * the next subbuffer header here because we want to be able to use
1204 * SWITCH_ACTIVE for periodical buffer flush, which must
1205 * guarantee that all the buffer content (records and header
1206 * timestamps) are visible to the reader. This is required for
1207 * quiescence guarantees for the fusion merge.
1209 if (mode
== SWITCH_FLUSH
|| off
> 0) {
1210 if (unlikely(off
== 0)) {
1212 * The client does not save any header information.
1213 * Don't switch empty subbuffer on finalize, because it
1214 * is invalid to deliver a completely empty subbuffer.
1216 if (!config
->cb
.subbuffer_header_size())
1219 * Need to write the subbuffer start header on finalize.
1221 offsets
->switch_old_start
= 1;
1223 offsets
->begin
= subbuf_align(offsets
->begin
, chan
);
1225 return -1; /* we do not have to switch : buffer is empty */
1226 /* Note: old points to the next subbuf at offset 0 */
1227 offsets
->end
= offsets
->begin
;
1232 * Force a sub-buffer switch. This operation is completely reentrant : can be
1233 * called while tracing is active with absolutely no lock held.
1235 * Note, however, that as a v_cmpxchg is used for some atomic
1236 * operations, this function must be called from the CPU which owns the buffer
1237 * for a ACTIVE flush.
1239 void lib_ring_buffer_switch_slow(struct lib_ring_buffer
*buf
, enum switch_mode mode
,
1240 struct shm_handle
*handle
)
1242 struct channel
*chan
= shmp(handle
, buf
->backend
.chan
);
1243 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
1244 struct switch_offsets offsets
;
1245 unsigned long oldidx
;
1251 * Perform retryable operations.
1254 if (lib_ring_buffer_try_switch_slow(mode
, buf
, chan
, &offsets
,
1256 return; /* Switch not needed */
1257 } while (v_cmpxchg(config
, &buf
->offset
, offsets
.old
, offsets
.end
)
1261 * Atomically update last_tsc. This update races against concurrent
1262 * atomic updates, but the race will always cause supplementary full TSC
1263 * records, never the opposite (missing a full TSC record when it would
1266 save_last_tsc(config
, buf
, tsc
);
1269 * Push the reader if necessary
1271 lib_ring_buffer_reserve_push_reader(buf
, chan
, offsets
.old
);
1273 oldidx
= subbuf_index(offsets
.old
, chan
);
1274 lib_ring_buffer_clear_noref(config
, &buf
->backend
, oldidx
, handle
);
1277 * May need to populate header start on SWITCH_FLUSH.
1279 if (offsets
.switch_old_start
) {
1280 lib_ring_buffer_switch_old_start(buf
, chan
, &offsets
, tsc
, handle
);
1281 offsets
.old
+= config
->cb
.subbuffer_header_size();
1285 * Switch old subbuffer.
1287 lib_ring_buffer_switch_old_end(buf
, chan
, &offsets
, tsc
, handle
);
1293 * -ENOSPC if event size is too large for packet.
1294 * -ENOBUFS if there is currently not enough space in buffer for the event.
1295 * -EIO if data cannot be written into the buffer for any other reason.
1298 int lib_ring_buffer_try_reserve_slow(struct lib_ring_buffer
*buf
,
1299 struct channel
*chan
,
1300 struct switch_offsets
*offsets
,
1301 struct lib_ring_buffer_ctx
*ctx
)
1303 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
1304 struct shm_handle
*handle
= ctx
->handle
;
1305 unsigned long reserve_commit_diff
;
1307 offsets
->begin
= v_read(config
, &buf
->offset
);
1308 offsets
->old
= offsets
->begin
;
1309 offsets
->switch_new_start
= 0;
1310 offsets
->switch_new_end
= 0;
1311 offsets
->switch_old_end
= 0;
1312 offsets
->pre_header_padding
= 0;
1314 ctx
->tsc
= config
->cb
.ring_buffer_clock_read(chan
);
1315 if ((int64_t) ctx
->tsc
== -EIO
)
1318 if (last_tsc_overflow(config
, buf
, ctx
->tsc
))
1319 ctx
->rflags
|= RING_BUFFER_RFLAG_FULL_TSC
;
1321 if (unlikely(subbuf_offset(offsets
->begin
, ctx
->chan
) == 0)) {
1322 offsets
->switch_new_start
= 1; /* For offsets->begin */
1324 offsets
->size
= config
->cb
.record_header_size(config
, chan
,
1326 &offsets
->pre_header_padding
,
1329 lib_ring_buffer_align(offsets
->begin
+ offsets
->size
,
1332 if (unlikely(subbuf_offset(offsets
->begin
, chan
) +
1333 offsets
->size
> chan
->backend
.subbuf_size
)) {
1334 offsets
->switch_old_end
= 1; /* For offsets->old */
1335 offsets
->switch_new_start
= 1; /* For offsets->begin */
1338 if (unlikely(offsets
->switch_new_start
)) {
1339 unsigned long sb_index
;
1342 * We are typically not filling the previous buffer completely.
1344 if (likely(offsets
->switch_old_end
))
1345 offsets
->begin
= subbuf_align(offsets
->begin
, chan
);
1346 offsets
->begin
= offsets
->begin
1347 + config
->cb
.subbuffer_header_size();
1348 /* Test new buffer integrity */
1349 sb_index
= subbuf_index(offsets
->begin
, chan
);
1350 reserve_commit_diff
=
1351 (buf_trunc(offsets
->begin
, chan
)
1352 >> chan
->backend
.num_subbuf_order
)
1353 - ((unsigned long) v_read(config
,
1354 &shmp_index(handle
, buf
->commit_cold
, sb_index
)->cc_sb
)
1355 & chan
->commit_count_mask
);
1356 if (likely(reserve_commit_diff
== 0)) {
1357 /* Next subbuffer not being written to. */
1358 if (unlikely(config
->mode
!= RING_BUFFER_OVERWRITE
&&
1359 subbuf_trunc(offsets
->begin
, chan
)
1360 - subbuf_trunc((unsigned long)
1361 uatomic_read(&buf
->consumed
), chan
)
1362 >= chan
->backend
.buf_size
)) {
1364 * We do not overwrite non consumed buffers
1365 * and we are full : record is lost.
1367 v_inc(config
, &buf
->records_lost_full
);
1371 * Next subbuffer not being written to, and we
1372 * are either in overwrite mode or the buffer is
1373 * not full. It's safe to write in this new
1379 * Next subbuffer reserve offset does not match the
1380 * commit offset. Drop record in producer-consumer and
1381 * overwrite mode. Caused by either a writer OOPS or too
1382 * many nested writes over a reserve/commit pair.
1384 v_inc(config
, &buf
->records_lost_wrap
);
1388 config
->cb
.record_header_size(config
, chan
,
1390 &offsets
->pre_header_padding
,
1393 lib_ring_buffer_align(offsets
->begin
+ offsets
->size
,
1396 if (unlikely(subbuf_offset(offsets
->begin
, chan
)
1397 + offsets
->size
> chan
->backend
.subbuf_size
)) {
1399 * Record too big for subbuffers, report error, don't
1400 * complete the sub-buffer switch.
1402 v_inc(config
, &buf
->records_lost_big
);
1406 * We just made a successful buffer switch and the
1407 * record fits in the new subbuffer. Let's write.
1412 * Record fits in the current buffer and we are not on a switch
1413 * boundary. It's safe to write.
1416 offsets
->end
= offsets
->begin
+ offsets
->size
;
1418 if (unlikely(subbuf_offset(offsets
->end
, chan
) == 0)) {
1420 * The offset_end will fall at the very beginning of the next
1423 offsets
->switch_new_end
= 1; /* For offsets->begin */
1429 * lib_ring_buffer_reserve_slow - Atomic slot reservation in a buffer.
1430 * @ctx: ring buffer context.
1432 * Return : -NOBUFS if not enough space, -ENOSPC if event size too large,
1433 * -EIO for other errors, else returns 0.
1434 * It will take care of sub-buffer switching.
1436 int lib_ring_buffer_reserve_slow(struct lib_ring_buffer_ctx
*ctx
)
1438 struct channel
*chan
= ctx
->chan
;
1439 struct shm_handle
*handle
= ctx
->handle
;
1440 const struct lib_ring_buffer_config
*config
= &chan
->backend
.config
;
1441 struct lib_ring_buffer
*buf
;
1442 struct switch_offsets offsets
;
1445 if (config
->alloc
== RING_BUFFER_ALLOC_PER_CPU
)
1446 buf
= shmp(handle
, chan
->backend
.buf
[ctx
->cpu
].shmp
);
1448 buf
= shmp(handle
, chan
->backend
.buf
[0].shmp
);
1454 ret
= lib_ring_buffer_try_reserve_slow(buf
, chan
, &offsets
,
1458 } while (unlikely(v_cmpxchg(config
, &buf
->offset
, offsets
.old
,
1463 * Atomically update last_tsc. This update races against concurrent
1464 * atomic updates, but the race will always cause supplementary full TSC
1465 * records, never the opposite (missing a full TSC record when it would
1468 save_last_tsc(config
, buf
, ctx
->tsc
);
1471 * Push the reader if necessary
1473 lib_ring_buffer_reserve_push_reader(buf
, chan
, offsets
.end
- 1);
1476 * Clear noref flag for this subbuffer.
1478 lib_ring_buffer_clear_noref(config
, &buf
->backend
,
1479 subbuf_index(offsets
.end
- 1, chan
),
1483 * Switch old subbuffer if needed.
1485 if (unlikely(offsets
.switch_old_end
)) {
1486 lib_ring_buffer_clear_noref(config
, &buf
->backend
,
1487 subbuf_index(offsets
.old
- 1, chan
),
1489 lib_ring_buffer_switch_old_end(buf
, chan
, &offsets
, ctx
->tsc
, handle
);
1493 * Populate new subbuffer.
1495 if (unlikely(offsets
.switch_new_start
))
1496 lib_ring_buffer_switch_new_start(buf
, chan
, &offsets
, ctx
->tsc
, handle
);
1498 if (unlikely(offsets
.switch_new_end
))
1499 lib_ring_buffer_switch_new_end(buf
, chan
, &offsets
, ctx
->tsc
, handle
);
1501 ctx
->slot_size
= offsets
.size
;
1502 ctx
->pre_offset
= offsets
.begin
;
1503 ctx
->buf_offset
= offsets
.begin
+ offsets
.pre_header_padding
;