X-Git-Url: http://git.lttng.org/?a=blobdiff_plain;f=libringbuffer%2Fring_buffer_frontend.c;h=76d6ec746c40ade48905bcfbf0cd3c81274d812e;hb=e56bb47c3af00db3bc9e2dea711bef4882e6ef4c;hp=eb4e48629ca3d26a08b3c015b069a0e3c29f7d68;hpb=80249235cb6560c6bb915f1cd486aa5017f87456;p=lttng-ust.git

diff --git a/libringbuffer/ring_buffer_frontend.c b/libringbuffer/ring_buffer_frontend.c
index eb4e4862..76d6ec74 100644
--- a/libringbuffer/ring_buffer_frontend.c
+++ b/libringbuffer/ring_buffer_frontend.c
@@ -52,6 +52,7 @@
  */
 
 #define _GNU_SOURCE
+#define _LGPL_SOURCE
 #include <sys/types.h>
 #include <sys/mman.h>
 #include <sys/stat.h>
@@ -59,9 +60,11 @@
 #include <fcntl.h>
 #include <signal.h>
 #include <time.h>
+#include <stdbool.h>
 #include <urcu/compiler.h>
 #include <urcu/ref.h>
 #include <urcu/tls-compat.h>
+#include <poll.h>
 #include <helper.h>
 
 #include "smp.h"
@@ -70,13 +73,9 @@
 #include "backend.h"
 #include "frontend.h"
 #include "shm.h"
-#include "tlsfixup.h"
+#include "rb-init.h"
 #include "../liblttng-ust/compat.h"	/* For ENODATA */
 
-#ifndef max
-#define max(a, b)	((a) > (b) ? (a) : (b))
-#endif
-
 /* Print DBG() messages about events lost only every 1048576 hits */
 #define DBG_PRINT_NR_LOST	(1UL << 20)
 
@@ -84,6 +83,14 @@
 #define LTTNG_UST_RB_SIG_READ		SIGRTMIN + 1
 #define LTTNG_UST_RB_SIG_TEARDOWN	SIGRTMIN + 2
 #define CLOCKID		CLOCK_MONOTONIC
+#define LTTNG_UST_RING_BUFFER_GET_RETRY		10
+#define LTTNG_UST_RING_BUFFER_RETRY_DELAY_MS	10
+#define RETRY_DELAY_MS				100	/* 100 ms. */
+
+/*
+ * Non-static to ensure the compiler does not optimize away the xor.
+ */
+uint8_t lttng_crash_magic_xor[] = RB_CRASH_DUMP_ABI_MAGIC_XOR;
 
 /*
  * Use POSIX SHM: shm_open(3) and shm_unlink(3).
@@ -144,6 +151,14 @@ static struct timer_signal_data timer_signal = {
 	.lock = PTHREAD_MUTEX_INITIALIZER,
 };
 
+int lttng_ust_blocking_retry_timeout =
+		CONFIG_LTTNG_UST_DEFAULT_BLOCKING_RETRY_TIMEOUT_MS;
+
+void lttng_ust_ringbuffer_set_retry_timeout(int timeout)
+{
+	lttng_ust_blocking_retry_timeout = timeout;
+}
+
 /**
  * lib_ring_buffer_reset - Reset ring buffer to initial values.
  * @buf: Ring buffer.
@@ -156,19 +171,32 @@ static struct timer_signal_data timer_signal = {
 void lib_ring_buffer_reset(struct lttng_ust_lib_ring_buffer *buf,
 			   struct lttng_ust_shm_handle *handle)
 {
-	struct channel *chan = shmp(handle, buf->backend.chan);
-	const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
+	struct channel *chan;
+	const struct lttng_ust_lib_ring_buffer_config *config;
 	unsigned int i;
 
+	chan = shmp(handle, buf->backend.chan);
+	if (!chan)
+		return;
+	config = &chan->backend.config;
 	/*
 	 * Reset iterator first. It will put the subbuffer if it currently holds
 	 * it.
 	 */
 	v_set(config, &buf->offset, 0);
 	for (i = 0; i < chan->backend.num_subbuf; i++) {
-		v_set(config, &shmp_index(handle, buf->commit_hot, i)->cc, 0);
-		v_set(config, &shmp_index(handle, buf->commit_hot, i)->seq, 0);
-		v_set(config, &shmp_index(handle, buf->commit_cold, i)->cc_sb, 0);
+		struct commit_counters_hot *cc_hot;
+		struct commit_counters_cold *cc_cold;
+
+		cc_hot = shmp_index(handle, buf->commit_hot, i);
+		if (!cc_hot)
+			return;
+		cc_cold = shmp_index(handle, buf->commit_cold, i);
+		if (!cc_cold)
+			return;
+		v_set(config, &cc_hot->cc, 0);
+		v_set(config, &cc_hot->seq, 0);
+		v_set(config, &cc_cold->cc_sb, 0);
 	}
 	uatomic_set(&buf->consumed, 0);
 	uatomic_set(&buf->record_disabled, 0);
@@ -205,6 +233,95 @@ void channel_reset(struct channel *chan)
 	/* Don't reset reader reference count */
 }
 
+static
+void init_crash_abi(const struct lttng_ust_lib_ring_buffer_config *config,
+		struct lttng_crash_abi *crash_abi,
+		struct lttng_ust_lib_ring_buffer *buf,
+		struct channel_backend *chanb,
+		struct shm_object *shmobj,
+		struct lttng_ust_shm_handle *handle)
+{
+	int i;
+
+	for (i = 0; i < RB_CRASH_DUMP_ABI_MAGIC_LEN; i++)
+		crash_abi->magic[i] = lttng_crash_magic_xor[i] ^ 0xFF;
+	crash_abi->mmap_length = shmobj->memory_map_size;
+	crash_abi->endian = RB_CRASH_ENDIAN;
+	crash_abi->major = RB_CRASH_DUMP_ABI_MAJOR;
+	crash_abi->minor = RB_CRASH_DUMP_ABI_MINOR;
+	crash_abi->word_size = sizeof(unsigned long);
+	crash_abi->layout_type = LTTNG_CRASH_TYPE_UST;
+
+	/* Offset of fields */
+	crash_abi->offset.prod_offset =
+		(uint32_t) ((char *) &buf->offset - (char *) buf);
+	crash_abi->offset.consumed_offset =
+		(uint32_t) ((char *) &buf->consumed - (char *) buf);
+	crash_abi->offset.commit_hot_array =
+		(uint32_t) ((char *) shmp(handle, buf->commit_hot) - (char *) buf);
+	crash_abi->offset.commit_hot_seq =
+		offsetof(struct commit_counters_hot, seq);
+	crash_abi->offset.buf_wsb_array =
+		(uint32_t) ((char *) shmp(handle, buf->backend.buf_wsb) - (char *) buf);
+	crash_abi->offset.buf_wsb_id =
+		offsetof(struct lttng_ust_lib_ring_buffer_backend_subbuffer, id);
+	crash_abi->offset.sb_array =
+		(uint32_t) ((char *) shmp(handle, buf->backend.array) - (char *) buf);
+	crash_abi->offset.sb_array_shmp_offset =
+		offsetof(struct lttng_ust_lib_ring_buffer_backend_pages_shmp,
+			shmp._ref.offset);
+	crash_abi->offset.sb_backend_p_offset =
+		offsetof(struct lttng_ust_lib_ring_buffer_backend_pages,
+			p._ref.offset);
+
+	/* Field length */
+	crash_abi->length.prod_offset = sizeof(buf->offset);
+	crash_abi->length.consumed_offset = sizeof(buf->consumed);
+	crash_abi->length.commit_hot_seq =
+		sizeof(((struct commit_counters_hot *) NULL)->seq);
+	crash_abi->length.buf_wsb_id =
+		sizeof(((struct lttng_ust_lib_ring_buffer_backend_subbuffer *) NULL)->id);
+	crash_abi->length.sb_array_shmp_offset =
+		sizeof(((struct lttng_ust_lib_ring_buffer_backend_pages_shmp *) NULL)->shmp._ref.offset);
+	crash_abi->length.sb_backend_p_offset =
+		sizeof(((struct lttng_ust_lib_ring_buffer_backend_pages *) NULL)->p._ref.offset);
+
+	/* Array stride */
+	crash_abi->stride.commit_hot_array =
+		sizeof(struct commit_counters_hot);
+	crash_abi->stride.buf_wsb_array =
+		sizeof(struct lttng_ust_lib_ring_buffer_backend_subbuffer);
+	crash_abi->stride.sb_array =
+		sizeof(struct lttng_ust_lib_ring_buffer_backend_pages_shmp);
+
+	/* Buffer constants */
+	crash_abi->buf_size = chanb->buf_size;
+	crash_abi->subbuf_size = chanb->subbuf_size;
+	crash_abi->num_subbuf = chanb->num_subbuf;
+	crash_abi->mode = (uint32_t) chanb->config.mode;
+
+	if (config->cb.content_size_field) {
+		size_t offset, length;
+
+		config->cb.content_size_field(config, &offset, &length);
+		crash_abi->offset.content_size = offset;
+		crash_abi->length.content_size = length;
+	} else {
+		crash_abi->offset.content_size = 0;
+		crash_abi->length.content_size = 0;
+	}
+	if (config->cb.packet_size_field) {
+		size_t offset, length;
+
+		config->cb.packet_size_field(config, &offset, &length);
+		crash_abi->offset.packet_size = offset;
+		crash_abi->length.packet_size = length;
+	} else {
+		crash_abi->offset.packet_size = 0;
+		crash_abi->length.packet_size = 0;
+	}
+}
+
 /*
  * Must be called under cpu hotplug protection.
  */
@@ -215,6 +332,9 @@ int lib_ring_buffer_create(struct lttng_ust_lib_ring_buffer *buf,
 {
 	const struct lttng_ust_lib_ring_buffer_config *config = &chanb->config;
 	struct channel *chan = caa_container_of(chanb, struct channel, backend);
+	struct lttng_ust_lib_ring_buffer_backend_subbuffer *wsb;
+	struct channel *shmp_chan;
+	struct commit_counters_hot *cc_hot;
 	void *priv = channel_get_private(chan);
 	size_t subbuf_header_size;
 	uint64_t tsc;
@@ -224,18 +344,12 @@ int lib_ring_buffer_create(struct lttng_ust_lib_ring_buffer *buf,
 	if (buf->backend.allocated)
 		return 0;
 
-	ret = lib_ring_buffer_backend_create(&buf->backend, &chan->backend,
-			cpu, handle, shmobj);
-	if (ret)
-		return ret;
-
 	align_shm(shmobj, __alignof__(struct commit_counters_hot));
 	set_shmp(buf->commit_hot,
 		 zalloc_shm(shmobj,
 			sizeof(struct commit_counters_hot) * chan->backend.num_subbuf));
 	if (!shmp(handle, buf->commit_hot)) {
-		ret = -ENOMEM;
-		goto free_chanbuf;
+		return -ENOMEM;
 	}
 
 	align_shm(shmobj, __alignof__(struct commit_counters_cold));
@@ -247,22 +361,47 @@ int lib_ring_buffer_create(struct lttng_ust_lib_ring_buffer *buf,
 		goto free_commit;
 	}
 
+	ret = lib_ring_buffer_backend_create(&buf->backend, &chan->backend,
+			cpu, handle, shmobj);
+	if (ret) {
+		goto free_init;
+	}
+
 	/*
 	 * Write the subbuffer header for first subbuffer so we know the total
 	 * duration of data gathering.
 	 */
 	subbuf_header_size = config->cb.subbuffer_header_size();
 	v_set(config, &buf->offset, subbuf_header_size);
-	subbuffer_id_clear_noref(config, &shmp_index(handle, buf->backend.buf_wsb, 0)->id);
-	tsc = config->cb.ring_buffer_clock_read(shmp(handle, buf->backend.chan));
+	wsb = shmp_index(handle, buf->backend.buf_wsb, 0);
+	if (!wsb) {
+		ret = -EPERM;
+		goto free_chanbuf;
+	}
+	subbuffer_id_clear_noref(config, &wsb->id);
+	shmp_chan = shmp(handle, buf->backend.chan);
+	if (!shmp_chan) {
+		ret = -EPERM;
+		goto free_chanbuf;
+	}
+	tsc = config->cb.ring_buffer_clock_read(shmp_chan);
 	config->cb.buffer_begin(buf, tsc, 0, handle);
-	v_add(config, subbuf_header_size, &shmp_index(handle, buf->commit_hot, 0)->cc);
+	cc_hot = shmp_index(handle, buf->commit_hot, 0);
+	if (!cc_hot) {
+		ret = -EPERM;
+		goto free_chanbuf;
+	}
+	v_add(config, subbuf_header_size, &cc_hot->cc);
+	v_add(config, subbuf_header_size, &cc_hot->seq);
 
 	if (config->cb.buffer_create) {
 		ret = config->cb.buffer_create(buf, priv, cpu, chanb->name, handle);
 		if (ret)
-			goto free_init;
+			goto free_chanbuf;
 	}
+
+	init_crash_abi(config, &buf->crash_abi, buf, chanb, shmobj, handle);
+
 	buf->backend.allocated = 1;
 	return 0;
 
@@ -299,6 +438,9 @@ void lib_ring_buffer_channel_switch_timer(int sig, siginfo_t *si, void *uc)
 		for_each_possible_cpu(cpu) {
 			struct lttng_ust_lib_ring_buffer *buf =
 				shmp(handle, chan->backend.buf[cpu].shmp);
+
+			if (!buf)
+				goto end;
 			if (uatomic_read(&buf->active_readers))
 				lib_ring_buffer_switch_slow(buf, SWITCH_ACTIVE,
 					chan->handle);
@@ -307,14 +449,128 @@ void lib_ring_buffer_channel_switch_timer(int sig, siginfo_t *si, void *uc)
 		struct lttng_ust_lib_ring_buffer *buf =
 			shmp(handle, chan->backend.buf[0].shmp);
 
+		if (!buf)
+			goto end;
 		if (uatomic_read(&buf->active_readers))
 			lib_ring_buffer_switch_slow(buf, SWITCH_ACTIVE,
 				chan->handle);
 	}
+end:
 	pthread_mutex_unlock(&wakeup_fd_mutex);
 	return;
 }
 
+static
+int lib_ring_buffer_poll_deliver(const struct lttng_ust_lib_ring_buffer_config *config,
+				 struct lttng_ust_lib_ring_buffer *buf,
+			         struct channel *chan,
+				 struct lttng_ust_shm_handle *handle)
+{
+	unsigned long consumed_old, consumed_idx, commit_count, write_offset;
+	struct commit_counters_cold *cc_cold;
+
+	consumed_old = uatomic_read(&buf->consumed);
+	consumed_idx = subbuf_index(consumed_old, chan);
+	cc_cold = shmp_index(handle, buf->commit_cold, consumed_idx);
+	if (!cc_cold)
+		return 0;
+	commit_count = v_read(config, &cc_cold->cc_sb);
+	/*
+	 * No memory barrier here, since we are only interested
+	 * in a statistically correct polling result. The next poll will
+	 * get the data is we are racing. The mb() that ensures correct
+	 * memory order is in get_subbuf.
+	 */
+	write_offset = v_read(config, &buf->offset);
+
+	/*
+	 * Check that the subbuffer we are trying to consume has been
+	 * already fully committed.
+	 */
+
+	if (((commit_count - chan->backend.subbuf_size)
+	     & chan->commit_count_mask)
+	    - (buf_trunc(consumed_old, chan)
+	       >> chan->backend.num_subbuf_order)
+	    != 0)
+		return 0;
+
+	/*
+	 * Check that we are not about to read the same subbuffer in
+	 * which the writer head is.
+	 */
+	if (subbuf_trunc(write_offset, chan) - subbuf_trunc(consumed_old, chan)
+	    == 0)
+		return 0;
+
+	return 1;
+}
+
+static
+void lib_ring_buffer_wakeup(struct lttng_ust_lib_ring_buffer *buf,
+		struct lttng_ust_shm_handle *handle)
+{
+	int wakeup_fd = shm_get_wakeup_fd(handle, &buf->self._ref);
+	sigset_t sigpipe_set, pending_set, old_set;
+	int ret, sigpipe_was_pending = 0;
+
+	if (wakeup_fd < 0)
+		return;
+
+	/*
+	 * Wake-up the other end by writing a null byte in the pipe
+	 * (non-blocking).  Important note: Because writing into the
+	 * pipe is non-blocking (and therefore we allow dropping wakeup
+	 * data, as long as there is wakeup data present in the pipe
+	 * buffer to wake up the consumer), the consumer should perform
+	 * the following sequence for waiting:
+	 * 1) empty the pipe (reads).
+	 * 2) check if there is data in the buffer.
+	 * 3) wait on the pipe (poll).
+	 *
+	 * Discard the SIGPIPE from write(), not disturbing any SIGPIPE
+	 * that might be already pending. If a bogus SIGPIPE is sent to
+	 * the entire process concurrently by a malicious user, it may
+	 * be simply discarded.
+	 */
+	ret = sigemptyset(&pending_set);
+	assert(!ret);
+	/*
+	 * sigpending returns the mask of signals that are _both_
+	 * blocked for the thread _and_ pending for either the thread or
+	 * the entire process.
+	 */
+	ret = sigpending(&pending_set);
+	assert(!ret);
+	sigpipe_was_pending = sigismember(&pending_set, SIGPIPE);
+	/*
+	 * If sigpipe was pending, it means it was already blocked, so
+	 * no need to block it.
+	 */
+	if (!sigpipe_was_pending) {
+		ret = sigemptyset(&sigpipe_set);
+		assert(!ret);
+		ret = sigaddset(&sigpipe_set, SIGPIPE);
+		assert(!ret);
+		ret = pthread_sigmask(SIG_BLOCK, &sigpipe_set, &old_set);
+		assert(!ret);
+	}
+	do {
+		ret = write(wakeup_fd, "", 1);
+	} while (ret == -1L && errno == EINTR);
+	if (ret == -1L && errno == EPIPE && !sigpipe_was_pending) {
+		struct timespec timeout = { 0, 0 };
+		do {
+			ret = sigtimedwait(&sigpipe_set, NULL,
+				&timeout);
+		} while (ret == -1L && errno == EINTR);
+	}
+	if (!sigpipe_was_pending) {
+		ret = pthread_sigmask(SIG_SETMASK, &old_set, NULL);
+		assert(!ret);
+	}
+}
+
 static
 void lib_ring_buffer_channel_do_read(struct channel *chan)
 {
@@ -334,6 +590,8 @@ void lib_ring_buffer_channel_do_read(struct channel *chan)
 			struct lttng_ust_lib_ring_buffer *buf =
 				shmp(handle, chan->backend.buf[cpu].shmp);
 
+			if (!buf)
+				goto end;
 			if (uatomic_read(&buf->active_readers)
 			    && lib_ring_buffer_poll_deliver(config, buf,
 					chan, handle)) {
@@ -344,12 +602,15 @@ void lib_ring_buffer_channel_do_read(struct channel *chan)
 		struct lttng_ust_lib_ring_buffer *buf =
 			shmp(handle, chan->backend.buf[0].shmp);
 
+		if (!buf)
+			goto end;
 		if (uatomic_read(&buf->active_readers)
 		    && lib_ring_buffer_poll_deliver(config, buf,
 				chan, handle)) {
 			lib_ring_buffer_wakeup(buf, handle);
 		}
 	}
+end:
 	pthread_mutex_unlock(&wakeup_fd_mutex);
 }
 
@@ -528,7 +789,7 @@ void lib_ring_buffer_channel_switch_timer_start(struct channel *chan)
 	}
 
 	its.it_value.tv_sec = chan->switch_timer_interval / 1000000;
-	its.it_value.tv_nsec = chan->switch_timer_interval % 1000000;
+	its.it_value.tv_nsec = (chan->switch_timer_interval % 1000000) * 1000;
 	its.it_interval.tv_sec = its.it_value.tv_sec;
 	its.it_interval.tv_nsec = its.it_value.tv_nsec;
 
@@ -582,7 +843,7 @@ void lib_ring_buffer_channel_read_timer_start(struct channel *chan)
 	}
 
 	its.it_value.tv_sec = chan->read_timer_interval / 1000000;
-	its.it_value.tv_nsec = chan->read_timer_interval % 1000000;
+	its.it_value.tv_nsec = (chan->read_timer_interval % 1000000) * 1000;
 	its.it_interval.tv_sec = its.it_value.tv_sec;
 	its.it_interval.tv_nsec = its.it_value.tv_nsec;
 
@@ -637,22 +898,25 @@ static void channel_print_errors(struct channel *chan,
 		for_each_possible_cpu(cpu) {
 			struct lttng_ust_lib_ring_buffer *buf =
 				shmp(handle, chan->backend.buf[cpu].shmp);
-			lib_ring_buffer_print_errors(chan, buf, cpu, handle);
+			if (buf)
+				lib_ring_buffer_print_errors(chan, buf, cpu, handle);
 		}
 	} else {
 		struct lttng_ust_lib_ring_buffer *buf =
 			shmp(handle, chan->backend.buf[0].shmp);
 
-		lib_ring_buffer_print_errors(chan, buf, -1, handle);
+		if (buf)
+			lib_ring_buffer_print_errors(chan, buf, -1, handle);
 	}
 }
 
 static void channel_free(struct channel *chan,
-		struct lttng_ust_shm_handle *handle)
+		struct lttng_ust_shm_handle *handle,
+		int consumer)
 {
 	channel_backend_free(&chan->backend, handle);
 	/* chan is freed by shm teardown */
-	shm_object_table_destroy(handle->table);
+	shm_object_table_destroy(handle->table, consumer);
 	free(handle);
 }
 
@@ -672,6 +936,8 @@ static void channel_free(struct channel *chan,
  *                         padding to let readers get those sub-buffers.
  *                         Used for live streaming.
  * @read_timer_interval: Time interval (in us) to wake up pending readers.
+ * @stream_fds: array of stream file descriptors.
+ * @nr_stream_fds: number of file descriptors in array.
  *
  * Holds cpu hotplug.
  * Returns NULL on failure.
@@ -684,7 +950,8 @@ struct lttng_ust_shm_handle *channel_create(const struct lttng_ust_lib_ring_buff
 		   void *priv_data_init,
 		   void *buf_addr, size_t subbuf_size,
 		   size_t num_subbuf, unsigned int switch_timer_interval,
-		   unsigned int read_timer_interval)
+		   unsigned int read_timer_interval,
+		   const int *stream_fds, int nr_stream_fds)
 {
 	int ret;
 	size_t shmsize, chansize;
@@ -698,6 +965,9 @@ struct lttng_ust_shm_handle *channel_create(const struct lttng_ust_lib_ring_buff
 	else
 		nr_streams = 1;
 
+	if (nr_stream_fds != nr_streams)
+		return NULL;
+
 	if (lib_ring_buffer_check_config(config, switch_timer_interval,
 					 read_timer_interval))
 		return NULL;
@@ -721,7 +991,8 @@ struct lttng_ust_shm_handle *channel_create(const struct lttng_ust_lib_ring_buff
 	shmsize += priv_data_size;
 
 	/* Allocate normal memory for channel (not shared) */
-	shmobj = shm_object_table_alloc(handle->table, shmsize, SHM_OBJECT_MEM);
+	shmobj = shm_object_table_alloc(handle->table, shmsize, SHM_OBJECT_MEM,
+			-1);
 	if (!shmobj)
 		goto error_append;
 	/* struct channel is at object 0, offset 0 (hardcoded) */
@@ -751,7 +1022,8 @@ struct lttng_ust_shm_handle *channel_create(const struct lttng_ust_lib_ring_buff
 	}
 
 	ret = channel_backend_init(&chan->backend, name, config,
-				   subbuf_size, num_subbuf, handle);
+				   subbuf_size, num_subbuf, handle,
+				   stream_fds);
 	if (ret)
 		goto error_backend_init;
 
@@ -767,7 +1039,7 @@ struct lttng_ust_shm_handle *channel_create(const struct lttng_ust_lib_ring_buff
 
 error_backend_init:
 error_append:
-	shm_object_table_destroy(handle->table);
+	shm_object_table_destroy(handle->table, 1);
 error_table_alloc:
 	free(handle);
 	return NULL;
@@ -799,7 +1071,7 @@ struct lttng_ust_shm_handle *channel_handle_create(void *data,
 	return handle;
 
 error_table_object:
-	shm_object_table_destroy(handle->table);
+	shm_object_table_destroy(handle->table, 0);
 error_table_alloc:
 	free(handle);
 	return NULL;
@@ -827,9 +1099,10 @@ unsigned int channel_handle_get_nr_streams(struct lttng_ust_shm_handle *handle)
 }
 
 static
-void channel_release(struct channel *chan, struct lttng_ust_shm_handle *handle)
+void channel_release(struct channel *chan, struct lttng_ust_shm_handle *handle,
+		int consumer)
 {
-	channel_free(chan, handle);
+	channel_free(chan, handle, consumer);
 }
 
 /**
@@ -861,7 +1134,7 @@ void channel_destroy(struct channel *chan, struct lttng_ust_shm_handle *handle,
 	 * sessiond/consumer are keeping a reference on the shm file
 	 * descriptor directly. No need to refcount.
 	 */
-	channel_release(chan, handle);
+	channel_release(chan, handle, consumer);
 	return;
 }
 
@@ -962,6 +1235,8 @@ void lib_ring_buffer_release_read(struct lttng_ust_lib_ring_buffer *buf,
 {
 	struct channel *chan = shmp(handle, buf->backend.chan);
 
+	if (!chan)
+		return;
 	CHAN_WARN_ON(chan, uatomic_read(&buf->active_readers) != 1);
 	cmm_smp_mb();
 	uatomic_dec(&buf->active_readers);
@@ -981,11 +1256,15 @@ int lib_ring_buffer_snapshot(struct lttng_ust_lib_ring_buffer *buf,
 			     unsigned long *consumed, unsigned long *produced,
 			     struct lttng_ust_shm_handle *handle)
 {
-	struct channel *chan = shmp(handle, buf->backend.chan);
-	const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
+	struct channel *chan;
+	const struct lttng_ust_lib_ring_buffer_config *config;
 	unsigned long consumed_cur, write_offset;
 	int finalized;
 
+	chan = shmp(handle, buf->backend.chan);
+	if (!chan)
+		return -EPERM;
+	config = &chan->backend.config;
 	finalized = CMM_ACCESS_ONCE(buf->finalized);
 	/*
 	 * Read finalized before counters.
@@ -1026,6 +1305,43 @@ nodata:
 		return -EAGAIN;
 }
 
+/**
+ * Performs the same function as lib_ring_buffer_snapshot(), but the positions
+ * are saved regardless of whether the consumed and produced positions are
+ * in the same subbuffer.
+ * @buf: ring buffer
+ * @consumed: consumed byte count indicating the last position read
+ * @produced: produced byte count indicating the last position written
+ *
+ * This function is meant to provide information on the exact producer and
+ * consumer positions without regard for the "snapshot" feature.
+ */
+int lib_ring_buffer_snapshot_sample_positions(
+			     struct lttng_ust_lib_ring_buffer *buf,
+			     unsigned long *consumed, unsigned long *produced,
+			     struct lttng_ust_shm_handle *handle)
+{
+	struct channel *chan;
+	const struct lttng_ust_lib_ring_buffer_config *config;
+
+	chan = shmp(handle, buf->backend.chan);
+	if (!chan)
+		return -EPERM;
+	config = &chan->backend.config;
+	cmm_smp_rmb();
+	*consumed = uatomic_read(&buf->consumed);
+	/*
+	 * No need to issue a memory barrier between consumed count read and
+	 * write offset read, because consumed count can only change
+	 * concurrently in overwrite mode, and we keep a sequence counter
+	 * identifier derived from the write offset to check we are getting
+	 * the same sub-buffer we are expecting (the sub-buffers are atomically
+	 * "tagged" upon writes, tags are checked upon read).
+	 */
+	*produced = v_read(config, &buf->offset);
+	return 0;
+}
+
 /**
  * lib_ring_buffer_move_consumer - move consumed counter forward
  * @buf: ring buffer
@@ -1036,9 +1352,12 @@ void lib_ring_buffer_move_consumer(struct lttng_ust_lib_ring_buffer *buf,
 				   struct lttng_ust_shm_handle *handle)
 {
 	struct lttng_ust_lib_ring_buffer_backend *bufb = &buf->backend;
-	struct channel *chan = shmp(handle, bufb->chan);
+	struct channel *chan;
 	unsigned long consumed;
 
+	chan = shmp(handle, bufb->chan);
+	if (!chan)
+		return;
 	CHAN_WARN_ON(chan, uatomic_read(&buf->active_readers) != 1);
 
 	/*
@@ -1064,12 +1383,16 @@ int lib_ring_buffer_get_subbuf(struct lttng_ust_lib_ring_buffer *buf,
 			       unsigned long consumed,
 			       struct lttng_ust_shm_handle *handle)
 {
-	struct channel *chan = shmp(handle, buf->backend.chan);
-	const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
+	struct channel *chan;
+	const struct lttng_ust_lib_ring_buffer_config *config;
 	unsigned long consumed_cur, consumed_idx, commit_count, write_offset;
-	int ret;
-	int finalized;
+	int ret, finalized, nr_retry = LTTNG_UST_RING_BUFFER_GET_RETRY;
+	struct commit_counters_cold *cc_cold;
 
+	chan = shmp(handle, buf->backend.chan);
+	if (!chan)
+		return -EPERM;
+	config = &chan->backend.config;
 retry:
 	finalized = CMM_ACCESS_ONCE(buf->finalized);
 	/*
@@ -1078,7 +1401,10 @@ retry:
 	cmm_smp_rmb();
 	consumed_cur = uatomic_read(&buf->consumed);
 	consumed_idx = subbuf_index(consumed, chan);
-	commit_count = v_read(config, &shmp_index(handle, buf->commit_cold, consumed_idx)->cc_sb);
+	cc_cold = shmp_index(handle, buf->commit_cold, consumed_idx);
+	if (!cc_cold)
+		return -EPERM;
+	commit_count = v_read(config, &cc_cold->cc_sb);
 	/*
 	 * Make sure we read the commit count before reading the buffer
 	 * data and the write offset. Correct consumed offset ordering
@@ -1103,14 +1429,66 @@ retry:
 
 	/*
 	 * Check that the subbuffer we are trying to consume has been
-	 * already fully committed.
+	 * already fully committed. There are a few causes that can make
+	 * this unavailability situation occur:
+	 *
+	 * Temporary (short-term) situation:
+	 * - Application is running on a different CPU, between reserve
+	 *   and commit ring buffer operations,
+	 * - Application is preempted between reserve and commit ring
+	 *   buffer operations,
+	 *
+	 * Long-term situation:
+	 * - Application is stopped (SIGSTOP) between reserve and commit
+	 *   ring buffer operations. Could eventually be resumed by
+	 *   SIGCONT.
+	 * - Application is killed (SIGTERM, SIGINT, SIGKILL) between
+	 *   reserve and commit ring buffer operation.
+	 *
+	 * From a consumer perspective, handling short-term
+	 * unavailability situations is performed by retrying a few
+	 * times after a delay. Handling long-term unavailability
+	 * situations is handled by failing to get the sub-buffer.
+	 *
+	 * In all of those situations, if the application is taking a
+	 * long time to perform its commit after ring buffer space
+	 * reservation, we can end up in a situation where the producer
+	 * will fill the ring buffer and try to write into the same
+	 * sub-buffer again (which has a missing commit). This is
+	 * handled by the producer in the sub-buffer switch handling
+	 * code of the reserve routine by detecting unbalanced
+	 * reserve/commit counters and discarding all further events
+	 * until the situation is resolved in those situations. Two
+	 * scenarios can occur:
+	 *
+	 * 1) The application causing the reserve/commit counters to be
+	 *    unbalanced has been terminated. In this situation, all
+	 *    further events will be discarded in the buffers, and no
+	 *    further buffer data will be readable by the consumer
+	 *    daemon. Tearing down the UST tracing session and starting
+	 *    anew is a work-around for those situations. Note that this
+	 *    only affects per-UID tracing. In per-PID tracing, the
+	 *    application vanishes with the termination, and therefore
+	 *    no more data needs to be written to the buffers.
+	 * 2) The application causing the unbalance has been delayed for
+	 *    a long time, but will eventually try to increment the
+	 *    commit counter after eventually writing to the sub-buffer.
+	 *    This situation can cause events to be discarded until the
+	 *    application resumes its operations.
 	 */
 	if (((commit_count - chan->backend.subbuf_size)
 	     & chan->commit_count_mask)
 	    - (buf_trunc(consumed, chan)
 	       >> chan->backend.num_subbuf_order)
-	    != 0)
-		goto nodata;
+	    != 0) {
+		if (nr_retry-- > 0) {
+			if (nr_retry <= (LTTNG_UST_RING_BUFFER_GET_RETRY >> 1))
+				(void) poll(NULL, 0, LTTNG_UST_RING_BUFFER_RETRY_DELAY_MS);
+			goto retry;
+		} else {
+			goto nodata;
+		}
+	}
 
 	/*
 	 * Check that we are not about to read the same subbuffer in
@@ -1126,12 +1504,23 @@ retry:
 	 * the writer is getting access to a subbuffer we were trying to get
 	 * access to. Also checks that the "consumed" buffer count we are
 	 * looking for matches the one contained in the subbuffer id.
+	 *
+	 * The short-lived race window described here can be affected by
+	 * application signals and preemption, thus requiring to bound
+	 * the loop to a maximum number of retry.
 	 */
 	ret = update_read_sb_index(config, &buf->backend, &chan->backend,
 				   consumed_idx, buf_trunc_val(consumed, chan),
 				   handle);
-	if (ret)
-		goto retry;
+	if (ret) {
+		if (nr_retry-- > 0) {
+			if (nr_retry <= (LTTNG_UST_RING_BUFFER_GET_RETRY >> 1))
+				(void) poll(NULL, 0, LTTNG_UST_RING_BUFFER_RETRY_DELAY_MS);
+			goto retry;
+		} else {
+			goto nodata;
+		}
+	}
 	subbuffer_id_clear_noref(config, &buf->backend.buf_rsb.id);
 
 	buf->get_subbuf_consumed = consumed;
@@ -1158,10 +1547,16 @@ void lib_ring_buffer_put_subbuf(struct lttng_ust_lib_ring_buffer *buf,
 				struct lttng_ust_shm_handle *handle)
 {
 	struct lttng_ust_lib_ring_buffer_backend *bufb = &buf->backend;
-	struct channel *chan = shmp(handle, bufb->chan);
-	const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
-	unsigned long read_sb_bindex, consumed_idx, consumed;
+	struct channel *chan;
+	const struct lttng_ust_lib_ring_buffer_config *config;
+	unsigned long sb_bindex, consumed_idx, consumed;
+	struct lttng_ust_lib_ring_buffer_backend_pages_shmp *rpages;
+	struct lttng_ust_lib_ring_buffer_backend_pages *backend_pages;
 
+	chan = shmp(handle, bufb->chan);
+	if (!chan)
+		return;
+	config = &chan->backend.config;
 	CHAN_WARN_ON(chan, uatomic_read(&buf->active_readers) != 1);
 
 	if (!buf->get_subbuf) {
@@ -1181,11 +1576,16 @@ void lib_ring_buffer_put_subbuf(struct lttng_ust_lib_ring_buffer *buf,
 	 * Can be below zero if an iterator is used on a snapshot more than
 	 * once.
 	 */
-	read_sb_bindex = subbuffer_id_get_index(config, bufb->buf_rsb.id);
-	v_add(config, v_read(config,
-			     &shmp(handle, shmp_index(handle, bufb->array, read_sb_bindex)->shmp)->records_unread),
-	      &bufb->records_read);
-	v_set(config, &shmp(handle, shmp_index(handle, bufb->array, read_sb_bindex)->shmp)->records_unread, 0);
+	sb_bindex = subbuffer_id_get_index(config, bufb->buf_rsb.id);
+	rpages = shmp_index(handle, bufb->array, sb_bindex);
+	if (!rpages)
+		return;
+	backend_pages = shmp(handle, rpages->shmp);
+	if (!backend_pages)
+		return;
+	v_add(config, v_read(config, &backend_pages->records_unread),
+			&bufb->records_read);
+	v_set(config, &backend_pages->records_unread, 0);
 	CHAN_WARN_ON(chan, config->mode == RING_BUFFER_OVERWRITE
 		     && subbuffer_id_is_noref(config, bufb->buf_rsb.id));
 	subbuffer_id_set_noref(config, &bufb->buf_rsb.id);
@@ -1221,10 +1621,18 @@ void lib_ring_buffer_print_subbuffer_errors(struct lttng_ust_lib_ring_buffer *bu
 {
 	const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
 	unsigned long cons_idx, commit_count, commit_count_sb;
+	struct commit_counters_hot *cc_hot;
+	struct commit_counters_cold *cc_cold;
 
 	cons_idx = subbuf_index(cons_offset, chan);
-	commit_count = v_read(config, &shmp_index(handle, buf->commit_hot, cons_idx)->cc);
-	commit_count_sb = v_read(config, &shmp_index(handle, buf->commit_cold, cons_idx)->cc_sb);
+	cc_hot = shmp_index(handle, buf->commit_hot, cons_idx);
+	if (!cc_hot)
+		return;
+	cc_cold = shmp_index(handle, buf->commit_cold, cons_idx);
+	if (!cc_cold)
+		return;
+	commit_count = v_read(config, &cc_hot->cc);
+	commit_count_sb = v_read(config, &cc_cold->cc_sb);
 
 	if (subbuf_offset(commit_count, chan) != 0)
 		DBG("ring buffer %s, cpu %d: "
@@ -1308,7 +1716,8 @@ void lib_ring_buffer_print_errors(struct channel *chan,
 /*
  * lib_ring_buffer_switch_old_start: Populate old subbuffer header.
  *
- * Only executed when the buffer is finalized, in SWITCH_FLUSH.
+ * Only executed by SWITCH_FLUSH, which can be issued while tracing is
+ * active or at buffer finalization (destroy).
  */
 static
 void lib_ring_buffer_switch_old_start(struct lttng_ust_lib_ring_buffer *buf,
@@ -1320,6 +1729,7 @@ void lib_ring_buffer_switch_old_start(struct lttng_ust_lib_ring_buffer *buf,
 	const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
 	unsigned long oldidx = subbuf_index(offsets->old, chan);
 	unsigned long commit_count;
+	struct commit_counters_hot *cc_hot;
 
 	config->cb.buffer_begin(buf, tsc, oldidx, handle);
 
@@ -1328,15 +1738,18 @@ void lib_ring_buffer_switch_old_start(struct lttng_ust_lib_ring_buffer *buf,
 	 * determine that the subbuffer is full.
 	 */
 	cmm_smp_wmb();
+	cc_hot = shmp_index(handle, buf->commit_hot, oldidx);
+	if (!cc_hot)
+		return;
 	v_add(config, config->cb.subbuffer_header_size(),
-	      &shmp_index(handle, buf->commit_hot, oldidx)->cc);
-	commit_count = v_read(config, &shmp_index(handle, buf->commit_hot, oldidx)->cc);
+	      &cc_hot->cc);
+	commit_count = v_read(config, &cc_hot->cc);
 	/* Check if the written buffer has to be delivered */
 	lib_ring_buffer_check_deliver(config, buf, chan, offsets->old,
 				      commit_count, oldidx, handle, tsc);
-	lib_ring_buffer_write_commit_counter(config, buf, chan, oldidx,
+	lib_ring_buffer_write_commit_counter(config, buf, chan,
 			offsets->old + config->cb.subbuffer_header_size(),
-			commit_count, handle);
+			commit_count, handle, cc_hot);
 }
 
 /*
@@ -1357,6 +1770,7 @@ void lib_ring_buffer_switch_old_end(struct lttng_ust_lib_ring_buffer *buf,
 	const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
 	unsigned long oldidx = subbuf_index(offsets->old - 1, chan);
 	unsigned long commit_count, padding_size, data_size;
+	struct commit_counters_hot *cc_hot;
 
 	data_size = subbuf_offset(offsets->old - 1, chan) + 1;
 	padding_size = chan->backend.subbuf_size - data_size;
@@ -1368,12 +1782,16 @@ void lib_ring_buffer_switch_old_end(struct lttng_ust_lib_ring_buffer *buf,
 	 * determine that the subbuffer is full.
 	 */
 	cmm_smp_wmb();
-	v_add(config, padding_size, &shmp_index(handle, buf->commit_hot, oldidx)->cc);
-	commit_count = v_read(config, &shmp_index(handle, buf->commit_hot, oldidx)->cc);
+	cc_hot = shmp_index(handle, buf->commit_hot, oldidx);
+	if (!cc_hot)
+		return;
+	v_add(config, padding_size, &cc_hot->cc);
+	commit_count = v_read(config, &cc_hot->cc);
 	lib_ring_buffer_check_deliver(config, buf, chan, offsets->old - 1,
 				      commit_count, oldidx, handle, tsc);
-	lib_ring_buffer_write_commit_counter(config, buf, chan, oldidx,
-			offsets->old + padding_size, commit_count, handle);
+	lib_ring_buffer_write_commit_counter(config, buf, chan,
+			offsets->old + padding_size, commit_count, handle,
+			cc_hot);
 }
 
 /*
@@ -1393,6 +1811,7 @@ void lib_ring_buffer_switch_new_start(struct lttng_ust_lib_ring_buffer *buf,
 	const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
 	unsigned long beginidx = subbuf_index(offsets->begin, chan);
 	unsigned long commit_count;
+	struct commit_counters_hot *cc_hot;
 
 	config->cb.buffer_begin(buf, tsc, beginidx, handle);
 
@@ -1401,15 +1820,17 @@ void lib_ring_buffer_switch_new_start(struct lttng_ust_lib_ring_buffer *buf,
 	 * determine that the subbuffer is full.
 	 */
 	cmm_smp_wmb();
-	v_add(config, config->cb.subbuffer_header_size(),
-	      &shmp_index(handle, buf->commit_hot, beginidx)->cc);
-	commit_count = v_read(config, &shmp_index(handle, buf->commit_hot, beginidx)->cc);
+	cc_hot = shmp_index(handle, buf->commit_hot, beginidx);
+	if (!cc_hot)
+		return;
+	v_add(config, config->cb.subbuffer_header_size(), &cc_hot->cc);
+	commit_count = v_read(config, &cc_hot->cc);
 	/* Check if the written buffer has to be delivered */
 	lib_ring_buffer_check_deliver(config, buf, chan, offsets->begin,
 				      commit_count, beginidx, handle, tsc);
-	lib_ring_buffer_write_commit_counter(config, buf, chan, beginidx,
+	lib_ring_buffer_write_commit_counter(config, buf, chan,
 			offsets->begin + config->cb.subbuffer_header_size(),
-			commit_count, handle);
+			commit_count, handle, cc_hot);
 }
 
 /*
@@ -1480,23 +1901,27 @@ int lib_ring_buffer_try_switch_slow(enum switch_mode mode,
 
 	if (caa_unlikely(off == 0)) {
 		unsigned long sb_index, commit_count;
+		struct commit_counters_cold *cc_cold;
 
 		/*
-		 * We are performing a SWITCH_FLUSH. At this stage, there are no
-		 * concurrent writes into the buffer.
+		 * We are performing a SWITCH_FLUSH. There may be concurrent
+		 * writes into the buffer if e.g. invoked while performing a
+		 * snapshot on an active trace.
 		 *
-		 * The client does not save any header information.  Don't
-		 * switch empty subbuffer on finalize, because it is invalid to
-		 * deliver a completely empty subbuffer.
+		 * If the client does not save any header information
+		 * (sub-buffer header size == 0), don't switch empty subbuffer
+		 * on finalize, because it is invalid to deliver a completely
+		 * empty subbuffer.
 		 */
 		if (!config->cb.subbuffer_header_size())
 			return -1;
 
 		/* Test new buffer integrity */
 		sb_index = subbuf_index(offsets->begin, chan);
-		commit_count = v_read(config,
-				&shmp_index(handle, buf->commit_cold,
-					sb_index)->cc_sb);
+		cc_cold = shmp_index(handle, buf->commit_cold, sb_index);
+		if (!cc_cold)
+			return -1;
+		commit_count = v_read(config, &cc_cold->cc_sb);
 		reserve_commit_diff =
 		  (buf_trunc(offsets->begin, chan)
 		   >> chan->backend.num_subbuf_order)
@@ -1554,12 +1979,17 @@ int lib_ring_buffer_try_switch_slow(enum switch_mode mode,
 void lib_ring_buffer_switch_slow(struct lttng_ust_lib_ring_buffer *buf, enum switch_mode mode,
 				 struct lttng_ust_shm_handle *handle)
 {
-	struct channel *chan = shmp(handle, buf->backend.chan);
-	const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
+	struct channel *chan;
+	const struct lttng_ust_lib_ring_buffer_config *config;
 	struct switch_offsets offsets;
 	unsigned long oldidx;
 	uint64_t tsc;
 
+	chan = shmp(handle, buf->backend.chan);
+	if (!chan)
+		return;
+	config = &chan->backend.config;
+
 	offsets.size = 0;
 
 	/*
@@ -1602,6 +2032,23 @@ void lib_ring_buffer_switch_slow(struct lttng_ust_lib_ring_buffer *buf, enum swi
 	lib_ring_buffer_switch_old_end(buf, chan, &offsets, tsc, handle);
 }
 
+static
+bool handle_blocking_retry(int *timeout_left_ms)
+{
+	int timeout = *timeout_left_ms, delay;
+
+	if (caa_likely(!timeout))
+		return false;	/* Do not retry, discard event. */
+	if (timeout < 0)	/* Wait forever. */
+		delay = RETRY_DELAY_MS;
+	else
+		delay = min_t(int, timeout, RETRY_DELAY_MS);
+	(void) poll(NULL, 0, delay);
+	if (timeout > 0)
+		*timeout_left_ms -= delay;
+	return true;	/* Retry. */
+}
+
 /*
  * Returns :
  * 0 if ok
@@ -1613,11 +2060,13 @@ static
 int lib_ring_buffer_try_reserve_slow(struct lttng_ust_lib_ring_buffer *buf,
 				     struct channel *chan,
 				     struct switch_offsets *offsets,
-				     struct lttng_ust_lib_ring_buffer_ctx *ctx)
+				     struct lttng_ust_lib_ring_buffer_ctx *ctx,
+				     void *client_ctx)
 {
 	const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
 	struct lttng_ust_shm_handle *handle = ctx->handle;
 	unsigned long reserve_commit_diff, offset_cmp;
+	int timeout_left_ms = lttng_ust_blocking_retry_timeout;
 
 retry:
 	offsets->begin = offset_cmp = v_read(config, &buf->offset);
@@ -1640,7 +2089,7 @@ retry:
 		offsets->size = config->cb.record_header_size(config, chan,
 						offsets->begin,
 						&offsets->pre_header_padding,
-						ctx);
+						ctx, client_ctx);
 		offsets->size +=
 			lib_ring_buffer_align(offsets->begin + offsets->size,
 					      ctx->largest_align)
@@ -1653,6 +2102,7 @@ retry:
 	}
 	if (caa_unlikely(offsets->switch_new_start)) {
 		unsigned long sb_index, commit_count;
+		struct commit_counters_cold *cc_cold;
 
 		/*
 		 * We are typically not filling the previous buffer completely.
@@ -1671,9 +2121,10 @@ retry:
 		 * are not seen reordered when updated by another CPU.
 		 */
 		cmm_smp_rmb();
-		commit_count = v_read(config,
-				&shmp_index(handle, buf->commit_cold,
-					sb_index)->cc_sb);
+		cc_cold = shmp_index(handle, buf->commit_cold, sb_index);
+		if (!cc_cold)
+			return -1;
+		commit_count = v_read(config, &cc_cold->cc_sb);
 		/* Read buf->commit_cold[sb_index].cc_sb before buf->offset. */
 		cmm_smp_rmb();
 		if (caa_unlikely(offset_cmp != v_read(config, &buf->offset))) {
@@ -1698,6 +2149,9 @@ retry:
 				>= chan->backend.buf_size)) {
 				unsigned long nr_lost;
 
+				if (handle_blocking_retry(&timeout_left_ms))
+					goto retry;
+
 				/*
 				 * We do not overwrite non consumed buffers
 				 * and we are full : record is lost.
@@ -1741,7 +2195,7 @@ retry:
 			config->cb.record_header_size(config, chan,
 						offsets->begin,
 						&offsets->pre_header_padding,
-						ctx);
+						ctx, client_ctx);
 		offsets->size +=
 			lib_ring_buffer_align(offsets->begin + offsets->size,
 					      ctx->largest_align)
@@ -1795,7 +2249,8 @@ retry:
  * -EIO for other errors, else returns 0.
  * It will take care of sub-buffer switching.
  */
-int lib_ring_buffer_reserve_slow(struct lttng_ust_lib_ring_buffer_ctx *ctx)
+int lib_ring_buffer_reserve_slow(struct lttng_ust_lib_ring_buffer_ctx *ctx,
+		void *client_ctx)
 {
 	struct channel *chan = ctx->chan;
 	struct lttng_ust_shm_handle *handle = ctx->handle;
@@ -1808,13 +2263,15 @@ int lib_ring_buffer_reserve_slow(struct lttng_ust_lib_ring_buffer_ctx *ctx)
 		buf = shmp(handle, chan->backend.buf[ctx->cpu].shmp);
 	else
 		buf = shmp(handle, chan->backend.buf[0].shmp);
+	if (!buf)
+		return -EIO;
 	ctx->buf = buf;
 
 	offsets.size = 0;
 
 	do {
 		ret = lib_ring_buffer_try_reserve_slow(buf, chan, &offsets,
-						       ctx);
+						       ctx, client_ctx);
 		if (caa_unlikely(ret))
 			return ret;
 	} while (caa_unlikely(v_cmpxchg(config, &buf->offset, offsets.old,
@@ -1866,6 +2323,158 @@ int lib_ring_buffer_reserve_slow(struct lttng_ust_lib_ring_buffer_ctx *ctx)
 	return 0;
 }
 
+static
+void lib_ring_buffer_vmcore_check_deliver(const struct lttng_ust_lib_ring_buffer_config *config,
+					  struct lttng_ust_lib_ring_buffer *buf,
+				          unsigned long commit_count,
+				          unsigned long idx,
+					  struct lttng_ust_shm_handle *handle)
+{
+	struct commit_counters_hot *cc_hot;
+
+	if (config->oops != RING_BUFFER_OOPS_CONSISTENCY)
+		return;
+	cc_hot = shmp_index(handle, buf->commit_hot, idx);
+	if (!cc_hot)
+		return;
+	v_set(config, &cc_hot->seq, commit_count);
+}
+
+/*
+ * The ring buffer can count events recorded and overwritten per buffer,
+ * but it is disabled by default due to its performance overhead.
+ */
+#ifdef LTTNG_RING_BUFFER_COUNT_EVENTS
+static
+void deliver_count_events(const struct lttng_ust_lib_ring_buffer_config *config,
+		struct lttng_ust_lib_ring_buffer *buf,
+		unsigned long idx,
+		struct lttng_ust_shm_handle *handle)
+{
+	v_add(config, subbuffer_get_records_count(config,
+			&buf->backend, idx, handle),
+		&buf->records_count);
+	v_add(config, subbuffer_count_records_overrun(config,
+			&buf->backend, idx, handle),
+		&buf->records_overrun);
+}
+#else /* LTTNG_RING_BUFFER_COUNT_EVENTS */
+static
+void deliver_count_events(const struct lttng_ust_lib_ring_buffer_config *config,
+		struct lttng_ust_lib_ring_buffer *buf,
+		unsigned long idx,
+		struct lttng_ust_shm_handle *handle)
+{
+}
+#endif /* #else LTTNG_RING_BUFFER_COUNT_EVENTS */
+
+void lib_ring_buffer_check_deliver_slow(const struct lttng_ust_lib_ring_buffer_config *config,
+				   struct lttng_ust_lib_ring_buffer *buf,
+			           struct channel *chan,
+			           unsigned long offset,
+				   unsigned long commit_count,
+			           unsigned long idx,
+				   struct lttng_ust_shm_handle *handle,
+				   uint64_t tsc)
+{
+	unsigned long old_commit_count = commit_count
+					 - chan->backend.subbuf_size;
+	struct commit_counters_cold *cc_cold;
+
+	/*
+	 * If we succeeded at updating cc_sb below, we are the subbuffer
+	 * writer delivering the subbuffer. Deals with concurrent
+	 * updates of the "cc" value without adding a add_return atomic
+	 * operation to the fast path.
+	 *
+	 * We are doing the delivery in two steps:
+	 * - First, we cmpxchg() cc_sb to the new value
+	 *   old_commit_count + 1. This ensures that we are the only
+	 *   subbuffer user successfully filling the subbuffer, but we
+	 *   do _not_ set the cc_sb value to "commit_count" yet.
+	 *   Therefore, other writers that would wrap around the ring
+	 *   buffer and try to start writing to our subbuffer would
+	 *   have to drop records, because it would appear as
+	 *   non-filled.
+	 *   We therefore have exclusive access to the subbuffer control
+	 *   structures.  This mutual exclusion with other writers is
+	 *   crucially important to perform record overruns count in
+	 *   flight recorder mode locklessly.
+	 * - When we are ready to release the subbuffer (either for
+	 *   reading or for overrun by other writers), we simply set the
+	 *   cc_sb value to "commit_count" and perform delivery.
+	 *
+	 * The subbuffer size is least 2 bytes (minimum size: 1 page).
+	 * This guarantees that old_commit_count + 1 != commit_count.
+	 */
+
+	/*
+	 * Order prior updates to reserve count prior to the
+	 * commit_cold cc_sb update.
+	 */
+	cmm_smp_wmb();
+	cc_cold = shmp_index(handle, buf->commit_cold, idx);
+	if (!cc_cold)
+		return;
+	if (caa_likely(v_cmpxchg(config, &cc_cold->cc_sb,
+				 old_commit_count, old_commit_count + 1)
+		   == old_commit_count)) {
+		/*
+		 * Start of exclusive subbuffer access. We are
+		 * guaranteed to be the last writer in this subbuffer
+		 * and any other writer trying to access this subbuffer
+		 * in this state is required to drop records.
+		 */
+		deliver_count_events(config, buf, idx, handle);
+		config->cb.buffer_end(buf, tsc, idx,
+				      lib_ring_buffer_get_data_size(config,
+								buf,
+								idx,
+								handle),
+				      handle);
+
+		/*
+		 * Increment the packet counter while we have exclusive
+		 * access.
+		 */
+		subbuffer_inc_packet_count(config, &buf->backend, idx, handle);
+
+		/*
+		 * Set noref flag and offset for this subbuffer id.
+		 * Contains a memory barrier that ensures counter stores
+		 * are ordered before set noref and offset.
+		 */
+		lib_ring_buffer_set_noref_offset(config, &buf->backend, idx,
+						 buf_trunc_val(offset, chan), handle);
+
+		/*
+		 * Order set_noref and record counter updates before the
+		 * end of subbuffer exclusive access. Orders with
+		 * respect to writers coming into the subbuffer after
+		 * wrap around, and also order wrt concurrent readers.
+		 */
+		cmm_smp_mb();
+		/* End of exclusive subbuffer access */
+		v_set(config, &cc_cold->cc_sb, commit_count);
+		/*
+		 * Order later updates to reserve count after
+		 * the commit cold cc_sb update.
+		 */
+		cmm_smp_wmb();
+		lib_ring_buffer_vmcore_check_deliver(config, buf,
+					 commit_count, idx, handle);
+
+		/*
+		 * RING_BUFFER_WAKEUP_BY_WRITER wakeup is not lock-free.
+		 */
+		if (config->wakeup == RING_BUFFER_WAKEUP_BY_WRITER
+		    && uatomic_read(&buf->active_readers)
+		    && lib_ring_buffer_poll_deliver(config, buf, chan, handle)) {
+			lib_ring_buffer_wakeup(buf, handle);
+		}
+	}
+}
+
 /*
  * Force a read (imply TLS fixup for dlopen) of TLS variables.
  */