Fix fork handling, where child shares shm with parent

[lttng-ust.git] / libringbuffer / ring_buffer_frontend.c

/*
 * ring_buffer_frontend.c
 *
 * (C) Copyright 2005-2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 *
 * Ring buffer wait-free buffer synchronization. Producer-consumer and flight
 * recorder (overwrite) modes. See thesis:
 *
 * Desnoyers, Mathieu (2009), "Low-Impact Operating System Tracing", Ph.D.
 * dissertation, Ecole Polytechnique de Montreal.
 * http://www.lttng.org/pub/thesis/desnoyers-dissertation-2009-12.pdf
 *
 * - Algorithm presentation in Chapter 5:
 *     "Lockless Multi-Core High-Throughput Buffering".
 * - Algorithm formal verification in Section 8.6:
 *     "Formal verification of LTTng"
 *
 * Author:
 *      Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 *
 * Inspired from LTT and RelayFS:
 *  Karim Yaghmour <karim@opersys.com>
 *  Tom Zanussi <zanussi@us.ibm.com>
 *  Bob Wisniewski <bob@watson.ibm.com>
 * And from K42 :
 *  Bob Wisniewski <bob@watson.ibm.com>
 *
 * Buffer reader semantic :
 *
 * - get_subbuf_size
 * while buffer is not finalized and empty
 *   - get_subbuf
 *     - if return value != 0, continue
 *   - splice one subbuffer worth of data to a pipe
 *   - splice the data from pipe to disk/network
 *   - put_subbuf
 *
 * Dual LGPL v2.1/GPL v2 license.
 */

#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <urcu/compiler.h>
#include <urcu/ref.h>

#include "smp.h"
#include <lttng/ringbuffer-config.h>
#include "backend.h"
#include "frontend.h"
#include "shm.h"

#ifndef max
#define max(a, b)       ((a) > (b) ? (a) : (b))
#endif

/*
 * Use POSIX SHM: shm_open(3) and shm_unlink(3).
 * close(2) to close the fd returned by shm_open.
 * shm_unlink releases the shared memory object name.
 * ftruncate(2) sets the size of the memory object.
 * mmap/munmap maps the shared memory obj to a virtual address in the
 * calling proceess (should be done both in libust and consumer).
 * See shm_overview(7) for details.
 * Pass file descriptor returned by shm_open(3) to ltt-sessiond through
 * a UNIX socket.
 *
 * Since we don't need to access the object using its name, we can
 * immediately shm_unlink(3) it, and only keep the handle with its file
 * descriptor.
 */

/*
 * Internal structure representing offsets to use at a sub-buffer switch.
 */
struct switch_offsets {
        unsigned long begin, end, old;
        size_t pre_header_padding, size;
        unsigned int switch_new_start:1, switch_new_end:1, switch_old_start:1,
                     switch_old_end:1;
};

__thread unsigned int lib_ring_buffer_nesting;

/*
 * TODO: this is unused. Errors are saved within the ring buffer.
 * Eventually, allow consumerd to print these errors.
 */
static
void lib_ring_buffer_print_errors(struct channel *chan,
                                  struct lttng_ust_lib_ring_buffer *buf, int cpu,
                                  struct lttng_ust_shm_handle *handle);

/**
 * lib_ring_buffer_reset - Reset ring buffer to initial values.
 * @buf: Ring buffer.
 *
 * Effectively empty the ring buffer. Should be called when the buffer is not
 * used for writing. The ring buffer can be opened for reading, but the reader
 * should not be using the iterator concurrently with reset. The previous
 * current iterator record is reset.
 */
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;
        unsigned int i;

        /*
         * 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);
        }
        uatomic_set(&buf->consumed, 0);
        uatomic_set(&buf->record_disabled, 0);
        v_set(config, &buf->last_tsc, 0);
        lib_ring_buffer_backend_reset(&buf->backend, handle);
        /* Don't reset number of active readers */
        v_set(config, &buf->records_lost_full, 0);
        v_set(config, &buf->records_lost_wrap, 0);
        v_set(config, &buf->records_lost_big, 0);
        v_set(config, &buf->records_count, 0);
        v_set(config, &buf->records_overrun, 0);
        buf->finalized = 0;
}

/**
 * channel_reset - Reset channel to initial values.
 * @chan: Channel.
 *
 * Effectively empty the channel. Should be called when the channel is not used
 * for writing. The channel can be opened for reading, but the reader should not
 * be using the iterator concurrently with reset. The previous current iterator
 * record is reset.
 */
void channel_reset(struct channel *chan)
{
        /*
         * Reset iterators first. Will put the subbuffer if held for reading.
         */
        uatomic_set(&chan->record_disabled, 0);
        /* Don't reset commit_count_mask, still valid */
        channel_backend_reset(&chan->backend);
        /* Don't reset switch/read timer interval */
        /* Don't reset notifiers and notifier enable bits */
        /* Don't reset reader reference count */
}

/*
 * Must be called under cpu hotplug protection.
 */
int lib_ring_buffer_create(struct lttng_ust_lib_ring_buffer *buf,
                           struct channel_backend *chanb, int cpu,
                           struct lttng_ust_shm_handle *handle,
                           struct shm_object *shmobj)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chanb->config;
        struct channel *chan = caa_container_of(chanb, struct channel, backend);
        void *priv = channel_get_private(chan);
        size_t subbuf_header_size;
        u64 tsc;
        int ret;

        /* Test for cpu hotplug */
        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;
        }

        align_shm(shmobj, __alignof__(struct commit_counters_cold));
        set_shmp(buf->commit_cold,
                 zalloc_shm(shmobj,
                        sizeof(struct commit_counters_cold) * chan->backend.num_subbuf));
        if (!shmp(handle, buf->commit_cold)) {
                ret = -ENOMEM;
                goto free_commit;
        }

        /*
         * 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));
        config->cb.buffer_begin(buf, tsc, 0, handle);
        v_add(config, subbuf_header_size, &shmp_index(handle, buf->commit_hot, 0)->cc);

        if (config->cb.buffer_create) {
                ret = config->cb.buffer_create(buf, priv, cpu, chanb->name, handle);
                if (ret)
                        goto free_init;
        }
        buf->backend.allocated = 1;
        return 0;

        /* Error handling */
free_init:
        /* commit_cold will be freed by shm teardown */
free_commit:
        /* commit_hot will be freed by shm teardown */
free_chanbuf:
        return ret;
}

#if 0
static void switch_buffer_timer(unsigned long data)
{
        struct lttng_ust_lib_ring_buffer *buf = (struct lttng_ust_lib_ring_buffer *)data;
        struct channel *chan = shmp(handle, buf->backend.chan);
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;

        /*
         * Only flush buffers periodically if readers are active.
         */
        if (uatomic_read(&buf->active_readers) || uatomic_read(&buf->active_shadow_readers))
                lib_ring_buffer_switch_slow(buf, SWITCH_ACTIVE, handle);

        //TODO timers
        //if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
        //      mod_timer_pinned(&buf->switch_timer,
        //                       jiffies + chan->switch_timer_interval);
        //else
        //      mod_timer(&buf->switch_timer,
        //                jiffies + chan->switch_timer_interval);
}
#endif //0

static void lib_ring_buffer_start_switch_timer(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;

        if (!chan->switch_timer_interval || buf->switch_timer_enabled)
                return;
        //TODO
        //init_timer(&buf->switch_timer);
        //buf->switch_timer.function = switch_buffer_timer;
        //buf->switch_timer.expires = jiffies + chan->switch_timer_interval;
        //buf->switch_timer.data = (unsigned long)buf;
        //if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
        //      add_timer_on(&buf->switch_timer, buf->backend.cpu);
        //else
        //      add_timer(&buf->switch_timer);
        buf->switch_timer_enabled = 1;
}

static void lib_ring_buffer_stop_switch_timer(struct lttng_ust_lib_ring_buffer *buf,
                           struct lttng_ust_shm_handle *handle)
{
        struct channel *chan = shmp(handle, buf->backend.chan);

        if (!chan->switch_timer_interval || !buf->switch_timer_enabled)
                return;

        //TODO
        //del_timer_sync(&buf->switch_timer);
        buf->switch_timer_enabled = 0;
}

#if 0
/*
 * Polling timer to check the channels for data.
 */
static void read_buffer_timer(unsigned long data)
{
        struct lttng_ust_lib_ring_buffer *buf = (struct lttng_ust_lib_ring_buffer *)data;
        struct channel *chan = shmp(handle, buf->backend.chan);
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;

        CHAN_WARN_ON(chan, !buf->backend.allocated);

        if (uatomic_read(&buf->active_readers) || uatomic_read(&buf->active_shadow_readers))
            && lib_ring_buffer_poll_deliver(config, buf, chan)) {
                //TODO
                //wake_up_interruptible(&buf->read_wait);
                //wake_up_interruptible(&chan->read_wait);
        }

        //TODO
        //if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
        //      mod_timer_pinned(&buf->read_timer,
        //                       jiffies + chan->read_timer_interval);
        //else
        //      mod_timer(&buf->read_timer,
        //                jiffies + chan->read_timer_interval);
}
#endif //0

static void lib_ring_buffer_start_read_timer(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;

        if (config->wakeup != RING_BUFFER_WAKEUP_BY_TIMER
            || !chan->read_timer_interval
            || buf->read_timer_enabled)
                return;

        //TODO
        //init_timer(&buf->read_timer);
        //buf->read_timer.function = read_buffer_timer;
        //buf->read_timer.expires = jiffies + chan->read_timer_interval;
        //buf->read_timer.data = (unsigned long)buf;

        //if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
        //      add_timer_on(&buf->read_timer, buf->backend.cpu);
        //else
        //      add_timer(&buf->read_timer);
        buf->read_timer_enabled = 1;
}

static void lib_ring_buffer_stop_read_timer(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;

        if (config->wakeup != RING_BUFFER_WAKEUP_BY_TIMER
            || !chan->read_timer_interval
            || !buf->read_timer_enabled)
                return;

        //TODO
        //del_timer_sync(&buf->read_timer);
        /*
         * do one more check to catch data that has been written in the last
         * timer period.
         */
        if (lib_ring_buffer_poll_deliver(config, buf, chan, handle)) {
                //TODO
                //wake_up_interruptible(&buf->read_wait);
                //wake_up_interruptible(&chan->read_wait);
        }
        buf->read_timer_enabled = 0;
}

static void channel_unregister_notifiers(struct channel *chan,
                           struct lttng_ust_shm_handle *handle)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        int cpu;

        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU) {
                for_each_possible_cpu(cpu) {
                        struct lttng_ust_lib_ring_buffer *buf = shmp(handle, chan->backend.buf[cpu].shmp);

                        lib_ring_buffer_stop_switch_timer(buf, handle);
                        lib_ring_buffer_stop_read_timer(buf, handle);
                }
        } else {
                struct lttng_ust_lib_ring_buffer *buf = shmp(handle, chan->backend.buf[0].shmp);

                lib_ring_buffer_stop_switch_timer(buf, handle);
                lib_ring_buffer_stop_read_timer(buf, handle);
        }
        //channel_backend_unregister_notifiers(&chan->backend);
}

static void channel_free(struct channel *chan, struct lttng_ust_shm_handle *handle,
                int shadow)
{
        if (!shadow)
                channel_backend_free(&chan->backend, handle);
        /* chan is freed by shm teardown */
        shm_object_table_destroy(handle->table);
        free(handle);
}

/**
 * channel_create - Create channel.
 * @config: ring buffer instance configuration
 * @name: name of the channel
 * @priv_data: ring buffer client private data area pointer (output)
 * @priv_data_size: length, in bytes, of the private data area.
 * @priv_data_init: initialization data for private data.
 * @buf_addr: pointer the the beginning of the preallocated buffer contiguous
 *            address mapping. It is used only by RING_BUFFER_STATIC
 *            configuration. It can be set to NULL for other backends.
 * @subbuf_size: subbuffer size
 * @num_subbuf: number of subbuffers
 * @switch_timer_interval: Time interval (in us) to fill sub-buffers with
 *                         padding to let readers get those sub-buffers.
 *                         Used for live streaming.
 * @read_timer_interval: Time interval (in us) to wake up pending readers.
 *
 * Holds cpu hotplug.
 * Returns NULL on failure.
 */
struct lttng_ust_shm_handle *channel_create(const struct lttng_ust_lib_ring_buffer_config *config,
                   const char *name,
                   void **priv_data,
                   size_t priv_data_align,
                   size_t priv_data_size,
                   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,
                   int *shm_fd, int *wait_fd, uint64_t *memory_map_size)
{
        int ret, cpu;
        size_t shmsize, chansize;
        struct channel *chan;
        struct lttng_ust_shm_handle *handle;
        struct shm_object *shmobj;
        struct shm_ref *ref;

        if (lib_ring_buffer_check_config(config, switch_timer_interval,
                                         read_timer_interval))
                return NULL;

        handle = zmalloc(sizeof(struct lttng_ust_shm_handle));
        if (!handle)
                return NULL;

        /* Allocate table for channel + per-cpu buffers */
        handle->table = shm_object_table_create(1 + num_possible_cpus());
        if (!handle->table)
                goto error_table_alloc;

        /* Calculate the shm allocation layout */
        shmsize = sizeof(struct channel);
        shmsize += offset_align(shmsize, __alignof__(struct lttng_ust_lib_ring_buffer_shmp));
        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
                shmsize += sizeof(struct lttng_ust_lib_ring_buffer_shmp) * num_possible_cpus();
        else
                shmsize += sizeof(struct lttng_ust_lib_ring_buffer_shmp);
        chansize = shmsize;
        shmsize += offset_align(shmsize, priv_data_align);
        shmsize += priv_data_size;

        shmobj = shm_object_table_append(handle->table, shmsize);
        if (!shmobj)
                goto error_append;
        /* struct channel is at object 0, offset 0 (hardcoded) */
        set_shmp(handle->chan, zalloc_shm(shmobj, chansize));
        assert(handle->chan._ref.index == 0);
        assert(handle->chan._ref.offset == 0);
        chan = shmp(handle, handle->chan);
        if (!chan)
                goto error_append;

        /* space for private data */
        if (priv_data_size) {
                DECLARE_SHMP(void, priv_data_alloc);

                align_shm(shmobj, priv_data_align);
                chan->priv_data_offset = shmobj->allocated_len;
                set_shmp(priv_data_alloc, zalloc_shm(shmobj, priv_data_size));
                if (!shmp(handle, priv_data_alloc))
                        goto error_append;
                *priv_data = channel_get_private(chan);
                memcpy(*priv_data, priv_data_init, priv_data_size);
        } else {
                chan->priv_data_offset = -1;
                *priv_data = NULL;
        }

        ret = channel_backend_init(&chan->backend, name, config,
                                   subbuf_size, num_subbuf, handle);
        if (ret)
                goto error_backend_init;

        chan->commit_count_mask = (~0UL >> chan->backend.num_subbuf_order);
        //TODO
        //chan->switch_timer_interval = usecs_to_jiffies(switch_timer_interval);
        //chan->read_timer_interval = usecs_to_jiffies(read_timer_interval);
        //TODO
        //init_waitqueue_head(&chan->read_wait);
        //init_waitqueue_head(&chan->hp_wait);

        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU) {
                /*
                 * In case of non-hotplug cpu, if the ring-buffer is allocated
                 * in early initcall, it will not be notified of secondary cpus.
                 * In that off case, we need to allocate for all possible cpus.
                 */
                for_each_possible_cpu(cpu) {
                        struct lttng_ust_lib_ring_buffer *buf = shmp(handle, chan->backend.buf[cpu].shmp);
                        lib_ring_buffer_start_switch_timer(buf, handle);
                        lib_ring_buffer_start_read_timer(buf, handle);
                }
        } else {
                struct lttng_ust_lib_ring_buffer *buf = shmp(handle, chan->backend.buf[0].shmp);

                lib_ring_buffer_start_switch_timer(buf, handle);
                lib_ring_buffer_start_read_timer(buf, handle);
        }
        ref = &handle->chan._ref;
        shm_get_object_data(handle, ref, shm_fd, wait_fd, memory_map_size);
        return handle;

error_backend_init:
error_append:
        shm_object_table_destroy(handle->table);
error_table_alloc:
        free(handle);
        return NULL;
}

struct lttng_ust_shm_handle *channel_handle_create(int shm_fd, int wait_fd,
                                        uint64_t memory_map_size)
{
        struct lttng_ust_shm_handle *handle;
        struct shm_object *object;

        handle = zmalloc(sizeof(struct lttng_ust_shm_handle));
        if (!handle)
                return NULL;

        /* Allocate table for channel + per-cpu buffers */
        handle->table = shm_object_table_create(1 + num_possible_cpus());
        if (!handle->table)
                goto error_table_alloc;
        /* Add channel object */
        object = shm_object_table_append_shadow(handle->table,
                        shm_fd, wait_fd, memory_map_size);
        if (!object)
                goto error_table_object;
        /* struct channel is at object 0, offset 0 (hardcoded) */
        handle->chan._ref.index = 0;
        handle->chan._ref.offset = 0;
        return handle;

error_table_object:
        shm_object_table_destroy(handle->table);
error_table_alloc:
        free(handle);
        return NULL;
}

int channel_handle_add_stream(struct lttng_ust_shm_handle *handle,
                int shm_fd, int wait_fd, uint64_t memory_map_size)
{
        struct shm_object *object;

        /* Add stream object */
        object = shm_object_table_append_shadow(handle->table,
                        shm_fd, wait_fd, memory_map_size);
        if (!object)
                return -1;
        return 0;
}

static
void channel_release(struct channel *chan, struct lttng_ust_shm_handle *handle,
                int shadow)
{
        channel_free(chan, handle, shadow);
}

/**
 * channel_destroy - Finalize, wait for q.s. and destroy channel.
 * @chan: channel to destroy
 *
 * Holds cpu hotplug.
 * Call "destroy" callback, finalize channels, decrement the channel
 * reference count. Note that when readers have completed data
 * consumption of finalized channels, get_subbuf() will return -ENODATA.
 * They should release their handle at that point. 
 */
void channel_destroy(struct channel *chan, struct lttng_ust_shm_handle *handle,
                int shadow)
{
        if (shadow) {
                channel_release(chan, handle, shadow);
                return;
        }

        channel_unregister_notifiers(chan, handle);

        /*
         * Note: the consumer takes care of finalizing and switching the
         * buffers.
         */

        /*
         * sessiond/consumer are keeping a reference on the shm file
         * descriptor directly. No need to refcount.
         */
        channel_release(chan, handle, shadow);
        return;
}

struct lttng_ust_lib_ring_buffer *channel_get_ring_buffer(
                                        const struct lttng_ust_lib_ring_buffer_config *config,
                                        struct channel *chan, int cpu,
                                        struct lttng_ust_shm_handle *handle,
                                        int *shm_fd, int *wait_fd,
                                        uint64_t *memory_map_size)
{
        struct shm_ref *ref;

        if (config->alloc == RING_BUFFER_ALLOC_GLOBAL) {
                ref = &chan->backend.buf[0].shmp._ref;
                shm_get_object_data(handle, ref, shm_fd, wait_fd,
                        memory_map_size);
                return shmp(handle, chan->backend.buf[0].shmp);
        } else {
                if (cpu >= num_possible_cpus())
                        return NULL;
                ref = &chan->backend.buf[cpu].shmp._ref;
                shm_get_object_data(handle, ref, shm_fd, wait_fd,
                        memory_map_size);
                return shmp(handle, chan->backend.buf[cpu].shmp);
        }
}

int lib_ring_buffer_open_read(struct lttng_ust_lib_ring_buffer *buf,
                              struct lttng_ust_shm_handle *handle,
                              int shadow)
{
        if (shadow) {
                if (uatomic_cmpxchg(&buf->active_shadow_readers, 0, 1) != 0)
                        return -EBUSY;
                cmm_smp_mb();
                return 0;
        }
        if (uatomic_cmpxchg(&buf->active_readers, 0, 1) != 0)
                return -EBUSY;
        cmm_smp_mb();
        return 0;
}

void lib_ring_buffer_release_read(struct lttng_ust_lib_ring_buffer *buf,
                                  struct lttng_ust_shm_handle *handle,
                                  int shadow)
{
        struct channel *chan = shmp(handle, buf->backend.chan);

        if (shadow) {
                CHAN_WARN_ON(chan, uatomic_read(&buf->active_shadow_readers) != 1);
                cmm_smp_mb();
                uatomic_dec(&buf->active_shadow_readers);
                return;
        }
        CHAN_WARN_ON(chan, uatomic_read(&buf->active_readers) != 1);
        cmm_smp_mb();
        uatomic_dec(&buf->active_readers);
}

/**
 * lib_ring_buffer_snapshot - save subbuffer position snapshot (for read)
 * @buf: ring buffer
 * @consumed: consumed count indicating the position where to read
 * @produced: produced count, indicates position when to stop reading
 *
 * Returns -ENODATA if buffer is finalized, -EAGAIN if there is currently no
 * data to read at consumed position, or 0 if the get operation succeeds.
 */

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;
        unsigned long consumed_cur, write_offset;
        int finalized;

        finalized = CMM_ACCESS_ONCE(buf->finalized);
        /*
         * Read finalized before counters.
         */
        cmm_smp_rmb();
        consumed_cur = 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).
         */
        write_offset = v_read(config, &buf->offset);

        /*
         * 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_cur, chan)
            == 0)
                goto nodata;

        *consumed = consumed_cur;
        *produced = subbuf_trunc(write_offset, chan);

        return 0;

nodata:
        /*
         * The memory barriers __wait_event()/wake_up_interruptible() take care
         * of "raw_spin_is_locked" memory ordering.
         */
        if (finalized)
                return -ENODATA;
        else
                return -EAGAIN;
}

/**
 * lib_ring_buffer_put_snapshot - move consumed counter forward
 * @buf: ring buffer
 * @consumed_new: new consumed count value
 */
void lib_ring_buffer_move_consumer(struct lttng_ust_lib_ring_buffer *buf,
                                   unsigned long consumed_new,
                                   struct lttng_ust_shm_handle *handle)
{
        struct lttng_ust_lib_ring_buffer_backend *bufb = &buf->backend;
        struct channel *chan = shmp(handle, bufb->chan);
        unsigned long consumed;

        CHAN_WARN_ON(chan, uatomic_read(&buf->active_readers) != 1
                        && uatomic_read(&buf->active_shadow_readers) != 1);

        /*
         * Only push the consumed value forward.
         * If the consumed cmpxchg fails, this is because we have been pushed by
         * the writer in flight recorder mode.
         */
        consumed = uatomic_read(&buf->consumed);
        while ((long) consumed - (long) consumed_new < 0)
                consumed = uatomic_cmpxchg(&buf->consumed, consumed,
                                           consumed_new);
}

/**
 * lib_ring_buffer_get_subbuf - get exclusive access to subbuffer for reading
 * @buf: ring buffer
 * @consumed: consumed count indicating the position where to read
 *
 * Returns -ENODATA if buffer is finalized, -EAGAIN if there is currently no
 * data to read at consumed position, or 0 if the get operation succeeds.
 */
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;
        unsigned long consumed_cur, consumed_idx, commit_count, write_offset;
        int ret;
        int finalized;

retry:
        finalized = CMM_ACCESS_ONCE(buf->finalized);
        /*
         * Read finalized before counters.
         */
        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);
        /*
         * Make sure we read the commit count before reading the buffer
         * data and the write offset. Correct consumed offset ordering
         * wrt commit count is insured by the use of cmpxchg to update
         * the consumed offset.
         */
        /*
         * Local rmb to match the remote wmb to read the commit count
         * before the buffer data and the write offset.
         */
        cmm_smp_rmb();

        write_offset = v_read(config, &buf->offset);

        /*
         * Check that the buffer we are getting is after or at consumed_cur
         * position.
         */
        if ((long) subbuf_trunc(consumed, chan)
            - (long) subbuf_trunc(consumed_cur, chan) < 0)
                goto nodata;

        /*
         * 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_cur, chan)
               >> chan->backend.num_subbuf_order)
            != 0)
                goto nodata;

        /*
         * 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_cur, chan)
            == 0)
                goto nodata;

        /*
         * Failure to get the subbuffer causes a busy-loop retry without going
         * to a wait queue. These are caused by short-lived race windows where
         * 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.
         */
        ret = update_read_sb_index(config, &buf->backend, &chan->backend,
                                   consumed_idx, buf_trunc_val(consumed, chan),
                                   handle);
        if (ret)
                goto retry;
        subbuffer_id_clear_noref(config, &buf->backend.buf_rsb.id);

        buf->get_subbuf_consumed = consumed;
        buf->get_subbuf = 1;

        return 0;

nodata:
        /*
         * The memory barriers __wait_event()/wake_up_interruptible() take care
         * of "raw_spin_is_locked" memory ordering.
         */
        if (finalized)
                return -ENODATA;
        else
                return -EAGAIN;
}

/**
 * lib_ring_buffer_put_subbuf - release exclusive subbuffer access
 * @buf: ring buffer
 */
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;

        CHAN_WARN_ON(chan, uatomic_read(&buf->active_readers) != 1
                        && uatomic_read(&buf->active_shadow_readers) != 1);

        if (!buf->get_subbuf) {
                /*
                 * Reader puts a subbuffer it did not get.
                 */
                CHAN_WARN_ON(chan, 1);
                return;
        }
        consumed = buf->get_subbuf_consumed;
        buf->get_subbuf = 0;

        /*
         * Clear the records_unread counter. (overruns counter)
         * Can still be non-zero if a file reader simply grabbed the data
         * without using iterators.
         * 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);
        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);

        /*
         * Exchange the reader subbuffer with the one we put in its place in the
         * writer subbuffer table. Expect the original consumed count. If
         * update_read_sb_index fails, this is because the writer updated the
         * subbuffer concurrently. We should therefore keep the subbuffer we
         * currently have: it has become invalid to try reading this sub-buffer
         * consumed count value anyway.
         */
        consumed_idx = subbuf_index(consumed, chan);
        update_read_sb_index(config, &buf->backend, &chan->backend,
                             consumed_idx, buf_trunc_val(consumed, chan),
                             handle);
        /*
         * update_read_sb_index return value ignored. Don't exchange sub-buffer
         * if the writer concurrently updated it.
         */
}

/*
 * cons_offset is an iterator on all subbuffer offsets between the reader
 * position and the writer position. (inclusive)
 */
static
void lib_ring_buffer_print_subbuffer_errors(struct lttng_ust_lib_ring_buffer *buf,
                                            struct channel *chan,
                                            unsigned long cons_offset,
                                            int cpu,
                                            struct lttng_ust_shm_handle *handle)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        unsigned long cons_idx, commit_count, commit_count_sb;

        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);

        if (subbuf_offset(commit_count, chan) != 0)
                DBG("ring buffer %s, cpu %d: "
                       "commit count in subbuffer %lu,\n"
                       "expecting multiples of %lu bytes\n"
                       "  [ %lu bytes committed, %lu bytes reader-visible ]\n",
                       chan->backend.name, cpu, cons_idx,
                       chan->backend.subbuf_size,
                       commit_count, commit_count_sb);

        DBG("ring buffer: %s, cpu %d: %lu bytes committed\n",
               chan->backend.name, cpu, commit_count);
}

static
void lib_ring_buffer_print_buffer_errors(struct lttng_ust_lib_ring_buffer *buf,
                                         struct channel *chan,
                                         void *priv, int cpu,
                                         struct lttng_ust_shm_handle *handle)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        unsigned long write_offset, cons_offset;

        /*
         * Can be called in the error path of allocation when
         * trans_channel_data is not yet set.
         */
        if (!chan)
                return;
        /*
         * No need to order commit_count, write_offset and cons_offset reads
         * because we execute at teardown when no more writer nor reader
         * references are left.
         */
        write_offset = v_read(config, &buf->offset);
        cons_offset = uatomic_read(&buf->consumed);
        if (write_offset != cons_offset)
                DBG("ring buffer %s, cpu %d: "
                       "non-consumed data\n"
                       "  [ %lu bytes written, %lu bytes read ]\n",
                       chan->backend.name, cpu, write_offset, cons_offset);

        for (cons_offset = uatomic_read(&buf->consumed);
             (long) (subbuf_trunc((unsigned long) v_read(config, &buf->offset),
                                  chan)
                     - cons_offset) > 0;
             cons_offset = subbuf_align(cons_offset, chan))
                lib_ring_buffer_print_subbuffer_errors(buf, chan, cons_offset,
                                                       cpu, handle);
}

static
void lib_ring_buffer_print_errors(struct channel *chan,
                                  struct lttng_ust_lib_ring_buffer *buf, int cpu,
                                  struct lttng_ust_shm_handle *handle)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        void *priv = channel_get_private(chan);

        DBG("ring buffer %s, cpu %d: %lu records written, "
                          "%lu records overrun\n",
                          chan->backend.name, cpu,
                          v_read(config, &buf->records_count),
                          v_read(config, &buf->records_overrun));

        if (v_read(config, &buf->records_lost_full)
            || v_read(config, &buf->records_lost_wrap)
            || v_read(config, &buf->records_lost_big))
                DBG("ring buffer %s, cpu %d: records were lost. Caused by:\n"
                       "  [ %lu buffer full, %lu nest buffer wrap-around, "
                       "%lu event too big ]\n",
                       chan->backend.name, cpu,
                       v_read(config, &buf->records_lost_full),
                       v_read(config, &buf->records_lost_wrap),
                       v_read(config, &buf->records_lost_big));

        lib_ring_buffer_print_buffer_errors(buf, chan, priv, cpu, handle);
}

/*
 * lib_ring_buffer_switch_old_start: Populate old subbuffer header.
 *
 * Only executed when the buffer is finalized, in SWITCH_FLUSH.
 */
static
void lib_ring_buffer_switch_old_start(struct lttng_ust_lib_ring_buffer *buf,
                                      struct channel *chan,
                                      struct switch_offsets *offsets,
                                      u64 tsc,
                                      struct lttng_ust_shm_handle *handle)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        unsigned long oldidx = subbuf_index(offsets->old, chan);
        unsigned long commit_count;

        config->cb.buffer_begin(buf, tsc, oldidx, handle);

        /*
         * Order all writes to buffer before the commit count update that will
         * determine that the subbuffer is full.
         */
        cmm_smp_wmb();
        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);
        /* Check if the written buffer has to be delivered */
        lib_ring_buffer_check_deliver(config, buf, chan, offsets->old,
                                      commit_count, oldidx, handle);
        lib_ring_buffer_write_commit_counter(config, buf, chan, oldidx,
                                             offsets->old, commit_count,
                                             config->cb.subbuffer_header_size(),
                                             handle);
}

/*
 * lib_ring_buffer_switch_old_end: switch old subbuffer
 *
 * Note : offset_old should never be 0 here. It is ok, because we never perform
 * buffer switch on an empty subbuffer in SWITCH_ACTIVE mode. The caller
 * increments the offset_old value when doing a SWITCH_FLUSH on an empty
 * subbuffer.
 */
static
void lib_ring_buffer_switch_old_end(struct lttng_ust_lib_ring_buffer *buf,
                                    struct channel *chan,
                                    struct switch_offsets *offsets,
                                    u64 tsc,
                                    struct lttng_ust_shm_handle *handle)
{
        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;

        data_size = subbuf_offset(offsets->old - 1, chan) + 1;
        padding_size = chan->backend.subbuf_size - data_size;
        subbuffer_set_data_size(config, &buf->backend, oldidx, data_size,
                                handle);

        /*
         * Order all writes to buffer before the commit count update that will
         * 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);
        lib_ring_buffer_check_deliver(config, buf, chan, offsets->old - 1,
                                      commit_count, oldidx, handle);
        lib_ring_buffer_write_commit_counter(config, buf, chan, oldidx,
                                             offsets->old, commit_count,
                                             padding_size, handle);
}

/*
 * lib_ring_buffer_switch_new_start: Populate new subbuffer.
 *
 * This code can be executed unordered : writers may already have written to the
 * sub-buffer before this code gets executed, caution.  The commit makes sure
 * that this code is executed before the deliver of this sub-buffer.
 */
static
void lib_ring_buffer_switch_new_start(struct lttng_ust_lib_ring_buffer *buf,
                                      struct channel *chan,
                                      struct switch_offsets *offsets,
                                      u64 tsc,
                                      struct lttng_ust_shm_handle *handle)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        unsigned long beginidx = subbuf_index(offsets->begin, chan);
        unsigned long commit_count;

        config->cb.buffer_begin(buf, tsc, beginidx, handle);

        /*
         * Order all writes to buffer before the commit count update that will
         * 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);
        /* Check if the written buffer has to be delivered */
        lib_ring_buffer_check_deliver(config, buf, chan, offsets->begin,
                                      commit_count, beginidx, handle);
        lib_ring_buffer_write_commit_counter(config, buf, chan, beginidx,
                                             offsets->begin, commit_count,
                                             config->cb.subbuffer_header_size(),
                                             handle);
}

/*
 * lib_ring_buffer_switch_new_end: finish switching current subbuffer
 *
 * The only remaining threads could be the ones with pending commits. They will
 * have to do the deliver themselves.
 */
static
void lib_ring_buffer_switch_new_end(struct lttng_ust_lib_ring_buffer *buf,
                                    struct channel *chan,
                                    struct switch_offsets *offsets,
                                    u64 tsc,
                                    struct lttng_ust_shm_handle *handle)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        unsigned long endidx = subbuf_index(offsets->end - 1, chan);
        unsigned long commit_count, padding_size, data_size;

        data_size = subbuf_offset(offsets->end - 1, chan) + 1;
        padding_size = chan->backend.subbuf_size - data_size;
        subbuffer_set_data_size(config, &buf->backend, endidx, data_size,
                                handle);

        /*
         * Order all writes to buffer before the commit count update that will
         * determine that the subbuffer is full.
         */
        cmm_smp_wmb();
        v_add(config, padding_size, &shmp_index(handle, buf->commit_hot, endidx)->cc);
        commit_count = v_read(config, &shmp_index(handle, buf->commit_hot, endidx)->cc);
        lib_ring_buffer_check_deliver(config, buf, chan, offsets->end - 1,
                                  commit_count, endidx, handle);
        lib_ring_buffer_write_commit_counter(config, buf, chan, endidx,
                                             offsets->end, commit_count,
                                             padding_size, handle);
}

/*
 * Returns :
 * 0 if ok
 * !0 if execution must be aborted.
 */
static
int lib_ring_buffer_try_switch_slow(enum switch_mode mode,
                                    struct lttng_ust_lib_ring_buffer *buf,
                                    struct channel *chan,
                                    struct switch_offsets *offsets,
                                    u64 *tsc)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        unsigned long off;

        offsets->begin = v_read(config, &buf->offset);
        offsets->old = offsets->begin;
        offsets->switch_old_start = 0;
        off = subbuf_offset(offsets->begin, chan);

        *tsc = config->cb.ring_buffer_clock_read(chan);

        /*
         * Ensure we flush the header of an empty subbuffer when doing the
         * finalize (SWITCH_FLUSH). This ensures that we end up knowing the
         * total data gathering duration even if there were no records saved
         * after the last buffer switch.
         * In SWITCH_ACTIVE mode, switch the buffer when it contains events.
         * SWITCH_ACTIVE only flushes the current subbuffer, dealing with end of
         * subbuffer header as appropriate.
         * The next record that reserves space will be responsible for
         * populating the following subbuffer header. We choose not to populate
         * the next subbuffer header here because we want to be able to use
         * SWITCH_ACTIVE for periodical buffer flush, which must
         * guarantee that all the buffer content (records and header
         * timestamps) are visible to the reader. This is required for
         * quiescence guarantees for the fusion merge.
         */
        if (mode == SWITCH_FLUSH || off > 0) {
                if (caa_unlikely(off == 0)) {
                        /*
                         * 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 (!config->cb.subbuffer_header_size())
                                return -1;
                        /*
                         * Need to write the subbuffer start header on finalize.
                         */
                        offsets->switch_old_start = 1;
                }
                offsets->begin = subbuf_align(offsets->begin, chan);
        } else
                return -1;      /* we do not have to switch : buffer is empty */
        /* Note: old points to the next subbuf at offset 0 */
        offsets->end = offsets->begin;
        return 0;
}

/*
 * Force a sub-buffer switch. This operation is completely reentrant : can be
 * called while tracing is active with absolutely no lock held.
 *
 * Note, however, that as a v_cmpxchg is used for some atomic
 * operations, this function must be called from the CPU which owns the buffer
 * for a ACTIVE flush.
 */
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 switch_offsets offsets;
        unsigned long oldidx;
        u64 tsc;

        offsets.size = 0;

        /*
         * Perform retryable operations.
         */
        do {
                if (lib_ring_buffer_try_switch_slow(mode, buf, chan, &offsets,
                                                    &tsc))
                        return; /* Switch not needed */
        } while (v_cmpxchg(config, &buf->offset, offsets.old, offsets.end)
                 != offsets.old);

        /*
         * Atomically update last_tsc. This update races against concurrent
         * atomic updates, but the race will always cause supplementary full TSC
         * records, never the opposite (missing a full TSC record when it would
         * be needed).
         */
        save_last_tsc(config, buf, tsc);

        /*
         * Push the reader if necessary
         */
        lib_ring_buffer_reserve_push_reader(buf, chan, offsets.old);

        oldidx = subbuf_index(offsets.old, chan);
        lib_ring_buffer_clear_noref(config, &buf->backend, oldidx, handle);

        /*
         * May need to populate header start on SWITCH_FLUSH.
         */
        if (offsets.switch_old_start) {
                lib_ring_buffer_switch_old_start(buf, chan, &offsets, tsc, handle);
                offsets.old += config->cb.subbuffer_header_size();
        }

        /*
         * Switch old subbuffer.
         */
        lib_ring_buffer_switch_old_end(buf, chan, &offsets, tsc, handle);
}

/*
 * Returns :
 * 0 if ok
 * -ENOSPC if event size is too large for packet.
 * -ENOBUFS if there is currently not enough space in buffer for the event.
 * -EIO if data cannot be written into the buffer for any other reason.
 */
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)
{
        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;

        offsets->begin = v_read(config, &buf->offset);
        offsets->old = offsets->begin;
        offsets->switch_new_start = 0;
        offsets->switch_new_end = 0;
        offsets->switch_old_end = 0;
        offsets->pre_header_padding = 0;

        ctx->tsc = config->cb.ring_buffer_clock_read(chan);
        if ((int64_t) ctx->tsc == -EIO)
                return -EIO;

        if (last_tsc_overflow(config, buf, ctx->tsc))
                ctx->rflags |= RING_BUFFER_RFLAG_FULL_TSC;

        if (caa_unlikely(subbuf_offset(offsets->begin, ctx->chan) == 0)) {
                offsets->switch_new_start = 1;          /* For offsets->begin */
        } else {
                offsets->size = config->cb.record_header_size(config, chan,
                                                offsets->begin,
                                                &offsets->pre_header_padding,
                                                ctx);
                offsets->size +=
                        lib_ring_buffer_align(offsets->begin + offsets->size,
                                              ctx->largest_align)
                        + ctx->data_size;
                if (caa_unlikely(subbuf_offset(offsets->begin, chan) +
                             offsets->size > chan->backend.subbuf_size)) {
                        offsets->switch_old_end = 1;    /* For offsets->old */
                        offsets->switch_new_start = 1;  /* For offsets->begin */
                }
        }
        if (caa_unlikely(offsets->switch_new_start)) {
                unsigned long sb_index;

                /*
                 * We are typically not filling the previous buffer completely.
                 */
                if (caa_likely(offsets->switch_old_end))
                        offsets->begin = subbuf_align(offsets->begin, chan);
                offsets->begin = offsets->begin
                                 + config->cb.subbuffer_header_size();
                /* Test new buffer integrity */
                sb_index = subbuf_index(offsets->begin, chan);
                reserve_commit_diff =
                  (buf_trunc(offsets->begin, chan)
                   >> chan->backend.num_subbuf_order)
                  - ((unsigned long) v_read(config,
                                            &shmp_index(handle, buf->commit_cold, sb_index)->cc_sb)
                     & chan->commit_count_mask);
                if (caa_likely(reserve_commit_diff == 0)) {
                        /* Next subbuffer not being written to. */
                        if (caa_unlikely(config->mode != RING_BUFFER_OVERWRITE &&
                                subbuf_trunc(offsets->begin, chan)
                                 - subbuf_trunc((unsigned long)
                                     uatomic_read(&buf->consumed), chan)
                                >= chan->backend.buf_size)) {
                                /*
                                 * We do not overwrite non consumed buffers
                                 * and we are full : record is lost.
                                 */
                                v_inc(config, &buf->records_lost_full);
                                return -ENOBUFS;
                        } else {
                                /*
                                 * Next subbuffer not being written to, and we
                                 * are either in overwrite mode or the buffer is
                                 * not full. It's safe to write in this new
                                 * subbuffer.
                                 */
                        }
                } else {
                        /*
                         * Next subbuffer reserve offset does not match the
                         * commit offset. Drop record in producer-consumer and
                         * overwrite mode. Caused by either a writer OOPS or too
                         * many nested writes over a reserve/commit pair.
                         */
                        v_inc(config, &buf->records_lost_wrap);
                        return -EIO;
                }
                offsets->size =
                        config->cb.record_header_size(config, chan,
                                                offsets->begin,
                                                &offsets->pre_header_padding,
                                                ctx);
                offsets->size +=
                        lib_ring_buffer_align(offsets->begin + offsets->size,
                                              ctx->largest_align)
                        + ctx->data_size;
                if (caa_unlikely(subbuf_offset(offsets->begin, chan)
                             + offsets->size > chan->backend.subbuf_size)) {
                        /*
                         * Record too big for subbuffers, report error, don't
                         * complete the sub-buffer switch.
                         */
                        v_inc(config, &buf->records_lost_big);
                        return -ENOSPC;
                } else {
                        /*
                         * We just made a successful buffer switch and the
                         * record fits in the new subbuffer. Let's write.
                         */
                }
        } else {
                /*
                 * Record fits in the current buffer and we are not on a switch
                 * boundary. It's safe to write.
                 */
        }
        offsets->end = offsets->begin + offsets->size;

        if (caa_unlikely(subbuf_offset(offsets->end, chan) == 0)) {
                /*
                 * The offset_end will fall at the very beginning of the next
                 * subbuffer.
                 */
                offsets->switch_new_end = 1;    /* For offsets->begin */
        }
        return 0;
}

/**
 * lib_ring_buffer_reserve_slow - Atomic slot reservation in a buffer.
 * @ctx: ring buffer context.
 *
 * Return : -NOBUFS if not enough space, -ENOSPC if event size too large,
 * -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)
{
        struct channel *chan = ctx->chan;
        struct lttng_ust_shm_handle *handle = ctx->handle;
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        struct lttng_ust_lib_ring_buffer *buf;
        struct switch_offsets offsets;
        int ret;

        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
                buf = shmp(handle, chan->backend.buf[ctx->cpu].shmp);
        else
                buf = shmp(handle, chan->backend.buf[0].shmp);
        ctx->buf = buf;

        offsets.size = 0;

        do {
                ret = lib_ring_buffer_try_reserve_slow(buf, chan, &offsets,
                                                       ctx);
                if (caa_unlikely(ret))
                        return ret;
        } while (caa_unlikely(v_cmpxchg(config, &buf->offset, offsets.old,
                                    offsets.end)
                          != offsets.old));

        /*
         * Atomically update last_tsc. This update races against concurrent
         * atomic updates, but the race will always cause supplementary full TSC
         * records, never the opposite (missing a full TSC record when it would
         * be needed).
         */
        save_last_tsc(config, buf, ctx->tsc);

        /*
         * Push the reader if necessary
         */
        lib_ring_buffer_reserve_push_reader(buf, chan, offsets.end - 1);

        /*
         * Clear noref flag for this subbuffer.
         */
        lib_ring_buffer_clear_noref(config, &buf->backend,
                                    subbuf_index(offsets.end - 1, chan),
                                    handle);

        /*
         * Switch old subbuffer if needed.
         */
        if (caa_unlikely(offsets.switch_old_end)) {
                lib_ring_buffer_clear_noref(config, &buf->backend,
                                            subbuf_index(offsets.old - 1, chan),
                                            handle);
                lib_ring_buffer_switch_old_end(buf, chan, &offsets, ctx->tsc, handle);
        }

        /*
         * Populate new subbuffer.
         */
        if (caa_unlikely(offsets.switch_new_start))
                lib_ring_buffer_switch_new_start(buf, chan, &offsets, ctx->tsc, handle);

        if (caa_unlikely(offsets.switch_new_end))
                lib_ring_buffer_switch_new_end(buf, chan, &offsets, ctx->tsc, handle);

        ctx->slot_size = offsets.size;
        ctx->pre_offset = offsets.begin;
        ctx->buf_offset = offsets.begin + offsets.pre_header_padding;
        return 0;
}