Cleanup: ring buffer: remove lib_ring_buffer_switch_new_end()

[lttng-modules.git] / lib / ringbuffer / ring_buffer_frontend.c

/*
 * ring_buffer_frontend.c
 *
 * Copyright (C) 2005-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; only
 * version 2.1 of the License.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 *
 *
 * 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
 */

#include <linux/delay.h>
#include <linux/module.h>
#include <linux/percpu.h>

#include "../../wrapper/ringbuffer/config.h"
#include "../../wrapper/ringbuffer/backend.h"
#include "../../wrapper/ringbuffer/frontend.h"
#include "../../wrapper/ringbuffer/iterator.h"
#include "../../wrapper/ringbuffer/nohz.h"

/*
 * 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_old_start:1, switch_old_end:1;
};

#ifdef CONFIG_NO_HZ
enum tick_nohz_val {
        TICK_NOHZ_STOP,
        TICK_NOHZ_FLUSH,
        TICK_NOHZ_RESTART,
};

static ATOMIC_NOTIFIER_HEAD(tick_nohz_notifier);
#endif /* CONFIG_NO_HZ */

static DEFINE_PER_CPU(spinlock_t, ring_buffer_nohz_lock);

DEFINE_PER_CPU(unsigned int, lib_ring_buffer_nesting);
EXPORT_PER_CPU_SYMBOL(lib_ring_buffer_nesting);

static
void lib_ring_buffer_print_errors(struct channel *chan,
                                  struct lib_ring_buffer *buf, int cpu);

/*
 * Must be called under cpu hotplug protection.
 */
void lib_ring_buffer_free(struct lib_ring_buffer *buf)
{
        struct channel *chan = buf->backend.chan;

        lib_ring_buffer_print_errors(chan, buf, buf->backend.cpu);
        kfree(buf->commit_hot);
        kfree(buf->commit_cold);

        lib_ring_buffer_backend_free(&buf->backend);
}

/**
 * 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 lib_ring_buffer *buf)
{
        struct channel *chan = buf->backend.chan;
        const struct lib_ring_buffer_config *config = &chan->backend.config;
        unsigned int i;

        /*
         * Reset iterator first. It will put the subbuffer if it currently holds
         * it.
         */
        lib_ring_buffer_iterator_reset(buf);
        v_set(config, &buf->offset, 0);
        for (i = 0; i < chan->backend.num_subbuf; i++) {
                v_set(config, &buf->commit_hot[i].cc, 0);
                v_set(config, &buf->commit_hot[i].seq, 0);
                v_set(config, &buf->commit_cold[i].cc_sb, 0);
        }
        atomic_long_set(&buf->consumed, 0);
        atomic_set(&buf->record_disabled, 0);
        v_set(config, &buf->last_tsc, 0);
        lib_ring_buffer_backend_reset(&buf->backend);
        /* 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;
}
EXPORT_SYMBOL_GPL(lib_ring_buffer_reset);

/**
 * 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.
         */
        channel_iterator_reset(chan);
        atomic_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 */
}
EXPORT_SYMBOL_GPL(channel_reset);

/*
 * Must be called under cpu hotplug protection.
 */
int lib_ring_buffer_create(struct lib_ring_buffer *buf,
                           struct channel_backend *chanb, int cpu)
{
        const struct lib_ring_buffer_config *config = &chanb->config;
        struct channel *chan = container_of(chanb, struct channel, backend);
        void *priv = chanb->priv;
        size_t subbuf_header_size;
        u64 tsc;
        int ret;

        /* Test for cpu hotplug */
        if (buf->backend.allocated)
                return 0;

        /*
         * Paranoia: per cpu dynamic allocation is not officially documented as
         * zeroing the memory, so let's do it here too, just in case.
         */
        memset(buf, 0, sizeof(*buf));

        ret = lib_ring_buffer_backend_create(&buf->backend, &chan->backend, cpu);
        if (ret)
                return ret;

        buf->commit_hot =
                kzalloc_node(ALIGN(sizeof(*buf->commit_hot)
                                   * chan->backend.num_subbuf,
                                   1 << INTERNODE_CACHE_SHIFT),
                        GFP_KERNEL, cpu_to_node(max(cpu, 0)));
        if (!buf->commit_hot) {
                ret = -ENOMEM;
                goto free_chanbuf;
        }

        buf->commit_cold =
                kzalloc_node(ALIGN(sizeof(*buf->commit_cold)
                                   * chan->backend.num_subbuf,
                                   1 << INTERNODE_CACHE_SHIFT),
                        GFP_KERNEL, cpu_to_node(max(cpu, 0)));
        if (!buf->commit_cold) {
                ret = -ENOMEM;
                goto free_commit;
        }

        init_waitqueue_head(&buf->read_wait);
        init_waitqueue_head(&buf->write_wait);
        raw_spin_lock_init(&buf->raw_tick_nohz_spinlock);

        /*
         * 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, &buf->backend.buf_wsb[0].id);
        tsc = config->cb.ring_buffer_clock_read(buf->backend.chan);
        config->cb.buffer_begin(buf, tsc, 0);
        v_add(config, subbuf_header_size, &buf->commit_hot[0].cc);

        if (config->cb.buffer_create) {
                ret = config->cb.buffer_create(buf, priv, cpu, chanb->name);
                if (ret)
                        goto free_init;
        }

        /*
         * Ensure the buffer is ready before setting it to allocated and setting
         * the cpumask.
         * Used for cpu hotplug vs cpumask iteration.
         */
        smp_wmb();
        buf->backend.allocated = 1;

        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU) {
                CHAN_WARN_ON(chan, cpumask_test_cpu(cpu,
                             chan->backend.cpumask));
                cpumask_set_cpu(cpu, chan->backend.cpumask);
        }

        return 0;

        /* Error handling */
free_init:
        kfree(buf->commit_cold);
free_commit:
        kfree(buf->commit_hot);
free_chanbuf:
        lib_ring_buffer_backend_free(&buf->backend);
        return ret;
}

static void switch_buffer_timer(unsigned long data)
{
        struct lib_ring_buffer *buf = (struct lib_ring_buffer *)data;
        struct channel *chan = buf->backend.chan;
        const struct lib_ring_buffer_config *config = &chan->backend.config;

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

        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);
}

/*
 * Called with ring_buffer_nohz_lock held for per-cpu buffers.
 */
static void lib_ring_buffer_start_switch_timer(struct lib_ring_buffer *buf)
{
        struct channel *chan = buf->backend.chan;
        const struct lib_ring_buffer_config *config = &chan->backend.config;

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

/*
 * Called with ring_buffer_nohz_lock held for per-cpu buffers.
 */
static void lib_ring_buffer_stop_switch_timer(struct lib_ring_buffer *buf)
{
        struct channel *chan = buf->backend.chan;

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

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

/*
 * Polling timer to check the channels for data.
 */
static void read_buffer_timer(unsigned long data)
{
        struct lib_ring_buffer *buf = (struct lib_ring_buffer *)data;
        struct channel *chan = buf->backend.chan;
        const struct lib_ring_buffer_config *config = &chan->backend.config;

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

        if (atomic_long_read(&buf->active_readers)
            && lib_ring_buffer_poll_deliver(config, buf, chan)) {
                wake_up_interruptible(&buf->read_wait);
                wake_up_interruptible(&chan->read_wait);
        }

        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);
}

/*
 * Called with ring_buffer_nohz_lock held for per-cpu buffers.
 */
static void lib_ring_buffer_start_read_timer(struct lib_ring_buffer *buf)
{
        struct channel *chan = buf->backend.chan;
        const struct 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;

        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;
}

/*
 * Called with ring_buffer_nohz_lock held for per-cpu buffers.
 */
static void lib_ring_buffer_stop_read_timer(struct lib_ring_buffer *buf)
{
        struct channel *chan = buf->backend.chan;
        const struct 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;

        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)) {
                wake_up_interruptible(&buf->read_wait);
                wake_up_interruptible(&chan->read_wait);
        }
        buf->read_timer_enabled = 0;
}

#ifdef CONFIG_HOTPLUG_CPU
/**
 *      lib_ring_buffer_cpu_hp_callback - CPU hotplug callback
 *      @nb: notifier block
 *      @action: hotplug action to take
 *      @hcpu: CPU number
 *
 *      Returns the success/failure of the operation. (%NOTIFY_OK, %NOTIFY_BAD)
 */
static
int lib_ring_buffer_cpu_hp_callback(struct notifier_block *nb,
                                              unsigned long action,
                                              void *hcpu)
{
        unsigned int cpu = (unsigned long)hcpu;
        struct channel *chan = container_of(nb, struct channel,
                                            cpu_hp_notifier);
        struct lib_ring_buffer *buf = per_cpu_ptr(chan->backend.buf, cpu);
        const struct lib_ring_buffer_config *config = &chan->backend.config;

        if (!chan->cpu_hp_enable)
                return NOTIFY_DONE;

        CHAN_WARN_ON(chan, config->alloc == RING_BUFFER_ALLOC_GLOBAL);

        switch (action) {
        case CPU_DOWN_FAILED:
        case CPU_DOWN_FAILED_FROZEN:
        case CPU_ONLINE:
        case CPU_ONLINE_FROZEN:
                wake_up_interruptible(&chan->hp_wait);
                lib_ring_buffer_start_switch_timer(buf);
                lib_ring_buffer_start_read_timer(buf);
                return NOTIFY_OK;

        case CPU_DOWN_PREPARE:
        case CPU_DOWN_PREPARE_FROZEN:
                lib_ring_buffer_stop_switch_timer(buf);
                lib_ring_buffer_stop_read_timer(buf);
                return NOTIFY_OK;

        case CPU_DEAD:
        case CPU_DEAD_FROZEN:
                /*
                 * Performing a buffer switch on a remote CPU. Performed by
                 * the CPU responsible for doing the hotunplug after the target
                 * CPU stopped running completely. Ensures that all data
                 * from that remote CPU is flushed.
                 */
                lib_ring_buffer_switch_slow(buf, SWITCH_ACTIVE);
                return NOTIFY_OK;

        default:
                return NOTIFY_DONE;
        }
}
#endif

#if defined(CONFIG_NO_HZ) && defined(CONFIG_LIB_RING_BUFFER)
/*
 * For per-cpu buffers, call the reader wakeups before switching the buffer, so
 * that wake-up-tracing generated events are flushed before going idle (in
 * tick_nohz). We test if the spinlock is locked to deal with the race where
 * readers try to sample the ring buffer before we perform the switch. We let
 * the readers retry in that case. If there is data in the buffer, the wake up
 * is going to forbid the CPU running the reader thread from going idle.
 */
static int notrace ring_buffer_tick_nohz_callback(struct notifier_block *nb,
                                                  unsigned long val,
                                                  void *data)
{
        struct channel *chan = container_of(nb, struct channel,
                                            tick_nohz_notifier);
        const struct lib_ring_buffer_config *config = &chan->backend.config;
        struct lib_ring_buffer *buf;
        int cpu = smp_processor_id();

        if (config->alloc != RING_BUFFER_ALLOC_PER_CPU) {
                /*
                 * We don't support keeping the system idle with global buffers
                 * and streaming active. In order to do so, we would need to
                 * sample a non-nohz-cpumask racelessly with the nohz updates
                 * without adding synchronization overhead to nohz. Leave this
                 * use-case out for now.
                 */
                return 0;
        }

        buf = channel_get_ring_buffer(config, chan, cpu);
        switch (val) {
        case TICK_NOHZ_FLUSH:
                raw_spin_lock(&buf->raw_tick_nohz_spinlock);
                if (config->wakeup == RING_BUFFER_WAKEUP_BY_TIMER
                    && chan->read_timer_interval
                    && atomic_long_read(&buf->active_readers)
                    && (lib_ring_buffer_poll_deliver(config, buf, chan)
                        || lib_ring_buffer_pending_data(config, buf, chan))) {
                        wake_up_interruptible(&buf->read_wait);
                        wake_up_interruptible(&chan->read_wait);
                }
                if (chan->switch_timer_interval)
                        lib_ring_buffer_switch_slow(buf, SWITCH_ACTIVE);
                raw_spin_unlock(&buf->raw_tick_nohz_spinlock);
                break;
        case TICK_NOHZ_STOP:
                spin_lock(&__get_cpu_var(ring_buffer_nohz_lock));
                lib_ring_buffer_stop_switch_timer(buf);
                lib_ring_buffer_stop_read_timer(buf);
                spin_unlock(&__get_cpu_var(ring_buffer_nohz_lock));
                break;
        case TICK_NOHZ_RESTART:
                spin_lock(&__get_cpu_var(ring_buffer_nohz_lock));
                lib_ring_buffer_start_read_timer(buf);
                lib_ring_buffer_start_switch_timer(buf);
                spin_unlock(&__get_cpu_var(ring_buffer_nohz_lock));
                break;
        }

        return 0;
}

void notrace lib_ring_buffer_tick_nohz_flush(void)
{
        atomic_notifier_call_chain(&tick_nohz_notifier, TICK_NOHZ_FLUSH,
                                   NULL);
}

void notrace lib_ring_buffer_tick_nohz_stop(void)
{
        atomic_notifier_call_chain(&tick_nohz_notifier, TICK_NOHZ_STOP,
                                   NULL);
}

void notrace lib_ring_buffer_tick_nohz_restart(void)
{
        atomic_notifier_call_chain(&tick_nohz_notifier, TICK_NOHZ_RESTART,
                                   NULL);
}
#endif /* defined(CONFIG_NO_HZ) && defined(CONFIG_LIB_RING_BUFFER) */

/*
 * Holds CPU hotplug.
 */
static void channel_unregister_notifiers(struct channel *chan)
{
        const struct lib_ring_buffer_config *config = &chan->backend.config;
        int cpu;

        channel_iterator_unregister_notifiers(chan);
        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU) {
#ifdef CONFIG_NO_HZ
                /*
                 * Remove the nohz notifier first, so we are certain we stop
                 * the timers.
                 */
                atomic_notifier_chain_unregister(&tick_nohz_notifier,
                                                 &chan->tick_nohz_notifier);
                /*
                 * ring_buffer_nohz_lock will not be needed below, because
                 * we just removed the notifiers, which were the only source of
                 * concurrency.
                 */
#endif /* CONFIG_NO_HZ */
#ifdef CONFIG_HOTPLUG_CPU
                get_online_cpus();
                chan->cpu_hp_enable = 0;
                for_each_online_cpu(cpu) {
                        struct lib_ring_buffer *buf = per_cpu_ptr(chan->backend.buf,
                                                              cpu);
                        lib_ring_buffer_stop_switch_timer(buf);
                        lib_ring_buffer_stop_read_timer(buf);
                }
                put_online_cpus();
                unregister_cpu_notifier(&chan->cpu_hp_notifier);
#else
                for_each_possible_cpu(cpu) {
                        struct lib_ring_buffer *buf = per_cpu_ptr(chan->backend.buf,
                                                              cpu);
                        lib_ring_buffer_stop_switch_timer(buf);
                        lib_ring_buffer_stop_read_timer(buf);
                }
#endif
        } else {
                struct lib_ring_buffer *buf = chan->backend.buf;

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

static void channel_free(struct channel *chan)
{
        channel_iterator_free(chan);
        channel_backend_free(&chan->backend);
        kfree(chan);
}

/**
 * channel_create - Create channel.
 * @config: ring buffer instance configuration
 * @name: name of the channel
 * @priv: ring buffer client 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 channel *channel_create(const struct lib_ring_buffer_config *config,
                   const char *name, void *priv, void *buf_addr,
                   size_t subbuf_size,
                   size_t num_subbuf, unsigned int switch_timer_interval,
                   unsigned int read_timer_interval)
{
        int ret, cpu;
        struct channel *chan;

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

        chan = kzalloc(sizeof(struct channel), GFP_KERNEL);
        if (!chan)
                return NULL;

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

        ret = channel_iterator_init(chan);
        if (ret)
                goto error_free_backend;

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

        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU) {
#if defined(CONFIG_NO_HZ) && defined(CONFIG_LIB_RING_BUFFER)
                /* Only benefit from NO_HZ idle with per-cpu buffers for now. */
                chan->tick_nohz_notifier.notifier_call =
                        ring_buffer_tick_nohz_callback;
                chan->tick_nohz_notifier.priority = ~0U;
                atomic_notifier_chain_register(&tick_nohz_notifier,
                                       &chan->tick_nohz_notifier);
#endif /* defined(CONFIG_NO_HZ) && defined(CONFIG_LIB_RING_BUFFER) */

                /*
                 * 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.
                 */
#ifdef CONFIG_HOTPLUG_CPU
                chan->cpu_hp_notifier.notifier_call =
                                lib_ring_buffer_cpu_hp_callback;
                chan->cpu_hp_notifier.priority = 6;
                register_cpu_notifier(&chan->cpu_hp_notifier);

                get_online_cpus();
                for_each_online_cpu(cpu) {
                        struct lib_ring_buffer *buf = per_cpu_ptr(chan->backend.buf,
                                                               cpu);
                        spin_lock(&per_cpu(ring_buffer_nohz_lock, cpu));
                        lib_ring_buffer_start_switch_timer(buf);
                        lib_ring_buffer_start_read_timer(buf);
                        spin_unlock(&per_cpu(ring_buffer_nohz_lock, cpu));
                }
                chan->cpu_hp_enable = 1;
                put_online_cpus();
#else
                for_each_possible_cpu(cpu) {
                        struct lib_ring_buffer *buf = per_cpu_ptr(chan->backend.buf,
                                                              cpu);
                        spin_lock(&per_cpu(ring_buffer_nohz_lock, cpu));
                        lib_ring_buffer_start_switch_timer(buf);
                        lib_ring_buffer_start_read_timer(buf);
                        spin_unlock(&per_cpu(ring_buffer_nohz_lock, cpu));
                }
#endif
        } else {
                struct lib_ring_buffer *buf = chan->backend.buf;

                lib_ring_buffer_start_switch_timer(buf);
                lib_ring_buffer_start_read_timer(buf);
        }

        return chan;

error_free_backend:
        channel_backend_free(&chan->backend);
error:
        kfree(chan);
        return NULL;
}
EXPORT_SYMBOL_GPL(channel_create);

static
void channel_release(struct kref *kref)
{
        struct channel *chan = container_of(kref, struct channel, ref);
        channel_free(chan);
}

/**
 * channel_destroy - Finalize, wait for q.s. and destroy channel.
 * @chan: channel to destroy
 *
 * Holds cpu hotplug.
 * Call "destroy" callback, finalize channels, and then 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.  Returns the private
 * data pointer.
 */
void *channel_destroy(struct channel *chan)
{
        int cpu;
        const struct lib_ring_buffer_config *config = &chan->backend.config;
        void *priv;

        channel_unregister_notifiers(chan);

        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU) {
                /*
                 * No need to hold cpu hotplug, because all notifiers have been
                 * unregistered.
                 */
                for_each_channel_cpu(cpu, chan) {
                        struct lib_ring_buffer *buf = per_cpu_ptr(chan->backend.buf,
                                                              cpu);

                        if (config->cb.buffer_finalize)
                                config->cb.buffer_finalize(buf,
                                                           chan->backend.priv,
                                                           cpu);
                        if (buf->backend.allocated)
                                lib_ring_buffer_switch_slow(buf, SWITCH_FLUSH);
                        /*
                         * Perform flush before writing to finalized.
                         */
                        smp_wmb();
                        ACCESS_ONCE(buf->finalized) = 1;
                        wake_up_interruptible(&buf->read_wait);
                }
        } else {
                struct lib_ring_buffer *buf = chan->backend.buf;

                if (config->cb.buffer_finalize)
                        config->cb.buffer_finalize(buf, chan->backend.priv, -1);
                if (buf->backend.allocated)
                        lib_ring_buffer_switch_slow(buf, SWITCH_FLUSH);
                /*
                 * Perform flush before writing to finalized.
                 */
                smp_wmb();
                ACCESS_ONCE(buf->finalized) = 1;
                wake_up_interruptible(&buf->read_wait);
        }
        ACCESS_ONCE(chan->finalized) = 1;
        wake_up_interruptible(&chan->hp_wait);
        wake_up_interruptible(&chan->read_wait);
        priv = chan->backend.priv;
        kref_put(&chan->ref, channel_release);
        return priv;
}
EXPORT_SYMBOL_GPL(channel_destroy);

struct lib_ring_buffer *channel_get_ring_buffer(
                                        const struct lib_ring_buffer_config *config,
                                        struct channel *chan, int cpu)
{
        if (config->alloc == RING_BUFFER_ALLOC_GLOBAL)
                return chan->backend.buf;
        else
                return per_cpu_ptr(chan->backend.buf, cpu);
}
EXPORT_SYMBOL_GPL(channel_get_ring_buffer);

int lib_ring_buffer_open_read(struct lib_ring_buffer *buf)
{
        struct channel *chan = buf->backend.chan;

        if (!atomic_long_add_unless(&buf->active_readers, 1, 1))
                return -EBUSY;
        kref_get(&chan->ref);
        smp_mb__after_atomic_inc();
        return 0;
}
EXPORT_SYMBOL_GPL(lib_ring_buffer_open_read);

void lib_ring_buffer_release_read(struct lib_ring_buffer *buf)
{
        struct channel *chan = buf->backend.chan;

        CHAN_WARN_ON(chan, atomic_long_read(&buf->active_readers) != 1);
        smp_mb__before_atomic_dec();
        atomic_long_dec(&buf->active_readers);
        kref_put(&chan->ref, channel_release);
}
EXPORT_SYMBOL_GPL(lib_ring_buffer_release_read);

/*
 * Promote compiler barrier to a smp_mb().
 * For the specific ring buffer case, this IPI call should be removed if the
 * architecture does not reorder writes.  This should eventually be provided by
 * a separate architecture-specific infrastructure.
 */
static void remote_mb(void *info)
{
        smp_mb();
}

/**
 * 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.
 * Busy-loop trying to get data if the tick_nohz sequence lock is held.
 */

int lib_ring_buffer_snapshot(struct lib_ring_buffer *buf,
                             unsigned long *consumed, unsigned long *produced)
{
        struct channel *chan = buf->backend.chan;
        const struct lib_ring_buffer_config *config = &chan->backend.config;
        unsigned long consumed_cur, write_offset;
        int finalized;

retry:
        finalized = ACCESS_ONCE(buf->finalized);
        /*
         * Read finalized before counters.
         */
        smp_rmb();
        consumed_cur = atomic_long_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 if (raw_spin_is_locked(&buf->raw_tick_nohz_spinlock))
                goto retry;
        else
                return -EAGAIN;
}
EXPORT_SYMBOL_GPL(lib_ring_buffer_snapshot);

/**
 * lib_ring_buffer_put_snapshot - move consumed counter forward
 *
 * Should only be called from consumer context.
 * @buf: ring buffer
 * @consumed_new: new consumed count value
 */
void lib_ring_buffer_move_consumer(struct lib_ring_buffer *buf,
                                   unsigned long consumed_new)
{
        struct lib_ring_buffer_backend *bufb = &buf->backend;
        struct channel *chan = bufb->chan;
        unsigned long consumed;

        CHAN_WARN_ON(chan, atomic_long_read(&buf->active_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 = atomic_long_read(&buf->consumed);
        while ((long) consumed - (long) consumed_new < 0)
                consumed = atomic_long_cmpxchg(&buf->consumed, consumed,
                                               consumed_new);
        /* Wake-up the metadata producer */
        wake_up_interruptible(&buf->write_wait);
}
EXPORT_SYMBOL_GPL(lib_ring_buffer_move_consumer);

/**
 * 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.
 * Busy-loop trying to get data if the tick_nohz sequence lock is held.
 */
int lib_ring_buffer_get_subbuf(struct lib_ring_buffer *buf,
                               unsigned long consumed)
{
        struct channel *chan = buf->backend.chan;
        const struct lib_ring_buffer_config *config = &chan->backend.config;
        unsigned long consumed_cur, consumed_idx, commit_count, write_offset;
        int ret;
        int finalized;

        if (buf->get_subbuf) {
                /*
                 * Reader is trying to get a subbuffer twice.
                 */
                CHAN_WARN_ON(chan, 1);
                return -EBUSY;
        }
retry:
        finalized = ACCESS_ONCE(buf->finalized);
        /*
         * Read finalized before counters.
         */
        smp_rmb();
        consumed_cur = atomic_long_read(&buf->consumed);
        consumed_idx = subbuf_index(consumed, chan);
        commit_count = v_read(config, &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.
         * smp_call_function_single can fail if the remote CPU is offline,
         * this is OK because then there is no wmb to execute there.
         * If our thread is executing on the same CPU as the on the buffers
         * belongs to, we don't have to synchronize it at all. If we are
         * migrated, the scheduler will take care of the memory barriers.
         * Normally, smp_call_function_single() should ensure program order when
         * executing the remote function, which implies that it surrounds the
         * function execution with :
         * smp_mb()
         * send IPI
         * csd_lock_wait
         *                recv IPI
         *                smp_mb()
         *                exec. function
         *                smp_mb()
         *                csd unlock
         * smp_mb()
         *
         * However, smp_call_function_single() does not seem to clearly execute
         * such barriers. It depends on spinlock semantic to provide the barrier
         * before executing the IPI and, when busy-looping, csd_lock_wait only
         * executes smp_mb() when it has to wait for the other CPU.
         *
         * I don't trust this code. Therefore, let's add the smp_mb() sequence
         * required ourself, even if duplicated. It has no performance impact
         * anyway.
         *
         * smp_mb() is needed because smp_rmb() and smp_wmb() only order read vs
         * read and write vs write. They do not ensure core synchronization. We
         * really have to ensure total order between the 3 barriers running on
         * the 2 CPUs.
         */
        if (config->ipi == RING_BUFFER_IPI_BARRIER) {
                if (config->sync == RING_BUFFER_SYNC_PER_CPU
                    && config->alloc == RING_BUFFER_ALLOC_PER_CPU) {
                        if (raw_smp_processor_id() != buf->backend.cpu) {
                                /* Total order with IPI handler smp_mb() */
                                smp_mb();
                                smp_call_function_single(buf->backend.cpu,
                                                         remote_mb, NULL, 1);
                                /* Total order with IPI handler smp_mb() */
                                smp_mb();
                        }
                } else {
                        /* Total order with IPI handler smp_mb() */
                        smp_mb();
                        smp_call_function(remote_mb, NULL, 1);
                        /* Total order with IPI handler smp_mb() */
                        smp_mb();
                }
        } else {
                /*
                 * Local rmb to match the remote wmb to read the commit count
                 * before the buffer data and the write offset.
                 */
                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));
        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 if (raw_spin_is_locked(&buf->raw_tick_nohz_spinlock))
                goto retry;
        else
                return -EAGAIN;
}
EXPORT_SYMBOL_GPL(lib_ring_buffer_get_subbuf);

/**
 * lib_ring_buffer_put_subbuf - release exclusive subbuffer access
 * @buf: ring buffer
 */
void lib_ring_buffer_put_subbuf(struct lib_ring_buffer *buf)
{
        struct lib_ring_buffer_backend *bufb = &buf->backend;
        struct channel *chan = bufb->chan;
        const struct lib_ring_buffer_config *config = &chan->backend.config;
        unsigned long read_sb_bindex, consumed_idx, consumed;

        CHAN_WARN_ON(chan, atomic_long_read(&buf->active_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,
                             &bufb->array[read_sb_bindex]->records_unread),
              &bufb->records_read);
        v_set(config, &bufb->array[read_sb_bindex]->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));
        /*
         * update_read_sb_index return value ignored. Don't exchange sub-buffer
         * if the writer concurrently updated it.
         */
}
EXPORT_SYMBOL_GPL(lib_ring_buffer_put_subbuf);

/*
 * 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 lib_ring_buffer *buf,
                                            struct channel *chan,
                                            unsigned long cons_offset,
                                            int cpu)
{
        const struct 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, &buf->commit_hot[cons_idx].cc);
        commit_count_sb = v_read(config, &buf->commit_cold[cons_idx].cc_sb);

        if (subbuf_offset(commit_count, chan) != 0)
                printk(KERN_WARNING
                       "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);

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

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

        /*
         * 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 = atomic_long_read(&buf->consumed);
        if (write_offset != cons_offset)
                printk(KERN_DEBUG
                       "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 = atomic_long_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);
}

static
void lib_ring_buffer_print_errors(struct channel *chan,
                                  struct lib_ring_buffer *buf, int cpu)
{
        const struct lib_ring_buffer_config *config = &chan->backend.config;
        void *priv = chan->backend.priv;

        if (!strcmp(chan->backend.name, "relay-metadata")) {
                printk(KERN_DEBUG "ring buffer %s: %lu records written, "
                        "%lu records overrun\n",
                        chan->backend.name,
                        v_read(config, &buf->records_count),
                        v_read(config, &buf->records_overrun));
        } else {
                printk(KERN_DEBUG "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))
                        printk(KERN_WARNING
                                "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);
}

/*
 * 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 lib_ring_buffer *buf,
                                      struct channel *chan,
                                      struct switch_offsets *offsets,
                                      u64 tsc)
{
        const struct 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);

        /*
         * Order all writes to buffer before the commit count update that will
         * determine that the subbuffer is full.
         */
        if (config->ipi == RING_BUFFER_IPI_BARRIER) {
                /*
                 * Must write slot data before incrementing commit count.  This
                 * compiler barrier is upgraded into a smp_mb() by the IPI sent
                 * by get_subbuf().
                 */
                barrier();
        } else
                smp_wmb();
        v_add(config, config->cb.subbuffer_header_size(),
              &buf->commit_hot[oldidx].cc);
        commit_count = v_read(config, &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);
        lib_ring_buffer_write_commit_counter(config, buf, chan, oldidx,
                                             offsets->old, commit_count,
                                             config->cb.subbuffer_header_size());
}

/*
 * 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 lib_ring_buffer *buf,
                                    struct channel *chan,
                                    struct switch_offsets *offsets,
                                    u64 tsc)
{
        const struct 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);

        /*
         * Order all writes to buffer before the commit count update that will
         * determine that the subbuffer is full.
         */
        if (config->ipi == RING_BUFFER_IPI_BARRIER) {
                /*
                 * Must write slot data before incrementing commit count.  This
                 * compiler barrier is upgraded into a smp_mb() by the IPI sent
                 * by get_subbuf().
                 */
                barrier();
        } else
                smp_wmb();
        v_add(config, padding_size, &buf->commit_hot[oldidx].cc);
        commit_count = v_read(config, &buf->commit_hot[oldidx].cc);
        lib_ring_buffer_check_deliver(config, buf, chan, offsets->old - 1,
                                      commit_count, oldidx);
        lib_ring_buffer_write_commit_counter(config, buf, chan, oldidx,
                                             offsets->old, commit_count,
                                             padding_size);
}

/*
 * 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 lib_ring_buffer *buf,
                                      struct channel *chan,
                                      struct switch_offsets *offsets,
                                      u64 tsc)
{
        const struct 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);

        /*
         * Order all writes to buffer before the commit count update that will
         * determine that the subbuffer is full.
         */
        if (config->ipi == RING_BUFFER_IPI_BARRIER) {
                /*
                 * Must write slot data before incrementing commit count.  This
                 * compiler barrier is upgraded into a smp_mb() by the IPI sent
                 * by get_subbuf().
                 */
                barrier();
        } else
                smp_wmb();
        v_add(config, config->cb.subbuffer_header_size(),
              &buf->commit_hot[beginidx].cc);
        commit_count = v_read(config, &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);
        lib_ring_buffer_write_commit_counter(config, buf, chan, beginidx,
                                             offsets->begin, commit_count,
                                             config->cb.subbuffer_header_size());
}

/*
 * Returns :
 * 0 if ok
 * !0 if execution must be aborted.
 */
static
int lib_ring_buffer_try_switch_slow(enum switch_mode mode,
                                    struct lib_ring_buffer *buf,
                                    struct channel *chan,
                                    struct switch_offsets *offsets,
                                    u64 *tsc)
{
        const struct 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 and CPU tick_nohz stop
         * 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 (unlikely(off == 0)) {
                        /*
                         * A final flush that encounters an empty
                         * sub-buffer cannot switch buffer if a
                         * reader is located within this sub-buffer.
                         * Anyway, the purpose of final flushing of a
                         * sub-buffer at offset 0 is to handle the case
                         * of entirely empty stream.
                         */
                        if (unlikely(subbuf_trunc(offsets->begin, chan)
                                        - subbuf_trunc((unsigned long)
                                                atomic_long_read(&buf->consumed), chan)
                                        >= chan->backend.buf_size))
                                return -1;
                        /*
                         * 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 lib_ring_buffer *buf, enum switch_mode mode)
{
        struct channel *chan = buf->backend.chan;
        const struct 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);

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

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

/*
 * 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 lib_ring_buffer *buf,
                                     struct channel *chan,
                                     struct switch_offsets *offsets,
                                     struct lib_ring_buffer_ctx *ctx)
{
        const struct lib_ring_buffer_config *config = &chan->backend.config;
        unsigned long reserve_commit_diff;

        offsets->begin = v_read(config, &buf->offset);
        offsets->old = offsets->begin;
        offsets->switch_new_start = 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 (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 (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 (unlikely(offsets->switch_new_start)) {
                unsigned long sb_index;

                /*
                 * We are typically not filling the previous buffer completely.
                 */
                if (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,
                                            &buf->commit_cold[sb_index].cc_sb)
                     & chan->commit_count_mask);
                if (likely(reserve_commit_diff == 0)) {
                        /* Next subbuffer not being written to. */
                        if (unlikely(config->mode != RING_BUFFER_OVERWRITE &&
                                subbuf_trunc(offsets->begin, chan)
                                 - subbuf_trunc((unsigned long)
                                     atomic_long_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 (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;
        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 lib_ring_buffer_ctx *ctx)
{
        struct channel *chan = ctx->chan;
        const struct lib_ring_buffer_config *config = &chan->backend.config;
        struct lib_ring_buffer *buf;
        struct switch_offsets offsets;
        int ret;

        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
                buf = per_cpu_ptr(chan->backend.buf, ctx->cpu);
        else
                buf = chan->backend.buf;
        ctx->buf = buf;

        offsets.size = 0;

        do {
                ret = lib_ring_buffer_try_reserve_slow(buf, chan, &offsets,
                                                       ctx);
                if (unlikely(ret))
                        return ret;
        } while (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));

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

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

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

int __init init_lib_ring_buffer_frontend(void)
{
        int cpu;

        for_each_possible_cpu(cpu)
                spin_lock_init(&per_cpu(ring_buffer_nohz_lock, cpu));
        return 0;
}

module_init(init_lib_ring_buffer_frontend);

void __exit exit_lib_ring_buffer_frontend(void)
{
}

module_exit(exit_lib_ring_buffer_frontend);