[linux-2.6-omap-h63xx.git] / kernel / trace / ring_buffer.c

/*
 * Generic ring buffer
 *
 * Copyright (C) 2008 Steven Rostedt <srostedt@redhat.com>
 */
#include <linux/ring_buffer.h>
#include <linux/spinlock.h>
#include <linux/debugfs.h>
#include <linux/uaccess.h>
#include <linux/module.h>
#include <linux/percpu.h>
#include <linux/mutex.h>
#include <linux/sched.h>        /* used for sched_clock() (for now) */
#include <linux/init.h>
#include <linux/hash.h>
#include <linux/list.h>
#include <linux/fs.h>

/* Up this if you want to test the TIME_EXTENTS and normalization */
#define DEBUG_SHIFT 0

/* FIXME!!! */
u64 ring_buffer_time_stamp(int cpu)
{
        /* shift to debug/test normalization and TIME_EXTENTS */
        return sched_clock() << DEBUG_SHIFT;
}

void ring_buffer_normalize_time_stamp(int cpu, u64 *ts)
{
        /* Just stupid testing the normalize function and deltas */
        *ts >>= DEBUG_SHIFT;
}

#define RB_EVNT_HDR_SIZE (sizeof(struct ring_buffer_event))
#define RB_ALIGNMENT_SHIFT      2
#define RB_ALIGNMENT            (1 << RB_ALIGNMENT_SHIFT)
#define RB_MAX_SMALL_DATA       28

enum {
        RB_LEN_TIME_EXTEND = 8,
        RB_LEN_TIME_STAMP = 16,
};

/* inline for ring buffer fast paths */
static inline unsigned
rb_event_length(struct ring_buffer_event *event)
{
        unsigned length;

        switch (event->type) {
        case RINGBUF_TYPE_PADDING:
                /* undefined */
                return -1;

        case RINGBUF_TYPE_TIME_EXTEND:
                return RB_LEN_TIME_EXTEND;

        case RINGBUF_TYPE_TIME_STAMP:
                return RB_LEN_TIME_STAMP;

        case RINGBUF_TYPE_DATA:
                if (event->len)
                        length = event->len << RB_ALIGNMENT_SHIFT;
                else
                        length = event->array[0];
                return length + RB_EVNT_HDR_SIZE;
        default:
                BUG();
        }
        /* not hit */
        return 0;
}

/**
 * ring_buffer_event_length - return the length of the event
 * @event: the event to get the length of
 */
unsigned ring_buffer_event_length(struct ring_buffer_event *event)
{
        return rb_event_length(event);
}

/* inline for ring buffer fast paths */
static inline void *
rb_event_data(struct ring_buffer_event *event)
{
        BUG_ON(event->type != RINGBUF_TYPE_DATA);
        /* If length is in len field, then array[0] has the data */
        if (event->len)
                return (void *)&event->array[0];
        /* Otherwise length is in array[0] and array[1] has the data */
        return (void *)&event->array[1];
}

/**
 * ring_buffer_event_data - return the data of the event
 * @event: the event to get the data from
 */
void *ring_buffer_event_data(struct ring_buffer_event *event)
{
        return rb_event_data(event);
}

#define for_each_buffer_cpu(buffer, cpu)                \
        for_each_cpu_mask(cpu, buffer->cpumask)

#define TS_SHIFT        27
#define TS_MASK         ((1ULL << TS_SHIFT) - 1)
#define TS_DELTA_TEST   (~TS_MASK)

/*
 * This hack stolen from mm/slob.c.
 * We can store per page timing information in the page frame of the page.
 * Thanks to Peter Zijlstra for suggesting this idea.
 */
struct buffer_page {
        u64              time_stamp;    /* page time stamp */
        unsigned         size;          /* size of page data */
        unsigned         write;         /* index for next write */
        unsigned         read;          /* index for next read */
        struct list_head list;          /* list of free pages */
        void *page;                     /* Actual data page */
};

/*
 * Also stolen from mm/slob.c. Thanks to Mathieu Desnoyers for pointing
 * this issue out.
 */
static inline void free_buffer_page(struct buffer_page *bpage)
{
        if (bpage->page)
                __free_page(bpage->page);
        kfree(bpage);
}

/*
 * We need to fit the time_stamp delta into 27 bits.
 */
static inline int test_time_stamp(u64 delta)
{
        if (delta & TS_DELTA_TEST)
                return 1;
        return 0;
}

#define BUF_PAGE_SIZE PAGE_SIZE

/*
 * head_page == tail_page && head == tail then buffer is empty.
 */
struct ring_buffer_per_cpu {
        int                             cpu;
        struct ring_buffer              *buffer;
        spinlock_t                      lock;
        struct lock_class_key           lock_key;
        struct list_head                pages;
        struct buffer_page              *head_page;     /* read from head */
        struct buffer_page              *tail_page;     /* write to tail */
        struct buffer_page              *reader_page;
        unsigned long                   overrun;
        unsigned long                   entries;
        u64                             write_stamp;
        u64                             read_stamp;
        atomic_t                        record_disabled;
};

struct ring_buffer {
        unsigned long                   size;
        unsigned                        pages;
        unsigned                        flags;
        int                             cpus;
        cpumask_t                       cpumask;
        atomic_t                        record_disabled;

        struct mutex                    mutex;

        struct ring_buffer_per_cpu      **buffers;
};

struct ring_buffer_iter {
        struct ring_buffer_per_cpu      *cpu_buffer;
        unsigned long                   head;
        struct buffer_page              *head_page;
        u64                             read_stamp;
};

#define RB_WARN_ON(buffer, cond)                        \
        if (unlikely(cond)) {                           \
                atomic_inc(&buffer->record_disabled);   \
                WARN_ON(1);                             \
                return -1;                              \
        }

/**
 * check_pages - integrity check of buffer pages
 * @cpu_buffer: CPU buffer with pages to test
 *
 * As a safty measure we check to make sure the data pages have not
 * been corrupted.
 */
static int rb_check_pages(struct ring_buffer_per_cpu *cpu_buffer)
{
        struct list_head *head = &cpu_buffer->pages;
        struct buffer_page *page, *tmp;

        RB_WARN_ON(cpu_buffer, head->next->prev != head);
        RB_WARN_ON(cpu_buffer, head->prev->next != head);

        list_for_each_entry_safe(page, tmp, head, list) {
                RB_WARN_ON(cpu_buffer, page->list.next->prev != &page->list);
                RB_WARN_ON(cpu_buffer, page->list.prev->next != &page->list);
        }

        return 0;
}

static unsigned rb_head_size(struct ring_buffer_per_cpu *cpu_buffer)
{
        return cpu_buffer->head_page->size;
}

static int rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
                             unsigned nr_pages)
{
        struct list_head *head = &cpu_buffer->pages;
        struct buffer_page *page, *tmp;
        unsigned long addr;
        LIST_HEAD(pages);
        unsigned i;

        for (i = 0; i < nr_pages; i++) {
                page = kzalloc_node(ALIGN(sizeof(*page), cache_line_size()),
                                    GFP_KERNEL, cpu_to_node(cpu_buffer->cpu));
                if (!page)
                        goto free_pages;
                list_add(&page->list, &pages);

                addr = __get_free_page(GFP_KERNEL);
                if (!addr)
                        goto free_pages;
                page->page = (void *)addr;
        }

        list_splice(&pages, head);

        rb_check_pages(cpu_buffer);

        return 0;

 free_pages:
        list_for_each_entry_safe(page, tmp, &pages, list) {
                list_del_init(&page->list);
                free_buffer_page(page);
        }
        return -ENOMEM;
}

static struct ring_buffer_per_cpu *
rb_allocate_cpu_buffer(struct ring_buffer *buffer, int cpu)
{
        struct ring_buffer_per_cpu *cpu_buffer;
        struct buffer_page *page;
        unsigned long addr;
        int ret;

        cpu_buffer = kzalloc_node(ALIGN(sizeof(*cpu_buffer), cache_line_size()),
                                  GFP_KERNEL, cpu_to_node(cpu));
        if (!cpu_buffer)
                return NULL;

        cpu_buffer->cpu = cpu;
        cpu_buffer->buffer = buffer;
        spin_lock_init(&cpu_buffer->lock);
        INIT_LIST_HEAD(&cpu_buffer->pages);

        page = kzalloc_node(ALIGN(sizeof(*page), cache_line_size()),
                            GFP_KERNEL, cpu_to_node(cpu));
        if (!page)
                goto fail_free_buffer;

        cpu_buffer->reader_page = page;
        addr = __get_free_page(GFP_KERNEL);
        if (!addr)
                goto fail_free_reader;
        page->page = (void *)addr;

        INIT_LIST_HEAD(&cpu_buffer->reader_page->list);
        cpu_buffer->reader_page->size = 0;

        ret = rb_allocate_pages(cpu_buffer, buffer->pages);
        if (ret < 0)
                goto fail_free_reader;

        cpu_buffer->head_page
                = list_entry(cpu_buffer->pages.next, struct buffer_page, list);
        cpu_buffer->tail_page
                = list_entry(cpu_buffer->pages.next, struct buffer_page, list);

        return cpu_buffer;

 fail_free_reader:
        free_buffer_page(cpu_buffer->reader_page);

 fail_free_buffer:
        kfree(cpu_buffer);
        return NULL;
}

static void rb_free_cpu_buffer(struct ring_buffer_per_cpu *cpu_buffer)
{
        struct list_head *head = &cpu_buffer->pages;
        struct buffer_page *page, *tmp;

        list_del_init(&cpu_buffer->reader_page->list);
        free_buffer_page(cpu_buffer->reader_page);

        list_for_each_entry_safe(page, tmp, head, list) {
                list_del_init(&page->list);
                free_buffer_page(page);
        }
        kfree(cpu_buffer);
}

/*
 * Causes compile errors if the struct buffer_page gets bigger
 * than the struct page.
 */
extern int ring_buffer_page_too_big(void);

/**
 * ring_buffer_alloc - allocate a new ring_buffer
 * @size: the size in bytes that is needed.
 * @flags: attributes to set for the ring buffer.
 *
 * Currently the only flag that is available is the RB_FL_OVERWRITE
 * flag. This flag means that the buffer will overwrite old data
 * when the buffer wraps. If this flag is not set, the buffer will
 * drop data when the tail hits the head.
 */
struct ring_buffer *ring_buffer_alloc(unsigned long size, unsigned flags)
{
        struct ring_buffer *buffer;
        int bsize;
        int cpu;

        /* Paranoid! Optimizes out when all is well */
        if (sizeof(struct buffer_page) > sizeof(struct page))
                ring_buffer_page_too_big();


        /* keep it in its own cache line */
        buffer = kzalloc(ALIGN(sizeof(*buffer), cache_line_size()),
                         GFP_KERNEL);
        if (!buffer)
                return NULL;

        buffer->pages = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
        buffer->flags = flags;

        /* need at least two pages */
        if (buffer->pages == 1)
                buffer->pages++;

        buffer->cpumask = cpu_possible_map;
        buffer->cpus = nr_cpu_ids;

        bsize = sizeof(void *) * nr_cpu_ids;
        buffer->buffers = kzalloc(ALIGN(bsize, cache_line_size()),
                                  GFP_KERNEL);
        if (!buffer->buffers)
                goto fail_free_buffer;

        for_each_buffer_cpu(buffer, cpu) {
                buffer->buffers[cpu] =
                        rb_allocate_cpu_buffer(buffer, cpu);
                if (!buffer->buffers[cpu])
                        goto fail_free_buffers;
        }

        mutex_init(&buffer->mutex);

        return buffer;

 fail_free_buffers:
        for_each_buffer_cpu(buffer, cpu) {
                if (buffer->buffers[cpu])
                        rb_free_cpu_buffer(buffer->buffers[cpu]);
        }
        kfree(buffer->buffers);

 fail_free_buffer:
        kfree(buffer);
        return NULL;
}

/**
 * ring_buffer_free - free a ring buffer.
 * @buffer: the buffer to free.
 */
void
ring_buffer_free(struct ring_buffer *buffer)
{
        int cpu;

        for_each_buffer_cpu(buffer, cpu)
                rb_free_cpu_buffer(buffer->buffers[cpu]);

        kfree(buffer);
}

static void rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer);

static void
rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned nr_pages)
{
        struct buffer_page *page;
        struct list_head *p;
        unsigned i;

        atomic_inc(&cpu_buffer->record_disabled);
        synchronize_sched();

        for (i = 0; i < nr_pages; i++) {
                BUG_ON(list_empty(&cpu_buffer->pages));
                p = cpu_buffer->pages.next;
                page = list_entry(p, struct buffer_page, list);
                list_del_init(&page->list);
                free_buffer_page(page);
        }
        BUG_ON(list_empty(&cpu_buffer->pages));

        rb_reset_cpu(cpu_buffer);

        rb_check_pages(cpu_buffer);

        atomic_dec(&cpu_buffer->record_disabled);

}

static void
rb_insert_pages(struct ring_buffer_per_cpu *cpu_buffer,
                struct list_head *pages, unsigned nr_pages)
{
        struct buffer_page *page;
        struct list_head *p;
        unsigned i;

        atomic_inc(&cpu_buffer->record_disabled);
        synchronize_sched();

        for (i = 0; i < nr_pages; i++) {
                BUG_ON(list_empty(pages));
                p = pages->next;
                page = list_entry(p, struct buffer_page, list);
                list_del_init(&page->list);
                list_add_tail(&page->list, &cpu_buffer->pages);
        }
        rb_reset_cpu(cpu_buffer);

        rb_check_pages(cpu_buffer);

        atomic_dec(&cpu_buffer->record_disabled);
}

/**
 * ring_buffer_resize - resize the ring buffer
 * @buffer: the buffer to resize.
 * @size: the new size.
 *
 * The tracer is responsible for making sure that the buffer is
 * not being used while changing the size.
 * Note: We may be able to change the above requirement by using
 *  RCU synchronizations.
 *
 * Minimum size is 2 * BUF_PAGE_SIZE.
 *
 * Returns -1 on failure.
 */
int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size)
{
        struct ring_buffer_per_cpu *cpu_buffer;
        unsigned nr_pages, rm_pages, new_pages;
        struct buffer_page *page, *tmp;
        unsigned long buffer_size;
        unsigned long addr;
        LIST_HEAD(pages);
        int i, cpu;

        size = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
        size *= BUF_PAGE_SIZE;
        buffer_size = buffer->pages * BUF_PAGE_SIZE;

        /* we need a minimum of two pages */
        if (size < BUF_PAGE_SIZE * 2)
                size = BUF_PAGE_SIZE * 2;

        if (size == buffer_size)
                return size;

        mutex_lock(&buffer->mutex);

        nr_pages = DIV_ROUND_UP(size, BUF_PAGE_SIZE);

        if (size < buffer_size) {

                /* easy case, just free pages */
                BUG_ON(nr_pages >= buffer->pages);

                rm_pages = buffer->pages - nr_pages;

                for_each_buffer_cpu(buffer, cpu) {
                        cpu_buffer = buffer->buffers[cpu];
                        rb_remove_pages(cpu_buffer, rm_pages);
                }
                goto out;
        }

        /*
         * This is a bit more difficult. We only want to add pages
         * when we can allocate enough for all CPUs. We do this
         * by allocating all the pages and storing them on a local
         * link list. If we succeed in our allocation, then we
         * add these pages to the cpu_buffers. Otherwise we just free
         * them all and return -ENOMEM;
         */
        BUG_ON(nr_pages <= buffer->pages);
        new_pages = nr_pages - buffer->pages;

        for_each_buffer_cpu(buffer, cpu) {
                for (i = 0; i < new_pages; i++) {
                        page = kzalloc_node(ALIGN(sizeof(*page),
                                                  cache_line_size()),
                                            GFP_KERNEL, cpu_to_node(cpu));
                        if (!page)
                                goto free_pages;
                        list_add(&page->list, &pages);
                        addr = __get_free_page(GFP_KERNEL);
                        if (!addr)
                                goto free_pages;
                        page->page = (void *)addr;
                }
        }

        for_each_buffer_cpu(buffer, cpu) {
                cpu_buffer = buffer->buffers[cpu];
                rb_insert_pages(cpu_buffer, &pages, new_pages);
        }

        BUG_ON(!list_empty(&pages));

 out:
        buffer->pages = nr_pages;
        mutex_unlock(&buffer->mutex);

        return size;

 free_pages:
        list_for_each_entry_safe(page, tmp, &pages, list) {
                list_del_init(&page->list);
                free_buffer_page(page);
        }
        return -ENOMEM;
}

static inline int rb_per_cpu_empty(struct ring_buffer_per_cpu *cpu_buffer)
{
        return cpu_buffer->reader_page->read == cpu_buffer->reader_page->size &&
                (cpu_buffer->tail_page == cpu_buffer->reader_page ||
                 (cpu_buffer->tail_page == cpu_buffer->head_page &&
                  cpu_buffer->head_page->read ==
                  cpu_buffer->tail_page->write));
}

static inline int rb_null_event(struct ring_buffer_event *event)
{
        return event->type == RINGBUF_TYPE_PADDING;
}

static inline void *__rb_page_index(struct buffer_page *page, unsigned index)
{
        return page->page + index;
}

static inline struct ring_buffer_event *
rb_reader_event(struct ring_buffer_per_cpu *cpu_buffer)
{
        return __rb_page_index(cpu_buffer->reader_page,
                               cpu_buffer->reader_page->read);
}

static inline struct ring_buffer_event *
rb_head_event(struct ring_buffer_per_cpu *cpu_buffer)
{
        return __rb_page_index(cpu_buffer->head_page,
                               cpu_buffer->head_page->read);
}

static inline struct ring_buffer_event *
rb_iter_head_event(struct ring_buffer_iter *iter)
{
        return __rb_page_index(iter->head_page, iter->head);
}

/*
 * When the tail hits the head and the buffer is in overwrite mode,
 * the head jumps to the next page and all content on the previous
 * page is discarded. But before doing so, we update the overrun
 * variable of the buffer.
 */
static void rb_update_overflow(struct ring_buffer_per_cpu *cpu_buffer)
{
        struct ring_buffer_event *event;
        unsigned long head;

        for (head = 0; head < rb_head_size(cpu_buffer);
             head += rb_event_length(event)) {

                event = __rb_page_index(cpu_buffer->head_page, head);
                BUG_ON(rb_null_event(event));
                /* Only count data entries */
                if (event->type != RINGBUF_TYPE_DATA)
                        continue;
                cpu_buffer->overrun++;
                cpu_buffer->entries--;
        }
}

static inline void rb_inc_page(struct ring_buffer_per_cpu *cpu_buffer,
                               struct buffer_page **page)
{
        struct list_head *p = (*page)->list.next;

        if (p == &cpu_buffer->pages)
                p = p->next;

        *page = list_entry(p, struct buffer_page, list);
}

static inline void
rb_add_stamp(struct ring_buffer_per_cpu *cpu_buffer, u64 *ts)
{
        cpu_buffer->tail_page->time_stamp = *ts;
        cpu_buffer->write_stamp = *ts;
}

static void rb_reset_head_page(struct ring_buffer_per_cpu *cpu_buffer)
{
        cpu_buffer->head_page->read = 0;
}

static void rb_reset_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
{
        cpu_buffer->read_stamp = cpu_buffer->reader_page->time_stamp;
        cpu_buffer->reader_page->read = 0;
}

static inline void rb_inc_iter(struct ring_buffer_iter *iter)
{
        struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;

        /*
         * The iterator could be on the reader page (it starts there).
         * But the head could have moved, since the reader was
         * found. Check for this case and assign the iterator
         * to the head page instead of next.
         */
        if (iter->head_page == cpu_buffer->reader_page)
                iter->head_page = cpu_buffer->head_page;
        else
                rb_inc_page(cpu_buffer, &iter->head_page);

        iter->read_stamp = iter->head_page->time_stamp;
        iter->head = 0;
}

/**
 * ring_buffer_update_event - update event type and data
 * @event: the even to update
 * @type: the type of event
 * @length: the size of the event field in the ring buffer
 *
 * Update the type and data fields of the event. The length
 * is the actual size that is written to the ring buffer,
 * and with this, we can determine what to place into the
 * data field.
 */
static inline void
rb_update_event(struct ring_buffer_event *event,
                         unsigned type, unsigned length)
{
        event->type = type;

        switch (type) {

        case RINGBUF_TYPE_PADDING:
                break;

        case RINGBUF_TYPE_TIME_EXTEND:
                event->len =
                        (RB_LEN_TIME_EXTEND + (RB_ALIGNMENT-1))
                        >> RB_ALIGNMENT_SHIFT;
                break;

        case RINGBUF_TYPE_TIME_STAMP:
                event->len =
                        (RB_LEN_TIME_STAMP + (RB_ALIGNMENT-1))
                        >> RB_ALIGNMENT_SHIFT;
                break;

        case RINGBUF_TYPE_DATA:
                length -= RB_EVNT_HDR_SIZE;
                if (length > RB_MAX_SMALL_DATA) {
                        event->len = 0;
                        event->array[0] = length;
                } else
                        event->len =
                                (length + (RB_ALIGNMENT-1))
                                >> RB_ALIGNMENT_SHIFT;
                break;
        default:
                BUG();
        }
}

static inline unsigned rb_calculate_event_length(unsigned length)
{
        struct ring_buffer_event event; /* Used only for sizeof array */

        /* zero length can cause confusions */
        if (!length)
                length = 1;

        if (length > RB_MAX_SMALL_DATA)
                length += sizeof(event.array[0]);

        length += RB_EVNT_HDR_SIZE;
        length = ALIGN(length, RB_ALIGNMENT);

        return length;
}

static struct ring_buffer_event *
__rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
                  unsigned type, unsigned long length, u64 *ts)
{
        struct buffer_page *tail_page, *head_page, *reader_page;
        unsigned long tail;
        struct ring_buffer *buffer = cpu_buffer->buffer;
        struct ring_buffer_event *event;

        tail_page = cpu_buffer->tail_page;
        tail = cpu_buffer->tail_page->write;

        if (tail + length > BUF_PAGE_SIZE) {
                struct buffer_page *next_page = tail_page;

                spin_lock(&cpu_buffer->lock);
                rb_inc_page(cpu_buffer, &next_page);

                head_page = cpu_buffer->head_page;
                reader_page = cpu_buffer->reader_page;

                /* we grabbed the lock before incrementing */
                WARN_ON(next_page == reader_page);

                if (next_page == head_page) {
                        if (!(buffer->flags & RB_FL_OVERWRITE)) {
                                spin_unlock(&cpu_buffer->lock);
                                return NULL;
                        }

                        /* count overflows */
                        rb_update_overflow(cpu_buffer);

                        rb_inc_page(cpu_buffer, &head_page);
                        cpu_buffer->head_page = head_page;
                        rb_reset_head_page(cpu_buffer);
                }

                if (tail != BUF_PAGE_SIZE) {
                        event = __rb_page_index(tail_page, tail);
                        /* page padding */
                        event->type = RINGBUF_TYPE_PADDING;
                }

                tail_page->size = tail;
                tail_page = next_page;
                tail_page->size = 0;
                tail = 0;
                cpu_buffer->tail_page = tail_page;
                cpu_buffer->tail_page->write = tail;
                rb_add_stamp(cpu_buffer, ts);
                spin_unlock(&cpu_buffer->lock);
        }

        BUG_ON(tail + length > BUF_PAGE_SIZE);

        event = __rb_page_index(tail_page, tail);
        rb_update_event(event, type, length);

        return event;
}

static int
rb_add_time_stamp(struct ring_buffer_per_cpu *cpu_buffer,
                  u64 *ts, u64 *delta)
{
        struct ring_buffer_event *event;
        static int once;

        if (unlikely(*delta > (1ULL << 59) && !once++)) {
                printk(KERN_WARNING "Delta way too big! %llu"
                       " ts=%llu write stamp = %llu\n",
                       *delta, *ts, cpu_buffer->write_stamp);
                WARN_ON(1);
        }

        /*
         * The delta is too big, we to add a
         * new timestamp.
         */
        event = __rb_reserve_next(cpu_buffer,
                                  RINGBUF_TYPE_TIME_EXTEND,
                                  RB_LEN_TIME_EXTEND,
                                  ts);
        if (!event)
                return -1;

        /* check to see if we went to the next page */
        if (cpu_buffer->tail_page->write) {
                /* Still on same page, update timestamp */
                event->time_delta = *delta & TS_MASK;
                event->array[0] = *delta >> TS_SHIFT;
                /* commit the time event */
                cpu_buffer->tail_page->write +=
                        rb_event_length(event);
                cpu_buffer->write_stamp = *ts;
                *delta = 0;
        }

        return 0;
}

static struct ring_buffer_event *
rb_reserve_next_event(struct ring_buffer_per_cpu *cpu_buffer,
                      unsigned type, unsigned long length)
{
        struct ring_buffer_event *event;
        u64 ts, delta;

        ts = ring_buffer_time_stamp(cpu_buffer->cpu);

        if (cpu_buffer->tail_page->write) {
                delta = ts - cpu_buffer->write_stamp;

                if (test_time_stamp(delta)) {
                        int ret;

                        ret = rb_add_time_stamp(cpu_buffer, &ts, &delta);
                        if (ret < 0)
                                return NULL;
                }
        } else {
                spin_lock(&cpu_buffer->lock);
                rb_add_stamp(cpu_buffer, &ts);
                spin_unlock(&cpu_buffer->lock);
                delta = 0;
        }

        event = __rb_reserve_next(cpu_buffer, type, length, &ts);
        if (!event)
                return NULL;

        /* If the reserve went to the next page, our delta is zero */
        if (!cpu_buffer->tail_page->write)
                delta = 0;

        event->time_delta = delta;

        return event;
}

/**
 * ring_buffer_lock_reserve - reserve a part of the buffer
 * @buffer: the ring buffer to reserve from
 * @length: the length of the data to reserve (excluding event header)
 * @flags: a pointer to save the interrupt flags
 *
 * Returns a reseverd event on the ring buffer to copy directly to.
 * The user of this interface will need to get the body to write into
 * and can use the ring_buffer_event_data() interface.
 *
 * The length is the length of the data needed, not the event length
 * which also includes the event header.
 *
 * Must be paired with ring_buffer_unlock_commit, unless NULL is returned.
 * If NULL is returned, then nothing has been allocated or locked.
 */
struct ring_buffer_event *
ring_buffer_lock_reserve(struct ring_buffer *buffer,
                         unsigned long length,
                         unsigned long *flags)
{
        struct ring_buffer_per_cpu *cpu_buffer;
        struct ring_buffer_event *event;
        int cpu;

        if (atomic_read(&buffer->record_disabled))
                return NULL;

        local_irq_save(*flags);
        cpu = raw_smp_processor_id();

        if (!cpu_isset(cpu, buffer->cpumask))
                goto out;

        cpu_buffer = buffer->buffers[cpu];

        if (atomic_read(&cpu_buffer->record_disabled))
                goto out;

        length = rb_calculate_event_length(length);
        if (length > BUF_PAGE_SIZE)
                return NULL;

        event = rb_reserve_next_event(cpu_buffer, RINGBUF_TYPE_DATA, length);
        if (!event)
                goto out;

        return event;

 out:
        local_irq_restore(*flags);
        return NULL;
}

static void rb_commit(struct ring_buffer_per_cpu *cpu_buffer,
                      struct ring_buffer_event *event)
{
        cpu_buffer->tail_page->write += rb_event_length(event);
        cpu_buffer->tail_page->size = cpu_buffer->tail_page->write;
        cpu_buffer->write_stamp += event->time_delta;
        cpu_buffer->entries++;
}

/**
 * ring_buffer_unlock_commit - commit a reserved
 * @buffer: The buffer to commit to
 * @event: The event pointer to commit.
 * @flags: the interrupt flags received from ring_buffer_lock_reserve.
 *
 * This commits the data to the ring buffer, and releases any locks held.
 *
 * Must be paired with ring_buffer_lock_reserve.
 */
int ring_buffer_unlock_commit(struct ring_buffer *buffer,
                              struct ring_buffer_event *event,
                              unsigned long flags)
{
        struct ring_buffer_per_cpu *cpu_buffer;
        int cpu = raw_smp_processor_id();

        cpu_buffer = buffer->buffers[cpu];

        rb_commit(cpu_buffer, event);

        local_irq_restore(flags);

        return 0;
}

/**
 * ring_buffer_write - write data to the buffer without reserving
 * @buffer: The ring buffer to write to.
 * @length: The length of the data being written (excluding the event header)
 * @data: The data to write to the buffer.
 *
 * This is like ring_buffer_lock_reserve and ring_buffer_unlock_commit as
 * one function. If you already have the data to write to the buffer, it
 * may be easier to simply call this function.
 *
 * Note, like ring_buffer_lock_reserve, the length is the length of the data
 * and not the length of the event which would hold the header.
 */
int ring_buffer_write(struct ring_buffer *buffer,
                        unsigned long length,
                        void *data)
{
        struct ring_buffer_per_cpu *cpu_buffer;
        struct ring_buffer_event *event;
        unsigned long event_length, flags;
        void *body;
        int ret = -EBUSY;
        int cpu;

        if (atomic_read(&buffer->record_disabled))
                return -EBUSY;

        local_irq_save(flags);
        cpu = raw_smp_processor_id();

        if (!cpu_isset(cpu, buffer->cpumask))
                goto out;

        cpu_buffer = buffer->buffers[cpu];

        if (atomic_read(&cpu_buffer->record_disabled))
                goto out;

        event_length = rb_calculate_event_length(length);
        event = rb_reserve_next_event(cpu_buffer,
                                      RINGBUF_TYPE_DATA, event_length);
        if (!event)
                goto out;

        body = rb_event_data(event);

        memcpy(body, data, length);

        rb_commit(cpu_buffer, event);

        ret = 0;
 out:
        local_irq_restore(flags);

        return ret;
}

/**
 * ring_buffer_record_disable - stop all writes into the buffer
 * @buffer: The ring buffer to stop writes to.
 *
 * This prevents all writes to the buffer. Any attempt to write
 * to the buffer after this will fail and return NULL.
 *
 * The caller should call synchronize_sched() after this.
 */
void ring_buffer_record_disable(struct ring_buffer *buffer)
{
        atomic_inc(&buffer->record_disabled);
}

/**
 * ring_buffer_record_enable - enable writes to the buffer
 * @buffer: The ring buffer to enable writes
 *
 * Note, multiple disables will need the same number of enables
 * to truely enable the writing (much like preempt_disable).
 */
void ring_buffer_record_enable(struct ring_buffer *buffer)
{
        atomic_dec(&buffer->record_disabled);
}

/**
 * ring_buffer_record_disable_cpu - stop all writes into the cpu_buffer
 * @buffer: The ring buffer to stop writes to.
 * @cpu: The CPU buffer to stop
 *
 * This prevents all writes to the buffer. Any attempt to write
 * to the buffer after this will fail and return NULL.
 *
 * The caller should call synchronize_sched() after this.
 */
void ring_buffer_record_disable_cpu(struct ring_buffer *buffer, int cpu)
{
        struct ring_buffer_per_cpu *cpu_buffer;

        if (!cpu_isset(cpu, buffer->cpumask))
                return;

        cpu_buffer = buffer->buffers[cpu];
        atomic_inc(&cpu_buffer->record_disabled);
}

/**
 * ring_buffer_record_enable_cpu - enable writes to the buffer
 * @buffer: The ring buffer to enable writes
 * @cpu: The CPU to enable.
 *
 * Note, multiple disables will need the same number of enables
 * to truely enable the writing (much like preempt_disable).
 */
void ring_buffer_record_enable_cpu(struct ring_buffer *buffer, int cpu)
{
        struct ring_buffer_per_cpu *cpu_buffer;

        if (!cpu_isset(cpu, buffer->cpumask))
                return;

        cpu_buffer = buffer->buffers[cpu];
        atomic_dec(&cpu_buffer->record_disabled);
}

/**
 * ring_buffer_entries_cpu - get the number of entries in a cpu buffer
 * @buffer: The ring buffer
 * @cpu: The per CPU buffer to get the entries from.
 */
unsigned long ring_buffer_entries_cpu(struct ring_buffer *buffer, int cpu)
{
        struct ring_buffer_per_cpu *cpu_buffer;

        if (!cpu_isset(cpu, buffer->cpumask))
                return 0;

        cpu_buffer = buffer->buffers[cpu];
        return cpu_buffer->entries;
}

/**
 * ring_buffer_overrun_cpu - get the number of overruns in a cpu_buffer
 * @buffer: The ring buffer
 * @cpu: The per CPU buffer to get the number of overruns from
 */
unsigned long ring_buffer_overrun_cpu(struct ring_buffer *buffer, int cpu)
{
        struct ring_buffer_per_cpu *cpu_buffer;

        if (!cpu_isset(cpu, buffer->cpumask))
                return 0;

        cpu_buffer = buffer->buffers[cpu];
        return cpu_buffer->overrun;
}

/**
 * ring_buffer_entries - get the number of entries in a buffer
 * @buffer: The ring buffer
 *
 * Returns the total number of entries in the ring buffer
 * (all CPU entries)
 */
unsigned long ring_buffer_entries(struct ring_buffer *buffer)
{
        struct ring_buffer_per_cpu *cpu_buffer;
        unsigned long entries = 0;
        int cpu;

        /* if you care about this being correct, lock the buffer */
        for_each_buffer_cpu(buffer, cpu) {
                cpu_buffer = buffer->buffers[cpu];
                entries += cpu_buffer->entries;
        }

        return entries;
}

/**
 * ring_buffer_overrun_cpu - get the number of overruns in buffer
 * @buffer: The ring buffer
 *
 * Returns the total number of overruns in the ring buffer
 * (all CPU entries)
 */
unsigned long ring_buffer_overruns(struct ring_buffer *buffer)
{
        struct ring_buffer_per_cpu *cpu_buffer;
        unsigned long overruns = 0;
        int cpu;

        /* if you care about this being correct, lock the buffer */
        for_each_buffer_cpu(buffer, cpu) {
                cpu_buffer = buffer->buffers[cpu];
                overruns += cpu_buffer->overrun;
        }

        return overruns;
}

/**
 * ring_buffer_iter_reset - reset an iterator
 * @iter: The iterator to reset
 *
 * Resets the iterator, so that it will start from the beginning
 * again.
 */
void ring_buffer_iter_reset(struct ring_buffer_iter *iter)
{
        struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;

        /* Iterator usage is expected to have record disabled */
        if (list_empty(&cpu_buffer->reader_page->list)) {
                iter->head_page = cpu_buffer->head_page;
                iter->head = cpu_buffer->head_page->read;
        } else {
                iter->head_page = cpu_buffer->reader_page;
                iter->head = cpu_buffer->reader_page->read;
        }
        if (iter->head)
                iter->read_stamp = cpu_buffer->read_stamp;
        else
                iter->read_stamp = iter->head_page->time_stamp;
}

/**
 * ring_buffer_iter_empty - check if an iterator has no more to read
 * @iter: The iterator to check
 */
int ring_buffer_iter_empty(struct ring_buffer_iter *iter)
{
        struct ring_buffer_per_cpu *cpu_buffer;

        cpu_buffer = iter->cpu_buffer;

        return iter->head_page == cpu_buffer->tail_page &&
                iter->head == cpu_buffer->tail_page->write;
}

static void
rb_update_read_stamp(struct ring_buffer_per_cpu *cpu_buffer,
                     struct ring_buffer_event *event)
{
        u64 delta;

        switch (event->type) {
        case RINGBUF_TYPE_PADDING:
                return;

        case RINGBUF_TYPE_TIME_EXTEND:
                delta = event->array[0];
                delta <<= TS_SHIFT;
                delta += event->time_delta;
                cpu_buffer->read_stamp += delta;
                return;

        case RINGBUF_TYPE_TIME_STAMP:
                /* FIXME: not implemented */
                return;

        case RINGBUF_TYPE_DATA:
                cpu_buffer->read_stamp += event->time_delta;
                return;

        default:
                BUG();
        }
        return;
}

static void
rb_update_iter_read_stamp(struct ring_buffer_iter *iter,
                          struct ring_buffer_event *event)
{
        u64 delta;

        switch (event->type) {
        case RINGBUF_TYPE_PADDING:
                return;

        case RINGBUF_TYPE_TIME_EXTEND:
                delta = event->array[0];
                delta <<= TS_SHIFT;
                delta += event->time_delta;
                iter->read_stamp += delta;
                return;

        case RINGBUF_TYPE_TIME_STAMP:
                /* FIXME: not implemented */
                return;

        case RINGBUF_TYPE_DATA:
                iter->read_stamp += event->time_delta;
                return;

        default:
                BUG();
        }
        return;
}

static struct buffer_page *
rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
{
        struct buffer_page *reader = NULL;
        unsigned long flags;

        spin_lock_irqsave(&cpu_buffer->lock, flags);

 again:
        reader = cpu_buffer->reader_page;

        /* If there's more to read, return this page */
        if (cpu_buffer->reader_page->read < reader->size)
                goto out;

        /* Never should we have an index greater than the size */
        WARN_ON(cpu_buffer->reader_page->read > reader->size);

        /* check if we caught up to the tail */
        reader = NULL;
        if (cpu_buffer->tail_page == cpu_buffer->reader_page)
                goto out;

        /*
         * Splice the empty reader page into the list around the head.
         * Reset the reader page to size zero.
         */

        reader = cpu_buffer->head_page;
        cpu_buffer->reader_page->list.next = reader->list.next;
        cpu_buffer->reader_page->list.prev = reader->list.prev;
        cpu_buffer->reader_page->size = 0;

        /* Make the reader page now replace the head */
        reader->list.prev->next = &cpu_buffer->reader_page->list;
        reader->list.next->prev = &cpu_buffer->reader_page->list;

        /*
         * If the tail is on the reader, then we must set the head
         * to the inserted page, otherwise we set it one before.
         */
        cpu_buffer->head_page = cpu_buffer->reader_page;

        if (cpu_buffer->tail_page != reader)
                rb_inc_page(cpu_buffer, &cpu_buffer->head_page);

        /* Finally update the reader page to the new head */
        cpu_buffer->reader_page = reader;
        rb_reset_reader_page(cpu_buffer);

        goto again;

 out:
        spin_unlock_irqrestore(&cpu_buffer->lock, flags);

        return reader;
}

static void rb_advance_reader(struct ring_buffer_per_cpu *cpu_buffer)
{
        struct ring_buffer_event *event;
        struct buffer_page *reader;
        unsigned length;

        reader = rb_get_reader_page(cpu_buffer);

        /* This function should not be called when buffer is empty */
        BUG_ON(!reader);

        event = rb_reader_event(cpu_buffer);

        if (event->type == RINGBUF_TYPE_DATA)
                cpu_buffer->entries--;

        rb_update_read_stamp(cpu_buffer, event);

        length = rb_event_length(event);
        cpu_buffer->reader_page->read += length;
}

static void rb_advance_iter(struct ring_buffer_iter *iter)
{
        struct ring_buffer *buffer;
        struct ring_buffer_per_cpu *cpu_buffer;
        struct ring_buffer_event *event;
        unsigned length;

        cpu_buffer = iter->cpu_buffer;
        buffer = cpu_buffer->buffer;

        /*
         * Check if we are at the end of the buffer.
         */
        if (iter->head >= iter->head_page->size) {
                BUG_ON(iter->head_page == cpu_buffer->tail_page);
                rb_inc_iter(iter);
                return;
        }

        event = rb_iter_head_event(iter);

        length = rb_event_length(event);

        /*
         * This should not be called to advance the header if we are
         * at the tail of the buffer.
         */
        BUG_ON((iter->head_page == cpu_buffer->tail_page) &&
               (iter->head + length > cpu_buffer->tail_page->write));

        rb_update_iter_read_stamp(iter, event);

        iter->head += length;

        /* check for end of page padding */
        if ((iter->head >= iter->head_page->size) &&
            (iter->head_page != cpu_buffer->tail_page))
                rb_advance_iter(iter);
}

/**
 * ring_buffer_peek - peek at the next event to be read
 * @buffer: The ring buffer to read
 * @cpu: The cpu to peak at
 * @ts: The timestamp counter of this event.
 *
 * This will return the event that will be read next, but does
 * not consume the data.
 */
struct ring_buffer_event *
ring_buffer_peek(struct ring_buffer *buffer, int cpu, u64 *ts)
{
        struct ring_buffer_per_cpu *cpu_buffer;
        struct ring_buffer_event *event;
        struct buffer_page *reader;

        if (!cpu_isset(cpu, buffer->cpumask))
                return NULL;

        cpu_buffer = buffer->buffers[cpu];

 again:
        reader = rb_get_reader_page(cpu_buffer);
        if (!reader)
                return NULL;

        event = rb_reader_event(cpu_buffer);

        switch (event->type) {
        case RINGBUF_TYPE_PADDING:
                WARN_ON(1);
                rb_advance_reader(cpu_buffer);
                return NULL;

        case RINGBUF_TYPE_TIME_EXTEND:
                /* Internal data, OK to advance */
                rb_advance_reader(cpu_buffer);
                goto again;

        case RINGBUF_TYPE_TIME_STAMP:
                /* FIXME: not implemented */
                rb_advance_reader(cpu_buffer);
                goto again;

        case RINGBUF_TYPE_DATA:
                if (ts) {
                        *ts = cpu_buffer->read_stamp + event->time_delta;
                        ring_buffer_normalize_time_stamp(cpu_buffer->cpu, ts);
                }
                return event;

        default:
                BUG();
        }

        return NULL;
}

/**
 * ring_buffer_iter_peek - peek at the next event to be read
 * @iter: The ring buffer iterator
 * @ts: The timestamp counter of this event.
 *
 * This will return the event that will be read next, but does
 * not increment the iterator.
 */
struct ring_buffer_event *
ring_buffer_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
{
        struct ring_buffer *buffer;
        struct ring_buffer_per_cpu *cpu_buffer;
        struct ring_buffer_event *event;

        if (ring_buffer_iter_empty(iter))
                return NULL;

        cpu_buffer = iter->cpu_buffer;
        buffer = cpu_buffer->buffer;

 again:
        if (rb_per_cpu_empty(cpu_buffer))
                return NULL;

        event = rb_iter_head_event(iter);

        switch (event->type) {
        case RINGBUF_TYPE_PADDING:
                rb_inc_iter(iter);
                goto again;

        case RINGBUF_TYPE_TIME_EXTEND:
                /* Internal data, OK to advance */
                rb_advance_iter(iter);
                goto again;

        case RINGBUF_TYPE_TIME_STAMP:
                /* FIXME: not implemented */
                rb_advance_iter(iter);
                goto again;

        case RINGBUF_TYPE_DATA:
                if (ts) {
                        *ts = iter->read_stamp + event->time_delta;
                        ring_buffer_normalize_time_stamp(cpu_buffer->cpu, ts);
                }
                return event;

        default:
                BUG();
        }

        return NULL;
}

/**
 * ring_buffer_consume - return an event and consume it
 * @buffer: The ring buffer to get the next event from
 *
 * Returns the next event in the ring buffer, and that event is consumed.
 * Meaning, that sequential reads will keep returning a different event,
 * and eventually empty the ring buffer if the producer is slower.
 */
struct ring_buffer_event *
ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts)
{
        struct ring_buffer_per_cpu *cpu_buffer;
        struct ring_buffer_event *event;

        if (!cpu_isset(cpu, buffer->cpumask))
                return NULL;

        event = ring_buffer_peek(buffer, cpu, ts);
        if (!event)
                return NULL;

        cpu_buffer = buffer->buffers[cpu];
        rb_advance_reader(cpu_buffer);

        return event;
}

/**
 * ring_buffer_read_start - start a non consuming read of the buffer
 * @buffer: The ring buffer to read from
 * @cpu: The cpu buffer to iterate over
 *
 * This starts up an iteration through the buffer. It also disables
 * the recording to the buffer until the reading is finished.
 * This prevents the reading from being corrupted. This is not
 * a consuming read, so a producer is not expected.
 *
 * Must be paired with ring_buffer_finish.
 */
struct ring_buffer_iter *
ring_buffer_read_start(struct ring_buffer *buffer, int cpu)
{
        struct ring_buffer_per_cpu *cpu_buffer;
        struct ring_buffer_iter *iter;
        unsigned long flags;

        if (!cpu_isset(cpu, buffer->cpumask))
                return NULL;

        iter = kmalloc(sizeof(*iter), GFP_KERNEL);
        if (!iter)
                return NULL;

        cpu_buffer = buffer->buffers[cpu];

        iter->cpu_buffer = cpu_buffer;

        atomic_inc(&cpu_buffer->record_disabled);
        synchronize_sched();

        spin_lock_irqsave(&cpu_buffer->lock, flags);
        ring_buffer_iter_reset(iter);
        spin_unlock_irqrestore(&cpu_buffer->lock, flags);

        return iter;
}

/**
 * ring_buffer_finish - finish reading the iterator of the buffer
 * @iter: The iterator retrieved by ring_buffer_start
 *
 * This re-enables the recording to the buffer, and frees the
 * iterator.
 */
void
ring_buffer_read_finish(struct ring_buffer_iter *iter)
{
        struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;

        atomic_dec(&cpu_buffer->record_disabled);
        kfree(iter);
}

/**
 * ring_buffer_read - read the next item in the ring buffer by the iterator
 * @iter: The ring buffer iterator
 * @ts: The time stamp of the event read.
 *
 * This reads the next event in the ring buffer and increments the iterator.
 */
struct ring_buffer_event *
ring_buffer_read(struct ring_buffer_iter *iter, u64 *ts)
{
        struct ring_buffer_event *event;

        event = ring_buffer_iter_peek(iter, ts);
        if (!event)
                return NULL;

        rb_advance_iter(iter);

        return event;
}

/**
 * ring_buffer_size - return the size of the ring buffer (in bytes)
 * @buffer: The ring buffer.
 */
unsigned long ring_buffer_size(struct ring_buffer *buffer)
{
        return BUF_PAGE_SIZE * buffer->pages;
}

static void
rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
{
        cpu_buffer->head_page
                = list_entry(cpu_buffer->pages.next, struct buffer_page, list);
        cpu_buffer->head_page->size = 0;
        cpu_buffer->tail_page = cpu_buffer->head_page;
        cpu_buffer->tail_page->size = 0;
        INIT_LIST_HEAD(&cpu_buffer->reader_page->list);
        cpu_buffer->reader_page->size = 0;

        cpu_buffer->head_page->read = 0;
        cpu_buffer->tail_page->write = 0;
        cpu_buffer->reader_page->read = 0;

        cpu_buffer->overrun = 0;
        cpu_buffer->entries = 0;
}

/**
 * ring_buffer_reset_cpu - reset a ring buffer per CPU buffer
 * @buffer: The ring buffer to reset a per cpu buffer of
 * @cpu: The CPU buffer to be reset
 */
void ring_buffer_reset_cpu(struct ring_buffer *buffer, int cpu)
{
        struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
        unsigned long flags;

        if (!cpu_isset(cpu, buffer->cpumask))
                return;

        spin_lock_irqsave(&cpu_buffer->lock, flags);

        rb_reset_cpu(cpu_buffer);

        spin_unlock_irqrestore(&cpu_buffer->lock, flags);
}

/**
 * ring_buffer_reset - reset a ring buffer
 * @buffer: The ring buffer to reset all cpu buffers
 */
void ring_buffer_reset(struct ring_buffer *buffer)
{
        int cpu;

        for_each_buffer_cpu(buffer, cpu)
                ring_buffer_reset_cpu(buffer, cpu);
}

/**
 * rind_buffer_empty - is the ring buffer empty?
 * @buffer: The ring buffer to test
 */
int ring_buffer_empty(struct ring_buffer *buffer)
{
        struct ring_buffer_per_cpu *cpu_buffer;
        int cpu;

        /* yes this is racy, but if you don't like the race, lock the buffer */
        for_each_buffer_cpu(buffer, cpu) {
                cpu_buffer = buffer->buffers[cpu];
                if (!rb_per_cpu_empty(cpu_buffer))
                        return 0;
        }
        return 1;
}

/**
 * ring_buffer_empty_cpu - is a cpu buffer of a ring buffer empty?
 * @buffer: The ring buffer
 * @cpu: The CPU buffer to test
 */
int ring_buffer_empty_cpu(struct ring_buffer *buffer, int cpu)
{
        struct ring_buffer_per_cpu *cpu_buffer;

        if (!cpu_isset(cpu, buffer->cpumask))
                return 1;

        cpu_buffer = buffer->buffers[cpu];
        return rb_per_cpu_empty(cpu_buffer);
}

/**
 * ring_buffer_swap_cpu - swap a CPU buffer between two ring buffers
 * @buffer_a: One buffer to swap with
 * @buffer_b: The other buffer to swap with
 *
 * This function is useful for tracers that want to take a "snapshot"
 * of a CPU buffer and has another back up buffer lying around.
 * it is expected that the tracer handles the cpu buffer not being
 * used at the moment.
 */
int ring_buffer_swap_cpu(struct ring_buffer *buffer_a,
                         struct ring_buffer *buffer_b, int cpu)
{
        struct ring_buffer_per_cpu *cpu_buffer_a;
        struct ring_buffer_per_cpu *cpu_buffer_b;

        if (!cpu_isset(cpu, buffer_a->cpumask) ||
            !cpu_isset(cpu, buffer_b->cpumask))
                return -EINVAL;

        /* At least make sure the two buffers are somewhat the same */
        if (buffer_a->size != buffer_b->size ||
            buffer_a->pages != buffer_b->pages)
                return -EINVAL;

        cpu_buffer_a = buffer_a->buffers[cpu];
        cpu_buffer_b = buffer_b->buffers[cpu];

        /*
         * We can't do a synchronize_sched here because this
         * function can be called in atomic context.
         * Normally this will be called from the same CPU as cpu.
         * If not it's up to the caller to protect this.
         */
        atomic_inc(&cpu_buffer_a->record_disabled);
        atomic_inc(&cpu_buffer_b->record_disabled);

        buffer_a->buffers[cpu] = cpu_buffer_b;
        buffer_b->buffers[cpu] = cpu_buffer_a;

        cpu_buffer_b->buffer = buffer_a;
        cpu_buffer_a->buffer = buffer_b;

        atomic_dec(&cpu_buffer_a->record_disabled);
        atomic_dec(&cpu_buffer_b->record_disabled);

        return 0;
}