Quelle relay.c Sprache: C

/*
* Public API and common code for kernel->userspace relay file support.
*
* See Documentation/filesystems/relay.rst for an overview.
*
* Copyright (C) 2002-2005 - Tom Zanussi (zanussi@us.ibm.com), IBM Corp
* Copyright (C) 1999-2005 - Karim Yaghmour (karim@opersys.com)
*
* Moved to kernel/relay.c by Paul Mundt, 2006.
* November 2006 - CPU hotplug support by Mathieu Desnoyers
* (mathieu.desnoyers@polymtl.ca)
*
* This file is released under the GPL.
*/
#include <linux/errno.h>
#include <linux/stddef.h>
#include <linux/slab.h>
#include <linux/export.h>
#include <linux/string.h>
#include <linux/relay.h>
#include <linux/vmalloc.h>
#include <linux/mm.h>
#include <linux/cpu.h>
#include <linux/splice.h>

/* list of open channels, for cpu hotplug */
static DEFINE_MUTEX(relay_channels_mutex);
static LIST_HEAD(relay_channels);

/*
* fault() vm_op implementation for relay file mapping.
*/
static vm_fault_t relay_buf_fault(struct vm_fault *vmf)
{
struct page *page;
struct rchan_buf *buf = vmf->vma->vm_private_data;
pgoff_t pgoff = vmf->pgoff;

if (!buf)
  return VM_FAULT_OOM;

page = vmalloc_to_page(buf->start + (pgoff << PAGE_SHIFT));
if (!page)
  return VM_FAULT_SIGBUS;
get_page(page);
vmf->page = page;

return 0;
}

/*
* vm_ops for relay file mappings.
*/
static const struct vm_operations_struct relay_file_mmap_ops = {
.fault = relay_buf_fault,
};

/*
* allocate an array of pointers of struct page
*/
static struct page **relay_alloc_page_array(unsigned int n_pages)
{
return kvcalloc(n_pages, sizeof(struct page *), GFP_KERNEL);
}

/*
* free an array of pointers of struct page
*/
static void relay_free_page_array(struct page **array)
{
kvfree(array);
}

/**
* relay_mmap_buf: - mmap channel buffer to process address space
* @buf: relay channel buffer
* @vma: vm_area_struct describing memory to be mapped
*
* Returns 0 if ok, negative on error
*
* Caller should already have grabbed mmap_lock.
*/
static int relay_mmap_buf(struct rchan_buf *buf, struct vm_area_struct *vma)
{
unsigned long length = vma->vm_end - vma->vm_start;

if (!buf)
  return -EBADF;

if (length != (unsigned long)buf->chan->alloc_size)
  return -EINVAL;

vma->vm_ops = &relay_file_mmap_ops;
vm_flags_set(vma, VM_DONTEXPAND);
vma->vm_private_data = buf;

return 0;
}

/**
* relay_alloc_buf - allocate a channel buffer
* @buf: the buffer struct
* @size: total size of the buffer
*
* Returns a pointer to the resulting buffer, %NULL if unsuccessful. The
* passed in size will get page aligned, if it isn't already.
*/
static void *relay_alloc_buf(struct rchan_buf *buf, size_t *size)
{
void *mem;
unsigned int i, j, n_pages;

*size = PAGE_ALIGN(*size);
n_pages = *size >> PAGE_SHIFT;

buf->page_array = relay_alloc_page_array(n_pages);
if (!buf->page_array)
  return NULL;

for (i = 0; i < n_pages; i++) {
  buf->page_array[i] = alloc_page(GFP_KERNEL | __GFP_ZERO);
  if (unlikely(!buf->page_array[i]))
   goto depopulate;
  set_page_private(buf->page_array[i], (unsigned long)buf);
}
mem = vmap(buf->page_array, n_pages, VM_MAP, PAGE_KERNEL);
if (!mem)
  goto depopulate;

buf->page_count = n_pages;
return mem;

depopulate:
for (j = 0; j < i; j++)
  __free_page(buf->page_array[j]);
relay_free_page_array(buf->page_array);
return NULL;
}

/**
* relay_create_buf - allocate and initialize a channel buffer
* @chan: the relay channel
*
* Returns channel buffer if successful, %NULL otherwise.
*/
static struct rchan_buf *relay_create_buf(struct rchan *chan)
{
struct rchan_buf *buf;

if (chan->n_subbufs > KMALLOC_MAX_SIZE / sizeof(size_t))
  return NULL;

buf = kzalloc(sizeof(struct rchan_buf), GFP_KERNEL);
if (!buf)
  return NULL;
buf->padding = kmalloc_array(chan->n_subbufs, sizeof(size_t),
         GFP_KERNEL);
if (!buf->padding)
  goto free_buf;

buf->start = relay_alloc_buf(buf, &chan->alloc_size);
if (!buf->start)
  goto free_buf;

buf->chan = chan;
kref_get(&buf->chan->kref);
return buf;

free_buf:
kfree(buf->padding);
kfree(buf);
return NULL;
}

/**
* relay_destroy_channel - free the channel struct
* @kref: target kernel reference that contains the relay channel
*
* Should only be called from kref_put().
*/
static void relay_destroy_channel(struct kref *kref)
{
struct rchan *chan = container_of(kref, struct rchan, kref);
free_percpu(chan->buf);
kfree(chan);
}

/**
* relay_destroy_buf - destroy an rchan_buf struct and associated buffer
* @buf: the buffer struct
*/
static void relay_destroy_buf(struct rchan_buf *buf)
{
struct rchan *chan = buf->chan;
unsigned int i;

if (likely(buf->start)) {
  vunmap(buf->start);
  for (i = 0; i < buf->page_count; i++)
   __free_page(buf->page_array[i]);
  relay_free_page_array(buf->page_array);
}
*per_cpu_ptr(chan->buf, buf->cpu) = NULL;
kfree(buf->padding);
kfree(buf);
kref_put(&chan->kref, relay_destroy_channel);
}

/**
* relay_remove_buf - remove a channel buffer
* @kref: target kernel reference that contains the relay buffer
*
* Removes the file from the filesystem, which also frees the
* rchan_buf_struct and the channel buffer.  Should only be called from
* kref_put().
*/
static void relay_remove_buf(struct kref *kref)
{
struct rchan_buf *buf = container_of(kref, struct rchan_buf, kref);
relay_destroy_buf(buf);
}

/**
* relay_buf_empty - boolean, is the channel buffer empty?
* @buf: channel buffer
*
* Returns 1 if the buffer is empty, 0 otherwise.
*/
static int relay_buf_empty(struct rchan_buf *buf)
{
return (buf->subbufs_produced - buf->subbufs_consumed) ? 0 : 1;
}

/**
* relay_buf_full - boolean, is the channel buffer full?
* @buf: channel buffer
*
* Returns 1 if the buffer is full, 0 otherwise.
*/
int relay_buf_full(struct rchan_buf *buf)
{
size_t ready = buf->subbufs_produced - buf->subbufs_consumed;
return (ready >= buf->chan->n_subbufs) ? 1 : 0;
}
EXPORT_SYMBOL_GPL(relay_buf_full);

/*
* High-level relay kernel API and associated functions.
*/

static int relay_subbuf_start(struct rchan_buf *buf, void *subbuf,
         void *prev_subbuf)
{
int full = relay_buf_full(buf);

if (full)
  buf->stats.full_count++;

if (!buf->chan->cb->subbuf_start)
  return !full;

return buf->chan->cb->subbuf_start(buf, subbuf,
        prev_subbuf);
}

/**
* wakeup_readers - wake up readers waiting on a channel
* @work: contains the channel buffer
*
* This is the function used to defer reader waking
*/
static void wakeup_readers(struct irq_work *work)
{
struct rchan_buf *buf;

buf = container_of(work, struct rchan_buf, wakeup_work);
wake_up_interruptible(&buf->read_wait);
}

/**
* __relay_reset - reset a channel buffer
* @buf: the channel buffer
* @init: 1 if this is a first-time initialization
*
* See relay_reset() for description of effect.
*/
static void __relay_reset(struct rchan_buf *buf, unsigned int init)
{
size_t i;

if (init) {
  init_waitqueue_head(&buf->read_wait);
  kref_init(&buf->kref);
  init_irq_work(&buf->wakeup_work, wakeup_readers);
} else {
  irq_work_sync(&buf->wakeup_work);
}

buf->subbufs_produced = 0;
buf->subbufs_consumed = 0;
buf->bytes_consumed = 0;
buf->finalized = 0;
buf->data = buf->start;
buf->offset = 0;
buf->stats.full_count = 0;
buf->stats.big_count = 0;

for (i = 0; i < buf->chan->n_subbufs; i++)
  buf->padding[i] = 0;

relay_subbuf_start(buf, buf->data, NULL);
}

/**
* relay_reset - reset the channel
* @chan: the channel
*
* This has the effect of erasing all data from all channel buffers
* and restarting the channel in its initial state.  The buffers
* are not freed, so any mappings are still in effect.
*
* NOTE. Care should be taken that the channel isn't actually
* being used by anything when this call is made.
*/
void relay_reset(struct rchan *chan)
{
struct rchan_buf *buf;
unsigned int i;

if (!chan)
  return;

if (chan->is_global && (buf = *per_cpu_ptr(chan->buf, 0))) {
  __relay_reset(buf, 0);
  return;
}

mutex_lock(&relay_channels_mutex);
for_each_possible_cpu(i)
  if ((buf = *per_cpu_ptr(chan->buf, i)))
   __relay_reset(buf, 0);
mutex_unlock(&relay_channels_mutex);
}
EXPORT_SYMBOL_GPL(relay_reset);

static inline void relay_set_buf_dentry(struct rchan_buf *buf,
     struct dentry *dentry)
{
buf->dentry = dentry;
d_inode(buf->dentry)->i_size = buf->early_bytes;
}

static struct dentry *relay_create_buf_file(struct rchan *chan,
         struct rchan_buf *buf,
         unsigned int cpu)
{
struct dentry *dentry;
char *tmpname;

tmpname = kasprintf(GFP_KERNEL, "%s%d", chan->base_filename, cpu);
if (!tmpname)
  return NULL;

/* Create file in fs */
dentry = chan->cb->create_buf_file(tmpname, chan->parent,
        S_IRUSR, buf,
        &chan->is_global);
if (IS_ERR(dentry))
  dentry = NULL;

kfree(tmpname);

return dentry;
}

/*
* relay_open_buf - create a new relay channel buffer
*
* used by relay_open() and CPU hotplug.
*/
static struct rchan_buf *relay_open_buf(struct rchan *chan, unsigned int cpu)
{
struct rchan_buf *buf;
struct dentry *dentry;

  if (chan->is_global)
  return *per_cpu_ptr(chan->buf, 0);

buf = relay_create_buf(chan);
if (!buf)
  return NULL;

if (chan->has_base_filename) {
  dentry = relay_create_buf_file(chan, buf, cpu);
  if (!dentry)
   goto free_buf;
  relay_set_buf_dentry(buf, dentry);
} else {
  /* Only retrieve global info, nothing more, nothing less */
  dentry = chan->cb->create_buf_file(NULL, NULL,
         S_IRUSR, buf,
         &chan->is_global);
  if (IS_ERR_OR_NULL(dentry))
   goto free_buf;
}

  buf->cpu = cpu;
  __relay_reset(buf, 1);

  if(chan->is_global) {
  *per_cpu_ptr(chan->buf, 0) = buf;
   buf->cpu = 0;
   }

return buf;

free_buf:
  relay_destroy_buf(buf);
return NULL;
}

/**
* relay_close_buf - close a channel buffer
* @buf: channel buffer
*
* Marks the buffer finalized and restores the default callbacks.
* The channel buffer and channel buffer data structure are then freed
* automatically when the last reference is given up.
*/
static void relay_close_buf(struct rchan_buf *buf)
{
buf->finalized = 1;
irq_work_sync(&buf->wakeup_work);
buf->chan->cb->remove_buf_file(buf->dentry);
kref_put(&buf->kref, relay_remove_buf);
}

int relay_prepare_cpu(unsigned int cpu)
{
struct rchan *chan;
struct rchan_buf *buf;

mutex_lock(&relay_channels_mutex);
list_for_each_entry(chan, &relay_channels, list) {
  if (*per_cpu_ptr(chan->buf, cpu))
   continue;
  buf = relay_open_buf(chan, cpu);
  if (!buf) {
   pr_err("relay: cpu %d buffer creation failed\n", cpu);
   mutex_unlock(&relay_channels_mutex);
   return -ENOMEM;
  }
  *per_cpu_ptr(chan->buf, cpu) = buf;
}
mutex_unlock(&relay_channels_mutex);
return 0;
}

/**
* relay_open - create a new relay channel
* @base_filename: base name of files to create
* @parent: dentry of parent directory, %NULL for root directory or buffer
* @subbuf_size: size of sub-buffers
* @n_subbufs: number of sub-buffers
* @cb: client callback functions
* @private_data: user-defined data
*
* Returns channel pointer if successful, %NULL otherwise.
*
* Creates a channel buffer for each cpu using the sizes and
* attributes specified.  The created channel buffer files
* will be named base_filename0...base_filenameN-1.  File
* permissions will be %S_IRUSR.
*/
struct rchan *relay_open(const char *base_filename,
    struct dentry *parent,
    size_t subbuf_size,
    size_t n_subbufs,
    const struct rchan_callbacks *cb,
    void *private_data)
{
unsigned int i;
struct rchan *chan;
struct rchan_buf *buf;

if (!(subbuf_size && n_subbufs))
  return NULL;
if (subbuf_size > UINT_MAX / n_subbufs)
  return NULL;
if (!cb || !cb->create_buf_file || !cb->remove_buf_file)
  return NULL;

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

chan->buf = alloc_percpu(struct rchan_buf *);
if (!chan->buf) {
  kfree(chan);
  return NULL;
}

chan->version = RELAYFS_CHANNEL_VERSION;
chan->n_subbufs = n_subbufs;
chan->subbuf_size = subbuf_size;
chan->alloc_size = PAGE_ALIGN(subbuf_size * n_subbufs);
chan->parent = parent;
chan->private_data = private_data;
if (base_filename) {
  chan->has_base_filename = 1;
  strscpy(chan->base_filename, base_filename, NAME_MAX);
}
chan->cb = cb;
kref_init(&chan->kref);

mutex_lock(&relay_channels_mutex);
for_each_online_cpu(i) {
  buf = relay_open_buf(chan, i);
  if (!buf)
   goto free_bufs;
  *per_cpu_ptr(chan->buf, i) = buf;
}
list_add(&chan->list, &relay_channels);
mutex_unlock(&relay_channels_mutex);

return chan;

free_bufs:
for_each_possible_cpu(i) {
  if ((buf = *per_cpu_ptr(chan->buf, i)))
   relay_close_buf(buf);
}

kref_put(&chan->kref, relay_destroy_channel);
mutex_unlock(&relay_channels_mutex);
return NULL;
}
EXPORT_SYMBOL_GPL(relay_open);

struct rchan_percpu_buf_dispatcher {
struct rchan_buf *buf;
struct dentry *dentry;
};

/**
* relay_switch_subbuf - switch to a new sub-buffer
* @buf: channel buffer
* @length: size of current event
*
* Returns either the length passed in or 0 if full.
*
* Performs sub-buffer-switch tasks such as invoking callbacks,
* updating padding counts, waking up readers, etc.
*/
size_t relay_switch_subbuf(struct rchan_buf *buf, size_t length)
{
void *old, *new;
size_t old_subbuf, new_subbuf;

if (unlikely(length > buf->chan->subbuf_size))
  goto toobig;

if (buf->offset != buf->chan->subbuf_size + 1) {
  size_t prev_padding;

  prev_padding = buf->chan->subbuf_size - buf->offset;
  old_subbuf = buf->subbufs_produced % buf->chan->n_subbufs;
  buf->padding[old_subbuf] = prev_padding;
  buf->subbufs_produced++;
  if (buf->dentry)
   d_inode(buf->dentry)->i_size +=
    buf->chan->subbuf_size -
    buf->padding[old_subbuf];
  else
   buf->early_bytes += buf->chan->subbuf_size -
         buf->padding[old_subbuf];
  smp_mb();
  if (waitqueue_active(&buf->read_wait)) {
   /*
* Calling wake_up_interruptible() from here
* will deadlock if we happen to be logging
* from the scheduler (trying to re-grab
* rq->lock), so defer it.
*/
   irq_work_queue(&buf->wakeup_work);
  }
}

old = buf->data;
new_subbuf = buf->subbufs_produced % buf->chan->n_subbufs;
new = buf->start + new_subbuf * buf->chan->subbuf_size;
buf->offset = 0;
if (!relay_subbuf_start(buf, new, old)) {
  buf->offset = buf->chan->subbuf_size + 1;
  return 0;
}
buf->data = new;
buf->padding[new_subbuf] = 0;

if (unlikely(length + buf->offset > buf->chan->subbuf_size))
  goto toobig;

return length;

toobig:
buf->stats.big_count++;
return 0;
}
EXPORT_SYMBOL_GPL(relay_switch_subbuf);

/**
* relay_subbufs_consumed - update the buffer's sub-buffers-consumed count
* @chan: the channel
* @cpu: the cpu associated with the channel buffer to update
* @subbufs_consumed: number of sub-buffers to add to current buf's count
*
* Adds to the channel buffer's consumed sub-buffer count.
* subbufs_consumed should be the number of sub-buffers newly consumed,
* not the total consumed.
*
* NOTE. Kernel clients don't need to call this function if the channel
* mode is 'overwrite'.
*/
void relay_subbufs_consumed(struct rchan *chan,
       unsigned int cpu,
       size_t subbufs_consumed)
{
struct rchan_buf *buf;

if (!chan || cpu >= NR_CPUS)
  return;

buf = *per_cpu_ptr(chan->buf, cpu);
if (!buf || subbufs_consumed > chan->n_subbufs)
  return;

if (subbufs_consumed > buf->subbufs_produced - buf->subbufs_consumed)
  buf->subbufs_consumed = buf->subbufs_produced;
else
  buf->subbufs_consumed += subbufs_consumed;
}
EXPORT_SYMBOL_GPL(relay_subbufs_consumed);

/**
* relay_close - close the channel
* @chan: the channel
*
* Closes all channel buffers and frees the channel.
*/
void relay_close(struct rchan *chan)
{
struct rchan_buf *buf;
unsigned int i;

if (!chan)
  return;

mutex_lock(&relay_channels_mutex);
if (chan->is_global && (buf = *per_cpu_ptr(chan->buf, 0)))
  relay_close_buf(buf);
else
  for_each_possible_cpu(i)
   if ((buf = *per_cpu_ptr(chan->buf, i)))
    relay_close_buf(buf);

list_del(&chan->list);
kref_put(&chan->kref, relay_destroy_channel);
mutex_unlock(&relay_channels_mutex);
}
EXPORT_SYMBOL_GPL(relay_close);

/**
* relay_flush - close the channel
* @chan: the channel
*
* Flushes all channel buffers, i.e. forces buffer switch.
*/
void relay_flush(struct rchan *chan)
{
struct rchan_buf *buf;
unsigned int i;

if (!chan)
  return;

if (chan->is_global && (buf = *per_cpu_ptr(chan->buf, 0))) {
  relay_switch_subbuf(buf, 0);
  return;
}

mutex_lock(&relay_channels_mutex);
for_each_possible_cpu(i)
  if ((buf = *per_cpu_ptr(chan->buf, i)))
   relay_switch_subbuf(buf, 0);
mutex_unlock(&relay_channels_mutex);
}
EXPORT_SYMBOL_GPL(relay_flush);

/**
* relay_stats - get channel buffer statistics
* @chan: the channel
* @flags: select particular information to get
*
* Returns the count of certain field that caller specifies.
*/
size_t relay_stats(struct rchan *chan, int flags)
{
unsigned int i, count = 0;
struct rchan_buf *rbuf;

if (!chan || flags > RELAY_STATS_LAST)
  return 0;

if (chan->is_global) {
  rbuf = *per_cpu_ptr(chan->buf, 0);
  if (flags & RELAY_STATS_BUF_FULL)
   count = rbuf->stats.full_count;
  else if (flags & RELAY_STATS_WRT_BIG)
   count = rbuf->stats.big_count;
} else {
  for_each_online_cpu(i) {
   rbuf = *per_cpu_ptr(chan->buf, i);
   if (rbuf) {
    if (flags & RELAY_STATS_BUF_FULL)
     count += rbuf->stats.full_count;
    else if (flags & RELAY_STATS_WRT_BIG)
     count += rbuf->stats.big_count;
   }
  }
}

return count;
}

/**
* relay_file_open - open file op for relay files
* @inode: the inode
* @filp: the file
*
* Increments the channel buffer refcount.
*/
static int relay_file_open(struct inode *inode, struct file *filp)
{
struct rchan_buf *buf = inode->i_private;
kref_get(&buf->kref);
filp->private_data = buf;

return nonseekable_open(inode, filp);
}

/**
* relay_file_mmap - mmap file op for relay files
* @filp: the file
* @vma: the vma describing what to map
*
* Calls upon relay_mmap_buf() to map the file into user space.
*/
static int relay_file_mmap(struct file *filp, struct vm_area_struct *vma)
{
struct rchan_buf *buf = filp->private_data;
return relay_mmap_buf(buf, vma);
}

/**
* relay_file_poll - poll file op for relay files
* @filp: the file
* @wait: poll table
*
* Poll implemention.
*/
static __poll_t relay_file_poll(struct file *filp, poll_table *wait)
{
__poll_t mask = 0;
struct rchan_buf *buf = filp->private_data;

if (buf->finalized)
  return EPOLLERR;

if (filp->f_mode & FMODE_READ) {
  poll_wait(filp, &buf->read_wait, wait);
  if (!relay_buf_empty(buf))
   mask |= EPOLLIN | EPOLLRDNORM;
}

return mask;
}

/**
* relay_file_release - release file op for relay files
* @inode: the inode
* @filp: the file
*
* Decrements the channel refcount, as the filesystem is
* no longer using it.
*/
static int relay_file_release(struct inode *inode, struct file *filp)
{
struct rchan_buf *buf = filp->private_data;
kref_put(&buf->kref, relay_remove_buf);

return 0;
}

/*
* relay_file_read_consume - update the consumed count for the buffer
*/
static void relay_file_read_consume(struct rchan_buf *buf,
        size_t read_pos,
        size_t bytes_consumed)
{
size_t subbuf_size = buf->chan->subbuf_size;
size_t n_subbufs = buf->chan->n_subbufs;
size_t read_subbuf;

if (buf->subbufs_produced == buf->subbufs_consumed &&
     buf->offset == buf->bytes_consumed)
  return;

if (buf->bytes_consumed + bytes_consumed > subbuf_size) {
  relay_subbufs_consumed(buf->chan, buf->cpu, 1);
  buf->bytes_consumed = 0;
}

buf->bytes_consumed += bytes_consumed;
if (!read_pos)
  read_subbuf = buf->subbufs_consumed % n_subbufs;
else
  read_subbuf = read_pos / buf->chan->subbuf_size;
if (buf->bytes_consumed + buf->padding[read_subbuf] == subbuf_size) {
  if ((read_subbuf == buf->subbufs_produced % n_subbufs) &&
      (buf->offset == subbuf_size))
   return;
  relay_subbufs_consumed(buf->chan, buf->cpu, 1);
  buf->bytes_consumed = 0;
}
}

/*
* relay_file_read_avail - boolean, are there unconsumed bytes available?
*/
static int relay_file_read_avail(struct rchan_buf *buf)
{
size_t subbuf_size = buf->chan->subbuf_size;
size_t n_subbufs = buf->chan->n_subbufs;
size_t produced = buf->subbufs_produced;
size_t consumed;

relay_file_read_consume(buf, 0, 0);

consumed = buf->subbufs_consumed;

if (unlikely(buf->offset > subbuf_size)) {
  if (produced == consumed)
   return 0;
  return 1;
}

if (unlikely(produced - consumed >= n_subbufs)) {
  consumed = produced - n_subbufs + 1;
  buf->subbufs_consumed = consumed;
  buf->bytes_consumed = 0;
}

produced = (produced % n_subbufs) * subbuf_size + buf->offset;
consumed = (consumed % n_subbufs) * subbuf_size + buf->bytes_consumed;

if (consumed > produced)
  produced += n_subbufs * subbuf_size;

if (consumed == produced) {
  if (buf->offset == subbuf_size &&
      buf->subbufs_produced > buf->subbufs_consumed)
   return 1;
  return 0;
}

return 1;
}

/**
* relay_file_read_subbuf_avail - return bytes available in sub-buffer
* @read_pos: file read position
* @buf: relay channel buffer
*/
static size_t relay_file_read_subbuf_avail(size_t read_pos,
        struct rchan_buf *buf)
{
size_t padding, avail = 0;
size_t read_subbuf, read_offset, write_subbuf, write_offset;
size_t subbuf_size = buf->chan->subbuf_size;

write_subbuf = (buf->data - buf->start) / subbuf_size;
write_offset = buf->offset > subbuf_size ? subbuf_size : buf->offset;
read_subbuf = read_pos / subbuf_size;
read_offset = read_pos % subbuf_size;
padding = buf->padding[read_subbuf];

if (read_subbuf == write_subbuf) {
  if (read_offset + padding < write_offset)
   avail = write_offset - (read_offset + padding);
} else
  avail = (subbuf_size - padding) - read_offset;

return avail;
}

/**
* relay_file_read_start_pos - find the first available byte to read
* @buf: relay channel buffer
*
* If the read_pos is in the middle of padding, return the
* position of the first actually available byte, otherwise
* return the original value.
*/
static size_t relay_file_read_start_pos(struct rchan_buf *buf)
{
size_t read_subbuf, padding, padding_start, padding_end;
size_t subbuf_size = buf->chan->subbuf_size;
size_t n_subbufs = buf->chan->n_subbufs;
size_t consumed = buf->subbufs_consumed % n_subbufs;
size_t read_pos = (consumed * subbuf_size + buf->bytes_consumed)
   % (n_subbufs * subbuf_size);

read_subbuf = read_pos / subbuf_size;
padding = buf->padding[read_subbuf];
padding_start = (read_subbuf + 1) * subbuf_size - padding;
padding_end = (read_subbuf + 1) * subbuf_size;
if (read_pos >= padding_start && read_pos < padding_end) {
  read_subbuf = (read_subbuf + 1) % n_subbufs;
  read_pos = read_subbuf * subbuf_size;
}

return read_pos;
}

/**
* relay_file_read_end_pos - return the new read position
* @read_pos: file read position
* @buf: relay channel buffer
* @count: number of bytes to be read
*/
static size_t relay_file_read_end_pos(struct rchan_buf *buf,
          size_t read_pos,
          size_t count)
{
size_t read_subbuf, padding, end_pos;
size_t subbuf_size = buf->chan->subbuf_size;
size_t n_subbufs = buf->chan->n_subbufs;

read_subbuf = read_pos / subbuf_size;
padding = buf->padding[read_subbuf];
if (read_pos % subbuf_size + count + padding == subbuf_size)
  end_pos = (read_subbuf + 1) * subbuf_size;
else
  end_pos = read_pos + count;
if (end_pos >= subbuf_size * n_subbufs)
  end_pos = 0;

return end_pos;
}

static ssize_t relay_file_read(struct file *filp,
          char __user *buffer,
          size_t count,
          loff_t *ppos)
{
struct rchan_buf *buf = filp->private_data;
size_t read_start, avail;
size_t written = 0;
int ret;

if (!count)
  return 0;

inode_lock(file_inode(filp));
do {
  void *from;

  if (!relay_file_read_avail(buf))
   break;

  read_start = relay_file_read_start_pos(buf);
  avail = relay_file_read_subbuf_avail(read_start, buf);
  if (!avail)
   break;

  avail = min(count, avail);
  from = buf->start + read_start;
  ret = avail;
  if (copy_to_user(buffer, from, avail))
   break;

  buffer += ret;
  written += ret;
  count -= ret;

  relay_file_read_consume(buf, read_start, ret);
  *ppos = relay_file_read_end_pos(buf, read_start, ret);
} while (count);
inode_unlock(file_inode(filp));

return written;
}

const struct file_operations relay_file_operations = {
.open  = relay_file_open,
.poll  = relay_file_poll,
.mmap  = relay_file_mmap,
.read  = relay_file_read,
.release = relay_file_release,
};
EXPORT_SYMBOL_GPL(relay_file_operations);

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