Commit 13c589d5 authored by Tejun Heo's avatar Tejun Heo Committed by Greg Kroah-Hartman
Browse files

sysfs: use seq_file when reading regular files

sysfs read path implements its own buffering scheme between userland
and kernel callbacks, which essentially is a degenerate duplicate of
seq_file.  This patch replaces the custom read buffering
implementation in sysfs with seq_file.

While the amount of code reduction is small, this reduces low level
hairiness and enables future development of a new versatile API based
on seq_file so that sysfs features can be shared with other

As write path was already converted to not use sysfs_open_file->page,
this patch makes ->page and ->count unused and removes them.

Userland behavior remains the same except for some extreme corner
cases - e.g. sysfs will now regenerate the content each time a file is
read after a non-contiguous seek whereas the original code would keep
using the same content.  While this is a userland visible behavior
change, it is extremely unlikely to be noticeable and brings sysfs
behavior closer to that of procfs.
Signed-off-by: default avatarTejun Heo <>
Cc: Kay Sievers <>
Signed-off-by: default avatarGreg Kroah-Hartman <>
parent 8ef445f0
......@@ -21,6 +21,7 @@
#include <linux/mutex.h>
#include <linux/limits.h>
#include <linux/uaccess.h>
#include <linux/seq_file.h>
#include "sysfs.h"
......@@ -31,7 +32,8 @@
* sysfs_dirent-> points to sysfs_open_dirent. is
* protected by sysfs_open_dirent_lock.
* filp->private_data points to sysfs_open_file which is chained at
* filp->private_data points to seq_file whose ->private points to
* sysfs_open_file. sysfs_open_files are chained at
* sysfs_open_dirent->files, which is protected by sysfs_open_file_mutex.
static DEFINE_SPINLOCK(sysfs_open_dirent_lock);
......@@ -47,13 +49,16 @@ struct sysfs_open_dirent {
struct sysfs_open_file {
struct sysfs_dirent *sd;
struct file *file;
size_t count;
char *page;
struct mutex mutex;
int event;
struct list_head list;
static struct sysfs_open_file *sysfs_of(struct file *file)
return ((struct seq_file *)file->private_data)->private;
* Determine ktype->sysfs_ops for the given sysfs_dirent. This function
* must be called while holding an active reference.
......@@ -66,40 +71,54 @@ static const struct sysfs_ops *sysfs_file_ops(struct sysfs_dirent *sd)
return kobj->ktype ? kobj->ktype->sysfs_ops : NULL;
* fill_read_buffer - allocate and fill buffer from object.
* @dentry: dentry pointer.
* @buffer: data buffer for file.
* Allocate @buffer->page, if it hasn't been already, then call the
* kobject's show() method to fill the buffer with this attribute's
* data.
* This is called only once, on the file's first read unless an error
* is returned.
* Reads on sysfs are handled through seq_file, which takes care of hairy
* details like buffering and seeking. The following function pipes
* sysfs_ops->show() result through seq_file.
static int fill_read_buffer(struct dentry *dentry, struct sysfs_open_file *of)
static int sysfs_seq_show(struct seq_file *sf, void *v)
struct sysfs_dirent *attr_sd = dentry->d_fsdata;
struct kobject *kobj = attr_sd->s_parent->s_dir.kobj;
struct sysfs_open_file *of = sf->private;
struct kobject *kobj = of->sd->s_parent->s_dir.kobj;
const struct sysfs_ops *ops;
int ret = 0;
char *buf;
ssize_t count;
if (!of->page)
of->page = (char *) get_zeroed_page(GFP_KERNEL);
if (!of->page)
return -ENOMEM;
/* acquire buffer and ensure that it's >= PAGE_SIZE */
count = seq_get_buf(sf, &buf);
if (count < PAGE_SIZE) {
seq_commit(sf, -1);
return 0;
/* need attr_sd for attr and ops, its parent for kobj */
if (!sysfs_get_active(attr_sd))
* Need @of->sd for attr and ops, its parent for kobj. @of->mutex
* nests outside active ref and is just to ensure that the ops
* aren't called concurrently for the same open file.
if (!sysfs_get_active(of->sd)) {
return -ENODEV;
of->event = atomic_read(&attr_sd->>event);
of->event = atomic_read(&of->sd->>event);
ops = sysfs_file_ops(attr_sd);
count = ops->show(kobj, attr_sd->s_attr.attr, of->page);
* Lookup @ops and invoke show(). Control may reach here via seq
* file lseek even if @ops->show() isn't implemented.
ops = sysfs_file_ops(of->sd);
if (ops->show)
count = ops->show(kobj, of->sd->s_attr.attr, buf);
count = 0;
if (count < 0)
return count;
* The code works fine with PAGE_SIZE return but it's likely to
......@@ -111,54 +130,8 @@ static int fill_read_buffer(struct dentry *dentry, struct sysfs_open_file *of)
/* Try to struggle along */
count = PAGE_SIZE - 1;
if (count >= 0)
of->count = count;
ret = count;
return ret;
* sysfs_read_file - read an attribute.
* @file: file pointer.
* @buf: buffer to fill.
* @count: number of bytes to read.
* @ppos: starting offset in file.
* Userspace wants to read an attribute file. The attribute descriptor
* is in the file's ->d_fsdata. The target object is in the directory's
* ->d_fsdata.
* We call fill_read_buffer() to allocate and fill the buffer from the
* object's show() method exactly once (if the read is happening from
* the beginning of the file). That should fill the entire buffer with
* all the data the object has to offer for that attribute.
* We then call flush_read_buffer() to copy the buffer to userspace
* in the increments specified.
static ssize_t
sysfs_read_file(struct file *file, char __user *buf, size_t count, loff_t *ppos)
struct sysfs_open_file *of = file->private_data;
ssize_t retval = 0;
* Fill on zero offset and the first read so that silly things like
* "dd bs=1 skip=N" can work on sysfs files.
if (*ppos == 0 || !of->page) {
retval = fill_read_buffer(file->f_path.dentry, of);
if (retval)
goto out;
pr_debug("%s: count = %zd, ppos = %lld, buf = %s\n",
__func__, count, *ppos, of->page);
retval = simple_read_from_buffer(buf, count, ppos, of->page, of->count);
return retval;
seq_commit(sf, count);
return 0;
......@@ -216,7 +189,7 @@ static int flush_write_buffer(struct sysfs_open_file *of, char *buf,
static ssize_t sysfs_write_file(struct file *file, const char __user *user_buf,
size_t count, loff_t *ppos)
struct sysfs_open_file *of = file->private_data;
struct sysfs_open_file *of = sysfs_of(file);
ssize_t len = min_t(size_t, count, PAGE_SIZE - 1);
char *buf;
......@@ -364,10 +337,7 @@ static int sysfs_open_file(struct inode *inode, struct file *file)
goto err_out;
* No error? Great, allocate a sysfs_open_file for the file, and
* store it it in file->private_data for easy access.
/* allocate a sysfs_open_file for the file */
error = -ENOMEM;
of = kzalloc(sizeof(struct sysfs_open_file), GFP_KERNEL);
if (!of)
......@@ -376,20 +346,34 @@ static int sysfs_open_file(struct inode *inode, struct file *file)
of->sd = attr_sd;
of->file = file;
file->private_data = of;
* Always instantiate seq_file even if read access is not
* implemented or requested. This unifies private data access and
* most files are readable anyway.
error = single_open(file, sysfs_seq_show, of);
if (error)
goto err_free;
/* seq_file clears PWRITE unconditionally, restore it if WRITE */
if (file->f_mode & FMODE_WRITE)
file->f_mode |= FMODE_PWRITE;
/* make sure we have open dirent struct */
error = sysfs_get_open_dirent(attr_sd, of);
if (error)
goto err_free;
goto err_close;
/* open succeeded, put active references */
return 0;
single_release(inode, file);
return error;
......@@ -397,12 +381,10 @@ static int sysfs_open_file(struct inode *inode, struct file *file)
static int sysfs_release(struct inode *inode, struct file *filp)
struct sysfs_dirent *sd = filp->f_path.dentry->d_fsdata;
struct sysfs_open_file *of = filp->private_data;
struct sysfs_open_file *of = sysfs_of(filp);
sysfs_put_open_dirent(sd, of);
if (of->page)
free_page((unsigned long)of->page);
single_release(inode, filp);
return 0;
......@@ -423,7 +405,7 @@ static int sysfs_release(struct inode *inode, struct file *filp)
static unsigned int sysfs_poll(struct file *filp, poll_table *wait)
struct sysfs_open_file *of = filp->private_data;
struct sysfs_open_file *of = sysfs_of(filp);
struct sysfs_dirent *attr_sd = filp->f_path.dentry->d_fsdata;
struct sysfs_open_dirent *od = attr_sd->;
......@@ -481,9 +463,9 @@ void sysfs_notify(struct kobject *k, const char *dir, const char *attr)
const struct file_operations sysfs_file_operations = {
.read = sysfs_read_file,
.read = seq_read,
.write = sysfs_write_file,
.llseek = generic_file_llseek,
.llseek = seq_lseek,
.open = sysfs_open_file,
.release = sysfs_release,
.poll = sysfs_poll,
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment