sysfs, kernfs: prepare read path for kernfs

We're in the process of separating out core sysfs functionality into
kernfs which will deal with sysfs_dirents directly.  This patch
rearranges read path so that the kernfs and sysfs parts are separate.

* Regular file read path is refactored such that
  kernfs_seq_start/next/stop/show() handle all the boilerplate work
  including locking and updating event count for poll, while
  sysfs_kf_seq_show() deals with interaction with kobj show method.

* Bin file read path is refactored such that kernfs_file_direct_read()
  handles all the boilerplate work including buffer management and
  locking, while sysfs_kf_bin_read() deals with interaction with
  bin_attribute read method.

kernfs_file_read() is added.  It invokes either the seq_file or direct
read path depending on the file type.  This will eventually allow
using the same file_operations for both file types, which is necessary
to separate out kernfs.

While this patch changes the order of some operations, it shouldn't
change any visible behavior.

v2: Dropped unnecessary zeroing of @count from sysfs_kf_seq_show().
    Add comments explaining single_open() behavior.  Both suggested by
    Pavel.

v3: seq_stop() is called even after seq_start() failed.
    kernfs_seq_start() updated so that it doesn't unlock
    sysfs_open_file->mutex on failure so that kernfs_seq_stop()
    doesn't try to unlock an already unlocked mutex.  Reported by
    Fengguang.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 126 insertions, 65 deletions

diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index 9b58d874c825..b695b8b229fc 100644
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -86,13 +86,13 @@ static const struct sysfs_ops *sysfs_file_ops(struct sysfs_dirent *sd)
  * details like buffering and seeking.  The following function pipes
  * sysfs_ops->show() result through seq_file.
  */
-static int sysfs_seq_show(struct seq_file *sf, void *v)
+static int sysfs_kf_seq_show(struct seq_file *sf, void *v)
 {
         struct sysfs_open_file *of = sf->private;
         struct kobject *kobj = of->sd->s_parent->priv;
-        const struct sysfs_ops *ops;
-        char *buf;
+        const struct sysfs_ops *ops = sysfs_file_ops(of->sd);
         ssize_t count;
+        char *buf;
 
         /* acquire buffer and ensure that it's >= PAGE_SIZE */
         count = seq_get_buf(sf, &buf);
@@ -102,33 +102,14 @@ static int sysfs_seq_show(struct seq_file *sf, void *v)
         }
 
         /*
-         * 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.
+         * Invoke show().  Control may reach here via seq file lseek even
+         * if @ops->show() isn't implemented.
          */
-        mutex_lock(&of->mutex);
-        if (!sysfs_get_active(of->sd)) {
-                mutex_unlock(&of->mutex);
-                return -ENODEV;
-        }
-
-        of->event = atomic_read(&of->sd->s_attr.open->event);
-
-        /*
-         * 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)
+        if (ops->show) {
                 count = ops->show(kobj, of->sd->priv, buf);
-        else
-                count = 0;
-
-        sysfs_put_active(of->sd);
-        mutex_unlock(&of->mutex);
-
-        if (count < 0)
-                return count;
+                if (count < 0)
+                        return count;
+        }
 
         /*
          * The code works fine with PAGE_SIZE return but it's likely to
@@ -144,68 +125,146 @@ static int sysfs_seq_show(struct seq_file *sf, void *v)
         return 0;
 }
 
-/*
- * Read method for bin files.  As reading a bin file can have side-effects,
- * the exact offset and bytes specified in read(2) call should be passed to
- * the read callback making it difficult to use seq_file.  Implement
- * simplistic custom buffering for bin files.
- */
-static ssize_t sysfs_bin_read(struct file *file, char __user *userbuf,
-                              size_t bytes, loff_t *off)
+static ssize_t sysfs_kf_bin_read(struct sysfs_open_file *of, char *buf,
+                                 size_t count, loff_t pos)
 {
-        struct sysfs_open_file *of = sysfs_of(file);
         struct bin_attribute *battr = of->sd->priv;
         struct kobject *kobj = of->sd->s_parent->priv;
-        loff_t size = file_inode(file)->i_size;
-        int count = min_t(size_t, bytes, PAGE_SIZE);
-        loff_t offs = *off;
-        char *buf;
+        loff_t size = file_inode(of->file)->i_size;
 
-        if (!bytes)
+        if (!count)
                 return 0;
 
         if (size) {
-                if (offs > size)
+                if (pos > size)
                         return 0;
-                if (offs + count > size)
-                        count = size - offs;
+                if (pos + count > size)
+                        count = size - pos;
         }
 
-        buf = kmalloc(count, GFP_KERNEL);
+        if (!battr->read)
+                return -EIO;
+
+        return battr->read(of->file, kobj, battr, buf, pos, count);
+}
+
+static void *kernfs_seq_start(struct seq_file *sf, loff_t *ppos)
+{
+        struct sysfs_open_file *of = sf->private;
+
+        /*
+         * @of->mutex nests outside active ref and is just to ensure that
+         * the ops aren't called concurrently for the same open file.
+         */
+        mutex_lock(&of->mutex);
+        if (!sysfs_get_active(of->sd))
+                return ERR_PTR(-ENODEV);
+
+        /*
+         * The same behavior and code as single_open().  Returns !NULL if
+         * pos is at the beginning; otherwise, NULL.
+         */
+        return NULL + !*ppos;
+}
+
+static void *kernfs_seq_next(struct seq_file *sf, void *v, loff_t *ppos)
+{
+        /*
+         * The same behavior and code as single_open(), always terminate
+         * after the initial read.
+         */
+        ++*ppos;
+        return NULL;
+}
+
+static void kernfs_seq_stop(struct seq_file *sf, void *v)
+{
+        struct sysfs_open_file *of = sf->private;
+
+        sysfs_put_active(of->sd);
+        mutex_unlock(&of->mutex);
+}
+
+static int kernfs_seq_show(struct seq_file *sf, void *v)
+{
+        struct sysfs_open_file *of = sf->private;
+
+        of->event = atomic_read(&of->sd->s_attr.open->event);
+
+        return sysfs_kf_seq_show(sf, v);
+}
+
+static const struct seq_operations kernfs_seq_ops = {
+        .start = kernfs_seq_start,
+        .next = kernfs_seq_next,
+        .stop = kernfs_seq_stop,
+        .show = kernfs_seq_show,
+};
+
+/*
+ * As reading a bin file can have side-effects, the exact offset and bytes
+ * specified in read(2) call should be passed to the read callback making
+ * it difficult to use seq_file.  Implement simplistic custom buffering for
+ * bin files.
+ */
+static ssize_t kernfs_file_direct_read(struct sysfs_open_file *of,
+                                       char __user *user_buf, size_t count,
+                                       loff_t *ppos)
+{
+        ssize_t len = min_t(size_t, count, PAGE_SIZE);
+        char *buf;
+
+        buf = kmalloc(len, GFP_KERNEL);
         if (!buf)
                 return -ENOMEM;
 
-        /* need of->sd for battr, 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.
+         */
         mutex_lock(&of->mutex);
         if (!sysfs_get_active(of->sd)) {
-                count = -ENODEV;
+                len = -ENODEV;
                 mutex_unlock(&of->mutex);
                 goto out_free;
         }
 
-        if (battr->read)
-                count = battr->read(file, kobj, battr, buf, offs, count);
-        else
-                count = -EIO;
+        len = sysfs_kf_bin_read(of, buf, len, *ppos);
 
         sysfs_put_active(of->sd);
         mutex_unlock(&of->mutex);
 
-        if (count < 0)
+        if (len < 0)
                 goto out_free;
 
-        if (copy_to_user(userbuf, buf, count)) {
-                count = -EFAULT;
+        if (copy_to_user(user_buf, buf, len)) {
+                len = -EFAULT;
                 goto out_free;
         }
 
-        pr_debug("offs = %lld, *off = %lld, count = %d\n", offs, *off, count);
-
-        *off = offs + count;
+        *ppos += len;
 
  out_free:
         kfree(buf);
-        return count;
+        return len;
+}
+
+/**
+ * kernfs_file_read - kernfs vfs read callback
+ * @file: file pointer
+ * @user_buf: data to write
+ * @count: number of bytes
+ * @ppos: starting offset
+ */
+static ssize_t kernfs_file_read(struct file *file, char __user *user_buf,
+                                size_t count, loff_t *ppos)
+{
+        struct sysfs_open_file *of = sysfs_of(file);
+
+        if (sysfs_is_bin(of->sd))
+                return kernfs_file_direct_read(of, user_buf, count, ppos);
+        else
+                return seq_read(file, user_buf, count, ppos);
 }
 
 /**
@@ -677,12 +736,14 @@ static int sysfs_open_file(struct inode *inode, struct file *file)
          * and readable regular files are the vast majority anyway.
          */
         if (sysfs_is_bin(attr_sd))
-                error = single_open(file, NULL, of);
+                error = seq_open(file, NULL);
         else
-                error = single_open(file, sysfs_seq_show, of);
+                error = seq_open(file, &kernfs_seq_ops);
         if (error)
                 goto err_free;
 
+        ((struct seq_file *)file->private_data)->private = of;
+
         /* seq_file clears PWRITE unconditionally, restore it if WRITE */
         if (file->f_mode & FMODE_WRITE)
                 file->f_mode |= FMODE_PWRITE;
@@ -697,7 +758,7 @@ static int sysfs_open_file(struct inode *inode, struct file *file)
         return 0;
 
 err_close:
-        single_release(inode, file);
+        seq_release(inode, file);
 err_free:
         kfree(of);
 err_out:
@@ -711,7 +772,7 @@ static int sysfs_release(struct inode *inode, struct file *filp)
         struct sysfs_open_file *of = sysfs_of(filp);
 
         sysfs_put_open_dirent(sd, of);
-        single_release(inode, filp);
+        seq_release(inode, filp);
         kfree(of);
 
         return 0;
@@ -816,7 +877,7 @@ void sysfs_notify(struct kobject *k, const char *dir, const char *attr)
 EXPORT_SYMBOL_GPL(sysfs_notify);
 
 const struct file_operations sysfs_file_operations = {
-        .read           = seq_read,
+        .read           = kernfs_file_read,
         .write          = sysfs_write_file,
         .llseek         = generic_file_llseek,
         .open           = sysfs_open_file,
@@ -825,7 +886,7 @@ const struct file_operations sysfs_file_operations = {
 };
 
 const struct file_operations sysfs_bin_operations = {
-        .read           = sysfs_bin_read,
+        .read           = kernfs_file_read,
         .write          = sysfs_write_file,
         .llseek         = generic_file_llseek,
         .mmap           = sysfs_bin_mmap,