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Date:	Wed, 06 Aug 2014 16:56:55 +0100
From:	Rob Jones <rob.jones@...ethink.co.uk>
To:	linux-fsdevel@...r.kernel.org
CC:	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-kernel@...ts.codethink.co.uk, viro@...IV.linux.org.uk,
	ebiederm@...ssion.com, ian.molton@...ethink.co.uk
Subject: Re: [PATCH] seq_file: Allow private data to be supplied on seq_open

Does anyone have any feedback on this?

I would have thought it was quite uncontroversial.

On 29/07/14 18:39, Rob Jones wrote:
> Create a function seq_open_priv() that is identical to seq_open() except
> that it accepts a void * parameter that it stores in the private field
> of the struct seq_file.
>
> Document seq_open_priv().
>
> Some consumers of the seq_file interface need to pass data to their
> iterators that is best obtained at the time the seq_file is opened.
>
> At the moment these consumers have to obtain the struct seq_file pointer
> (stored by seq_open() in file->private_data) and then store a pointer to
> their own data in the private field of the struct seq_file so that it
> can be accessed by the iterator functions.
>
> Although this is not a long piece of code it is unneccessary boilerplate.
>
> seq_open() remains in place and its behaviour remains unchanged so no
> existing code should be broken by this patch.
>
> Signed-off-by: Ian Molton <ian.molton@...ethink.co.uk>
> Signed-off-by: Rob Jones <rob.jones@...ethink.co.uk>
> ---
>   Documentation/filesystems/seq_file.txt |    9 +++++++++
>   fs/seq_file.c                          |   30 ++++++++++++++++++++++++++++--
>   include/linux/seq_file.h               |    1 +
>   3 files changed, 38 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/filesystems/seq_file.txt b/Documentation/filesystems/seq_file.txt
> index a1e2e0d..128ffee 100644
> --- a/Documentation/filesystems/seq_file.txt
> +++ b/Documentation/filesystems/seq_file.txt
> @@ -226,6 +226,15 @@ be used for more than one file, you can store an arbitrary pointer in the
>   private field of the seq_file structure; that value can then be retrieved
>   by the iterator functions.
>
> +There is also a function seq_open_priv() which behaves identically to
> +seq_open() except that it takes an additional void * parameter that it
> +stores in the private field of the seq_file structure, thereby making it
> +available to the start function and thus all subsequent iterator functions
> +Note that a corresponding wrapper function for seq_release() may need to
> +be created to free any resources allocated by an open function that uses
> +this capability (although, for simple cases, seq_release_private() may
> +suffice).
> +
>   The other operations of interest - read(), llseek(), and release() - are
>   all implemented by the seq_file code itself. So a virtual file's
>   file_operations structure will look like:
> diff --git a/fs/seq_file.c b/fs/seq_file.c
> index 1d641bb..9a0db94 100644
> --- a/fs/seq_file.c
> +++ b/fs/seq_file.c
> @@ -31,9 +31,10 @@ static void seq_set_overflow(struct seq_file *m)
>   }
>
>   /**
> - *	seq_open -	initialize sequential file
> + *	seq_open_priv -	initialize sequential file with private data
>    *	@file: file we initialize
>    *	@op: method table describing the sequence
> + *	@d: private data to be made available to the iterator functions
>    *
>    *	seq_open() sets @file, associating it with a sequence described
>    *	by @op.  @op->start() sets the iterator up and returns the first
> @@ -43,8 +44,12 @@ static void seq_set_overflow(struct seq_file *m)
>    *	ERR_PTR(error).  In the end of sequence they return %NULL. ->show()
>    *	returns 0 in case of success and negative number in case of error.
>    *	Returning SEQ_SKIP means "discard this element and move on".
> + *
> + *	Supplying @d allows data that is only available at the time the file
> + *	is opened to be supplied to @op->start() (and thereby to @op->next()
> + *	as well).
>    */
> -int seq_open(struct file *file, const struct seq_operations *op)
> +int seq_open_priv(struct file *file, const struct seq_operations *op, void *d)
>   {
>   	struct seq_file *p = file->private_data;
>
> @@ -57,6 +62,7 @@ int seq_open(struct file *file, const struct seq_operations *op)
>   	memset(p, 0, sizeof(*p));
>   	mutex_init(&p->lock);
>   	p->op = op;
> +	p->private = d;
>   #ifdef CONFIG_USER_NS
>   	p->user_ns = file->f_cred->user_ns;
>   #endif
> @@ -80,6 +86,26 @@ int seq_open(struct file *file, const struct seq_operations *op)
>   	file->f_mode &= ~FMODE_PWRITE;
>   	return 0;
>   }
> +EXPORT_SYMBOL(seq_open_priv);
> +
> +/**
> + *	seq_open -	initialize sequential file
> + *	@file: file we initialize
> + *	@op: method table describing the sequence
> + *
> + *	seq_open() sets @file, associating it with a sequence described
> + *	by @op.  @op->start() sets the iterator up and returns the first
> + *	element of sequence. @op->stop() shuts it down.  @op->next()
> + *	returns the next element of sequence.  @op->show() prints element
> + *	into the buffer.  In case of error ->start() and ->next() return
> + *	ERR_PTR(error).  In the end of sequence they return %NULL. ->show()
> + *	returns 0 in case of success and negative number in case of error.
> + *	Returning SEQ_SKIP means "discard this element and move on".
> + */
> +int seq_open(struct file *file, const struct seq_operations *op)
> +{
> +	return seq_open_priv(file, op, NULL);
> +}
>   EXPORT_SYMBOL(seq_open);
>
>   static int traverse(struct seq_file *m, loff_t offset)
> diff --git a/include/linux/seq_file.h b/include/linux/seq_file.h
> index 52e0097..fce87af 100644
> --- a/include/linux/seq_file.h
> +++ b/include/linux/seq_file.h
> @@ -95,6 +95,7 @@ static inline void seq_setwidth(struct seq_file *m, size_t size)
>   void seq_pad(struct seq_file *m, char c);
>
>   char *mangle_path(char *s, const char *p, const char *esc);
> +int seq_open_priv(struct file *, const struct seq_operations *, void *);
>   int seq_open(struct file *, const struct seq_operations *);
>   ssize_t seq_read(struct file *, char __user *, size_t, loff_t *);
>   loff_t seq_lseek(struct file *, loff_t, int);
>

-- 
Rob Jones
Codethink Ltd
mailto:rob.jones@...ethink.co.uk
tel:+44 161 236 5575
--
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