Re: [PATCH] seq_file: Allow private data to be supplied on seq_open

[ebiederm@xmission.com (Eric W. Biederman)]

Rob Jones <rob.jones@codethink.co.uk> writes:

> On 06/08/14 17:02, Al Viro wrote:
>> On Tue, Jul 29, 2014 at 06:39:53PM +0100, Rob Jones wrote:
>>
>>> 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.
>>
>> How many of those do we actually have?
>
> A quick grep (I didn't examine them all) showed what looked like at
> least 80 instances of the work around.

I took a quick look as well and what I saw was that if we were to
implement the helpers: seq_open_PDE_DATA, and seq_open_i_private.  That
would cover essentially all of seq_open that set seq_file->private.  So
my gut feel is that a seq_open_priv is the wrong helper.

In the vast majority of the cases either seq_open is good enough,
we want seq_open_private, or seq_file->private is set to PDE_DATA
or i_private.

I think there may be 5 cases in the kernel that do something different,
and those cases probably need a code audit.

>>> seq_open() remains in place and its behaviour remains unchanged so no
>>> existing code should be broken by this patch.
>>
>> I have no objections against such helper, but I's rather have it
>> implemented via seq_open() (and as a static inline, not an export),
>> not the other way round.  Oh, and conversion of at least some users would
>> be nice to have as well...

I have no significant objection but having both seq_open_private
and seq_open_priv seems confusing name wise.

Eric