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

Rob Jones
2014-08-07T14:30:18Z

On 07/08/14 15:22, Steven Whitehouse wrote:
> Hi,
>
> On 07/08/14 15:16, Rob Jones wrote:
>>
>>
>> On 07/08/14 15:09, Rob Jones wrote:
>>> Hi Steve,
>>>
>>> On 07/08/14 14:32, Steven Whitehouse wrote:
>>>> Hi,
>>>>
>>>> On 07/08/14 13:58, Rob Jones wrote:
>>>> [snip]
>>>>>
>>>>> On a related subject, Having looked at a few uses of seq_file, I must
>>>>> say that some users seem to make assumptions about the internal
>>>>> workings of the module. Dangerous behaviour as only some behaviours
>>>>> are
>>>>> documented.
>>>>>
>>>>> e.g. The behaviour that "struct seq_file" pointer is stored in
>>>>> file->private_data is documented and can therefore be relied upon but
>>>>> the fact that the output buffer and its size are only defined at the
>>>>> first output (and can therefore be pre-defined and pre-allocated by
>>>>> user code) is not documented and could therefore change without
>>>>> warning.
>>>>>
>>>>> This second behaviour is assumed in, for example, module
>>>>> fs/gfs2/glock.c
>>>>> which could, therefore, stop working properly without warning if the
>>>>> internal behaviour was changed.
>>>>>
>>>> While it is undocumented, it is I understand, how this feature was
>>>> intended to be used, so I think that it is safe to do this in the GFS2
>>>> case. Here is a ref to the thread which explains how it landed up like
>>>> that:
>>>> https://www.redhat.com/archives/cluster-devel/2012-June/msg00000.html
>>>
>>> No criticism was intended of that particular piece of code, It has been
>>> there for a couple of years and is, presumably, still working :-)
>>>
>>> It was just a general point about things needing to be written down. A
>>> behaviour such as you were relying on can be a very positive thing but
>>> it would be of much greater use if it was written down in the file docs.
>>>
>>> I completely missed seq_file_private() because I was looking at the
>>
>> Sorry, that should be seq_open_private()
>>
>> Why does one never see the mistake until *after* hitting send?
>>
> Always the way, unfortunately!
>
>>> docs more than the code. If it had been written down in the docs it
>>> would have saved me quite a bit of time, similarly, if the buffer
>>> allocation behaviour was documented, changes to seq_file.c would not be
>>> made that could break your code.
>>>
>>> God knows, I'm not a fan of unnecessary documentation but where it's
>>> useful I'm all for it.
>>>
> Yes, very much agreed, and no doubt it would be useful in this case. I
> hoped that the earlier thread might be a useful starting point, since it
> explained some of the whys and wherefores,

Well, I'm making a start by documenting seq_open_private(). Small
steps :-)

>
> Steve.
>
>
>

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