linux-kernel - Re: [PATCH] seq_file: Allow private data to be supplied on seq

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Message-ID: <53E38BC3.40500@redhat.com>
Date:	Thu, 07 Aug 2014 15:22:59 +0100
From:	Steven Whitehouse <swhiteho@...hat.com>
To:	Rob Jones <rob.jones@...ethink.co.uk>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	linux-kernel@...ts.codethink.co.uk, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, Al Viro <viro@...IV.linux.org.uk>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] seq_file: Allow private data to be supplied
 on seq_open

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,

Steve.

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