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Date:	Thu, 07 Aug 2014 15:30:12 +0100
From:	Rob Jones <>
To:	Steven Whitehouse <>,
	"Eric W. Biederman" <>
CC:,,, Al Viro <>,
Subject: Re: [PATCH] seq_file: Allow private data to be supplied
 on seq_open

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:
>>> 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
tel:+44 161 236 5575
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