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Date:	Thu, 07 Aug 2014 15:16:26 +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 15:09, 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?

> 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.
>> Steve.

Rob Jones
Codethink Ltd
tel:+44 161 236 5575
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