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

[Rob Jones]

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?

> 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
mailto:rob.jones@xxxxxxxxxxxxxxx
tel:+44 161 236 5575
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