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

[Rob Jones]

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@xxxxxxxxxxxxxxx
tel:+44 161 236 5575
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