From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rob Jones Subject: Re: [PATCH] seq_file: Allow private data to be supplied on seq_open Date: Thu, 07 Aug 2014 15:09:13 +0100 Message-ID: <53E38889.3000603@codethink.co.uk> References: <1406655593-12626-1-git-send-email-rob.jones@codethink.co.uk> <20140806160259.GR18016@ZenIV.linux.org.uk> <53E254F1.30605@codethink.co.uk> <87sil9sa50.fsf@x220.int.ebiederm.org> <53E37810.1050109@codethink.co.uk> <53E37FDF.50102@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Al Viro , linux-fsdevel@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kernel@lists.codethink.co.uk, ian.molton@codethink.co.uk To: Steven Whitehouse , "Eric W. Biederman" Return-path: In-Reply-To: <53E37FDF.50102@redhat.com> Sender: linux-doc-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org 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 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@codethink.co.uk tel:+44 161 236 5575