From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rob Jones Subject: Re: [Linux-kernel] [PATCH] seq_file: Allow private data to be supplied on seq_open Date: Thu, 07 Aug 2014 15:30:12 +0100 Message-ID: <53E38D74.4030004@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> <53E38889.3000603@codethink.co.uk> <53E38A3A.5060605@codethink.co.uk> <53E38BC3.40500@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: linux-kernel@lists.codethink.co.uk, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Al Viro , linux-fsdevel@vger.kernel.org To: Steven Whitehouse , "Eric W. Biederman" Return-path: In-Reply-To: <53E38BC3.40500@redhat.com> Sender: linux-doc-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org 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@codethink.co.uk tel:+44 161 236 5575