linux-kernel - Re: [PATCH] locks: export device name

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Message-Id: <20120215123929.6888c867.akpm@linux-foundation.org>
Date:	Wed, 15 Feb 2012 12:39:29 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Davidlohr Bueso <dave@....org>, Matthew Wilcox <matthew@....cx>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] locks: export device name

On Wed, 15 Feb 2012 07:42:30 -0500
"J. Bruce Fields" <bfields@...ldses.org> wrote:

> > > Perhaps safest would be to replace /proc/locks by another interface and
> > > deprecate this one.
> > 
> > If exporting the name in the current /proc/locks file is out of the
> > question, then IMHO I don't think it would be worth adding a new
> > interface just for such a small change.
> 
> OK.
> 
> If you want to just change this over, I guess the thing to do would be
> to stick something in feature-removal-schedule.txt saying "we'll switch
> this in 2 years" (or however long you think before there are
> realistically no more lslk users left), then do it then.
> 
> Switching to a new api would be better as we could warn users of the old
> api then.  Maybe it'd be worth it if there was some other change we'd
> been wanting to make?   Can't think of anything off the top of my head.
> 
> We may be adding more lock types--will lslk and lslocks handle that
> gracefully?

Adding a whole new interface is pretty attractive.  It lets us get it
right this time.  In particular, something which is extensible given
certain simple rules.  As we've learned, the current /proc/locks didn't
get that right!

We can eventually remove the old code - it may take longer than two
years, but whatever.  If we go this way, we should arrange for the
kernel to emit a warning (printk_once) into the logs the first time
someone accesses the old file.  This will help to prompt people to
migrate off the deprecated interface.  After a while, we can add a
config option to make the old interface go away.  Distros will start to
disable the feature.  Later, we zap it altogether.

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