linux-kernel - Re: [PATCH 14/39] union-mount: Union mounts documentation

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Date:	Wed, 25 Aug 2010 04:59:01 +0200
From:	Christian Stroetmann <stroetmann@...olinux.com>
To:	Valerie Aurora <vaurora@...hat.com>
CC:	"J. R. Okajima" <hooanon05@...oo.co.jp>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 14/39] union-mount: Union mounts documentation

  Aloha Everybody;

On the 24.08.2010 22:48, Valerie Aurora wrote:
> On Tue, Aug 24, 2010 at 11:28:37AM +0900, J. R. Okajima wrote:
>> Thank you for explanation, very much.

Me too

> You are welcome!
>
>> When a rename happens on a layer directly, aufs receives a
>> inotify/fsnotify event. Following the event type, aufs makes the cached
>> dentry/inode obsoleted and they will be lookup-ed again in the
>> succeeding access. Finally aufs will know the upper parent_dir1 is not
>> covering the lower parent_dir2 anymore.
>> This notification is the main purpose of the strict option which is
>> called "udba=notify" (User's Direct Branch Access).
> No, that's not a sufficient description and leaves open questions
> about all sorts of deadlocks and race conditions.  For example,
> inotify events occur while holding locks only on one layer.  You
> obviously need to lock the top layer to update the inheritance and
> parent-child relationships.  Now you are locking the lower layer first
> and the top layer second, which is the reverse of the usual order.
> Also, it should not be an option.
>
> If Al Viro says it's wrong, you need a very detailed explanation of
> why it is right.  See Documentation/filesystem/directory-locking for
> an example of the argument you have to make to show that moving things
> around on the lower layer is safe.  In general, your first task is to
> show a global lock ordering to prove lack of deadlocks (which I don't
> think you should spend time on because most VFS experts think it is
> impossible to do with two read-write layers).

This all reminds me of the 5/dining philosophers problem and its 
solutions, especially the waiter and the resource hierarchy solutions 
(see [1]).
And I do think that such problems can always be solved in a real world 
context, but often the solutions are very time and/or space consuming.

> I'm not going to explain any more how aufs is wrong; it's the
> maintainer's job to convince Al Viro and other maintainers that aufs
> is right.  But I hope this gave you a start and showed why union
> mounts is a preferred approach for many people.
>
> Thanks,
>
> -VAL

[1] http://en.wikipedia.org/wiki/Dining_philosophers_problem

Have fun
Christian
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