linux-kernel - Re: Union mount and lockdep design issues

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Date:	Tue, 12 Jul 2011 20:49:13 +0200
From:	Michal Suchanek <hramrach@...trum.cz>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	Ian Kent <ikent@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	David Howells <dhowells@...hat.com>,
	Ric Wheeler <rwheeler@...hat.com>,
	Alexander Viro <aviro@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Jeff Moyer <jmoyer@...hat.com>
Subject: Re: Union mount and lockdep design issues

On 12 July 2011 13:45, Miklos Szeredi <miklos@...redi.hu> wrote:
> Michal Suchanek <hramrach@...trum.cz> writes:
>
>> On 12 July 2011 10:30, Miklos Szeredi <miklos@...redi.hu> wrote:
>>> Michal Suchanek <hramrach@...trum.cz> writes:
>>>
>>>> The locking order is likely determined by the structure of the union
>>>> and not some system-wide order of filesystems so assuming the readonly
>>>> layers are locked as well you will probably get a deadlock with
>>>> technically correct mount:
>>>>
>>>> mount -t overlayfs overlayfs -olowerdir=/lower2,upperdir=/upper /tmpoverlay
>>>> mount -t overlayfs overlayfs -olowerdir=/lower1,upperdir=/tmpoverlay /overlay
>>>>
>>>> mount -t overlayfs overlayfs -olowerdir=/lower1,upperdir=/upper2 /tmpoverlay2
>>>> mount -t overlayfs overlayfs -olowerdir=/lower2,upperdir=/tmpoverlay2 /overlay2
>>>>
>>>> because now lower1 and lower2 are differently ordered in the two
>>>> overlays.
>>>
>>> Overlayfs never locks both upper and lower at the same time, which means
>>> there's no AB-BA locking dependency.  The lock orderings are:
>>>
>>> -> /overlay
>>>  -> /lower1
>>>  -> /tmpoverlay
>>>    -> /lower2
>>>    -> /upper
>>> -> /overlay2
>>>  -> /lower2
>>>  -> /tmpoverlay2
>>>    -> /lower1
>>>    -> /upper2
>>>
>>> As you can see there's no nesting of lower2 and lower1 into each other.
>>>
>>> When you combine two filesystems, a completely new ordering is created
>>> each time, there's no possibility to make an AB-BA nesting.  At least I
>>> cannot see one.
>>
>> Except you can get in situation where overlay locks lower1 and
>> tmpoverlay waits for lower2
>
> Note: tmpoverlay lock does *not* nest into lower1 lock, they are both on
> the same nesting level.  There's no dependency between the two.
>
>>  which is held by overlay2 waiting for
>> lower1 in tmpoverlay2.
>
> No deadlock there.
>

That's nice.

You can still do

mount --bind /lower1 /lower2/lower1
mount --bind /lower2 /lower1/lower2

Which is technically not against usage guidelines, unlike mount --bind
/upper /lower1/upper

If crossing mount boundaries is forbidden try with symlinks or hardlinks.

Thanks

Michal
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