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

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Message-ID: <CAOMqctQauGk0XszoUo61k4GQ5gWWWgyRXY9x9J=D+K4kMJ_CUw@mail.gmail.com>
Date:	Tue, 12 Jul 2011 11:58:35 +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 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 which is held by overlay2 waiting for
lower1 in tmpoverlay2.

Thanks

Michal
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