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

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Date:	Tue, 12 Jul 2011 10:30:19 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Michal Suchanek <hramrach@...trum.cz>
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

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.

Thanks,
Miklos
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