lkml.org 
[lkml]   [2011]   [Jul]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: Union mount and lockdep design issues
Date
Michal Suchanek <hramrach@centrum.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


\
 
 \ /
  Last update: 2011-07-12 10:31    [from the cache]
©2003-2011 Jasper Spaans