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SubjectRe: Union mount and lockdep design issues
Michal Suchanek <> 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.


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