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    SubjectRe: Union mount and lockdep design issues
    On 12 July 2011 10:30, Miklos Szeredi <miklos@szeredi.hu> wrote:
    > 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.

    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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