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    SubjectRe: [PATCH 21/32] union-mount: Make lookup work for union-mounted file systems
    On Tue, May 19, 2009 at 06:15:52PM +0200, Miklos Szeredi wrote:
    > On Mon, 18 May 2009, Jan Blunck wrote:
    > > On union-mounted file systems the lookup function must also visit lower layers
    > > of the union-stack when doing a lookup. This patches add support for
    > > union-mounts to cached lookups and real lookups.
    > >
    > > We have 3 different styles of lookup functions now:
    > > - multiple pathname components, follow mounts, follow union, follow symlinks
    > > - single pathname component, doesn't follow mounts, follow union, doesn't
    > > follow symlinks
    > > - single pathname component doesn't follow mounts, doesn't follow unions,
    > > doesn't follow symlinks
    >
    > Ugh... I do wonder if this could be done in a less complicated way,
    > there does seem to be a fair amount of duplication between these
    > functions.

    Yeah, I agree. My best idea so far is not very good - have one
    skeleton function and pass in function pointers for the
    lookup_topmost() and build_union() functions. Do you have any ideas?

    > Worse, it looks like there are still i_mutex lock ordering issues
    > (__hash_lookup_topmost()/__hash_lookup_build_union()). What happens
    > if two separate unions of two filesystems are built where the order of
    > branches is reversed?

    We have a similar problem in union_copyup_dir(). Hm, thinking about
    this, only one of the file systems can actually change while we are
    doing work. That might help us get out of the lock ordering problems.
    Thoughts?

    -VAL


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