Re: [PATCH 21/32] union-mount: Make lookup work for union-mounted file systems

[Miklos Szeredi]

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.

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?

Thanks,
Miklos
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