Re: a take at cache coherency

["Simon 'corecode' Schubert" <corecode@xxxxxxxxxxxx>]

Csaba Henk wrote:

>   - Unidirectonal synchronization. Only upper layers keep a reference to
>     lower ones, a lower ("shadowed") layer doesn't need to be aware of
>     being overlayed.
>   - Namecache API got messed up a bit, too. Some attributes can't be
>     accessed directly: you can't use "ncp->nc_vp", you need to do
>     "cache_grphead(ncp)->nc_vp". Eventually some kind of wrapper should
>     be made around namecache entries, so that you could write eg.
>     "NC_VP(ncp)". At this stage I didn't do this.
>   - Some attributes of namecache entries were transformed into shadow
>     group attributes: these are
>
>       nc_vp, nc_error, nc_timeout, nc_exlocks, nc_locktd, and some of
>       the flags:
>         NCF_LOCKED, NCF_WHITEOUT, NCF_UNRESOLVED, NCF_ISSYMLINK, NCF_ISDIR,
>         NCF_ISDESTROYED
>       -- ie, attributes which refer the underlying vnode, and locking.

i'm sorry but i don't like the patch for those reasons.  general vfs 
code gets cluttered with namecache interal going-ons (the shadow groups) 
and makes it super-easy to introduce a mistake.

I think if we already carry the fields around in the namecache structs 
we should also populate them.

>     * Unsolved issues. It's not handled properly when the lower layer
>       gets unmounted. There are two possible behaviours:
>
>         - keep lower fs busy, don't let it go.
> 	- let lower fs go; when it's gone, return ENXIO from calls.

+ renames on the lower layer don't trigger namecache 
renames/invalidations on a higher layer.  this is what the coherency 
should be about, i think.

maybe we should discuss design before actually implementing it? :)

cheers
  simon

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