Re: nullfs stabilization I

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

Matthew Dillon wrote:
>     Let me suggest an alternative approach.  The real problem here is
>     always going to be locking of the namecache record(s).  The creation
>     aspect of the namecache record(s) (the shadow chain) is trivial since
>     an upper layer can simply do the translation and ask for the lower
>     layer.  In the absense of locking, we do not have any deadlock issues
>     with this approach.  The code would also be more readable.
>
>     So perhaps the solution here is to create an auxillary locking structure
>     that all related namecache records can share.  This locking structure
>     would serve to 'glue' the namecache records in the chain together.

I still don't understand at which point we want to associate the upper 
layer with the lower layer.  I think this has to be done upon creation 
of the upper namecache record, which means in cache_lookup().

Otherwise cache_lookup() will return a namecache record which is a) 
locked but b) not sharing the lock with lower layers, because it doesn't 
have the association yet.  I don't think this will work.  Especially if 
we want to have one shared locking structure, we need to associate every 
record with this structure from the beginning (== in cache_lookup()).

But doing this in cache_lookup() requires a VFS upcall there.  Either by 
resolving the namecache directly (slow) or by just associating it with a 
lower layer namecache record (and the glue lock, at the same time), via 
a vfs_getshadow() upcall, for instance.

maybe i'm just missing something quite obvious how to get this locking 
right without needing this upcall in cache_lookup(), but i just don't 
see it.

cheers
  simon

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