Reference counting in pnfs:
==========================

The are several inter-related caches.  We have layouts which can
reference multiple devices, each of which can reference multiple data servers.
Each data server can be referenced by multiple devices.  Each device
can be referenced by multiple layouts.  To keep all of this straight,
we need to reference count.


struct pnfs_layout_hdr
----------------------
The on-the-wire command LAYOUTGET corresponds to struct
pnfs_layout_segment, usually referred to by the variable name lseg.
Each nfs_inode may hold a pointer to a cache of of these layout
segments in nfsi->layout, of type struct pnfs_layout_hdr.

We reference the header for the inode pointing to it, across each
outstanding RPC call that references it (LAYOUTGET, LAYOUTRETURN,
LAYOUTCOMMIT), and for each lseg held within.

Each header is also (when non-empty) put on a list associated with
struct nfs_client (cl_layouts).  Being put on this list does not bump
the reference count, as the layout is kept around by the lseg that
keeps it in the list.

deviceid_cache
--------------
lsegs reference device ids, which are resolved per nfs_client and
layout driver type.  The device ids are held in a RCU cache (struct
nfs4_deviceid_cache).  The cache itself is referenced across each
mount.  The entries (struct nfs4_deviceid) themselves are held across
the lifetime of each lseg referencing them.

RCU is used because the deviceid is basically a write once, read many
data structure.  The hlist size of 32 buckets needs better
justification, but seems reasonable given that we can have multiple
deviceid's per filesystem, and multiple filesystems per nfs_client.

The hash code is copied from the nfsd code base.  A discussion of
hashing and variations of this algorithm can be found at:
http://groups.google.com/group/comp.lang.c/browse_thread/thread/9522965e2b8d3809

data server cache
-----------------
file driver devices refer to data servers, which are kept in a module
level cache.  Its reference is held over the lifetime of the deviceid
pointing to it.

lseg
----
lseg maintains an extra reference corresponding to the NFS_LSEG_VALID
bit which holds it in the pnfs_layout_hdr's list.  When the final lseg
is removed from the pnfs_layout_hdr's list, the NFS_LAYOUT_DESTROYED
bit is set, preventing any new lsegs from being added.
