In fs/nfsd/nfsfh.c:fh_verify() we do this:

	exp = exp_find(rqstp->rq_client, 0, tfh, &rqstp->rq_chandle);

In the following lines we have:

	error = nfserr_dropit;
	if (IS_ERR(exp) && PTR_ERR(exp) == -EAGAIN)
		goto out;

The nfserr_dropit return will cause us to drop this request and revisit it
later, after an upcall has been made and a corresponding downcall received from
userspace.

Note that this revisitation can cause incorrect behavior in the NFSv4 case:
compound requests may appear to be processed out of order, and non-idempotent
operations may be performed more than once.

There are two caches defined in fs/nfsd/export.c:
	the "export map": "maps client+vfsmnt+dentry to export options"
		.name = "nfsd.export"
		svc_export_request: upcall request has form
			"clientname path"
		svc_export_parse: downcall has form
			"clientname path expiry flags"
				(flags omitted for negative reponse)
		exp_get_by_name: clp, mnt, dentry --> svc_export
		exp_parent: clp, mnt, dentry -->svc_export
			note exp_get_by_name will return NULL if the dentry
			corresponds to the child of an exported directory;
			this function actually searches up the tree for a
			parent that's been exported and returns that if it
			finds one.
		note that this is the *only* cache that uses in-place
			updates; why?
	the "expkey map": "maps client+filehandle-fragment to export options"
			(Well, actually upcall itself just maps to a pathname
			to be used in the above upcall)
		.name = "nfsd.fh"
		note that the filehandle-fragment is the fsid:
			either type 0: 8 bytes, device and inode number, or
			       type 1: 4 bytes, fsid: described in nfsfh.h as
			       		"possible future encoding..." currently
					used to identify pseudoroot.
		expkey_request: upcall request has form
			"clientname fsidtype fsid(in hex)"
		expkey_parse: downcall has form
			"clientname fsidtype fsid(in hex) expiry pathame"
				(pathname omitted for negative responses)
			also does an exp_get_by_name, which may of course
			trigger another upcall.
		exp_find_key: clnt, fsid_type, fsid --> svc_expkey
		exp_get_key: clnt, dev, ino --> svc_expkey
		exp_get_fsid_key: clp, fsid --> svc_expkey
		exp_set_key: adds clp, fsid_type, fsid, exp to expkey hash