nfsd dropping requests after lazy umount

[Malahal Naineni <malahal@us.ibm.com>]

Hi All,

I have found an issue with lazy/forced unmounts with nfsd. Let us say
/nfs is a mount point, and we are exporting /nfs/exp directory.

After mounting from a client, this export will have a cache entry
"svc_export" in the kernel. The exports pathname
d_path(svc_export->ex_path) correctly resolves to "/nfs/exp"

After the /nfs is lazy unmounted, the d_path(svc_export->ex_path)
resolves to "/exp" only.

Now, if the cache entry needs revalidation, you will see "/exp" in the
nfsd.export/channel file that never gets cleared as that cache entry
will be in a forever CACHE_PENDING state. We will also see
svc_export_parse() failing with -2 err as such a pathname doesn't exist
in the system any more.

I noticed that svc_export has two fields: ex_path and ex_pathname.  Is
it possible to make a string comparison of d_path(ex_path) and
ex_pathname? If so, we should be able to fail the upcall without making
the upcall if those two don't match.

Regards, Malahal.