We have a management product that can manage a Linux-based NFS server to 
allocate and export storage to managed servers.  (Then automatically 
mount the storage on those servers so it can be used)

I was seeing a problem where sometimes the directory I was exporting 
would not be mountable on the server it was exported to.

 >From the server machine's /var/log/messages:
Mar 1 10:49:53 fedora3 rpc.mountd: refused mount request from 
172.31.183.2 for
/mnt/NAS/exports/Levanta-1109702993-18 (/): not exported

But according to exportfs:

[root@fedora3 ~]# exportfs
/mnt/NAS/exports/Levanta-1109702992-17
172.31.183.2
/mnt/NAS/exports/Levanta-1109702993-18
172.31.183.2
/mnt/NAS/exports/Levanta-0
172.31.183.2

It looks like exportfs updates /var/lib/nfs/etab to inform mountd that 
there are new exports.

mountd decides whether to re-read etab based on whether the data 
modification time has changed since it was last read.  This fails if 
there are multiple updates in a second - quite possible on fast new 
machines if the NFS server is being programmatically controlled.  This 
patch updates mountd to track when it actually read the file, and 
compare that with the file's modification time.  If it last read the 
file at the same time it was last modified, re-read the file again in 
case there were other modifications later that second.  (Strictly 
speaking, the modification time equal check isn't necessary... only 
comparing the modification time to the current time, but I guess isn't a 
bad idea as added protection against someone playing with the clock).

-Nate