preupgrade: Generating metadata hang

We're seeing a number of hangs using preupgrade at the "Generating 
metadata ..." stage.  The hangs are so tight that they corrupt the 
mdraid arrays which were probably in the middle of a write.  The causes 
the arrays to resync on next boot which takes a couple hours.  The hang 
always occurs on a disk access as we can see the array activity light 
stays on after the hang.  I suspect some type of uninterrupible infinite 
loop is going on.  Once the hang occurs the server completely stop 
responding to inputs: network, keyboard, mouse, etc.  I did not find any 
core files upon reboot.

We're on our fourth attempt with one machine to get preupgrade to 
complete without this hang.

Regards,
Gerry