Andrew,

Here's a patch to fix a race condition in nbd that was causing struct 
request corruption (requests were being freed while still in use). This 
patch improves on the previous one, which admittedly was a bit dodgy, 
using struct request's ref_count field (I should have listened to Jens 
in the first place :). This should fix all the corner cases related to 
struct request usage/freeing in nbd. My stress tests do a lot better 
with this patch applied.

Thanks,
Paul