> This appears to be the scenario:
> 
> 	- call vndstrategy
> 	- vnode gets locked
> 	- ufs_bmap is called
> 	- an available buffer is looked for
> 	- while doing this, a DELWRI buffer on the same vnode is found
> 	- a bawrite() is done on it
> 	- vndstrategy again
> 	- try to lock the vnode as in step 2, but it's already locked.
> 	  -> panic

Actually, since the DELWRI buffer is passed to vndstrategy(), it must
have been associated with a vnode on the "vnd" filesystem. Once in
vndstrategy() again, a lock is requested on the underlying vnode (which
is on ffs, in this case).

So, the problem is that the VOP_LOCK/VOP_UNLOCK pair precludes recursive
calls to vndstrategy() through VOP_BMAP. This was also a problem in
the new swap code that deals with swapping on regular files; there,
the VOP_LOCK calls were omitted, which should be Ok as long as the file
you're trying to "bmap()" isn't changing in size.

-pk