> > deal with two write operations happening at once, we can just let 
> > VOP_WRITE() not need a lock and thus we won't need the _RANGE calls. :-)
> 
> we can't ever do away with locking entirely for VOP_READ and VOP_WRITE,
> since POSIX requires that for file, read() and write() must be atomic
> with respect to each other (and write() with respect to other write()s).

really?  from susv3 (pwrite, write):

> This volume of IEEE Std 1003.1-2001 does not specify behavior of concurrent
> writes to a file from multiple processes.

YAMAMOTO Takashi