Re: vnode lock and v_numoutput

[Taylor R Campbell <campbell+netbsd-tech-kern%mumble.net@localhost>]

   Date: Mon, 26 Jan 2015 21:01:58 +0100
   From: Manuel Bouyer <bouyer%antioche.eu.org@localhost>

   On Mon, Jan 26, 2015 at 07:33:18PM +0000, Taylor R Campbell wrote:
   > We should probably do the same thing in genfs_do_io and anywhere else
   > that uses nestiobuf for writes, and invent a better way to express the
   > nestiobuf API so it's less ridiculously error-prone.

   Yes, I'll look at this (but I don't know why it does +2 here).

genfs_do_io does +2 because it needs to count the nestiobuf master
biodone and the caller-supplied (*iodone)(mbp) callback, which calls
biodone or vwakeup (see uvm_aio_biodone or genfs_dio_iodone).

Counting the nestiobuf master biodone can be done like we're doing
with vnd.  I would guess (and hope) that counting the caller-supplied
iodone callback can be done after biowait and before (*iodone)(mbp).

   But I guess genfs_do_io() is safe as is: it's probably called
   with the vnode locked, as VOP_BMAP() is called without taking and
   releasing the lock.

Maybe...  I think there's a great deal too much blocking (i.e., any)
while the vnode lock is held, but I suppose cleaning that up can be
left for another day.