The following reply was made to PR kern/34882; it has been noted by GNATS.

From: "Julio M. Merino Vidal" <jmmv84@gmail.com>
To: "YAMAMOTO Takashi" <yamt@mwd.biglobe.ne.jp>
Cc: tech-kern@netbsd.org, gnats-bugs@NetBSD.org
Subject: Re: Use vn_rdwr in vnd (Re: kern/34882)
Date: Fri, 10 Nov 2006 15:08:16 +0100

 On 11/10/06, YAMAMOTO Takashi <yamt@mwd.biglobe.ne.jp> wrote:
 > > On 11/9/06, YAMAMOTO Takashi <yamt@mwd.biglobe.ne.jp> wrote:
 > > > > +     /* Flush the vnode if requested. */
 > > > > +     if (obp->b_flags & B_VFLUSH)
 > > > > +             vflushbuf(vp, 0);
 > > > > +
 > > > > +     /* We need to increase the number of outputs on the vnode if
 > > > > +      * there was any write to it (either due to a real write or due
 > > > > +      * to a flush). */
 > > > > +     if (!doread || obp->b_flags & B_VFLUSH)
 > > > > +             vp->v_numoutput++;
 > > >
 > > > can you explain these B_VFLUSH handling?
 > >
 > > For the first part I just tried to flush the vnode.  I don't remember
 > > how I ended exactly in vflushbuf, but it *seemed* reasonable...
 > > Hmm... maybe one ought to call VOP_FSYNC instead?  (And maybe this
 > > handles the case below too.)
 >
 > why do you want to flush the underlying vnode here at all?
 > if you want to preserve stable storage semantics or such,
 > it should be done regardless of B_VFLUSH.
 
 Because in the non-rw case, B_VFLUSH is passed down to the disk driver
 and I guess that this will do a sync, isn't it?
 
 > > Aside that, the driver was getting read requests that had the B_VFLUSH
 > > bit active. Later on a "neg numoutput" panic happened which I was able
 > > to fix by incrementing v_numoutput.  Yes, that looks extremely ugly
 > > (at least to me) so, please, if you have any better idea on how to fix
 > > it tell me and I'll take a look.
 >
 > i don't know how read requests with B_VFLUSH happens
 
 Just try a newfs over the vnd device and you'll get into
 handle_with_rw with B_READ and B_VFLUSH set.
 
 > or how it can be fixed
 > by incrementing v_numoutput.  can you explain?
 
 If you remove that, as soon as you run the newfs command above the
 system will panic with a "neg numoutput".
 
 I checked other drivers and they seem to do similar things.  See
 ccd.c:770.  Oh, and now I see there is a V_INCR_NUMOUTPUT macro to do
 this, although it is not always used.  It is also preceded by a
 "useful" comment.
 
 -- 
 Julio M. Merino Vidal <jmmv84@gmail.com>
 The Julipedia - http://julipedia.blogspot.com/