linux-kernel - RE: [Xen-devel] [PATCH 2/4] xenbus: limit when state is forced to closed

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Date:   Mon, 9 Dec 2019 12:19:18 +0000
From:   "Durrant, Paul" <pdurrant@...zon.com>
To:     Jürgen Groß <jgross@...e.com>,
        Roger Pau Monné <roger.pau@...rix.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "Stefano Stabellini" <sstabellini@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: RE: [Xen-devel] [PATCH 2/4] xenbus: limit when state is forced to
 closed

> -----Original Message-----
> From: Jürgen Groß <jgross@...e.com>
> Sent: 09 December 2019 12:09
> To: Durrant, Paul <pdurrant@...zon.com>; Roger Pau Monné
> <roger.pau@...rix.com>
> Cc: linux-kernel@...r.kernel.org; xen-devel@...ts.xenproject.org; Stefano
> Stabellini <sstabellini@...nel.org>; Boris Ostrovsky
> <boris.ostrovsky@...cle.com>
> Subject: Re: [Xen-devel] [PATCH 2/4] xenbus: limit when state is forced to
> closed
> 
> On 09.12.19 13:03, Durrant, Paul wrote:
> >> -----Original Message-----
> >> From: Jürgen Groß <jgross@...e.com>
> >> Sent: 09 December 2019 11:55
> >> To: Roger Pau Monné <roger.pau@...rix.com>; Durrant, Paul
> >> <pdurrant@...zon.com>
> >> Cc: linux-kernel@...r.kernel.org; xen-devel@...ts.xenproject.org;
> Stefano
> >> Stabellini <sstabellini@...nel.org>; Boris Ostrovsky
> >> <boris.ostrovsky@...cle.com>
> >> Subject: Re: [Xen-devel] [PATCH 2/4] xenbus: limit when state is forced
> to
> >> closed
> >>
> >> On 09.12.19 12:39, Roger Pau Monné wrote:
> >>> On Thu, Dec 05, 2019 at 02:01:21PM +0000, Paul Durrant wrote:
> >>>> Only force state to closed in the case when the toolstack may need to
> >>>> clean up. This can be detected by checking whether the state in
> >> xenstore
> >>>> has been set to closing prior to device removal.
> >>>
> >>> I'm not sure I see the point of this, I would expect that a failure to
> >>> probe or the removal of the device would leave the xenbus state as
> >>> closed, which is consistent with the actual driver state.
> >>>
> >>> Can you explain what's the benefit of leaving a device without a
> >>> driver in such unknown state?
> >>
> >> And more concerning: did you check that no frontend/backend is
> >> relying on the closed state to be visible without closing having been
> >> set before?
> >
> > Blkfront doesn't seem to mind and I believe the Windows PV drivers cope,
> but I don't really understand the comment since this patch is actually
> removing a case where the backend transitions directly to closed.
> 
> I'm not speaking of blkfront/blkback only, but of net, tpm, scsi, pvcall
> etc. frontends/backends. After all you are modifying a function common
> to all PV driver pairs.
> 
> You are removing a state switc to "closed" in case the state was _not_
> "closing" before.

Yes, which AFAIK is against the intention of the generic PV protocol such that it ever existed anyway.

> So any PV driver reacting to "closed" of the other end
> in case the previous state might not have been "closing" before is at
> risk to misbehave with your patch.

Well, they will see nothing now. If the state was not closing, it gets left alone, so the frontend shouldn't do anything. The only risk that I can see is that some frontend/backend pair needed a direct 4 -> 6 transition to support 'unbind' before but AFAIK nothing has ever supported that, and blk and net crash'n'burn if you try that on upstream as it stands. A clean unplug would always set state to 5 first, since that's part of the unplug protocol.

  Paul

> 
> Juergen