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

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Message-ID: <20191209122537.GV980@Air-de-Roger>
Date:   Mon, 9 Dec 2019 13:25:37 +0100
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     "Durrant, Paul" <pdurrant@...zon.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "Juergen Gross" <jgross@...e.com>,
        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 Mon, Dec 09, 2019 at 12:01:38PM +0000, Durrant, Paul wrote:
> > -----Original Message-----
> > From: Roger Pau Monné <roger.pau@...rix.com>
> > Sent: 09 December 2019 11:39
> > To: Durrant, Paul <pdurrant@...zon.com>
> > Cc: linux-kernel@...r.kernel.org; xen-devel@...ts.xenproject.org; Juergen
> > Gross <jgross@...e.com>; 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 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?
> > 
> 
> If probe fails then I think it should leave the state alone. If the
> state is moved to closed then basically you just killed that
> connection to the guest (as the frontend will normally close down
> when it sees this change) so, if the probe failure was due to a bug
> in blkback or, e.g., a transient resource issue then it's game over
> as far as that guest goes.

But the connection can be restarted by switching the backend to the
init state again.

> The ultimate goal here is PV backend re-load that is completely transparent to the guest. Modifying anything in xenstore compromises that so we need to be careful.

That's a fine goal, but not switching to closed state in
xenbus_dev_remove seems wrong, as you have actually left the frontend
without a matching backend and with the state not set to closed.

Ie: that would be fine if you explicitly state this is some kind of
internal blkback reload, but not for the general case where blkback
has been unbound. I think we need someway to difference a blkback
reload vs a unbound.

Thanks, Roger.