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 13:08:41 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     "Durrant, Paul" <pdurrant@...zon.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

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. 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.


Juergen