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Re: [Xen-devel] [Qemu-block] [PATCH] xen-block: treat XenbusStateUnknown the same as XenbusStateClosed
- To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, "qemu-devel@xxxxxxxxxx" <qemu-devel@xxxxxxxxxx>, "qemu-block@xxxxxxxxxx" <qemu-block@xxxxxxxxxx>
- From: John Snow <jsnow@xxxxxxxxxx>
- Date: Mon, 23 Sep 2019 13:08:43 -0400
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- Cc: Kevin Wolf <kwolf@xxxxxxxxxx>, Anthony Perard <anthony.perard@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Mark Syms <Mark.Syms@xxxxxxxxxx>, Max Reitz <mreitz@xxxxxxxxxx>
- Delivery-date: Mon, 23 Sep 2019 17:08:56 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 9/23/19 5:38 AM, Paul Durrant wrote:
>> -----Original Message-----
>> From: John Snow <jsnow@xxxxxxxxxx>
>> Sent: 20 September 2019 22:11
>> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx;
>> qemu-devel@xxxxxxxxxx;
>> qemu-block@xxxxxxxxxx
>> Cc: Kevin Wolf <kwolf@xxxxxxxxxx>; Stefano Stabellini
>> <sstabellini@xxxxxxxxxx>; Max Reitz
>> <mreitz@xxxxxxxxxx>; Anthony Perard <anthony.perard@xxxxxxxxxx>; Mark Syms
>> <Mark.Syms@xxxxxxxxxx>
>> Subject: Re: [Qemu-block] [PATCH] xen-block: treat XenbusStateUnknown the
>> same as XenbusStateClosed
>>
>>
>>
>> On 9/18/19 7:57 AM, Paul Durrant wrote:
>>> When a frontend gracefully disconnects from an offline backend, it will
>>> set its own state to XenbusStateClosed. The code in xen-block.c correctly
>>> deals with this and sets the backend into XenbusStateClosed. Unfortunately
>>> it is possible for toolstack to actually delete the frontend area
>>> before the state key has been read, leading to an apparent frontend state
>>> of XenbusStateUnknown. This prevents the backend state from transitioning
>>> to XenbusStateClosed and hence leaves it limbo.
>>>
>>
>> Does the 0 come from a read into de-allocated memory?
>
> No, it comes from the fact that the xenstore state key is not present.
> Conventionally a missing state key means the state is reported as
> XenbusStateUnknown.
>
> Paul
>
Good enough for me, just had to confirm.
Reviewed-by: John Snow <jsnow@xxxxxxxxxx>
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