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Date: Wed, 7 Dec 2016 10:55:34 +0800
From: Fam Zheng <famz@redhat.com>
Message-ID: <20161207025457.GB30227@lemon>
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Subject: Re: [Qemu-devel] Meeting notes on -blockdev,
 dynamic backend reconfiguration
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To: Markus Armbruster <armbru@redhat.com>
Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org

On Mon, 12/05 13:03, Markus Armbruster wrote:
> == Basic dynamic reconfiguration operation ==
> 
> The basic operation is "replace child".
> 
> Beware of race conditions.  Consider:
> 
>           BB
>           |
>     mirror-filter
>           |
>          BDS
> 
> Add a throttle filter under BB while the mirror job is running.  First
> step, create the filter:
> 
>           BB    throttle-filter
>           |     /
>     mirror-filter
>           |
>          BDS
> 
> Second step, replace child of BB by the new filter:
> 
>           BB
>           |
>    throttle-filter
>           |
>     mirror-filter
>           |
>          BDS
> 
> But: if mirror-filter goes away between the two steps, the replace
> brings it right back!
> 
> To guard against such races, we need to specify both ends of the edge
> being replaced, i.e. parent, child name, actual child.  Then the replace
> step fails if the mirror-filter has gone away.  We can either fail the
> whole operation, or start over.
> 
> Alternatively, transactions, but that feels much more complex.
> 

Isn't it easy to make creating throttle-filter and replacing child happen in the
same critical section of BQL, without any coroutine yield?  If so I think there
is no race to worry about, mirror-filter should go away only after a QMP command.

Fam