Re: [Qemu-devel] Drainage in bdrv_replace_child_noperm()

[Max Reitz <mreitz@redhat.com>]

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On 2017-11-07 06:21, Fam Zheng wrote:
> On Mon, 11/06 19:49, Max Reitz wrote:

[...]

>> I have two ideas:
>>
>> One is to assume that (un-)draining a parent will always (un-)drain all
>> children, including the one the (un-)drain comes from.  This assumption
>> seems wrong, see [1], but maybe it isn't.  Anyway, if so, we could just
>> explicitly drain the new child in bdrv_replace_child_noperm() after
>> having drained the parent and thus get a consistent state again.
>>
>> The other is to declare (A) wrong.  Maybe when
>> BdrvChildRole.drained_{begin,end}() is invoked, we should not drain that
>> child because we can declare it the caller's responsibility to make sure
>> it's drained.  This seems logical to me because usually those methods
>> are invoked when the child is drained anyway.  But maybe I'm wrong. :-)
> 
> I'm in favor of asking the caller to make sure all nodes involved in the graph
> manupulation are drained, it feels comparably easier to do, than fixing the
> problem in bdrv_append().

I guess my main question was whether any of the two approaches is
evidently wrong in some way, but if you don't say that (and Kevin says
the first is wrong), I'm going to assume doing the second is OK. :-)

Max


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