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Date: Thu, 8 Dec 2016 21:47:59 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [Qemu-block]  Meeting notes on -blockdev,
 dynamic backend reconfiguration
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To: Markus Armbruster <armbru@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-block@nongnu.org, qemu-devel@nongnu.org

On Thu, 12/08 13:46, Markus Armbruster wrote:
> Fam Zheng <famz@redhat.com> writes:
> 
> > On Wed, 12/07 10:48, Kevin Wolf wrote:
> >> > If so I think there is no race to worry about, mirror-filter should go
> >> > away only after a QMP command.
> >> 
> >> Currently, a mirror job goes away whenever it is done. This is not
> >> directly tied to a QMP command.
> >
> > Ah right, block-job-complete is only "start to complete" and the job goes away
> > at some later point. I thought this is "the" QMP command but it is not.
> >
> >> 
> >> Of course, in the new job API we want an explicit job-delete, so in
> >> that case it wouldn't happen, but we need to keep the old case for
> >> compatibility.
> >
> > Another possibility is to add a placeholder node in the right location first
> > then fill in the actual throttling node once created. QMP owns the placeholder
> > node so it won't suddenly vanish when mirror job goes away.
> 
> I'm not sure I understand this idea.  Could you explain it in a bit more
> detail, perhaps even with a bit of ASCII art?

OK, I'll base on your version:

== Basic dynamic reconfiguration operation ==

The two primitives are "insert placeholder" and bdrv_replace_child;

A placeholder BDS is a dummy "pass through everything" filter. Inserting
placeholder is something like:

    void bdrv_insert_placeholder(BdrvChild *child) {
        BlockDriverState *ph = bdrv_new_placeholder();
        ph->file->bs = child->bs;
        child->bs = ph;
    }

Consider:

          BB
          |
    mirror-filter
          |
         BDS

Add a throttle filter under BB while the mirror job is running.  First step,
create the dummy _placeholder_ node and throttle-filter node (the existence of
placeholder node can be an implementation detail hidden from user):

    placeholder = bdrv_insert_placeholder(BB->root);
    throttle_filter = bdrv_create_throttle_filter(placeholder);

we get:

          BB     throttle-filter
          |     /
     placeholder
          |
    mirror-filter
          |
         BDS

Second step, move throttle filter in:

    bdrv_replace_child(BB->root, throttle_filter);
    bdrv_replace_child(throttle_filter->file, throttle_filter->file->bs->file->bs);
    bdrv_unref(placeholder);

we get:

          BB
          |
   throttle-filter
          |
    mirror-filter
          |
         BDS

Does this make us safe from racing with mirror-filter disappearing?

Fam