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Re: [Qemu-block] [RFC] block-insert-node and block-job-delete

From: Manos Pitsidianakis
Subject: Re: [Qemu-block] [RFC] block-insert-node and block-job-delete
Date: Wed, 26 Jul 2017 21:23:20 +0300
User-agent: NeoMutt/20170609-57-1e93be (1.8.3)

On Wed, Jul 26, 2017 at 04:12:21PM +0100, Stefan Hajnoczi wrote:
On Wed, Jul 26, 2017 at 05:19:24PM +0300, Manos Pitsidianakis wrote:
This proposal follows a discussion we had with Kevin and Stefan on filter
node management.

With block filter drivers arises a need to configure filter nodes on runtime
with QMP on live graphs. A problem with doing live graph modifications is
that some block jobs modify the graph when they are done and don't operate
under any synchronisation, resulting in a race condition if we try to insert
a filter node in the place of an existing edge.

But maybe you are thinking about higher level race conditions between
QMP commands.  Can you give an example of the race?

Exactly, synchronisation between the QMP/user actions. Here's an example, correct me if I'm wrong:

User issues a block-commit to this tree to commit A onto B:
   BB -> A -> B

This inserts the dummy mirror node at the top since this is a mirror job (active commit)
   BB -> dummy -> A -> B

User wants to add a filter node F below the BB. First we create the node:
   BB -> dummy -> A -> B
      F /

Commit finishes before we do block-insert-node, dummy and A is removed from the BB's chain, mirror_exit calls bdrv_replace_node()

   BB ------------> B
               F -> /

Now inserting F under BB will error since dummy does not exist any more.

The proposal doesn't guard against the following:
User issues a block-commit to this tree to commit B onto C:
   BB -> A -> B -> C

The dummy node (commit's top bs is A):
   BB -> A -> dummy -> B -> C

blockdev-add of a filter we wish to eventually be between A and C:
   BB -> A -> dummy -> B -> C

if commit finishes before we issue block-insert-node (commit_complete()
calls bdrv_set_backing_hd() which only touches A)
   BB -> A --------------> C
          F -> dummy -> B /
   resulting in error when issuing block-insert-node,

else if we set the job to manual-delete and insert F:
   BB -> A -> F -> dummy -> B -> C
deleting the job will result in F being lost since the job's top bs wasn't updated.

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