Re: [Qemu-devel] [PATCH 07/19] block: Really pause block jobs on drain

[Paolo Bonzini]

On 12/04/2018 13:11, Kevin Wolf wrote:
>> Well, there is one gotcha: bdrv_ref protects against disappearance, but
>> bdrv_ref/bdrv_unref are not thread-safe.  Am I missing something else?
>
> Apart from the above, if we do an extra bdrv_ref/unref we'd also have
> to keep track of all the nodes that we've referenced so that we unref
> the same nodes again, even if the graph has changes.
>
> So essentially you'd be introducing a new list of BDSes that we have to
> manage and then check for every reachable node whether it's already in
> that list or not, and for every node in the list whether it's still
> reachable.

That would be a hash table (a set), not a list, so easy to check.  But
the thread-safety is a bigger issue.

The problem I have is that there is a direction through which I/O flows
(parent-to-child), so why can't draining follow that natural direction.
Having to check for the parents' I/O, while draining the child, seems
wrong.  Perhaps we can't help it, but I cannot understand the reason.

Paolo