Re: [Qemu-block] [PATCH 0/5] blockjobs: Fix transactional race condition

[John Snow]

On 09/29/2016 07:33 AM, Kevin Wolf wrote:
> Am 28.09.2016 um 14:16 hat Vladimir Sementsov-Ogievskiy geschrieben:
> > > I think jobs will need to remain "one coroutine, one job" for now,
> > > but there's no reason why drive-backup or blockdev-backup can't
> > > just create multiple jobs each if that's what they need to do.
> > > (The backup job object could, in theory, just have another job
> > > pointer to a helper job if it really became necessary.)
>
> What's the problem with a job spawning additional coroutines internally?
> Jobs already do this all the time (AIO requests are just coroutines in
> disguise.)

Mostly I was just pushing back against baking in multi-coroutines 
per-job as a default. If you want to add them in your own extensions, 
e.g. MultiJob : BlockJob {
  Coroutine *extra;
}

then I'm okay with that and realize we already do exactly that. Beyond 
that I think complicates the job layer a little more than necessary... 
but maybe I am being too pre-fearful.

> A job is basically just the user interface for managing a background
> process, so helper jobs that are managed internally rather than by the
> user don't seem to make that much sense.
>
> Kevin