Re: QMP (without OOB) function running in thread different from the main thread as part of aio_poll

[Juan Quintela <quintela@redhat.com>]

Kevin Wolf <kwolf@redhat.com> wrote:

>> 
>> I am perhaps a bit ingenuous here, but it is there a way to convince
>> qemu that snapshot_save_job_bh *HAS* to run on the main thread?
>
> I believe we're talking about a technicality here. I asked another more
> fundamental question that nobody has answered yet:
>
> Why do you think that it's ok to call bdrv_writev_vmstate() without
> holding the BQL?

I will say this function starts by bdrv_ (i.e. block layer people) and
endes with _vmstate (i.e. migration people).

To be honest, I don't know.  That is why I _supposed_ you have an idea.

> Because if we come to the conclusion that it's not ok (which is what I
> think), then it doesn't matter whether we violate the condition in the
> main thread or a vcpu thread. It's wrong in both cases, just the failure
> mode differs - one crashes spectacularly with an assertion failure, the
> other has a race condition. Moving from the assertion failure to a race
> condition is not a proper fix.

Fully agree there.  Just that I don't know the answer.

Later, Juan.