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

[Kevin Wolf]

Am 28.04.2023 um 10:38 hat Juan Quintela geschrieben:
> 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.

My idea is that bdrv_*() can only be called when you hold the BQL, or
for BlockDriverStates in an iothread the AioContext lock.

Apparently dropping the BQL in migration code was introduced in Paolo's
commit 9b095037527. I'm not sure what this was supposed to improve in
the case of snapshots because the VM is stopped anyway.

Would anything bad happen if we removed the BQL unlock/lock section in
qemu_savevm_state() again?

Kevin