Re: [PATCH 0/3] monitor: only run coroutine commands in qemu_aio_context

[Peter Maydell]

On Tue, 16 Jan 2024 at 19:01, Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> Several bugs have been reported related to how QMP commands are rescheduled in
> qemu_aio_context:
> - https://gitlab.com/qemu-project/qemu/-/issues/1933
> - https://issues.redhat.com/browse/RHEL-17369
> - https://bugzilla.redhat.com/show_bug.cgi?id=2215192
> - https://bugzilla.redhat.com/show_bug.cgi?id=2214985
>
> The first instance of the bug interacted with drain_call_rcu() temporarily
> dropping the BQL and resulted in vCPU threads entering device emulation code
> simultaneously (something that should never happen). I set out to make
> drain_call_rcu() safe to use in this environment, but Paolo and Kevin 
> discussed
> the possibility of avoiding rescheduling the monitor_qmp_dispatcher_co()
> coroutine for non-coroutine commands. This would prevent monitor commands from
> running during vCPU thread aio_poll() entirely and addresses the root cause.
>
> This patch series implements this idea. qemu-iotests is sensitive to the exact
> order in which QMP events and responses are emitted. Running QMP handlers in
> the iohandler AioContext causes some QMP events to be ordered differently than
> before. It is therefore necessary to adjust the reference output in many test
> cases. The actual QMP code change is small and everything else is just to make
> qemu-iotests happy.

Hi; we have a suspicion that this change has resulted in a flaky-CI
test: iotest-144 sometimes fails, apparently because a "return"
result from QMP isn't always returned at the same place in relation
to other QMP events. Could you have a look at it?

https://gitlab.com/qemu-project/qemu/-/issues/2126

thanks
-- PMM