Re: [PATCH v2] monitor/qmp: fix race on CHR_EVENT_CLOSED without OOB

[Kevin Wolf <kwolf@redhat.com>]

Am 08.04.2021 um 11:21 hat Markus Armbruster geschrieben:
> Kevin Wolf <kwolf@redhat.com> writes:
> 
> > Am 22.03.2021 um 16:40 hat Stefan Reiter geschrieben:
> >> The QMP dispatcher coroutine holds the qmp_queue_lock over a yield
> >> point, where it expects to be rescheduled from the main context. If a
> >> CHR_EVENT_CLOSED event is received just then, it can race and block the
> >> main thread on the mutex in monitor_qmp_cleanup_queue_and_resume.
> >> 
> >> monitor_resume does not need to be called from main context, so we can
> >> call it immediately after popping a request from the queue, which allows
> >> us to drop the qmp_queue_lock mutex before yielding.
> >> 
> >> Suggested-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
> >> Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
> >> ---
> >> v2:
> >> * different approach: move everything that needs the qmp_queue_lock mutex before
> >>   the yield point, instead of moving the event handling to a different context
> >
> > The interesting new case here seems to be that new requests could be
> > queued and the dispatcher coroutine could be kicked before yielding.
> > This is safe because &qmp_dispatcher_co_busy is accessed with atomics
> > on both sides.
> >
> > The important part is just that the first (conditional) yield stays
> > first, so that the aio_co_wake() in handle_qmp_command() won't reenter
> > the coroutine while it is expecting to be reentered from somewhere else.
> > This is still the case after the patch.
> >
> > Reviewed-by: Kevin Wolf <kwolf@redhat.com>
> 
> Thanks for saving me from an ugly review headache.
> 
> Should this go into 6.0?

This is something that the responsible maintainer needs to decide.

If it helps you with the decision, and if I understand correctly, it is
a regression from 5.1, but was already broken in 5.2.

Kevin