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

[Markus Armbruster <armbru@redhat.com>]

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?