Kevin Wolf <kw...@redhat.com> writes:
> Am 02.03.2020 um 15:22 hat Markus Armbruster geschrieben:
>> Marc-André Lureau <marcandre.lur...@gmail.com> writes:
>>
>> > Hi
>> >
>> > On Fri, Feb 21, 2020 at 5:50 PM Markus Armbruster <arm...@redhat.com>
>> > wrote:
>> >>
>> >> Kevin Wolf <kw...@redhat.com> writes:
>> >>
>> >> > Am 20.02.2020 um 17:01 hat Markus Armbruster geschrieben:
>> >> >> >> > void qmp_screendump(const char *filename, bool has_device, const
>> >> >> >> > char *device,
>> >> >> >> > bool has_head, int64_t head, Error **errp)
>> >> >> >> > {
>> >> >> >> > QemuConsole *con;
>> >> >> >> > DisplaySurface *surface;
>> >> >> >> > + g_autoptr(pixman_image_t) image = NULL;
>> >> >> >> > int fd;
>> >> >> >> >
>> >> >> >> > if (has_device) {
>> >> >> >> > @@ -365,7 +375,15 @@ void qmp_screendump(const char *filename,
>> >> >> >> > bool has_device, const char *device,
>> >> >> >> > }
>> >> >> >> > }
>> >> >> >> >
>> >> >> >> > - graphic_hw_update(con);
>> >> >> >> > + if (qemu_in_coroutine()) {
>> >> >> >> > + assert(!con->screendump_co);
>> >> >> >> > + con->screendump_co = qemu_coroutine_self();
>> >> >> >> > + aio_bh_schedule_oneshot(qemu_get_aio_context(),
>> >> >> >> > + graphic_hw_update_bh, con);
>> >> >> >> > + qemu_coroutine_yield();
>> >> >> >> > + con->screendump_co = NULL;
>> >> >> >> > + }
>> >> >> >>
>> >> >> >> What if multiple QMP monitors simultaneously screendump? Hmm, it
>> >> >> >> works
>> >> >> >> because all execute one after another in the same coroutine
>> >> >> >> qmp_dispatcher_co. Implicit mutual exclusion.
>> >> >> >>
>> >> >> >> Executing them one after another is bad, because it lets an
>> >> >> >> ill-behaved
>> >> >> >> QMP command starve *all* QMP monitors. We do it only out of
>> >> >> >> (reasonable!) fear of implicit mutual exclusion requirements like
>> >> >> >> the
>> >> >> >> one you add.
>> >> >> >>
>> >> >> >> Let's not add more if we can help it.
>> >> >> >
>> >> >> > The situation is not worse than the current blocking handling.
>> >> >>
>> >> >> Really?
>> >> >>
>> >> >> What makes executing multiple qmp_screendump() concurrently (in
>> >> >> separate
>> >> >> threads) or interleaved (in separate coroutines in the same thread)
>> >> >> unsafe before this patch?
>> >> >
>> >> > QMP command handlers are guaranteed to run in the main thread with the
>> >> > BQL held, so there is no concurrency. If you want to change this, you
>> >> > would have much more complicated problems to solve than in this handler.
>> >> > I'm not sure it's fair to require thread-safety from one handler when
>> >> > no other handler is thread safe (except accidentally) and nobody seems
>> >> > to plan actually calling them from multiple threads.
>> >>
>> >> "Let's not [...] if we can help it." is hardly a "change this or else no
>> >> merge" demand. It is a challenge to find a more elegant solution.
>> >>
>> >> >> >> Your screendump_co is per QemuConsole instead of per QMP monitor
>> >> >> >> only
>> >> >> >> because you need to find the coroutine in graphic_hw_update_done().
>> >> >> >> Can
>> >> >> >> we somehow pass it via function arguments?
>> >> >> >
>> >> >> > I think it could be done later, so I suggest a TODO.
>> >> >>
>> >> >> We should avoid making our dependence on implicit mutual exclusion
>> >> >> worse. When we do it anyway, a big, fat, ugly comment is definitely
>> >> >> called for.
>> >> >
>> >> > Anyway, what I really wanted to add:
>> >> >
>> >> > This should be easy to solve by having a CoQueue instead of a single
>> >>
>> >> Ah, challenge accepted! Exactly the outcome I was hoping for :)
>> >>
>> >> > Coroutine pointer. The coroutine would just call qemu_co_queue_wait(),
>> >> > which adds itself to the queue before it yields and the update
>> >> > completion would wake up all coroutines that are currently queued with
>> >> > qemu_co_queue_restart_all().
>> >> >
>> >> > qemu_co_queue_wait() takes a lock as its second parameter. You don't
>> >> > need it in this context and can just pass NULL. (This is a lock that
>> >> > would be dropped while the coroutine is sleeping and automatically
>> >> > reacquired afterwards.)
>> >> >
>> >> >> >> In case avoiding the mutual exclusion is impractical: please
>> >> >> >> explain it
>> >> >> >> in a comment to make it somewhat less implicit.
>> >> >>
>> >> >> It is anything but: see appended patch.
>> >> >
>> >> > This works, too, but it requires an additional struct. I think the queue
>> >> > is easier. (Note there is a difference in the mechanism: Your patch
>> >> > waits for the specific update it triggered, while the CoQueue would wait
>> >> > for _any_ update to complete. I assume effectively the result is the
>> >> > same.)
>> >>
>> >> Your idea sounds much nicer to me. Thanks!
>> >
>> > Similar to the NULL check you asked to remove,
>> > having a CoQueue there would lead to think that several concurrently
>> > running screendump are possible.
>> >
>> > Is this a direction we are willing to take?
>>
>> Let's take a step back.
>>
>> The actual problem is to find the coroutine in graphic_hw_update_done(),
>> so you can wake it.
>>
>> Your solution stores the coroutine in the QemuConsole, because that's
>> readily available in graphic_hw_update_done().
>>
>> However, it really, really doesn't belong there, it belongs to the
>> monitor. Works anyway only because QMP commands execute one after the
>> other.
>>
>> Kevin suggested using a CoQueue to avoid this unspoken dependency. You
>> object, because it could make readers assume multiple screendump
>> commands could run concurrently, which is not the case.
>>
>> Alright, let's KISS: since there's just one main loop, there's just one
>> coroutine: @qmp_dispatcher_co. Let's use that, so the dependency on
>> "one command after the other" is explicit and obvious.
>
> Ugh... If you choose that this is the way to go, please add an assertion
> at least that we are indeed in qmp_dispatcher_co before yielding.
No objection.
To apply the QMP coroutine infrastructure for 5.0, I need a user. We
have two: block_resize from Kevin, and screendump from Marc-André.
Neither is quite ready, yet. I'll wait for a respin of either one.
