Re: [KVM PATCH v2 2/2] kvm: use POLLHUP to close an irqfd instead of an explicit ioctl

Gregory Haskins
2009-06-19T16:07:33Z

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Michael S. Tsirkin wrote:
> On Thu, Jun 18, 2009 at 12:29:31PM -0400, Gregory Haskins wrote:
>
>> Michael S. Tsirkin wrote:
>>
>>> On Thu, Jun 18, 2009 at 10:03:36AM -0400, Gregory Haskins wrote:
>>>
>>>
>>>> Michael S. Tsirkin wrote:
>>>>
>>>>
>>>>> On Thu, Jun 18, 2009 at 08:00:39AM -0400, Gregory Haskins wrote:
>>>>>
>>>>>
>>>>>
>>>>>> Michael S. Tsirkin wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>> BTW, Gregory, this can be used to fix the race in the design: create a
>>>>>>> thread and let it drop the module reference with module_put_and_exit.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>> I had thought of doing something like this initially too, but I think
>>>>>> its racy as well. Ultimately, you need to make sure the eventfd
>>>>>> callback is completely out before its safe to run, and deferring to a
>>>>>> thread would not change this race. The only sane way I can see to do
>>>>>> that is to have the caller infrastructure annotate the event somehow
>>>>>> (either directly with a module_put(), or indirectly with some kind of
>>>>>> state transition that can be tracked with something like
>>>>>> synchronize_sched().
>>>>>>
>>>>>>
>>>>>>
>>>>> Here's what one could do: create a thread for each irqfd, and increment
>>>>> module ref count, put that thread to sleep. When done with
>>>>> irqfd, don't delete it and don't decrement module refcount, wake thread
>>>>> instead. thread kills irqfd and calls module_put_and_exit.
>>>>>
>>>>> I don't think it's racy
>>>>>
>>>>>
>>>> I believe it is. How would you prevent the thread from doing the
>>>> module_put_and_exit() before the eventfd callback thread is known to
>>>> have exited the relevant .text section?
>>>>
>>>>
>>> Right.
>>>
>>>
>>>
>>>> All this talk does give me an idea, tho. Ill make a patch.
>>>>
>>>>
>>> OK, but ask yourself whether this bag of tricks is worth it, and whether
>>> we'll find another hole later. Let's reserve the trickiness for
>>> fast-path, where it's needed, and keep at least the assign/deassign simple.
>>>
>>>
>> Understood. OTOH, going back to the model where two steps are needed
>> for close() is ugly too, so I don't want to just give up and revert that
>> fix too easily. At some point we will call it one way or the other, but
>> I am not there quite yet.
>>
>>>
>>>
>>>>>
>>>>>
>>>>>
>>>>>>> Which will work, but I guess at this point we should ask ourselves
>>>>>>> whether all the hearburn with srcu, threads and module references is
>>>>>>> better than just asking the user to call and ioctl.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>> I am starting to agree with you, here. :)
>>>>>>
>>>>>> Note one thing: the SRCU stuff is mostly orthogonal from the rest of the
>>>>>> conversation re: the module_put() races. I only tied it into the
>>>>>> current thread because the eventfd_notifier_register() thread gave me a
>>>>>> convenient way to hook some other context to do the module_put(). In
>>>>>> the long term, the srcu changes are for the can_sleep() stuff. So on
>>>>>> that note, lets see if I can convince Davide that the srcu stuff is not
>>>>>> so evil before we revert the POLLHUP patches, since the module_put() fix
>>>>>> is trivial once that is in place.
>>>>>>
>>>>>>
>>>>>>
>>>>> Can this help with DEASSIGN as well? We need it for migration.
>>>>>
>>>>>
>>>>>
>>>> No, but afaict you do not need this for migration anyway. Migrate the
>>>> GSI and re-call kvm_irqfd() on the other side. Would the fd even be
>>>> relevant across a migration anyway? I would think not, but admittedly I
>>>> know little about how qemu/kvm migration actually works.
>>>>
>>>>
>>> Yes but that's not live migration. For live migration, the trick is that
>>> you are running locally but send changes to remote guest. For that, we
>>> need to put qemu in the middle between the device and the guest, so it
>>> can detect activity and update the remote side.
>>>
>>> And the best way to do that is to take poll eventfd that device assigns
>>> and push eventfd that kvm polls. To switch between this setup
>>> and the one where kvm polls the ventfd from device directly,
>>> you need deassign.
>>>
>>>
>> So its still not clear why the distinction between
>> deassign-the-gsi-but-leave-the-fd-valid is needed over a simple
>> close(). Can you elaborate?
>>
>
>
> The fd needs to be left assigned to the device, so that we can poll
> the fd and get events, then forward them to kvm.
>

Ah, ok. Now I get what you are trying to do.

Well, per the PM I sent you this morning, I figured out the magic to
resolve the locking issues. So we should be able to add DEASSIGN logic
soon, I hope.

-Greg

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