Re: [RFC v2 2/3] vfio-ccw: Prevent quiesce function going into an infinite loop

Farhan Ali
2019-04-15T14:25:00Z

On 04/15/2019 10:18 AM, Cornelia Huck wrote:
> On Mon, 15 Apr 2019 09:38:37 -0400
> Farhan Ali <alifm@linux.ibm.com> wrote:
>
>> On 04/15/2019 04:13 AM, Cornelia Huck wrote:
>>> On Fri, 12 Apr 2019 10:38:50 -0400
>>> Farhan Ali <alifm@linux.ibm.com> wrote:
>>>
>>>> On 04/12/2019 04:10 AM, Cornelia Huck wrote:
>>>>> On Thu, 11 Apr 2019 16:30:44 -0400
>>>>> Farhan Ali <alifm@linux.ibm.com> wrote:
>>>>>
>>>>>> On 04/11/2019 12:24 PM, Cornelia Huck wrote:
>>>>>>> On Mon, 8 Apr 2019 17:05:32 -0400
>>>>>>> Farhan Ali <alifm@linux.ibm.com> wrote:
>>>
>>>>>>> Looking at the possible return codes:
>>>>>>> * -ENODEV -> device is not operational anyway, in theory you should even
>>>>>>> not need to bother with disabling the subchannel
>>>>>>> * -EIO -> we've run out of retries, and the subchannel still is not
>>>>>>> idle; I'm not sure if we could do anything here, as disable is
>>>>>>> unlikely to work, either
>>>
>>> (...)
>>>
>>>> Thinking a little bit more about EIO, if the return code is EIO then it
>>>> means we have exhausted all our options with cancel_halt_clear and the
>>>> subchannel/device is still status pending, right?
>>>
>>> Yes.
>>>
>>>>
>>>> I think we should still continue to try and disable the subchannel,
>>>> because if not then the subchannel/device could in some point of time
>>>> come back and bite us. So we really should protect the system from this
>>>> behavior.
>>>
>>> I think trying to disable the subchannel does not really hurt, but I
>>> fear it won't succeed in that case...
>>>
>>>>
>>>> I think for EIO we should log an error message, but still try to
>>>> continue with disabling the subchannel. What do you or others think?
>>>
>>> Logging an error may be useful (it's really fouled up at that time), but...
>>>
>>>>
>>>>
>>>>
>>>>
>>>>>>
>>>>>>>> + flush_workqueue(vfio_ccw_work_q);
>>>>>>>> + spin_lock_irq(sch->lock);
>>>>>>>> ret = cio_disable_subchannel(sch);
>>>
>>> ...there's a good chance that we'd get -EBUSY here, which would keep us
>>> in the loop. We probably need to break out after we got -EIO from
>>> cancel_halt_clear, regardless of which return code we get from the
>>> disable.
>>
>> Okay, for EIO we can log an error message and break out of the loop.
>>
>> I will send a v3. Are you going to queue patch 1 or patch 3 soon? If you
>> are then I will just send this patch separately.
>
> Yes, please do send it separately. I'm currently testing patch 1 and 3
> on top of my patchset, will queue either with or without the halt/clear
> patches proper, depending on how soon I get acks/r-bs (hint, hint :)

I am going through your patches 4 and 6 and hopefully will get back to
you by the end of the day :).

>
>>
>> Thanks
>> Farhan
>>
>>>
>>> (It will be "interesting" to see what happens with such a stuck
>>> subchannel in the calling code; but I don't really see many options.
>>> Panic seems way too strong; maybe mark the subchannel as "broken; no
>>> idea how to fix"? But that would be a follow-on patch; I think if we
>>> avoid the endless loop here, this patch is a real improvement and
>>> should just go in.)
>>>
>>>>>>>> } while (ret == -EBUSY);
>>>>>>>> out_unlock:
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>
>>>
>>>
>>
>
>