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

Farhan Ali
2019-04-15T13:38:37Z

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.

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:
>>>>>
>>>>>
>>>>
>>>
>>>
>>
>
>