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

[Cornelia Huck <cohuck@redhat.com>]

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 :)

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