Message-ID: <87r1dif7v9.fsf@redhat.com>
Date:   Tue, 21 Sep 2021 14:09:14 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     Vineeth Vijayan <vneethv@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>,
        Michael Mueller <mimu@...ux.ibm.com>,
        linux-s390@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, bfu@...hat.com,
        Halil Pasic <pasic@...ux.ibm.com>
Subject: Re: [PATCH 1/1] virtio/s390: fix vritio-ccw device teardown

On Tue, Sep 21 2021, Halil Pasic <pasic@...ux.ibm.com> wrote:

> On Mon, 20 Sep 2021 12:07:23 +0200
> Cornelia Huck <cohuck@...hat.com> wrote:
>
>> On Mon, Sep 20 2021, Vineeth Vijayan <vneethv@...ux.ibm.com> wrote:
>> 
>> > On Mon, 2021-09-20 at 00:39 +0200, Halil Pasic wrote:  
>> >> On Fri, 17 Sep 2021 10:40:20 +0200
>> >> Cornelia Huck <cohuck@...hat.com> wrote:
>> >>   
>> > ...snip...  
>> >> > > 
>> >> > > Thanks, if I find time for it, I will try to understand this
>> >> > > better and
>> >> > > come back with my findings.
>> >> > >    
>> >> > > > > * Can virtio_ccw_remove() get called while !cdev->online and 
>> >> > > > >   virtio_ccw_online() is running on a different cpu? If yes,
>> >> > > > > what would
>> >> > > > >   happen then?      
>> >> > > > 
>> >> > > > All of the remove/online/... etc. callbacks are invoked via the
>> >> > > > ccw bus
>> >> > > > code. We have to trust that it gets it correct :) (Or have the
>> >> > > > common
>> >> > > > I/O layer maintainers double-check it.)
>> >> > > >     
>> >> > > 
>> >> > > Vineeth, what is your take on this? Are the struct ccw_driver
>> >> > > virtio_ccw_remove and the virtio_ccw_online callbacks mutually
>> >> > > exclusive. Please notice that we may initiate the onlining by
>> >> > > calling ccw_device_set_online() from a workqueue.
>> >> > > 
>> >> > > @Conny: I'm not sure what is your definition of 'it gets it
>> >> > > correct'...
>> >> > > I doubt CIO can make things 100% foolproof in this area.    
>> >> > 
>> >> > Not 100% foolproof, but "don't online a device that is in the
>> >> > progress
>> >> > of going away" seems pretty basic to me.
>> >> >   
>> >> 
>> >> I hope Vineeth will chime in on this.  
>> > Considering the online/offline processing, 
>> > The ccw_device_set_offline function or the online/offline is handled
>> > inside device_lock. Also, the online_store function takes care of
>> > avoiding multiple online/offline processing. 
>> >
>> > Now, when we consider the unconditional remove of the device,
>> > I am not familiar with the virtio_ccw driver. My assumptions are based
>> > on how CIO/dasd drivers works. If i understand correctly, the dasd
>> > driver sets different flags to make sure that a device_open is getting
>> > prevented while the the device is in progress of offline-ing.   
>> 
>> Hm, if we are invoking the online/offline callbacks under the device
>> lock already, 
>
> I believe we have a misunderstanding here. I believe that Vineeth is
> trying to tell us, that online_store_handle_offline() and
> online_store_handle_offline() are called under the a device lock of
> the ccw device. Right, Vineeth?
>
> Conny, I believe, by online/offline callbacks, you mean
> virtio_ccw_online() and virtio_ccw_offline(), right?

Whatever the common I/O layer invokes.

>
> But the thing is that virtio_ccw_online() may get called (and is
> typically called, AFAICT) with no locks held via:
> virtio_ccw_probe() --> async_schedule(virtio_ccw_auto_online, cdev)
> -*-> virtio_ccw_auto_online(cdev) --> ccw_device_set_online(cdev) -->
> virtio_ccw_online()

That's the common I/O layer in there again?

>
> Furthermore after a closer look, I believe because we don't take
> a reference to the cdev in probe, we may get virtio_ccw_auto_online()
> called with an invalid pointer (the pointer is guaranteed to be valid
> in probe, but because of async we have no guarantee that it will be
> called in the context of probe).
>
> Shouldn't we take a reference to the cdev in probe? BTW what is the
> reason for the async?

I don't know.

>
>
>> how would that affect the remove callback?
>
> I believe dev->bus->remove(dev) is called by 
> bus_remove_device() with the device lock held. I.e. I believe that means
> that virtio_ccw_remove() is called with the ccw devices device lock
> held. Vineeth can you confirm that?
>
>
> The thing is, both virtio_ccw_remove() and virtio_ccw_offline() are
> very similar, with the notable exception that offline assumes we are
> online() at the moment, while remove() does the same only if it
> decides based on vcdev && cdev->online that we are online.
>
>
>> Shouldn't they
>> be serialized under the device lock already? I think we are fine.
>
> AFAICT virtio_ccw_remove() and virtio_ccw_offline() are serialized
> against each other under the device lock. And also against
> virtio_ccw_online() iff it was initiated via the sysfs, and not via
> the auto-online mechanism.
>
> Thus I don't think we are fine at the moment.

I don't understand this, sorry.

>
>> 
>> For dasd, I think they also need to deal with the block device
>> lifetimes. For virtio-ccw, we are basically a transport that does not
>> know about devices further down the chain (that are associated with the
>> virtio device, whose lifetime is tied to online/offline processing.) I'd
>> presume that the serialization above would be enough.
>> 
>
> I don't know about dasd that much. For the reasons stated above, I don't
> think the serialization we have right now is entirely sufficient.

I'm not sure it makes sense to discuss this further right now, I feel I
currently can't really provide any meaningful contribution.

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