Message-ID: <05b1ac0e4aa4a1c7df1a8994c898630e9b2e384d.camel@linux.ibm.com>
Date:   Tue, 21 Sep 2021 15:31:03 +0200
From:   Vineeth Vijayan <vneethv@...ux.ibm.com>
To:     Halil Pasic <pasic@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>
Cc:     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
Subject: Re: [PATCH 1/1] virtio/s390: fix vritio-ccw device teardown

On Tue, 2021-09-21 at 05:25 +0200, Halil Pasic 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?
Yes. I wanted to bring-out both the scenario.The set_offline/_online()
calls and the unconditional-remove call. 
For the set_online The virtio_ccw_online() also invoked with ccwlock
held. (ref: ccw_device_set_online)
> 
> Conny, I believe, by online/offline callbacks, you mean
> virtio_ccw_online() and virtio_ccw_offline(), right?
> 
> 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()
> 
> 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?
We just had a quick look at the virtio_ccw_probe() function.
Did you mean to have a get_device() during the probe() and put_device()
just after the virtio_ccw_auto_online() ?

> reason for the async?
> 
> 
> > 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?
This is what my understanding too.
When we disconnect a working/online device, the CIO layer gets a CRW
which indicates this disconnection. Then the subchannel driver endup
un-registering the ccw-device. This ccw_device_unregister() then
invokes device_del(), which invokes the bus->driver->remove calls which
is called with @dev-lock held.
> 
> 
> 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.
> 
> > 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.
> 
> Regards,
> Halil

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