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From:   Cornelia Huck <cohuck@redhat.com>
To:     Vineeth Vijayan <vneethv@linux.ibm.com>,
        Halil Pasic <pasic@linux.ibm.com>
Cc:     Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Pierre Morel <pmorel@linux.ibm.com>,
        Michael Mueller <mimu@linux.ibm.com>,
        linux-s390@vger.kernel.org,
        virtualization@lists.linux-foundation.org, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, bfu@redhat.com
Subject: Re: [PATCH 1/1] virtio/s390: fix vritio-ccw device teardown
In-Reply-To: <88b514a4416cf72cda53a31ad2e15c13586350e4.camel@linux.ibm.com>
Organization: Red Hat GmbH
References: <20210915215742.1793314-1-pasic@linux.ibm.com>
 <87pmt8hp5o.fsf@redhat.com> <20210916151835.4ab512b2.pasic@linux.ibm.com>
 <87mtobh9xn.fsf@redhat.com> <20210920003935.1369f9fe.pasic@linux.ibm.com>
 <88b514a4416cf72cda53a31ad2e15c13586350e4.camel@linux.ibm.com>
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Date:   Mon, 20 Sep 2021 12:07:23 +0200
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On Mon, Sep 20 2021, Vineeth Vijayan <vneethv@linux.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@redhat.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, how would that affect the remove callback? Shouldn't they
be serialized under the device lock already? I think we are fine.

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.