Re: [PATCH 1/1] virtio/s390: fix vritio-ccw device teardown

[Halil Pasic <pasic@linux.ibm.com>]

On Thu, 16 Sep 2021 10:59:15 +0200
Cornelia Huck <cohuck@redhat.com> wrote:

> > Since commit 48720ba56891 ("virtio/s390: use DMA memory for ccw I/O and
> > classic notifiers") we were supposed to make sure that
> > virtio_ccw_release_dev() completes before the ccw device, and the
> > attached dma pool are torn down, but unfortunately we did not.
> > Before that commit it used to be OK to delay cleaning up the memory
> > allocated by virtio-ccw indefinitely (which isn't really intuitive for
> > guys used to destruction happens in reverse construction order).
> >
> > To accomplish this let us take a reference on the ccw device before we
> > allocate the dma_area and give it up after dma_area was freed.
> >
> > Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
> > Fixes: 48720ba56891 ("virtio/s390: use DMA memory for ccw I/O and
> > classic notifiers")
> > Reported-by: bfu@redhat.com
> > ---
> >
> > I'm not certain this is the only hot-unplug and teardonw related problem
> > with virtio-ccw.
> >
> > Some things that are not perfectly clear to me:
> > * What would happen if we observed an hot-unplug while we are doing
> >   wait_event() in ccw_io_helper()? Do we get stuck? I don't thin we
> >   are guaranteed to receive an irq for a subchannel that is gone.  
> 
> Hm. I think we may need to do a wake_up during remove handling.

My guess is that the BQL is saving us from ever seeing this with QEMU
as the hypervisor-userspace. Nevertheless I don't think we should rely
on that. 

> 
> > * cdev->online seems to be manipulated under cdev->ccwlock, but
> >   in virtio_ccw_remove() we look at it to decide should we clean up
> >   or not. What is the idea there? I guess we want to avoid doing
> >   if nothing is there or twice. But I don't understand how stuff
> >   interlocks.  
> 
> We only created the virtio device when we onlined the ccw device. Do you
> have a better idea how to check for that? (And yes, I'm not sure the
> locking is correct.)
> 

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.

> >  
> > The main addresse of these questions is Conny ;).

In any case, I think we can go step by step. I would like the issue
this patch intends to address, addressed first. Then we can think
about the rest.

> >
> > An alternative to this approach would be to inc and dec the refcount
> > in ccw_device_dma_zalloc() and ccw_device_dma_free() respectively.  
> 
> Yeah, I also thought about that. This would give us more get/put
> operations, but might be the safer option.

My understanding is, that having the ccw device go away while in a
middle of doing ccw stuff (about to submit, or waiting for a channel
program, or whatever) was bad before. So my intuition tells me that
drivers should manage explicitly. Yes virtio_ccw happens to have dma
memory whose lifetime is more or less the lifetime of struct virtio_ccw,
but that may not be always the case.

Thanks for your comments!

Regards,
Halil