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

On Thu, Sep 16 2021, Halil Pasic [off-list ref] wrote:

On Thu, 16 Sep 2021 10:59:15 +0200
Cornelia Huck [off-list ref] wrote:

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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
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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.

I agree. Let's do that via a separate patch.

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* 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.

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* 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.

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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.

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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.

What do you mean with "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.

I'm not sure what you're getting at here. Regardless of the lifetime of
the dma memory, it depends on the presence of the ccw device to which it
is tied. This means that the ccw device must not be released while the
dma memory is alive. We can use the approach in your patch here due to
the lifetime of the dma memory that virtio-ccw allocates when we start
using the device and frees when we stop using the device, or we can use
get/put with every allocate/release dma memory pair, which should be
safe for everyone?

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