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

[Cornelia Huck <cohuck@redhat.com>]

On Wed, Sep 15 2021, Halil Pasic <pasic@linux.ibm.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.

> * 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.)

> * 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.)

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

>
> ---
>  drivers/s390/virtio/virtio_ccw.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/drivers/s390/virtio/virtio_ccw.c b/drivers/s390/virtio/virtio_ccw.c
> index d35e7a3f7067..99141df3259b 100644
> --- a/drivers/s390/virtio/virtio_ccw.c
> +++ b/drivers/s390/virtio/virtio_ccw.c
> @@ -1006,10 +1006,12 @@ static void virtio_ccw_release_dev(struct device *_d)
>  {
>  	struct virtio_device *dev = dev_to_virtio(_d);
>  	struct virtio_ccw_device *vcdev = to_vc_device(dev);
> +	struct ccw_device *cdev = READ_ONCE(vcdev->cdev);
>  
>  	ccw_device_dma_free(vcdev->cdev, vcdev->dma_area,
>  			    sizeof(*vcdev->dma_area));
>  	kfree(vcdev);
> +	put_device(&cdev->dev);
>  }
>  
>  static int irb_is_error(struct irb *irb)
> @@ -1262,6 +1264,7 @@ static int virtio_ccw_online(struct ccw_device *cdev)
>  	struct virtio_ccw_device *vcdev;
>  	unsigned long flags;
>  
> +	get_device(&cdev->dev);
>  	vcdev = kzalloc(sizeof(*vcdev), GFP_KERNEL);
>  	if (!vcdev) {
>  		dev_warn(&cdev->dev, "Could not get memory for virtio\n");
> @@ -1315,6 +1318,7 @@ static int virtio_ccw_online(struct ccw_device *cdev)
>  				    sizeof(*vcdev->dma_area));
>  	}
>  	kfree(vcdev);
> +	put_device(&cdev->dev);
>  	return ret;
>  }
>  
>
> base-commit: 3ca706c189db861b2ca2019a0901b94050ca49d8
> -- 
> 2.25.1