linux-kernel - Re: [PATCH] virtio: decrement dev_index when device is unregistered

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Message-ID: <BANLkTi=DGfKuapLhvt62UVG_h7fzsRRo3g@mail.gmail.com>
Date:	Tue, 5 Apr 2011 07:21:56 +0100
From:	Stefan Hajnoczi <stefanha@...il.com>
To:	Takuma Umeya <tumeya@...hat.com>
Cc:	virtualization@...ts.linux-foundation.org,
	Stephen Hemminger <shemminger@...tta.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] virtio: decrement dev_index when device is unregistered

On Tue, Apr 5, 2011 at 5:49 AM, Takuma Umeya <tumeya@...hat.com> wrote:
> When virtio device is removed, dev_index does not get decremented.
> The next device hotplug event results in consuming the next pci to
> the one that is suppose to be available.
>
> Signed-off-by: Takuma Umeya <tumeya@...hat.com>
>
> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> index efb35aa..67fe71d 100644
> --- a/drivers/virtio/virtio.c
> +++ b/drivers/virtio/virtio.c
> @@ -216,6 +216,7 @@ EXPORT_SYMBOL_GPL(register_virtio_device);
>  void unregister_virtio_device(struct virtio_device *dev)
>  {
>        device_unregister(&dev->dev);
> +       dev_index--;

I don't think there is any guarantee that virtio devices are
added/removed in first-in-last-out order.

That means I could add a virtio-net device (index 0) followed by a
virtio-blk device (index 1).  Now I remove the virtio-net device
(index 0) which causes me to decrement dev_index and hand index 1 out
again to the next device.  This leaves us with virtio-blk (index 1)
and the new device with index 1, which is not unique.

Perhaps I missed a constraint which prevents this from occurring?

Stefan
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