linux-kernel - Re: [PATCH] virtio: make PCI devices take a virtio

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Message-Id: <1228489626.3858.37.camel@blaa>
Date:	Fri, 05 Dec 2008 15:07:06 +0000
From:	Mark McLoughlin <markmc@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, Anthony Liguori <aliguori@...ibm.com>,
	Michael Tokarev <mjt@....msk.ru>,
	Jesse Barnes <jbarnes@...tuousgeek.org>
Subject: Re: [PATCH] virtio: make PCI devices take a virtio_pci module ref

Hi Rusty,

On Fri, 2008-12-05 at 10:43 +1030, Rusty Russell wrote:
> On Thursday 04 December 2008 23:14:31 Mark McLoughlin wrote:
> > Nothing takes a ref on virtio_pci, so even if you have
> > devices in use, rmmod will attempt to unload the module.
> > 
> > Fix by simply making each device take a ref on the module.
> 
> Hi Mark,
> 
>    Taking a reference to oneself is almost always wrong.

Yeah, it certainly seems fairly unorthodox, alright. But then again,
virtio_pci is an odd creature anyway :-)

My thinking was that the virtio abstraction is preventing there being an
explicit dependency between e.g. virtio_net and virtio_pci. If we didn't
have the abstraction, virtio_net would be calling directly into
virtio_pci and we'd have an explicit dep. So, I was just trying to
artificially mimic that.

Another example of a lack of an explicit dependency causing problems is
Fedora's mkinitrd having this hack:

    if echo $PWD | grep -q /virtio-pci/ ; then
        findmodule virtio_pci
    fi

which basically says "if this is a virtio device, don't forget to
include virtio_pci in the initrd too!". Now, mkinitrd is full of hacks,
but this is a particularly unusual one.

Í'm thinking that maybe we should default to having virtio_pci built-in
if e.g. CONFIG_KVM_GUEST is set.

> I'm a little surprised that a successful call to pci_device->probe 
> doesn't bump the module count though.

Nah, removing a module for device should actually work fine.

Anyway, with the root cause of Michael's traceback fixed, rmmod-ing
virtio_pci and re-loading it works just fine, so ...

Cheers,
Mark.

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