Re: [KVM PATCH v10] kvm: add support for irqfd

From: Gregory Haskins <ghaskins@novell.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	avi@redhat.com, davidel@xmailserver.org, mtosatti@redhat.com
Subject: Re: [KVM PATCH v10] kvm: add support for irqfd
Date: Wed, 27 May 2009 10:06:50 -0400	[thread overview]
Message-ID: <4A1D48FA.9080403@novell.com> (raw)
In-Reply-To: <20090527130447.GA11643@redhat.com>

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Michael S. Tsirkin wrote:
> On Wed, May 20, 2009 at 10:30:49AM -0400, Gregory Haskins wrote:
>   
>> +static int
>> +kvm_assign_irqfd(struct kvm *kvm, int fd, int gsi)
>> +{
>> +	struct _irqfd *irqfd;
>> +	struct file *file = NULL;
>> +	int ret;
>> +
>> +	irqfd = kzalloc(sizeof(*irqfd), GFP_KERNEL);
>> +	if (!irqfd)
>> +		return -ENOMEM;
>> +
>> +	irqfd->kvm = kvm;
>> +	irqfd->gsi = gsi;
>> +	INIT_LIST_HEAD(&irqfd->list);
>> +	INIT_WORK(&irqfd->work, irqfd_inject);
>> +
>> +	/*
>> +	 * Embed the file* lifetime in the irqfd.
>> +	 */
>> +	file = fget(fd);
>> +	if (IS_ERR(file)) {
>> +		ret = PTR_ERR(file);
>> +		goto fail;
>> +	}
>>     
>
> So we get a reference to a file, and unless the user is nice to us, it
> will only be dropped when kvm char device file is closed?
> I think this will deadlock if the fd in question is the open kvm char device.
>
>
>   
Hmm...I hadn't considered this possibility, though I am not sure if it
would cause a deadlock in the pattern you suggest.  It seems more like
it would result in, at worst, an extra reference to itself (and thus a
leak) rather than a deadlock...

I digress.  In either case, perhaps I should s/fget/eventfd_fget to at
least limit the type of fd to eventfd.  I was trying to be "slick" by
not needing the eventfd_fget() exported, but I am going to need to
export it later anyway for iosignalfd, so its probably a moot point.

Thanks Michael,
-Greg

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